Protective Relays

A MINI PROJECT REPORT ON

“STUDY OF PROTECTIVE RELAYS”

Report submitted for MINI PROJECT

For

BACHELOR OF TECHNOLOGY

IN

ELECTRICAL & ELECTRONICS ENGINEERING

By

1. AKASH SINGH (09P61A0202)

2. K.RAJASHEKAR REDDY (09P61A0224)

3. B.KIRAN (10P65A0202)

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

VIGNANA BHARATHI INSTITUTE OF TECHNOLOGY

(Aushapur Village, Ghatkesar Mandal, Hyderabad – 501301)

CERTIFICATE

This is to certify that the following students of VIGNANA BHARATHI INSTITUTE OF

TECHNOLOGY have successfully completed their project titled “STUDY OF

PROTECTIVE RELAYS” in GACHIBOWLI POWER STATION report submitted for

MINIPROJECT for Bachelor of Technology in ELECTRICAL AND ELECTRONICS

ENGINEERING.

Project Associates:



3. B. KIRAN (10P65A0202)

Under the external Guidance of under the internal Guidance of

Sri U. RAMULU, B. SARVESHWAR REDDY,

Assistant Divisional Engineer, professor of EEE A.P.Transco: 220KVSS : Gachibowli

ACKNOWLEDGEMENT

This project titled “STUDY OF PROTECTIVE RELAYS” was carried out in

220KVSS, Gachibowli.

We are expressing our sincere and heartfelt thanks to our guides Sri

U.RAMULU, Assistant divisional engineer, A.P.TRANSCO: 220KVSS, Gachibowli,

and Sri S.V.SOMAYAJULU, for their support and spending their valuable time to

analyze and review our project at every stage. We are extremely thankful towards

them for their wonderful suggestions and constant critiquing.

Before ending, I would like to acknowledge all those people who helped me to

come out with this project, whose names cannot be penned here, but without whose

help this project would never have been completed successfully.



3. B.KIRAN (10P65A0202)

ABSTRACT

Protective relays are the devices that detect abnormal conditions in electrical

circuits by constantly measuring the electrical quantities which are different under

normal and fault conditions. The basic quantities which may change during fault

conditions are voltage, current, phase angle (direction) and frequency. Having

detected the fault the relay operates to complete the trip circuit which results in the

opening of circuit breaker and therefore in the disconnection of the faulty circuit.

The 220KV SWITCHYARD AT Gachibowli, which consists of double bus

system and 6 No’s. 220KV incoming, 2No’s. Outgoing feeders through which can

send or receive power, 6 No’s 132 KV, and 14No’s 33KV feeders and four 3KV

feeders through which we can send or receive power.

The 220KV switchyard also consists of protection of transmission lines like

distance protection, current carrying protection and maintenance of CVT’s and CT’s.

CONTENTS

1. INTRODUCTION

1.1 DESCRIPTION.

1.2 WORKING PRINCIPLE.

1.3 FUNCTIONS OF RELAY.

1.4 FUNCTIONAL CHARACTERICSTIC OF RELAYS

2. CLASSIFICATION OF RELAYS

2.1 CLASSIFICATION BY STRUCTURE.

2.1.1 HINGED RELAYS

2.1.2 PLUNGER RELAYS

2.1.3 REED RELAYS

2.2 CLASSIFICATION BY OPERATNG PRINCIPLE.

3. LINE PROTECTION RELAY SCHEMES.

3.1 DISTANCE RELAYS.

3.1.1 INTRODUCTION

3.1.2 OPERATING PRINCIPLE OF DISTANCE RELAY

3.1.3 ZONES OF PROTECTION

3.1.3.1 ZONE 1 SETTING



3.2 RAZOA

3.2.1 DESIGNs.

3.2.2 MODE OF OPERATION.

3.2.3 FEEDER PROTECTION TYPE RAZOA.

3.2.4 TECHNICAL DATA.

4. CONVENTIONAL RELAYS

4.1 FUSE FAILURE RELAY.

4.2 HIGH SPEED TRIPPING RELAY.

4.3 POWER SWING BLOCKING RELAY.

4.4 TRIP SUPERVISION RELAY.

4.5 EASUN REYROLLE.

5. TRANSFORMER PROTECTION RELAYS

5.1. GAS OPERATED RELAYS

5.1.1 BUCHHOLZ RELAY

5.1.2 OIL SURGE RELAYs.

5.1.3 PRESSURE RELIEF VALVE

5,.2 DIFFERENTIAL PROTECTION RELAYS

5.3 OVERCURRENT AND EARTH FAULT PROTECTION RELAY

6. OTHER PRACTICAL RELAY EXAMPLES

6.1 ABB REALYS.

6.1.1 READK

6.1.2 GPU200R

6.2 EASUN REYROLLE RELAYS

. 6.2.1 NUMERICAL OVERCURRENT RELAY [MIT 103/104/113/114] 6.2.2 RESTRICTED EARTH FAULT RELAY [ EB3 ]

7. CONCLUSION

8. REFERENCES

PROTECTION RELAYS1. Introduction:

In order to generate electric power and transmit it to customers millions of rupees must be spent on power system equipments. However a short circuit may occur due to failure of insulation caused by over-voltages due to switching or lightning stroke or breakdown of insulation. These short circuit currents may cause heavy damage to equipment and would also cause intolerable interruption of service to customers.

In modern power system to minimize damage to equipment two alternatives are open to designer, one is to design the system so that faults cannot occur and the other is to accept the possibility of faults and take steps to guard against the effect of these faults. Although it is possible to eliminate faults to a large degree by careful systems design, careful insulation co-ordination, current operation and maintenance, it is obviously not possible to ensure cent percent reliability and therefore the possibility of faults must be accepted.

Protective relays are the devices that detect abnormal conditions in electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The basic quantities which may change during fault conditions are voltage, current, phase angle (direction) and frequency. Having detected the fault the relay operates to complete the trip circuit which results in the opening of circuit breaker and therefore in the disconnection of the faulty circuit

Description: A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays".

