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Department Of Electrical And Electronics Engineering

EE2402 PROTECTION AND SWITCHGEAR

Question fand answers

For

Two mark and 16,8,4 mark questions

UNIT I INTRODUCTION

UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS

UNIT III APPARATUS PROTECTION

UNIT IV THEORY OF CIRCUIT INTERRUPTION

UNIT V CIRCUIT BREAKERS

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EE2402 PROTECTION AND SWITCHGEAR L T P C 3 0 0 3

AIM:

To introduce the students to the various abnormal operating conditions in power system and

describe the apparatus and system protection schemes. Also to describe the phenomena of current interruption to study the

various switchgears.

OBJECTIVES:

i. To discuss the causes of abnormal operating conditions (faults, lightning and switching

surges) of the apparatus and system.

ii. To understand the characteristics and functions of relays and protection schemes.

iii. To understand the problems associated with circuit interruption by a circuit breaker.

UNIT I INTRODUCTION 9

Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults and fault

currents - relay terminology – definitions - and essential qualities of protection. Protection against over voltages due to

lightning and switching - arcing grounds - Peterson Coil - ground wires - surge absorber and diverters

Power System earthing – neutral Earthing - basic ideas of insulation coordination.

UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS 9

Electromagnetic relays – over current, directional and non-directional, distance, negative sequence, differential and under

frequency relays – Introduction to static relays.

UNIT III APPARATUS PROTECTION 9

Main considerations in apparatus protection - transformer, generator and motor protection -

protection of bus bars. Transmission line protection - zones of protection. CTs and PTs and their applications in protection

schemes.

UNIT IV THEORY OF CIRCUIT INTERRUPTION 9

Physics of arc phenomena and arc interruption. DC and AC circuit breaking - restriking voltage and recovery voltage -

rate of rise of recovery voltage - resistance switching - current chopping - interruption of capacitive current.

UNIT V CIRCUIT BREAKERS 9

Types of circuit breakers – air blast, air break, oil, SF6 and vacuum circuit breakers – comparative merits of different

circuit breakers – testing of circuit breakers.

TOTAL : 45 PERIODS

TEXT BOOKS:

1. M.L. Soni, P.V. Gupta, V.S. Bhatnagar, A. Chakrabarti, ‘A Text Book on Power System

Engineering’, Dhanpat Rai & Co., 1998. (For All Chapters 1, 2, 3, 4 and 5).

2. R.K.Rajput, A Tex book of Power System Engineering. Laxmi Publications, First

Edition Reprint 2007.

REFERENCES:

1. Sunil S. Rao, ‘Switchgear and Protection’, Khanna publishers, New Delhi, 1986.

2. C.L. Wadhwa, ‘Electrical Power Systems’, Newage International (P) Ltd., 2000.

3. B. Ravindranath, and N. Chander, ‘Power System Protection & Switchgear’, Wiley Eastern Ltd.,1977.

4. Badri Ram, Vishwakarma, ‘Power System Protection and Switchgear’, Tata McGraw Hill, 2001.

5. Y.G. Paithankar and S.R. Bhide, ‘Fundamentals of Power System Protection’, Prentice Hall of India Pvt. Ltd., New

Delhi–110001, 2003.

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

EE2402 PROTECTION AND SWITCHGEAR

UNIT I INTRODUCTION 9

Importance of protective schemes for electrical apparatus and power system. Qualitative review of faults and

fault currents - relay terminology – definitions - and essential qualities of protection. Protection against over

voltages due to lightning and switching - arcing grounds - Peterson Coil - ground wires - surge absorber and

diverters

Power System earthing – neutral Earthing - basic ideas of insulation coordination.

Short Answers 2 Marks

1] List the types of faults in power system

Active Faults

Passive Faults

Transient & Permanent Faults

Symmetrical & Asymmetrical Faults

A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, and represents a

steady state condition.

An asymmetrical fault displays a d.c. offset, transient in nature and decaying to the steady state of the

symmetrical fault after a period of time:

Faults on a Three Phase System

Types of Faults on a Three Phase System.

(A) Phase-to-earth fault

(B) Phase-to-phase fault

(C) Phase-to-phase-to-earth fault

(D) Three phase fault

(E) Three phase-to-earth fault

(F) Phase-to-pilot fault *

(G) Pilot-to-earth fault *

2] What is the need for protection zones in the system?

Any fault occurring within the given zone will provide necessary tripping of relays or disconnecting or opening

of circuit breakers and thus the healthy section is safe guarded.

If a fault occurs in the overlapping zone in a proper protected scheme, more circuit breakers than the minimum

necessary to isolate the faulty part of the system would trip.

3] what is surge absorber? How do they differ from surge diverter?

Surge Absorber: it is a protective device used to reduce the steepness of the wave front of a surge and

absorbs energy contained in the travelling wave.

Surge Diverter

It will divert excess voltages from an electrical surge to earth. It measures the volts coming in and once

it gets above a set amount (normally 260 volts), will divert the excess volts to earth.

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An Electrical Surge Diverter is a great way to ensure adequate lightning protection for your valuable electronic

equipment. Unlike the more common Surge Protector Powerboards that simply switch off if there is spike in

volts, a Surge Diverter will just divert the excess volts away. It is also installed on your main switchboard,

thereby protecting all powerpoints.

4] Define the term “Insulation Coordination”.

Insulation Coordination is the process of determining the proper insulation levels of various components

in a power system as well as their arrangements. It is the selection of an insulation structure that will withstand

voltage stresses to which the system, or equipment will be subjected to, together with the proper surge arrester.

The process is determined from the known characteristics of voltage surges and the characteristics of surge

arresters.

5] Write any two functions of protective relaying?

(i) The function of a protective relay is to detect and locate a fault and issue a command to the circuit

breaker to disconnect the faulty element.

(ii)It is a device which senses abnormal conditions on a power system by constantly monitoring

electrical quantities of the system which differ under normal and abnormal conditions.

6] What are the desirable qualities of protective relaying? Or Mention the essential features of the power

system protection. Or List the essential features of switchgear.

1. Selectivity 2. Speed & time 3. Sensitivity

4. Reliability 5. Simplicity 6. Economy

7] What is meant by switchgear?

The apparatus used for switching, controlling and protecting the electrical circuits and equipment is

known as switchgear.

8] What are the functions of protection relaying?

The principal function of protective relaying is to cause the prompt removal from service of any element

of the power system when it starts to operate in an abnormal manner or interface with the operation of rest of

the system.

9] What are the causes of faults in power system?

(i) Internal causes.

(ii) Heavy short circuit current may cause damage to damage equipment or other element of the system

of the system due to overheating and high mechanical forces set up due to heavy current.

(iii) Arc associated with short circuits may cause fire hazards. Such fires resulting from arcing may

destroy the fault element of the system. There is also possibility of firing spreading to the other devices if the

fault is not isolated quickly.

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10] What are the different types of fault in power system transmission lines?

1. Symmetrical faults 3 phase faults

2. Unsymmetrical faults Single phase to ground, single phase to open circuit, Two phase to ground

fault; phase to phase short circuit.

11] List out the types of faults in power system

A].Single phase to ground B] phase to phase faults C] Two phase to ground fault and D] Three phase

short circuit faults.

12] Explain the need for overlapping the zones of protection.

1. The circuit breakers are located in the connection to each power element.

2. This provision makes it possible to disconnect only the faulty element from the system.

13] Differentiate between primary and back – up protection.

No Primary protection Back – Up Protection

1 It is designed to protect the components

of the power system. [main protection]

It is second line of protection in case main

protection fails.

2 It is for instantaneous protection It is designed to operate with enough time

delay

3 Only faulty element will be removed. Larger part of the power system is removed.

14] What are the causes of faults in power system?

1. Internal causes of the equipment.

2. Heavy short circuit current may cause s damage the equipment or other element of the system due to

overheating and high mechanical forces set up due to heavy current.

3. Deterioration of insulation.

15] What are the functions of protective relays

To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from

the rest of the system, thereby protecting the system from damages consequent to the fault.

16. Give the consequences of short circuit.

Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If

protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing

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its contacts. The trip circuit is then closed , the circuit breaker opens and the fault is isolated from the rest of the

system. Also, a low voltage may be created which may damage systems connected to the supply.

17. Define protected zone.

Are those which are directly protected by a protective system such as relays, fuses or switchgears. If a fault

occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that

particular zone.

18. What are unit system and non-unit system?

A unit protective system is one in which only faults occurring within its protected zone are isolated.

Faults occurring elsewhere in the system have no influence on the operation of a unit system.

A non-unit system is a protective system which is activated even when the faults are external to its

protected zone.

19.What is primary protection?

Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit

breaker. It serves as the first line of defence.

20. What is back up protection?

Is the second line of defence, which operates if the primary protection fails to activate within a definite

time delay.

21. Name the different kinds of over current relays.

Induction type non-directional over current relay, Induction type directional over current relay & current

differential relay.

22. Define energizing quantity.

It refers to the current or voltage which is used to activate the relay into operation.

23. Define operating time of a relay.

It is defined as the time period extendind from the occurrence of the fault through the relay detecting the

fault to the operation of the relay.

24. Define resetting time of a relay.

It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the

fault is removed and the relay can be reset.

25. What are over and under current relays?

Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg:

Induction type non-directional/directional overcurrent relay, differential overcurrent relay)whereas undercurrent

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relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg:

differential over-voltage relay)

26. Mention any two applications of differential relay.

Protection of generator & generator transformer unit; protection of large motors and busbars .

27. What is biased differential bus zone reduction?

The biased beam relay is designed to respond to the differential current in terms of its fractional relation

to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with

an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating

force.

16 Marks Questions

1] Discuss and compare the various methods of neutral earthing.

[Any two type may ask each carries 8marks or brief all the five divisions] [16]

Types of Neutral Earthing in Power Distribution:

Introduction:

In the early power systems were mainly Neutral ungrounded due to the fact that the first ground fault did

not require the tripping of the system. An unscheduled shutdown on the first ground fault was particularly

undesirable for continuous process industries. These power systems required ground detection systems, but

locating the fault often proved difficult. Although achieving the initial goal, the ungrounded system provided no

control of transient over-voltages.

A capacitive coupling exists between the system conductors and ground in a typical distribution system.

As a result, this series resonant L-C circuit can create over-voltages well in excess of line-to-line voltage when

subjected to repetitive re-strikes of one phase to ground. This in turn, reduces insulation life resulting in

possible equipment failure.

Neutral grounding systems are similar to fuses in that they do nothing until something in the system

goes wrong. Then, like fuses, they protect personnel and equipment from damage. Damage comes from two

factors, how long the fault lasts and how large the fault current is. Ground relays trip breakers and limit how

long a fault lasts and Neutral grounding resistors limit how large the fault current is.

Importance of Neutral Grounding: [seven points alone 2marks]

There are many neutral grounding options available for both Low and Medium voltage power systems. The

neutral points of transformers, generators and rotating machinery to the earth ground network provides a

reference point of zero volts. This protective measure offers many advantages over an ungrounded system, like,

1. Reduced magnitude of transient over voltages

2. Simplified ground fault location

3. Improved system and equipment fault protection

4. Reduced maintenance time and expense

5. Greater safety for personnel

6. Improved lightning protection

7. Reduction in frequency of faults.

Method of Neutral Farthing:

There are five methods for Neutral earthing.

1. Unearthed Neutral System

2. Solid Neutral Earthed System.

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3. Resistance Neutral Earthing System.

1. Low Resistance Earthing.

2. High Resistance Earthing.

4. Resonant Neutral Earthing System.

5. Earthing Transformer Earthing.

(1) Ungrounded Neutral Systems:

In ungrounded system there is no internal connection between the conductors and earth. However, as

system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces.

Consequently, the “ungrounded system” is, in reality, a “capacitive grounded system” by virtue of the

distributed capacitance.

Under normal operating conditions, this distributed capacitance causes no problems. In fact, it is

beneficial because it establishes, in effect, a neutral point for the system; As a result, the phase

conductors are stressed at only line-to-neutral voltage above ground.

But problems can rise in ground fault conditions. A ground fault on one line results in full line-to-line

voltage appearing throughout the system. Thus, a voltage 1.73 times the normal voltage is present on all

insulation in the system. This situation can often cause failures in older motors and transformers, due to

insulation breakdown.

Advantage:

1. After the first ground fault, assuming it remains as a single fault, the circuit may continue in operation,

permitting continued production until a convenient shut down for maintenance can be scheduled.

Disadvantages: 1. The interaction between the faulted system and its distributed capacitance may cause transient over-

voltages (several times normal) to appear from line to ground during normal switching of a circuit

having a line-to ground fault (short). These over voltages may cause insulation failures at points other

than the original fault.

