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ELECTRIC POWER AND MACHINES DEPARTMENT

Faculty of Engineering

Cairo University

Protection Systems and Devices

(Relays)

Prepared by

Prof. Hany M. Amin Elghazaly

2009


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Protection Systems & Devices (Relays)

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CHAPTER 1Introduction

Protective relays are the devices that provide protection against faults, such as shortcircuits, and abnormal system conditions, such as low frequency, to avoid seriousdamage to vital pieces of equipment such as lines, transformers, and generators. Theprotective relay system detects the fault and sends trip signals to circuit breakers andother switchgear, while the switchgear clears the fault by interrupting it and isolating thefaulty equipment. Although it is desirable to limit damage to the equipment subjected tothe fault, the overriding concern is to protect the rest of the system from the fault. Forexample, a line subjected to a short circuit will often suffer damage if the short circuit isnot promptly cleared. Relays that are fast, selective, and reliable along with fast, reliablecircuit breakers will often prevent such damage, and even more importantly will preventthe damage from spreading to the substation bus and the transformer. Obviously, theprotective relay system must be carefully designed to achieve the proper balance among

factors such as reliability, speed, selectivity, and economics.

One aspect of protection, namely transient overvoltage protection by application of surgearresters, is not covered in this course. Rather, the present topic focuses on protectiverelay systems for overcurrent protection and protection from other abnormal conditions.An overvoltage relay might be used to protect some apparatus from sustainedovervoltage condition, but not to protect the apparatus from a transient surge due toswitching or lightning.

Importance of protection

Protection of the system from damaging short circuit currents is obviously important, but tounderscore its importance consider that an extensive overhead transmission system may besubject to temporary faults due to lightning-induced flashover and permanent faults due tophysical damage from ice and wind loading as well as accidental destruction of poles andtowers.The frequency of these faults is obviously a function of the lines' overall exposure to thedamage, but faults may occur several times a day during normal conditions. In the worstcases of storm damage, hundreds of faults may occur in a few hours.

Also of great importance is protection from various abnormal system conditions. Forexample, severe damage to steam turbine blades can occur at low system frequencies,generator step-up transformers may be overexcited at very low frequency operation as theunit starts up, certain relays may respond to transient generator swings during disturbancesresulting in transmission line tripping. All these, and other, conditions must be foreseen bythe protective relay engineer.

Electric power equipment is designed to work under specific normal conditions. However,short circuit or failure may happen due to: Over-voltages due to switching. Over-voltages due to lightning strokes. Bridging of conductors by birds.

Breakdown due to decrease of dielectric strength. Mechanical damage of the equipment.


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Protective Systems

It continuously monitors the power system to ensure maximum continuity of electricitysupply with minimum damage to life, equipment & properties.

It consists of : 1. Battery and DC supply.2. Circuit Breakers.3. Relay.4. Current Transformers and Potential Transformers.

The Protective Relay

Its a device, which detects abnormal conditions in a part of a power system and gives asignal to isolate that part from the healthy system or gives an alarm to the operator.The relays are: compact, self-contained devices, which respond to abnormal conditions.

Basic Requirements of Protective Relaying

A well designed and efficient protective relaying should have:

1. Speed:

Protective relaying should disconnect a faulty element as quickly as possible to:

improve power system stability. decrease the amount of damage.


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Clearing time: It is the time interval within which a faulty system element is disconnectedfrom the system.

Fault clearing time = Relay time + Circuit breaker time

Relay time: operating time of the protection relay from the instant of fault up to the closureof contacts in the trip circuit of the circuit breaker.

CB time: is the time from the closing of the trip circuit up to the time when the current isinterrupted (final arc extinction of the circuit breaker)

Relaying Classifications:

1. Instantaneous: these relays operate as soon as a secure decision made. Nointentional time delay is introduced to slow down the relay response (1 6 cycles).

2. Time delay: an intentional time delay is inserted between the relay decision time andthe initiation of the trip action.

a) High speed: a relay that operates in less than a specified time. The specified time inpresent practice is 50 milliseconds (2 3 cycles).

b) Ultra high speed: a relay that operates in 4 milliseconds or less.

I) Definite time relay (Instantaneous): the time of operation is fixed and not function ofthe quantity causing operation.