Working principle Electromechanical protective relays operate by either magnetic attraction, or magnetic induction. "Armature"-type relays have a pivoted lever supported on a hinge or knife-edge pivot, which carries a moving contact. These relays may work on either alternating or direct current, but for alternating current, a shading coil on the pole is used to maintain contact force throughout the alternating current cycle. Because the air gap between the fixed coil and the moving armature becomes much smaller when the relay has operated, the current required to maintain the relay closed is much smaller than the current to first operate it. The "returning ratio" or "differential" is the measure of how much the current must be reduced to reset the relay. A variant application of the attraction principle is the plunger-type or solenoid operator. A reed relay is another example of the attraction principle. "Induction" disk meters work by inducing currents in a disk that is free to rotate; the rotary motion of the disk operates a contact. Induction relays require alternating current; if two or more coils are used, they must be at the same frequency otherwise no net operating force is produced. Protective relays can also be classified by the type of measurement they make. A protective relay may respond to the magnitude of a quantity such as voltage or current. Induction types of relay can respond to the product of two quantities in two field coils, which could for example represent the power in a circuit. Although an electromechanical relay calculating the ratio of two quantities is not practical, the same effect can be obtained by a balance between two operating coils, which can be arranged to effectively give the same result. Several operating coils can be used to provide "bias" to the relay, allowing the sensitivity of response in one circuit to be controlled by another. Various combinations of "operate torque" and "restraint torque" can be produced in the relay.By use of a permanent magnetic in the magnetic circuit, a relay can be made to respond differently to current in one direction than in another. Such polarized relays are used on direct-current circuits to detect, for example, reverse current into a generator. These

relays can be made bistable, maintaining a contact closed with no coil current and requiring reverse current to reset. For AC circuits, the principle is extended with a polarizing winding connected to a reference voltage source.Light weight contacts make for sensitive relays that operate quickly, but small contacts can't carry or break heavy currents. Often auxiliary telephone-type armature relays are triggered by the measuring relay.In a large installation of electromechanical relays, it would be difficult to determine which device originated the signal that tripped the circuit. This information is useful to operating personnel to determine the likely cause of the fault and to prevent its re-occurrence. Relays may be fitted with a "target" or "flag" unit, which is released when the relay operates, to display a distinctive colored signal when the relay has tripped

Functions of Protective relaying

a) Prompt removal from service any element of power system when it suffers a short circuit, or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system

b) Secondary function, to provide indication of location and type of failure.

Functional Characteristics of Protective relaying

Selectivity, discrimination Sensitivity, power consumption

Reliability Adequateness Speed, time Stability System security

Selectivity - ensure that only the unhealthy part of the system is disconnected

Speed - to prevent or minimize damage and risk of instability of rotating plants

Reliability - to ensure proper action even after long periods of inactivity and also after repeated operations under severe conditions

Sensitive - detection of short circuit or abnormal condition

2. Classification of relays

Relays are classified according to 1) based on structure 2) based on operating principle

2.1 Classification of Relays by Structure:

2.1.1 Hinged Relays:

With hinged relays, the armature of the electromagnet rotates around a fulcrum. This action directly or indirectly opens and closes a contact.

2.1.2 Plunger Relays:

Plunger relays use mainly the power of a plunger-shaped electromagnet as the armature section to open and close contacts.

2.1.3 Reed Relays:

Reed relays consist of a pair of magnetic strips sealed within a glass envelope. These reeds are the contacts. Magnetic flux applied to a coil wrapped around the glass envelope moves these reeds, which opens and closes the contacts.

2.2 Classification by operating principles:

2.2.1 Attracted Armature relay:

These are simplest type of relays. These relays have coil or an electromagnet energized by coil. The coil is energized by the operating quantity which may be proportional to circuit current or volt-age. A plunger or rotating iron vane is subjected to the action of magnetic field produced by the operating quantity. It is basically a single actuating quantity relay.

Features:

1) Attracted armature relays respond to both AC and DC, because torque is proportional to square of current.

2) These are fast relays. They have fast operating and fast reset because of small length of travel and light moving parts.

3) Ratio of reset to pick-up can be as high as 90-95% for AC relays and 60-90% for DC relays.

4) Modern attraction relays are Compact, Robust, and Reliable.

OPERATING PRINCIPLE:

The electromagnetic force exerted on the moving element is proportional to the square of the flux in the air gap. If we neglect the effect of saturation, the total actuating force may be expressed:

Where F = net force.

K1 = a force-conversion constant.

I = the R.M.S magnitude of the current in the actuating coil.

K2 = the restraining force (including friction).

When the relay is on the verge of picking up, the net force is zero, and the operating characteristic is:

DIRECTIONAL RELAYS OF THE ELECTROMAGNETIC ATTRACTION TYPE:

Directional relays of the electromagnetic-attraction type are actuated by dc or by rectified ac quantities. The most common use of such relays is for protection of dc circuits where the actuating quantity is obtained either from a shunt or directly from the circuit. Ip and Ia, are assumed to flow through the coils in such directions that a pickup force is produced, as in Figure. It will be evident that, if the direction of either Ip or Ia (but not of both) is reversed, the direction of the force will be reversed. Therefore, this relay gets its name from its ability to distinguish between opposite directions of actuating-coil current flow, or opposite polarities. If the relative directions are correct for operation, the relay will pick up at a constant magnitude of the product of the two currents.

If permanent-magnet polarization is used, or if the polarizing coil is connected to a source that will cause a constant magnitude of current to flow, the operating characteristic becomes:

Ia still must have the correct polarity, as well as the correct magnitude, for the relay to pick up.

2.2.2 INDUCTION-TYPE RELAYS-GENERAL OPERATING PRINCIPLES

Induction-type relays are the most widely used for protective-relaying purposes involving ac quantities. They are not usable with d-c quantities, owing to the principle of operation. An induction-type relay is a split-phase induction motor with contacts. Actuating force is developed in a movable element, which may be a disc or other form of rotor of non-magnetic current-conducting material, by the interaction of electromagnetic fluxes with eddy currents that are induced in the rotor by these fluxes.

The different types of structure that have been used are commonly called: (1) the "shaded-pole" structure; (2) the "watt-house meter" structure; (3) the "induction-cup" structure.

a) Shaded-Pole Structure:

The shaded-pole structure, illustrated in Figure is generally actuated by current flowing in a single coil on a magnetic structure containing an air gap. The air-

gap flux produced by this current is split into two out-of-phase components by a so called "shading ring," generally of copper, that encircles part of the pole face of each pole at the air gap. The rotor, shown edgewise in Figure, is a copper or aluminum disc, pivoted so as to rotate in the air gap between the poles. The phase angle between the fluxes piercing the disc is fixed by design, and consequently it does not enter into application considerations.

The shading rings may be replaced by coils if control of the operation of a shaded-pole relay is desired. If the shading coils are short-circuited by a contact of some other relay, torque will be produced; but, if the coils are open-circuited, no torque will be produced because there will be no phase splitting of the flux. Such torque control is employed where "directional control" is desired, which will be described later.