2. A second fault on another phase may occur before the first fault can be cleared. This can result in very

high line-to-line fault currents, equipment damage and disruption of both circuits.

3. The cost of equipment damage.

4. Complicate for locating fault(s), involving a tedious process of trial and error: first isolating the correct

feeder, then the branch, and finally, the equipment at fault. The result is unnecessarily lengthy and

expensive down downtime.

(2) Solidly Neutral Grounded Systems:

Solidly grounded systems are usually used in low voltage applications at 600 volts or less.

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In solidly grounded system, the neutral point is connected to earth.

Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the

ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of the

system three phase fault current. However, if the reactance of the generator or transformer is too great,

the problem of transient over voltages will not be solved.

While solidly grounded systems are an improvement over ungrounded systems, and speed up the

location of faults, they lack the current limiting ability of resistance grounding and the extra protection

this provides.

To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must be

extend from the source to the furthest point of the system within the same raceway or conduit. Its

purpose is to maintain very low impedance to ground faults so that a relatively high fault current will

flow thus insuring that circuit breakers or fuses will clear the fault quickly and therefore minimize

damage. It also greatly reduces the shock hazard to personnel

If the system is not solidly grounded, the neutral point of the system would “float” with respect to

ground as a function of load subjecting the line-to-neutral loads to voltage unbalances and instability.

The single-phase earth fault current in a solidly earthed system may exceed the three phase fault current.

The magnitude of the current depends on the fault location and the fault resistance. One way to reduce

the earth fault current is to leave some of the transformer neutrals unearthed.

Advantage: 1. The main advantage of solidly earthed systems is low over voltages, which makes the earthing design

common at high voltage levels (HV).

Disadvantage: 1. This system involves all the drawbacks and hazards of high earth fault current: maximum damage and

disturbances.

2. There is no service continuity on the faulty feeder.

3. The danger for personnel is high during the fault since the touch voltages created are high.

Applications:

1. Distributed neutral conductor.

2. 3-phase + neutral distribution.

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3. Use of the neutral conductor as a protective conductor with systematic earthing at each transmission

pole.

4. Used when the short-circuit power of the source is low.

(3) Resistance earthed systems:

Resistance grounding has been used in three-phase industrial applications for many years and it resolves

many of the problems associated with solidly grounded and ungrounded systems.

Resistance Grounding Systems limits the phase-to-ground fault currents. The reasons for limiting the

Phase to ground Fault current by resistance grounding are:

1. To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers,

cables, and rotating machines.

2. To reduce mechanical stresses in circuits/Equipments carrying fault currents.

3. To reduce electrical-shock hazards to personnel caused by stray ground fault.

4. To reduce the arc blast or flash hazard.

5. To reduce the momentary line-voltage dip.

6. To secure control of the transient over-voltages while at the same time.

7. To improve the detection of the earth fault in a power system.

Grounding Resistors are generally connected between ground and neutral of transformers, generators

and grounding transformers to limit maximum fault current as per Ohms Law to a value which will not

damage the equipment in the power system and allow sufficient flow of fault current to detect and

operate Earth protective relays to clear the fault. Although it is possible to limit fault currents with high

resistance Neutral grounding Resistors, earth short circuit currents can be extremely reduced. As a result

of this fact, protection devices may not sense the fault.

Therefore, it is the most common application to limit single phase fault currents with low resistance

Neutral Grounding Resistors to approximately rated current of transformer and / or generator.

In addition, limiting fault currents to predetermined maximum values permits the designer to selectively

coordinate the operation of protective devices, which minimizes system disruption and allows for quick

location of the fault.

There are two categories of resistance grounding:(1) Low resistance Grounding.

(2) High resistance Grounding.

Ground fault current flowing through either type of resistor when a single phase faults to ground will

increase the phase-to-ground voltage of the remaining two phases. As a result, conductor insulation and

surge arrestor ratings must be based on line-to-line voltage. This temporary increase in phase-to-

ground voltage should also be considered when selecting two and three pole breakers installed on

resistance grounded low voltage systems.

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Neither of these grounding systems (low or high resistance) reduces arc-flash hazards associated with

phase-to-phase faults, but both systems significantly reduce or essentially eliminate the arc-flash hazards

associated with phase-to-ground faults. Both types of grounding systems limit mechanical stresses and

reduce thermal damage to electrical equipment, circuits, and apparatus carrying faulted current.

The difference between Low Resistance Grounding and High Resistance Grounding is a matter of

perception and, therefore, is not well defined. Generally speaking high-resistance grounding refers to a

system in which the NGR let-through current is less than 50 to 100 A. Low resistance grounding

indicates that NGR current would be above 100 A. A better distinction between the two levels might be alarm only and tripping. An alarm-only system

continues to operate with a single ground fault on the system for an unspecified amount of time. In a

tripping system a ground fault is automatically removed by protective relaying and circuit interrupting

devices. Alarm-only systems usually limit NGR current to 10 A or less.

Rating of The Neutral grounding resistor:

1. Voltage: Line-to-neutral voltage of the system to which it is connected.

2. Initial Current: The initial current which will flow through the resistor with rated voltage applied.

3. Time: The “on time” for which the resistor can operate without exceeding the allowable temperature

rise.

(A).Low Resistance Grounded:

Low Resistance Grounding is used for large electrical systems where there is a high investment in

capital equipment or prolonged loss of service of equipment has a significant economic impact and it is

not commonly used in low voltage systems because the limited ground fault current is too low to

reliably operate breaker trip units or fuses. This makes system selectivity hard to achieve. Moreover, low

resistance grounded systems are not suitable for 4-wire loads and hence have not been used in

commercial market applications

A resistor is connected from the system neutral point to ground and generally sized to permit only 200A

to 1200 amps of ground fault current to flow. Enough current must flow such that protective devices can

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detect the faulted circuit and trip it off-line but not so much current as to create major damage at the

fault point.

Since the grounding impedance is in the form of resistance, any transient over voltages are quickly

damped out and the whole transient overvoltage phenomena is no longer applicable. Although

theoretically possible to be applied in low voltage systems (e.g. 480V),significant amount of the system

voltage dropped across the grounding resistor, there is not enough voltage across the arc forcing current

to flow, for the fault to be reliably detected. For this reason, low resistance grounding is not used for

low voltage systems (under 1000 volts line to-line).

Advantages:

1. Limits phase-to-ground currents to 200-400A.

2. Reduces arcing current and, to some extent, limits arc-flash hazards associated with phase-to-ground

arcing current conditions only.

3. May limit the mechanical damage and thermal damage to shorted transformer and rotating machinery

windings.

Disadvantages:

1. Does not prevent operation of over current devices.

2. Does not require a ground fault detection system.

3. May be utilized on medium or high voltage systems.

4. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral

loads must be served through an isolation transformer.

Used: Up to 400 amps for 10 sec are commonly found on medium voltage systems.

(B).High Resistance Grounded:

High resistance grounding is almost identical to low resistance grounding except that the ground fault

current magnitude is typically limited to 10 amperes or less. High resistance grounding accomplishes

two things.

The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable

damage is done at the fault point. This means that the faulted circuit need not be tripped off-line when

the fault first occurs. Means that once a fault does occur, we do not know where the fault is located. In

this respect, it performs just like an ungrounded system.

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The second point is it can control the transient overvoltage phenomenon present on ungrounded

systems if engineered properly.

Under earth fault conditions, the resistance must dominate over the system charging capacitance but not

to the point of permitting excessive current to flow and thereby excluding continuous operation

High Resistance Grounding (HRG) systems limit the fault current when one phase of the system shorts

or arcs to ground, but at lower levels than low resistance systems.

In the event that a ground fault condition exists, the HRG typically limits the current to 5-10A.

HRG’s are continuous current rated, so the description of a particular unit does not include a time rating.

Unlike NGR’s, ground fault current flowing through a HRG is usually not of significant magnitude to

result in the operation of an over current device. Since the ground fault current is not interrupted, a

ground fault detection system must be installed.

These systems include a bypass contactor tapped across a portion of the resistor that pulses (periodically

opens and closes). When the contactor is open, ground fault current flows through the entire resistor.

When the contactor is closed a portion of the resistor is bypassed resulting in slightly lower resistance

and slightly higher ground fault current.

To avoid transient over-voltages, an HRG resistor must be sized so that the amount of ground

fault current the unit will allow to flow exceeds the electrical system’s charging current. As a rule of

thumb, charging current is estimated at 1A per 2000KVA of system capacity for low voltage systems

and 2A per 2000KVA of system capacity at 4.16kV.

These estimated charging currents increase if surge suppressors are present. Each set of suppressors

installed on a low voltage system results in approximately 0.5A of additional charging current and each

set of suppressors installed on a 4.16kV system adds 1.5A of additional charging current.

A system with 3000KVA of capacity at 480 volts would have an estimated charging current of

1.5A.Add one set of surge suppressors and the total charging current increases by 0.5A to 2.0A. A

standard 5A resistor could be used on this system. Most resistor manufacturers publish detailed

estimation tables that can be used to more closely estimate an electrical system’s charging current.

Advantages:

1. Enables high impedance fault detection in systems with weak capacitive connection to earth

2. Some phase-to-earth faults are self-cleared.

3. The neutral point resistance can be chosen to limit the possible over voltage transients to 2.5 times the

fundamental frequency maximum voltage.

4. Limits phase-to-ground currents to 5-10A.

5. Reduces arcing current and essentially eliminates arc-flash hazards associated with phase-to-ground

arcing current conditions only.

6. Will eliminate the mechanical damage and may limit thermal damage to shorted transformer and

rotating machinery windings.

7. Prevents operation of over current devices until the fault can be located (when only one phase faults to

ground).

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8. May be utilized on low voltage systems or medium voltage systems up to 5kV. IEEE Standard 141-1993

states that “high resistance grounding should be restricted to 5kV class or lower systems with charging

currents of about 5.5A or less and should not be attempted on 15kV systems, unless proper grounding

relaying is employed”.

9. Conductor insulation and surge arrestors must be rated based on the line to-line voltage. Phase-to-neutral

loads must be served through an isolation transformer.

Disadvantages:

1. Generates extensive earth fault currents when combined with strong or moderate capacitive connection

to earth Cost involved.

2. Requires a ground fault detection system to notify the facility engineer that a ground fault condition has

occurred.

(4) Resonant earthed system:

Adding inductive reactance from the system neutral point to ground is an easy method of limiting the

available ground fault from something near the maximum 3 phase short circuit capacity (thousands of

amperes) to a relatively low value (200 to 800 amperes).

To limit the reactive part of the earth fault current in a power system a neutral point reactor can be

connected between the transformer neutral and the station earthing system.

A system in which at least one of the neutrals is connected to earth through an

1. Inductive reactance.

2. Petersen coil / Arc Suppression Coil / Earth Fault Neutralizer.

The current generated by the reactance during an earth fault approximately compensates the capacitive

component of the single phase earth fault current, is called a resonant earthed system.

The system is hardly ever exactly tuned, i.e. the reactive current does not exactly equal the capacitive

earth fault current of the system.

A system in which the inductive current is slightly larger than the capacitive earth fault current is over

compensated. A system in which the induced earth fault current is slightly smaller than the

capacitiveearth fault current is under compensated

However, experience indicated that this inductive reactance to ground resonates with the system shunt

capacitance to ground under arcing ground fault conditions and creates very high transient over voltages

on the system.

To control the transient over voltages, the design must permit at least 60% of the 3 phase short circuit

current to flow underground fault conditions.

Example. A 6000 amp grounding reactor for a system having 10,000 amps 3 phase short circuit capacity

available. Due to the high magnitude of ground fault current required to control transient over voltages,

inductance grounding is rarely used within industry.

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Petersen Coils:

A Petersen Coil is connected between the neutral point of the system and earth, and is rated so that the

capacitive current in the earth fault is compensated by an inductive current passed by the Petersen

Coil. A small residual current will remain, but this is so small that any arc between the faulted phase and

earth will not be maintained and the fault will extinguish. Minor earth faults such as a broken pin

insulator, could be held on the system without the supply being interrupted. Transient faults would not

result in supply interruptions.

Although the standard ‘Peterson coil’ does not compensate the entire earth fault current in a network due

to the presence of resistive losses in the lines and coil, it is now possible to apply ‘residual current

compensation’ by injecting an additional 180° out of phase current into the neutral via the Peterson coil.

The fault current is thereby reduced to practically zero. Such systems are known as ‘Resonant earthing

with residual compensation’, and can be considered as a special case of reactive earthing.

