NATIONAL TRANSMISSION & DESPATCH COMPANY LTD.

GSO TRAINING CENTRE TARBELA PROTECTION & INSTRUMENTATION

1st Semester

By:Muhammad Mustafa Additional Manager Protection & Instrumentation

March, 2013

In the name of Almighty Allah, The most merciful and the most beneficial

Dedicated To:

All those who helped mein preparation of this

book

TABLE OF CONTENTS

1. General 1

1-1 Safety Principles

1-2 Electric Shock Hazards

1.2.1 Shock Current Equation

1.2.2 Effects of Electrical Shock

1.2.3 Path of Shock Current

1-3 FIRST AID

1.3.1 AIMS OF FIRST AID

1.3.2 First Aid for Electric Shock

1.3.3 Artificial respiration

1.3.4 Cardiopulmonary Resuscitation (CPR)

1-4 Low voltage hazards

1-5 Touch and Step Potential

1.6 Grounding & Bonding

1-7 Absolute limits of approach

1.7.1 Position of the Workman

1.7.2 Platforms and Aerial Devices

1-8 Clothing and Use of Personal Protective Equipment (PPE)

1-9 Fire Safety

1.9.1 Fire Extinguishers

1.9.2 In-Plant Training

1.9.3 Extinguisher Maintenance Tips

1.9.4 Hazardous Locations

2. Fundamentals of Electricity 20

2-1 AC and DC Supply

2-2 Types of Load

2-3 Ohm’s Law

2-4 Series and Parallel Combination of Resistors

2.4.1 Resistivity and its Dependence upon Temperature

2.4.2 Color Code for Carbon Resistances

2.4.3 Rheostat

2.4.4 Thermistors

2.4.5 Varastors

2.4.6 Electrical Power and Power Dissipation in Resistors

2-5 Single Phase and Three Phase Supply

2-6 Star and Delta Connections

2-7 Electrostatic and Electromagnetic Induction

3. Basic Concept of an Electrical Power System and Basic Components of Power

System 35

3-1 Power System Concept

3-2 Components of Power System

4. Electrical Measuring Instruments 38

4-1 Galvanometer

4-2 Voltmeter

4-3 Ammeter

4-4 Ohmmeter

4-5 Multi-meters

4-6 Clip-on ammeters

4-7 Cathode Ray Oscilloscopes

4-8 Safety Precautions

4-9 Important Points when Using Insulation Resistance Tester

4.10 Temperature Correction of Ri Readings

5. Basic Requirements Control Circuits 53

5.1 Auxiliary Switches

5.2 Device Function Numbers

5.3 Basic Requirements Control Circuits

5.3.1 Control the Closing5.3.2 Control the Tripping5.3.3 Trip Free Feature5.3.4 Anti-Pumping Feature 5.3.5 Anti-Slam Feature5.3.6 Reliability5.3.7 General Maintenance of Breaker Control Relays5.3.8 ASA Definitions

5.4 Overload Protection5.5 Over Current or Short Circuit Protection 5.6 Contactor5.7 Maintenance of Contactor

6. P&I Tools & Plant (T&P) 60

6.1 P&I Tools

6.1.1 P&I Personnel Tools

6.1.2 P&I Test Equipments

6.2 Shop Rules

6.3 Care and Up Keep of Tools

6.4 Safety Precautions when Applying Voltage from Test Equipment Sources

6.4.1 Testing Apparatus in its In-Service Location6.4.2 Testing in a Location other in the In-Service Position 6.4.3 Signs and Guards Required during Voltage Testing of Apparatus6.4.4 Special Precautions

7. Introduction of Grid Station Main and Auxiliary Equipment 67

7-1 Transformers

7-1-1 Use of Power Transformer

7-1-2 Types of Transformer

7-2 Circuit Breakers

7.2.1 Working Principle of Circuit Breaker

7.2.2 Types of Circuit Breaker

7-3 Disconnect Switches/Isolators

7-4 Lightning Arrester

7-5 Batteries and Battery Chargers

7-6 Station Grounding System

7-7 AC&DC Supply System

7-8 Power and Control Cables

7.8.1 Purposes of Shielding / Shield Grounding

7-9 Bus Bars

7.9.1 Bus Bar Schemes

8. Transformers 79

8.1 Fundamental Theory

8.1.2 Main Constructional Parts of Transformer

8.1.3 Ideal Transformer

8.1.4 Ideal Transformer Model

8.1.5 Theory of Transformer

8.1.6 Equivalent Circuit of Transformer

8.1.7 Losses in Transformer

8.2 Auto Transformer

8.3 Tertiary Winding of Transformer

8.4 Transformer Connections/Transformer Bank Connections, and Winding

Connections/Vector Groups

8.5 Phase sequence

8.6 Parallel operation of transformers

8.7 Transformer Tests

8.7.1 Polarity Test

8.7.2 Insulation Resistance Test

8.7.3 Transformer Turn Ratio Test

8.7.4 Open Circuit Test

8.7.5 Short Circuit Test

8.7.6 Verification of Vector Group

8.8 Cooling Systems

9. Current Transformers 125

9-1 Fundamental Theory

9-2 Types of Current Transformers

9-3 Errors in Current Transformers

9-4 Current Transformers Connections

9-5 Current Transformer Parameters

9-6 Accuracy Limit Factor and Instrument Security Factor of Current Transformer

9-7 Current Transformer Tests

9.6.1 Continuity Test

9.6.2 Insulation Resistance Test

9.6.3 Current Ratio Test

9.6.4 Polarity Test

9.6.5 Saturation Test

9.6.6 Circuit Verification Test

10. Potential transformers (PT) and Capacitor voltage transformers (CVT) 141

10.1 Fundamental Theory

10.2 Types of Potential Transformers

10.3 Errors in Potential Transformers

10.4 Potential Transformer Parameters

10.5 Potential Transformers Connections

10.6 Potential Transformer Tests

10.6.1 Continuity Test

10.6.2 Polarity test

10.6.3 Insulation resistance test

10.6.4 Voltage ratio test

10.6.5 Circuit verification

10.7 Potential Transformer Supply Supervision

11. Introduction to Protection 149

11-1 Introduction

11-2 Purpose of Protection Relaying11-3 Principles of Protection Relaying

11-4 Functions of Protective Relaying

11-5 Protection Equipment

11-6 The Functional Requirements of the Relay

11.6.1 Reliability

11.6.2 Selectivity

11.6.3 Stability

11.6.4 Speed

11.6.6 Sensitivity

11.7 Relaying Terminology

11.7.1 Relaying Operation

11.7.2 Relay Resetting

11.7.3 Relay Pickup, Relay Dropout

11.7.4 Normally Open, Normally Closed Contacts

11.7.5 Pallet Switches

11.7.6 Relay Seal-In

11.7.7 Inverse Time and Definite Time Relays

11.7.8 Relay Target

11.7.9 Reach

11.7.10 Direct Under Reach

11.7.11 Permissive Overreach

11.7.12 Echo

11.7.13 Automatic Reclosing

11-8 Device Numbers and their Universal Nomenclature

11-9 Relay Protective Schemes

12. Over-Current Protection 162

12-1 Over Current Relays

12-2 Types of Over Current Relays

12-3 Operating Principals of Over Current Relays

12-4 Setting Calculations

12-5 Over Current Relay Testing

12-5-1 Pick- up/Drop off

12-5-2 Operating Time

13. Differential Protection 177

13-1 Faults in Power Transformer

13-2 Differential Protection

13-3 Principle of Differential Protection

13-4 Types of differential relays

13-5 Magnetizing Inrush Current

13-6 Balance of Differential Relay for Various Vector Groups

13-7 Differential relay testing

13-7-1 Pickup/ Drop off

13-7-2 Operating time

13-7-3 Percent slope

13-7-4 Percent Second Harmonics

13-8 Practical Connections of Differential on a Power Transformer

14. Under-Frequency relay 185

14-1 Under-Frequency Protection

14-2 Operating Principles

14-4 Under Frequency Relay Testing

14-4-1 Pickup/Drop off

14-4-2 Operating Time

15. Over-fluxing Relay 188

15-1 Causes of Over-fluxing in Transformer

15.2 Effect of Over Fluxing in Transformers

15-3 Operating Principles

16. Trip Circuit Supervision Relay 193

16-1 Trip Circuit Supervision Protection

16-2 Operating Principles

17. Restricted Earth Fault Relay 195

17-1 Restricted Earth fault Protection

17-2 Operating Principles

18. Breaker failure protection 198

18-1 Breaker Fail Protection

18-2 Operating Principles

19. Bus Bar Protection 203

19-1 Introduction

19-1 Bus Bar Faults

19-3 Bus Bar Protection Requirements

19-4 Types of Bus Bar Protection Systems

1.

General

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1.1 THE SAFETY PRINCIPLES

Consider a work place. If, these places look familiar, you are right. However, we are not interested in places, but the people who work in them. People like you and me. This program then, is about safety and how to make it a part of everything you do.

A good place to begin our program is with the Safety Backs or as we will refer to them from now on the “Safety Principles”.

There are five Safety Principles and it is expected at the end of this session that you will be able to: State all five principles. Explain how the principles are applied in the work place. Demonstrate their application in a work assignment selected by your supervisor.

Now, what are the five Safety Principles? Well, here they are:

Safety Principles: 1. Know and identify the hazards.2. Eliminate the hazards wherever practical.3. Control the hazards when they cannot be eliminated.4. Prevent or minimize the injuries when controlling the hazards.5. Minimize the severity of injury if an injury has occurred.

The Safety Principles are logical and straightforward and it is very, very important that they are applied in sequence to every job. The details of each principle are as follows:

Safety Principle - 1: Know and identify the hazard.

What do we mean? Well the requirement here is to ensure that you are knowledgeable of the various hazards in the work place. The physical work environment is designed with the intent of ensuring that unnecessary hazards are not present. However, sometimes it’s just not possible to ensure that any given local work environment is hazard free. It’s of the utmost importance that you be fully aware of the hazards associated with any work you are assigned.

Being knowledgeable of the hazard is not enough you must also be able to identify the hazards. In other words, “knowing the hazard does not necessarily mean that you can identify them”.

Here is an example. You may know that asbestos is a potential hazard, but can you identify asbestos? Probably not!

Although you know that asbestos is a hazard, you may have to get the help of someone qualified to identify it.

No doubt you can think of other hazardous materials which might be difficult to identify. So, identifying the hazard may mean having the hazard identified if you can’t do it yourself.

Safety Principle - 2: Eliminate the hazards wherever practical.

In many cases, it will be practical to eliminate or remove the hazards that have been identified.

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Let’s suppose that you are a mechanical maintainer and you are given a job to replace some tubes in the boiler. The boiler will be shut down, and the tubes allowed cooling and draining. Thus we have in effect eliminated two hazards. They are:

The heat, and The pressurized water in the tubes

Now other hazards may still remain but we eliminated those that were practical to eliminate.

Ok, let’s try it from a different point of view. This time an E&I Maintainer is given a job to change a wall receptacle in the business office. One of the steps the Maintainer will take is to isolate and de-energize the receptacle circuit from the power source. By doing so, he has eliminated the electrical hazard.

Again, other hazards may remain in his immediate work environment but he has eliminated a hazard that was practical to eliminate.

Safety Principle - 3: Control the hazards when they cannot be eliminated.

Control the hazards when they cannot be eliminated. Sometimes it’s not practical to eliminate or remove all the hazards associated with a particular job. When that is the case, you must take the steps to control those hazards that remain.

For example, you are to perform a routine job in an area of the plant where the noise is considered to be excessive.

You have identified the hazard to be noise. In this case elimination was investigated and found to be impractical. So, now you must control the hazard. Period

But how do you control noise? Well control of the hazard can be achieved by following a work procedure that will limit your exposure to the noise to an acceptably safe period of time.

Again take the example of drilling into a piece of steel. Certainly you can’t eliminate the potential hazards due to the mechanical motion of the drill bit or the flying metal particles. However, you can control the hazards by using a proper technical work procedure.

Safety Principle - 4: Prevent or minimize injuries when controlling the hazards.

This principle recognizes the need for a back-up control, measure to support Safety Principle#3. Which is? That’s right, controlling the hazards that cannot be eliminated. What do we mean by back-up control measure?

Well, we can control a noise hazard by ensuring that the time spent in an excessively noisy environment is limited to a safe value, which in essence, is Safety Principle - 3. We can now apply Safety Principle - 4 by making sure that the worker wears adequate hearing protection during his entire stay in the noisy environment. That’s the back-up control measure.

Safety Principle - 5: Minimize the severity of the injury if an injury has occurred.

The principle dictates that you must know what to do to minimize the injury to yourself or a fellow worker after an injury has occurred.

This may require: Applying first aid to the injured person if you are qualified. Getting someone who is qualified to perform first aid to the injured. Providing emergency rescue assistance, if necessary.

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Precisely what needs to be done depends a great deal on the extent of the injury and the consequences of not taking immediate action.

And there you have it! Really what could be more logical and straight forward?

Yes, the five Safety Principles.1. Know and identify the hazards.2. Eliminate the hazards wherever practical.3. Control the hazards when they cannot be eliminated.4. Prevent or minimize the injuries when controlling the hazards.5. Minimize the severity of injury if an injury has occurred.

We expect you to know and apply them on every job. As a matter of fact, you would be wise to apply them off the job as well.

Looking at the safety principles in sequence, which, by the way, is how they must always be applied, you will observe that principles - 1, 2, and 3 concentrate on the identification, elimination and/or control of hazards.

Principle - 4 and 5 are directed toward the minimization of injuries.

You must certainly be aware that your primary safety objective is to prevent human injury. This can be done through hazard identification, elimination and/or control. However, if human injuries do occur, you must be able to minimize those injuries.

The five safety principles are designed to create and maintain a safer working environment for all of us. Learn and apply the “Safety Principles” in everything you do.

1.2 ELECTRICAL SHOCK HAZARD

The basic shock hazards presented by electricity to human being are:1. Physical movement caused by involuntary muscular reactions stimulated by the passage

of current through the body, or reactions caused by the sensation of the passage of current. At high current values a person might be “thrown” from the circuit, at medium currents he may not be able to let go, and at low “perceptible” currents he might pull back or “jump”. Spark discharges, although not dangerous in themselves, can cause involuntary body movement. Unplanned physical movement can cause falls, slips and others injuries involving body mechanics.

2. Actual physical damage to the body caused by the passage of an electrical current. Tissue distraction occurs due to the heat produced by the current flowing through the body resistance.

3. Cassation of the proper functioning of vital organs due to the passage of current through the body.

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All involve the passage of an electric current through the body. Let us first examine, then, how this is likely to happen.

1.2.1 SHOCK CURRENT EQUATION

When a voltage is impressed across any two points of the body, a resultant current will flow between these points. This condition is setup by bodily proximity or contact with an electric circuit or component that is “alive”. A circuit or component that is alive is one that has an electric potential impressed upon it at some level above that of ground potential. The most usual type of contact is between the circuit and ground, i.e. the hand touching a live circuit while standing on the ground. This situation is shown in diagrammatically in Fig (1). The circuit formed is a relatively simple one to which Ohm’s Law can readily be applied. The resultant current that will pass through the body is of vital importance in determining the severity of the damage to the body; therefore, the circuit should be examined with the object of determining this current:

Where, V = Voltage to ground of source

RL = Resistance of circuit

RC = Contact resistance

RB = Resistance of body

RN = Resistance of ground

The source voltage V to ground is readily determined and can be considered to be constant. RL, the resistance of the circuit is constant and can be determined with some difficulty. RN, the resistance of the ground return path is constant and can be determined accurately only with great difficulty. The contact resistance RL is a variable depending upon the area of the contact, the resistance of gloves and soles of shoes, the condition of the skin, i.e. moist or dry. RB, the body resistance varies considerably with the individual, with his condition at the instant of contact, with the magnitude and frequency of the voltage that is applied and with the duration of contact. The body resistance is the greatest unknown in the equation.

It is very difficult to determine the current that will pass through the body for any particular situation; yet, it is the current that determines the severity of the damage of the individual and whether he will live or die.

Unfortunately, the best known factor in our life-or-death equation is the source voltage and, therefore, we commonly talk in terms of voltage and tend to relate it to a measurement of hazard, i.e. “220 000 V circuits are more dangerous than 220 V circuits”. This is because live HV circuits are generally inaccessible and are conspicuously dangerous, i.e. intense electrostatic field warns these approaching, whereas, LV circuits give no warning prior to contact and are generally easily accessible. THIS CAN LEAD ONE TO BE CARELESS AROUND LOWER VOLTAGES. Individuals have been killed by contact with ordinary house circuits of 110 V AC and by electrical apparatus in industry using as 42 V DC.

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Fig (1)

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1.2.2 EFFECTS OF ELECTRICAL SHOCK

The chart in Fig (2) outlines the probable effects of various magnitudes of shock current. Any amount of current over 10 mA (0.01 A) is capable of producing a painful to severe shock. As the current is increased the shock becomes increasingly severe. When a current flows through the body, the muscles in its path tend to contract. This contraction may be so severe that the victim cannot release his grasp on the live circuit.

The fact that the victim cannot let go is very important because in a few seconds blisters will form on the skin at the contact points and the skin resistance reduces substantially thus allowing the current through the body to increase.

At values as low as 20 mA (0.020 A) breathing become labored, finally ceasing completely at values even below 75 mA (0.75 A).

At values approaching 100 mA (0.10 A) ventricular fibrillation of the heart occurs. This is an uncoordinated twitching of the walls of the heart’s ventricles; in this condition the heart is not pumping and death will most certainly occur unless the victim receives specialized medical treatment which is not readily available.

Strangely enough, 200 mA (0.2 A) the muscular contractions are so severe that the heart is forcibly clamped during the period that the current is flowing. This clamping action prevents the heart from going into ventricular fibrillation and the victim has a chance for a survival.

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Fig (2)

1.2.3 PATH OF SHOCK CURRENT

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The path of the current through the body as well as the magnitude of the current is important factors in shock damage. For instance, if the current does not pass through any vital organs, the victim will no doubt be somewhat better off. The current limiting effect of the outer skin can be considerably greater on a calloused palm than a sensitive area on another part of the body. The outer layer of the skin provides most of the body’s current limiting resistance. Between the ears, for example, the internal resistance (less the skin resistance) is only about 100 ohms, while from hand to foot it is about 500 ohms. The outer skin resistance may vary from 1000 ohms for wet skin to over 1000, 000 ohms for dry skin.

1.3 FIRST AID

First aid is the provision of initial care for an illness or injury. It is usually performed by non-expert, but trained personnel to a sick or injured person until definitive medical treatment can be accessed. Certain self-limiting illnesses or minor injuries may not require further medical care past the first aid intervention. It generally consists of a series of simple and in some cases, potentially life-saving techniques that an individual can be trained to perform with minimal equipment.

While first aid can also be performed on all animals, the term generally refers to care of human patients.

1.3.1 AIMS OF FIRST AID

The key aims of first aid can be summarized in three key points:

Preserve life: The overriding aim of all medical care, including first aid, is to save lives Prevent further harm: Also sometimes called prevent the condition from worsening, or

danger of further injury, this covers both external factors, such as moving a patient away from any cause of harm, and applying first aid techniques to prevent worsening of the condition, such as applying pressure to stop a bleed becoming dangerous.

Promote recovery: First aid also involves trying to start the recovery process from the illness or injury, and in some cases might involve completing a treatment, such as in the case of applying a plaster to a small wound

First aid training also involves the prevention of initial injury and responder safety, and the treatment phases.

1.3.2 FIRST AID FOR ELECTRIC SHOCK

Shock is a common occupational hazard associated with working with electricity. A person who has stopped breathing is not necessarily dead but is in immediate danger. Life is dependent on oxygen, which is breathed into the lungs and then carried by the blood to each and every body cell. Since body cells cannot store oxygen and since the blood can hold only a limited amount (and only for a short time), death will surely result from continued lack of breathing.

However, the heart may continue to beat for some time after breathing has stopped, and the blood may still be circulated to the body cells. Since the blood will, for a short time, contain a

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small supply of oxygen, the body cells will not die immediately. For a very few minutes, there is some chance that the person's life may be saved.

The only logical, permissible delay is that required to free the victim from contact with the electricity in the quickest, safest way. This step, while it must be taken quickly, must be done with great care; otherwise, there may be two victims instead of one.

In the case of portable electric tools, lights, appliances, equipment, or portable outlet extensions, the victim should be freed from contact with the electricity by turning off the supply switch or by removing the plug from its receptacle. If the switch or receptacle cannot be quickly located, the suspected electrical device may be pulled free of the victim. Other persons arriving on the scene must be clearly warned not to touch the suspected equipment until it is reenergized.

The injured person should be pulled free of contact with stationary equipment (such as a bus bar) if the equipment cannot be quickly reenergized or if the survival of others relies on the electricity and prevents immediate shutdown of the circuits. This can be done quickly and easily by carefully applying the following procedures:

1. Protect yourself with dry insulating material.2. Use a dry board, belt, clothing, or other available nonconductive material to free the

victim from electrical contact. DO NOT touch the victim until the source of electricity has been removed.

Once the victim has been removed from the electrical source, it should be determined whether the person is breathing. If the person is not breathing, a method of artificial respiration is used.

1.3.3 ARTIFICIAL RESPIRATION

The process by which a person who has stopped breathing can be saved is called artificial respiration (ventilation). The purpose of artificial respiration is to force air out of the lungs and into the lungs, in rhythmic alternation, until natural breathing is reestablished. Records show that seven out of ten victims of electric shock were revived when artificial respiration was started in less than three minutes. After three minutes, the chances of revival decrease rapidly.

Artificial respiration should be given only when the breathing has stopped. Do not give artificial respiration to any person who is breathing naturally. You should not assume that an individual who is unconscious due to electrical shock has stopped breathing. To tell if someone suffering from an electrical shock is breathing, place your hands on the person's sides at the level of the lowest ribs. If the victim is breathing, you will usually be able to feel movement.

Once it has been determined that breathing has stopped, the person nearest the victim should start

the artificial respiration without delay and send others for assistance and medical aid.

Practical demonstration will be made by using SCHAFER method of artificial respiration

practice received by an electrical short victim.

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1.3.4 CARDIOPULMONARY RESUSCITATION (CPR):

Sometimes victims of electrical shock suffer cardiac arrest or heart stoppage as well as loss of breathing. Artificial respiration alone is not enough in cases where the heart has stopped. A technique known as CPR has been developed to provide aid to a person who has stopped breathing and suffered a cardiac arrest. Because you are working with electricity, the risk of electrical shock is higher than in other occupations. You should, at the earliest opportunity, take a course to learn the latest techniques used in CPR. The techniques are relatively easy to learn and are taught in courses available through the Civil Defense Organization.

A heart that is in fibrillation cannot be restricted by closed chest cardiac massage. A special device called a defibrillator is available in some medical facilities and ambulance services.

Muscular contractions are so severe with 200 mA and over that the heart is forcibly clamped during the shock. This clamping prevents the heart from going into ventricular fibrillation, making the victim's chances for survival better.

1.4 LOW VOLTGE HAZARD

Signs are often placed on electrical equipment displaying the words “Danger – High – Voltage”. In our minds, we would do well to register another sign “Danger – Low – Voltage!” From experience, the victims of high voltage shock usually respond to artificial respiration more readily than to the victims of low voltage shock. The only conclusion to be drawn here is that 220 V can be just as lethal as 11,000 V. Why Low Voltage is considered more dangerous than H.V? Generally electric hazard occurs due to low voltage, rather than high voltage because high voltages are inaccessible as they give warning to approaching persons prior to contact due to their electrostatic fields. But low voltages give no such warning and are felt only when they are approached or touched. Therefore, low voltages are considered more dangerous than high voltages.

1.5 TOUCH AND STEP POTENCIALS

Fig (3A) and (3B) illustrate the potential gradient that can exist along the earth during heavy fault currents into a ground rod (electrode).

The heavy faults currents flowing down through the portable-temporary grounds cause the tower, the ground rod and the earth in its immediate vicinity to rise well above ground potential. Since the earth some distance away remains at normal ground potential a voltage gradient exists across the surface of the earth in the immediate area.

In Fig (3A) touch potential hazard would be experienced by man ‘A’. It is the voltage experienced by a person standing on earth and touching the structure, while a ground fault is occurring.

The man ‘B’ in this figure would experience step potential hazard. The situation is much the same as that described for touch potential, except in this case the person experiences the potentials from foot to foot when he straddles between the ground grid and the earth.

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If man ‘A’ was standing on a ground gradient mat fastened to the ground rod or tower, no potential difference would exist between his hands and feet and he would be protected during the fault condition.

Furthermore, if a ground grid system with a perimeter extending beyond the work area, such as used in station ground networks, replaced the single ground rod, both man ‘A’ and ‘B’ would be protected.

Station designers consider “touch and step potential” hazards in station ground network design and incorporate gradient control methods into individual station network design.

A well designed station ground network provides low overall impedance to ground, a current carrying capacity consistent with fault currents obtainable and a uniform or near uniform potential of all earth surfaces within its perimeter during heavy fault current conditions.

Fig (3)

1.6 GROUNDING AND BONDING

The distinction between “grounding,” to clear the fault, and “bonding”, to save the man, can be illustrated by referring to Fig (4). For the purpose of illustration only, one phase on a three-phase circuit is shown; the other two phases would be similarly grounded. The work includes checking the HV bushing connection, the LV bushing connection, the disconnect switch, and the bus connector which will not be opened.

The portable-temporary ground on the HV side will provide a high capacity circuit to station ground, and as well bond the workman because the transformer case he is standing on is also solidly fastened to station ground, The same applies to the LV bushing connection, to the bus and the blade side of the disconnect switch. The third ground on the jaw, side of the switch provides both grounding and bonding for the man on the switch structure and bus are grounded to station ground.

If the circuit becomes accidentally energized, the portion of the fault current which returns to its source through the ground system and the earth would cause a potential difference between the station ground networks including everything connected to it and the surrounding earth. This potential difference could achieve several thousand volts for the duration of the fault.

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However, since the station ground network is designed so there will be no appreciable difference of potential in its various parts during the fault conditions, the greatest voltage gradient will exist in the immediate vicinity of the buried conductors around the perimeter of the ground network where the concentration of current is highest.

This gradient will produce a potential difference between the scaffold, touching the earth, and the grounded bus above it. For this reason, the man on the scaffold must be bonded by connecting the scaffold to either the bus or station ground by means of a portable-temporary ground. The safety of this man is ensured by bonding the scaffold he is standing on to the bus he is touching. The man is “outside” the grounds, but he is safely bonded and the circuit is bonded.

Fig (4)1.7 ABSOLUTE LIMITS OF APPROACH

Of prime importance is the maintenance of the absolute limits of approach in using live line tools. This is defined here as the length of clean dry epoxiglas insulation (or polypropylene rope on the mannen stick) between the live component of the tool attached to the conductor and the closest part of the person using the tool. The tools selected must be at least long enough to ensure that these clearances are maintained. The table of absolute limits is shown hereunder as in Table “A”.

Table A

Nominal Voltage Range Absolute Limit of Approach

750 up to 15,000 0.31 m (1foot) Over 15,000 up to 50,000 0.46 m (1.5 feet) Over 50,000 up to 150,000 0.92 m (3 feet) Over 150,000 up to 250,000 1.22 m (4 feet)Over 250,000 up to 550,000 2.75 m (9 feet)

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Caution: It is absolutely necessary to consider other live equipment or circuits that may be in the work area when selecting live line tools and planning the work procedures. Additional clearance should always be provided to allow for unplanned or inadvertent movement.

1.7.1 POSITION OF THE WORKMAN

Usually the workman will be standing on a grounded surface and be at ground potential when performing live line operations. He should position himself in a convenient location where he has good footing, easy access to the work and from where he can maintain, at all times, the absolute limits of approach to all live parts in the work area.

In many station situations; accesses to equipment is difficult or awkward; work clearances are limited and the fault current capability is very high. Live line operations in these circumstances must be strictly limited to those which are absolutely essential and every precaution must be taken not only for electric shock hazard protection but also to avoid an arc from forming, which could have disastrous results.

1.7.2 PLATFORMS AND AERIAL DEVICES

When working from a non-insulated work platform including a ladder truck, the same general principles will apply in the use of live line tools. Additional hazards exist, however, in these circumstances, particularly in high voltage switchyards.

The work platform may be less stable and provide poorer footing than when standing on the ground or belted onto a structure. Additional care must be taken to achieve firm balance and control of the live line tools by the correct positioning of the workman with respect to the work platform and the conductor being worked on.

In addition, the workmen will likely be within the influence of the HV electrostatic field of the circuit being worked on of or a neighboring circuit. The capacitive coupling effect will tend to cause his body to become charged at a voltage somewhat above ground potential. The resulting shocks that he feels when his body comes in contact with ground potential, although not dangerous in themselves, could be seriously distracting or even cause him to lose his balance. To avoid this hazard, the workman should wear approved conductive boots and ensure that the metal surface that he is standing on is grounded, thus continuously draining the charge that tends to build up in his body.

When the absolute limit of approach to high voltage conductors is maintained, the electrostatic field is not strong enough to cause a continuous current flow through his body to ground, to exceed the “perception threshold” value (the minimum continuity with a feeling) provided that the worker maintains continuity with a grounded surface.

15

There are two very important factors to consider when positioning the Ariel device:

It must be positioned so that those working from it will maintain the absolute limits of approach as described above and, in addition, it is also necessary to maintain more stringent clearances between the aerial devices itself and all live circuits. These further restrictions are outlined in Table B.

Table-B

Nominal Voltage Range Limit of Approach750 up to 15,000 0.92 m (3 feet)Over 15,000 up to 50,000 1.22 m (4 feet)Over 50,000 up to 150,000 2.44 m (8 feet)Over 150,000 up to 250,000 3.05 m (10 feet)Over 250,000 up to 550,000 4.58 m (15 feet)

Station crews also use insulated aerial devices or bucket trucks. They are used primarily as work

platforms to gain to access to station structures and equipment. Table B does not apply to the use

of insulated bucket trucks. The general rule for work in stations is THE ABSOLUTE LIMITS

OF APPROACH (see table A) FROM THE LIVE APARATUS TO THE MAN IN THE

BUCKET SHALL BE MAINTAINED AT ALL TIMES.

Exception might be made from time to time in specific instances for men who receive

specialized training and become qualified to work within these distances, e.g. bare hand work.

The continuous current flow through the body due to electrostatic induction increases as one

comes closer to an energized high voltage conductor. Special techniques must sometimes be

employed to continuously bypass this current around the worker or shield him from the

electrostatic field. This effect becomes much stronger as the voltage of the circuit increases.

1.8 CLOTHING AND USE OF PERSONAL PROTECTIVE EQUIPMENT (PPE)

Clothing should fit snugly to avoid danger of becoming entangled in moving machinery or creating a tripping or stumbling hazard. See Fig (5). Clothing should fit snugly to avoid danger of becoming entangled in moving machinery or creating a tripping or stumbling hazard.

16

Fig (5)

Recommended safe work clothes include:

1. Thick-soled work shoes for protection against sharp objects such as nails. Wear work with safety toes if the job required. Make sure the soles are oil resistant if the shoes are subject to oil and grease.

2. Rubber boots for damp locations3. A hat or cap. Wear an approved safety helmet (hard hat) if the job requires

Do not wear Tie, confine long hair or keep hair trimmed and avoid placing the head in close proximity to rotating machinery. Do not wear jewelry. Gold and silver are excellent conductors of electricity.

1.9 FIRE SAFETY

The chance of fire is greatly decreased by good housekeeping. Keep rags containing oil, gasoline, alcohol, shellac, paint, and varnish in a covered metal container. Keep debris in a designated area away from the building. Sound an alarm if a fire occurs. Alert all workers on the job and then call the fire department. After calling the fire department, make a reasonable effort to contain the fire.

1.9.1 FIRE EXTINGUISHERS

Always read instructions before using a fire extinguisher.

Always use the correct fire extinguisher for the class of fire.

See Fig (6). Fire extinguishers are normally red. Fire

17

extinguishers may be located on a red background so they can be easily located. Always use the

correct fire extinguisher for the class of fire.