Shaded-pole structure.

a) Watthour-Meter Structure:

This structure gets its name from the fact that it is used for watt-hour meters. As shown in Figure, this structure contains two separate coils on two different magnetic circuits, each of which produces one of the two necessary fluxes for driving the rotor, which is also a disc. It consists of E-shaped electromagnet and a U-shaped electromagnet with a disc free to rotate in between. The E-shaped produces flux which is displaced from the flux of U-shaped magnet is adjusted by a reactance in parallel with the secondary winding. The relay coil is tapped at several points. The current setting is selected by inserting a knob to take desired number of turns of coil in the circuit

The ratio of reset of pick-up is high because operation does not involve any change in air gap. The ratio is above 95%.

The operating time of this relay is 10-60 seconds.

Watthour-meter structure

b) Induction-cup Relay:

This relay has two, four or more electro magnets, in stator. These are energized by the relay coils. A stationary iron core is placed as shown in figure. The rotor consists of a hollow metallic cylindrical cup. The rotor is free to rotate in the air gap between the stationary iron and the electromagnets. Eddy currents are produced in metallic cup by the principle of “Faraday’s law of electromagnetic induction”. These currents interact with the flux produced by the other electromagnet and torque is produced. Two fluxes are produced in the air-gap which are at right angles and produce eddy currents in the cup. There by torque is produced.

The operating time characteristic depends on the type of structure. The relays have inverse time characteristic. Modern induction cup relay may have an operating time of the order of 0.01 second.

Electromagnet

3. EHV LINE PROTECTION RELAY SCHEMES

3.1 DISTANCE RELAYS:

3.1.1 Introduction:

The operation of distance relays is not governed by the current or power in the protected circuit but by the ratio of applied voltage to current. The relays therefore effectively measure the impedance of the circuit they are protecting Under fault conditions the voltage is depressed and the current flow greatly increases. The impedance therefore falls during the fault; this is sensed by the distance relay and a trip initiated. Such relays are used on power network over-head lines and feeder cables when the required operating times cannot be achieved by conventional current operated relays. Since the impedance of an overhead line or cable is proportional to its length such impedance measuring relays are known as distance relays or distance impedance (DI) protection.

3.1.2 Basic principles:

When a fault occurs on a feeder the fault current, If, is dependent upon the sum of the fault impedance, Zf, from the ‘relay point’ to the fault and the source impedance, Zs

where E is the line to earth voltage.

The voltage at the relaying point is proportional to the ratio of the fault impedance to total source-to-fault impedance.

If the relay is designed to measure and compare both voltage and current then Vf /If = Zf and the relay measurement of fault impedance is effectively independent of the source impedance. If a fault occurs on the feeder a long way from the relaying point then there may be insufficient fault current to operate the relay and the relay will not trip. The point at which a fault occurs which just fails to cause relay operation is known as the ‘balance point’ or relay ‘reach’.

3.1.3 Zones of protection:

Careful selection of the reach settings and tripping times for the various zones of

measurement enables correct coordination between distance relays on a power system.

Basic distance protection will comprise instantaneous directional Zone 1

protection and one or more time delayed zones.

Digital and numerical distance relays may have up to five zones, some set to

measure in the reverse direction. Typical settings for three forward-looking zones of

basic distance protection are given in the following sub-sections. To determine the

settings for a particular relay design the relay manufacturer’s instructions should be

referred to.

3.1.3.1 Zone 1 Setting:

Reach setting of up to 80% of the protected line impedance for

instantaneous includes in Zone 1 protection. The resulting 15-20% safety margin

ensures that there is no risk of the Zone 1 protection over-reaching the protected line

due to errors in the current and voltage transformers, inaccuracies in line impedance

data provided for setting purposes and errors of relay setting and measurement.

Otherwise, there would be a loss of discrimination with fast operating protection on the

following line section. Zone 2 of the distance protection must cover the remaining 15-

20% of the line.


The reach setting of the Zone 2 protection should be at least 120% of the

protected line impedance. In many applications it is common practice to set the Zone 2

reach to be equal to the protected line section +50% of the shortest adjacent line.

This ensures that the resulting maximum effective Zone 2 reach does not extend beyond

the minimum effective Zone 1 reach of the adjacent line protection. This avoids the need

to grade the Zone 2 time settings between upstream and downstream relays.

Zone 2 tripping must be time-delayed to ensure grading with the primary

relaying applied to adjacent circuits that fall within the Zone 2 reach. Thus complete

coverage of a line section is obtained, with fast clearance of faults in the first 80-85% of

the line and somewhat slower clearance of faults in the remaining section of the line.


Remote back-up protection for all faults on adjacent lines can be provided by a

third zone of protection that is time delayed to discriminate with Zone 2 protection plus

circuit breaker trip time for the adjacent line. Zone 3 reach should be set to at least

1.2 times the impedance presented to the relay for a fault at the remote end of the

second line section.

On interconnected power systems, the effect of fault current infeed at the

remote busbars will cause the impedance presented to the relay to be much greater

than the actual impedance to the fault and this needs to be taken into account when

setting Zone 3.

3.2 RAZOA:

RAZOA is a three-phase distance relay of switched scheme type where the choice

of measuring quantities is carried out by means of static circuits. The distance relay is

intended primarily for overhead transmission lines and cables up to approx. 150KV but

can also be adapted to the requirements imposed for lines with higher voltages. In

isolated or high impedance grounded networks, RAZOA operates for short circuits and

double ground faults. In solidly grounded networks the relay also operates for single

ground faults. The minimum operating time of the relay is 20msec.

The relay has three directional impedance measuring zones and a fourth zone

that follows the setting of the starting elements. The starting relays can be either

impedance or over current relays. The first measuring zone is instantaneous and is

normally set to under reach i.e., its reach covers less than 100% of the line. The other

measuring zones function as back up protections, their operation being based on time

selectively where the reach is extended incrementally after a give time lag which is

governed by the selective time settings T2, T3 and T4.

Since RAZOA is a switched scheme type distance relay and consequently has only

one measuring unit in the basic version. In the event of a fault on the line, the starting

elements control the phase selector unit so that depending on the type of fault,

predetermined measuring quantities are fed into the measuring unit. The measuring

quantities are switched on the secondary side of the input transformers of the distance

relay where as the switching is performed without interruption in the current circuits.

3.2.1 DESIGN:

The distance relay is built up of COMBIFLEX modular units.