Resonant earthing can reduce EPR to a safe level. This is because the Petersen coil can often effectively

act as a high impedance NER, which will substantially reduce any earth fault currents, and hence also

any corresponding EPR hazards (e.g. touch voltages, step voltages and transferred voltages, including

any EPR hazards impressed onto nearby telecommunication networks).

Advantages:

1. Small reactive earth fault current independent of the phase to earth capacitance of the system.

2. Enables high impedance fault detection.

Disadvantages:

1. Risk of extensive active earth fault losses.

2. High costs associated.

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(5) Earthing Transformers:

For cases where there is no neutral point available for Neutral Earthing (e.g. for a delta winding), an

earthing transformer may be used to provide a return path for single phase fault currents

In such cases the impedance of the earthing transformer may be sufficient to act as effective earthing

impedance. Additional impedance can be added in series if required. A special ‘zig-zag’ transformer is

sometimes used for earthing delta windings to provide a low zero-sequence impedance and high positive

and negative sequence impedance to fault currents.

Conclusion:

Resistance Grounding Systems have many advantages over solidly grounded systems including arc-flash hazard

reduction, limiting mechanical and thermal damage associated with faults, and controlling transient over

voltages.

High resistance grounding systems may also be employed to maintain service continuity and assist with

locating the source of a fault.

When designing a system with resistors, the design/consulting engineer must consider the specific

requirements for conductor insulation ratings, surge arrestor ratings, breaker single-pole duty ratings,

and method of serving phase-to-neutral loads.

2] Discuss the essential qualities of protective relaying. 16

Protective Relaying (Part11)

A protective relaying scheme should have certain important qualities. Such an essential qualities of

protective relaying are,

1. Reliability

2. Selectivity and Discrimination

3. Speed and Time

4. Sensitivity

5. Stability

6. Adequateness

7. Simplicity and Economy

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1.1 Reliability

A protective relaying should be reliable is its basic quality. It indicates the ability of the relay system to

operate under the predetermined conditions. There are various components which go into the operation before a

relay operates. Therefore every component and circuit which is involved in the operation of a relay plays an

important role. The reliability of a protection system depends on the reliability of various components like

circuit breakers, relays, current transformers (C.T.s), potential transformers (P.T.s), cables, trip circuits etc. The

proper maintenance also plays an important role in improving the reliable operation of the system. The

reliability can not be expressed in the mathematical expressions but can be adjusted from the statistical data.

The statistical survey and records give good idea about the reliability of the protective system. The inherent

reliability is based on the design which is based on the long experience. This can be achieved by the factors like,

i) Simplicity ii) Robustness

iii) High contact pressure iv) Dust free enclosure

iv) Good contact material vi) Good workmanship and

vii) Careful Maintenance

1.2 Selectivity and Discrimination

The selectivity id the ability of the protective system to identify the faulty part correctly and disconnect that

part without affecting the rest of the healthy part of system. The discrimination means to distinguish between.

The discrimination quality of the protective system is the ability to distinguish between normal condition and

abnormal condition and also between abnormal condition within protective zone and elsewhere. The protective

system should operate only at the time of abnormal condition and not at the time of normal condition. Hence it

must clearly discriminate between normal and abnormal condition. Thus the protective system should select the

fault part and disconnect only the faulty part without disturbing the healthy part of the system.

The protective system should not operate for the faults beyond its protective zone. For example, consider

the portion of a typical power system shown in the Fig. 1.

Fig. 1

It is clear from the Fig. 1 that if fault F2 occurs on transmission line then the circuit breakers 2 and 3 should

operate and disconnect the line from the remaining system. The protective system should be selective in

selecting faulty transmission line only for the fault and it should isolate it without tripping the adjacent

transmission line breakers or the transformer.

If the protective system is not selective then it operates for the fault beyond its protective zones and

unnecessary the large part of the system gets isolated. This causes a lot of inconvenience to the supplier and

users.

1.3 Speed and Time

a protective system must disconnect the faulty system as fast as possible. If the faulty system is not

disconnect for a long time then,

1. The devices carrying fault currents may get damaged.

2. The failure leads to the reduction in system voltage. Such low voltage may affect the motors and generators

running on the consumer sude.

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3. If fault persists for long time, then subsequently other faults may get generated.

The high speed protective system avoids the possibility of such undesirable effects.

The total time required between the instant of fault and the instant of final arc interruption in the circuit

breaker is called fault clearing time. It is the sum of relay time and circuit breaker time. The relay time is the

time between the instant of fault occurrence and the instant of closure of relay contacts. The circuit breaker

times is the time taken by the circuit breaker to operate to open the contacts and to extinguish the arc

completely. The fault clearing time should be as small as possible to have high speed operation of the protective

system.

Though the small fault clearing time is preferred, in practice certain time lag is provided. This is because,

1. To have clear discrimination between primary and backup protection

2. To prevent unnecessary operation of relay under the conditions such as transient, starting inrush of current

etc.

Thus fast protective system is an important quality which minimises the damage and it improves the overall

stability of the power system.

1.4 Sensitivity

The protective system should be sufficiently sensitive so that it can operate reliably when required. The

sensitivity of the system is the ability of the relay system to operate with low value of actuating quantity.

It indicates the smallest value of the actuating quantity at which the protection starts operating in relation

with the minimum value of the fault current in the protected zone.

The relay sensitivity is the function of the volt-amperes input to the relay coil necessary to cause its

operation. Smaller the value of volt-ampere input, more sensitive is the relay. Thus 1 VA input relay is more

sensitive than the 5VA input relay.

Mathematically the sensitivity is expressed by a factor called sensitivity factor . It is the ratio of minimum

short circuit current in the protected zone to the minimum operating current required for the protection to start.

Ks = Is/Io

where Ks = sensitivity factor

Is = minimum short circuit current in the zone

Io= minimum operating current for the protection

1.5 Stability

The stability is the quality of the protective system due to which the system remains inoperative and stable

under certain specified conditions such as transients, disturbance, through faults etc. For providing the stability,

certain modifications are required in the system design. In most of the cases time delays, filter circuits,

mechanical and electrical bias are provided to achieve stable operation during the disturbances.

1.6 Adequateness

There are variety of faults and disturbance those may practically exists in a power system. It is impossible

to provide protection against each and every abnormal condition which may exist in practice, due to economical

reasons. But the protective system must provide adequate protection for any element of the system. The

adequateness of the system can be assessed by considering following factors,

1. Ratings of various equipments

2. Cost of the equipments

3. Locations of the equipments

4. Probability of abnormal condition due to internal and external causes.

5. Discontinuity of supply due to the failure of the equipment

1.7 Simplicity and Economy

In addition to all the important qualities, it is necessary that the cost of the system should be well within

limits. In practice sometimes it is not necessary to use ideal protection scheme which is economically

unjustified. In such cases compromise is done. As a rule, the protection cost should not be more than 5% of the

total cost. But if the equipments to be protected are very important, the economic constrains can be relaxed.

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The protective system should be as simple as possible so that it can be easily maintained. The complex

system are difficult from the maintenance point of view. The simplicity and reliability are closely related to

each other. The simpler system are always more reliable.

3] Discuss the nature and causes of different faults in a power system. (16)

Nature and causes of Faults: Any faults in electrical apparatus are nothing but the defect in its electrical

circuit which makes current path directed from its intended path. Normally due to breaking of conductors or

failure of insulation, these faults occur. The other reasons for occurrence of fault include mechanical failure,

accidents. Excessive internal and external stresses. The impedance of the path in the fault is low and the fault

currents are comparatively large. The induction of insulation is not considered as a fault until it shows some

effect sucj as excessive current flow or reduction of impedance between conductors or between conductors and

earth.

When a fault occurs on a system, the voltage of the three phases become unbalanced. As the fault currents are

large, the apparatus may get damaged. The flow of power is diverted towards the fault which affects the supply

to the neighboring zone.

A power system consists of generators, transformers, switchgear, transmission and distribution circuits. There

is always a possibility in such a large network that some fault will occur in some part of the system. The

maximum possibility of fault occurrence is on the transmission lines due to their greater lengths and exposure to

atmospheric conditions.

The faults cannot be classified according to the causes of their incidence. The breakdown may occur at normal

voltage due to deterioration of insulation. The breakdown may also occur due to damage on account of

unpredictable causes which include perching of birds, accidental short circuiting by snakes, kite strings, three

branches etc. The breakdown may occur at abnormal voltages due to switching surges or surges caused by

lighting.sss

4] List the types of faults in power system

Active Faults

The “Active” fault is when actual current flows from one phase conductor to another

(phase-to-phase) or alternatively from one phase conductor to earth (phase-to-earth).

This type of fault can also be further classified into two areas, namely the “solid”

fault and the “incipient” fault.

Passive Faults

Passive faults are not real faults in the true sense of the word but are rather conditions

that are stressing the system beyond its design capacity, so that ultimately active

faults will occur.

Typical examples are:

Overloading - leading to overheating of insulation (deteriorating quality,

reduced life and ultimate failure).

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Overvoltage - stressing the insulation beyond its limits.

under frequency - causing plant to behave incorrectly.

Power swings - generators going out-of-step or synchronism with each other

Transient & Permanent Faults

Transient faults are faults which do not damage the insulation permanently and allow

the circuit to be safely re-energized after a short period of time. A typical example would be an insulator

flashover following a lightning strike, which would be successfully cleared on opening of the circuit breaker,

which could then be automatically reclosed.

Transient faults occur mainly on outdoor equipment where air is the main insulating medium.

Permanent faults, as the name implies, are the result of permanent damage to the insulation. In this case, the

equipment has to be repaired and reclosing must not be entertained.

Symmetrical & Asymmetrical Faults

A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, and represents a

steady state condition.

An asymmetrical fault displays a d.c. offset, transient in nature and decaying to the steady state of the

symmetrical fault after a period of time:

Faults on a Three Phase System

The types of faults that can occur on a three phase A.C. system are as follows:

Types of Faults on a Three Phase System.

(A) Phase-to-earth fault

(B) Phase-to-phase fault

(C) Phase-to-phase-to-earth fault

(D) Three phase fault

(E) Three phase-to-earth fault

(F) Phase-to-pilot fault *

(G) Pilot-to-earth fault *

5] What is a surge absorber? Write a short note on Ferranti surge absorber. [8]

Surge absorbers are protective devices used to absorb the complete surge i,e. due to lightening surge or any

transient surge in the system..........unlike the

lightening arrestor in which a non-linear resistor is provided which provides a low resistance path to the

dangerously high voltages on the system to the earth...

Ferranti surge absorber

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6. What are the causes of over voltage on a power system? (8)

Overvoltage - Causes and Protection

Over voltages occur in a system when the system voltage rises over 110% of the nominal rated voltage.

Overvoltage can be caused by a number of reasons, sudden reduction in loads, switching of transient loads,

lightning strikes, failure of control equipment such as voltage regulators, neutral displacement,. Overvoltage can

cause damage to components connected to the power supply and lead to insulation failure, damage to electronic

components, heating, flashovers, etc.

Overvoltage relays can be used to identify overvoltages and isolate equipment. These relays operate when the

measured voltage exceeds a predetermined set-point. The voltage is usually measured using a Potential

Transformers. The details of the ratio of the potential transformer are also entered into the relay. These relays

are usually provided with a time delay. The time delay can be either instantaneous, fixed time or for IDMT

(inverse definite minimum time) curves.

Generally, overvoltage relays are provided with sufficient time delay in order to avoid unwanted trippings due

to transients (See article on Transients).

These relays can be used to isolate feeders and other equipment connected to the network. In the case of

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generators, these relay also switch off the excitation system to the generators thereby preventing voltage build-

up.

7. Describe the phenomenon of lightning. (16)

Lightning, an awesome and terrifying natural phenomenon, is really nothing more than an electrical

discharge that happens to be at an enormous voltage. Lightning equalizes a difference of electrical potential,

whether it is between a cloud and the ground, between two clouds, or between two different areas of the same

cloud. Due to various mechanisms in storm cloud formation, such as the rising and falling of air currents

carrying moisture and ice particles, the storm cloud generates an electrical charge at its base. Charged regions in

a thundercloud create conductive ionization channels through the atmosphere called “stepped leaders,” while an

opposite electrical charge or “shadow” accumulates on the ground below.

These leaders jump and branch ever downward in a quest to connect with their oppositely charged image

or “shadow” on the earth’s surface. As the fiercely charged electrical field builds, streamers rise from objects on

the ground. When a stepped leader from the sky chooses to meet the strongest rising streamer from below, a

lightning “circuit” is completed. The main lightning stroke erupts up the ionized path. Numerous additional

strokes along the same channel are common, creating a mesmerizing strobe light effect. We are most interested

in protecting our clients from the “shadow” and its resulting electrical discharge—a lightning strike.