Be ready to direct firefighters to the fire. Inform them of any special problems or conditions that

exist, such as downed electrical wires or leaks in gas lines. Report any accumulations of rubbish

or unsafe conditions that could be fire hazards. Also, if a portable tool bin is used on the job, a

good practice is to store a CO2 extinguisher in it.

1.9.2 IN-PLANT TRAINING Fig (6)

A selected group of personnel (if not all personnel) should be acquainted with all extinguisher

types and sizes available in a plant or work area. Training should include a tour of the facility

indicating special fire hazard operations.

In addition, it is helpful to periodically practice a dry run, discharging each type of extinguisher.

Such practice is essential in learning how to activate each type, knowing the discharge ranges,

realizing which types are affected by winds and drafts, familiarizing oneself with discharge

duration, and learning of any precautions to take as noted on the nameplate.

1.9.3 EXTINGUISHER MAINTENANCE TIPS

Inspect extinguishers at least once a month. It is common to find units that are missing, damaged,

or used. Consider contracting for such a service. Make contract for annual maintenance with a

qualified service agency. Never attempt to make repairs to extinguishers. This is the chief cause

of dangerous shell ruptures.

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1.9.4 HAZARDOUS LOCATIONS

The use of electrical equipment in areas where explosion hazards are present can lead to an

explosion and fire. This danger exists in the form of escaped flammable gases such as naphtha,

benzene, propane, and others. Coal, grain, and other dust suspended in air can also cause an

explosion. Any hazardous location requires the maximum in safety and adherence to local,

provincial, and federal guidelines and laws, as well as in-plant safety rules.

To sum it all up:

“Working with electricity can be dangerous. However,

electricity can be safe if properly respected”

So Be Careful Out There!

19

2.

Fundamental of

Electricity

20

2.1 DC AND AC SUPPLY

Electricity is a form of energy called Electrical Energy. There are two types of electricity, Static and Dynamic. Dynamic Electricity can be either Direct Current (DC) or Alternating Current (AC).

STATIC ELECTRICITY

When two non-conductors such as silk cloth and glass rod are rubbed together, some electrons are freed. Both materials become electrically charged. One is lacking electron and is positively charged. The other has extra electrons and is negatively charged. These charges remain on the surface of the material and do not move unless the two materials touch or are connected by a conductor. Since there is no electricity flow, this is called Static Electricity.

DYNAMIC ELECTRICITY

When electrons are freed from their atoms and flow in a material, this is called dynamic electricity.

DC SUPPLY

If the free electrons flow in one direction, the electricity is called Direct Current (DC). This is the type of current produced by the vehicle’s battery. So an electrical supply whose amplitude remains constant with respect to time is called Direct Current supply. Voltages and currents remain constant over time (subject to ripple & transients).

AC SUPPLY

If the free electrons change direction from one positive to negative and back repeatedly with time, the electricity is called alternating Current (AC). This is the type of current produced by the vehicle’s alternator. So an electrical supply whose amplitude varies with respect to time and

repeats its shape is called alternating current supply. Voltages and currents are sinusoidal (subject to higher frequency harmonics and transients). In Pakistan, all AC has a fixed frequency of 50 Hz.

2.2 TYPES OF LOAD

Loads are classified as follows:

TYPES BY NATURE

21

Resistive Inductive Capacitive

TYPES BY CONNECTIONS Single Phase Poly Phase

2.3 OHM’S LAW

When a battery is connected across a conductor, an electric current begins to flow through it.

How much current flows through a conductor when a certain potential difference is set up across

its ends?

The answer to this question was given by German Physicist George Simon Ohm. He showed by

elaborate experiments that the current through a metallic conductor is directly proportional to the

potential difference across its ends. This fact is known as Ohm’s law which states that

“The current flowing through a conductor is directly proportional to the potential difference

applied across its ends provided the physical state such as temperature etc. is kept constant”.

Symbolically Ohm’s law can be written as

I α V

It implies that V=IR (1)

Where R is the constant of proportionality, is called the resistance of the conductor. The value of

the resistance depends upon the nature of the conductor, dimensions and the physical state of the

conductor. In fact the resistance is the measure of the opposition to the motion of electrons due to

their continuous bumping with lattice atoms. The unit of resistance is ohm. A conductor has a

resistance of one ohm if a current of one ampere flow through it when a potential difference of

one volt is applied across its ends. The symbol of ohm is Ω. If I is measured in ampere, V in

volts, then R is measured in ohms i.e.

R (ohms) = V (volts) / I (amperes)

A sample of conductor is used to obey Ohm’s law if its resistance R remains constant that is,

graph of its V verses I is exactly a straight line Fig (1). A conductor which strictly obeys Ohm’s

22

law is called Ohmic Conductor. However, there are certain devices, which do not obey Ohms

law i.e., they are Non-Ohmic Conductors. The examples of such devices are filament of a bulb

and semiconductor diodes.

Let us apply a certain potential difference across the terminals of a filament lamp and measure

the resulting current passing through it. If we repeat the measurement for the different values of

potential difference and draw a graph of voltage V verses current I, it will be seen that the graph

is not a straight line Fig (2). It means that a filament is a non- Ohmic device. This deviation of

I-V graph from a straight line is due to increase in resistance of the filament with temperature-a

topic which will be discussed in next section.

As the current passing through the filament is increased from zero, the graph is a straight line in

the initial stage because the change in the resistance of the filament with temperature due to

small current is not appreciable. As the current is further increased, the resistance of the filament

continues to increase due to rise in its temperature.

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Fig (2)

Another example of non-Ohmic device is a semiconductor diode. The current-voltage plot of

such a diode is shown in Fig (3). The graph is not a straight line so semiconductor is also non-

Ohmic device.

2.4 SERIES AND PARALLEL COMBINATION OF RESISTORS

In an electric circuit, usually, a number of resistors are connected together. There are two

arrangements in which resistors can be connected with each other; one is known as series

arrangement and other one as parallel arrangement.

If the resistors are connected end to end such that the same current flow through all of them, they

are said to be connected in series as shown in Fig (4).

24

There equivalent resistance Re is given by

Re=R1 + R2 + R3 + ------ (2)

In parallel arrangement a number of resistors are

connected side by side with their ends joined

together at two common points as shown in Fig (5). Fig (4)

The equivalent resistance Re of the arrangement is given by

1/Re= 1/R1+1/R2+1/R3+ ------ (3)

Fig (5)

2.4.1 RESISTIVITY AND ITS DEPENDENCE UPON TEMPERATURE

It has been experimentally seen that the resistance R of a wire is directly proportional to its

length L and inversely proportional to its cross sectional area A. Expressing mathematically we

have

R α L/A

or R=ρL/A (4)

Where ρ is the constant of proportionality, known as resistivity or

specific resistance of the material of the wire. It may be noted that

resistance is the characteristic of a particular wire whereas the

resistivity is the property of the material of which the wire is made.

From Equation (4), we have

25

ρ = RA/L (5)

The above equation gives the definition of resistivity as the resistance of a meter cube of a

material. The SI unit of resistivity is ohm-meter (Ω-m)

Conductance is another quantity used to describe the electrical properties of materials. In fact

conductance is the reciprocal of resistance i.e.

Conductance = 1/Resistance

The SI unit of conductance is mho or siemens.

Likewise conductivity, σ is the reciprocal of resistivity i.e.

σ= 1/ρ Table (1)

The SI unit of conductivity is ohm_1 m_1 or mho m_1. Resistivity of various materials is given in

Table (1).

It may be noted from Table (1) that silver and copper are two best conductors. That is the reason

that most electric wires are made of copper.

The resistivity of a substance depends upon the temperature also. It can be explained by recalling

that the resistance offered by a conductor to the flow of electric current is due to collisions,

which the free electrons encounter with atoms of lattice. As the temperature of the conductor

rises, the amplitude of vibrations of the atoms in the lattice increases and hence, the probability

of their collisions with free electrons also increases. One may say that the atoms then offer a

bigger target, i.e., the collision cross-section of the atoms increases with temperature. This makes

the collisions between the free electrons and the atoms of the lattice more frequent and hence, the

resistance of the conductor increases. Experimentally the change in resistance of a metallic

conductor with temperature is found to be nearly linear over a considerable range of temperature.

26

Above and below 0oC as shown in Fig (6).

Over such a range the fractional change in resistance per Kelvin is known as the temperature

coefficient of resistance i.e.

α = (Rt – Ro)/ Rot (6)

Where Ro and Rt are resistances at temperature 0oC and toC. As resistivity ρ depends on the

temperature, Eq. (6) gives

Rt=ρL/A and Ro=ρL/A

Substituting the values of Rt and Ro in Eq. (6) we get

As α = ρt – ρo/ ρot (7)

Where ρo is the resistivity of the conductor at 0oC and ρt is the resistivity at toC.

There are some substances like silicon, germanium etc., whose resistance decreases with increase

in temperature i.e., these substances have negative temperature coefficients.

2.4.2 COLOR CODE FOR CARBON RESISTANCES

Carbon resistors are more common in electronic equipments. These consists of high grade

ceramic rod or cone (called the substrate) on which is deposited resistive thin film of carbon. The

numerical values of their resistances are indicated by a color code which consists of bands of

different colors printed on the body of the resistor. The color used in this code and the digit

represented by them are given in Table (2).

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Usually the code consists of four bands, as shown in Fig (7). Starting from left to right, the color bands are interpreted as follows:

1. The first band indicates the first digit in the numerical value of the resistance.

2. The second band gives the second digit.

3. The third band is decimal multiplier i.e., it gives the number of zeroes after two digits.

4. The fourth band gives resistance tolerance. Its color is either gold or silver.

Silver band indicates tolerance of 10%, a gold shows a tolerance of 5%. If there is no fourth band, tolerance is understood to be 20%. By tolerance, we mean the possible variation from the marked value. For example, a 1000 resistor with a tolerance of 10% will have an actual resistance anywhere between 900Ω and 1100Ω.

Fig (7)

Color Value

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Gray 8

White 9

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Table (2)

2.4.3 RHEOSTAT

It is a wire wound variable resistance. It consists of a bare Manganine wire wound over an

insulating cylinder. The ends of the wire are connected to two fixed terminals A and B as in Fig

(8). A third terminal is connected to a sliding contact C which can also be moved over the wire.

A rheostat can be used as a variable resistor as well as potential divider. To use it as a variable

resistor one of the fixed terminals such as A and the sliding terminal C is used. If the sliding

contact is shifted away from the terminal A, the length and hence the resistance included in the

circuit is increased and vice versa.

Fig (8)

A rheostat is also used as potential divider. This is illustrated in Fig (9). A potential difference

V is applied across the ends A and B of the rheostat with the help of a battery. If R is the

resistance of the wire AB, the current I passing through it is given by I=V/R.

Fig (9)

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The potential difference between the portions BC of the wire AB is given by

VBC = current X resistance

= (V/R) r = (r/R) V (8)

Where r, is the resistance of the portion BC of the wire. The circuit shown in the Fig (8) is

known as potential divider. Eq. (8) shows that this circuit can provide at its output terminals a

potential difference varying from zero to the full potential difference of the battery depending on

the position of the sliding contact. As the sliding contact C is moved towards end B, the length

and hence the resistance r of the portion of the wire decreases which according to Eq. (8),

decreases VBC. On the other hand if the sliding contact C is moved towards the end A, the output

voltage VBC increases.

2.4.4 THERMISTORS

A thermistor is a heat sensitive resistor. Most thermistors have negative temperature of resistance

i.e. the resistance of such thermistors decreases with increase in temperature. Thermistors with

positive temperature of coefficient are also available.

Thermistors are made by heating under high pressure semiconductor ceramic made from

mixtures of metallic oxides of manganese, nickel, copper, cobalt, iron etc. these are pressed into

desired shapes and then baked at high temperatures. Different types of thermistors are shown in

Fig (10) they may be in the form of beads, rods or washers.

Fig (10)

Thermistors with high negative temperature of resistance are very accurate for measuring low

temperatures especially near 10K. The higher resistance at low temperature enables more

accurate measurement possible.

Thermistors have wide applications as temperature sensors i.e., they convert changes of

temperature into electrical voltages which are duly processed.

30

2.4.5 VARISTORS

A varistor is an electronic component with diode-like non-linear voltage-current characteristics. The name is a portmanteau of variable resistor. Varistors are often used to protect circuits against excessive transient over-voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from sensitive components. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive.

Only non-ohmic variable resistors are usually called varistors. Other, ohmic types of variable resistor include the potentiometer and the rheostat.

The most common type of varistor is the metal-oxide varistor (MOV). This contains a ceramic mass of Zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a minute current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and Electron Tunneling, and a large current flow. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

A varistor remains non-conductive as a shunt-mode device during normal operation when the voltage across it remains well below its "clamping voltage", so varistors are typically used to suppress line voltage surges. However, a varistor may not be able to successfully limit a very large surge from an event such as a lightning strike where the energy involved is many orders of magnitude greater than it can handle. Follow-through current as a result of a strike may generate excessive current that completely destroys the varistor. Lesser surges still degrade it, however. Degradation is defined by manufacturer's life-expectancy charts that relate current, time and number of transient pulses. The main parameter affecting varistor life expectancy is its energy (Joule) rating. As the energy rating increases, its life expectancy typically increases exponentially, the number of transient pulses that it can accommodate increases and the "clamping voltage" it provides during each transient decreases.

The probability of catastrophic failure can be reduced by increasing the rating, either by using a single varistor of higher rating or by connecting more devices in parallel. A varistor is typically deemed to be fully degraded when its "clamping voltage" has changed by 10%. In this condition it is not visibly damaged and it remains functional (no catastrophic failure).

In general, the primary case of varistor breakdown is localized heating caused as an effect of thermal runaway. This is due to a lack of conformity in individual grain-boundary junctions,

31

which leads to the failure of dominant current paths under thermal stress. If the energy in a transient pulse (normally measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This (catastrophic) failure occurs when "Absolute Maximum Ratings" in manufacturer's data-sheet are significantly exceeded.

HIGH VOLTAGE VARISTOR

Important parameters are the varistor's energy rating in joules, operating voltage, response time, maximum current, and breakdown (clamping) voltage. Energy rating is often defined using standardized transients such as 8/20 microseconds or 10/1000 microseconds, where 8 microseconds is the transient's front time and 20 microseconds is the time to half value. To protect communication lines (such as telephone lines) transient suppression devices such as 3 mil carbon blocks (IEEE C62.32), ultra-low capacitance varistors or avalanche diodes are used. For higher frequencies such as radio communication equipment, a gas discharge tube (GDT) may be utilized.

A typical surge protector power strip is built using MOVs. The cheapest kind may use just one varistor, from hot (live, active) to neutral. A better protector would contain at least three varistors; one across each of the three pairs of conductors (hot-neutral, hot-ground, neutral-ground). A power strip protector in the United States should have a UL1449 3rd edition approval so that catastrophic MOV failure would not create a fire hazard.

2.4.6 ELECTRICAL POWER AND POWER DISSIPATION IN RESISTORS

Consider a circuit consisting of a battery E connected in series with a resistance R Fig (11).

Fig (11)

A steady current flow through the circuit and a steady potential difference V exists between the

terminals A and B of the resistor R. terminal A, connected to the positive pole of the battery, and

is at higher potential than the terminal B. In this circuit the battery is continuously lifting charge

32

uphill through the potential difference V. using the meaning of potential difference, the work

done in moving a charge ΔQ up through the potential difference V is given by

Work done =ΔW= V X ΔQ (9)

This is the energy supplied by the battery. The rate at which the battery is supplying energy is the

power output or electrical power of the battery. Using the definition of power, we have

Electrical Power = Energy supplied/Time taken = V (ΔQ/ΔT) (10)

Since I= ΔQ/ΔT (11)

Electrical power = V x I (12)

Eq. (12) is a general equation relation for power delivered from a source of current I operating on

a voltage V. in the circuit shown in Fig (11) the power supplied by the battery is expended or

dissipated in the resistor R. the principle of the conservation of electrical energy tells us that the

power dissipated in the resistor is also given by Eq. (12)

Power dissipated (P) = V x I (13)

Alternative equation for calculating power can be found by substituting V=IR, I=V/R in turn in

Eq. (13)

P=V x I = IR x I = I2R

P = V x I = V x (V/R) = V2/R

Thus we have three equations for calculating the power dissipated in resistor.

P= V x I = I2R = V2/R (14)

If V is expressed in volts and I in amperes, the power is expressed in Watts.

2-5 SINGLE PHASE AND THREE PHASE SUPPLY

SINGLE PHASE SUPPLY

It is that system in which a single phase and two wire system is used. The first wire is used for

phase and the second wire is used for the completion of current path.

33

THREE PHASE SUPPLY

It is that system in which three phases are used instead of a single phase system. Such systems

are used where bulk load is available and more rotational torque is needed.

2-6 STAR AND DELTA CONNECTIONS

Three single phases are connected together in such a manner that either we get a Delta (Δ) or

Star (Y) connection.

DELTA (Δ) CONNECTION

In a three phase system, three single phases are connected in such a manner that the head of one

phase is joined with the tail of the other and so on such that all the three phases are connected

together and finally we get a closed loop like a delta. Supply is taken from the three corners of

the delta. The only constraint with such a system is that it cannot be use for single phase loads.

In a Delta Supply System:

VLL = VPh

IL = √3 IPh

STAR (Y) CONNECTION

In a three phase system, three single phases are connected in such a manner that the heads or tails

of all the single phases are connected together and finally we get a common point called Neutral

or Star point and three branches. Three phase load is connected between the three branches and

single phase load is connected between any phase and the Neutral point.

In a Star Supply System:

34

VLL =√3 VPh

IL = IPh

2-7 Electrostatic and Electromagnetic Induction

As soon as Oersted discovered that electric currents produce magnetic fields, many scientists

began to look for the reverse effect, that is, to cause an electric current by means of a magnetic

field. In 1831, Michel Faraday in England and at the same time Joseph Henry in USA observed

that an EMF is set up in a conductor when it moves across a magnetic field. If the moving

conductor was connected to a sensitive galvanometer, it would show an electric current flowing

through the circuit as long as the conductor is kept moving in the magnetic field. The EMF

produced in the conductor is induced EMF, and the current generated is called the induced

current. This phenomenon is known as electromagnetic induction.

35

3.

Basic Concept of an

Electrical Power System

36

and Basic Components

of Power System

3.1 POWER SYSTEM CONCEPT

In a simplest power system, a generator and load is required. The need of transferring power

from generating station to load centre requires an additional transmission system of suitable

voltage level.

Interaction of these power components including generation, transmission and distribution give

rise to a power system. This may be a three phase balanced system energized at different voltage

levels for a convenience of maximum availability, minimum losses and safety of the system. In

an AC power system active power always flow from a leading power angle towards a lagging

power angle.

Generation power is transmitted to the load obeying power equation.

P = (V1 V2 Sin θ) /X1

Where, V1 = sending end voltage

V2 = Receiving and voltage

37

X1 = Reactance of the line

θ = Power angle, Phase difference between V1 and V2

For an efficient transmission, sending end voltage must lead the receiving end voltage or there

must be a potential difference. Operating a power system includes a large number of control

functions.

1. Control of frequency and thus control of active power.2. Control of voltage and thus control of reactive power.3. Control of network switching under normal operating conditions.

In a three phase AC system, generator produces three balanced voltages having 120o phase

difference, called positive sequence system. In a normal system three phase currents and voltages

have equal magnitudes and same phase angle between them.

Generator produces this set of voltages system, sometime called a positive sequence system. In

power system generator can produce both active and reactive power, it can also absorb reactive

power. Other sources of reactive power may be static capacitors and synchronous condensers.

The angle between the voltage and current is referred as phase angle between similar voltages

due to impedances of the power system is termed as power angle or load angle. Each type of

power may have different direction of flow on the same transmission line.

A normal system is always treated as a balanced three phase network with given characteristics:

1. Same phase angle between phases2. Same nominal voltages3. Same normal rated current4. Frequency within limit

During an abnormal condition the balance of system may disturb, resulting in unbalanced

currents and voltages appearing in the system, due to any of the following causes

1. Symmetrical faults 2. Un-symmetrical faults3. Switching over voltages4. Ferranti effects on long lines5. Lightning over voltages6. Shifting of system neutral

38

3.2 COMPONENTS OF POWER SYSTEM

GENERATION

Hydel

Thermal

Nuclear

Solar

Wind

Tidal

TRANSMISSION

Extra High Voltage

High Voltage

Medium Voltage

Low Voltage

LOAD

Industrial

Commercial

Agricultural

Domestic

39

4.

Electrical Measuring

Instruments

40

4-1 GALVANOMETER

A galvanometer is an electrical instrument use to detect the passage of a current. Its working

depends upon the fact that when a conductor is placed in an electric field, it experiences a force

as soon as the current passes through it. Due to this force, a torque τ acts upon the conductor if it

is in the form of a coil or loop.

τ = NIBA cosα (1)

Where N is the number of turns in the coil, A is its area, I is the current passing through it, B is

the magnetic field in which the coil is placed such that its plane makes an angle α with the

direction of B. due to action of the torque, the coil rotates and thus it detects the current. The

construction of a moving coil galvanometer is shown in Fig (1).

41

Fig (1)

A rectangular coil C is suspended between the concave shaped poles N and P of a U-shaped

magnet with the help of a fine metallic suspension wire. The rectangular coil is made of

enameled copper wire. It is wounded on a frame of non-magnetic material. The suspension wire

F is also used as one current lead of the coil. The other terminal of the coil is connected to a

loosely wound spiral E which serves as the second current lead. A soft iron cylinder D is place

inside the coil to make the field radial and stronger near the coil as shown in Fig (1).

When a current is passed through the coil, it is acted upon by a couple which tends to rotate the

coil. This couple is known as deflecting couple and is given by NIBA cosα. As the coil is placed

in a radial magnetic field in which the plane of the coil is always parallel to the field Fig (1), so α

is always zero. This makes cosα =1 and thus,

Deflecting couple = NIBA

As the coil turns under the action of deflecting couple, the suspension wire Fig (1) is twisted

which gives rise to a torsional couple. It tends to untwist the suspension and restore the coil to its

original positions this couple is known as restoring couple. The restoring couple of the

suspension wire is proportional to the angle of deflection θ as long as the suspension wire obeys

Hooke’s law. Thus

Restoring torque = cθ (1)

Whereas, the constant c of the suspension wire is known as torsional couple and is defined as

couple for unit twist.

Under the effect of these two couples, coil comes to rest when

Deflecting torque = Restoring torque

NIBA = cθ

Or, I = (cθ)/BAN (2)

Thus I α θ, since c/BAN = Constant

Thus the current passing through the coil is directly proportional to the angle of deflection.

There are two methods commonly used for observing the angle of deflection of the coil. In

sensitive galvanometers the angle of deflection is observed by means of small mirror attached to

42

the coil along with lamp and scale arrangement Fig (2). A beam of light from the lamp is

directed towards the mirror of the galvanometer. After reflection from the mirror it produces a

spot on a translucent scale placed at a distance of one meter from the galvanometer. When the

coil rotates, the mirror attached to coil also rotates and spot of light moves on the scale. The

displacement of the spot of light on the scale is proportional to the angle of deflection (provided

the angle of deflection is small).

Fig (2)

The galvanometer used in school and college laboratories is a pivoted type galvanometer. In this

type of galvanometer, the coil is pivoted between two jeweled bearings. The restoring torque is

provided by two hair springs which also serve as current leads. A light Aluminum pointer is

attached to the coil which moves over a scale as shown in Fig (3). It gives angle of the deflection

of the coil.

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Fig (3)

It is obvious from Eq.(2) that a galvanometer can be made more sensitive (to give large

deflection for a given current) if c/BAN is made small. Thus, to increase sensitivity of a

galvanometer, c can be increased or B, A and N may be increased. The couple c for unit twist of

the suspension wire can be decreased by increasing length and by decreasing its diameter. This

process, however, cannot be taken too far, as the suspension must be strong enough to support

the coil.

Another method to increase the sensitivity of galvanometer is to increase N, the number of turns

of the coil. In case of suspended coil type galvanometer, the number of turns cannot be increased

beyond a limit because it will make the coil heavy. To compensate for the loss of sensitivity, in

case fewer turns are used in the coil, we increase the value of the magnetic field employed. We

define current sensitivity of a galvanometer as the current, in microamperes, required to produce

one millimeter deflection on a scale placed one meter away from the mirror of the galvanometer.

When the current passing through the galvanometer is discontinued, the coil will not come to rest

as soon as the current flowing through the coil is stopped. It keeps on oscillating about its mean

position before coming to rest. In the same way if the current is established suddenly in a

galvanometer, the coil will shoot beyond its final equilibrium position and will oscillate several

times before coming to rest at its equilibrium position. As it is annoying and time consuming to

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wait for the coil to come to rest, artificial ways are employed to make the coil come to rest

quickly after the current passed through it or the current is stopped from flowing through it, is

called stable or a dead beat galvanometer.

4-2 VOLTMETER

A voltmeter is an electrical device which measures the potential difference in volts between two

points. This, too, is made by modifying galvanometer. Since a voltmeter is always connected in

parallel, it must have a very high resistance so that it will not short the circuit across which the

voltage is to be measured. This is achieved by connecting a very high resistance R h placed in

series with the meter-movement Fig (4). Suppose we have a meter-movement whose resistance is

Rg and which deflects full scale with a current Ig. In order to make a voltmeter from it which has

a range of V volts, the value of the high resistance Rh should be such thus full scale deflection

will be obtained when it is connected across V volts. Under this condition the current through the

meter-movement is Ig. Applying Ohms law Fig (4) we have

V = Ig(Rg+Rh)

Rh = (V/Ig) - Rg (3)

Fig (4)

If the scale of the galvanometer is calibrated from 0 to V volts, the combination of galvanometer

and the series resistor as a voltmeter with range 0-V volts by properly arranging the resistance Rh

any voltage can be measured. Thus, we see that a voltmeter possesses high resistance.

It may be noted that a voltmeter is always connected across the two points between which the

potential difference is to be measured. Before connecting a voltmeter, it should be assured that

its resistance is very high in comparison with the resistance of the circuit across which it is

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connected otherwise it will load a circuit and will alter the potential difference which is required

to be measured.

4-3 AMMETER

An ammeter is an electrical instrument which is used to measure current in amperes. This is

basically a galvanometer. The portion of the galvanometer whose motion causes the needle of

the device to move across a scale is usually known as meter-movement. Most (meter movements

are very sensitive and full scale deflection is obtained with a current of few milli amperes only.

So an ordinary galvanometer cannot be used for measuring large currents without proper

modification.

Suppose we have a galvanometer whose meter-movement (coil) has a resistant Rg and which

gives full scale deflection when current Ig is passed through it. From Ohm’s law, we know that

the potential difference Vg which causes a current Ig to pass through the galvanometer is given by

Vg=IgRg

If we want to convert this galvanometer into an ammeter which can measure a maximum current

I, it is necessary to connect a low value bypass resistor called shunt. The shunt resistance is of

such a value so that the current Ig for full scale deflection of the galvanometer passes through

galvanometer and the remaining current (I – Ig) passes through the shunt in this situation as

shown in Fig (5)

The shunt resistance Rs can be calculated from the fact that as the meter-movement and the shunt

are connected in parallel with each other, the potential difference across the meter-movement is

equal to the potential difference across the shunt. Therefore,

IgRg = (I – Ig)Rs

Or Rs = (Ig Rg)/ (I – Ig) (4)

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Fig (5)

The resistance of the shunt is usually so small that a piece of copper wire serves the purpose. The

resistance of the ammeter is the combined resistance of the galvanometer meter-movement and

the shunt. Usually it is very small. An ammeter must have a very low resistance so that it does

not disturb the circuit in which it is connected in series in order to measure the current.

4-4 OHMMETER

It is a useful device for rapid measurement of resistance consists of a galvanometer, and

adjustable resistance rs and a cell connected in series Fig (6). The series resistance rs is so

adjusted that when terminals c and d are short circuited, i.e., when R is equal to 0, the

galvanometer gives full scale deflection. So the extreme graduation of the usual scale of the

galvanometer is marked 0 for resistance measurement. When terminals c and d are not joined, no

current passes through the galvanometer and its deflection is zero. Thus zero of the scale is

marked as infinity as shown in Fig (6).

Now a known resistance R is connected across the terminals c and d. the galvanometer deflects

to some intermediate point. This point is calibrated as R. in this way the whole scale is calibrated

into resistance. The resistance to be measured is connected across the terminals c and d. the

deflection on the calibrated scale reads the value of the resistance directly.

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Fig (6)

4-5 MULTIMETER-AVO METER It is an instrument which can measure current in

amperes, potential difference in volts and resistance in ohms. It basically consist of a sensitive

moving coil galvanometer which is converted into a multi-range ammeter, voltmeter or

ohmmeter accordingly as measuring circuit or a voltage measuring circuit or a resistance

measuring circuit is connected with galvanometer with the help of a switch known as function

switch as shown in Fig (7). Here X and Y are the main terminals of the AVO meter which are

connected with the circuit in which measurement is required. FS is the function selector switch

which connects the galvanometer with relevant measuring circuit.

Fig (7)

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VOLTAGE MEASURING PART OF AVO METER

The voltage measuring part of the AVO meter is actually multi-range voltmeter. It consist of a

number of resistance of which can be connected in series with the moving coil galvanometer as

shown in Fig (8). The value of each resistance depends upon the range of the voltmeter which it

controls

Alternating voltages are also measured by AVO meter. AC voltage is first converted into DC

voltage by using diode as rectifier and then measured as usual.

Fig (8)

CURRENT MEASURING PART OF AVO METER

The current measuring part of the AVO meter is actually a multi-range ammeter. It consists of a

number of low resistances connected in parallel with the galvanometer. The values of these

resistances depend upon the range of the ammeter as shown in Fig (9).

The circuit also has a range selection switch RS which is used to select particular range of the

current.

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Fig (9)

RESISTANCE MEASURING PART OF AVO METER

The resistance measuring part of AVO meter is, in fact, a multi-range ohm meter. Circuit for

each range of this meter consists of a battery of emf Vo and a variable resistance rs connected in

series with galvanometer of resistance Rg. when the function switch is switched to position X3,

this circuit is connected with the terminals X, Y of the AVO meter as shown in Fig (10).

Before measuring an unknown resistance by an ohmmeter it is first zeroed which means that we

short circuit the terminals X and Y and adjust rs, to produce full scale deflection.

Fig (10)

DIGITAL MULTIMETER (DMM)

Another useful device to measure resistance, current and voltage is an

electronic instrument called digital multi-meter. It is a digital version of an

AVO meter. It has become a very popular testing device because the digital

values are displayed automatically with decimal point, polarity and the unit

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for V, A or Ω. These meters are generally easier to use because they eliminate the human error

that often occurs in reading the dial of an ordinary AVO meter. A portable DMM is shown in Fig

(11).

Fig (11)

4-6 CLIP-ON AMMETERS

Clip-on Ammeters work on the principal that an Ammeter is connected to the circuit through a

Current Transformer, thus avoiding the breakage of the circuit needed for current measurement.

The Clip-on Ammeter may be of Analogue or Digital type.

4-7 CATHODE RAY OSCILLOSCOPES

Cathode Ray Oscilloscopes (CRO) is a very versatile electronic instrument which is, in fact, a

high speed graph plotting device. It works by the deflected beam of electrons as they pass

through a uniform electric field between the two sets of parallel plates as shown in the Fig (12).

The deflected beam then falls on a fluorescent screen where it makes a visible spot

Fig (12)

It can display graphs of functions which rapidly vary with time. It is called cathode ray

oscilloscope because it traces the desired waveform with a beam of electrons which are also

called cathode rays.

The beam of the electrons is provided by an electron gun which consists of an indirectly heated

cathode, a grid and three anodes. The filament F heats the cathode C which emits electrons. The

anodes A1, A2, A3 with high positive potential with respect to cathode, accelerate as well as focus

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the electron beam to fix spot on the screen S. the grid G is at negative potential with respect to

cathode. It controls the number of electrons which are accelerated by anodes, and thus it controls

the brightness of the spot formed on the screen.

Now we would explain how the waveform of various voltages is formed in CRO.