The units incorporated in RAZOA are:

DEIGNATION TYPE

Test Switch REXP18

DC-DC Converter RXTUG

Input Unit RGKC070

Under Impedance or over current unit RGZB030, RGIC 030

Phase selector unit RGGB 030

Current setting unit RGAA 030

Voltage setting unit RGAB 030

Time lag unit RGTA 030

Extra measuring unit RGZA 030

Memory circuit unit RGLA 030

Measuring and indicating unit RGSB 030

Output unit RGKD 050

Transformer Unit RTTG

3.2.2 Mode of operation:

The distance relay receives its power via a test switch and input transformers.

The transformers convert the measured quantities to a level suitable to the electronic

units. In the event of under impedance on a protected line, the under impedance

elements of the relevant phases and the zero sequence current element will start the

timing elements and also initiate the phase selector unit which then ensures that the

correct current and voltage measuring and the correct direction are used.

The start indicators and the signal relays of the relevant phases will also be

energized. The determine the direction the distance relay uses healthy phase

polarization in the case of single and two phase faults.

The reach of each respective measuring zone is determined by the settings on

the current and voltage setting units. Four measuring signals are generated and these

are filtered and converted to digital signals. These signals are then used in the

measuring unit for determining the direction to and the position of the fault. If the fault

has occurred in the first zone, the output unit is simultaneously activated If the fault lies

outside the first zone, the reach is increased successively according to the selective

times T2, T3 and T4 by reconnection in the voltage circuits.

In conjunction with the programming, the function of the distance relay can be

influenced via the input relays, information from the distance relays relating to e.g. co

operation with the communication equipment and tripping of the breaker is obtained

via the output relays.

The distance relay has a dc-dc converter, which adapts the stations dc supply to t

level suitable for the electronic units. If the distance relay incorporates over current

starting instead of under impedance starting these starting signals will influence the

timing circuit and the phase selection unit in the same manner as the under impedance

unit.

3.2.3 Feeder Protection Type RAZOA:

The details of protection of a particular feeder through RAZOA are given below:

Name of the Feeders : Shapurnagar-I&II, Yeddumilaram,

Thandoor.

Station : Gachibowli 220KVSS.

The Basic double circuit data as follows:

Rated line voltage : 220 KV

Load MVA : 350 MVA

Line Length : 68.23 Km.

Positive sequence impedance of the line : 1.69+j15.29 ohms/km/phase.

Zero sequence impedance of the line : 11.56+j66.83 ohms/km/phase.

CT Ratio : 1600/1

PT Ratio : 220000/110

Positive sequence impedance of the : 1.69+j15.29 ohms/km/phase

shortest adjacent line

Positive sequence impedance of the longest : 1.69+j15.29 ohms/km/phase.

line

Percentage of line for 1-zone : 80

Percentage of the adjacent line for 2-zone : 120

Percentage of the adjacent line for 3-zone : 200

Relay particulars:

The particulars of the relay are given as follows :

Rated current : 1 Amp.

Rated AC voltage : 110V

Setting calculations :

The distance protection operated is an under reaching mode with the

following settings of the first three zones.

Zone-1 : 80 percent of the protected line length.

Zone-2 : 100 percent of the protected line length + 120 percent of the

shortest adjoining line length.

Zone-3 : 100 percent of the protected line length + 200 percent of the

longest

adjoining line length.

The time delays will be chosen accordingly to selectively plan and they will be

equal to:

T2 = 300msec.

T3 = 600msec.

The under impedance starting is set to cover beyond zone-3 reach. However the

load impedance UU/s should be outside the reach within a certain margin. The distance

protection zone reaches vary with the switching state of the parallel line configuration.

The important cases are:

1. Parallel line switched off and earthed at both ends.

2. Parallel switches off and unearthed or earthed only at one end.

3. Both lines in service.

In case 1, lowest impedance is measured and in case 3 highest impedance is

measured.

For relay with common Kn setting following are CIGRE recommendations:

1. Avoid earthing at both ends of the disconnected line (earthing only at one

side).

2. Set zone 1 to 80%.

With the above setting, the reach for 1 phase to ground fault will be 70% for case

3. Zone 2 should have safety margin of about 15 to 20% over reach over next station.

In the case of a parallel line switched off and earthed at both ends is to be

considered, Kn will have to be different from the above value.

3.2.4 Technical data:

Rated frequency Fn : 50 Hz.

Rated voltage Vn : 100 – 110V+/- 10%.

Rated current Cn : 1A, 2A or 5A.

Maximum permitted continuous current : 3 x in.

Operating time (over current starting) : min 20msec.

Weight : 11 Kg.

4. Conventional Relays

4.1 FUSE FAILURE REALY:

The FFR relay operates for single phase two and three phase fuse failures in the supervised

circuits and for an interruption in any of the cables or their associated fuses. When a fuse blows the

relay operates fast enough to inhibit the undesired operation of high speed distance relays that

could other wise occur. The FFR can be connected to a voltage transformer secondary.

Features:

1. Detects single, two or three phase fuse failure in less than 8 msec.

2. Operates fast to block high speed distance relay tripping in case of fuse failure.

3. Needs no auxiliary power supply.

4. Output contacts for alarm.

5. LED Indicated operation.

DESIGN:

FFR makes use of the principle of instantaneous amplitude comparison of two voltages

this is achieved with two voltages, one before the fuse and the other after the fuse. These two are

compared in a transformer having a common magnetic loop.

Under normal operating conditions the two voltages give rise to zero resultant voltage,

thus no operation of the relay. When the fuse blows out, the neutralizing effect is lost hence the

relay operates. This operation is achieved by means of a special transformer, which not only

provides isolation between the main PT circuit and the electronic circuit, but also becomes an

instantaneous amplitude comparator, it has two primary windings and secondary stepped down to

the required operating voltage of the auxiliary relay. The common point of the two windings is

connected to the neutral through a series resistor.

4.2 HIGH SPEED TRIPPING RELAY

The RXMS1 auxiliary relay is especially suited for ultra high speed circuit breaker tripping.

The six contact relay is recommended for multiple tripping applications like in bus or transformer

protection where more than one breaker must be tripped. The negligible bounce time is important

in breaker failure initiation application to assure consistency of timing.

DESIGN

The RXMX1 relay is high speed, hinged armature design with up to 6 medium duty contacts.

This relay includes a resister in series with the coil, so no external series resistor is required. It is

available in a 6.8W(3.5 – 4ms) standard version A and a low power 3.3W (5 – 5.5ms) version B the

relay has an additional terminal brought out between the coil and the series resistor. This terminal

can also be used for dropout delay by connecting a diode across the coil.