Whatever lightning protection method is implemented, the importance of the grounding system

supporting it cannot be overemphasized. A well designed, correctly installed, low impedance and low resistance

connection from the earth to the components of the lightning protection system is essential. With the addition of

transient voltage surge suppression, a comprehensive facility protection approach addressing all concerns of

safety and power quality finally is made possible. Only then may we watch in unflinching wonder as the sky

explodes in its timeless electrical display.

8. (a) What is necessity of protecting electrical equipment against traveling waves? (6)

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9. Describe the construction and principle of operation of

(i) expulsion type lightning arrester, (8) (ii) Value type lightning arrester. (8)

(i) expulsion type lightning arrester, (8)

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10. Write short notes on the following.

(i) klydonograph and magnetic link (4)

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(ii) Rod gap (4)

It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in

between as shown in Fig 8. One rod is connected to the line circuit and the other rod is connected to earth.

The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so

that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown

should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the

insulators. The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap

across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the

gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and

the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly

conducted to earth

Limitations:

(i) After the surge is over, the arc n the gap is maintained by the normal supply voltage, leading to short-circuit

on the system.

(ii) The rods may melt or get damaged due to excessive heat produced by the arc.

(iii) The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester.

(iv) The polarity of the f the surge also affects the performance of this arrester.

Due to the above limitations, the rod gap arrester is only used as a back-up protection in case of main

arresters.

It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in

between as shown in Fig 8. One rod is connected to the line circuit and the other rod is connected to earth.

The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so

that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown

should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the

insulators. The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap

across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the

gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and

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the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly

conducted to earth

(iii) Arcing horns (4)

Transmission and other electrical equipment can be exposed to overvoltages. Overvoltages can be

caused by a number of reasons such as lightning strikes, transient surges, sudden load fluctuation, etc. In the

event of an overvoltage, the insulating equipment such as the insulators on a transmission line or bushings in a

transformer can be exposed to high voltages which may lead to their failure.

Arcing horns are protective devices that are constructed in the form of projections in the conducting

materials on both sides of an insulator. Arcing horns are fitted in pairs. Thus in transmission lines they are

found on the conducting line and the transmission tower across the insulators. In transmission lines, in the

event of a lightning strike on the tower, the tower potential rises to dangerous levels and can result in flashovers

across the insulators causing their failure. Arcing horns prevent this by conducting the arc across the air gap

across them.

Arcing horns function by bypassing the high voltage across the insulator using air as a conductive

medium. The small gap between the horns ensures that the air between them breaks down resulting in a

flashover and conducts the voltage surge rather than cause damage to the insulator.The horns are constructed in

pair so that one horn is on the line side and the other is on the ground side.

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Arcing Horns are also used along with air insulated switchgear equipment. Air insulated switchgear are

vulnerable to damage due to arcing. Arcing horns serve to divert the arc towards themselves thus protecting the

switching equipment. The arcing horns serve to move the arc away from the bushings or the insulators.

Figure shows the horn gap arrester. It consists of a horn shaped metal rods A and B separated by a small air

gap. The horns are so constructed that distance between them gradually increases towards the top as shown. The

horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and

choke coil L while the other end is effectively grounded. The resistance R helps in limiting the follow current to

a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very

high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be

protected. The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc

across the gap.

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Under normal conditions, the gap is non-conducting i.e. normal supply voltage is insufficient to initiate the arc

between the gap. On the occurrence of an over voltage, spark-over takes place across the small gap G. The

heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves

progressively into positions 1,2 and 3. At some position of the arc (position 3), the distance may be too great for

the voltage to maintain the arc; consequently, the arc is extinguished. The excess charge on the line is thus

conducted through the arrester to the ground.

(iv) Basic impulse insulation level (4)

BIL or basic impulse insulation level is the dielectric insulation gradient of a material tested to withstand the

voltage stress at a voltage impressed between the material and a conductive surface beyond the BIL rating, an

electric tracking starts to occur which will then result into an arcing flashover to the conductive surface. In

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addition it is the capacity of an equipment to withstand mechanical stress like lightning strike without causing

any damage to the equipment...

11. What is Peterson coil? What protective functions are performed by this device? (16)

[Ref Q.No 1 IN 16 MARKS]

ASSIGNMENT

1. What are the requirements of a ground wire for protecting power conductors against direct lightning

stroke? Explain how they are achieved in practice. (16)

2. Determine the inductance of Peterson coil to be connected between the neutral and ground to

neutralize the charging current of overhead line having the line to ground capacitive of 0.15μf. If the

supply frequency is 50Hz and the operating voltage is 132 KV, find the KVA rating of the coil. (16)

3. (a) Explain the term insulation coordination. (8)

(b) Describe the constn of volt-time curve and the terminology associated with impulse-testing. (8)

4. Explain the operation of various types of surge absorbers (16)

5. Why is it necessary to protect the lines and other equipment of the power system against over voltages? REF Q.No

6.Describe in brief the protective devices used for protection of equipment against such waves? (10)

7. What protective measures are taken against lightning over voltages? (16)

8. (a) What is tower-footing resistance? (4)

(b) Why is it required to have this resistance as low as economically possible? (4)

(c) What are the methods to reduce this resistance? (8)

9. Describe the protection of stations and sub-stations against direct lightning stroke. (16)

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UNIT 2

UNIT II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS 9

Electromagnetic relays – over current, directional and non-directional, distance, negative sequence, differential and under

frequency relays – Introduction to static relays.

1] Draw the block diagram of a static differential relay.

2] What are the advantages of over current relays over electromagnetic relays?

o Low power consumption as low as 1mW

o No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of

contacts

o No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc.

o A single relay can perform several functions like over current, under voltage, single phasing protection

by incorporating respective functional blocks. This is not possible in electromagnetic relays

o Static relay is compact

o Superior operating characteristics and accuracy

o Static relay can think , programmable operation is possible with static relay

o Effect of vibration is nil, hence can be used in earthquake-prone areas

o Simplified testing and servicing. Can convert even non-electrical quantities to electrical

in conjunction with transducers.

3] Compare static and electromagnetic relay.

Electromagnetic relays are1st generation relays they use principle of electromagnetic principle. They

depend upon gravitation also and the value changes to the surrounding magnetic fields also.

Static relays are 2nd generation relays. In this relays transistors and IC's r been used. There value may

vary with respect to temperature also.

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Numerical relays are present generation relays. They use Microprocessor. Within built software and

predefined values if any values extend the predefined values then relay gets on by this relay we can find

distance of fault also.

4] A relay is connected to 400/5 ratio current transformer with current setting of 150%. Calculate the

Plug Setting Multiplier when circuit carries a fault current of 4000A.

5] What are the features of directional relay?

• Dual polarized directional element capability

• Zero sequence (Residual) voltage (3V0)

• Zero sequence (Neutral) current (I0)

• Directional element is insensitive to third or higher harmonics.

• 12 Field selectable, inverse, definite time, and British Standard (BS142) time over current curves.

• Independent time and instantaneous over current elements.

• Optional independent directional and non-directional instantaneous over current functions.

• Wide continuous setting ranges:

• Current pickup accuracy +2%

• Timing accuracy +5%

• Time Over current Reset – Instantaneous

• Optional Auxiliary Output Contact follows operation of user defined function (TOC, Dir INST, Non

Dir INST)

• Draw out construction, testable-in-case

• Provision for trip circuit testing

• Less than 0.1 Ohm burden for all sensing inputs.

• Standard magnetically latched targets for each trip function.

6] What are the advantages and disadvantages of static relays?

a. Low burden on CTs & PTs.

b. No moving contacts

c. Fast operation and long life.;

d. Sensitivity is more.

e. Fast reset & no overshoot.

f. Size of the relay is small since measuring circuit needs very low current.

7] Classify the different types of over current relays based on inverse time characteristics?

a. Static instantaneous over current relay

b. Directional static over current relay

c. Inverse time over current relay

8] Mention any two applications of differential relay.

It is comparing miniature to heavy loads.

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Fault current can be easily detected.

Used for protecting generator, generator unit transformer unit, large motors and bus bars

9] Derive and draw the characteristics of an impedance relay.

Operating time α Z α distance

Time of operation of relay α 𝑉

𝐼 α distance

where V- voltage in volts Z – impedance in ohms

I – current in amperes

10] For what purpose distance relays used?

It is used in the main transmission lines or sub transmission lines of 33KV,66KV,& 132KV.

It measures in terms of impedance, reactance etc., from relay to the point of fault.

11] Define the terms a] pick up value b]plug setting multiplier.

a] pick up value

As the current or voltage on an un operated relay is increased, the value at or below which all contacts function

b]Plug Setting Multiplier

The actual r.m.s current flowing in the relay expressed as a multiple of the setting current (pick up

current) is known as plug setting multiplier.

mathematically ,

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PSM= SECONDARY CURRENT/RELAY CURRENT SETTING

or

PSM= (primary current during fault)/(relay current setting *C.T ratio)

PSM = current through operating coil

plug setting current =

CT secondary current

plug setting current

PSM = line current 𝐼 CT ratio

plug setting current =

line current

CT ratio ×plug setting current

12] What type of relay is best suited for long distance , very high voltage transmissions?

Impedance relay

13] What type of relay is best suited for generator protection?

Differential relay

14] Give any two applications of electromagnetic relays.

To protect the ac & dc equipments.

Over/under current and over/under voltage protection of various ac & dc equipments.

15. What is the need of relay coordination?

The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest

possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be

sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can

always be isolated quickly without serious disturbance to the rest of the system.

16. Mention the short comings of Merz Price scheme of protection applied to a power transformer.

In a power transformer, currents in the primary and secondary are to be compared. As these two currents

are usually different, the use of identical transformers will give differential current, and operate the relay under

no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three

phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay

under normal condition

.

17. What are the various faults to which a turbo alternator is likely to be subjected?

Failure of steam supply; failure of speed; overcurrent; over voltage; unbalanced loading; stator winding

fault .

18. What is an under frequency relay?

An under frequency relay is one which operates when the frequency of the system (usually an alternator

or transformer) falls below a certain value.

19. Define the term pilot with reference to power line protection.

Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as

part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.

20. Mention any two disadvantage of carrier current scheme for transmission line only.

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The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response

time of band pass filter; capacitance phase-shift of the transmission line

21. What are the features of directional relay?

High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal

ratio; burden must not be excessive.

22. What are the causes of over speed and how alternators are protected from it? Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are

provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime

mover when a dangerous over-speed occurs.

23. What are the main types of stator winding faults? Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same

phase winding.

23. Give the limitations of Merz Price protection. Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes

impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the

voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In

addition to this, there always an inherent phase difference between the primary and the secondary quantities and

a possibility of current through the relay even when there is no fault.

24. What are the uses of Buchholz’s relay? Bucholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer

and to connect the transformer from the supply in the event of severe internal faults. It is usually used in oil

immersion transformers with a rating over 750KVA.

16 Mark questions

1] What are the different types of electromagnetic relays? Discuss their field of applications. [16]

Type of protection · Over current.

· Directional over current.

· Distance.

· Over voltage.

· Differential.

· Reverse power.

Electromagnetic relays

Electromagnetic relays are constructed with electrical, magnetic and mechanical components, have

an operating coil and various contacts and are very robust and reliable. The construction characteristics

can be classified in three groups, as detailed below.

Attraction relays

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Attraction relays can be supplied by AC or DC, and operate by the movement of a piece of metal

when it is attracted by the magnetic field produced by a coil. There are two main types of relay in this class.

The attracted armature relay, which is shown in figure 1, consists of a bar or plate of metal which

pivots when it is att racted towards the coil.

The armature carries the moving part of the contact, which is closed or opened according to the

design when the armature is attracted to the coil. The other type is the piston or solenoid relay, illustrated in

Figure 2, in which α bar or piston is attracted axially within the field of the solenoid. In this case, the piston also

carries the operating contacts.

It can be shown that the force of attraction is equal to K1I2 - K2, where Κ1 depends upon the number of turns on

the operating solenoid, the air gap, the effective area and the reluctance of the magnetic circuit, among other

factors. K2 is the restraining force, usually produced by a spring. When the relay is balanced, the resultant force is

zero and therefore Κ112 = K2,

So that

In order to control the value at which the relay starts to operate, the restraining tension of the spring or

the resistance of the solenoid circuit can be varied, thus modifying the restricting force. Attraction relays

effectively have no time delay and, for that reason, are widely used when instantaneous operations are

required.

Relays with moveable coils This type of relay consists of a rotating movement with a small coil suspended or pivoted with the freedom to

rotate between the poles of a permanent magnet. The coil is restrained by two springs which also serve as

connections to carry the current to the coil.