The two sets of deflecting plates, shown in Fig (12) are usually referred as x and y deflection

plates because a voltage between the x plates deflects the beam horizontally on the screen i.e.,

parallel to x-axis. A voltage applied across the y plates deflects the beam vertically on the screen

i.e., along the y-axis. The voltage that is applied across the x-plates is usually provided by a

circuit that is built in the CRO. It is known as sweep or time base generator. Its output waveform

is a saw tooth voltage of period T Fig (13).

The voltage increases linearly with time for period T and then drops to zero. As this voltage is

impressed across the x-plates, the spot is directed linearly, with time along with x-axis for a time

T. then the spot returns to its starting point on the screen very quickly because a saw tooth

voltage rapidly falls to its initial value at the end of each period. We can actually see the spot

moving on the x-axis. If the time period T is very short, we just see a bright line on the screen.

If a sinusoidal voltage is applied across the y plates when, simultaneously, the time base voltage

is impressed across the x plates, the sinusoidal voltage, which itself gives rise to a vertical line,

will now spread out and will now appear as a sinusoidal trace on the screen. The pattern will

appear stationary only if the time T is equal to or some multiple of the time of one cycle of the

voltage on y plates. It is thus necessary to synchronize the frequency of the time base generator

with the frequency of the voltage at y plates. This is possible by adjusting the synchronization

controls provided on the front panel of the CRO.

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USES OF CRO

The CRO is used for displaying the waveform of a given voltage. Once the waveform is

displayed, we can measure the voltage, its frequency and phase. For example, Fig (14) shows the

waveform of an alternating voltage. As the y-axis is calibrated in volts and the x-axis in time, we

can easily find the instantaneous value and the peak value of the voltage. The time period can

also be determined by using the time calibration of x-axis. Information about the phase

difference between two voltages can be obtained by simultaneously displaying their waveforms.

For example, the waveforms of two voltages are shown in Fig (15). These waveforms show that

when the voltage of I is increasing, that of II is decreasing and vice versa. Thus the phase

difference between these voltages is 180o.

Fig (14) Fig (15)

4-8 SAFETY PRECAUTIONS

Proper Use of an Ammeter

1. Avoid careless handling2. Always connect ammeter in series in the circuit whose current is to be measured.3. Always first set the range selector switch at the highest range and then stepwise decrease the

range until suitable scale reading is obtained.

Proper Use of a VOLTMeter

1. Avoid careless handling2. Always connect voltmeter in parallel to the circuit, whose voltage is to be measured.3. Always first set the range selector switch at the highest range and then stepwise decrease the

range until suitable scale reading is obtained.

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4. Leads of voltmeter must be free of defects.

Proper Use of an ohmMeter

1. Never use ohmmeter on energized circuit2. Zero adjustment must be made for each range. For this, short its leads together and then bring

pointer to zero by variable resistor by using screw driver (or by turning the knob provided).3. Avoid careless handling4. For checking continuity of wires etc. it is better to use smallest range of ohmmeter.

PROPER USE OF A MULTIMETER OR (AVO METER)

1. Adjust the selector switch to the quantity which is to be measured (i.e. current, voltage or resistance and A.C voltage or DC voltage etc.).

2. When multi-meter is not in use, put its selector switch to off position because of following reasons.a. Meter damage is avoided if it is connected to high voltages by someone.b. Needle movement is damped and hence needle is protected from damage during

transportation etc. Note: If off position is not provided, then return the selector switch to the highest voltage range.

3. Avoid careless handling4. When measuring V and I, always set range selection switch at the highest range and then

switch successively to lower ranges until a suitable range is obtained.5. When measuring resistance (or continuity) of a circuit with multi-meter, the circuit must be

de-energized before inserting the meter leads in it.6. Leads of multi-meter must be clean, dry and free of defects.

4.9 IMPORTANT POINTS WHEN USING INSULATION RESISTANCE TESTER

1. Insulation Resistance Tester must never be used in energized circuit.2. Accuracy of Insulation Resistance Tester must be checked before using it. Accuracy of

Insulation Resistance Tester is checked by performing following two tests:a. Zero Check: For this short Insulation Resistance Tester leads together and crank it,

the pointer of it must deflect towards zero position.b. Infinity Check: For this leave Insulation Resistance Tester leads open circuited and

crank it, its needle must deflects towards infinity position. 3. The equipment under test must be discharged and grounded after test to discharge the

capacitor (insulation) and to release the energy which it may have absorbed due to the dielectric absorption phenomenon.

4. Insulation Resistance Tester leads must be clean, dry and free of defects.5. Guard terminal of Insulation Resistance Tester must never be touched during test.6. The rating of Insulation Resistance Tester should not be more than the rating of equipment

under test.

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PROCEDURE:

The H (or line or +) terminal of Insulation Resistance Tester is connected to the equipment whose insulation resistance is to be measured; the E (Earth) terminal of it is connected to the terminal of equipment which is grounded. Then it is cranked until its needle is stabilized and reading is taken / recorded.

Insulation Resistance Tester readings are always corrected to 200C standard temperature because Ri varies inversely with temperature. To convert Ri to 200C following rule is adopted:

a. For every 100C rise of temperature from 200C, Ri becomes half of its value.b. For every 100C fall of temperature from 200C, Ri becomes double of its value.

One Mega-ohm reading for 1kv rated equipment at 20 C0 is considered satisfactory.

4.10 TEMPERATURE CORRECTION OF Ri READINGS

Example

Ri reading of 66KV equipment at 400C was measured to be 20 Mega-ohms. Can this equipment be energized, explain.

Solution

Ri at 400C = 20 Mega-ohms

Ri at 300C = 40 Mega-ohms

Ri at 200C = 80 Mega-ohms

The equipment can be energized, since for 66KV equipment R i reading of 66 Mega-ohms is satisfactory. This reading is 80 Mega-ohms.

Example

Ri reading of 220KV insulation measured at –50C was 1000 Mega-ohms. Is this good reading? Explain.

Solution

Ri at –50C = 1000 Mega-ohms

Ri at 50C = 500 Mega-ohms

Ri at 150C = 250 Mega-ohms

Ri at 250C = 125 Mega-ohms

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Ri at 200C = (250 + 125)/2 = 375/2 = 187.5 Mega-ohmsThis is not a good reading as the satisfactory reading for 220kV equipment is 220 Mega-ohms at 200C.

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5.

Basic Control Circuits

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5.1 AUXILIARY SWITCHES

The names of various automatic switches are:

1. Level switch 2. Flow switch 3. Position or limit switch4. Pressure switch 5. Temperature switch 6. Speed switch (centrifugal switch)

IMPORTANT LETTERS

Letters ‘a’, ‘b’, ‘aa’ and ‘bb’ are used in diagrams to represent switches or auxiliary switches.a- It is closed when main device (Circuit Breaker, Isolator or Contactor/Relay) is closed

/energized and it is opened when main device is open or de-energized. Sometimes, ‘a’ is also called normally open contact of a device.

b- It is closed when main device is opened and vice versa. ‘b’ is also called normally closed contact of a device.

aa- It is always open. It only closes for a very short time when the driving force; (air pressure, hydraulic pressure or spring) operating the main device is in action. It returns to its original position when driving force ceases.

bb- It is opposite to ‘aa’ i.e it is always closed. It opens for a very short time when driving force operating the main device is in action. It re-closes or re-sets when driving force is ceased

5.2 DEVICE FUNCTION NUMBERS

Function No.

Type of Relay

2 Time delay relay

3 Interlocking relay

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21 Distance relay

25 Check synchronizing relay

27 Under voltage relay

30 Enunciator relay

32 Directional power (Reverse power) relay

37 Low forward power relay

40 Field failure (loss of excitation) relay

46 Negative phase sequence relay

49 Machine or Transformer Thermal relay

50 Instantaneous Over current relay

51 A.C IDMT over current relay

52 Circuit breaker

52a Circuit breaker Auxiliary switch “Normally open” (‘a’ contact)

52b Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact)

55 Power Factor relay

56 Field Application relay

59 Overvoltage relay

60 Voltage or current balance relay

64 Earth fault relay

67 Directional relay

68 Locking relay

74 Alarm relay

76 D.C Over current relay

78 Phase angle measuring or out of step relay

79 AC Auto reclose relay

81 Frequency relay

81U under frequency relay

81O over frequency relay

83 Automatic selective control or transfer relay

85 Carrier or pilot wire receive relay

86 Tripping Relay

87 Differential relay

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87G Generator differential relay

87GT overall differential relay

87U UAT differential relay

87NT Restricted earth fault relay (provided on HV side of Generator transformer)

95 Trip circuit supervision relay

99 Over flux relay

186A Auto reclose lockout relay

186B Auto recluse lockout relay

5.3 BASIC REQUIRMENTS OF CONTROL CIRCUITS

For circuit breakers there are a number of basic requirements which are desirable in the control circuit. These features can be found in the motor, solenoid, spring (stored energy) and pneumatically-operated Circuit Breakers.

A good understanding in these control circuit features will allow an intelligent approach to trouble-shooting. Examination of the circuit diagram of the modern breakers will relatively complicated network of switches, contactors and coils, the correct functioning of which is essential. Each individual component of a circuit has a definite function to perform, thus removing any one element will cause some type of faulty operation.

When the maintenance electrician knows the function of each component, he also knows what type of faulty operation to expect when that component is inoperative. Conversely when mal-operation of a breaker is found, the likely component or components at fault will be known.

5.3.1 CONTROL THE CLOSING

The closing circuit must do more than merely close the breaker, it must control this closing. To do this, the following features are necessary.

Initiate The Closing Stroke: Means must be provided in the circuit to energize the closing device, for example, the solenoid of the solenoid-operated breaker or the motor of a motor-operated breaker.

Cut-Off: The closing power must be cut-off or disconnected automatically at the end of the closing stroke. This is necessary to prevent overheating of the closing device. Solenoid coils used

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on circuit breakers have only a short time rating, thus if a closing coil is left energized for more than 15 seconds. It will overheat and suffer damage to the insulation. For this reason it cannot be left to the operator to decide when to cut off the closing power since it left on too long, damage will result. The alternative where the closing power could be left on for too short a time is covered in Seal-In.

Seal-In: It is desirable to have the control circuit ensure that the breaker will fully close each time that closing operation is initiated, if the breaker is closed by a simple switch. Simply speaking; it completes the operation automatically started by us manually so as not to hold the push button all the times.

5.3.2 CONTROL THE TRIPPING

Initiate Trip Stroke: Means must be provided to trip the breaker. This may involve energizing a solenoid coil to trip a latch or in case of air blast breakers, to admit air to the blast valves and contacts.

Cut-Off: For the reasons noted in above, means must be provided to automatically disconnect the trip coil.

5.3.3 TRIP FREE FEATURE

When closing a breaker, the closing device (for example, the solenoid in a solenoid-operated breaker) is energized and the plunger operates through the linkage to close the breaker contacts. At the end of the closing stroke, appreciable time is required to de-energize the solenoid coil. In the event that the breaker has been close on a faulted circuit, it must be reopened as quickly as possible. If a breaker can trip automatically upon receiving a trip signal before closing operation is complete, it is said to be “trip free”.

Various arrangements are provided to obtained trip-free action. Solenoid-operated breakers are sometimes provided with action. Solenoid-operated breakers are sometimes provided with a collapsible linkage. Pneumatically-operated breakers may be equipped with two latches, one of which is unlatched during a normal trip operation and the other is only unlatched for a trip signal while the breaker is closing. Other breakers use a large dump valve to quickly exhaust the air under the closing piston. Many motor-operated breakers obtain a fast trip-free action by use of a relay energized from the trip circuit to open the closing circuit. These methods would be known as mechanically trip free, pneumatically trip free and electrically trip free respectively.

5.3.4 ANTI-PUMPING FEATURE

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When a breaker is closed and a trip-free operation results, the close and trip stroke will be completed in a very short time for a modern pneumatically-operated high voltage breaker, the complete operation will take less than one-half of one second, thus it is quite likely that the operator will still have the control switch in the closed position. Means must therefore be provided to prevent the breaker from closing a second time, even though the operator is holding the control switch in the closed position. This is usually accomplished by the use of a sealed in relay which can only be released which in the closed position. This is usually accomplished by the use of a sealed in relay which can only be released by opening the closing control switch. When this feature is incorporated in the control circuit, the breaker is said to be “pump free” or “anti-pumping”. Following a trip-free operation of the breaker, the operator must release the control switch before a second attempt to close the breaker can be made.

5.3.5 ANTI-SLAM FEATURE

This feature prevents the energization of closing coil or tripping coil of an already closed breaker or tripped breaker respectively.

1. In closing circuit this feature is mostly achieved through auxiliary switch b.2. In opening circuit, this feature is mostly achieved through auxiliary switch a.

5.3.6 RELIABILITY

A circuit breaker is a protective device. It will be called upon to open faulty circuits infrequently, however while it may stand inoperative for long periods; it must be relied upon to operate correctly in time of trouble. Reliability for such a protective device is essential. For this reason a battery supply is always used to provide the tripping power and in most cases for closing.

The control circuits usually have a separate trip and close bus. This is to give extra reliability to the trip circuit. On 115 kV and above circuit breakers there are dual trip buses thus, if a closing control circuit fuse fails during a closing stroke, the trip circuit or circuits are not affected.

The above requirement of extra reliability during tripping is also seen in the size of fuses used in the trip and closed circuits, for example, on a solenoid-operated breaker the fuses in the closing circuit will be rated at slightly less than value of current obtained by dividing the voltage by resistance. The fuses must be so rated to provide a measure of protection for the short time rated closing coil. During a normal closing operation, the closing coil will be energized for less than one second. To have the closing fuses blow in approximately six seconds, it is necessary to use a size which is actually less than the maximum current that the closing coil will normally draw. Conversely, in the trip circuit the fuses will be rated at several times the current obtained by voltage to resistance ratio. If the trip coil is not automatically disconnected at the end of the trip stroke, the trip coil may carry current for a long period and being are short time rated coil, it will be damaged. The fuses in this circuit will open only due to some fault condition. In order to gain more reliable tripping, we do not protect the short time rated trip coil. The trip fuses are not put in the breaker but are located in the control building.

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5.3.7 GENERAL MAINTENANCE OF BREAKER CONTROL RELAYS

Frequent reference is made in this reference material to relays. These are control relays located in the operating mechanism housing. They are concerned entirely with the sequence of the mechanism of the breaker. The control circuit relays are all located on the breaker side of the four-pole isolating switch. Such relays are a part of the breaker, being required for the breaker’s correct functioning as much as possible, for example, a trip coil or interrupter and as such are the responsibility of the maintenance electrician.

Other relays remote from the breaker determine under what system conditions the breaker will be tripped. These protective relays, together with the interposing relays where such are used, form the protective network and are the responsibility of the Meter and Relay Department. The dividing line between the breaker control circuit relays and protective relays is well defined and there should be not confusion in this regard.

5.3.8 ASA DEFINATIONS

RELAY: A relay is a device that is operative by a variation in the conditions of one electric circuit to effect the operations of other devices in the same or another electric circuit.

CONTROL RELAY: A control relay is a relay which functions to initiate or to permit the next desired operation in a control circuit or scheme.

PROTECTIVE RELAY: A protective relay is a relay, the principal function of which is to protect service from interruption by removing defective components or to prevent or limit damage to apparatus.

5.4 OVERLOAD PROTECTION

In order to avoid damage to motors etc. due to temperature rise because of overloading and defective bearings etc., overload protection features are incorporated in motor control circuits. It should be noted that over load relay or element is always incorporated in the power circuit but its contact is installed in the control circuit. Due to this, the life of contact increases as it breaks small current because in control circuit current is small. Over load relays are mostly operated thermally and may be of bimetallic strip type or solder pot type.

5.5 OVER CURRENT OR SHORT CIRCUIT PROTECTION

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The function of over current protective devices is to protect motors and its circuit elements etc. from damage in case of phase-phase short circuits or phase-ground faults etc. The over-current device must be capable of carrying the starting current of motors. Mostly fuses and magnetic devices are used as over current protective devices. Rating of fuse should not exceed 300% of full load current.

5.6 CONTACTOR

Contractor is a device which is operated electrically and controlling the operation of other circuits magnetically. Contactor may also be called as an ON-OFF Switch. Contactor has two types of contacts:

1. Main Contacts: These are used in power circuits and hence must be strong to carry the full load current of motor continuously without undue heating.

2. Auxiliary Contacts: These are small and used in control circuits only. These may be NO or NC and are used as seal in contact (NO), interlocking contact (NC) and for indications etc.

5.7 MAINTENANCE OF CONTACTOR

Contactor maintenance mainly includes

1. Removal of rust or deposits etc. from contacts with emery paper and dry cloth. Never file the contacts as it will remove the elkonite from the contacts.

2. Checking of contacts alignment 3. Free movement of moving contacts assembly with binding etc.4. Checking of connections at terminal points.

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6.

P&I Tools & Plant

(T&P)

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6.1 P&I TOOLS

The following is a list of tools that will be required by a P & I man when maintaining the equipment within his area of responsibility. It is broken into two areas.

1 Personnel Tools: These are the tools each man should have at his end only on his disposal. They should be his responsibility alone.

2 Common Test Equipments: These equipments are purchased by WAPDA/NTDC/GENCOs/DISCOs and kept in divisional stores for use by individuals when required. The responsibility for this equipment is that of the XEN in-charge and a definite method of checking the equipment’s in/out should be established along with a system of periodic inventory check.

3 It is vital to maintenances program that these test equipments must be kept in a perfect condition and any broken or worn parts be replaced or repaired as soon as possible.

4 All test equipments should be kept in an enclosed cabinet free from dust, moisture and excessive vibration.

6.1.1 P&I PERSONNEL TOOLS

1. Electronic Digital Meter2. Leather Tool Carrying Case, 20” x 10” x 12” high.3. Soldering Iron-220 volt 40-50 Watt:4. 1 roll 60/40 Flux Core Solders.5. Insulating Tap (Black).6. Continuity Tester (Buzzer).7. Jeweler Screwdrivers Set (7 pieces / set).8. Wire Strippers And Crimpers.9. Nut Drivers 5 / 16”, ¼”, 11/32”, 3/16”, ½” and 7/16.10. L-Type, Hex Head Set-9 Pieces.11. Set of Screw Drivers Slot Head, Phillips. 12. Pliers, Side Cutting, Long Nose.13. Potential Test Indicators (750 volts AC / DC) Neon Bulb Type.14. Various Assortments of Length of Clip Cords (RED + BLACK) (6” To 6’) with Alligator

Clips and Insulated Rubbers Covers Wire should be No: 18 AWG.15. Multiplex Scrappers 7½” length – 1 “wide.16. Hacksaw, 10” length and Spare Blades.17. Center Punch.18. Slip Joint Pliers, 5” Length.19. Curved Nose Pliers – 6” Length.20. Adjustable Wrenches 100 mm, 200 mm.21. Heavy Duty Screw Driver Set (Slot, Phillips).22. Vise Grip Pliers 7”.

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23. Utility Knife.24. Screw Holding Screw Driver 4”, 6”

P&I TEST EQUIPMENTS

1. CT Analyzer2. Universal Relay Test Set3. Secondary Injection Relay Test Set4. Primary Current Injection Test Set5. Digital Clamp Meter6. AC/DC Clamp on Meter7. Digital Multi-Meter8. Phase Angle Meter9. Oscilloscope Digital, Dual Channel10. Variac, Single Phase11. Variac, Three Phase12. Analog Multi-Meter

6.2 SHOP RULES

1. Do not use shop machinery until you are trained.2. Grease / oil on floor must be removed immediately to avoid slipping / falling.3. Keep all tools in their proper place.4. Wear eye protection when necessary5. All the tools must be kept clean.6. After the completion of job with tools, dry and clean them and return them to their proper

location.7. Do not use damaged tools.8. Long hairs / loose clothing should be confined when working around rotating machines.

6.2 CARE AND UP KEEP OF TOOLS

1. All the tools should be stored in a proper toolbox and each should be placed in their own compartment.

2. Tools should not be thrown in toolbox in order to avoid damage to itself and to other tools.

3. Tools must be kept away from heat because heat reduces hardness of tools.4. All the tools should be kept clean. Dust or rust must never be allowed to accumulate on

tools.5. After completion of work the tools should be wiped with a clean cloth moistened with

machine oil and then each tool should be stored in its proper place.6. Each tool must be used only for that job for which it is made.7. Ordinary plastic insulated tools must never be relied upon for electrical insulation.8. Always purchase best quality tools regardless of their cost because good quality tools last

for long time and give good continuous service.

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6.3 SAFETY PRECAUTIONS WHEN APPLYING VOLTAGE FROM TEST

EQUIPMENT SOURCES

Employees, particularly maintenance staff, quite often have occasion to perform tests on isolated apparatus by application of an external voltage source over which such maintenance staff have exclusive control.

This information outlines the general and special precautions required during testing of apparatus which involves application of a potential from a voltage source external to the system, i.e., from a test equipment source. Written procedures based on these principles should be developed for all repetitive tests performed by maintenance personnel, and other special or non-repetitive tests where this is practical.

The external voltage source may consist of a high DC voltage test set, AC high voltage set, “megger” type insulation resistance testers up to 10 kV range, etc. Tests involving a low AC voltage source may also produce high voltage by inductive action.

6.4.1 TESTING APPARATUS IN ITS IN-SERVICE LOCATION

Observe the following precautions:

1. Where work safety depends on the isolation of apparatus, the apparatus to be tested shall be de-energized prior to the connection of the test equipment.

2. This condition should be guaranteed by the appropriate form of work protection (where such is required by the Work Protection Code) to ensure safe working conditions for those operating the test equipment and for those working outside the immediate test area.

3. The equipment shall remain in the de-energized condition throughout the work period accept as may be required to allow authorized testing.

4. In the case where isolation of apparatus is necessary to carry out a test, but de-energizing is not (e.g., testing of instrument transformers from the secondary windings) other precautions, as detailed in specific work practices and procedures, must be followed to ensure safe working conditions.

6.4.2 TESTING IN A LOCATION OTHER IN THE IN-SERVICE POSITION

When electrical apparatus has been moved from its normal position to a shop or other area, safety precautions appropriate to the new location must be taken before such equipment is energized in any way for testing. The person in charge of any voltage testing shall be responsible for establishing proper safe working conditions of the test or testing.

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6.4.2.1 SIGNS AND GUARDS REQUIRED DURING VOLTAGE TESTING OF APPARATUS RESPONSIBILITY OF PERSON IN CHARGE

The person in charge of any voltage testing is responsible for assuring that unauthorized persons are barred from the test area while testing is in progress.

TYPES OF BARRIERS

Apparatus under voltage testing must be barricaded by suitable barriers displaying approved warning signs, or alternatively approved written procedures must be followed for specific repetitive tests.

Rigid-type barriers or rope mesh barriers may be employed. This type of safety barrier is available from Central Stores. Cotton tape, when supplemented by warning signs and the posting of observers, may be used as an alternative to such barriers.

Wherever, a complete barricade is not practical, as for example, when testing a high voltage cable from one junction to another, then adequate alternative must be devised. One alternative for this case would be to have each end monitored and observed by test personnel who are in radio contact.

It is deemed not a requirement to barricade the equipment for Insulation Resistance tests at 2500 V or less.

WARNING SIGNS

An approved sign, “Caution HV Testing Keep Away”, is available from Stores, for use where, warning signs are required.

WHEN BARRIERS AND SIGNS ARE REQUIRED

Barriers and signs should only be in place while actual testing is in progress and should be removed promptly upon completion of the test. Adherence to this practice promotes respect for such warnings.

TEST OPERATOR

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Test shall be undertaken only by persons qualified by experience or training to do this type of work.

Where possible, the controls for the test equipment should be located outside the main barrier so that the person conducting the testing will be clear of the test area while the test is in progress.

6.4.3 SPECIAL PRECAUTIONS

TEST EQUIPMET LEADS

When special leads are not provided with the test equipment, a limp bare braided copper conductor shall be used for high voltage connections. An approved bare conductor is available from Central Stores. Solid drawn wire of the so-called “binding wire” type shall not be used for high voltage test leads. This type of wire is not considered safe because of its tendency to kink and break, thereby permitting a loose end to spring back through the air while still at high potential.

Bare test leads are preferable to insulated leads in that the bare wire encourages more respect. When insulated leads are necessary to be allowed the test to be performed, the insulation should be capable of withstanding the maximum test voltage.

GROUNDING

Particular attention should be paid to how the test set and apparatus are connected to ground. The frame of the apparatus must be connected to the ground of the test equipment, and both must be attached to a station ground. Alternative procedures to ensure adequate safety shall be instituted where such ground connections would make performance of the test impractical. Where a station ground is not available, a temporary driven ground shall be used.

An approved grounding device shall be used to ground the accessible high voltage test supply when the actual testing is not being performed.

TERMINATION POINTS

Under no circumstances shall an external supply be applied to a circuit without all termination points of the circuit being known and isolated or protected.

RESIDUAL CHARGE FOLLOWING HIGH DC VOLTAGE TEST

The characteristics of large generator or motor windings and power cables are such that they may retain dangerous charges for long charges for long times of periods of following application of a

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high DC voltage. Equipment which has been tested with high DC voltage shall be grounded and left grounded for a significant period of time or until by test it has been proven safe, since otherwise there may be a voltage buildup over a period of time. A significant period of time is defined as no less than four hours.

To prevent possible damage due to high peak discharge currents, the equipment which has been tested should be grounded initially through an approved grounding device incorporating a resistance between its connection to the equipment and the ground, by an approved procedure for the specific application. The resistance grounding device should remain in service for not less than 15 minutes, or alternatively, until the voltage has been reduced to a level of ten percent or less of the test potential prior to making a solid connection to ground. After the solid ground connection has been connected, the temporary working ground shall be reattached.

When it is necessary to reassemble the permanent connections to the equipment before expiration of the recommended discharge time, work may be carried out on these connections provided they are kept grouped solidly during the course of the work by means of approved grounding devices. When it is impractical to make the permanent connection while the grounding device is in place, an approved procedure shall be devised which may include special tests and/or experience for that specific application.

TRANSFORMER WINDINGS

To carry out voltage tests on major equipment, all voltage transformer windings shall be disconnected from the apparatus and grounded prior to test. All current transformer secondary windings are to be short circuited and grounded prior to test.

It should be noted that application of even a low voltage AC supply to the low voltage terminals of voltage transformation apparatus, such as voltage transformers and power transformers, could result in the induction of a voltage of sufficient magnitude to cause an unsafe condition. Similarly, the application of either a DC or AC voltage to the primary terminals of a current transformer can cause high voltage at the terminals if these are left open circuited.

NOTE: DC voltages are not recommended for use on transformers, where DC currents pass through a winding. This could cause a high voltage hazard.

Where DC currents must be passed through a winding, the opposite winding on the same core leg (phase) must be short circuited.

GROUNDING OF COMPONENTS NOT UNDER TEST

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Care shall be taken to ensure that all un-energized metallic parts in the vicinity, such as conductors or shields not subjected to the test voltage, are connected to the ground. This must be emphasized since charges can build up on ungrounded metallic equipment

7.72

Introduction of Grid

Station Main and

Auxiliary Equipment

7-1 TRANSFORMERSElectrical transformer is a static device which transforms electrical energy from one circuit to another without any direct electrical connection and with the help of mutual induction between to windings. It transforms power from one circuit to another without changing its frequency but may be in different voltage level.

7-1-1 USE OF POWER TRANSFORMER

Generation of Electrical Power in low voltage level is very much cost effective. Hence Electrical Power is generated in low voltage level. Theoretically, this low voltage leveled power can be transmitted to the receiving end. But if the voltage level of a power is increased, the current of the power is reduced which causes reduction in ohmic or I2R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low leveled power must be

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stepped up for efficient. This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with help of step down transformer. These are the use of electrical power transformer in the electrical power system.

7-1-2 TYPES OF TRANSFORMER

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. The types of transformer are as follows:

Step Up Transformer & Step Down Transformer - Generally used for stepping up and down the voltage level of power in transmission and distribution power network.

Three phase transformer & Single Phase Transformer - Former is generally used in three phase power system as it is cost effective than later but when size matters it is preferable to use three phase transformer as it is easier to transport three single phase unit separately than one single three phase unit.

Electrical Power Transformer, Distribution Transformer & Instrument Transformer - Transformer generally used in transmission network is normally known as Power Transformer, Distribution Transformer is used in distribution network and this is lower rating transformer and Current Transformer & Potential Transformer, we use for relay and protection purpose in electrical power system and in different instruments in industries are called instrument transformer.

Two Winding Transformer & Auto-Transformer - Former is generally used where ratio between High Voltage and Low Voltage is greater than 2. It is cost effective to use later where the ratio between High Voltage and Low Voltage is less than 2.

Outdoor Transformer & Indoor Transformer - Transformers designed for installing at outdoor is Outdoor Transformer and Transformers designed for installing at indoor is Indoor Transformer.

7-2 CIRCUIT BREAKERS

Circuit Breaker is a switching device which can be operated manually as well as automatically for controlling and protection of electrical power system respectively. As the modern power system deals with huge currents, special attention should be given during designing of circuit breaker to safe interruption of arc produced during the operation of circuit breaker

The modern power system deals with huge power network and huge numbers of associated electrical equipment. During short circuit fault or any other types of electrical fault these equipment as well as the power network suffer a high stress of fault current in them which may damage the equipment and networks permanently. For saving these equipments and the power networks the fault current should be cleared from the system as quickly as possible. Again after

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the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to the receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed. So for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices which can be operated safely under huge current carrying condition. During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench these arcs in safe manner.

Circuit breaker is the special device which does all the required switching operations during current carrying condition.

7.2.1 WORKING PRINCIPLE OF CIRCUIT BREAKER

Circuit breaker mainly consists of fixed contacts and moving contacts. In normal "ON" condition of circuit breaker, these two contacts are physically connected to each other due to applied mechanical pressure on the moving contacts. There is an arrangement stored potential energy in the operating mechanism of circuit breaker which is realized if switching signal given to the breaker. The potential energy can be stored in the circuit breaker by different ways like by deforming metal spring, by compressed air, or by hydraulic pressure. But whatever the source of potential energy, it must be released during operation. Release of potential energy makes sliding of the moving contact at extremely fast manner.

All circuit breaker have operating coils (tripping coils and close coil), whenever these coils are energized by switching pulse, and the plunger inside them displaced. This operating coil plunger is typically attached to the operating mechanism of circuit breaker, as a result the mechanically stored potential energy in the breaker mechanism is released in forms of kinetic energy, which makes the moving contact to move as these moving contacts mechanically attached through a gear lever arrangement with the operating mechanism. After a cycle of operation of circuit breaker the total stored energy is released and hence the potential energy again stored in the operating mechanism of circuit breaker by means of spring charging motor or air compressor or by any other means.

Till now we have discussed about mechanical working principle of circuit breaker. But there are electrical characteristics of a circuit breaker which also should be considered in this discussion of operation of circuit breaker.

The circuit breaker has to carry large rated or fault power. Due to this large power there is always dangerously high arcing between moving contacts and fixed contact during operation of circuit breaker.

Again as we discussed earlier the arc in circuit breaker can be quenched safely if the dielectric strength between the current carrying contacts of circuit breaker increases rapidly during every current zero crossing of the alternating current. The dielectric strength of the media in between contacts can be increased in numbers of ways, like by compressing the ionized arcing media since compressing accelerates the deionization process of the media, by cooling the arcing media since cooling increase the resistance of arcing path or by replacing the ionized arcing media by fresh gasses. Hence a numbers of arc quenching processes should be involved in operation of circuit breaker.

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7.2.2 TYPES OF CIRCUIT BREAKER

According to different criteria there are different types of circuit breaker

Classification Based on Arc Quenching Media:

1. Oil Circuit Breaker2. Air Circuit Breaker3. SF6 Circuit Breaker4. Vacuum Circuit Breaker

Classification Based on Service:

1. Outdoor Circuit Breaker2. Indoor Circuit Breaker

Classification Based on Operating Mechanism of circuit breaker:

1. Spring Operated Circuit Breaker2. Pneumatic Circuit Breaker3. Hydraulic Circuit Breaker

Classification Based on Voltage level of installation:

1. High Voltage Circuit Breaker2. Medium Voltage Circuit Breaker3. Low Voltage Circuit Breaker

7.3 DISCONNECT SWITCHES/ISOLATORS

In electrical engineering, a disconnector or isolator switch or disconnect switch is used to make sure that an electrical circuit can be completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications where machinery must have its source of driving power removed for adjustment or repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers and transformers, and transmission lines, for maintenance. Often the isolation switch is not intended for normal control of the circuit and is used only for isolation.