4.3 POWER SWING BLOCKING RELAY

During a power swing the change of impedance is lower than during a fault on the power

system. The RANZP relay has two impedance-measuring elements; ZP2 and ZP1 in case of power

swing, the RANZP measures the time difference between the impedance operating characteristic ZP2

and ZP1. If the time is longer than 50ms the power swing blocking relay operates and the output

signal is maintaining for approximately 2 sec. The RANZP relay is normally used for blocking of the

distance relay, or if desirable, for direct tripping.

DESIGN

The RANZP single phase power swing blocking relay consists of a dc-dc converter type

RXTUG2H, an impedance relay type RXZF21 and a power swing blocking unit type RXDA2 the

impedance relay type RXZF21 is provided with an 80 characteristics the ratio between the major axis

and the minor axis is 2/1 the relay provides input signals to the power swing blocking unit as well as

its own impedance measuring element. The set operate value on the impedance relay corresponds

to the outer oval ZP2 the inner oval ZP1 is fixed at 0.8 times the set value of ZP2 the power swing

blocking unit contains a timing circuit and an electromechanical output relay with a target and 2

medium duty contacts; 1 transfer and 1no. the necessary auxiliary voltage is obtained from the dc-dc

converter.

TECHNICAL DATA

Rated Frequency - 50/60 Hz

Rated Voltage - 110 V

Current setting - Is Re-connectable 1,2/5A

Setting Range:

Outer Oval – ZP2

Resistive reach

Is = 1A 10-87

2A 5-43.5

5A 2-17.5

Ratio:

Major Axis/ Minor Axis = 2/1

Inner Oval ZP1 = 0.8 times ZP2

Characteristic Angle = 800

Consistency = 10%

Reset Ratio < 105%

Minimum Operating Current = 0.2Is

Power Consumption:

Voltage Circuit < 1.2 VA

Current Circuit < .15 VA

Auxiliary Voltage Circuit < 7W

Auxiliary Voltage DC - 110, 125, 220/250 Volts

Permitted Ambient Temperature -50 to 550 C

4.4 TRIP CIRCUIT SUPERVISION RELAY [TSR]:

The trip circuit supervision relay (TSR) is used to supervise the trip circuit and this provides

alarm for:

1. Loss of DC Supply.

2. Faults on the trip coil or cables independent of the breaker position.

3. For faults on the breaker auxiliary contacts.

4. For faults on the supervision relay itself.

Design:

The TSR relay consists of an electromechanical auxiliary relay with an RC circuit in parallel for

drop out delay. In series with the auxiliary relay coil there is a light emitting diode which lights up

when the trip circuit is healthy. When the circuit breaker is opened, a current of about 15ma flows

through the LED and the auxiliary relay via terminal 13 and one contact of the circuit breaker trip

coil. When this circuit is open or if the DC supply is lost, the auxiliary relay will drop out after 0.3 sec

delay and give an alarm via the contacts of the relay.

When the breaker is closed the TSR will supervise the trip circuit vi the terminal 21. If

the protective relay trips the breaker, the protective relay contact will short circuit the supply to TSR.

The circuit breaker will open after a certain time and its auxiliary contacts will switch over. The

protective relay contact will also open TSR will be supplied via terminal 13 and additional supervision

wire.

Technical data :

Rated voltage : 220/ 110 VDC.

Voltage variation : 80-110% of Un.

Maximum system voltage : 250 VAC/ 300VDC.

Maximum load current : 5 Amps.

Weight : 0.4 Kg.

5. TRANSFORMER PROTECTION RELAYS

A power transformer is subjected to various faults throughout its life time. The protection of power transformer should be effective enough to isolate the power transformer from internal and external faults of the transformer minimizing the fault clearing time as well as increasing the life time of transformer. The protection incorporated for an oil immersed type power transformer is as below.

5.1 Gas operated relays: 5.1.1 Buchholz relay: The gas actuated Buchholz relay is a protective device designed to give indication of

faults occurring in oil field conservator type transformers, on load tap changers. All types of

faults occurring within an oil filled transformers are accompanied by gas generation. This

phenomenon has been effectively utilized by buchholz relays to provide the best known

protection arrangement for transformers.

The high sensitivity and capability of the relay has been successfully proved to

detect faults stated below.

Defective core laminations.

Breakdown of core insulations.

Local overheating of windings.

Phase to phase, phase to earth or internal short circuits.

Insulation breakdown of major nature.

Protection of transformer

Against Internal Faults Against Through Faults

Over current and Earth Fault Protection

Differential Protection Gas and Surge Operated Relays Buchholz relay Oil Surge Relay Pressure Relief Valve

Construction:

The relay comprises of flanged housing detachable front cover, terminal, box,

toughened glass windows, alarm and tripping device with mercury switches and valves for

venting and relay setting. The alarm and the tripping device consist of 2 counter

balanced aluminum buckets which are hinged and the tilting of these buckets operates the

mercury switches.

Air release valve is provided at the top of the housing for releasing the trapped air

and for taking out the gas samples. One more valve is provided for introducing air inside the

relay to carry out the gas volume and surge tests

The front inspection glass is provided with scale to allow reading the accumulated

volume of the gas and observe color of the gas for fault analysis.IN Service/Test lock can be

selected in the form of movement of red indicator on the reading scale.

Fig 5.3: Buchholz relayOperation:

When any fault occurs the gas formation takes place in the transformer and

accumulates in the Buchholz relay on its way to conservator .In consequence, oil level in the

relay drops and the upper counter balanced and hinged bucket moves down tilting the

mercury switch to activate alarm circuit. If the oil level drops further lower bucket also

operates and closes the contact for tip circuit.

Commissioning:

While mounting the relay care should be taken to see that the arrow on the relay

is pointing towards the conservator and the air vent valve at top. Ensure that pipe ascends to

conservator at angle between 10 -90 as per specification of manufacturer & the relay is kept in

service position.

After installation of the relay and when the transformer has been filled with oil

up to the oil conservator the air trapped in the gas chamber must be allowed to escape

through the air vent valve at top.

Gas Analysis:

Depending on the nature of fault i.e., with winding, paper insulation, oil flashover in

the transformer the nature of gases formed will vary. The type of fault formed in the

transformer can be detected by diagnosing the gases collected in the Buchholz relay.

Gases collected in the Buchholz relay is allowed to pass through a test tube filled with

Silver Nitrate (AgNO3) solution by releasing the Air release plug .The precipitate formed on

the walls of the test tube is observed for diagnosing the fault in the transformer as stated

below.

Diagnosing the fault of transformer by precipitate formed

Nature of gas Probable fault

Colorless and odorless Air trapped in oil or insulation.