The torque produced in the coil is given by:

T = B.l.a.N.i

Where:

T= torque

B = flux density

L =length of the coil

a = diameter of the coil

N = number of turns on the coil

i = current flowing through the coil

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Figure 2 Solenoid-type relay

Figure 3 Inverse time characteristic

From the above equation it will be noted that the torque developed is proportional to the current. The speed of

movement is controlled by the damping action, which is proportional to the torque. It thus follows that the relay

has an inverse time characteristic similar to that illustrated in Figure 3. The relay can be designed so that the

coil makes a large angular movement, for example 80º.

Induction relays An induction relay works only with alternating current. It consists of an electromagnetic system which operates on

a moving conductor, generally in the form of a disc or cup, and functions through the interaction of

electromagnetic fluxes with the parasitic Fault currents which are induced in the rotor by these fluxes. These

two fluxes, which are mutually displaced both in angle and in position, produce a torque that can be expressed

by

T= Κ1.Φ1.Φ2 .sin θ,

Where Φ1 and Φ2 are the interacting fluxes and θ is the phase angle between Φ1 and Φ2. It should be noted that

the torque is a maximum when the fluxes are out of phase by 90º, and zero when they are in phase.

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Figure 4 Electromagnetic forces in induction relays

It can be shown that Φ1= Φ1sin ωt, and Φ2= Φ2 sin (ωt+ θ ) , where θ is the angle by which Φ2 leads Φ1. Then:

And

Figure 4 shows the interrelationship between the currents and the opposing forces. Thus:

F = ( F 1 - F 2 ) α (Φ2 iΦ1+ Φ1

iΦ2 )

F α Φ2 Φ1

sin θ α T

Induction relays can be grouped into three classes as set out below.

Shaded-pole relay

In this case a portion of the electromagnetic section is short-circuited by means of a copper ring or coil.

This creates a flux in the area influenced by the short circuited section (the so-called shaded section) which

lags the flux in the nonshaded section, see Figure 5.

Figure 5 Shaded-pole relay

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Figure 6 Wattmetric-type relay

In its more common form, this type of relay uses an arrangement of coils above and below the disc with the upper and

lower coils fed by different values or, in some cases, with just one supply for the top coil, which induces an out-of-

phase f lux in the lower coil because of the air gap. Figure 6 illust r ates a typical arrangement.

Cup-type relay

This type of relay has a cylinder similar to a cu which can rotate in the annular air gap between the poles of the coils,

and has a fixed central core, see Figure 7. The operation of this relay is very similar to that

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Figure 7Cup-type relay

Of an induction motor with salient poles for the windings of the stator. Configurations with four or eight poles

spaced symmetrically around the circumference of the cup are often used. The movement of the cylinder is

limited to a small amount by the contact and the stops. Α special spring provides the restraining torque.

The torque is a function of the product of the two currents through the coils and the cosine of the angle

between them. The torque equation is

T= ( KI1I2 cos (θ12 – Φ) – Ks ),

Where K, .Κs and Φ are design constants, Ι1 and I2 are the currents through the two coils and θ12 is the angle

between I1 and I2.

In the first two types of relay mentioned above, which are provided with a disc, the inertia of the disc

provides the time-delay characteristic. The time delay can be increased by the addition of a permanent magnet.

The cup-type relay has a small inertia and is therefore principally used when high speed operation is required,

for example in instantaneous units.

2.What are the various types of over current relays? Discuss their area of application. (16)

STATIC INSTANTANEOUS OVER-CURRENT RELAY

The block diagram of an instantaneous over-current relay is shown in fig 21. The same construction may

be used for under-voltage, over-voltage and earth fault relays too.

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The secondaries of the line CT’s are connected to a summation circuit (not shown in the fig). The output of this

summation CT is fed to an auxiliary CT, whose output is rectified smoothened and supplied to the measuring

unit (level detector). The measuring unit determines whether the quantity has attained the threshold value (set

value) or not. When the input to measuring unit is less than the threshold value, the output of the level detector

is zero.

For an over-current relay

For I input < I threshold; Ioutput = 0

For I input > I threshold; Ioutput = Present

In an actual relay I threshold can be adjusted.

After operation of the measuring unit, the amplifier amplifies the output. Amplified output is given to the output

circuit to cause trip/alarm. If time-delay is desired, a timing circuit is introduced before the level detector.

Smoothing circuit and filters are introduced in the output of the bridge rectifier. Static over-current relay is

made in the form of a single unit in which diodes, transistors, resistors, capacitors etc., are arranged on printed

board and are bolted with epoxy resin.

STATIC OVER-CURRENT TIME RELAY

The block diagram of static over current time relay is shown in fig

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The current from the line CT is reduced to 1/1000 th by an auxiliary CT, the auxiliary has taps on the primary

for selecting the desired pick-up and current range and its rectified output is supplied to level detector I (over-

load level detector) and an R-C timing circuit. When the voltage on the timing capacitor Vc attains the threshold

value of the level detector II, tripping occurs. Time delay given by the timing circuit shown in fig 22 b is given

as Tc = RC log e E/(E – VT) .

Where VT is the threshold value of the level detector II. By varying values of R and C the time can be varied

without difficulties

3.Describe the operating principle, constructional features and area of applications of reverse power or

directional relay. (16)

Directionalized relays are relays that use a polarizing circuit to determine which "direction" (in the zone of

protection, or out of the zone protection) a fault is. There are many different types and different polarizing

methods - ground polarizing, voltage polarizing, zero sequence voltage polarizing, negative sequence

polarizing, etc.

The basic operation of this relay is just like any nondirectional relay, but with an added torque control - the

directionalizing element. This element allows the relay to operate when it is satisfied that the fault is within the

zone of protection (ie not behind where the relay is looking).

A reverse power relay is a directional power relay that is used to monitor the power from a generator running in

parallel with another generator or the utility. The function of the reverse power relay is to prevent a reverse

power condition in which power flows from the bus bar into the generator. This condition can occur when there

is a failure in the prime mover such as an engine or a turbine which drives the generator.

Causes of Reverse Power

The failure can be caused to a starvation of fuel in the prime mover, a problem with the speed controller or an

other breakdown. When the prime mover of a generator running in a synchronized condition fails. There is a

condition known as motoring, where the generator draws power from the bus bar, runs as a motor and drives the

prime mover. This happens as in a synchronized condition all the generators will have the same frequency. Any

drop in frequency in one generator will cause the other power sources to pump power into the generator. The

flow of power in the reverse direction is known as the reverse power relay.

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Another cause of reverse power can occur during synchronization. If the frequency of the machine to be

synchronized is slightly lesser than the bus bar frequency and the breaker is closed, power will flow from the

bus bar to the machine. Hence, during synchronization(forward), frequency of the incoming machine is kept

slight higher than that of the bus bar i.e. the synchroscope is made to rotate in the "Too fast" direction. This

ensures that the machine takes on load as soon as the breaker is closed.

Setting the Reverse Power Relay The reverse power relay is usually set to 20% to 50% of the motoring power required by prime mover. By

motoring power we mean the power required by the generator to drive the prime mover at the rated rpm. This is

usually obtained from the manufacturer of the prime mover (turbine or engine).

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4. Describe the construction and principle of operation of an induction type directional over

current relay. (16)

INDUCTION TYPE DIRECTIONAL OVER CURRENT RELAY

The directional power relay is not suitable under short circuit conditions because as short circuit occurs the

system voltage falls to a low value resulting in insufficient torque to cause relay operations. This difficulty is

overcome in the directional over current relay, which is independent of system voltage and power factor.

Constructional details: –

Figure shows the constructional details of a typical induction type directional over current relay. It consists of

two relay elements mounted on a common case viz. (i) directional element and (ii) non-directional element.

(i) Directional element: It is essentially a directional power relay, which operates when power flows in a

specific direction. The potential of this element is connected through a potential transformer (PT.) to the system

voltage. The current coil of the element is energized through a CT by the circuit current. This winding is carried

over the upper magnet of the non-directional element. The trip contacts (1 and 2) of the directional element are

connected in series with secondary circuit of the over current element. The latter element cannot start to operate

until its secondary circuit is completed. In other words, the directional element must first operate (ie. contacts 1

and 2 should close) in order to operate the over current element.

(ii) Non-directional element: – It is an over current element similar in all respects to a non-directional over

current relay. The spindle of the disc of this element carries a moving contact which closes the fixed contact

after the operation of directional element. Plug setting bridge is provided for current setting. The tappings are

provided on the upper magnet of over current element and are connected to the bridge.

Operation:-

Under normal operating conditions, power flows in the normal direction in the circuit operated by the relay.

Therefore, directional power relay does not operate, thereby keeping the (lower element) un-energized.

However, when a short circuit occurs, there is a tendency for the current or power to flow in the reverse

direction. The disc of the upper element rotates to bridge the fixed contacts 1 and 2. This completes the circuit

for over current element. The disc of this element rotates and the moving contact attached to closes the trip

circuit. This operates the circuit breaker which isolates the faulty section.

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5. Write a detailed note on differential relays. (16)

CURRENT DIFFERENTIAL PROTECTION

Differential protection is a very reliable method of protecting generators, transformers, buses, and transmission

lines from the effects of internal faults.

Figure: Differential Protection of a Generator

In a differential protection scheme in the above figure, currents on both sides of the equipment are compared.

The figure shows the connection only for one phase, but a similar connection is usually used in each phase of

the protected equipment. Under normal conditions, or for a fault outside of the protected zone, current I1 is

equal to current I2 . Therefore the currents in the current transformers secondaries are also equal, i.e. i1 = i2 and

no current flows through the current relay.

If a fault develops inside of the protected zone, currents I1 and I2 are no longer equal, therefore i1 and i2 are not

equal and there is a current flowing through the current relay.

Differential Protection of a Station Bus

The principle of the differential protection of a station bus is the same as for generators.

The sum of all currents entering and leaving the bus must be equal to zero under normal conditions or if the

fault is outside of the protected zone. If there is a fault on the bus, there will be a net flow of current to the bus

and the differential relay will operate.

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BIASED BEAM RELAY or PERCENTAGE DIFFERENTIAL RELAY

The biased beam relay also called percentage differential relay is designed to respond to the differential current

in terms of its fractional relation to the current flowing through the protected section. It’s called percentage

differential relay because the ratio of differential operating current to average restraining current is a fixed

percentage. It’s called bias relay because restraining known as biased coil produces the bias force. Fig 17 a,

shows the schematic arrangements of biased beam relay. It is essentially an over current balanced beam type

relay with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to

the operating force.

Under normal and through load conditions, the bias force due to restraining coil is greater than operating force.

Therefore, the relay remains inoperative. When an internal fault occurs, the operating force exceeds the bias

force. Consequently the trip contacts are closed to open the circuit breaker. The bias force can be adjusted by

varying the number of turns on the restraining coil.

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The equivalent circuit of a biased beam relay is shown in fig 17 b. The differential current in operating coil is

proportional to i2 – i1 and the equivalent current in the restraining coil is proportional to (i2 – i1)/2 since the

operating coil is connected to the mid-point of the restraining coil. It is clear that greater the current flowing

through the restraining coil, the higher the value of current required in the operating winding to trip the relay.

Thus under heavy load, a greater differential current through the relay operating coil is required for operation,

than under light load conditions

VOLTAGE DIFFERENTIAL RELAY

Fig. 18 shows the arrangement of voltage balance protection. In this scheme of protection, two similar current

transformers are connected at either end of the element to be protected (e.g. an alternator winding) by means of

pilot of wires. The secondaries of current transformers are connected in series with a relay in such a way that

under normal conditions, their induced e.m.f’s are in opposition

Under healthy conditions, equal currents will flow in both primary windings. Therefore, the secondary voltages

of the two transformers are balanced against each other and no current will flow through the relay-operating

coil. When a fault occurs in they protected zone, the currents in the two primaries will differ from one another

and their secondary voltages will no longer be in balance. This voltage difference will cause a current to flow

through the operating coil of the relay, which closes the trip circuit

Disadvantages The voltage balance system suffers from the following drawbacks

(i) A multi-gap transformer construction is required to achieve the accurate balance between current transformer

pairs.

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(i) The system is suitable for protection of cables of relatively short, lengths due to the capacitance of pilot

wires.

.