Isolator switches have provisions for a Padlock so that inadvertent operation is not possible. In high voltage or complex systems, these padlocks may be part of a trapped-key interlocked to ensure proper sequence of operation. In some designs the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which inter-connect power systems where both end of the circuit need to be isolated.

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The major difference between an isolator and a circuit breaker is that an isolator is an off-load device intended to be opened only after current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the disconnector while it supplies a circuit.

7-4 LIGHTNING ARRESTER

A lightning arrester (in Europe: surge arrester) is a device used on electrical power system and communications systems to protect the insulation and conductors of the system from the damaging effects lightning. The typical lightning arrester has a high voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.

If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-produced extreme voltage spikes in incoming power lines can damage electrical home appliances.

A lightning arrester may be a spark gap or may have a block of a semiconducting material such

as Silicon Carbide or Zinc Oxide. Some spark gaps are open to the air, but most modern varieties

are filled with a precision gas mixture, and have a small amount of radioactive material to

encourage the gas to ionize when the voltage across the gap reaches a specified level. Other

designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp)

connected between the protected conductor and ground, or voltage-activated solid-state switches

called varistors or MOVs.

Lightning arresters built for power system consist of a porcelain tube several feet long and several inches in diameter, typically filled with disks of zinc oxide. A safety port on the side of the device vents the occasional internal explosion without shattering the porcelain cylinder.

Lightning arresters are rated by the peak current they can withstand the amount of energy they can absorb, and the break over voltage that they require to begin conduction. They are applied as part of a lightning protection system, in combination with air terminals and bonding.

7-5 BATTERIES AND BATTERY CHARGERS

Supply of power to protection and control circuits is provided from storage batteries due to

reliability point of view.

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The simplest operating unit to produce emf chemically is called a cell, whereas several cells

constitute a battery. Electrochemical devices consist of two dissimilar electrodes immersed in a

conducting solution, normally known as electrolyte that is capable of storing electrical energy.

The voltage of the cell depends upon the material of electrolyte, while the current and power

capacity of a cell depends upon the plate area and weight of active material in the electrodes.

Main types of storage batteries are:1. Lead Acid Batteries

2. Alkaline Batteries

Active Parts of Lead Acid Battery:1. Grid (Lead Antimony)

2. Positive Plates (Lead Per Oxide- PbO2)

3. Negative Plates (Lead- Pb)

4. Electrolyte (Sulphuric Acid-H2SO4)

Chemical Reactions

At Anode: PbO2 + H2SO4↔ PbSO4 + H2O + ½O2

At Cathode: Pb + H2SO4↔ PbSO4 + H2O

7-6 STATION GROUNDING SYSTEM

Earthing or grounding is the term used for electrical connection to general mass of earth in such a manner as to ensure, at all times, an immediate discharge of energy without danger. A grounding system to be totally effective must satisfy the following conditions:

A. Provide a low impedance path to ground for personnel and equipment protection and effective circuit relaying.

B. Withstand and dissipate repeated fault and surge currents.C. Provide corrosion allowance or corrosion resistance to various soil chemicals to insure

continuous performance during the life of the equipment being protected.

Types of Earthing:A. Solid or Effective Earthing B. Resistance Earthing/Reactance Earthing

Classification of EarthingA. System or Neutral Earthing : The neutral point of generator, transformer, transmission and

distribution system or circuit, rotating machines etc. is connected to earth either directly or through a resistance, or a reactance.

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B. Equipment Earthing : Equipment Earthing means connecting the non current carrying metallic parts in the neighborhood of electrical circuits to earth.

Resistance to current through an earth electrode system has the following three components:

A. Resistance of the ground rod itself and connections to it.B. Contact resistance between the ground rod and earth adjacent to it.C. Resistance of the surrounding earth.

7-7 AC & DC Supply System

In any substation AC and DC supply system plays a very important role for protection, control

and for all auxiliary services.

AC Supply System

For AC supply, normally a dedicated panel is specified in a substation which is only for the

substation and no external load is connected to it in order to avoid interruptions on it. On the LT

side two transformers are provided exclusively for the substation auxiliary services. For

reliability purposes, load is fed from one transformer; however in case load can be shifted to the

other transformer either from HT or LT side. Then we have distribution panels, from where load

is distributed throughout the substation through appropriate Circuit Breakers/ Miniature Circuit

Breakers.

DC Supply System

For DC supply system, Rectifiers, Batteries and Distribution Panels are provided in the

substation. In important substations, normally Two sets of Batteries along with Three Rectifiers

(One as standby) are provided for reliability purposes.

110 Volts Batteries Two Sets

110 Volts Rectifiers Three Sets

220 Volts Batteries Two Sets

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220 Volts Rectifiers Three Sets

In 500 kV substations, Four sets of Batteries and Six Rectifiers (One as standby for Two banks).

Even, in case of emergency, loads of the same rating can be coupled with one Rectifier/Battery.

7.8 POWER CABLES

There are four main parts of cable:-

1. Conductor 2. Insulation 3. Shield or Sheath4. Protective Covering

7.8.1 PURPOSES OF SHIELDING / SHIELD GROUNDING

The application of conducting (copper etc) and semiconducting (metabolized paper tap or containing carbon or silicon etc) materials over the conductor insulation is called shielding. The main purpose of shield is to keep even voltage gradient across the insulation in order to avoid damage to insulation by corona or ionization.

Now shield may have induced voltages in it, so shield must be grounded in order to discharge these induced voltages. When shield is grounded, it provides some more advantages as well, which are:

1. Provides earth return path in case of phase to ground fault2. Human safety 3. Protects the cable from external high voltages, produced by lightening etc

Shield must be grounded at one place only (especially in single phase cable) in order to avoid

flow of current in shield and hence damage to it due to overheating. Shielding idea was given by

Martin Hochsadter in 1915. He gave the idea that put shield around the conductor of each phase

and then ground all shields. Such cables are called H-cables. Such cable fails phase to ground. In

these cables it is very rare that cable may fail phase to phase.

In a very long cable, sectionalized are used. In sectionalized shield each section is insulated from

each other and then each section is grounded at one place only.

7.9 BUS-BARS

There are two types of bus bars used in grid station, which are:

1. The Flexile or Stranded Bus Bar

2. The Rigid Bus Bar (may be tubular or solid)

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1. Flexible or Stranded Bus Bar: It is used where:A. Longer spans are involved.B. Where sufficient clearances are needed to allow for conductor sways and.C. It is used as a long drop from horizontal bus to equipment bushing.

In the flexible bus bar sag must be enough to account for temperature variations without affecting the clearances between phases and phases to ground.

2. Rigid Bus Bar: It is used where:

A. Heavy currents are involved

B. Short or less Spacing is available

To account for thermal expansion /contraction of rigid bus provision must be made by means of expansion joints and clamps to permit bus to slide both ways in order to avoid damage to equipment bushing and isolators etc.

7.9.1 BUS BAR SCHEMES

SINGLE BUS SYSTEM

Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer bay are connected to only one single bus as shown.

Fig (1)

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SINGLE BUS SYSTEM WITH BUS SECTIONALIZER

FIG) (2

DOUBLE BUS SYSTEM

FIG (3)

DOUBLE BREAKER BUS SYSTEM

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Fig (4)

ONE AND A HALF BREAKER BUS SYSTEM

FIG (5)

MAIN AND TRANSFER BUS SYSTEM

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FIG (6)

DOUBLE BUS SYSTEM WITH BYPASS ISOLATORS

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FIG (7)

RING BUS SYSTEM

Fig (8)

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8.

Transformers

8.1 BASIC THEORY OF TRANSFORMER

The working principle of transformer is very simple. It depends upon Faraday's laws of Electromagnetic Induction. Actually mutual induction between two or more winding is responsible for transformation action in an electrical transformer. According to Faraday's laws, "Rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or coil".

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Say you have one winding which is supplied by an alternating electrical source. The alternating current through the winding produces a continually changing flux or alternating flux surrounds the winding. If any other winding is brought nearer to the previous one, obviously some portion of this flux will link with the second. As this flux is continually changing in its amplitude and direction, there must be a change in flux linkage in the second winding or coil. According to Faraday's laws of Electromagnetic Induction, there must be an EMF induced in the second. If the circuit of the latter winding is closed, there must be a current flow through it. This is the simplest form of electrical transformer and this is most basic of working principle of transformer.

For better understanding we are trying to repeat the above explanation in more brief here. Whenever we apply alternating current to an electric coil, there will be an alternating flux surrounding that coil. Now if we bring another coil nearby this first one, there will be an alternating flux linkage with that second coil. As the flux is alternating, there will be obviously a rate of change of flux linkage with respect to time in the second coil. Naturally EMF will be induced in it as per Faraday's laws of electromagnetic induction. This is the most basic concept of working principle of transformer.

The winding which takes electrical power from the source, is generally known as Primary Winding of transformer. Here in our above example, it is first winding.

Fig (1)

The winding which gives the desired output voltage due to mutual induction in the transformer, is commonly known as Secondary Winding of Transformer. Here in our example it is second winding

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Fig (2)

The above mentioned form of transformer is theoretically possible but not practically, because in open air very tiny portion of the flux of the first winding will link with second winding so the current flows through the closed circuit of latter, will be so small that it may be difficult to measure.

The rate of change of flux linkage depends upon the amount of linked flux, with the second winding. So it is desired to link almost all flux of primary winding, to the secondary winding. This is effectively and efficiently done by placing one low reluctance path common to both the winding. This low reluctance path is transformer core, through which maximum number of flux produced by the primary is passed through and linked with the secondary winding. This is most basic electrical transformer.

8.2 MAIN CONSTRUCTIONAL PARTS OF TRANSFORMER

Three main parts of a transformer are,

1. Primary Winding of transformer - which produces magnetic flux when it is connected to electrical source.

2. Magnetic transformer core - the magnetic flux produced by the primary winding, will pass through this low reluctance path linked with secondary winding and creates a closed magnetic circuit.

3. Secondary Winding of transformer - the flux, produced by primary winding, passes through the core, will link with the secondary winding. This winding is also wound on the same core and gives the desired output of the transformer.

8.1.3 IDEAL TRANSFORMER

An Ideal Transformer is an imaginary transformer which does not have any loss in it, means no core losses, copper losses and any other losses in transformer. Efficiency of this transformer is considered as 100%.

8.1.4 IDEAL TRANSFORMER MODEL

Ideal Transformer Model is developed by considering a transformer which does not have any loss. That means the windings of the transformer are purely inductive and core of transformer is loss free. There is zero leakage reactance of transformer. As we said, whenever we place a low reluctance core inside the windings, maximum amount of flux passes through this core; but still there is some flux which does not pass through the core but passes through the insulation used in the transformer. This flux does not take part in the transformation action of the transformer. This flux is called leakage flux of transformer.

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In an Ideal Transformer, this leakage flux is considered as nil. That means 100% flux passes through the core and linked with both primary and secondary windings of transformer. Although every winding is supposed to be purely inductive, but it has some resistance which causes voltage drop and I2R loss in it. In such ideal transformer model, the winding are also considered, ideal that means resistance of the winding is zero.

Fig (3)

Now if an alternating source voltage V1 is applied in the primary winding of that Ideal Transformer, there will be a counter self EMF, E1 induced in the primary winding which is purely 180o in phase opposition with supply voltage V1.

Fig (4)

For developing counter EMF, E1 across the primary winding it draws current from the source to produces required magnetizing flux. As the primary winding is purely inductive, that current is in 90o lags from the supply voltage V1. This current is called magnetizing current of transformer Iμ

Fig (5)

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This alternating current, Iμ produces an alternating magnetizing flux Φ which is proportional to that current and hence in phase with it. As this flux is also linked with secondary winding through the transformer core, there will be another EMF, E2 induced in the secondary winding, this is mutually induced EMF. As the secondary is placed on the same core where the primary winding is placed, the EMF induced in the secondary winding of transformer, E2 is in the phase with primary EMF, E1 and in phase opposition with source voltage, V1.

8.1.5 THEORY OF TRANSFORMER

We have discussed about theory of ideal transformer for better understanding of actual elementary theory of transformer. Now we will go through one by one practical aspects of an electrical transformer and try to draw vector diagram of transformer in every step. As we said that in ideal transformer, there are no core losses in transformer. i.e. loss free transformer core. But in practical transformer there are hysteresis and eddy current losses in transformer core.

THEORY OF TRANSFORMER ON NO-LOAD, AND HAVING NO WINDING RESISTANCE AND NO LEAKAGE REACTANCE OF TRANSFORMER

Let us consider one electrical transformer with only core losses. That means that it has only core losses but no copper lose and no leakage reactance of transformer. When an alternating source is applied in the primary, the source will supply the current for magnetizing the transformer core. But this current is not the actual magnetizing current, little bit greater than actual magnetizing current. Actually total current supplied from the source has two components one is magnetizing current which is merely utilized for magnetizing the core and other component of the source current, is consumed for compensating the core losses in transformer. Because of this core loss component, the source current in transformer on no-load condition, supplied from the source as source current is not exactly at 90o lags of supply voltage but it lags behind an angle θ, which is less than 90o.

If total current supplied from source is Io, it will have one component in phase with supply voltage V1 and this component of the current Iw is core loss component. This component is taken in phase with source voltage, because it is associated with active or working losses in transformer. Other component of the source current is denoted as Iμ. This component produces the alternating magnetic flux in the core, so it is watt-less means it is reactive part of the transformer source current. Hence Iμ will be in quadrature with V1 and in phase with alternating flux Φ.

Hence, total primary current in transformer on no-load condition can be represented as

Io = Iμ + Iw and,

|Iμ| = |Io|cosθ

|Iw| = |Io|sinθ

|Io| = ( |Iμ|2 + |Iw|2 )½

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Now you have seen how simple to explain the theory of transformer on no-load.

Fig (6)

THEORY OF TRANSFORMER ON LOAD BUT HAVING NO WINDING RESISTANCE AND LEAKAGE REACTANCE

Now we will examine the behavior of above said transformer on load that means load is connected to the secondary terminals. Consider, transformer having core loss but no copper loss and leakage reactance. Whenever load is connected to the secondary winding, load current will start to flow through the load as well as secondary winding. This load current solely depends upon the characteristics of the load and also upon secondary voltage of the transformer. This current is called secondary current or load current; here it is denoted as I2. As I2 is flowing through the secondary, a self MMF in secondary winding will be produced. Here it is N2I2, where, N2 is the number of turns of the secondary winding of transformer.

Fig (7)

This MMF or magneto motive force in the secondary winding produces flux φ2. This φ2 will oppose the main magnetizing flux and momentarily weakens the main flux and tries to reduce

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primary self induced EMF, E1. If E1 falls down below the primary source voltage V1, there will be extra current flow from source to primary winding. This extra primary current I2′ produces extra flux φ′ in the core which will neutralize the secondary counter flux φ2. Hence the main magnetizing flux of core, Φ remains unchanged irrespective of load.

So the total current drawn by this transformer from source can be divided into two components, first one is utilized for magnetizing the core and compensate the core loss i.e. Io. It is no load component of the primary current. Second one is utilized for compensating the counter flux of the secondary winding. It is known as load component of the primary current. Hence total no load primary current I1 of an electrical transformer having no winding resistance and leakage reactance can be represented as follows

I1 = Io + I2′

Where θ2, is the angle between Secondary Voltage and Secondary Current of a transformer. Now we will precede one further step towards more practical aspects of a transformer.

THEORY OF TRANSFORMER ON LOAD, WITH RESISTIVE WINDING, BUT NO LEAKAGE REACTANCE

Now, consider the winding resistance of transformer but no leakage reactance. So far we have discussed about the transformer which has ideal windings means winding with no resistance and leakage reactance, but now we will consider one transformer which has internal resistance in the winding but no leakage reactance. As the windings are resistive, there would be a voltage drop in the windings.

Fig (8)

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We have proved earlier that total primary current from the source on load is I1. The voltage drop in the primary winding with resistance, R1 is R1I1. Obviously induced EMF across primary winding E1, is not exactly equal to source voltage V1. E1 is less than V1 by voltage drop I1R1.

V1 = E1 + I1R1

Again in the case of secondary, the voltage induced across the secondary winding, E2 does not totally appear across the load since it also drops by an amount I2R2, where R2 is the secondary winding resistance and I2 is secondary current or load current.

Similarly voltage equation of the secondary side of the transformer will be

V2 = E2 − I2R2

THEORY OF TRANSFORMER ON LOAD, WITH RESISTANCE AS WELL AS LEAKAGE REACTANCE IN TRANSFORMER WINDINGS

Now we will consider the condition, when there is leakage reactance of transformer as well as winding resistance.

Fig (9)

Let leakage reactance of primary and secondary windings of the transformer is X1 and X2

respectively.

Hence total impedance of primary and secondary winding with resistance R1 and R2 respectively, can be represented as,

Z1 = R1 + jX1 (impedance of primary winding)

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Z2 = R2 + jX2 (impedance of secondary winding)

We have already established the voltage equation of a transformer on load, with only resistances in the windings; where voltage drops in the windings occur only due to resistive voltage drop. But when we consider leakage reactance of transformer windings, voltage drop occurs in the winding not only because of resistance, it is because of transformer windings. Hence, actual voltage equation of a transformer can easily be determined by just replacing resistances R1 & R2

in the previously established voltage equations by Z1 and Z2.

Therefore, the voltage equations are,

V1 = E1 + I1Z1 & V2 = E2 − I2Z2

V1 = E1 + I1(R1 + jX1) ⇒ V1 = E1 + I1R1 + jI1X1

V2 = E2 - I2(R2 + jX2) ⇒ V2 = E2 - I2R2 − jI2X2

Resistance drops are in the direction of current vector but reactive drop will be in perpendicular to the current vector as shown in the above vector diagram of transformer.

8.1.6 EQUIVALENT CIRCUIT OF TRANSFORMER

Equivalent impedance of Transformer is essential to be calculated as because the electrical transformer is an electrical power system equipment so for estimating different parameters of electrical power system, it may be required to calculate total internal impedance of an electrical transformer viewing from primary side or secondary side as per requirement. This calculation requires equivalent circuit of transformer referred to primary or equivalent circuit of transformer referred to secondary sides respectively. Percentage impedance is also very essential parameter of transformer. Special attention is to be given to this parameter during installing a transformer in an existing electrical power system. Percentage impedance of different power transformers should be properly matched during parallel operation of these transformers.

The percentage impedance can be derived from equivalent impedance of transformer so it can be said that equivalent circuit of transformer is also required during calculation of % impedance.

EQUIVALENT CIRCUIT OF TRANSFORMER REFERRED TO PRIMARY

For drawing equivalent circuit of transformer referred to primary, first we have to establish general equivalent circuit of transformer then we will modify it for referring from primary side. For doing this we first recall the complete vector diagram of a transformer which is shown in the figure below.

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Fig.(10)

Let us consider the transformation ratio be

K = N1/N2 = E1/E2

The applied voltage to the primary is V1 and voltage across the primary winding is E1. Total current supplied to primary is I1. So the voltage V1 applied to the primary is partly dropped by I1Z1 or I1R1 + j.I1X1 before it appears across primary winding. The voltage appeared across winding is countered by primary induced EMF E1.

So voltage equation of this portion of transformer can be written as

V1 - (I1R1 + j.I1X1) = E1

The equivalent circuit for that equation can be drawn as below,

Fig (11)

From the vector diagram above it is found that total primary current I1 has two components one is no - load component Io and other is load component I2′. As this primary current have two components or branches so there must be a parallel path with primary winding of transformer. This parallel path of current is known as excitation branch of equivalent circuit of transformer. The resistive and reactive branches of the excitation circuit can be represented as Ro = E1 / Iw and Xo = E1 / Iμ.

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Fig (12)

The load component I2′ flows through the primary winding of transformer and induced voltage across the winding is E1 as shown in the figure. This induced voltage E1 transforms to secondary and it is E2 and load component of primary current I2′ is transformed to secondary as secondary current I2. Current of secondary is I2. So the voltage E2 across secondary winding is partly dropped by I2Z2 or I2R2 + j.I2X2 before it appears across load. The load voltage is V2.

The complete equivalent circuit of transformer is shown below.

Fig (13)2

Now if we see the voltage drop in secondary from primary side then it would be K-times greater and would be written as K.Z2.I2.

Again I2′.N1 = I2.N2⇒ I2 = I2′.N1 / N2⇒ I2 = K.I2′

Therefore, K.Z2.I2 = K.Z2.K.I2′

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= K2.Z2.I2′

From above equation, Secondary impedance of transformer referred to primary is,

Z2′ = K2.Z2

Hence, R2′ = K2.R2

and X2′ = K2.X2

So the complete equivalent circuit of transformer referred to primary is shown in the figure below,

Fig (14)

APPROXIMATE EQUIVALENT CIRCUIT OF TRANSFORMER

Since Io is very small compared to I1, it is less than 5% of full load primary current, Io changes the voltage drop insignificantly. Hence, it is good approximation to ignore the excitation circuit in approximate equivalent circuit of transformer. The winding resistance and reactance being in series can now be combined into equivalent resistance and reactance of transformer referred to any particular side. In this case it is side 1 or primary side. Here V2′ = K.V2

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Fig (15)

EQUIVALENT CIRCUIT OF TRANSFORMER REFERRED TO SECONDARY

In similar way approximate equivalent circuit of transformer referred to secondary can be drawn. Where, equivalent impedance of transformer referred to secondary, can be derived as

Z1′ = Z1 / K2

Therefore,R1′ = R1 / K2 and X1′ = X1 / K2

Here, V1′ = V1 / K

Fig(16)

8.1.7 LOSSES IN TRANSFORMER

As the electrical transformer is a static device, mechanical loss in transformer normally does not come into picture. We generally consider only electrical losses in transformer. Loss in any machine is broadly defined as difference between input power and output power.

When input power is supplied to the primary of transformer, some portion of that power is used to compensate core losses in transformer i.e. Hysteresis loss in transformer and Eddy Current loss in transformer core and some portion of the input power is lost as I2R loss and dissipated as heat in the primary and secondary winding, as because these windings have some internal resistance in them. The first one is called core loss or iron loss in transformer and later is known

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as ohmic loss or copper loss in transformer. Another loss occurs in transformer, known as Stray Loss, due to Stray fluxes link with the mechanical structure and winding conductors.

COPPER LOSS IN TRANSFORMER

Copper loss is I2R loss, in primary side it is I12R1 and in secondary side it is I2

2R2 loss, where I1 & I2 are primary & secondary current of transformer and R1 & R2 are resistances of primary & secondary winding. As the both primary & secondary currents depend upon load of transformer, so copper loss in transformer vary with load.

CORE LOSSES IN TRANSFORMER

Hysteresis loss and eddy current loss both depend upon magnetic properties of the materials used to construct the transformer core and its design. So these losses in transformer are fixed and do not depend upon the load current. So core losses in transformer which is alternatively known as iron loss in transformer and can be considered as constant for all range of load.

Hysteresis loss in transformer is denoted as, Wh = KhfBm1.6 watts

Eddy Current loss in transformer is denoted as, We = Kef2Kf2Bm

2 wattsWhere, Kh = Hysteresis Constant.

Ke = Eddy Current Constant.Kf = form Constant.

Copper loss can simply be denoted as, IL2R2′ + Stray loss

Where, IL = I2 = load of transformer, and R2′ is the resistance of transformer referred to secondary.

Now we will discuss Hysteresis loss and Eddy Current loss in little bit more details for better understanding the topic of losses in transformer

HYSTERESIS LOSS IN TRANSFORMER

Hysteresis loss in transformer can be explained in different ways. We will discuss two of them, one is physical explanation other is mathematical explanation.

PHYSICAL EXPLANATION OF HYSTERESIS LOSS

The magnetic transformer is made of ′Cold Rolled Grain Oriented Silicon Steel′. Steel is very good ferromagnetic material. Such kinds of materials are very sensitive to be magnetized. That means whenever magnetic flux passes through, it will behave like magnet. Ferromagnetic substances have numbers of domains in their structure. Domains are very small region in the material structure, where all the dipoles are paralleled to same direction. In other words, the domains are like small permanent magnet situated randomly in the structure of substance. These domains are arranged inside the material structure in such a random manner, that net resultant magnetic field of the said material is zero. Whenever external magnetic field or MMF is applied to that substance, these randomly directed domains are arranged themselves in parallel to the axis of applied MMF. After removing this external MMF, maximum numbers of domains again come

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to random positions, but few of them still remain in their changed position. Because of these unchanged domains the substance becomes slightly magnetized permanently. This magnetism is called "Spontaneous Magnetism".

To neutralize this magnetism some opposite MMF is required to be applied. The magneto motive force or MMF applied in the transformer core is alternating. For every cycle, due to this domain reversal there will be extra work done. For this reason, there will be a consumption of electrical energy which is known as Hysteresis loss of transformer.

MATHEMATICAL EXPLANATION OF HYSTERESIS LOSS IN TRANSFORMER

DETERMINATION OF HYSTERESIS LOSS

Consider a ring of ferromagnetic specimen of circumference L meter, cross - sectional area a m2

and N turns of insulated wire as shown in the picture beside,

Fig (17)

Let us consider, the current flowing through the coil is I A,

Magnetizing force,

H = NI/L or I = HL/N

Let, the flux density at this instant is B,Therefore, total flux through the ring, Φ = BXa Wb

As the current flowing through the solenoid is alternating, the flux produced in the iron ring is also alternating in nature, so the EMF (e′) induced will be expressed as,

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Fig (18)

e = - N (dφ/dt)

e = -N (dBa/dt)

e = -Na (dB/dt)

According to Lenz’s law this induced EMF will oppose the flow of current, therefore, in order to

maintain the current I in the coil, the source must supply an equal and opposite EMF. Hence

applied EMF,

e = e’ = Na (dB/dt)

Energy consumed in short time dt, during which the flux density has changed,

= e.I.dt

= Na (dB/dt) x I x dt

= Na (dB/dt) x (HL/N) x dt

= aLH.dB Joules

Thus, total work done or energy consumed during one complete cycle of magnetism,

W = aL ʃ H.dB, where limits of integration are from zero to Bmax

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Now aL is the volume of the ring and H.dB is the area of elementary strip of B - H curve shown in the figure above,

ʃ H.dB = total area enclosed by Hysteresis Loop.

Therefore, Energy consumed per cycle = volume of the ring X area of hysteresis loop.

In the case of transformer, this ring can be considered as magnetic core of transformer. Hence this work done is nothing but electrical energy loss in transformer core and this is known as hysteresis loss in transformer.

EDDY CURRENT LOSS

In transformer we supply alternating current in the primary, this alternating current produces alternating magnetizing flux in the core and as this flux links with secondary winding there will be induced voltage in secondary, resulting current to flow through the load connected with it. Some of the alternating fluxes of transformer may also link with other conducting parts like steel core or iron body of transformer etc. As alternating flux links with these parts of transformer, locally induced EMF will result. Due to these EMFs there will be currents which will circulate locally in those parts of the transformer. These circulating current will not contribute in output of the transformer and dissipated as heat. This type of energy loss is called eddy current loss of transformer.

8.2 AUTO TRANSFORMER

THEORY OF AUTO TRANSFORMER

Auto transformer is kind of electrical transformer where primary and secondary shares same common single winding. In Auto Transformer, one single winding is used as primary winding as well as secondary winding. But in two windings transformer two different windings are used for primary and secondary purpose. A diagram of auto transformer is shown below, winding AB of total turns N1 is considered as primary winding. This winding is tapped from point C and the portion BC is considered as secondary. Let's assume the number of turns in between point B and C is N2.

If V1 voltage is applied across the winding i.e. in between A and C

So voltage per turn in this winding is V1/N1

Hence, the voltage across the portion BC of the winding, will be

(V1/N1) x N2 = V2

V2/V1 =N2/N1 = Constant = k

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As BC portion of the winding is considered as secondary, it can easily be understood that value of constant k-is nothing but turns ratio or voltage ratio of that Auto Transformer.

When load is connected between secondary terminals i.e. between B and C, load current I2 starts to flow. The current in the secondary winding or common winding is the difference of I2 & I1.

Fig (19)

COPPER SAVINGS IN AUTO TRANSFORMER

Now we will discuss savings of copper in an auto transformer compared to conventional two windings electrical transformer. We know that weight of copper of any winding depends upon its length and cross-sectional area. Again length of conductor in winding is proportional to its number of turns and cross-sectional area varies with rated current.So weight of copper in winding is directly proportional to product of number of turns and rated current of the winding.

Therefore, weight of copper in the section AC proportional to

(N1 − N2)I1

and similarly, weight of copper in the section BC proportional to

N2( I2 − I1)

Hence, total weight of copper in the winding of Auto Transformer proportional to

(N1 − N2)I1 + N2( I2 − I1)= N1I1 − N2I1 + N2I2 − N2I1

= N1I1 + N2I2 − 2N2I1

= 2N1I1 − 2N2I1 (Since, N1I1 = N2I2)= 2(N1I1 − N2I1)

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In similar way it can be proved, the weight of copper in two winding transformer is proportional to,

N1I1 + N2I2 = 2N1I1 (Since, in a transformer N1I1 = N2I2)

Let's assume, Wa and Wtw are weight of copper in auto transformer and two winding transformer respectively,

Hence, Wa /Wtw = 2(N1I1 − N2I1)/2(N1I1)

= N1I1 − N2I1/N1I1

= 1 – N2I1/N1I1

= 1 – N2/N1

= 1 – k

∴ Wa = Wtw(1 − k)⇒Wa = Wtw − kWtw∴ Saving of copper in auto transformer compared to two winding transformer,⇒Wtw − Wa = kWtw

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Fig (20)

Auto transformer employs only single winding per phase as against two distinctly separate windings in a conventional power transformer.

ADVANTAGES OF USING AUTO TRANSFORMER

For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the corresponding size of two winding transformer. For transformation ratio say 20 however the size would be 95%. The saving in cost is of course not in the same proportion. The saving of cost is appreciable when the ratio of transformer is low, that is lower than 2.

DISADVANTAGES OF USING AUTO TRANSFORMER

But auto transformer has the following disadvantages:

1. Because of electrical conductivity of the primary and secondary windings the lower voltage circuit is liable to be impressed upon by higher voltage. To avoid breakdown in the lower voltage circuit, it becomes necessary to design the low voltage circuit to withstand higher voltage.

2. The leakage flux between the primary and secondary windings is small and hence the impedance is low. This results into severer short circuit currents under fault conditions.

3. The connections on primary and secondary sides have necessarily to be same, except when using interconnected starring connections. This introduces complications due to changing primary and secondary phase angle particularly in the case-by-case of delta / delta connection.

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4. Because of common neutral in a star / star connected auto transformer it is not possible to earth neutral of one side only. Both their sides have to have their neutrality either earth or isolated.

5. It is more difficult to preserve the electromagnetic balance of the winding when voltage adjustment tapping are provided. It should be known that the provision of adjusting tapping on an auto transformer increases considerably the frame size of the transformer. If the range of tapping is very large, the advantages gained in initial cost is lost to a great event

8.3 TERTIARY WINDING OF TRANSFORMER

In some high rating transformer, one winding, in addition to its primary and secondary winding, is used. This additional winding, apart from primary and secondary windings, is known as Tertiary Winding of Transformer. Because of this third winding, the transformer is called Three Winding Transformer or 3 Winding Transformer.

ADVANTAGES OF USING TERTIARY WINDING IN TRANSFORMER

Tertiary Winding is provided in electrical transformer to meet one or more of the following requirements

1. It reduces the unbalancing in the primary due to unbalancing in three phase load2. It redistributed the flow of fault current3. Sometime it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.4. As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

STABILIZATION BY TERTIARY WINDING OF TRANSFORMER

In star - star transformer comprising three single units or a single unit with 5 limb core offers high impedance to the flow of unbalanced load between the line and neutral. This is because, in both of these transformers, there is very low reluctance return path of unbalanced flux.