Grayish white with pungent smell, non

flammableover heating of insulation, press board

Yellowish inflammable Decomposing of wood insulation

Dark grey inflammable

Flash over in oil or due to excessive

overheating of oil caused by a fault in the

winding or core.

5.1.2 Oil surge relay:

This relay is sensitive to both low oil level and oil surge conditions. It is usually

installed for protection against faults in the On Load Tap Changer.

The housing is provided with two flanges for connecting pipes leading to the tap

changer head and to the oil conservator. The flap valve position can be checked through the

inspection glass on front of the housing.

For checking the tripping function of the relays well as subsequently resting the flap

valve, two push buttons are installed in the terminal box .One pushbutton for actuating the

trip contact manually and the other pushbutton for resetting the trip contact.

Fig 5.4: Oil Surge Relay

Operation:

The protective relay is energized by an oil surges from the tap changer to the oil

conservator only. The oil flow operates the flap valve being trapped in to the OFF position.

At that moment the contact is actuated, the circuit breakers are operated and the transformer

is switched OFF the line. It is not energized by the tap changer being subject to nominal load

or permissible overload.

Commissioning:

The protective relay has to be mounted in the pipe leading from the tap changer head

to the oil conservator. The relay must be located as near as possible to the tap changer head.

Pipe work rising to the conservator should be arranged at an angle of 50 above the horizontal

to ensure the effective operation of the protective relay with the test push buttons in the top of

the housing. The arrow on the terminal box cover must point towards the oil conservator.

5.1.3 Pressure relief valve:

The pressure relief valve is a protective device for oil filled transformers. It is

designed to relieve the excessive pressures which may build up by a fault or an arcing inside

the transformer.

Parts of Pressure Relief Valve

Fig 5.3: Pressure Relief Valve

When the pressure inside the tank exceeds a preset limit, acts on the diaphragm

inside which in turn causes the diaphragm to lift up 4m from its seat. A movable rod attached

to this diaphragm operates a micro switch and makes the transformer to switch OFF. This

lifting is instantaneous and allows venting off the excess pressures inside the tank

It also gives a visual indication of the valve operation by raising a flag.

The diaphragm resets to its original position as soon as the pressure inside the tank

drops below the set limit.

S.No Description1 Base2 Gasket3 O Ring4 Diaphragm5 Cover6 Springs7 Rod retaining spring8 Lock nut9 Switch operating rod10 Visual indicator11 Stopper12 Visual indicator

5.2 Differential Protection:

Differential protection also called as unit protection, is the Main Protection of Power Transformer as it only operates for faults on the unit it is protecting, which is situated between the CTs. The relay therefore can be instantaneous in operation, as it does not have to coordinate with any other relay on the network. This Differential protection, as its name implies, compares currents entering and leaving the protected zone and operates when the differential current between these currents exceed a pre-determined level. Differential relays that compare voltage inputs are used for EHV capacitor protection.

The type of differential scheme normally applied to a transformer is called the current

balance or circulating current scheme as shown in Figure 5.6

Circulating current scheme in the case of through faults

The CTs and relay are connected in such a way that the relay is operated by

circulating current between secondary of CT. Differential current does not arise and hence

the relay does not operate for external.

Under internal fault conditions (i.e. faults between the CTs) the relay operates since

both the CT secondary currents add up and pass through the relay as seen in Fig: 5.7

Circulating current scheme in the case of internal faults

Unfortunately the circulating current protection mentioned above may operate even

for through faults due to the following factors which need careful consideration:

(a) Transformer vector group (i.e. phase shift between HV & LV) (b) The possibility of zero sequence current entering the relay may destabilize

the differential for an External earth fault. (c) Magnetizing in-rush currents (from one side only) (d) Mismatch of HV and LV CTs (e) Varying currents due to on-load tap changer (OLTC).

Factor (a) can be overcome by connecting the HV and LV CTs in star/delta

respectively (Or vice versa) opposite to the vector group connections of the primary

windings, so counteracting the effect of the phase shift through the transformer.

The delta connection of CTs provides a path for circulating zero sequence current,

thereby stabilizing the protection for an external earth fault as required by factor (b).

NOTE: To counter factors (a) & (b), the Thumb rule for differential protection is that

the CTs are to be connected in star for delta connected winding, and vice versa.

As the magnetizing current in-rush is predominantly 2nd, harmonic filters are utilized

to stabilize the protection for this condition (c).

And Finally, It is necessary to bias the differential relay to overcome the current

unbalances caused by factor (d) & (e) i.e. mismatch of CTs and taps of OLTC respectively.

And Hence a Percentage Biased Differential relay is preferred instead of a circulating current

principle.

The Percentage Bias Differential Protection applied for an YNyn0 Power Transformer

is mentioned in figure 1 :

Figure 1: differential protection of a transformer Figure 2: .Differential Realy opearting chareceristcs

Under-load or through-fault conditions, the CT secondary currents circulate, passing

through the bias windings to stabilize the relay, whilst only small out-of-balance spill currents

will flow through the operate coil, not enough to cause operation. In fact the higher the

circulating current the higher will be the spill current required for tripping the realy

Most transformer differential relays have a bias slope setting of 20%, 30% and 40%

as shown in figure 2. The desired setting is dictated by the operating range of the OLTC,

which is responsible for the biggest current unbalance under healthy conditions;

BACKUP PROTECTION:

5.3 Over Current and Earth Fault Protection:

Over current and Earth Fault protection is the Back Up protection for power

transformer. Unlike Differential Protection which operates for internal faults (between CTs),

Over current protection will operate even for through faults, above the pickup current setting,

thus isolating the transformer from feeding the fault.

Over current and earth fault protection is provided on both HV and LV Side of the

winding and hence proper time gradation is to be provided for relay operation starting from

the feeders connected to LV side of transformer to the Over Current protection on HV Side,

to avoid complete black out for an fault on feeder connected to LV Side of transformer.

To achieve selectivity and coordination by time grading two philosophies are

available, namely:

1. Definite time lag (DTL), or

2. Inverse definite minimum time (IDMT).

(1).Definite Time Characteristic:

The relays are graded using a definite time interval of approximately 0.5 s. The relay

R3 at the extremity of the network is set to operate in the fastest possible time, whilst its

upstream relay R2 is set 0.5 s higher. Relay operating times increase sequentially at 0.5 s

intervals on each section moving back towards the source as shown in Fig.5.10

Definite Time Relay protection

Draw Back:

The problem with this philosophy is, the closer the fault to the source the higher the

Fault current, the slower the clearing time – exactly the opposite to what we should be Trying

to achieve.