5. Explain the working principle of distance relays. (16)

Basic operation of Impedance relay

Reactance relay:

Here the operating torque is obtained by current while the restraining torque due to a current voltage

directional relay. The over current elements develops the positive torque and directional unit produces negative

torque. Thus the reactance relay is an overcurrent relay with the directional restraint. The directional element is

so designed that the maximum torque angle is 900

Construction

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The structure used for reactance relay can be of induction cup type. It is a four pole structure. It has operating

coil, polarizing coil and a restraining coil. The diagram is given above. The current I flows from pole 1, through

iron core stacking to lower pole3. The winding on pole 4 is fed from voltage V. the operating torque is produced

by interaction of fluxes due to the windings carrying current coils i.e., interaction of fluxes produced by poles

1,2 and 3. While restraining torque is developed due to interaction of fluxes due to poles 1,3 and 4. Hence

operating torque is proportional to the square of the current [ I2] while the restraining torque is proportional to

the product of V and I [V×I]. The desired maximum torque angle is obtained with the help of RC network.

Torque equation:

The operating torque is proportional to the square of the current [ I2] while the restraining torque is proportional

to the product of V and I [V×I].

Hence the net torque neglecting the effect of spring is given by

T = K1 I2 – K2VI cos [Ø-τ]

At the balance point net torque is zero

0 = K1 I2 – K2VI cos [Ø-τ]

K1 I2 = K2VI cos [Ø-τ]

K1 = K2 𝑉

𝐼 cos [Ø-τ]

K1 = K2 𝑍 cos [Ø-τ]

Adding capacitor, the torque angle is adjusted as 900

K1 = K2 𝑍 cos [Ø-900]

K1 = K2 𝑍 sinØ

𝑍 sinØ = K1

K2

Consider an impedance triangle

𝑍 sinØ = X = reactance

𝑍 cosØ = R = resistance

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𝑋 = K1

K2 = constant

Thus the relay operates on the reactance only. The constant X means a straight line parallel to X- axis on R-X

diagram

ASSIGNMENT

7. Describe the realization of a directional over current relay using a microprocessor. (16)

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8. Derive a generalized mathematical model of distance relays for digital protection. (16)

9. (a) How can digital distance relaying algorithm be implemented on the 8086 Micro

processor? (8)

(b) It is possible to implement these algorithms on the 8085micro processor? (8)

10. Explain with sketches and their R-X diagrams for the following distance relays.

(i) Impedance relay (5)

(ii) Mho relay (5)

(iii) Reactance relay (6)

11. (a) Explain the applications of microprocessors in power system protection. (8)

(b) Explain microprocessor based inverse time over current relay. (8)

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UNIT V CIRCUIT BREAKERS 9

Types of circuit breakers – air blast, air break, oil, SF6 and vacuum circuit breakers – comparative merits of

different circuit breakers – testing of circuit breakers.

CIRCUIT BREAKERS

1. What is circuit breaker? It is a piece of equipment used to break a circuit automatically under fault conditions. It breaks a circuit

either manually or by remote control under normal conditions and under fault conditions.

2. Write the classification of circuit breakers based on the medium used for arc

extinction? _ Air break circuit breaker

_ Oil circuit breaker

_ Minimum oil circuit breaker

_ Air blast circuit breaker

_ SF6 circuit breaker

_ Vacuum circuit breaker

3. What is the main problem of the circuit breaker? When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current

interruption process and also generates enormous heat which may cause damage to the system or to the breaker

itself. This is the main problem.

4. What are demerits of MOCB? _ Short contact life

_ Frequent maintenance

_ Possibility of explosion

_ Larger arcing time for small currents

_ Prone to restricts

5. What are the advantages of oil as arc quenching medium?

• It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties

• It acts as an insulator and permits smaller clearance between line conductors and earthed components

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6.What are the hazards imposed by oil when it is used as an arc quenching medium?

There is a risk of fire since it is inflammable. It may form an explosive mixture with arc. so oil is preferred

as an arc quenching medium

7. What are the disadvantages of MOCB over a bulk oil circuit breaker?

The degree of carbonization is increased due to smaller quantity of oil o There is difficulty of removing

the gases from the contact space in time o The dielectric strength of the oil deteriorates rapidly due to high

degree of carbonization.

8. What are the types of air blast circuit breaker?

_ Arial-blast type

_ Cross blast

_ Radial-blast

9. What are the advantages of air blast circuit breaker over oil circuit breaker?

o The risk of fire is diminished

o The arcing time is very small due to rapid buildup of dielectric strength between contacts

o The arcing products are completely removed by the blast whereas oil deteriorates with successive

operations

10. What are the demerits of using oil as an arc quenching medium?

• The air has relatively inferior arc quenching properties

• The air blast circuit breakers are very sensitive to variations in the rate of rise of restriking voltage

• Maintenance is required for the compression plant which supplies the air blast

11. What is meant by electro negativity of SF6 gas?

SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas molecule,

the electron is absorbed by the neutral gas molecule and negative ion is formed. This is called as electro

negativity of SF6 gas.

12. What are the characteristic of SF6 gas?

It has good dielectric strength and excellent arc quenching property. It is inert, non

toxic, no inflammable and heavy. At atmospheric pressure, its dielectric strength is 2.5 times that of air. At three

times atmospheric pressure, its dielectric strength is equal to that of the transformer oil.

13. Write the classifications of test conducted on circuit breakers. _ Type test

_ Routine test

_ Reliability test

_ Commissioning test

14. What are the indirect methods of circuit breaker testing? o Unit test

o Synthetic test

o Substitution testing

o Compensation testing

o Capacitance testing

15. What are the advantages of synthetic testing methods? • The breaker can be tested for desired transient recovery voltage and RRRV.

• Both test current and test voltage can be independently varied. This gives flexibility to the test

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• The method is simple

• With this method a breaker capacity (MVA) of five time of that of the capacity of the test plant can be

tested.

16. How does the over voltage surge affect the power system? The over voltage of the power system leads to insulation breakdown of the equipment’s. It causes the

line insulation to flash over and may also damage the nearby transformer, generators and the other equipment

connected to the line.

17. What are the advantages of MOCB over a bulk oil circuit breaker?

• It requires lesser quantity of oil

• It requires smaller space

• There is a reduced risk of fire

• Maintenance problem are reduced

14. What is the need of relay coordination?

The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible

time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently

quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be

isolated quickly without serious disturbance to the rest of the system.

15. Mention the short comings of Merz Price scheme of protection applied to a power transformer.

In a power transformer, currents in the primary and secondary are to be compared. As these two currents

are usually different, the use of identical transformers will give differential current, and operate the relay under

no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three

phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay

under normal condition.

16. What are the various faults to which a turbo alternator is likely to be subjected?

Failure of steam supply; failure of speed; overcurrent; over voltage; unbalanced loading; stator winding

fault .

17. What is an under frequency relay?

An under frequency relay is one which operates when the frequency of the system (usually an alternator

or transformer) falls below a certain value.

18. Define the term pilot with reference to power line protection.

Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as

part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.

19. Mention any two disadvantage of carrier current scheme for transmission line only.

The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response

time of band pass filter; capacitance phase-shift of the transmission line

20. What are the features of directional relay?

High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal

ratio; burden must not be excessive.

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21. What are the causes of over speed and how alternators are protected from it? Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are

provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime

mover when a dangerous over-speed occurs.

22. What are the main types of stator winding faults? Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same

phase winding.

23. Give the limitations of Merz Price protection. Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes

impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the

voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In

addition to this, there always an inherent phase difference between the primary and the secondary quantities and

a possibility of current through the relay even when there is no fault.

24. What are the uses of Buchholz’s relay? Bucholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer and to

connect the transformer from the supply in the event of severe internal faults. It is usually used in oil immersion

transformers with a rating over 750KVA.

16 Mark Questions

1]Explain the function of Air Blast Circuit Breaker

Air Blast Circuit Breaker Working

In the air blast circuit breakers the arc interruption takes place to direct a blast of air, at high pressure

and velocity, to the arc. Dry and fresh air of the air blast will replace the ionized hot gases within the arc zone

and the arc length is considerably increased.Consequently the arc may be interrupted at the first natural current

zero. In air blast circuit breakers, the contacts are surrounded by compressed air. When the contacts are opened

the compressed air is released in forced blast through the arc to the atmosphere extinguishing the arc in the

process.A compressor plant is necessary to maintain high air pressure in the receiver.

The air blast circuit breakers are especially suitable for railways and arc furnaces, where the breaker

operates repeatedly. Air blast circuit breakers is used for interconnected lines and important lines where rapid

operation is desired. In air blast circuit breaker (also called compressed air circuit breaker) high pressure air is

forced on the arc through a nozzle at the instant of contact separation. The ionized medium between the contacts

is blown away by the blast of the air. After the arc extinction the chamber is filled with high pressure air, which

prevents restrike. In some low capacity circuit breakers, the isolator is an integral part of the circuit breaker. The

circuit breaker opens and immediately after that the isolator opens, to provide addition gap.

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In the air reservoir there is a high pressure air stored between 20 to 30 kg/cm2. And that air is taken

from compressed air system. On the reservoir there are three hollow insulator columns mounted with valves at

their base. On the top of the hollow insulator chambers there are double arc extinguishing chambers mounted .

The current carrying parts connect the three arc extinction chambers to each other in series and the pole to the

neighboring equipment. since there exist a very high voltage between the conductor and the air reservoir, the

entire arc extinction chamber assembly is mounted on insulators. Since there are three double arc extinction

poles in series, there are six breakers per pole. Each arc extinction chamber consists of one twin fixed contact.

There are two moving contacts. The moving contacts can move axially so as to open or close. Its opening or

closing mechanism depends on spring pressure and air pressure.

The operation mechanism operates the rods when it gets a pneumatic or electrical signal. The valves

open so as to send the high pressure air in the hollow of the insulator. The high pressure air rapidly enters the

double arc extinction chamber. As the air enters into the arc extinction chamber the pressure on the moving

contacts becomes more than spring pressure and it causes the contacts to be open.The contacts travel through a

short distance against the spring pressure. At the end of contacts travel the part for outgoing air is closed by the

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moving contacts and the entire arc extinction chamber is filled with high pressure air, as the air is not allowed to

go out. However, during the arcing period the air goes out through the openings and takes away the ionized air

of arc.

While closing, the valve is turned so as to close connection between the hollow of the insulator and the

reservoir. The valve lets the air from the hollow insulator to the atmosphere. As a result the pressure of air in the

arc extinction chamber is dropped down to the atmospheric pressure and the moving contacts close over the

fixed contacts by virtue of the spring pressure. the opening is fast because the air takes a negligible time to

travel from the reservoir to the moving contact. The arc is extinguished within a cycle. Therefore, air blast

circuit breaker is very fast in breaking the current.Closing is also fast because the pressure in the arc extinction

chamber drops immediately as the value operates and the contacts close by virtue of the spring pressure.

Advantages: How air blast circuit breaker is better than oil circuit breaker:

1. The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very

small. this reduces the size of device.

2. The risk of fire is eliminated.

3. Due to lesser arc energy, air blast circuit breakers are very suitable for conditions where frequent

operation is required.

4. The arcing products are completely removed by the blast whereas the oil deteriorates with successive

operations; the expense of regular oil is replacement is avoided.

5. The energy supplied for arc extinction is obtained from high pressure air and is independent of the

current to be interrupted.

6. The arcing time is very small due to the rapid build up of dielectric strength between contacts.

Therefore, the arc energy is only a fraction that in oil circuit breakers, thus resulting in less burning of contacts.

Disadvantages: 1. Considerable maintenance is required for the compressor plant which supplies the air blast.

2. Air blast circuit breakers are very sensitive to the variations in the rate of restriking voltage.

3. Air blast circuit breakers are finding wide applications in high voltage installations.Majority of circuit

breakers for voltages beyond 110 kV are of this type.

2] Describe the operating principle of air vast circuit breakers.

Two types of air vast circuit breakers are

i. Axial air blast circuit breaker

ii. Cross Blast air circuit breaker

I] Axial-blast air circuit breaker

breaker. The fixed and

moving contacts are held in closed position by spring pressure under normal conditions. The air reservoir is

connected to the arcing chamber through an air valve. This valve remains closed under normal conditions but

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opens automatically by tripping impulse when a fault occurs on the system.

tripping impulse causes the opening of the air valve which connects the circuit

breaker reservoir to the arcing chamber.The high pressure air entering the arcing chamber pushes away the

moving contact against spring pressure.

rated and an arc is struck.At the same time,high pressure air blast flows along the

arc and takes away the ionised gases along with it.Consequently,the arc is extinguished and current flow is

interrupted.

e contact separation required for interruption is generally

small about 1.75 cm. Such a small gap may constitute inadequate clearance for the normal service

voltage.Therefore,an isolating switch is incorporated as part of this type of circuit breaker.This switch opens

immediately after fault interruption to provide necessary clearance for insulation.