If any transformer has N - turns in winding and reluctance of the magnetic path is RL, then,

MMF = N.I = ΦRL ....... (1)

Where I and Φ are current and flux in the transformer.Again, induced voltage V = 4.44ΦfN⇒ V ∝ Φ⇒ Φ = K.V (Where K is constant)....... (2)

Now, from equation (1) & (2), it can be rewritten as,

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N.I = K.V.RL ⇒ V/I = N/(K.RL) ⇒ Z = N/(K.RL)⇒ Z ∝ 1/RL

From, this above mathematical expression it is found that impedance is inversely proportional to reluctance. The impedance offered by the return path of unbalanced load current, is very high where very low reluctance return path is provided for unbalanced flux.

Fig (21)

In other words, very high impedance to the flow of unbalanced current in 3 phases system between line and neutral. Any unbalanced current in three phase system can be divided in to three sets of components likewise positive sequence, negative sequence and zero sequence components. The zero sequence current actually co-phasial current in three lines. If value of co-phasial current in each line is Io, then total current flows through the neutral of secondary side of transformer is In = 3.Io. This current cannot be balanced by primary current as the zero sequence current cannot flow through the isolated neutral star connected primary. Hence the said current in the secondary side set up a magnetic flux in the core.

As we discussed earlier in this chapter low reluctance path is available for the zero sequence flux in a bank of single phase units and in the 5 limb core consequently the impedance offered to the zero sequence current is very high. The delta connected tertiary winding of transformer permits the circulation of zero sequence current in it. This circulating current in this delta winding balance the zero sequence component of unbalance load, hence prevent unnecessary development of unbalance zero sequence flux in the transformer core. In few words it can be said that, placement of tertiary winding in star - star - neutral transformer considerably reduces the zero sequence impedance of transformer.

RATING OF TERTIARY WINDING OF TRANSFORMER

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Rating of tertiary winding of transformer depends upon its use. If it has to supply additional load, its winding cross-section and design philosophy is decided as per load and three phase dead short circuit on its terminal with power flow from both sides of HV & MV.

In case it is to be provided for stabilizing purpose only, its cross - section and design has to be decided from thermal and mechanical consideration for the short duration fault currents during various fault conditions single line-to-ground fault being the most onerous.

8.4 TRANSFORMER CONNECTIONS/TRANSFORMER BANK CONNECTIONS, AND

WINDING CONNECTIONS/VECTOR GROUPS

It is found that generation, transmission and distribution of electrical power are more economical in three phase system than single phase system. For three phase system three single phase transformers are required. Three phase transformation can be done in two ways, by using single three phase transformer or by using a bank of three single phase transformers. Both are having some advantages over other. Single three phase transformer costs around 15% less than bank of three single phase transformers. Again former occupies less space than later. For very big transformer, it is impossible to transport large three phase transformer to the site and it is easier to transport three single phase transformers which is erected separately to form a three phase unit. Another advantage of using bank of three single phase transformers is that, if one unit of the bank becomes out of order, then the bank can be run as open delta.

A Varity of connection of three phase transformer are possible on each side of both a single three phase transformer or a bank of three single phase transformers.

MARKING OR LABELING THE DIFFERENT TERMINALS OF TRANSFORMER

Terminals of each phase of HV side should be labeled as capital letters, A, B, C, and those of LV side should be labeled as small letters, a, b, c. Terminal polarities are indicated by suffixes 1 & 2. Suffix 1’s indicates similar polarity ends and so do 2’s.

STAR-STAR TRANSFORMER

Star-Star Transformer is formed in a three phase transformer by connecting one terminal of each phase of individual side, together. The common terminal is indicated by suffix 1 in the figure below. If terminal with suffix 1 in both primary and secondary are used as common terminal, voltages of primary and secondary are in same phase. That is why this connection is called zero degree connection or 0o - connection.

If the terminals with suffix 1 are connected together in HV side as common point and the terminals with suffix 2 in LV side are connected together as common point, the voltages in primary and secondary will be in opposite phase. Hence, Star-Star Transformer connection is called 180o - Connection, of three phase transformer.

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Fig (22)

DELTA-DETLA TRANSFORMER

In delta-delta transformer, 1 suffixed terminals of each phase primary winding will be connected with 2 suffixed terminal of next phase primary winding.

Fig (23)

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If primary is HV side, then A1 will be connected to B2, B1 will be connected to C2 and C1 will be connected to A2. Similarly in LV side 1 suffixed terminals of each phase winding will be connected with 2 suffixed terminals of next phase winding. That means, a1 will be connected to b2, b1 will be connected to c2 and c1 will be connected to a2. If transformer leads are taken out from primary and secondary 2 suffixed terminals of the winding, then there will be no phase difference between similar line voltages in primary and secondary. This delta-delta transformer connection is zero degree connection or 0o - Connection.

But in LV side of transformer, if, a2 is connected to b1, b2 is connected to c1 and c2 is connected to a1. The secondary leads of transformer are taken out from 2 suffixed terminals of LV windings, and then similar line voltages in primary and secondary will be in phase opposition. This connection is called 180o connection, of three phase transformer.

STAR-DELTA TRANSFORMER

Here in star delta transformer, star connection in HV side is formed by connecting all the 1 suffixed terminals together as common point and transformer primary leads are taken out from 2 suffixed terminals of primary windings.

The delta connection in LV side is formed by connecting 1 suffixed terminals of each phase LV winding with 2 suffixed terminal of next phase LV winding. More clearly, a1 is connected to b2, b1 is connected to c2 and c1 is connected to a2. The secondary (here it considered as LV) leads are taken out from

Fig (24)

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2 suffixed ends of the secondary windings of transformer. The transformer connection diagram is shown in the figure beside.

It is seen from the figure that the sum of the voltages in delta side is zero. This is a must as otherwise closed delta would mean a short circuit. It is also observed from the phasor diagram that, phase to neutral voltage (equivalent star basis) on the delta side lags by −30o to the phase to neutral voltage on the star side; this is also the phase relationship between the respective line to line voltages. This star delta transformer connection is therefore known as −30o connection.

STAR–DELTA +30O CONNECTION IS ALSO POSSIBLE BY CONNECTING

SECONDARY TERMINALS IN FOLLOWING SEQUENCE. A2 IS CONNECTED TO B1,

B2 IS CONNECTED TO C1 AND C2 IS CONNECTED TO A1. THE SECONDARY

LEADS OF TRANSFORMER ARE TAKEN OUT FROM 2 SUFFIXED TERMINALS OF

LV WINDINGS,

DELTA-STAR TRANSFORMER

Delta-Star transformer connection of three phase transformer is similar to star–delta connection. If anyone interchanges HV side and LV side of star–delta transformer in diagram, it simply becomes delta–star connected three phase transformer. That means all small letters of star delta connection should be replaced by capital letters and all small letters by capital in delta star transformer connection.

Fig (25)

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DELTA-ZIGZAG TRANSFORMER

The winding of each phase on the star connected side is divided into two equal halves in delta zig-zag transformer. Each leg of the core of transformer is wound by half winding from two different secondary phases in addition to its primary winding.

STAR-ZIGZAG TRANSFORMER

The winding of each phase on the secondary star in a star zigzag transformer is divided into two equal halves. Each leg of the core of transformer is wound by half winding from two different secondary phases in addition to its primary winding.

CHOICE BETWEEN STAR CONNECTION AND DELTA CONNECTION OF THREE

PHASE TRANSFORMER

In star connection with earthed neutral, phase voltage i.e. phase to neutral voltage, is 1/√3 times of line voltage i.e. line to line voltage. But in the case of delta connection phase voltage is equal to line voltage. Star connected high voltage side electrical transformer is about 10% cheaper than that of delta connected high voltage side transformer.

Let’s explain, let, the voltage ratio of transformer between HV & LV is K, voltage across HV winding is VH and voltage across LV winding is VL and voltage across transformer leads in HV side say Vp and in LV say Vs.

IN STAR-STAR TRANSFORMER

VH = Vp/√3 and VL = Vs/√3⇒ Vp / Vs = VH / VL = K⇒ VH = K. VL

Voltage difference between HV & LV winding,VH − VL = Vp − Vs = (K − 1).Vs

IN STAR-DELTA TRANSFORMERVH = Vp/√3 and VL = Vs

Voltage difference between HV & LV winding,VH − VL = Vp/√3 − Vs = (K/√3 − 1).Vs

IN DELTA-STAR TRANSFORMER

VH = Vp and VL = Vs/√3Voltage difference between HV & LV winding,

VH − VL = Vp − Vs/√3 = (K − 1/√3) .Vs

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For 132/33KV transformer K = 4, Therefore,

Case – 1:Voltage difference between HV & LV winding,(4 − 1) Vs = 3. Vs

Case – 2:Voltage difference between HV & LV winding,(4/√3 − 1) Vs = 1.3 Vs

Case – 3:Voltage difference between HV & LV winding,(4 − 1/√3) Vs = 3.42 Vs

In case 2 voltage differences between HV and LV winding is minimum therefore potential stresses between them is minimum, implies insulation cost in between these windings is also less. Hence for step down purpose Star–Delta transformer connection is most economical, design for transformer. Similarly it can be proved that for Step up purpose Delta - Star three phase transformer connection is most economical.

8.5 PHASE SEQUENCE

In three phase system, we have all the three phasors, 120o apart. There are three types of phase

sequences, which are

1. Positive Sequence2. Negative Sequence3. Zero Sequence

Fig (26)

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8.6 PARALLEL OPERATION OF TRANSFORMERS It is economical to install a numbers of smaller rated transformers in parallel than installing a bigger rated electrical transformer.

Fig (27)

This has mainly the following advantages,

1. To maximize electrical power system efficiency: Generally electrical transformer gives the maximum efficiency at full load. If we run numbers of transformers in parallel, we can switch on only those transformers which will give the total demand by running nearer to its full load rating for that time. When load increases we can switch on, one by one other transformers connected in parallel to fulfill the total demand. In this way we can run the system with maximum efficiency.

2. To maximize electrical power system availability: If numbers of transformers run in parallel we can take shutdown on any one of them for maintenance purpose. Other parallel transformers in system will serve the load without total interruption of power.

3. To maximize power system reliability: If any one of the transformers run in parallel, is tripped due to fault, other parallel transformers is the system will share the load hence power supply may not be interrupted if the shared loads do not make other transformers to over load.

4. To maximize electrical power system flexibility: Always there is a chance of increasing or decreasing future demand of power system. If it is predicted that power demand will be increased in future, there must be a provision of connecting transformers in system in parallel to fulfill the extra demand because it is not economical from business point of view to install a bigger rated single transformer by forecasting the increased future demand as it is unnecessary investment of money. Again if future demand is decreased,

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transformers running in parallel can be removed from system to balance the capital investment and its return.

CONDITIONS FOR PARALLEL OPERATION OF TRANSFORMERS

When two or more transformers are run in parallel they must satisfy the following conditions for satisfactory performance. These are the conditions for parallel operation of transformers.

1. Same voltage ratio of transformer2. Same percentage impedance3. Same polarity4. Same phase sequence

1. Same Voltage RatioIf two transformers of different voltage ratios are connected in parallel with same primary supply voltage, there will be a difference in secondary voltages. Now say the secondary of these transformers are connected to same bus, there will be a circulating current between secondary’s and therefore between primary’s also. As the internal impedance of transformer is small, a small voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R loss.2. Same Percentage ImpedanceThe current shared by two transformers running in parallel should be proportional to their MVA ratings. Again, electric current carried by these transformers are inversely proportional to their internal impedance. From these two statements it can be said that impedance of transformers running in parallel are inversely proportional to their MVA ratings. In other words percentage impedance or per unit values of impedance should be identical for all the transformers running in parallel.

3. Same PolarityPolarity of all transformers running in parallel should be same otherwise huge circulating current flows in the transformer but no load will be fed from these transformers. Polarity of transformer means the instantaneous direction of induced EMF in secondary. If the instantaneous directions of induced secondary EMF in two transformers are opposite to each other when same input power is fed to the both of the transformers, the transformers are said to be in opposite polarity. If the instantaneous directions of induced secondary EMF in two transformers are same when same input power is fed to both the transformers, the transformers are said to be in same polarity.

4. Same Phase SequenceThe phase sequence or the order in which the phases reach their maximum positive voltage must be identical for two parallel transformers. Otherwise, during the cycle, each pair of phases will be short circuited.

The above said conditions must be strictly followed for parallel operation of transformers but totally identical percentage impedance of two different transformers is difficult to achieve practically that is why the transforms run in parallel may not have exactly same percentage impedance but the values would be as nearer as possible.

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8.5 TRANSFORMER TESTS

TYPES OF TRANSFORMER TESTING

Tests Done At FactoryType TestsRoutine TestsSpecial Tests

Type Test Of Transformer: To prove that the transformer meets customer’s specifications and design expectations, the transformer has to go through different testing procedures in manufacturer premises. Some transformer tests are carried out for confirming the basic design expectation of that transformer. These tests are done mainly on a randomly selected unit units in a lot. Type test of transformer confirms main and basic design criteria of a production lot.

Routine Tests of Transformer: Routine tests of transformer are mainly for confirming operational performance of individual unit in a production lot. Routine tests are carried out on every unit manufactured.

Special Tests Of Transformer: Special tests of transformer are done as per customer requirement to obtain information useful to the user during operation or maintenance of the transformer.

TESTS DONE AT SITE

1. Pre Commissioning Tests2. Periodic/ Condition Monitoring Tests3. Emergency Tests

1. PRE-COMMISSIONING TEST OF TRANSFORMER

In addition to these, the transformer also goes through some other tests, performed on it, before actual commissioning of the transformer at site. These tests are done to assess the condition of transformer after installation and compare the test results of all the low voltage tests with the factory test reports

8.7.1 POLARITY TEST

Polarity of a transformer is defined as the relative instantaneous direction of current in its

terminals. Terminals have same polarity if current entering in the primary terminal and that

leaving from the secondary terminal is such that they form a continuous circuit. Terminals

having same polarity are marked with same number i.e. 1 or 2 e.g. H1, X1 and H2, X2 or with

same color.

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Polarity of Transformer may be changed by interchanging the connection of either HV or LV

side. Polarity of Transformer is needed when:

1. Transformers are connected in parallel for load sharing2. When single phase transformers are connection in Delta-Star, Star-Star etc. 3. When Transformer is under ratio test.

Polarity of 3-phase Transformer is indicated by phase marking or vector diagram or vector

groups.

POLARITY MEASUREMENT OF SINGLE PHASE TRANSFORMERS

It is carried out to know about the physical location of H1, H2, X1 and X2 Terminals etc.

Polarity of Single Phase Transformers can be measured by one of the following three methods:

1. D.C Method2. A.C Method 3. Ratio Meter Method

PROCEDUTRE FOR POLARITY CHECKING OF SINGLE PHASE TRANSFORMER BY D.C. METHOD

On HV side of transformer, select H1 and H2 terminals according to your own choice and connect them with 1.5 volts Dry Cell through a Switch. Then connect DC voltmeter to LV side in such a way that its positive terminal is connected to that terminal which is adjacent opposite to that HV Terminal to which positive of Cell is connected. After making connections as directed above, close switch “S”. If Voltmeter needle deflects towards clockwise direction (or upscale or right side) then polarity of transformer is “subtractive” (i.e. H1 and X1 terminals of transformer are adjacent opposite to each other).If voltmeter needle deflects towards anticlockwise direction (or down scale or left side), then polarity of transformer is “additive” (i.e. H1 and X1 terminals of transformer are diagonally opposite to each other).

1. Always energize HV side of transformer 2. This method is limited to low capacity transformer only because in high capacity

transformers danger of induction kick is present due to collapsing of flux when switch is put off.

3. For reducing the danger of inductive kick4. Use low capacity and low voltage battery i.e. always use 1.5v dry cell. Never use station/

truck battery.5. Don’t touch the leads during testing. After completion of test immediately disconnect

battery from the circuit

PROCEDURE FOR POLARITY CHECKING OF SINGLE PHASE TRANSFORMERS BY A.C. METHOD

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On HV side of transformer, select H1 and H2 terminals according to your own choice and connect them with a 220 volts AC supply through a Switch. Jumper any two adjacent HV and LV terminals, while connect the remaining HV and LV terminal through an AC voltmeter-V1. Also connect another AC voltmeter-V2 across the H1 and H2. After making connections, close switch “S”. If voltmeter-V1 reading is less than voltmeter-V2 reading, then polarity of transformer is “subtractive” (i.e. H1 and X1 terminals of transformer are adjacent opposite to each other). If voltmeter-V1 reading is more than voltmeter-V2 reading, then polarity of transformer is “additive” (i.e. H1 and X1 terminals of transformer are diagonally opposite to each other).

1. Always energize HV side of transformer2. Jumper is put to those HV and LV terminals which are adjacent opposite to each other3. Voltmeters V1 and V2 must be calibrated before the start of test

PROCEDURE FOR POLARITY TEST OF SINGLE PHASE TRANSFORMER BY RATIOMETER

Select H1 and H2 terminals of transformer on HV side according to your own choice. Connect H1

and H2 terminals of Ratio-meter to H1 and H2 terminals of transformer respectively. Then connect X1 terminal of Ratio meter to that LV terminal of transformer which is adjacent opposite to its H1 terminal and connect X2 terminal of Ratio-meter to the other LV terminal of transformer. Give supply to Ratio meter and follow the Ratio-meter manual for polarity checking, and mark the polarity accordingly.

8.7.2 INSULATION RESISTANCE TEST The main purpose of this test is to detect major faults of major insulation. To assess that transformer can be energized or not i.e. to know about the condition of transformer major insulations. For this Ri reading of 1 Mega-ohm/1kV at 20oC oil temperature is considered satisfactory. Insulation Resistance Test is performed on transformer when

1. Newly installed 2. Under fault

Insulation resistance test of transformer is essential type test. This test is carried out to ensure the healthiness of overall insulation system of an electrical power transformer.

PROCEDURE OF INSULATION RESISTANCE TEST OF TRANSFORMER1. First disconnect all the line and neutral terminals of the transformer.2. Test Set leads to be connected to LV and HV bushing studs to measure Insulation

Resistance, Ri value in between the LV and HV windings.3. Test Set leads to be connected to HV bushing studs and transformer tank earth point to

measure Insulation Resistance, Ri value in between the HV windings and earth.4. Test Set leads to be connected to LV bushing studs and transformer tank earth point to

measure Insulation Resistance, Ri value in between the LV windings and earth.

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It is unnecessary to perform insulation resistance test of transformer per phase wise in three phase transformer. Ri values are taken between the windings collectively as because all the windings on HV side are internally connected together to form either star or delta and also all the windings on LV side are internally connected together to form either star or delta.

MEASUREMENTS ARE TO BE TAKEN AS FOLLOWS

For Auto Transformer: HV-IV to LV, HV-IV to E, LV to E

For Two Winding Transformer: HV to LV, HV to E, LV to E

For Three Winding Transformer: HV to IV, HV to LV, IV to LV, HV to E, IV to E, LV to E

Oil temperature should be noted at the time of insulation resistance test of transformer. Since the Ri value of transformer insulating oil may vary with temperature. Ri values to be recorded at intervals of 15 seconds, 1 minute and 10 minutes.

With the duration of application of voltage, Ri value increases. The increase in Ri is an indication of dryness of insulation.

Absorption Coefficient = 1 minute value/ 15 sec valuePolarization Index = 10 minutes value / 1 minute value

8.7.3 TRANSFORMER TURN RATIO TEST

The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So Transformer Ratio Test is an essential type test of transformer. This test also performed as routine test of transformer. So for ensuring proper performance of electrical power transformer, voltage and turn ratio test of transformer are one of the vital tests. Actually the no load voltage ratio of transformer is equal to the turn ratio. So ratio test of transformer is same as no load voltage ratio.

PROCEDURE OF TRANSFORMER RATIO TEST

The procedure of Transformer Ratio Test is simple. We just apply three phase 415 V supply to HV winding, with keeping LV winding open. Then we measure the induced voltages at HV and LV terminals of transformer to find out actual voltage ratio of transformer.

We repeat the test for all tap position separately.

1. First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open.

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2. Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each phase (Phase-Phase) on HV and induced voltages at LV terminals simultaneously.

3. After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repeat test.

4. Repeat the same for each of the tap position separately.

The above transformer ratio test can also be performed by portable Transformer Turns Ratio (TTR) Meter. They have an in built power supply, with the voltages commonly used being very low, such as 8-10 V and 50 Hz. The HV and LV windings of one phase of a transformer are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.

Let's have a discussion on Transformer Turns Ratio (TTR) Meter method of turn ratio test of transformer.

A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio of induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1 %

Fig (28)

Theoretical Turn Ratio = HV Winding Voltage/ LV Winding Voltage

This theoretical turn ratio is adjusted on the transformer turn ratio tested or TTR by the adjustable transformer as shown in the figure above and it should be changed until a balance occurs in the percentage error indicator. The reading on this indicator implies the deviation of measured turn ratio from expected turn ratio in percentage.

Deviation in % = (Measured Turn Ratio - Expected Turn Ratio)/ Expected Turn Ratio x 100

Out-of-tolerance, ratio test of transformer can be due to shorted turns, especially if there is an associated high excitation current.

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Open turns in HV winding will indicate very low exciting current and no output voltage since open turns in HV winding causes no excitation current in the winding means no flux hence no induced voltage. But open turn in LV winding causes, low fluctuating LV voltage but normal excitation current in HV winding. Hence open turns in LV winding will be indicated by normal levels of exciting current, but very low levels of unstable output voltage.

The turn ratio test of transformer also detects high resistance connections in the lead circuitry or high contact resistance in tap changers by higher excitation current and a difficulty in balancing the bridge.

8.7.4 OPEN CIRCUIT TEST

The connection diagram for open circuit test on transformer is shown in the figure. A voltmeter, wattmeter, and an ammeter are connected in LV side of the transformer as shown.

Fig (29)

The voltage at rated frequency is applied to that LV side with the help of a variac of variable ratio auto transformer. The HV side of the transformer is kept open. Now with help of variac applied voltage is slowly increase until the voltmeter gives reading equal to the rated voltage of the LV side. After reaching at rated LV side voltage, all three instruments reading (Voltmeter, Ammeter and Wattmeter) are recorded. The ammeter reading gives the no load current I e. As no load current Ie is quite small compared to rated current of the transformer, the voltage drops due to this current then can be taken as negligible. Since, voltmeter reading V1 can be considered equal to secondary induced voltage of the transformer. The input power during test is indicated by wattmeter reading.

As the transformer is open circuited, there is no output hence the input power here consists of core losses in transformer and copper loss in transformer during no load condition. But as said earlier, the no load current in the transformer is quite small compared to full load current so copper loss due to the small no load current can be neglected. Hence the wattmeter reading can be taken as equal to core losses in transformer. Let us consider wattmeter reading is Po.

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Po = V1 2/Rm, Where Rm is shunt branch resistance of transformer

If, Zm is shunt branch impedance of transformer, then,

Zm = V1/ Ie,

therefore, if shunt branch reactance of transformer is Xm, then,

(1/ Xm)2 = (1/ Zm)2 - (1/ Rm)2

These values are referred to the LV side of transformer as because the test is conducted on LV side of transformer. These values could easily be referred to HV side by multiplying these values with square of transformation ratio. Therefore it is seen that the open circuit test on transformer is used to determine core losses in transformer and parameters of shunt branch of the equivalent circuit of transformer.

8.7.5 SHORT CIRCUIT TEST

The connection diagram for short circuit test on transformer is shown in the figure. A voltmeter, wattmeter, and an ammeter are connected in HV side of the transformer as shown. The voltage at rated frequency is applied to that HV side with the help of a variac of variable ratio auto transformer.

Fig (30)

The LV side of the transformer is short circuited. Now with help of variac applied voltage is slowly increase until the ammeter gives reading equal to the rated current of the HV side. After

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reaching at rated current of HV side, all three instruments reading (Voltmeter, Ammeter and Wattmeter) are recorded. The ammeter reading gives the primary equivalent of full load current IL. As the voltage, applied for full load current in short circuit test on transformer, is quite small compared to rated primary voltage of the transformer, the core loss in transformer can be taken as negligible here. Let’s, voltmeter reading is Vsc. The input power during test is indicated by wattmeter reading. As the transformer is short circuited, there is no output hence the input power here consists of copper losses in transformer. Since, the applied voltage Vsc is short circuit voltage in the transformer and hence it is quite small compared to rated voltage so core loss due to the small applied voltage can be neglected. Hence the wattmeter reading can be taken as equal to copper losses in transformer. Let us consider wattmeter reading is Psc.

Psc = Re.IL2, Where Re is equivalent resistance of transformer

If, Ze is equivalent impedance of transformer, then

Ze = Vsc/ IL, therefore, if equivalent reactance of transformer is Xe, then

Xe2 = Ze

2 - Re2

These values are referred to the HV side of transformer as because the test is conducted on HV side of transformer. These values could easily be referred to LV side by dividing these values with square of transformation ratio.

Therefore it is seen that the Short Circuit test on transformer is used to determine losses in transformer at full load and parameters of approximate equivalent circuit of transformer.

8.7.6 VERIFICATION OF VECTOR GROUP

In three phase transformers, it is essential to carry out a Vector Group Test of transformer. Proper vector grouping in a transformer is an essential criterion for parallel operation of transformers.

There are several internal connections of transformers available in market. These connections give various magnitudes and phase of the secondary voltage; the magnitude can be adjusted for parallel operation by suitable choice of turn ratio, but the phase divergence cannot be compensated. So we have to choose those transformers for parallel operation whose phase sequence and phase divergence are same. All the transformers with same vector group have same phase sequence and phase divergence between primary and secondary. So before procuring one electrical power transformer, one should ensure the vector group of the transformer, whether it will be matched with the existing system or not. The Vector Group Test of transformer confirms the requirements.

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The vector group of transformer is an essential property for successful parallel operation of transformer. Hence every transformer must undergo Vector Group Test at factory site for ensuring the customer specified vector group of transformer.

The phase sequence or the order, in which the phases reach their maximum positive voltages, must be identical for two paralleled transformers. Otherwise, during the cycle, each pair of phases will be short circuited.

Several secondary connections are available in respect of various primary three phase connections in a three phase transformer. So for same primary applied three phase voltage there may be different three phase secondary voltages with various magnitudes and phases for different internal connection of the transformer. Let's have a discussion in detail by example for better understanding.

We know that, the primary and secondary coils on any one limb have induced EMFs that are in time-phase. Let's consider two transformers of same number primary turns and the primary windings are connected in star. The secondary numbers of turns per phase in both transformers are also same. But the first transformer has star connected secondary and other transformer has delta connected secondary. If same voltages are applied in primary of both transformers, the secondary induced EMF in each phase will be in same time-phase with that of respective primary phase, as because the primary and secondary coils of same phase are wound on the same limb in the transformer core. In first transformer, as the secondary is star connected, the secondary line voltage is √3 times of induced voltage per secondary phase coil.

But in case of second transformer, where secondary is delta connected, the line voltage is equal to induced voltage per secondary phase coil. If we go through the vector diagram of secondary line voltages of both transformers, we will easily find that there will be a clear 30o angular difference between the line voltages of these transformers. Now, if we try to run these transformers in parallel then there will be circulating current flows between the transformers as because there is a phase angle difference between their secondary line voltages. This phase difference cannot be compensated. Thus two sets of connections giving secondary voltages with a phase displacement cannot be intended for parallel operation of transformer.

The following table gives the connections for which from the view point of phase sequence and angular divergences, transformer can be operated parallel. According to their vector relation, all three phase transformers are divided into different vector group of transformer. All transformers of a particular vector group can easily be operated in parallel if they fulfill other condition for parallel operations of transformers.

GROUP Connection Connection

0(0o)

Yy0 Dd0

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1( 30o)

Yd1 Dy1

GROUP Connection Connection

11( - 30o)

Yd11 Dy11

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Fig (31)

PROCEDURE OF VECTOR GROUP TEST OF TRANSFORMER

Let’s have a YNd11 transformer.

1. Connect neutral point of star connected winding with earth.2. Join 1U of HV and 2W of LV together.

3. Apply 415V, three phase supply to HV terminals.

4. Measure voltages between terminals

2U–1N, 2V–1N, 2W–1N, which means voltages

between each LV terminal and HV neutral 5. Also measure voltages between terminals 2V–1V,

2W–1W and 2V–1W.

Fig (32)

For YNd11 transformer, we will find,

2U–1N > 2V–1N > 2W–1N2V–1W > 2V–1V or 2W–1W

The Vector Group Test of transformer for other group can also be done in similar way.

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8.8 TRANSFORMER COOLING SYSTEM

The main source of heat generation in transformer is its copper loss or I2R loss. Although there are other factors contributing heat in transformer such as hysteresis & eddy current losses but contribution of I2R loss dominates them. If this heat is not dissipated properly, the temperature of the transformer will rise continually which may cause damages to paper insulation and liquid insulation of transformer. So it is essential to control the temperature within permissible limit to ensure the long life of transformer by reducing thermal degradation of its insulation system. In electrical transformer we use external transformer cooling system to accelerate the dissipation rate of heat of transformer.

DIFFERENT TRANSFORMER COOLING METHODS

For accelerating cooling different transformer cooling methods are used depending upon their size and ratings. We will discuss these one by one below,

ONAN COOLING OF TRANSFORMER

This is the simplest transformer cooling system. The full form of ONAN is "Oil Natural Air Natural". Here natural convectional flow of hot oil is utilized for cooling. In convectional circulation of oil, the hot oil flows to the upper portion of the transformer tank and the vacant place is occupied by cold oil. This hot oil which comes to upper side will dissipate heat in the atmosphere by natural conduction, convection & radiation in air and will become cold. In this way the oil in the transformer tank continually circulate when the transformer is loaded.

Fig (33)

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As the rate of dissipation of heat in air depends upon dissipating surface of the oil tank, it is essential to increase the effective surface area of the tank, so additional dissipating surface in the form of tubes or radiators are connected to the transformer tank. This is known as radiator of transformer or radiator bank of transformer. We have shown below a simplest form of Natural Cooling or ONAN Cooling arrangement of an earthing transformer.

Fig (34)

ONAF COOLING OF TRANSFORMER

Heat dissipation can obviously be increased, if dissipating surface is increased but it can be made further faster by applying forced air flow on that dissipating surface. Fans blowing air on cooling surface is employed. Forced air takes away the heat from the surface of radiators and provides better cooling than natural air. The full form of ONAF is "Oil Natural Air Forced". As the heat dissipation rate is faster and more in ONAF transformer cooling method than ONAN cooling system, electrical transformer can be loaded more without crossing the permissible temperature limits.

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Fig (35)

OFAF COOLING OF TRANSFORMER

In Oil Forced Air Natural cooling system of transformer, the heat dissipation is accelerated by using forced air on the dissipating surface but circulation of the hot oil in transformer tank is natural convectional flow.

Fig (36)

The heat dissipation rate can be still increased further if this oil circulation is accelerated by applying some force. In OFAF cooling system the oil is forced to circulate within the closed loop of transformer tank by means of oil pumps. OFAF means "Oil Forced Air Forced" cooling methods of transformer.

The main advantage of this system is that it is compact system and for same cooling capacity OFAF occupies much less space than farmer two systems of transformer cooling. Actually in Oil Natural cooling system, the heat comes out from conducting part of the transformer is displaced from its position, in slower rate due to convectional flow of oil but in forced oil cooling system the heat is displaced from its origin as soon as it comes out in the oil, hence rate of cooling becomes faster.

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OFWF COOLING OF TRANSFORMER

We know that ambient temperature of water is much less than the atmospheric air in same weather condition. So water may be used as better heat exchanger media than air. In OFWF cooling system of transformer, the hot oil is sent to oil to water heat exchanger by means of oil pump and there the oil is cooled by applying showers of cold water on the heat exchanger's oil pipes. OFWF means "Oil Forced Water Forced" cooling in transformer.

ODAF COOLING OF TRANSFORMER ODAF or Oil Directed Air Forced Cooling of Transformer can be considered as the improved version of OFAF. Here forced circulation of oil directed to flow through predetermined paths in transformer winding. The cool oil entering the transformer tank from cooler or radiator is passed through the winding where gaps for oil flow or pre-decided oil flowing paths between insulated conductors are provided for ensuring faster rate of heat transfer. ODAF or Oil Directed Air Forced Cooling of Transformer is generally used in very high rating transformer.