(2)Inverse Time Characteristic:

A relay having IDMT Characteristic will incorporate lower operating time for higher

fault currents. Various inverse characteristic curves can be selected among Normal Inverse,

Very Normal Inverse, and Extreme inverse curves. For a radial feeder, time grading by means

of relays having IDMT characteristics is shown in fig: 5.11

IDMT relay protection

IEC Standard Normal Inverse 3.0 sec curve is used for the Over current and Earth

Fault protection of power transformer and the characteristic curve can be expressed as:

Relay Operating time (in secs) = (0.14*TL) / (PSM0.02-1)

Where TL=Time Setting Multiplier adopted.

Plug Setting Multiplier = Fault current on CT Primary

(CTRatioAdopted)*(CurrentSettingAdoptedon relay)

OverCurrent & Earth Fault protection for a transformer

For an HV winding fault the HV breaker is tripped but the fault can continue to be

fed via the Low voltage side, the back-feed coming from the adjacent transformer(s) as

shown in Fig 5.12,where the LV Protection is set high to coordinate with downstream

requirements. Thus, the Transformer Protection should always trip both HV and LV circuit

breakers for operation of HV Side IDMT Relay.

Protection should trip HV and LV breakers

6. PRACTICAL RELAYS

6.1 ABB RELAYS

6.1.1 RAEDK

The RAEDK is an over/under voltage protection assembly and its features include: Wide setting range High setting resolution Low burden on the input voltage transformer Built in timing ranges Selectable inverse or definite time High reset ratio High continuous overload rating and built in LED indication of operation Starting and tripping of the two stages. The possibility of specifying built-in frequency filters enhances the application range. E.g., there are versions to measure voltages of specific frequencies occurring in the power system, e.g. 150-180 Hz. A band-pass filter for 50-60 Hz is also available. Products based on the RXEDK 2H measuring relays are available as assemblies of measurement relays. Versions having optional additional devices are also available. E.g. a test switch type RTXP 18 is available as an option, a DC-DC converter RXTUG 22H and a tripping relay RXME 18 are other standard options. Design:The over/under-voltage protection type RAEDK is designed in a number of variants for one, two- or three-phase voltage protection. Each protection assembly is available with or without test switch RTXP 18, DC-DC converter RXTUG 22H or tripping relay RXME 18. All protection assemblies are built up by modules in the COMBIFLEX modular system and mounted on apparatus bars. The electrical connections to the protection assemblies are made by leads equipped with COMBIFLEX sockets. The over/under-voltage protection type RAEDK is designed in a number of variants for one-, two- or three-phase voltage protection.DC-DC converter: The DC-DC converter RXTUG 22H converts the applied battery voltage to an alternating voltage which is then transformed, rectified, smoothed and in this application regulated to ±24 V DC. The auxiliary voltage is in that way adapted to the measuring relays. In addition, the input and output voltages will be galvanically separated, which contributes to damping of possible transients in the auxiliary voltage supply to the measuring relays.Measuring relay: The time over/under-voltage relay RXEDK 2H is a static microprocessor based relay with two voltage stages U1 and U2. It consists mainly of an input transformer for voltage adaption and isolation, filter circuits, digital- analog converter, microprocessor, MMI consisting of programming switches and potentiometers and LEDs for start, trip and in

service indications, and three output relays, each with a change-over contact, for the start and trip functions of stage U1 and for the trip function of stage U2 respectively. The relay has also a binary input for remote resetting of the LED indications for “U1 Trip” and “U2 Trip”.Tripping relay: The auxiliary relay RXME 18 can be included as a tripping relay when heavy duty contacts are required. It has two heavy duty make contacts and a red flag. The flag will be visible when the armature picks-up. The flag is manually reset by a knob in the front of the relay. Typical relay operate time is 35 ms.

Applications: Voltage relays are sometimes used to switch capacitors in and out (VAR control) for voltage regulation in power systems. It is not uncommon to use voltage relays for load-shedding applications. Upon a given percentage undervoltage a certain load is shed in order to try to maintain the normal system operating voltage level and thus preventing a system collapse. Two voltage relays may be used to provide selection of neutral point voltage for example when there is a double bus system and to connect the selected voltage to protective relays requiring neutral point voltage for operation. A single voltage relay may be used to prevent motor starting in case of low voltage on the bus. The RXEDK 2H relay provides two setting levels which may be used independently for different “control” functions.

Equipment Ratings:

6.1.2 GPU2000R The GPU2000R is a Generator Protection Unit in the proven line of 2000R series relays. The 589T series is designed to provide primary and/or backup protection for small to medium size generators, and the 589V/589W series are suitable for synchronous generators of any size. Utilizing three advanced microprocessors, the GPU2000R provides multifunction protection, expansive fault and operations records, detailed metering, programmable inputs and outputs, and advanced communications options. These new series units also provide improved performance compared to the earlier 589R series: The sensitivity of the reverse

power anti-motoring element is now down to 0.2% of rated machine power; the loss-of-excitation function is now accomplished by a mho-circle impedance element.Features• Complete Multifunction Protection• Programmable logic inputs (8) and outputs (6)• A 4-line by 20-character liquid crystal display provides easy access to metering, records, testing and settings• Electrically Isolated Communication Ports provide superior remote communications• Simultaneous communication through front and rear ports via dedicated microprocessors• Continuous self-diagnostics• Machine Running Timers and Alarms• Flash memory technology provides for quick and easy updating to latest software enhancements.

Ratings and TolerancesThe following are the ratings and tolerances of the GPU-2000R. Current Input Circuits• 5-A input rating, 16 A continuous and 450 A for 1 second• 1-A input rating, 3 A continuous and 100 A for 1 second• Input burden 0.245 VA at 5 A (2 - 8A range)• Input burden 0.014 VA at 1 A (0.4 - 1.6A range)• Frequency 50 or 60 Hz Contact Input Circuits Voltage Range• 24 Vdc model: 12 V to 140 Vdc• Other models: 24 V to 280 Vdc Voltage Input Circuits Voltage ratings based on the VT connection configuration setting.BURDEN• 0.04 VA for V(A-N) at 120 VacVOLTAGE• Wye Connection: 160 V continuous and 480 V for 10 seconds• Open-Delta Connection: 260 V continuous and 480 V for 10 seconds• Vo Input (termininals 35-36) 160 V continuous and 480 V for 10 secondsContact Input Circuits (Input Burden)• 2.10 VA at 220 Vdc and 250 Vdc• 0.52 VA at 125 Vdc and 110 Vdc• 0.08 VA at 48 Vdc• 0.02 VA at 24 Vdc Control Power Requirements• 48 Vdc model, range = 38 to 58 Vdc• 110/125/220/250 Vdc models, range = 70 to 280 Vdc• 24 Vdc model, range = 14 to 29 Vdc Control Power Burden• 24 Vdc = 0.7A max @ 19 V