II] Cross Blast air circuit breaker

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In this type of circuit breaker,an air blast is directed at right angles to the arc.The cross-blast lengthens

and forces the arc into a suitable chute for arc extinction.Figure below shows the parts of a typical cross-

blast air circuit breaker

When the moving contact is withdrawn,am arc is struck between the fixed and moving contacts.the high

pressure cross-blast forces into a chute consisting of an arc splitters and baffles.The splitters serve to

increase the length of the arc and baffles give improved cooling.The result is that arc is extinguished and

flow of current is interrupted.Since the blast pressure is same for all currents,the inefficiency at low

currents is eliminated.The final gap for interruption is great enough to give normal insulation clearance

so that series isolating switch is not necessary.

3]With the neat sketch explain the construction, principle of operation and application of Air break

circuit breaker

Air break circuit breaker These circuit breakers employ high resistance interruption principle. The arc is rapidly lengthened by

means of the arc runners and arc chutes and the resistance of the arc is increased by cooling, lengthening and

spilitting the arc. The arc resistance increases to such an extent that the voltage drop across the arc becomes

more than the supply voltage and the arc extinguished.

Air breaker circuit breakers are used in d.c circuits and a.c circuits upto 12 kV.

Magnetic field is utilized for lengthening the arc in high voltage air break circuit breaker.

The arc resistance is increased to such an extent that the system voltage cannot maintain the arc and the arc gets

extinguished.

There are two set of contacts: Main contacts (1) and Arching contacts (2).

Main contacts conduct the current in closed position of the breaker. They have low contact resistance and are

silver plated. The arching contacts (2) are hard, heat resistance and usually made of copper alloy. While opening

the contact, the main contacts dislodge first. The current is shifted to the arching contacts. The arching contacts

dislodge later and arc is drawn between them (3). This arc is forced upwards by the electromagnetic force and

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thermal action. The arc ends travel along the Arc Runner (Arcing horns). The arc moves upwards and is split by

arc splitter plates (5). The arc is extinguished by lengthening, cooling, splitting etc. In some breakers the arc is

drawn in the direction of the splitter by magnetic field.

Operating Mechanisms for Air Break Circuit Breakers The operating mechanisms are generally operating spring. The closing force is obtained from the

following means:

a. Solenoid

b. Spring charged manually or by motor

c. Pneumatic

The solenoid mechanisms drive power from battery supply or rectifiers. The solenoid energized by the

direct current gives the necessary force for the closing of the circuit breaker.

The springs used for closing operation can be charged either manually or by motor driven gears. At the

time of closing operation the energy stored in the spring is released by unlatching of the spring and is utilized in

closing of the circuit breaker.

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4] I] What are the properties of SF6 gas that makes it more useful for circuit breaking? II] With the neat

sketch explain the construction, principle of operation and application of SF6 circuit breaker.

At this point we are aware that the medium in which arc extinction of the circuit breaker takes place greatly

influences the important characteristics and life of the circuit breaker. In the last article the working of a

vacuum circuit breaker was illustrated. We already know that the use of vacuum circuit breaker is mainly

restricted to system voltage below 38 kV. The characteristics of vacuum as medium and cost of the vacuum CB

does not makes it suitable for voltage exceeding 38 kV. In the past for higher transmission voltage Oil Circuit

Breaker (OCB) and Air Blast Circuit Breaker (ABCB) were used. These days for higher transmission voltage

levels SF6 Circuit Breakers are largely used. OCB and ABCB have almost become obsolete. In fact in many

installations SF6 CB is used for lower voltages like 11 kV, 6 kV etc..

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i] Sulphur Hexafluoride symbolically written as SF6 is a gas which satisfy the requirements of an ideal arc

interrupting medium. So SF6 is extensively used these days as an arc interrupting medium in circuit breakers

ranging from 3 kv upto 765 kv class. In addition to this SF6 is used in many electrical equipments for

insulation. Here first we discuss in brief, some of the essential properties of SF6 which is the reason of it's

extensive use in circuit breakers

SF6 gas has high dielectric strength which is the most important quality of a material for use in electrical

equipments and in particular for breaker it is one of the most desired properties. Moreover it has high

Rate of Rise of dielectric strength after arc extinction. This characteristics is very much sought for a

circuit breaker to avoid restriking.

SF6 is colour less, odour less and non toxic gas.

SF6 is an inert gas. So in normal operating condition the metallic parts in contact with the gas are not

corroded. This ensures the life of the breaker and reduces the need for maintenance.

SF6 has high thermal conductivity which means the heat dissipation capacity is more. This implies

greater current carrying capacity when surrounded by SF6 .

The gas is quite stable. However it disintegrates to other fluorides of Sulphur in the presence of arc. but

after the extinction of the arc the SF6 gas is reformed from the decomposition.

SF6 being non-flammable so there is no risk of fire hazard and explosion.

A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc.

They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated

switchgear. In cold climates, supplemental heating or de-rating of the circuit breakers may be required due to

liquefaction of the SF6 gas.

Advantages:

Due to superior arc quenching property of sf6 , such breakers have very short arcing time

Dielectric strength of sf6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.

Gives noiseless operation due to its closed gas circuit

Closed gas enclosure keeps the interior dry so that there is no moisture problem

There is no risk of fire as sf6 is non-inflammable

There are no carbon deposits

Low maintenance cost, light foundation requirements and minimum auxiliary equipment

sf6 breakers are totally enclosed and sealed from atmosphere, they are particularly suitable where

explosion hazard exists

Disadvantages:

sf6 breakers are costly due to high cost of sf6

sf6 gas has to be reconditioned after every operation of the breaker, additional equipment is

required for this purpose

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CONSTRUCTION, PRINCIPLE OF OPERATION

The construction and working principles of SF6 circuit breaker varies from manufacturer to

manufacturer. In the past double pressure type of SF6 breakers were used. Now these are obsolete. Another type

of SF6 breaker design is the self blast type, which is usually used for medium transmission voltage. The Puffer

type SF6 breakers of single pressure type are the most favoured types prevalent in power industry. Here the

working principle of Puffer type breaker is illustrated (Fig-A).

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As illustrated in the figure the breaker has a cylinder and piston arrangement. Here the piston is fixed but the

cylinder is movable. The cylinder is tied to the moving contact so that for opening the breaker the cylinder

along with the moving contact moves away from the fixed contact (Fig-A(b)). But due to the presence of fixed

piston the SF6 gas inside the cylinder is compressed. The compressed SF6 gas flows through the nozzle and

over the electric arc in axial direction. Due to heat convection and radiation the arc radius reduces gradually

and the arc is finally extinguished at current zero. The dielectric strength of the medium between the separated

contacts increases rapidly and restored quickly as fresh SF6 gas fills the space. While arc quenching, small

quantity of SF6 gas is broken down to some other fluorides of sulphur which mostly recombine to

form SF6 again. A filter is also suitably placed in the interrupter to absorb the remaining decomposed

byproduct.

The gas pressure inside the cylinder is maintained at around 5 kgf per sq. cm. At higher pressure the dielectric

strength of the gas increases. But at higher pressure the SF6 gas liquify at higher temperature which is

undesired. So heater is required to be arranged for automatic control of the temperature for circuit breakers

where higher pressure is utilised. If the SF6 gas will liquify then it loses the ability to quench the arc.

Like vacuum breaker, SF6 breakers are also available in modular design form so that two modules connected in

series can be used for higher voltage levels. SF6 breakers are available as both live tank and dead tank types. In

Fig-B above a live tank outdoor type 400 kV SF6 breaker is shown.

5] Explain the construction and operation vacuum circuit breaker with neat diagram.

Vacuum Circuit Breakers( VCB )

In this breaker, vacuum is being used as the arc quenching medium. Vacuum offers highest insulating

strength, it has far superior arc quenching properties than any other medium. When contacts of a breaker are

opened in vacuum, the interruption occurs at first current zero with dielectric strength between the contacts

building up at a rate thousands of times that obtained with other circuit breakers.

Principle:

When the contacts of the breaker are opened in vacuum (10 -7 to 10 -5 torr), an arc is produced between

the contacts by the ionization of metal vapours of contacts. The arc is quickly extinguished because the metallic

vapours, electrons, and ions produced during arc condense quickly on the surfaces of the circuit breaker

contacts, resulting in quick recovery of dielectric strength. As soon as the arc is produced in vacuum, it is

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quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.

Construction:

Fig shows the parts of a typical vacuum circuit breaker. It consists of fixed contact, moving contact and

arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by

stainless steel bellows .This enables the permanent sealing of the vacuum chamber so as to eliminate the

possibility of leak .A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents

the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of

the outer insulating cover.

Working:

When the breaker operates the moving contacts separates from the fixed contacts and an arc is struck

between the contacts. The production of arc is due to the ionization of metal ions and depends very much upon

the material of contacts. The arc is quickly extinguished because the metallic vapours, electrons and ions

produced during arc are diffused in short time and seized by the surfaces of moving and fixed members and

shields. Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum

breaker occurs with a short contact separation.

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Advantages: a. They are compact, reliable and have longer life.

b. There are no fire hazards

c. There is no generation of gas during and after operation

d. They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy

fault current perfectly just before the contacts reach the definite open position.

e. They require little maintenance and are quiet in operation

f. Can withstand lightning surges

g. Low arc energy

h. Low inertia and hence require smaller power for control mechanism.

Applications:

For outdoor applications ranging from 22 kV to 66 kV. Suitable for majority of applications in rural

area.

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6] Explain the construction and operation bulk oil circuit breaker with neat diagram.

Types Of Oil Circuit Breakers

Oil circuit breakers can be classified into following types:

1)Bulk oil circuit breakers

which use a large quantity of oil. In this circuit breaker the oil serves two purposes. Firstly it

extinguishes the arc during opening of contacts and secondly it insulates the current conducting

parts from one another and from the earthed tank. Such circuit breakers are classified into:

a)Plain oil circuit breakers

b)Arc control circuit breakers

In the former type no means is available for controlling the arc and the contacts are exposed to the whole of the

oil in the tank. In the latter special arc control devices are employed to get the beneficial action of the arc as

efficiently as possible

2)Low oil circuit breakers, which use minimum amount of oil. In such circuit breakers oil is used only for arc

extinction, the current conducting parts are insulated by air or porcelain or organic insulating material.

Construction

There are two chambers in a low oil circuit breaker,the oil in each chamber is separated from each other.The

main advantage of this is that low oil is required and oil in second chamber wont get polluted.Upper chamber is

called the circuit breaker chamber and lower one is called the supporting chamber.Circuit breaking chamber

consists of moving contact and fixed contact.Moving contact is connected with a piston its just for the

movement of the contact and no pressure build due to its motion.There are two vents on fixed contact they are

axial vent for small current produced in oil due to heating of arc and radial vents for large currents.The whole

device is covered using Bakelite paper and porcelain for protection.Vents are placed in a turbulator.

Operation Under normal operating conditions,the moving contacts remain engaged with the upper fixed contact.When a

fault occurs,the moving contact is pulled down by the tripping springs and an arc is struck.The arc vapourises

oil and produces gases under high pressure.This action constrains the oil to pass through a central hole in the

moving contact and results in forcing series of oil through the respective passages of the turbulator.The process

of turbulation is orderly one,in which the sections of arc are successively quenched by the effectof separate

streams of oil ,moving across each section in turn and bearing away its gases

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Advantages A low oil circuit breaker has following advantages compared to bulk oil circuit breaker

1. It requires lesser quantity of oil

2. It requires smaller space

3. There is reduced risk of fire

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4. Maintenance problems are reduced

Disadvantages A low oil circuit breaker has following disadvantages compared to bulk oil circuit breaker

1. Due to smaller quantity of oil,the degree of carbonisation is increased

2. There is a difficulty of removing the gases from the contact space in time

3. The dielectric strength of oil deteriorates rapidly due to high degree of carbonisation.

7. I) What are the advantages and disadvantages of air blast circuit breakers? [8]

II) Discuss the synthetic testing of circuit breakers. [8]

I]Advantages: 1. The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very

small. this reduces the size of device.

. The risk of fire is eliminated.

3. Due to lesser arc energy, air blast circuit breakers are very suitable for conditions where frequent

operation is required.

4. The arcing products are completely removed by the blast whereas the oil deteriorates with successive

operations; the expense of regular oil is replacement is avoided.

5. The energy supplied for arc extinction is obtained from high pressure air and is independent of the

current to be interrupted.

6. The arcing time is very small due to the rapid build up of dielectric strength between contacts.

Therefore, the arc energy is only a fraction that in oil circuit breakers, thus resulting in less burning of contacts.

Disadvantages: 1. Considerable maintenance is required for the compressor plant which supplies the air blast.

2. Air blast circuit breakers are very sensitive to the variations in the rate of restriking voltage.

3. Air blast circuit breakers are finding wide applications in high voltage installations.Majority of circuit

breakers for voltages beyond 110 kV are of this type.