ODWF COOLING OF TRANSFORMER ODAF or Oil Directed Water Forced Cooling of Transformer is just like ODAF only difference is that here the hot oil is cooled in cooler by means of forced water instead of air. Both of these transformer cooling methods are called Forced Directed Oil Cooling of transformer

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9.

Current Transformers

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9-1 FUNDAMENTAL THEORY

Relay and meters cannot be connected directly to the power system (i.e. Primary of the Power System, the value of voltage may be 132 KV, 220 KV and 500 KV on the primary side while current may be also in hundreds of value. To prepare such relays having high values of V or I is not possible. Therefore such type of devices have been invented which can give a replica of actual primary side voltage or current on secondary side within safe Limits Such devices are known as Instrument Transformers, because those work in conjunction with meters and relays.

Fig (1)

Instrument transformers means current transformers & Voltage transformers are used in electrical power system for stepping down currents and voltages of the system for metering and protection purpose. Actually relays and meters used for protection and metering, are not designed for high currents and voltages.

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High currents or voltages of the system cannot be directly fed to relays and meters. Current Transformer steps down rated system current to 1 A or 5 A; similarly Voltage Transformers steps down system voltages to 110V. The relays and meters are generally designed for 1 A, 5 A and 110V.

A current transformer (CT) is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree.

A current transformer functions with the same basic working principle of electric power transformer as we discussed earlier, but here is some difference. If an electrical power transformer or other general purpose transformer, primary current varies with load or secondary current. In case of current transformer, primary current is the system current and this primary current or system current transforms to the CT secondary, hence secondary current or burden current depends upon primary current of the current transformer.

In a power transformer, if load is disconnected, there will be only magnetizing current flows in the primary. The primary of the power transformer takes current from the source proportional to the load connected with secondary. But in case of Current transformer, the primary is connected in series with power line. So current through its primary is nothing but the current flows through that power line. The primary current of the CT, hence does not depend upon whether the load or burden is connected to the secondary or not or what is the impedance value of burden. Generally current transformer has very few turns in primary where as secondary turns are large in number. Say Np is number of turns in CT primary and Ip is the current through primary. Hence the primary AT is equal to NpIp AT.

If number of turns in secondary and secondary current in that CT are Ns and Is respectively then Secondary AT is equal to NsIs AT.

In an ideal CT the primary AT is exactly is equal in magnitude to secondary AT.

So from the above statement it is clear that if a CT has one turn in primary and 400 turns in secondary winding, if it has 400 A current in primary then it will have 1A in secondary burden.Thus the turn ratio of the CT is 400/1A.

There are different cores in a CT e.g. if 132 KV CT has three core

1st full core is 1S1…………………... 1S3 (15 VA Metering Core)2nd full core is 2S1………………….. 2S3 (30 VA for-51)3rd full core is 3S1 ………………….. 3S3 (60 VA for-87)

1st core is used for metering (for energy meters) and has 15 AV burden. 2nd core has 30 VA burden and is used to give CT to over-current relay.

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3rd core has more VA equal to 60 VA and used for differential protection and impedance relay.

The characteristics of protection core and metering core are different; have different designs and cannot be interchanged with each other when more fault current occurs then mechanical system of meters pivot, jewel etc. damages and hence metering core is made smaller and should saturate earlier therefore. But protection core should give exact replica of system current at fault conditions protection core would saturate later.

9.2 TYPES OF CURRENT TRANSFORMERS

BAR PRIMARY TYPE

Its primary is in the form of solid straight rod, and its core on which winding is done is of rectangular shape. Such type of CTs are also called window type CTs, it has only one winding in its primary.

Fig (2)

RING TYPE

It is similar to window type CTs with the difference that its core is round shaped. Its primary is in the form of a rod like bar primary type. Bushing CT of a power transformer is ring type. Bushing CTs in a bush of a power transformer are also called free standing CTs.

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Fig (3)

WOUND TYPE

Fig (4)

It is that CT that has both primary and secondary windings. The primary winding should have at least one or more turns but the secondary winding of CT has many turns.

MULTI RATIO OR MULTI TAP

Such CTs having different CT ratios

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Fig (5)

MULTI WOUND OR MULTICORE

CTs having different windings in, secondary are called multi core CTs. Some winding are used

for metering and some for protection.

Fig (6)

In this figure a CT having 3 cores in shown. This CT’S has one winding for each core. Hence in

this case No of winding = No for core. This CT is multi wound and multi core.

A CT which is multi ratio as well as multi core can have its CT ratio as e.g. 100-200-400/5/5/5

TO CONVERT SIMPLE CT INTO MULTI RATIO CT

From Secondary Winding

Tap off the secondary winding.

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Fig (7)

If secondary windings are two in number then if secondary winding is tapped off from center then 50/5A CT ratio is obtained.

Fig (8)

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TO FORM MULTI RATIO CT

From Primary SideIn some high voltage CTs ratio can be changed from primary also. CT in this figure has two conductors on primary side. If P1 connected with C1 and P2 with C2 i.e. parallel connection then CT ratio becomes 600/5. When parallel connection is done then N1 = 1.

Fig (9)

For series connection, connect C1 C2, so that two turns are available on primary sides and ratio becomes 300/5A.

According to the relation N2/N1 = I1/ I2, N1 and I1 are inversely proportional, hence if N1 increases, I1 decreases and vice versa.

9.3 ERROR IN CURRENT TRANSFORMERS

In an actual current transformer, errors with which we are connected can best be considered through a study of phasor diagram for a CT,

Fig (10)

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Is - Secondary CurrentEs - Secondary induced EMFIp - primary CurrentEp - primary induced EMFKT - turns ratio = numbers of secondary turns/number of primary turnsIo - Excitation CurrentIm - magnetizing component of Io

Iw - core loss component of Io

Φm - main flux.

Let us take flux as reference. EMF Es and Ep lags behind the flux by 90o. The magnitude of the phasors Es and Ep are proportional to secondary and primary turns. The excitation current Io

which is made up of two components Im and Iw. The secondary current Io lags behind the secondary induced EMF Es by an angle Φ s. The secondary current is now transferred to the primary side by reversing Is and multiplied by the turns ratio KT. The total current flows through the primary Ip is then vector sum of KT Is and Io.

RATIO ERROR IN CURRENT TRANSFORMER

From the above phasor diagram it is clear that primary current Ip is not exactly equal to the secondary current multiplied by turn ratio, i.e. KTIs. This difference is due to the primary current is contributed by the core excitation current. The error in current transformer introduced due to this difference is called Current Error of CT or sometimes Ratio Error in Current Transformer.

Hence, the Percentage Current Error = (Ip – KT. Is)/ IP

PHASE ANGLE ERROR IN CURRENT TRANSFORMER

For an ideal current transformer the angle between the primary and reversed secondary current vector is zero. But for an actual current transformer there is always a difference in phase between two due to the fact that primary current has to supply the component of the exiting current. The angle between the above two phases in termed as Phase Angle Error in CT. Here in the pharos diagram it is β, the phase angle error is usually expressed in minutes.

CAUSE OF ERROR IN CURRENT TRANSFORMER

The total primary current is not actually transformed in CT. One part of the primary current is consumed for core excitation and remaining is actually transformed with turn ratio of CT so there is error in current transformer means there are both Ratio Error in Current Transformer as well as a Phase Angle Error in Current Transformer.

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HOW TO REDUCE ERROR IN CURRENT TRANSFORMER

It is desirable to reduce these errors, for better performance. For achieving minimum error in current transformer, one can adopt the following,

1. Using a core of high permeability and low hysteresis loss magnetic materials2. Keeping the rated burden to the nearest value of the actual burden3. Ensuring minimum length of flux path and increasing cross–sectional area of the core,

minimizing joint of the core4. Lowering the secondary internal impedance

9.4 CURRENT TRANSFORMERS CONNECTIONS

Current Transformers are either Delta Δ connection or Star or Y connection

DELTA CONNECTION

Fig (11)

If compared 1 and 2 current become 180o out of phase

STAR CONNECTION

Fig (12)

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If transformation is ∆ to Y (DY1 or DY11) or transformation is Y to ∆ (YD1 or YD11), then 30o

phase shift takes place.

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9.5 CURRENT TRANSFORMER PARAMETERS

RATED PRIMARY CURRENT OF CURRENT TRANSFORMER

This is the value of rated primary current of Current Transformer on which it is designed to perform best. Hence rated primary current of Current Transformer is an optimum value of primary current at which, error of the Current Transformer is minimum and losses are also less that means in few words, performance of the Current Transformer is best; with optimum heating of the transformer

RATED SECONDARY CURRENT OF CURRENT TRANSFORMER

Like rated primary current, this is the value of secondary current due to which errors in the Current Transformer is minimum. In other words, Rated Secondary Current of Current Transformer is the value of secondary current on which the best performance of the Current Transformer is based

RATED BURDEN OF CURRENT TRANSFORMER

It is the impedance connected to a Current Transformer. It is the total VA values connected to a

Current Transformer. This includes VA of Current Transformer’s winding (internal VA), VA of

the cable from Current Transformer to relays, meters, plus VA of the instruments connected to

Current Transformer.

Whatever is connected externally with the secondary of a Current Transformer is called burden

of Current Transformer. Rated burden of Current Transformer is the value of the burden to be

connected with the secondary of Current Transformer including connecting load resistance

expressed in VA or ohms on which accuracy requirement is based.

Fig (13)

This total VA value should not increase than the capacity of CT.

Usually capacity of metering core = 15 VA

Capacity of core for O/C relay = 30 VA

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Capacity of core for different relay = 60 VA

RATED FREQUENCY OF CURRENT TRANSFORMER

The value of the system frequency on which the Current Transformer operates

RATED SHORT CIRCUIT CURRENT OF A CURRENT TRANSFORMER

In some abnormal condition like huge short circuit fault, the Current Transformer faces a huge current, flow through the Current Transformer primary. Although this fault current will not continue for long time as because every fault in the system is cleared by electrical protection system within very short time. So Current Transformer should be designed in such a way that it can withstand this huge fault current at least for certain amount of time. It is unnecessary to design any equipment for withstanding short circuit current for long period of time since the short circuit fault is cleared by protection switch gear within fraction of second.

For Current Transformer Rated Short Circuit Current is defined as the RMS value of primary current which the Current Transformer will withstand for a rated time with its secondary winding short circuited without suffering harmful effects.

RATED VOLTAGE FOR CURRENT TRANSFORMER

The RMS value of the voltage used to designate the Current Transformer for a particular highest system voltage is Rated Voltage for Current Transformer. The voltage of electrical power system is increased if load of the system is reduced. As per standard, the system voltage can be raised up to 10% above the normal voltage during no load condition. So every electrical equipment is such designed so that it can withstand this highest voltage. As Current Transformer is an electrical equipment, it should also be designed to withstand highest system voltage.

INSTRUMENT SECURITY FACTOR

ISF or Instrument Security Factor is the ratio of Instrument Limit Primary Current to the rated Primary Current. Instrument Limit Current of a metering Current Transformer is the maximum value of primary current beyond which Current Transformer core becomes saturated. Instrument Security Factor of Current Transformer is the significant factor for choosing the metering Instruments which to be connected to the secondary of the Current Transformer. Security or Safety of the measuring unit is better, if ISF is low. If we go through the example below it would be clear to us.

Suppose one Current Transformer has rating 100/1A and ISF is 1.5 and another Current Transformer has same rating with ISF 2. That means, in first Current Transformer, the metering core would be saturated at 1.5x100 or 150 A, whereas is second Current Transformer, core will be saturated at 2x100 or 200A. That means whatever may be the primary current of both Current Transformers; secondary current will not increase further after 150 & 200A of primary current of the Current Transformers respectively. Hence maximum secondary current of the Current Transformers would be 1.5 & 2.0 A.

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As the maximum current can flow through the instrument connected to the first Current Transformers is 1.5A which is less than the maximum value of current can flow through the instrument connected to the second Current Transformers i.e. 2A. Hence security or safety of the instruments of first Current Transformers is better than later.

Another significance of ISF is during huge electrical fault, the short circuit current, flows through primary of the Current Transformers does not affect destructively, the measuring instrument attached to it as because, the secondary current of the Current Transformers will not rise above the value of rated secondary current multiplied by ISF

ACCURACY LIMIT FACTOR

For protection core of a Current Transformer, the ratio of Accuracy Limit Primary Current to the Rated Primary Current is called Accuracy Limit Factor of Current Transformer. Broadly, Accuracy Limit Primary Current is the maximum value of primary current, beyond which core of the Protection Core of Current Transformer starts saturating. The value of Rated Accuracy Limit Primary Current is always many times more than the value of Limit Primary Current. Actually Current Transformer transforms the fault current of the electrical power system for operation of the protection relays connected to the secondary of that Current Transformer.

If the core of the Current Transformer becomes saturated at lower value of primary current, as in the case of Metering core of Current Transformer, the system fault will not reflect properly to the secondary, which may cause, the relays remain inoperative even the fault level of the system is large enough. That is why the core of the protection Current Transformer is made in such a way that saturation level of that core must be high enough. But still there is a limit, because it is impossible to make one magnetic core with infinitely high saturation level and secondly most important reason is that although the protection core should have high saturation level but that must be limited up to certain level otherwise total transformation of primary current during huge fault may badly damage the protective relays. So it is clear from above explanation, Rated Accuracy Limit Primary Current, should not be so less, that it will not at all help the relays to be operated on the other hand this value must not be so high that it can damage the relays.

So, Accuracy Limit Factor or ALF should not have the value nearer to unit and at the same time it should not be as high as 100. The standard values of ALF as per IS-2705 are 5, 10, 15, 20 & 30.

KNEE POINT VOLTAGE OF CURRENT TRANSFORMER

Knee Point Voltage of Current Transformer is significance of saturation level of a Current Transformer core mainly used for protection purposes. The sinusoidal voltage of rated frequency is applied to the secondary terminals of Current Transformer, with other winding being open circuited which when increased by10%, cause the exiting current to increase by 50%. The CT core is made of CRGO steel. It has its own saturation level.

The EMF induced in the CT secondary windings is E2 = 4.44φfT2

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Where, f is the system frequency, φ is the maximum magnetic flux in Wb. T2 is the number of turns of the secondary winding. The flux in the core, is produced by excitation current I e. We have a non-liner relationship between excitation current and magnetizing flux. After certain value of excitation current, flux will not further increase so rapidly with increase in excitation current. This non-liner relation curve is also called B-H curve. Again from the equation above, it is found that, secondary voltage of a Current Transformer is directly proportional to flux φ. Hence one typical curve can be drawn from this relation between secondary voltage and excitation current as shown below,

It is clear from the curve that, linear relation between V & Ie is maintained from point A & K. The point A is known as Ankle Point and point K is known as Knee Point.

Fig (14)

In Differential and Restricted Earth Fault (REF) protection scheme, accuracy class and ALF of the Current Transformer may not ensure the reliability of the operation. It is desired that, Differential and REF relays should not be operated when fault occurs outside the protected transformer. When any fault occurs outside the Differential protection zone, the faulty current flows through Current Transformers of both sides of Electrical Power Transformer. Both LV & HV Current Transformers have magnetizing characteristics. Beyond the Knee Point, for slight increase in secondary EMF a large increase in excitation current is required. So after this knee point excitation current of both Current Transformers will be extremely high, this may cause mismatch between secondary current of LV & HV Current Transformers. This phenomenon may cause unexpected tripping of Power Transformer.

So the magnetizing characteristics of both LV & HV sides Current Transformers, should be same that means they have same knee point voltage Vk as well as same excitation current Ie at Vk/2. It can be again said that, if both knee point voltage of current transformer and magnetizing characteristic of Current Transformers of both sides of Power Transformer differ, there must be a mismatch in high excitation currents of the Current Transformers during fault which ultimately causes the unbalancing between secondary current of both groups of Current Transformers and transformer trips.

So for choosing Current Transformers for Differential Protection of Transformer, one should consider Current Transformer PS Class rather its convectional protection class. PS stands for

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Protection Special which is defined by Knee Point voltage of current transformer Vk and excitation current Ie at Vk/2.

WHY CT SECONDARY SHOULD NOT BE KEPT OPEN

The electrical power system load current always flows through current transformer primary; irrespective of whether the Current Transformer is open circuited or connected to burden at its secondary.

Fig (15)

If Current Transformer secondary is open circuited, all the primary current will behave as excitation current, which ultimately produce huge voltage. Every Current Transformer has its own Non-Linear magnetizing curve, because of which secondary open circuit voltage should be limited by saturation of the core. If the RMS voltage across the secondary terminals is measured, the value may not appear to be dangerous. As the Current Transformer primary current is sinusoidal in nature, it zero 100 times per second, as frequency of the current is 50Hz. The rate of change of flux at every current zero is not limited by saturation and is high indeed. This develops extremely high peaks or pulses of voltage. These high peaks of voltage may not be measured by conventional voltmeter. But these high peaks of induced voltage may breakdown the Current Transformer insulation, and may case accident to personnel. The actual open circuit voltage peak is difficult to measure accurately because of its very short peaks. That is why Current Transformer secondary should not be kept open.

9.6 CURRENT TRANSFORMER TESTS

9.6.1 CONTINUITY TEST

Check the continuity of the Secondary side of the CT with an ohmmeter.

9.6.2 INSULATION RESISTANCE TEST

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The main purpose of this test is to detect major faults of major insulation. To assess that Current Transformer can be energized or not i.e. to know about the condition of Current Transformer major insulations. For this Ri reading of 1 Mega-ohm/1kV at 20oC oil temperature is considered satisfactory. Insulation Resistance Test is performed on transformer when,

1. Newly installed 2. Under fault

Check the insulation of Current Transformer with an appropriate voltage of the Insulation Resistance Tester: Measurements are to be taken as follows:

1. Primary to Earth2. Secondary to Earth3. Primary to Secondary

9.6.3 RATIO TEST

It is the ratio between primary windings turns and secondary winding turns. In other words it is the ratio between primary winding current and secondary winding current. As current ratio and turns ratio are inversely proportional. So write it as

I1/I2 = N2/ N1

For example, a Current Transformer of ratio 100/5 means that

I2 = 100A; and N1 = 1 turn

I2 = 5A; N2 = 20 turns

Check the ratio of the Current Transformer with the help of a Current Injection Test Set.

9.6.4 POLARITY OF CURRENT TRANSFORMERS

Polarity of a transformer is defined as the relative instantaneous direction of current in its terminals. Terminals have same polarity if current entering in the primary terminal and that leaving from the secondary terminal is such that they form a continuous circuit. Terminals having same polarity are marked with same number i.e. 1 or 2 e.g. P1, S1 and P2, S2

147

Fig (16)

Assume the polarity marks on the Current Transformer are correct. Primary as P1-P2 and Secondary as S1-S2. Connect a DC Voltmeter to the Secondary S1-S2, such that Positive and Negative terminals of the Voltmeter are towards S1 and S2 of the Current Transformer respectively. Connect negative of the 1.5 Volts DC cell to the P2 and touch P1 terminal of the Current Transformer with the Positive terminal of the cell. The DC Voltmeter will show one of the following deflections:

Forward or upscale, if the polarity is correctReverse or downscale, if the polarity is reversed.

The correct polarity is opposite of the marked polarity.

9.6.5 CURRENT TRANSFORMER SATURATIONCurrent Transformer saturation is a point where the excitation impedance collapses and whole of the primary current is utilized in exciting the core of Current Transformer i.e. IP becomes, the Iexe

and secondary output of the Current Transformer ceases (reduces/finishes). The cause of Current Transformer saturation is fault current which flows on fault or if Current Transformer is opened accidentally at secondary side. DC transients are present in the fault current which superimpose on AC quantity having less time but high magnitude. These transients saturate Current Transformer core.

ANKLE POINT

Ankle point is a point where VE and IE are linear, i.e. VE and IE are direct proportional. Operating point of CT should be in linear portion.

KNEE POINT

It is a point where increase of 10% in Vexc causes an increase of 50% in Iexc and this is the point of the saturation of CT. Measuring CT operates between zero and ankle point and saturation level is low. Protection CT operates satisfactory up to knee point and saturation level is high. Protection CT cannot be used instead of measuring CT and vice versa.

148

Fig (17)

9.6.6 CIRCUIT VERIFICATION TEST

To be done in Laboratory.

10.

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Potential

Transformers

10.1 FUNDAMENTAL THEORY

Potential Transformers or Voltage Transformers are used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltages.

Fig (1)

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A Voltage Transformer theory or Potential Transformer theory is just like theory of general purpose step down transformer. Primary of this transformer is connected across the phases or and ground depending upon the requirement. Just like the transformer, used for stepping down purpose, Potential Transformer has fewer turns winding at its secondary. The system voltage is applied across the terminals of primary winding of that transformer, and then proportionate secondary voltage appears across the secondary terminals of the Potential Transformer. The secondary voltage of the Potential Transformer is generally 110V. In an ideal Potential Transformer or Voltage Transformer when rated burden is connected across the secondary the ratio of primary and secondary voltages of transformer is equal to the turn ratio and furthermore the two terminal voltages are in precise phase opposite to each other. But in actual transformer there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages.

The errors in Potential Transformer or Voltage Transformer can best be explained by phasor diagram.

10.2 TYPES OF POTENTIAL TRANSFORMERS

There are two types of Potential Transformer:

1. Conventional Voltage Transformer2. Coupling Voltage Transformer

10.3 ERROR IN POTENTIAL TRANSFORMER

151

Fig(2)

Is - Secondary CurrentEs - Secondary induced emfVs - Secondary terminal voltageRs - Secondary winding resistanceXs - Secondary winding reactanceIp - Primary currentEp - primary induced emfVp - Primary terminal voltageRp - Primary winding resistanceXp - Primary winding reactanceKT - turns ratio = numbers of primary turns/number of secondary turnsIo - Excitation CurrentIm - magnetizing component of Io

Iw - core loss component of Io

Φm - main fluxβ - phase angle error

As in the case of Current Transformer and other purpose electrical power transformer, total primary current Ip is the vector sum of excitation current and the current equal to reversal of secondary current multiplied by the ratio 1/KT

Hence, Ip = Io + Is/KT

If Vp is the system voltage applied to the primary of the PT then voltage drops due to resistance and reactance of primary winding due to primary current Ip will comes into picture. After subtracting this voltage drop from Vp, Ep will appear across the primary terminals. This Ep is equal to primary induced emf. This primary emf will transform to the secondary winding by

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mutual induction and transformed emf is Es. Again this Es will be dropped by secondary winding resistance and reactance, and resultant will actually appear across the burden terminals and it is denoted as Vs

So if system voltage is Vp, ideally Vp/KT should be the secondary voltage of PT, but in reality actual secondary voltage of PT is Vs.

Ratio Error In Potential Transformer

The difference between the ideal value Vp/KT and actual value Vs is the voltage error or ratio error in a potential transformer, it can be expressed as ,

% voltage error = (Vp − KT.Vs

) /Vp X 100 %

Phase Angle Error In Potential Transformer

The angle ′β′ between the primary system voltage Vp and the reversed secondary voltage vectors KT.Vs is the phase error

CAUSE OF ERROR IN POTENTIAL TRANSFORMER

Fig (3)

The voltage applied to the primary of the potential transformer first drops due to internal impedance of primary. Then it appears across the primary winding and then transformed proportionally to its turns ratio, to secondary winding. This transformed voltage across secondary winding will again drops due to internal impedance of secondary, before appearing across burden terminals. This is the reason of errors in potential transformer.

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10.4 POTENTIAL TRANSFORMER PARAMETERS

Rated Voltage For Potential Transformer The RMS value of the voltage used to designate the Potential Transformer for a particular highest system voltage is Rated Voltage for Potential Transformer. The voltage of electrical power system is increased if load of the system is reduced. As per standard, the system voltage can be raised up to 10% above the normal voltage during no load condition. So every electrical equipment; is such designed so that it can withstand this highest voltage. As Potential Transformer is electrical equipment, it should also be designed to withstand highest system voltage.

Rated Burden Of Potential Transformer

Rated burden of Voltage Transformer is the value of the burden to be connected with the secondary of Voltage Transformer including connecting load resistance expressed in VA or ohms on which accuracy requirement is based.

Rated Frequency Of Potential Transformer

The value of the system frequency on which the Voltage Transformer operates.

10.5 POTENTIAL TRANSFORMERS CONNECTIONS

Open Delta Connection

In case of balanced toads on RYB phases the voltage between open delta i.e. pts A and B is equal to zero volts.

Voltage across AB pt is called corner voltage which is usually 4 or 5 volts.

In balanced condition, VRL0o + VYL120o + VBL240o = 0

If unbalanced condition, VRL0o + VYL120o + VBL240o = 0

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Fig (4)

Broken Delta Connection

The above arrangement is called broken delta arrangement. We can get 3-phase supply from Two Potential Transformers (from Two-phases). It means two single phase Potential Transformers are used while one Potential Transformers is saved, so that much saving of cost.

Fig (5)

This situation is used in 11KV feeder system only for metering because metering is in normal healthy condition. This phenomenon is not used for protection purposes, means that this type of Potential Transformers is not used for protective relays.

The Potential Transformers used for protective relay must have three windings. But when ground fault occurs then zero sequence voltages cannot be generated if used for protection purposes having two windings. This type of connection can be used for low loads only.

10.6 POTENTIAL TRANSFORMER TESTS

10.6.1 CONTINUITY TEST

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Check the continuity of the Primary and Secondary side of the Potential Transformer with an ohmmeter.

10.6.2 POLARITY TEST

It is carried out to know about the physical location of H1, H2, X1 and X2 Terminals etc. On HV

side of Potential Transformer, select H1 and H2 terminals according to your own choice and

connect them with 1.5 volts Dry Cell through a Switch. Then connect DC voltmeter to LV side

in such a way that its positive terminal is connected to that terminal which is adjacent opposite to

that HV Terminal to which positive of Cell is connected. After making connections as directed

above, close switch “S”.

If Voltmeter needle deflects towards clockwise direction (or upscale or right side) then polarity of transformer is “subtractive” (i.e. H1 and X1 terminals of transformer are adjacent opposite to each other).

If voltmeter needle deflects towards anticlockwise direction (or down scale or left side), then polarity of transformer is “additive” (i.e. H1 and X1 terminals of transformer are diagonally opposite to each other).

10.6.3 INSULATION RESISTANCE TEST

Check the insulation of Potential Transformer with an appropriate voltage of the Insulation Resistance Tester:

(i) Primary to earth

(ii) Secondary to earth

(iii) Primary to Secondary

10.6.4 VOLTAGE RATIO TEST

Check the ratio of the Potential Transformer with one of the following methods

1. Back To Back Testing Method

2. Single Phase Testing Method

1. Back To Back Testing Method

In this method there are to Potential Transformers, one with known ratio and the other one with

unknown ratio.

Connect Primary sides of both the Potential Transformers through a jumper wire. Now voltage is

applied through a variac to the secondary of that Potential Transformer whose ratio is known.

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Then voltage at the secondary of unknown Potential Transformer is measured. Voltage Ratio of

the unknown Potential can now easily be calculated.

2. Single Phase Testing Method

If a Potential Transformer of known Ratio is not available, then single phase to ground supply is

applied to the Potential Transformer. Then voltage at the secondary of under test Potential

Transformer is measured. Measured and Calculated Voltage Ratio can then be compared.

10.6.5 CIRCUIT VERIFICATION

To be done in Laboratory

10.7 POTENTIAL TRANSFORMER SUPPLY SUPERVISION

The Potential Transformer supply supervision is carried out with one of the following methods:

1. Balanced Beam Method

2. Logic Gates

Both the methods will be discussed during studies of the Distance Relays

10.8 CAPACITOR VOLTAGE TRANSFORMERS

The basic electrical diagram for a typical CCVT is shown in Fig. 1. The primary side consists of two capacitive elements C1and C2connected in series. The Potential Transformer provides a secondary voltage vo for protective relays and measuring instruments. The inductance Lc is chosen to avoid phase shifts between viand vo at power frequency. However, small errors may occur due to the exciting current and the CCVT burden (Zb)

157

Fig (6)

11.

158

Introduction to

Protection

11.1 INTRODUCTION

The purpose of an electrical power system is to generate and supply electrical energy to

consumers. The system should be designed and managed to deliver this energy to the utilization

points with both reliability and economy. Severe disruption to the normal routine of modern

society is likely if power outages are frequent or prolonged, placing an increasing emphasis on

reliability and security of supply. As the requirements of reliability and economy are largely

opposed, power system design is inevitably a compromise. A power system comprises many

diverse items of equipment. Fig (1) illustrates the diversity of equipment that is found.

159

Fig (1)

Many items of equipment are very expensive, and so the complete power system represents a very large capital investment. To maximize the return on this outlay, the system must be utilized as much as possible within the applicable constraints of security and reliability of supply. More fundamental, however, is that the power system should operate in a safe manner at all times. No matter how well designed, faults will always occur on a power system, and these faults may represent a risk to life and/or property. Fig (2) shows the onset of a fault on an overhead line.

The destructive power of a fault arc carrying a high current is very great; it can burn through copper conductors or weld together core laminations in a transformer or machine in a very short time, some tens or hundreds of milliseconds. Even away from the fault arc itself, heavy fault currents can cause damage to plant if they continue for more than a few seconds. The provision of adequate protection to detect and disconnect elements of the power system in the event of fault is therefore an integral part of power system design. Only by so doing can the objectives of the power system be met and the investment protected. Fig (3) provides an illustration of the consequences of failure to provide appropriate protection. This is the measure of the importance of protection systems as applied in power system practice and of the responsibility vested in the Protection Engineer.

160

Fig (2) Fig (3)

11.2 PURPOSE OF PROTECTION RELAYING

The purpose of protection system is like a brain in human bodies. As brain is informed through five senses and then it decides the action, in a similar manner, relay senses through Current Transformer and/or Potential Transformer and then decides what to do.

11.3 PRINCIPLES OF PROTECTION SYSTEM

An electrical relay is a device which operates when the electrical quantity to which it responds changes in a prescribed manner. If such a relay is used in protection of electrical equipment or components of power system, it is called a protective relay.

Relays are used in three purposes

1. Protection2. Control 3. Regulation

11.4 FUNCTIONS OF PROTECTIVE RELAYING

1. To detect the presence of a fault

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2. To identify the faulted components3. To initiate the appropriate circuit breaker4. To remove the defective components from service

11.5 PROTECTION EQUIPMENT

The definitions that follow are generally used in relation to power system protection:

1. Protection System: a complete arrangement of protection equipment and other devices required to achieve a specified function based on a protection principal (IEC 60255-20)

2. Protection Equipment: a collection of protection devices (relays, fuses, etc.). Excluded are devices such as Current Transformers, Circuit Breakers, Contactors, etc.

3. Protection Scheme: a collection of protection equipment providing a defined function and including all equipment required to make the scheme work (i.e. relays, Current Transformers, Circuit Breakers, Batteries, etc.)

In order to fulfill the requirements of protection with the optimum speed for the many different configurations, operating conditions and construction features of power systems, it has been necessary to develop many types of relay that respond to various functions of the power system quantities. For example, observation simply of the magnitude of the fault current suffices in some cases but measurement of power or impedance may be necessary in others. Relays frequently measure complex functions of the system quantities, which are only readily expressible by mathematical or graphical means. Relays may be classified according to the technology used:

1. Electromechanical2. Static3. Digital4. Numerical

The different types have somewhat different capabilities, due to the limitations of the technology used.

In many cases, it is not feasible to protect against all hazards with a relay that responds to a single power system quantity. An arrangement using several quantities may be required. In this case, either several relays, each responding to a single quantity, or, more commonly, a single relay containing several elements, each responding independently to a different quantity may be used.

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11.6 THE FUNCTIONAL REQUIREMENTS OF THE RELAY

11.6.1 RELIABILITY

The most important requisite of protective relay is reliability since they supervise the circuit for a long time before a fault occurs; if a fault then occurs, the relays must respond instantly and correctly. Incorrect operation can be attributed to one of the following classifications:

1. Incorrect design/settings2. Incorrect installation/testing3. Deterioration in service

DesignThe design of a protection scheme is of paramount importance. This is to ensure that the system will operate under all required conditions, and (equally important) refrain from operating when so required (including, where appropriate, being restrained from operating for faults external to the zone being protected). Due consideration must be given to the nature, frequency and duration of faults likely to be experienced, all relevant parameters of the power system (including the characteristics of the supply source, and methods of operation) and the type of protection equipment used. Of course, no amount of effort at this stage can make up for the use of protection equipment that has not itself been subject to proper design.