• 48 Vdc = 0.35A max @ 38 V• 110/125 Vdc = 0.25A max @ 70 V• 220/250 Vdc = 0.10A max @ 250 VOutput Contacts Ratings 125 Vdc 250 Vdc• 30 A tripping 30 A tripping• 6 A continuous 6 A continuous• 0.25 A break inductive 0.1 A break inductive

6.2 EASUN REYROLLE Relays

6.2.1 Numerical Overcurrent Protection relay [ MIT 103/104/113/114 ]

The MIT overcurrent combine the power and flexibility of microcontroller technology with decades of experience in the field of protection. MIT has three pole version with two phase & one earth element and four pole version with 3 phase and one earth element. MIT is housed in a compact 2/3V draw-out vendetta case. Settings can be done by a readable Human Machine Interface which also serves for fault indication.

Design:

The MIT 103/104/113/114 Protection unit consists of the following modules within its compact dimensions.

• Input Module

• Power Supply and Output Relay module

• Measuring Module

• Front Fascia

The three modules viz. Input, Power supply and Measuring modules are plugged into the Front fascia which houses switches, LEDs and LED display for the human machine interface. All PCBs are well protected from one another and from external environment with best shielding for better electromagnetic compatibility and housed in the enclosed chassis, which is with drawable from the outer case. The drawout case is provided with the required CT shorting contacts. The relay has 3 or 4 input current transformers. The output from the current transformer is transformed to an equivalent voltage and sampled at the rate of 16 samples per cycle and digitized by means of analog to digital converter.

The digitized signals are processed numerically by means of microcontroller to effectively remove the DC components and to drive the Root Mean Square (RMS) value of the input signal. Based on this and the settings, decisions are taken to either elapse the time for inverse characteristics or definite time to operate the trip output relay. Where highset is included (which can be set OFF) an instantaneous tripping is effected once the current exceeds the set value.

Control:

• The basic relay has output relay with 2 N/O and 1 N/C contacts as standard

Trip 1 N/O

Alarm 1 N/O

Protection unhealthy 1 N/C

• Additional 5 output contacts can be given as follows: Starter 1 C/O

IDMTL Phase fault 1 N/O

IDMTL Earth fault 1 N/O

Highset Phase fault 1 N/O

Highset Earth fault 1 N/O

Other Features:

• 3 pole and 4 pole versions

• Both 1A & 5A CT inputs in one relay

• Wide range for current and time settings

• Both DC & AC auxiliary supply available

• Drawout modular case

• Compact design

• Non-volatile memory for trip indication

Wiring diagram:;

6.1.2 Restricted Earth Fault Relay [ EB3 ]

The relay uses a type B61 attracted armature element energized via a low pass filter circuit and a full wave rectifier. The relay has a preset setting of 15V. Other resistors are introduced into the circuit to provide the voltage setting range up to 270V in increments of 5V using heavy duty DIL switches. Included within the relays, are the essential non-linear resistors to limit the peak voltage output from saturated CT’s, these resistors protect the C.T. insulation and secondary wiring.

Current Transformer Requirements

Experience has shown that most protective C.T.s are suitable for use with the high impedance relays and where the C.T.s are specially designed for this protection, their overall size may be smaller than that required for an alternative current balance protection. The basic requirements are:

a) All C.T.s should have identical turns ratios, if possible.

b) The knee point voltage of each C.T. should be at least 2xVs. The knee point voltage is expressed as the voltage applied to the secondary circuit with the primary open circuit which when increased by 10% causes the magnetizing current to increase by 50%.

c) C.T.s should be of the low leakage reactance type. Most modern C.T.s are of this type and there is no difficulty in meeting this requirement. A low leakage reactance C.T. has a jointless ring type core, the secondary winding evenly distributed along the whole length of the magnetic circuit and the primary conductor passes through the approximate centre of the core.

Applications

Type 5B3 electromagnetic relay is ideal for restricted earth fault protection of transformer windings or phase and earth fault protection of reactors and the stator windings of large machines. This relay may also be used for high impedance busbar protection. High impedance schemes have the advantages over low impedance schemes that, a more sensitive setting can be obtained without any loss of stability and the primary fault setting calculation is simpler.

Current operated schemes are more susceptible to mal-operations from through faults, unless greater care is taken with the selection of the current transformer. For some restricted earth fault applications the primary fault setting needs to be greater at harmonic frequencies than the setting at the fundamental frequency. The 5B3 relay uses a low pass filter circuit to achieve this. No adverse reduction in fault setting can occur with the high frequency current, which may be produced during switching.

Stability:

For stability the voltage setting of the relay must be made equal to or exceed the highest value of V calculated below:

V=I(Rct + RL)

Where, RL = The largest value of pilot-loop resistance between the C.T.s and the relay

Rct = The secondary winding resistance of the C.T

I = The C.T. secondary current corresponding to the maximum steady state through fault current of the protected equipment.

Technical Information:

Frequency fn. : 50Hz.

Settings

Current Is : Fixed at 20mA.

Voltage Vs : 15V to 270V in steps of 5V

Thermal withstand : Continuous 1.25 x Vs

Burden : Vs x 20mA. (approx)

Operating Time : 45ms maximum at 3 x Vs

Indication : Hand reset flag.

Contact Arrangement : 3 normally open self reset.

Contact Rating : Contacts are capable of making and carrying 6.6 KVA for 0.2 seconds with a maximum of 30A. Contacts are intended for use in circuits where a circuit breaker auxiliary switch breaks the trip coil current.

Wiring Diagram:

7. CONCLUSION:

By making use of the above mentioned relays we can ensure reliable and efficient protection to our Transmission lines, Transformers and Generators. From the above discussion we came to know that Relays are HEART of the protection equipment. Here we discussed about different types of possible protections and the operating principles of

protection relays. By using the above discussed protection Relays, short circuit currents which may cause heavy damage to equipment and would also cause intolerable interruption of service to customers can be prevented. Thereby the reliability of the equipment used in transmission and generation can be increased which in turn decreases the cost occurred due to failure of equipment.

8. REFERENCES:1) GOOGLE2) “Switchgear protection and power systems” by SUNIL S.RAO3) “Electrical power systems” by C.L. WADHWA

Protective Relays

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