II]Synthetic testing

The principle of synthetic testing is given below. The current source provides short circuit current. The

voltage source gives restriking and recovery voltage. The test observations are proceeded by L,R & C. the

circuit current is feed by closing the switch S1 [IG]. final current becomes zero when switch S2 is closed and

voltage contains transient as it contains I and C .

Advantages

The breakers can be tested for desaired TRV and RRRV

The shor circuit generator has to supply currents at less voltage.

It is flexible because of independent voltage test and current test

It is very simple & it can applied to unit test also.

Upto five times of plant capacity can be tested.

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Synthetic testing

Types of Synthetic test circuits. a.Parallel current injection

b.Series current injection

a.Parallel current injection method.

This method is used for testing circuit breakers. It gives high frequency voltage as given by the standards. It is

given by the graph.

Here the voltage circuit is effectively connected in parallel with current circuit and test breaker before main IG

in test breaker current is properly simulated.

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b. Series current injection method

Here the voltage circuit [2] is connected to current circuit in series before main current zero. Due to this IG and

IH are in opposition. The stresses produced in synthetic test and those in actual network must be same but it is

not the actual case because of several factors like high current, high voltage , instant of applying voltage etc.,

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ASSIGNMENT

1. (a) Enumerate various types of ratings of a circuit breaker. (4)

(b) Discuss symmetrical and asymmetrical breaking capacity, (4)

(c) Making capacity (4)

(d) Short-time current capacity. (4)

2. What are the different methods of testing of circuit breakers? Discuss their merits and

demerits. (16)

3. What is the difficulty in the development of HVDC circuit breaker?

Describe its construction and operating principle. (16)

4. (a) What are the physical chemical and dielectric properties of SF6 Gas (8)

(b) Define switchgear. What are the essential features of switchgear s (8)

5. A 3-phase alternator has the line voltage of 11kV. The generator is connected to a circuit breaker. The inductive

reactance upto the circuit breaker is 5_/phase. the distributed capacitance upto circuit breaker between phase and

neutral is 0.001 μF. Determine peak restriking voltage across the CB, frequency of restriking voltage, average

rate of restriking voltage upto peak restriking voltage, maximum RRRV. (16)

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UNIT III APPARATUS PROTECTION 9

Main considerations in apparatus protection - transformer, generator and motor protection -

protection of bus bars. Transmission line protection - zones of protection. CTs and PTs and their applications in protection

schemes.

1. What are the types of graded used in line of radial relay feeder?

Definite time relay and inverse-definite time relay.

2. What are the various faults that would affect an alternator?

(a) Stator faults 1, Phase to phase faults 2, Phase to earth faults 3, Inter turn faults

(b) 1, Earth faults 2, Fault between turns 3, Loss of excitation due to fuel failure

(c) 1, Over speed 2, Loss of drive 3, Vacuum failure resulting in condenser pressure rise, resulting in

shattering of the turbine low pressure casing

(d) 1, Fault on lines 2, Fault on busbars

3. Why neutral resistor is added between neutral and earth of an alternator?

In order to limit the flow of current through neutral and earth a resistor is introduced between them.

4. What is the backup protection available for an alternator?

Overcurrent and earth fault protection is the backup protections.

5. What are faults associated with an alternator?

(a) External fault or through fault (b) Internal fault 1, Short circuit in transformer winding and connection 2,

Incipient or slow developing faults

6. What are the main safety devices available with transformer?

Oil level guage, sudden pressure delay, oil temperature indicator, winding temperature indicator .

7. What are the limitations of Buchholz relay?

(a) Only fault below the oil level are detected.

(b) Mercury switch setting should be very accurate, otherwise even for vibration, there can be a false

operation.

(c) The relay is of slow operating type, which is unsatisfactory

8. What are the problems arising in differential protection in power transformer and how are they overcome?

1. Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable

resistors to pilot wires to get equipotential points on the pilot wires.

2. Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate

even for external or through faults. This is overcome by introducing bias coil.

3. Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and

act. Bias coil will solve this.

4. Magnetizing inrush current appears wherever a transformer is energized on its primary side producing

harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in

current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when

the transformer is energized.

9. What is REF relay? It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the

voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the

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relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to

cover only around 85%. Hence the relay is called REF relay.

10. What is over fluxing protection in transformer? If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the

transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing.

11. Why busbar protection is needed? (a) Fault level at busbar is high

b) The stability of the system is affected by the faults in the bus zone.

(c) A fault in the bus bar causes interruption of supply to a large portion of the system network.

12. What are the merits of carrier current protection? Fast operation, auto re-closing possible, easy discrimination of simultaneous faults .

13. What are the errors in CT? (a) Ratio error

Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100

The value of transformation ratio is not equal to the turns ratio.

(b) Phase angle error:

Phase angle _ =180/_[(ImCos _-I1Sin _)/nIs]

14. What is field suppression? When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault

continues to fed because EMF is induced in the generator itself.Hence the field circuit breaker is opened and stored

energy in the field winding is discharged through another resistor. This method is known as field suppression.

15. What are the causes of bus zone faults?

_ Failure of support insulator resulting in earth fault

_ Flashover across support insulator during over voltage

_ Heavily polluted insulator causing flashover

_ Earthquake, mechanical damage etc.

16. What are the problems in bus zone differential protection?

_ Large number of circuits, different current levels for different circuits for external faults.

_ Saturation of CT cores due to dc component and ac component in short circuit currents. The saturation

introduces ratio error.

_ Sectionalizing of the bus makes circuit complicated.

_ Setting of relays need a change with large load changes.

17. What is static relay?

It is a relay in which measurement or comparison of electrical quantities is made in a static network which is

designed to give an output signal when a threshold condition is passed which operates a tripping device.

18. What is power swing?

During switching of lines or wrong synchronization surges of real and reactive power flowing in

transmission line causes severe oscillations in the voltage and current vectors. It is represented by curves

originating in load regions and traveling towards relay characteristics.

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19. What is a programmable relay?

A static relay may have one or more programmable units such as microprocessors or microcomputers in

its circuit.

20. What is CPMC?

It is combined protection, monitoring and control system incorporated in the static system.

21. What are the advantages of static relay over electromagnetic relay? Low power consumption as low as 1mW

No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts

No gravity effect on operation of static relays. Hence can be used in

vessels ie, ships, aircrafts etc.

A single relay can perform several functions like over current, under

voltage, single phasing protection by incorporating respective functional blocks. This is not possible in

electromagnetic relays

Static relay is compact

Superior operating characteristics and accuracy

Static relay can think , programmable operation is possible with static relay

Effect of vibration is nil, hence can be used in earthquake-prone areas

Simplified testing and servicing. Can convert even non-electrical quantities to electrical in conjunction with

transducers.

16 arks Questions

1.What are the various faults that may occur in an alternator. Give the diagram for circulating current

protection in alternator. [8]

The fault occurs in an alternator are classified as follows

A] stator faults B] rotor faults

1. stator inter-turn faults 1. Field ground fault

2. stator over heating 2. Loss of exitation

3. stator external faults 3. Rotor over heating because of unbalanced 3Q stator

Current.

ALTERNATOR WITH PERCENTAGE DIFFERENTIAL PROTECTION

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The diagram show the alternator stator protection with circulating protection scheme. During the normal condition or

external fault, the current flowing through the two CT’s are same. So no current flows through the restraining coil

and operating coil so relay cannot operate. During the inter turn fault, the winding of stator will be grounded and

hence the current will flow to the ground. So differential current will flow through the two CT’s and hence the

resultant current will flow through the operating coil.

Due to the current flowing the operating coil, the relay operates. The relay senses second and gives signal to the CB

and CB gets opened and breaks the current flow to the alternator.

II] A 5000KVA, 6600v star connected alternator has a synchronous reactance of 2 Ω per phase and 0.5Ω resistance.

It is protected by Merz prize balanced current system which operates when the out of balanced current exceeds

30% of the load current. Determine what proportion of the alternator winding is unprotected if the star point is

earthed through a resistor of 6.5Ω?

Given:

Rating 5000 KVA

Voltage 6600V

Xs = 2 Ω /ph R = 0.5 Ω

Operating current = 30% of load current

Earth resistor [Rh] = 6.5Ω

Solution

Primary earth fault current at which the relay operates

Fault current [or] operating current = 5000×103

√3×6600×

30

100

= 131.22 Amps.

Percentage of winding unprotected = √3 𝐼 𝑓 𝑅𝑛

𝑉 × 100

= √3×131.22×6.5

6600

% of winding unprotected = 22.38%

Therefore the % of winding protected is

= 100 – 22.38

= 77.62%

1. Explain the time graded over current for radial feeders.

1. Enumerate the relaying schemes which are employed for the protection of a modern

alternator. (16)

2. (a) What is transverse or split phase protection of an alternator? (4)

(b) What type of fault is this scheme of protection employed? (4)

(c) With a neat sketch discuss the working principle of this scheme. (8)

3. What type of a protective device is used for the protection of an alternator against

Overheating of its (i) stator (ii) rotor? Discuss them in brief. (8+8)

4. What type of a protective scheme is employed for the protection of the field winding of

the alternator against ground faults? (16)

5. Discuss the protection employed against loss of excitation of an alternator. (16)

6. (a) What do you understand by field suppression of an alternator? (8)

(b) How is it achieved? (8)

7. Briefly discuss the protection of an alternator against.

(i) Vibration of distortion of motor (4)

(ii) Bearing overheating (4)

(iii) Auxiliary failure (4)

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(iv) Voltage regulator failure (4)

8. What type of pilot protection is used for EHV and UHV transmission lines. (16)

9. (a) What is carrier protection? (4)

(b) For what voltage range is it used for the protection of transmission line? (4)

(c) What are its merits and demerits? (8)

10. (a) What is carrier aided distance protection. (4)

(b) What are its different types? (4)

(c) Discuss the permissive under-reach transfer tripping scheme of protection. (8)

11. (a) Draw and explain the merz-price protection of alternator stator winding. (10)

(b) A generator is protected by restricted earth fault protection. The generator ratings

are 13.2kv, 10MVA. The percentage of winding protected against phase to ground fault

is 85%. The relay setting is such that it trips for 20% out of balance calculate the

resistance to be added in the neutral to ground connection. (6)

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FRAME LEAKAGE PROTECTION

EXTERNAL FAULT

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Phase and earth fault protection

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s

Merz-prize protection for star-star power transformer

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s

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Magnetizing Inrush Current Wave Forms

Magnetizing Current Compensation

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FRAME LEAKAGE PROTECTION

BASIC ARRANGEMENT OF BUCH-HOLZ RELAY

CONSTRUCTION OF BUCH-HOLOZ RELAY

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Phase fault protection

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UNIT IV THEORY OF CIRCUIT INTERRUPTION 9

Physics of arc phenomena and arc interruption. DC and AC circuit breaking - restriking voltage and recovery voltage -

rate of rise of recovery voltage - resistance switching - current chopping - interruption of capacitive current.

UNIT IV THEORY OF CIRCUIT INTERRUPTION

1. What is resistance switching?

It is the method of connecting a resistance in parallel with the contact space(arc). The resistance reduces the

restriking voltage frequency and it diverts part of the arc current. It assists the circuit breaker in

interruptingthemagnetizing current and capacity current.

2. What do you mean by current chopping?

When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid

deionization of the contact space and blast effect may cause the current to be interrupted before the natural current

zero. This phenomenon of interruption of the current before its natural zero is called current chopping.

3. What are the methods of capacitive switching?

• Opening of single capacitor bank

• Closing of one capacitor bank against another

4. What is an arc?

Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or

short circuit condition.

5. Give the two methods of arc interruption?

High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully

extinguished _ Current zero method:-The arc is interrupted at current zero position that occurs100 times a second in

case of 50Hz power system frequency in ac.

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6.What is restriking voltage?

It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished.

7. What is meant by recovery voltage?

The power frequency rms voltage appearing across the breaker contacts after the arc is extinguished and

transient oscillations die out is called recovery voltage.

8. What is RRRV?

It is the rate of rise of restriking voltage, expressed in volts per microsecond. It is closely associated with

natural frequency of oscillation.

. 16 Marks Questions

1. (a) What is resistance switching? (4)

(b) Derive the expression for critical resistance. (12)

2. (a) Explain the phenomenon of current chopping in a circuit breaker. (12)

(b) What measures are taken to reduce it? ` (4)

3. Discuss the problem associated with the interruption of (i) Low inductive current (5)

(ii) Capacitive current and (5)

(iii) Fault current if the fault is very near the substation.

(6)

4. Explain in detail about RRRV.

(16)

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