SettingsIt is essential to ensure that settings are chosen for protection relays and systems which take into account the parameters of the primary system, including fault and load levels, and dynamic performance requirements etc. The characteristics of power systems change with time, due to changes in loads, location, type and amount of generation, etc. Therefore, setting values of relays may need to be checked at suitable intervals to ensure that they are still appropriate. Otherwise, unwanted operation or failure to operate when required may occur.

InstallationThe need for correct installation of protection systems is obvious, but the complexity of the interconnections of many systems and their relationship to the remainder of the installation may make checking difficult. Site testing is therefore necessary; since it will be difficult to reproduce all fault conditions correctly, these tests must be directed to proving the installation. The tests should be limited to such simple and direct tests as will prove the correctness of the connections, relay settings, and freedom from damage of the equipment. No attempt should be made to 'type test' the equipment or to establish complex aspects of its technical performance

163

TestingComprehensive testing is just as important, and this testing should cover all aspects of the protection scheme, as well as reproducing operational and environmental conditions as closely as possible. Type testing of protection equipment to recognized standards fulfils many of these requirements, but it may still be necessary to test the complete protection scheme (relays, Current Transformers, Potential Transformer and other ancillary items) and the tests must simulate fault conditions realistically.

Deterioration In ServiceSubsequent to installation in perfect condition, deterioration of equipment will take place and may eventually interfere with correct functioning. For example, contacts may become rough or burnt owing to frequent operation, or tarnished owing to atmospheric contamination; coils and other circuits may become open-circuited, electronic components and auxiliary devices may fail, and mechanical parts may seize up. The time between operations of protection relays may be years rather than days. During this period defects may have developed unnoticed until revealed by the failure of the protection to respond to a power system fault. For this reason, relays should be regularly tested in order to check for correct functioning.

Testing should preferably be carried out without disturbing permanent connections. This can be achieved by the provision of test blocks or switches. The quality of testing personnel is an essential feature when assessing reliability and considering means for improvement. Staff must be technically competent and adequately trained, as well as self-disciplined to proceed in a systematic manner to achieve final acceptance. Important circuits that are especially vulnerable can be provided with continuous electrical supervision; such arrangements are commonly applied to circuit breaker trip circuits and to pilot circuits.

Modern digital and numerical relays usually incorporate self testing/ diagnostic facilities to assist in the detection of failures. With these types of relay, it may be possible to arrange for such failures to be automatically reported by communications link to a remote operations centre, so that appropriate action may be taken to ensure continued safe operation of that part of the power system and arrangements put in hand for investigation and correction of the fault.

Protection PerformanceProtection system performance is frequently assessed statistically. For this purpose each system fault is classed as an incident and only those that are cleared by the tripping of the correct circuit breakers are classed as 'correct'. The percentage of correct clearances can then be determined. This principle of assessment gives an accurate evaluation of the protection of the system as a whole, but it is severe in its judgment of relay performance. Many relays are called into operation for each system fault, and all must behave correctly for a correct clearance to be recorded. Complete reliability is unlikely ever to be achieved by further improvements in construction. If the level of reliability achieved by a single device is not acceptable, improvement can be achieved through redundancy, e.g. duplication of equipment. Two complete, independent, main protection systems are provided, and arranged so that either by itself can carry out the required function. If the probability of each equipment failing is x/unit, the resultant probability of both equipments failing simultaneously, allowing for redundancy, is x2. Where x is small the

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resultant risk (x2) may be negligible. Where multiple protection systems are used, the tripping signal can be provided in a number of different ways. The two most common methods are:

All protection systems must operate for a tripping operation to occur (e.g. ‘two-out-of-two’ arrangement)

Only one protection system need operate to cause a trip (e.g. ‘one-out-of two’ arrangement)

The former method guards against palpation while the latter guards against failure to operate due to an unrevealed fault in a protection system. Rarely, three main protection systems are provided, configured in a ‘two-out-of three’ tripping arrangement, to provide both reliability of tripping, and security against unwanted tripping. It has long been the practice to apply duplicate protection systems to bus bars, both being required to operate to complete a tripping operation. Loss of a busbar may cause widespread loss of supply, which is clearly undesirable. In other cases, important circuits are provided with duplicate main protection systems, either being able to trip independently.

On critical circuits, use may also be made of a digital fault simulator to model the relevant section of the power system and check the performance of the relays used.

11.6.2 SELECTIVITY

The relay must be able to discriminate (select) between those conditions for which prompt operation is required and those for which no operation, or time delayed operation is required. When a fault occurs, the protection scheme is required to trip only those circuit breakers whose operation is required to isolate the fault. This property of selective tripping is also called 'discrimination' and is achieved by two general methods.

Time GradingProtection systems in successive zones are arranged to operate in times that are graded through the sequence of equipments so that upon the occurrence of a fault, although a number of protection equipments respond, only those relevant to the faulty zone complete the tripping function. The others make incomplete operations and then reset. The speed of response will often depend on the severity of the fault, and will generally be slower than for a unit system.

Unit SystemsIt is possible to design protection systems that respond only to fault conditions occurring within a clearly defined zone. This type of protection system is known as 'unit protection'. Certain types of unit protection are known by specific names, e.g. restricted earth fault and differential

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protection. Unit protection can be applied throughout a power system and, since it does not involve time grading, is relatively fast in operation. The speed of response is substantially independent of fault severity. Unit protection usually involves comparison of quantities at the boundaries of the protected zone as defined by the locations of the current transformers. This comparison may be achieved by direct hard-wired connections or may be achieved via a communications link.

However certain protection systems derive their 'restricted' property from the configuration of the power system and may be classed as unit protection, e.g. earth fault protection applied to the high voltage delta winding of a power transformer. Whichever method is used, it must be kept in mind that selectivity is not merely a matter of relay design. It also depends on the correct coordination of Current Transformers and relays with a suitable choice of relay settings, taking into account the possible range of such variables as fault currents, maximum load current, system impedances and other related factors, where appropriate.

11.6.3 STABILITY

The term ‘stability’ is usually associated with unit protection schemes and refers to the ability of the protection system to remain unaffected by conditions external to the protected zone, for example through load current and external fault conditions.

11.6.4 SPEED

The relay must operate at the required speed. It should neither be too slow which may not result in damage to the equipment nor should it be too fast which may result in undesired operation. The function of protection systems is to isolate faults on the power system as rapidly as possible. The main objective is to safeguard continuity of supply by removing each disturbance before it leads to widespread loss of synchronism and consequent collapse of the power system.

As the loading on a power system increases, the phase shift between voltages at different bus bars on the system also increases, and therefore so does the probability that synchronism will be lost when the system is disturbed by a fault. The shorter the time a fault is allowed to remain in the system, the greater can be the loading of the system. It worth to note that phase faults have a more marked effect on the stability of the system than a simple earth fault and therefore require faster clearance.

System stability is not, however, the only consideration. Rapid operation of protection ensures that fault damage is minimized, as energy liberated during a fault is proportional to the square of the fault current times the duration of the fault. Protection must thus operate as quickly as possible but speed of operation must be weighed against economy. Distribution circuits, which do not normally require a fast fault clearance, are usually protected by time-graded systems. Generating plant and EHV systems require protection gear of the highest attainable speed; the

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only limiting factor will be the necessity for correct operation, and therefore unit systems are normal practice.

11.6.6 SENSITIVITY

The relaying equipment must be sufficiently sensitive so that it operates reliably when required under the actual conditions that produces least operating tendency. Sensitivity is a term frequently used when referring to the minimum operating level (current, voltage, power etc.) of relays or complete protection schemes. The relay or scheme is said to be sensitive if the primary operating parameter(s) is low. With older electromechanical relays, sensitivity was considered in terms of the sensitivity of the measuring movement and was measured in terms of its volt-ampere consumption to cause operation. With modern digital and numerical relays the achievable sensitivity is seldom limited by the device design but by its application and Current Transformer/Potential Transformer parameters.

11.7 RELAYING TERMINOLOGY

11.7.1 RELAYING OPERATION

An electromechanical relay is said to have operated when sufficient current has passed through

the operating coil to cause movement of the mechanical components and move the contacts to

open or close, depending on the design and purpose of the relay. For solid sated relays, the relay

is said to have operated when the quantity to which it responds has reached the value where the

logic circuit initiates action to cause a set of contacts to open or close, depending on the purpose

of the relay.

11.7.2 RELAY RESETTING

Most electromechanical relays operate against a restraint spring or gravity, with the result that

when the actuating quantity disappears, or is reduced below preset pickup value, the relays will

reset. Theses relays are called “self-resetting”. However, some relays once they have operated

will not reset themselves. These are known as “manually-resetting” or “lockout relays”. Solid

state relays are similar, in that once the actuating quantity disappears or drops below the pickup

value, the logic circuit allow resetting of the contacts.

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11.7.3 RELAY PICKUP, RELAY DROPOUT

If the actuating quantity applied to a relay is gradually increased, a point will be reached at which

the relay will operate. This minimum operating value is called the “relay pickup” value. If the

actuating quantity is then gradually decreased, a point will be reached where the relay contacts

reopen. This value is called the “relay dropout” value.

11.7.4 NORMALLY OPEN, NORMALLY CLOSED CONTACTS

A contact which is open if the relay has not operated is called a “normally open” contact; if it is

closed when the relay has not operated; it is called a “normally closed” contact. On electrical

drawings, all contacts are shown “open” or “closed” as they are when the relay is not operated,

even though in normal operation it may be that the relay is picked up.

11.7.5 PALLET SWITCHES

Pallet switches are auxiliary switches provided in circuit breakers and in certain disconnect

switches and linked to the operating mechanisms in such a way that they are opened or closed by

the operation of the main device. Those switches which open when the device opens are called

“a” pallet switch; those which open when the device closes are called “b” pallet switch.

11.7.6 RELAY SEAL-IN

Under certain circumstances, it may be desirable to insure that once a relay as operated, it

remains in the operated position (or picked up) for a definite period of time or until certain other

events have occurred. In such cases, a relay “seal-in” is provided.

11.7.7 INVERSE TIME AND DEFINITE TIME RELAYS

A definite time relay is one in which the time delay introduced remains constant from one

operation to the next regardless of the severity of fault conditions.

An inverse time relay is one in which the rate of travel of the moving contact assembly increases

with an increase in magnitude of the actuating quantity, i.e. the time required to close the

contacts decreases as the fault current increases.

11.7.8 RELAY TARGET

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To analyze relay scheme performance during fault condition, it is necessary to know which

relays operated. This information is obtained from relay targets. These targets are operation

indicators which are “flagged” either mechanically by the movement of the moving contact

assembly, or electrically by the flow current through coil in series with the main contact. The

targets are generally brightly colored devices, mostly fluorescent red or orange, but some older

installations may have white targets.

11.7.9 REACH

This is a term applied to transmission lines and implies an impedance setting which is equivalent

to a certain line distance (impedance per line kilometer is a calculable quantity). The relay is set

according to the formula:

Z = E/I

11.7.10 DIRECT UNDER REACH

In this scheme, the fault detector is set at both ends of the line to cover only 85% of the line

distance. When the relays operate at either end, they initiate a transfer trip signal to the remote

end.

11.7.11 PERMISSIVE OVERREACH

This is a term applied to a relay scheme where the fault detectors at either end of a transmission

line are set to see more than 100% of the line and are, therefore, subject to operation for external

faults. To prevent operation on external faults, a permissive signal must be received from the

terminal before tripping can take place.

11.7.12 ECHO

This is a term used with a permissive overreach scheme when a line is open at one terminal and

the fault detectors at that terminal do not detect (or see) a fault and therefore, no permission to

trip is sent. To insure fast tripping, the permissive signal is echoed back to the detecting terminal

from the open terminal.

11.7.13 AUTOMATIC RECLOSING

Most faults on a power system are transient, lightning account for most of these. In order to limit

customer outages and to maintain a stable system, automatic reclosure schemes are employed to

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get lines back in service after a transient fault. If a fault is permanent, the line will trip and stay

out after the reclose operation. Automatic reclosing may be short or long time. Reclosure may be

selected in the following modes using the newer type of reclosure schemes:

1. No reclosure2. Under voltage – reclosure is permitted after the circuit breaker is tripped if no voltage is

present on the element tripped. 3. Synchrocheck – relosure is permitted if the voltages across the breaker to be reclosed are

in synchronism. 4. Voltage presence – reclosure is permitted after the breaker is tripped if voltage is present

on the element previously isolated.

11.8 DEVICE NUMBERS AND THEIR UNIVERSAL NOMENCLATURE

2 Time delay relay

3 Interlocking relay

21 Distance relay

25 Check synchronizing relay

27 Under voltage relay

30 Enunciator relay

32 Directional power (Reverse power) relay

37 Low forward power relay

40 Field failure (loss of excitation) relay

46 Negative phase sequence relay

49 Machine or Transformer Thermal relay

50 Instantaneous Over current relay

51 A.C IDMT over current relay

52 Circuit breaker

52a Circuit breaker Auxiliary switch “Normally open” (‘a’ contact)

52b Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact)

55 Power Factor relay

56 Field Application relay

59 Overvoltage relay

60 Voltage or current balance relay

64 Earth fault relay

67 Directional relay

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68 Locking relay

74 Alarm relay

76 D.C Over current relay

78 Phase angle measuring or out of step relay

79 AC Auto reclose relay

81 Frequency relay

81U under frequency relay

81O over frequency relay

83 Automatic selective control or transfer relay

85 Carrier or pilot wire receive relay

86 Tripping Relay

87 Differential relay

87G Generator differential relay

87GT overall differential relay

87U UAT differential relay

87NT Restricted earth fault relay (provided on HV side of Generator transformer)

95 Trip circuit supervision relay

99 Over flux relay

186A Auto reclose lockout relay

186B Auto recluse lockout relay

11.9 RELAY PROTECTIVE SCHEMES

Primary Protection

Transformers, lines, reactors, buses and generators are protested by at least one sensitive relay

package which will trip quickly (about 20 ms) when a fault occurs. These relays are first line of

defance against damage to the system.

Backup Protection

All power circuits are protected by a second or a backup relay package which is more or less

independent of the other set (primary protection). The backup operates with an intentional time

delay.

Duplicate Protection

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Now-a-days relay scheme have back up relaying as such. The new standard to protect a power

system consists of two independent relay schemes where neither of them has intentional time

delay.

Fig (4)

In addition Breaker Failure Protection is provided on all high voltage and some low voltage

breakers.

Fig (5)

Zone Protection

Power system is divided into zones which can be protected by a specialized group of relays and

which can also be separated at the rest of the system. If a zone is protected by two zone relays, it

will be called on Overlay Zone.

DETERMINING THE TYPE OF THE FAULT

Protective relay must be able to distinguish between and abnormal fault values. Disturbances to

the normal operating conditions, give rise to following changes in system parameters’ (some or

all).

1. Change in current2. Change in voltage3. Change in impedance (fall)

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4. Voltage vector displacement5. Temperature rise6. Frequency deviation

FAULT DETECTING RELAYING

Purpose of these relays is to detect the presence of faults and then initiating the tripping scheme.

These relays may be of solid state or electromechanical type

13.Over-Current

Protection

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12.1 OVER CURRENT RELAYS

During short circuits on the power system, fault current is many fold of normal current and if not removed from the system, it will cause damage or total collapse of the power system. Protection against an abnormal current is the earliest protective system, developed and used till now all over the world.

Over current relay basically sense the increase in flow of power system current, compare it with a preset value decided during the fault calculation and relay setting process, then generate a trip command for the related circuit breaker to isolate the faulty portion of the power system. Usually the magnitude of current increases during fault, hence such relays are generally as over current instead of a current relay. Under IEC (International Electro-technical Commission) standards these types of relays are allocated with a designation number “51”, in all protection references which include system drawings and manuals. Over current relays are connected to the power system with help of current transformers.

12.2 GENERATIONS AND TYPES OF OVER CURRENT RELAYS

Since their development, four generation of these relays are in simultaneous use around the world including power utilities in our country without any generation gap.

1. Electromechanical, the first generation relays.2. Electronic with analogue design, the second generation.3. Digital with micro-processor design, the third generation.4. Numerical with software based design.

Depending on the time characteristics, these relays are further divided into four major types, which include:

1. Instantaneous over current protection without any time delay, only include relay reaction time, usually 10ms.

2. Definite over current relay with real time setting.3. Inverse definite minimum time, IDMT, operating time varies inversely with magnitude of

fault current, hence called inverse over current relays.4. Directional over current relays.

Over current relays have wide range of applications in big motors, generators, transformers, and feeder/ transmission lines protection.

12.3 OPERATING PRINCIPALS OF OVER CURRENT RELAYS

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ELECTROMACHENICAL - THE FIRST GENERATION

These relays remained popular in earlier power system as well as today. A large number of relays and designs having robust characteristics and tropical weather condition resistant qualities, make these are ready choice of protection engineers. These includes CDG (GEC), S type (BBC) etc.

These electromechanical designs are popular in all time characteristics categories including:

1. Inverse time or IDMT.2. Instantaneous3. Definite time

OPERATING PRINCIPLES

ELECTROMECHANICAL CURRRENT DETECTING RELAYS

There may be a great variation in the physical appearance of the electromechanical relays. Their operation is based upon one of the two fundamental principles.

1. Electromagnetic attraction 2. Electromagnetic induction

1. ELECTROMAGNETIC ATTRACTION

A. PLUNGER TYPE

Fig (1)When a current is passed through an operating coil a magnetic field produced which, if the current is sufficient, will draw the iron plunger in the solenoid. The moving contact moves upward and completes the trip circuit. In the relay, operating current can be varied by adjusting screw. This screw changes the magnetic field strength by moving soft iron sleeve inward or out ward. These relay has a fix time.

B. HINGED ARMATURE OR ATTRACTED ARMATURE TYPE

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Figure below illustrates the essential components of hinged armature relay. Principle is similar to plunger type.

Fig (2)

C. INDUCTION DISC TYPE

This relay is basically a single phase induction motor which develops enough torque to turn its disc when the current is of the sufficient magnitude. These relays are of two basic types:

i. WOUND TYPE

This relay consists of two windings like a single phase capacitor motor. In this principle current is split into two components by adding a capacitor in a starting winding.

ii. SHADED POLE TYPE

In this method, instead of splitting the current, magnetic field is split in to two components to get a couple for rotary motion the important components are shown in the figure. When current is more than pickup value, disc start rotating and closes normally open contacts. Operating time depends upon the speed of the disc rotating. There is inverse relation in current and time. Large current means more torque and thus fast speed. Thus moving contact takes less time to close the contact because of high speed. Distance, between moving and fix contact subdivided into ten equal parts and is called calibrated on a dial, called a Time Dial. The settings are called time dial settings. Therefore, such relays are called inverse time relays or IDMT (Inverse Definite Minimum Time) relays.

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Fig (3)

If we want to operate that the different current values, the operating coil is tapped off in such a

manner that the Ampere Turns remain constant at all such values. Hence the characteristics

remain same throughout the current range. Induction disc relays are therefore both time varying

and current varying relays.

Fig (4)

Winding taps are connected to Current Transformer circuit via a PLUG. Therefore these currents

settings are called PLUG SETTING. Its’ current time characteristics are shown in the relay

literature for different time dial settings (TDS). To avoid number of graphs for different plug

settings; time is plotted against plug setting multiples.

ELECTRONIC WITH ANALOGUE DESIGN - THE SECOND GENERATION

Early solid state design permits all the processing in analog form, for decision making level

detector and comparator circuitry is used. In analog electronic and digital relays, input

transformers are used in three phase gating circuits mainly in two configurations.

1. Output dependent on highest instantaneous voltage.2. Output dependent on highest instantaneous current.

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DIGITAL WITH MICRO-PROCESSOR DESIGN - THE THIRD GENERATION

These relays use the numerical techniques to derive the protection and control functions. With

the flexibility of the microprocessor, many characteristics of the inverse type relays including

Very Long Inverse, Normal Inverse, Very Inverse, Extremely Inverse, Definite Time and

Instantaneous elements are designed with in the product using independent algorithms.

In Digital relays four analog derived inputs, inclusive of neutral are multiplexed, then sampled

eight or less times per frequency cycle. The Fourier derived power frequency component returns

the RMS value of the measured quantity for further use.

These values are processed in analog to digital (A/D) convertor which is synchronized to the

power frequency measurement. In addition current is measured once per power frequency cycle

and Fourier is used to extract the fundamental component. The logic inputs are then filtered to

ensure that induced AC current in the external wiring to these inputs does not cause an incorrect

response. Sometimes opto-isolation is used in input and output circuits.

NUMERICAL WITH SOFTWARE BASED DESIGN

The numerical over-current time protection is usually equipped with a 16-bit microprocessor. This provides fully digital processing of all functions for data acquisition of measured values.

The transducers of the measured value input section transforms the current from the current transformers of the switchgear being protected. This unit then matches them to the internal processing level of the relay. Apart from the galvanic and low capacitive isolation provided by the input transformers, filters are provided for the suppression of interference. Filters are designed according to bandwidth and processing speed to suit the measured value processing.

The matched analog values are then passed to the analog input section. This section contains

input amplifiers for each input, analog to digital convertors, and memory circuit for the data

transfer to the microprocessor. Apart from control and supervision of measured values, the

microprocessor processes the actual protective functions.

These include:

1. Scanning limit values and time sequences2. Filtering and formation of the measured quantities3. Calculation of negative and positive sequence currents for unbalanced load detection4. Calculation of RMS values for overload detection5. Decision regarding trip and re-close commands

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6. Storage of measured quantities during a fault for analysis

Binary inputs and outputs to and from the processor are channeled via the input/output elements. From these the processor receives information from the switchgear or from other equipments about remote resetting, blocking signals and membrane key pad.

Output includes trip and re-close commands to the circuit breakers and visual indications i.e. LEDs and alphanumerical display on the front. Usually integrated membrane key board in connection with a built in alphanumerical LCD display enables communication with the processor. All setting values and plant data are entered into the protection from this panel. The dialog with the relay can also be carried out via the serial interface by means of personal computer using RS232 Serial Bus or a Universal Serial Bus in modern relays.

In Numerical relays configuration is in software, some relays are provided with fixed

configuration. Some advance designs have more flexibility where user can make changes to the

internal logic of the relay by setting “software” links provided in software and called “Flexible

logic”, where you can generate your own inverse curves according to a specific requirement by

changing the variables of a particular algorithm. This process is sometimes called “Marshalling

of Numerical Relay”

12-4 SETTING CALCULATIONS

A-II AA-1 B-II BB-1C-II C C-1 D

600-1200-2400/1A 600-1200-2400/1A 300-600-1200/5A 400-800-1600/1A D-

1

~ωω ωω ωωω

400-800-1600/5A 400-800-1600/1A 300-600-1200/5A

150A 250A 200A

150A

Fig (5)

D-1:-Load=150A

10% of Over Load=1.1 x 150 =165A

CT Ratio= 300/5A

Plug Setting= 165 x 5/300 = 2.75A

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~

ATIf = 4000A

Ifsec = 4000x5/300 = 66.67A

M.O.P.S. =Ifsec/P.S = 66.67/2.75 =24.24

LetToper. =0.1 Sec

Toper. =0.14 x T.D.S/( MOPS)0.02 - 1

T.D.S = 0.1[ (24.24)0.02 - 1/ 0.14 ]

= 0.1 [ 1.066 – 1/0.14 ]

= 0.1 [ 0.066/0.14 ] = 0.1 x 0.47

T.D.S = 0.047 Sec

ATIf = 6000A

Ifsec =6000x5/300 =100A

MOPS = Ifsec/P.S

MOPS = 100/2.75 = 36.36

Toper.= 0.14 x TDS/ (MOPS)0.02 - 1

Toper.= 0.14 x 0.047 /(36.36)0.02 - 1 = 0.00658 / 0.0745 = 0.09 Sec

AT If = 1000A

Ifsec = 1000 x 5 /300 =16.67A

MOPS = 16.67/2.75 =6.06

Toper.=0.14 x TDS/(MOPS)0.02 - 1 = 0.14 x 0.047 /(6.06)0.02 – 1

Toper. = 0.00658 / 1.0367 – 1 = 0.00658 /0.0367 =0.18 Sec

C-1:-

Load = 150A

10 % overload = 1.1 x 150 = 165A

C.T Ratio = 400/1A

Plug Setting = 165 x 1/400 = 0.41A

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ATIf = 4000A

Ifsec =4000 x 1/400 =10A

MOPS = Ifsec/P.S = 10/0.41 =24.39

Let

Toper.= Toper. D-1 + D-1 C.B Mechanism operating time + D-1 C.B Arc Quenching Media + Safety Margin

Toper.C-1 = 0.1 + 0.05 + 0.02 + 0.05 =0.22Sec

Toper.=0.14 x TDS/(MOPS)0.02 - 1

T.D.S = 0.22 [(24.39)0.02 - 1/0.14 ]

= 0.22[1.0659 – 1/0.14 ] = 0.22 x 0.471

T.D.S = 0.104 Sec

ATIf = 6000A

Ifsec = 6000 x 1/400 = 15A

MOPS= 15/0.41 = 36.59

Toper.= 0.14 xTDS/(MOPS)0.02 - 1

Toper.= 0.14 x 0.104/(36.59)0.02 -1 = 0.0145 / 0.0746 = 0.19Sec

ATIf =1000A

Ifsec= 1000 x 1/400 =2.5A

MOPS = 2.5/0.41 = 6.097

Toper.= 0.14 x 0.104/(6.097)0.02 - 1

Toper.=0.0146/0.0368 = 0.397Sec

C-II:-

Load = 350A

10% overload =110% x 350 = 385A

C.T. Ratio = 400/1A

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Plug Setting = 385 x 1/400 =0.96A

ATIf = 4000A

Ifsec = 4000 x 1/400 = 10A

MOPS = 10/0.96 = 10.42

Toper = Toper C-1 + C.B Mechanism operating time + C-1 C.B Arc Quenching Media + Safety Margin

Toper = 0.22 + 0.05 + 0.02 + 0.05 = 0.34 Sec

Toper = 0.14 x TDS/(MOPS)0.02 - 1

T.D.S = 0.34 [(10.42)0.02 - 1/0.14 = 0.34[0.0479/0.14] = 0.34 x 0.342 = 0.116 Sec

ATIf= 6000A

Ifsec= 6000 x 1/400 =15A

MOPS = 15/0.96 =15.63

Toper = 0.14 x TDS/(MOPS)0.02- 1= 0.14 x 0.116/(15.63)0.02- 1

Toper = 0.0162 / 0.0565 = 0.287 Sec

ATIf = 1000A

Ifsec = 1000 x 1/400 = 2.5A

MOPS = 2.5/0.96 = 2.6

Toper = 0.14 x 0.116/(2.6)0.02- 1

Toper = 0.0162/0.0193 = 0.839 Sec

B-1:-

Load =350A

10% overload =110% x 350 = 385A

C.T Ratio = 600/5A

Plug Setting = 385 x 5/600 = 3.21A

ATIf = 4000A

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Ifsec = 4000 x 5/600 = 33.33A

MOPS =33.33/3.21 = 10.38

Toper B-1= Toper C-II+ C-II C.B Mechanism Opening time +C-II C.B Arc Quenching Media + Safety Margin

Toper B-1 =0.34 + 0.05 + 0.02 + 0.05 =0.46 Sec

AT If=6000A

Ifsec =6000 x 5/600 =50A

MOPS = 50/3.21 =15.58

Toper =0.14 x 0.16/(15.58)0.02- 1

Toper =0.0224/0.0564 = 0.397 Sec

ATIf =1000A

Ifsec= 1000 x 5/600 = 8.33A

MOPS = 8.33/3.21 = 2.6

Toper = 0.14 x 0.16/(2.6)0.02- 1 = 0.0224/1.0193

Toper = 0.0224/0.0193 = 1.16 Sec

B-II:-

Load =600A

10% overload =110% x 600 =660A

C.T Ratio = 800/5A

Plug Setting =660 x 5/800 =4.125A

ATIf = 4000A

Ifsec= 4000 x 5/800 =25A

MOPS =25/4.125 = 6.06

Toper B-II = Toper B-1 + B-1 C.B Mechanism opening time + B-1 Arc Quenching Media + Safety Margin

Toper B-II =0.46 + 0.05 + 0.02 + 0.05 = 0.58 Sec

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Toper = 0.14 x TDS /(MOPS)0.02- 1

T.D.S = 0.58[(6.06)0.02- 1/0.14] = 0.58[0.0367/0.14]

T.D.S = 0.58 x 0.262 = 0.152 Sec

ATIf=6000A

Ifsec= 6000 x 5/800 = 37.5A

MOPS = 37.5/4.125 =9.09

Toper = 0.14 x 0.152/(9.09)0.02 - 1 = 0.02128/0.045 = 0.47 Sec

ATIf= 1000A

Ifsec = 1000 x 5/800 = 6.25A

MOPS = 6.25/4.125 = 1.52

Toper = 0.14 x 0.152/(1.52)0.02 -1 = 0.02128/1.0084 – 1

Toper = 0.02128/0.0084 = 2.53 Sec

A-1:-

Load = 600A

10% overload = 110% x 600 = 660A

C.T. Ratio = 1200/1A

Plug Setting = 660 x 1/1200 = 0.55A

AT If = 4000A

Ifsec = 4000 x 1/1200 = 3.33A

MOPS = 3.33/0.55 = 6.06

Toper A-1 = Toper B-II + B-II C.B Mechanism operating time + B-II C.B Arc Quenching Media + Safety Margin

Toper A-1 = 0.58 + 0.05 +0.02 + 0.05 = 0.7 Sec

Toper = 0.14 x T.D.S/(MOPS)0.02 – 1

T.D.S = 0.7[(6.06)0.02 – 1/0.14] = 0.7[0.03669/0.14]

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T.D.S = 0.7 x 0.262 = 0.18 Sec

AT If = 6000A

Ifsec = 6000 x 1/1200 = 5A

MOPS = 5/0.55 = 9.09

Toper = 0.14 0.18/(9.09)0.02 – 1 = 0.0252/0.0451= 0.56 Sec

AT If = 1000A

Ifsec = 1000 x 1/1200 = 0.83A

MOPS = 0.83/0.55 = 1.51

Toper =0.14 x 0.18/(1.51)0.02 – 1 = 0.0252/0.0083 = 3.04 Sec

A-II:-

Load = 750A

10%overload = 110% x 750 = 825A

C.T ratio = 1200/1A

Plug Setting = 825 x 1/1200 = 0.687A

AT If = 4000A

Ifsec = 4000 x 1/1200 = 3.33A

MOPS = 3.33/0.687 = 4.85

Toper A-II = Toper A-1 + A-1 C.B Mechanism operating time + A-1 C.B Arc Quenching Media + Safety Margin

Toper A-II = 0.7 + 0.05 +0.02 + 0.05 = 0.82 Sec

Toper = 0.14 x T.D.S/(MOPS)0.02 – 1

T.D.S = 0.82[(4.85)0.02 – 1/0.14]= 0.82[0.0321/0.14]

T.D.S = 0.82 x 0.229 = 0.188 Sec

AT If = 6000A

Ifsec = 6000 x 1/1200 = 5A

MOPS = 5/0.687 = 7.28

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Toper = 0.14 x 0.188/(7.28)0.02 – 1 = 0.0263/0.0405 = 0.65 Sec

AT If =1000A

Ifsec= 1000 x 1/1200 = 0.83A

MOPS = 0.83/0.687 = 1.21

Toper = 0.14 x 0.188/(1.21)0.02 – 1 = 0.0263/0.0038 = 6.92 Sec

12-5 OVER CURRENT RELAY TESTING

The following Tests are to be carried out on various Over Current Relays in Laboratory

12-5-1 Pick- Up/Drop Off

12-5-2 Operating Time

13.

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Differential Protection

Relay

13.1 FAULTS IN POWER TRANSFORMERS

Transformer should be protected against.1. Internal faults2. Through faults

Internal fault occurs as a result of failure of the insulation, providing a short circuit path between phases and often to the ground icon core. The heavy fault current can cause damage to winding can even burn the core. Differential protection provides the best against internal faults. Through faults are cleared by protective devices downstream. However, failure of downstream relays could place a serve over load on the transformer. These currents due to through faults can cause mechanical and thermal damage. All the protections for through faults must operate before the transformer reaches at thermal limit according to transformer damage curve. Normally

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differential Protection is applied to all transformers of 10MVA and above and smaller transformers of importance.

13.2 DIFFERENTIAL PROTECTION

The Differential Protection of Transformer has many advantages over other schemes of protection.

The faults occur in the transformer inside the insulating oil can be detected by Buchholz relay. But if any fault occurs in the transformer but not in oil then it cannot be detected by Buchholz relay. Any flash over at the bushings are not adequately covered by Buchholz relay. Differential relays can detect such type of faults. Moreover Buchholz relay is provided in transformer for detecting any internal fault in the transformer but Differential Protection scheme detects the same in faster way.

The differential relays normally response to those faults which occur inside the differential protection zone of transformer.

13.3 PRINCIPLE OF DIFFERENTIAL PROTECTION

Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.

Suppose you have one transformer which has primary rated current Ip and secondary current Is. If you install Current Transformer of ratio Ip/1A at primary side and similarly, Current Transformers of ratio Is/1A at secondary side of the transformer. The secondary of these both Current Transformers are connected together in such a manner that secondary currents of both Current Transformers will oppose each other. In other words, the secondary of both Current Transformers should be connected to same current coil of differential relay in such a opposite manner that there will be no resultant current in that coil in normal working condition of the transformer. But if any major fault occurs inside the transformer due to which the normal ratio of the transformer is disturbed then the secondary current of both transformer will not remain the same and one resultant current will flow through the current coil of the differential relay, which will actuate the relay and inter trip both the primary and secondary circuit breakers.

To correct phase shift of current because of star-delta connection of transformer winding in case of three phase transformer, the Current Transformer secondary should be connected in delta and star respectively. If the equipment within the “Protection Zone” is functioning correctly, then the sum of currents entering the Zone must equal the sum of currents leaving i.e. their difference must be zero, and the relay will be inoperative.

13.4 TYPES OF DIFFERENTIAL RELAYS

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There are two types of differential relays:

1. Plain Differential Relay2. Percentage Differential relay

These relays can be best understood by an example. Consider a 132/11kV; YY connected Power Transformer having no phase shift. Full load current ratings are 100 A and 1200 A respectively Current Transformer ratio on HV side is taken as 100/5 and on LV side 1200/5. If polarities of transformer are taken as subtractive, the single phase schematic circuit will be as under.

Fig (1)

The current in primary side Current Transformer loop is 5A with the direction according to convention that “if primary current is entering ‘P1’ then secondary current leaves ‘S1’.

It is clear that 5A primary Current Transformer current balanced 5A secondary side Current Transformer current. Current flowing through differential relay is

I operate = I1 – I2

I operate = 10 – 10

I operate = 0

Hence Differential scheme is perfectly balanced. However, if current direction in any loop is reversed, the

I operate = I1 – (–I2)

I operate = I1 + I2

I operate = 5+5

I operate = 10 A, and the scheme will trip.

This shows the importance of current direction in differential schemes. Now the scheme is put under first test i.e.

THROUGH FAULT TEST: Consider an out of Zone Fault of 2400A,

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Fig (2)I operate = I1- I2

I operate = 10-10

I operate = 0, and the schemes remains still balanced.

INTERNAL FAULT TEST (WITH SINGLE FEED): Consider an Internal Fault of 2400A,

Fig (3)

I operate = I1- I2

I operate = 10-0

I operate = 10

The difference of current flows through relay because I2 = 0, as no current flows in the secondary of LV Current Transformers and the scheme will trip.

INTERNAL FAULT TEST (WITH DOUBLE FEED): Consider an Internal Fault, assume that impedance on both side of fault is same and both sources share equal current.

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Fig (4)

I operate = I1– 12

I operate = 10– (–10)

I operate =10 + 10

I operate = 20A, hence the scheme will trip too.

Consider such a relay having different character Current Transformers with two ampere mismatch current. In case of plain differential scheme relay should operate.

There are many factors still left that can affect the balance of scheme.

1. If CTs are not of identical design. Difference can flow to unbalance the scheme. 2. Saturation of one of the two CT’S.3. The magnetizing in rush currents in T/FS.4. Phase shifting in star delta connected transformers:5. Connections of CT’S, phase.6. Selection of phase if HV and LV side CT currents are not equal.7. Effect of transformer taps.

The relay discussed is called a plain differential relay. To avoid problems of Current

Transformer saturation or varying character Current Transformers, solution is to employ two

restrain coils and one operating coil in each current loop. The restraining coils produce Restrain

Torque or a Negative Torque while the operating coil produces an Operating Torque or Positive

Torque. Restrain Torque strength can be changed by using a tapped winding.

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Fig (5)

Consider such a relay having different character Current Transformers with 2 A mismatch current.

Fig (6)

In case of plain differential scheme, relay should operate, but in modified scheme a restraining current of

Ires = (I1 + I2)/2

Ires = (13+15)/2

Ires = 14A, flows in restraining coil and

I operate = I1 – I2

I operate = 15-13

I operate = 2 A, flows in operating coil

The stronger restraining field keeps the relay un-operative. Similarly if any CT saturates at such

high fault currents relay will behave similarly.

%age = Iop /I rest × 100

%age = 2/14 × 100

%age = 14.3%

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Relay will restrain if set more than 14.3% slope. Normal relays are provided with 15, 30 and 45

percent slope. Higher the slope lesser the sensitivity of relay at 10 times fault current.

I res = (130 + 150)/2

I res = 140A

I operate = 150 – 130

I operate = 20

% age = (20/140) × 100

% age = 14.3%

Such relays are, therefore, termed as Percentage Differential relays because operating current is a

fix percentage of restrain current for same slope.

Fig (7)

From the figure it is clear that for a fixed restraining current of 14 A operating coil pick up

currents are different. At 15 % slope, relay is more sensitive than at 45% slope.

13.5 MAGNETIZING INRUSH CURRENT

When a power transformer is energized, transient magnetizing current flows for few cycles

having instantaneous peaks of 8 to 20 times those of full load current. Duration of which depends

upon

1. Size of transformer2. Size of power system3. Source resistance4. Residual flux level5. Core material

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On analysis it revealed it Second Harmonic components are dominant.

Component 2nd Harmonic 3rd Harmonic 4th Harmonic

%age 63% 26% 5.1%

Since inrush current flows with only one side of transformer energized. The effect is similar to the fault on the system. Therefore, Harmonic restraint circuitry is essential to avoid mal-operation of differential relays. This can be achieved by two means:

1. Time delay: One method for preventing tripping due to inrush the operating coil of the differential relay is de-sensitized for a few milliseconds when the primary breaker is closed.

2. Second Harmonic Restraint Circuit: Second harmonic in the inrush current are filtered out using band pass filters then applied in restraining circuits to restrain the relay further, when inrush current is flowing.

Fig (8)But still exits a problem, whether these harmonics are generated by a power transformer or a saturated Current Transformer to avoid malfunction in such conditions, relay should differentiate between these conditions by Zero Detection Method. The wave shape of second harmonic currents of a Current Transformer compared with a fault current wave from suggest that magnetizing in rush wave normally stays at zero crossing for quite a sometime. This zero time is different for a Current Transformer wave form and a Power Transformer wave form.

Hence relay is made to restrain if zero is detected in a cycle for more than a certain period (typically for 1/4th of a cycle).

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Fig (9)

13-6 BALANCE OF DIFFERENTIAL RELAY FOR VARIOUS VECTOR GROUPS

For correct application of differential protection requires Current Transformer ratio and winding connections of Current Transformer secondary (Phasing) to match those of transformer. For this purpose polarity convention used is, if current is entering P1 (dot) then it should leave from S1

(dot) i.e. Subtractive Polarity. The Hard Rule is that Current Transformer Secondary circuit should be a Replica of Primary System, to consider

1. Minimizing of Difference in Current Magnitudes2. Phase Shift Compensation3. Zero Sequence Currents Trapping

All these requirements can only be achieved using matching or interposing Current

Transformers. It is a small transformer usually with four independent primary and secondary

windings.

Fig (10)

Using required turn on a matching Current Transformer, loop currents can be increased or

decreased depending on the difference of current at the relay. Phase shifting connections are also

be carried out of matching Current Transformers instead of main Current Transformers, to lower

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the Current Transformer secondary burden, because matching Current Transformers are located

near the relay. By doing so I2R losses in a delta connection are avoided.

Delta-Star Power Transformer: To compensate for Delta–Star transformer phase shift of 30o and Zero Sequence Current trap, the rule is to connect: Delta side Current Transformers of Power Transformer in Star andStar side Current Transformers of Power Transformer in Delta

Fig(11)

Delta-Delta Power Transformer: Star connected Current Transformers are used on both sides to reduce I2R losses and hence burden on Current Transformers. As

Total Current Transformer burden = Relay Resistance + Lead Resistance + Current Transformers Secondary Resistance

Star-Star Power Transformer: Current Transformers are now connected in delta on both sides to provide Zero Sequence Current trap on both side to reduce Current Transformers burden. Matching Current Transformers are used near differential relay in 1:1 ratio (if current change is not required), and connect their relay side winding in delta.

13-7 DIFFERENTIAL RELAY TESTING

The following Tests are to be carried out on various Differential Relays in Laboratory

13-7-1 Pickup/ Drop off

13-7-2 Operating time

13-7-3 Percent slope

13-7-4 Percent Second Harmonics

13-8 PRACTICAL CONNECTIONS OF DIFFERENTIAL ON A POWER TRANSFORMER

Practical demonstration will be carried out with the 66 kV/ 11 kV, 1MVA Power Transformer

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14.

Under Frequency

Relay

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14.1 UNDER FREQUENCY PROTECTION

System stability is an important objective, whenever there is an overloading on the power

system; load on the system is needed to be reduced.

As, Ns = (120*f)/P

Or f = (Ns*P)/120

Due to increase in load, the speed of the generator goes down, and so does the frequency. In

order to improve the frequency, the speed of the generator needs to be increased. This can be

achieved through different means depending upon the type of generation. However, this process

has certain limits. Similarly, sometimes, a situation will be encountered when due to sudden

rejection of load, the frequency of the system increase. In order to monitor the variation of

frequency from a predetermined range, Frequency Protection is used.

Normally, in our system, we face under frequency problem or overloading. In order to control

the load, load shedding is carried out. Load shedding can be carried out either manually or

automatic.

In manual load shedding, Power Demand and Power Generated are compared. If Power Demand

is more than Power Generation, then load shedding of the deficit amount is carried out by

dividing the power to be shed among different areas according to priority.

Sometimes due to system constraints or climatic conditions, predictions become difficult, and

forced load shedding is carried out. However there are situations, when there is no time for

manual load shedding and the frequency is supposed to cross a critical stage, where total system

might collapse. In such situations, automatic load shedding is carried out.

An under frequency load shedding protection system is incorporated for stably managing a

power system by recovering a power system frequency to within a predetermined range when the

power system frequency drops. An under frequency level detection unit judges an under

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frequency level of the power system frequency when the power system frequency drops resulting

from power generation shortage in the power system and a load shedding unit sequentially sheds

loads determined in advance based on a staying time when the power system frequency stays at

any one of the under frequency levels judged by the under frequency level detection unit and

sheds on this occasion more loads quickly when the under frequency level at which the power

system frequency stays is large.

14.2 OPERATING PRINCIPLES

Potential Transformer supply is applied to the relay. A rate of change of frequency detection unit

is provided, which judges, when the power system frequency deviates resulting from load

rejection, loading or a fault on the power system. When the power system frequency varies, the

frequency relay detects this change it acts accordingly. The relay might be over frequency or

under frequency or featuring both. Similarly the relay be single stage or multistage, depending

upon the requirements. Frequency relays are generally applied:

1. To monitor continuously the frequency of an electrical power system2. Used in graded load shedding system

14-3 UNDER FREQUENCY RELAY TESTING

The following Tests are to be carried out on various Under Frequency Relays in Laboratory

14-3-1 PICKUP/DROP OFF

14-3-2 OPERATING TIME

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15.

Over Fluxing Relay

200

15.1 CAUSES OF OVER FLUXING IN TRANSFORMER

As per present day transformer design practice, the peak rated value of the flux density is kept about 1.7 to 1.8 Tesla, while the saturation flux density of CRGD steel sheet of core of electrical power transformer is of the order of 1.9 to 2 Tesla which corresponds to about 1.1 times the rated value. If during operation, an electrical power transformer is subjected to carry rather swallow more than above mentioned flux density as per its design limitations, the transformer is said to have faced over fluxing problem and consequent bad effects towards its operation and life.

Depending upon the design and saturation flux densities and the thermal time constants of the heated component parts, a transformer has some over excitation capacity. I.S. specification for an electrical power transformer does not stipulate the short time permissible over excitation, though in a roundabout way it does indicate that the maximum over fluxing in transformer shall not exceed 110%.

The flux density in a transformer can be expressed by

B = C V/f, where, C = A constant, V = Induced voltage, f = Frequency.

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The magnetic flux density is, therefore, proportional to the quotient of voltage and frequency (V/f). Over fluxing can, therefore, occur either due to increase in voltage or decrease in-frequency of both.

The probability of over fluxing is relatively high in step up transformers in Power stations compared to step down transformers in Sub-Stations, where voltage and frequency usually remain constant. However, under very abnormal system condition, over-fluxing trouble can arise in step-down Sub-Station transformers as well.

15.2 EFFECT OF OVER FLUXING IN TRANSFORMERS

The flux in a transformer, under normal conditions is confined to the core of transformer because of its high permeability compared to the surrounding volume. When the flux density in the core of transformer increases beyond saturation point, a substantial amount of flux is diverted to steel structural parts and into the air. At saturation flux density the core steel will over heat.

Structural steel parts which are un-laminated and are not designed to carry magnetic flux will heat rapidly. Flux flowing in unplanned air paths may link conducing loops in the windings, loads, tank base at the bottom of the core and structural parts and the resulting circulating currents in these loops can cause dangerous temperature increase. Under conditions of excessive over fluxing the heating of the inner portion of the windings may be sufficiently extreme as the exciting current is rich in harmonies. It is obvious that the levels of loss which occur in the winding at high excitation cannot be tolerated for long if the damage is to be avoided.

Physical evidences of damage due to over fluxing will vary with the degree of over excitation, the time applied and the particular design of transformer. The Table given below summarizes such physical damage and probable consequences.

Sr. No. Component involved Physical evidences Consequences

1Metallic support and surfaces structure for

core and coils

Discoloration or metallic parts and adjacent insulation.

Possible carbonized material in oil. Evolution of combustible gas.

Contamination of oil and surfaces of insulation. Mechanical weakening of insulation. Loosening of Mechanical structure

2 Windings Discoloration winding insulation. Evolution of gas.

Electrical and mechanical. Weakening of winding insulation

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3 Lead conductors.

Discoloration of conductor insulation or support.

Evolution of gas.

Electrical and mechanical weakening of insulation. Mechanical Weakening of support.

4 Core laminations.

Discoloration of insulating material in contact with core.

Discoloration and carbonization of organic/lamination insulation.

Evaluation of gas.

Electrical weakening of major insulation (winding to core). Increased inter-laminar eddy loss.

5 Tank Blistering of paints Contamination of oil if paint inside tank is blistered.

It may be seen that metallic support structures for core & coil, windings, lead conductors, core laminations, tank etc. may attain sufficient temperature with the evolution of combustible gas in each case due to over-fluxing of transformer and the same gas may be collected in Buchholz Relay with consequent Alarm/Trip depending upon the quantity of gas collected which again depends upon the duration, the transformer is subjected to over fluxing.

Due to over fluxing in transformer its core becomes saturated as such induced voltage in the primary circuit becomes more or less constant. If the supply voltage to the primary is increased to abnormal high value, there must be high magnetizing current in the primary circuit. Under such magnetic state of condition of transformer core linear relations between primary and secondary quantities (viz. for voltage and currents) are lost. So there may not be sufficient and appropriate reflection of this high primary magnetizing current to secondary circuit as such mismatching of primary currents and secondary currents is likely to occur, causing differential relay to operate as we do not have over fluxing protection for sub-Station transformers.

STIPULATED WITHSTAND-DURATION OF OVER FLUXING IN TRANSFORMERS

Over fluxing in transformer has sufficient harmful effect towards its life which has been explained. As over fluxing protection is not generally provided in step-down transformers of Sub-Station, there must be a stipulated time which can be allowed matching with the transformer design to withstand such over fluxing without causing appreciable damage to the transformer and other protections shall be sensitive enough to trip the transformer well within such stipulated time, if cause of over fluxing is not removed by this time.

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It is already mentioned that the flux density 'B' in transformer core is proportional to V/f ratio. Power transformers are designed to withstand (Vn/fn x 1.1) continuously, where Vn is the normal highest RMS voltage and fn is the standard frequency. Core design is such that higher V/f causes higher core loss and core heating. The capability of a transformer to withstand higher V/f values i.e. over-fluxing effect, is limited to a few minutes as furnished below in the Table

F = (V/f)/(Vn/fn) 1.1 1.2 1.25 1.3 1.4

Duration of with stand limit (minutes) continuous 2 1 0.5 0

From the table above it may be seen that when over fluxing due to system hazards reaches such that the factor F attains a values 1.4, the transformer shall be tripped out of service instantaneously otherwise there may be a permanent damage.

15.3 OPERATING PRINCIPLES

The condition arising out of over fluxing does not call for high speed tripping. Instantaneous operation is undesirable as this would cause tripping on momentary system disturbances which can be borne safely but the normal condition must be restored or the transformer must be isolated within one or two minutes at the most.

Flux density is proportional to V/f and it is necessary to detect a ratio of V/f exceeding unity, V and f being expressed in per unit value of rated quantities. In a typical scheme designed for over fluxing protection, the system voltage as measured by the Voltages Transformer is applied to a resistance to produce a proportionate current; this current on being passed through a capacitor, produces a voltage drop which is proportional to the functioning in question i.e. V/f and hence to the flux in the power transformer. This is accompanied with a fixed reference D.C. voltage obtained across a Zener diode. When the peak A.C. signal exceeds the D.C. reference it triggers a transistor circuit which operates two electromechanical auxiliary elements. One is initiated after a fixed time delay, the other after an additional time delay which is adjustable. The over fluxing protection operates when the ratio of the terminal voltage to frequency exceeds a predetermined setting and resets when the ratio falls below 95 to 98% of the operating ratio. By adjustment of a potentiometer, the setting is calibrated from 1 to 1.25 times the ratio of rated volts to rated frequency.

The output from the first auxiliary element, which operates after fixed time delay available between 20 to 120 seconds second output relay operates and performs the tripping function.

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It is already pointed out that high V/f occurs in Generator Transformers and Unit-Auxiliary Transformers if full excitation is applied to generator before full synchronous speed is reached. V/f relay is provided in the automatic voltage regulator of generator. This relay blocks and prevents increasing excitation current before full frequency is reached.

When applying V/f relay to step down transformer it is preferable to connect it to the secondary

(L.V. side of the transformer so that change in tap position on the H.V. is automatically taken

care of. Further the relay should initiate an Alarm and the corrective operation is done / got done

by the operator. On extreme eventuality the transformer controlling breaker may be allowed to

trip.

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16.

Trip Circuit

Supervision Relay

16.1 TRIP CIRCUIT SUPERVISION PROTECTION

In a protection system the trip circuit of the circuit breaker is crucial. If an interruption occurs in the trip circuit a possible network fault will not be disconnected and would have to be cleared by another protection upstream in the power system. The supervision function is particularly important when there is only one tripping coil and circuit breaker tripping is vital. For instance, for generator’s circuit breakers or any other important circuit breaker in distribution networks. The supervision relay type Trip Circuit Supervision is intended for a continuous supervision of circuit breaker trip circuit and to give an alarm for loss of auxiliary supply, faults on the trip-coil

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or its wires independent of the breaker position, faults on the breaker auxiliary contacts and faults in the supervision relay itself.

This guide describes methods to protect a power system from faults that are not cleared because of failure of a power circuit breaker to operate or interrupt when called upon. The discussion is limited to those instances where the breaker does not clear the fault after a protective relay has issued a command to open or trip the circuit.

16-2 OPERATING PRINCIPLES

Trip Circuit Supervision Relay monitors the healthiness of the trip circuit of breaker. Basically

the circuit drives a small current through the breaker trip coil and monitors it continuously. If

there is any failure in the trip circuit or trip coil circuit, the relay will sense and operate its output

contacts. Certain time delay is introduced to prevent false indication during breaker

operation.

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17.

Restricted

Earth Fault Relay

208

17.1 RESTRICTED EARTH FAULT PROTECTION OF TRANSFORMER

An external fault in the star side will result in current flowing in the line Current Transformer of the affected phase and at the same time balancing current flows in the neutral Current Transformer; hence the resultant electric current in the relay is therefore zero. So this REF relay will not be actuated for external earth fault. But during internal fault the neutral Current Transformer only carries the unbalance fault current and operation of Restricted Earth Fault Relay takes place. This scheme of Restricted Earth Fault Protection is very sensitive for internal earth fault of electrical power transformer. The protection scheme is comparatively cheaper than differential protection scheme

17.2 OPERATING PRINCIPLES

Restricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer. In this scheme the Current Transformer secondary of each phase of electrical power transformer are connected together as shown in the figure. Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT. The Current Transformer connected to the neutral of power transformer is called Neutral Current Transformer or simply NCT. Whenever there is an unbalancing in between three phases of the power transformer, a resultant unbalance current flow through the close path connected to the common terminals of the Current Transformer secondary. An unbalance current will also flow through the neutral of power transformer and hence there will be a secondary current in NCT because of this unbalance neutral current. In Restricted Earth Fault scheme the common terminals of phase Current Transformers are connected to the secondary of NCT in such a manner that secondary unbalance current of phase Current Transformers and the secondary current of NCT will oppose each other. If these both currents are equal in amplitude there will not be any resultant current circulate through the said close path. The Restricted Earth Fault Relay is connected in this close path. Hence the relay will not respond even there is an unbalancing in phase current of the power transformer.

209

Fig (1)

Fig (2)

210

Fig (3)

18.

211

Breaker Failure

Protection

18-1 BREAKER FAIL PROTECTION

A failure occurs when the circuit-breaker fails to correctly open and clear the fault after single or three-pole trip commands have been issued by the protection unit. It is then necessary to trip the relevant bus bar zone (section) to ensure fault clearance.

One of the primary considerations in the design and application of a breaker failure protection system is that it should be biased toward security. Since a true breaker failure occurrence is so rare, the breaker failure protection scheme will be called upon to not trip many more times than it will be called upon to trip. Also, since the failure of a breaker generally requires tripping out all adjacent circuits, the consequences of mal-operation and over trip of the system are many times worse than mal-operation and over trip of nearly any other protective scheme on the power system. For this reason, there are several features that are commonly included to enhance security from mal-operation. A separate breaker failure protection system is required for each breaker. Backup tripping systems such as lockout relays can be common within a substation depending on the circumstances.

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18-2 OPERATING PRINCIPLES

At its most elemental level, a breaker failure protection system consists of a timer. The timer is started at the same time that the trip signal is sent to the breaker and is used to precisely time the period that is allowed for the breaker to interrupt the fault. If the breaker does not operate by the expiration of the time delay, the breaker is determined to have failed and tripping of backup breakers occurs. The breaker failure protection system can determine if the breaker has tripped by monitoring a contact mounted to the breaker operating mechanism; however, it generally includes a current detector to confirm that the current flowing in the tripped breaker has been successfully interrupted.

Beyond this simple concept, the timers and fault detectors in a breaker failure protection system can be combined in many different ways; but all can be simplified into a few common logic schemes. Figure 1 shows three of these basic logic schemes. The timing diagrams associated with these logic schemes are shown in Figure 2.

Figure 1a shows a scheme where both the BFI (breaker failure initiate) and the fault detector must be true to start the timer. Successful interruption is indicated by either the fault detector dropping out or the protective relays dropping out and removing the BFI signal. The breaker failure fault detector is important here because it has a high dropout ratio and fast reset characteristic whereas the protective relays do not have such a constraint put upon them. They may be slow to drop out after the fault is cleared.

213

214

Fig (1)

215

Figure 1b shows a scheme where the importance of the dropout/reset characteristics of the fault detector is minimized. The timer is initiated by the BFI signal from the protective relays. If the timer expires before the protective relays drop out, the fault detector is then started. If the breaker has interrupted successfully, the fault detector will not pick up at all. In this case, the fault detector should have a fast pickup time because that will be added to the time required to trip backup in the event of a failed breaker.

Fig (2)

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When considering the two timing diagrams, it is important to call attention to the time marked as the margin. The margin is the difference between normal clearing time and when the breaker failure protection system will cause backup tripping to occur. A larger margin will improve security from incorrect backup tripping. For Figure 2-b, it can be seen that the margin can be improved by the difference between the fault detector’s pickup and drop-out time for a given backup tripping time, or the backup tripping time can be reduced by this same amount for a given margin time.

Figure 1c is a subtle variation on scheme 1a. In this case, the timer is started by the BFI signal alone as in scheme 1b, so fault detector pickup time is not a factor in starting the timer. Breaker failure trip will occur if the timer expires and the fault detector is still picked up. The difference with this logic is that the effect of timer over travel (the timer continues for a short period after the input is removed) is minimized.

These basic schemes can be modified to accommodate additional situations. The most important modification to note is the need to also accommodate breaker failure protection with breaker status contact supervision instead of fault detector supervision. This modification would be used in situations where the faults being detected by the initiating relays may not involve high current. For example, initiation by transformer differential or sudden pressure relays or remote transfer trip.

With modern, solid state and numerical breaker failure relays, issues such as fault detector and timer performance are minimized over breaker failure schemes built up using discrete electromechanical components. Programmable logic in solid state and numerical relays also

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19.

Bus-Bar Protection

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19.1 INTRODUCTION

The protection scheme for a power system should cover the whole system against all probable types of fault. Unrestricted forms of line protection, such as over current and distance systems, meet this requirement, although faults in the bus bar zone are cleared only after some time delay. But if unit protection is applied to feeders and plant, the bus bars are not inherently protected. Bus bars have often been left without specific protection, for one or more of the following reasons:

1. The bus bars and switchgear have a high degree of reliability, to the point of being regarded as intrinsically safe

2. It was feared that accidental operation of bus bar protection might cause widespread dislocation of the power system, which, if not quickly cleared, would cause more loss than would the very infrequent actual bus faults

3. It was hoped that system protection or back-up protection would provide sufficient bus protection if needed.

4. It is true that the risk of a fault occurring on modern metal clad gear is very small, but it cannot be entirely ignored

19.2 BUSBAR FAULTS

The majority of bus faults involve one phase and earth, but faults arise from many causes and a significant number are inter-phase clear of earth. In fact, a large proportion of bus bar faults result from human error rather than the failure of switchgear components. With fully phase-segregated metal clad switchgear, only earth faults are possible, and a protection scheme need have earth fault sensitivity only. In other cases, an ability to respond to phase faults clear of earth is an advantage, although the phase fault sensitivity need not be very high.

19.3 BUS BAR PROTECTION REQUIREMENTS

Although not basically different from other circuit protection, the key position of the bus bar intensifies the emphasis put on the essential requirements of speed and stability. The special features of bus bar protection are discussed below.

SPEED

Bus bar protection is primarily concerned with:

1. Limitations of consequential damage

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2. Removal of bus bar faults in less time than could be achieved by back-up line protection, with the object of maintaining system stability

Some early bus bar protection schemes used a low impedance differential system having a relatively long operation time, of up to 0.5 seconds. The basis of most modern schemes is a differential system using either low impedance biased or high impedance unbiased relays capable of operating in a time of the order of one cycle at a very moderate multiple of fault setting. To this must be added the operating time of any tripping relays, but an overall tripping time of less than two cycles can be achieved. With high-speed circuit breakers, complete fault clearance may be obtained in approximately 0.1 seconds. When a frame-earth system is used, the operating speed is comparable.

STABILITY

The stability of bus protection is of paramount importance. Bearing in mind the low rate of fault incidence, amounting to no more than an average of one fault per bus bar in twenty years, it is clear that unless the stability of the protection is absolute, the degree of disturbance to which the power system is likely to be subjected may be increased by the installation of bus protection. The possibility of incorrect operation has, in the past, led to hesitation in applying bus protection and has also resulted in application of some very complex systems. Increased understanding of the response of differential systems to transient currents enables such systems to be applied with confidence in their fundamental stability. The theory of differential protection is given later in section 15.7. Notwithstanding the complete stability of a correctly applied protection system, dangers exist in practice for a number of reasons. These are:

1. Interruption of the secondary circuit of a current transformer will produce an unbalance, which might cause tripping on load depending on the relative values of circuit load and effective setting. It would certainly do so during a through fault, producing substantial fault current in the circuit in question

2. A mechanical shock of sufficient severity may cause operation, although the likelihood of this occurring with modern numerical schemes is reduced

3. Accidental interference with the relay, arising from a mistake during maintenance testing, may lead to operation

In order to maintain the high order of integrity needed for bus bar protection, it is an almost invariable practice to make tripping depend on two independent measurements of fault quantities. Moreover, if the tripping of all the breakers within a zone is derived from common measuring relays, two separate elements must be operated at each stage to complete a tripping operation. The two measurements may be made by two similar differential systems, or one differential system may be checked by a frame-earth system, by earth fault relays energized by current transformers in the transformer neutral-earth conductors or by voltage or over current relays.

Alternatively, a frame-earth system may be checked by earth fault relays. If two systems of the unit or other similar type are used, they should be energized by separate current transformers in

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the case of high impedance unbiased differential schemes. The duplicate ring CT cores may be mounted on a common primary conductor but independence must be maintained throughout the secondary circuit. In the case of low impedance, biased differential schemes that cater for unequal ratio CTs, the scheme can be energized from either one or two separate sets of main current transformers. The criteria of double feature operation before tripping can be maintained by the provision of two sets of ratio matching interposing CTs per circuit. When multi-contact tripping relays are used, these are also duplicated, one being energized from each discriminating relay; the contacts of the tripping relay are then series-connected in pairs to provide tripping outputs.

Separate tripping relays, each controlling one breaker only, are usually preferred. The importance of such relays is then no more than that of normal circuit protection, so no duplication is required at this stage. Not least among the advantages of using individual tripping relays is the simplification of trip circuit wiring, compared with taking all trip circuits associated with a given bus section through a common multi-contact tripping relay.

In double bus bar installations, a separate protection system is applied to each section of each bus bar. An overall check system is also provided, covering all sections of both bus bars. The separate zones are arranged to overlap the bus bar section switches, so that a fault on the section switch trips both the adjacent zones. This has sometimes been avoided in the past by giving the section switch a time advantage; the section switch is tripped first and the remaining breakers delayed by 0.5 seconds. Only the zone on the faulty side of the section switch will remain operated and trip, the other zone resetting and retaining that section in service. This gain, applicable only to very infrequent section switch faults, is obtained at the expense of seriously delaying the bus protection for all other faults. This practice is therefore not generally favored. Some variations are dealt with later under the more detailed scheme descriptions. There are many combinations possible, but the essential principle is that no single accidental incident of a secondary nature shall be capable of causing an unnecessary trip of a bus section.

Security against mal-operation is only achieved by increasing the amount of equipment that is required to function to complete an operation; and this inevitably increases the statistical risk that a tripping operation due to a fault may fail. Such a failure, leaving aside the question of consequential damage, may result in disruption of the power system to an extent as great, or greater, than would be caused by an unwanted trip. The relative risk of failure of this kind may be slight, but it has been thought worthwhile in some instances to provide a guard in this respect as well.

Security of both stability and operation is obtained by providing three independent channels (say X, Y and Z) whose outputs are arranged in a ‘two-out-of three’ voting arrangement, as shown in Fig (1).

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Fig (1)

19.4 TYPES OF BUS BAR PROTECTION SYSTEMS

A number of bus bar protection systems have been devised:

1. System protection used to cover bus bars2. Frame-earth protection3. Differential protection4. Phase comparison protection5. Directional blocking protection.

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