II) Inverse time lag relay: the time operation is inversely proportional to the magnitude ofthe quantity causing operation. The relay must separate the meaningful and significant

information with the necessary degree of certainty. The relationship between the relayresponse time and its degree of certainty is an inverse one.

Time-current characteristics of various families of overcurrent relays


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2. Selectivity, discrimination

Protective relay systems that are well designed will always exhibit selectivity, which meansthat the fewest possible numbers of relays will operate for a given fault. To assure that theprotective system is selective, relay coordination studies must be performed.

Much of the time expended by the relay engineer consists of coordinating the operation ofadjacent relays to ensure that the proper relays operate, but that those covering adjacentzones do not. To complicate this coordination, often-nearby relays will have a secondaryrole as backup protection. Much more will be said about backup protection and itscoordination in the course.

Selectivity is the ability of the system to determine the point at which the fault occurs andselect the nearest of the circuit-breakers tripping of which will lead to clearing of fault withminimum or no damage to the system.The protective system should operate under normal conditions and abnormal conditions. It

should select and disconnect only faulty part without disconnecting the remaining healthyparts.

Discrimination

The protection of any zone is said to discriminate when it can distinguish between aninternal fault in that zone and an external (through) fault in any other zone. The protectionshould trip for an internal fault but restrain from tripping for an external fault. The protectionshould not trip for any load current.

Zones of protection:

In the event of a fault in a zone, the protection of that zone should initiate the tripping of thenecessary circuit breakers to isolate that zone, and only that zone, from all live supplies

The zone of protection of a relay consists of that part of the system covered by the relay.One basic tenet of good protective relay practice is to maintain overlapping zones ofprotection over the entire system. As an example, consider a generator, its step-uptransformer, a line, and a substation. Note that the zones of protection and the switchgearlocations are interrelated. As in many design problems, the exact arrangement of the zonesof protection will depend on the design philosophy of the engineers involved.

The power system is divided into protective zones, which can adequately be protected with

minimum part of the system disconnection. Any failure occurring within a given zone willcause the opening of all breakers within that zone.


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The system can be divided into the following protective zones:

1. Generators. 2. Transformers. 3. Bus bars.4. Transmission lines. 5. Distribution circuits.

The boundaries of the protective zone are decided by C.T. locations.In order to cover all power equipment by their protection systems, the zones of protection

must meet these requirements:

a. All power system elements must be covered by at least one zone.b. Zones of protection must overlap to prevent any system element from being

unprotected.

A zone of protection may be closed or open. When the zone is closed, all power elementsinside the zone are protected. All the circuit breakers inside the zone must trip.

Consider a fault at F1. This fault lies in a closed zone and will cause C.B.s ( B1 ) and ( B2 ) totrip. The fault at F2, being inside the overlap between the zones of protection of the T.L. and

the B.B. , will cause B1, B2, B3 & B4 to trip.Now consider a fault at F3. This fault lies in two open zones. The fault should cause B6 to

trip. B5 is the backup and will trip if B6 fails to clear the fault (fails to trip).

3. Sensitivity

A protective system is said to be sensitive when it will operate for very small internal faultcurrents. If an overhead conductor breaks and falls on dry ground or hedges, the faultcurrent can be very small, and it is quite a problem to provide a protection sufficiently

current-sensitive to detect this fault condition.

Sensitivity is the smallest value of actuating quantity at which the protection startsoperating in relation with the minimum value of fault current in the protected zone.

Sensitivity can be defined in terms of sensitivity factor Ks where:Ks = Is or Vs

Io Vo

Where Is= minimum short circuit current in the zone

Io = minimum operating current of protection

The operating current should not be kept too small for the following reasons: The protection should not operate on maximum loads.


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The protection should not operate for faults somewhere else in the system.

Stability

A protective system is said to be stable when it will restrain from tripping for a large external

fault current. The system i should be stable up to the maximum fault current liable to flowthrough the zone. A typical specification might require stability up to 20 times the ratedprimary current of the C. T .s. Rated stability limit is given in terms of the r.m.s. value of thesymmetrical component of external fault current.

The terms stability and sensitivity are relative terms. A protective system can be verysensitive but not stable enough. Ideally it should be very sensitive and very stable (up toreasonable limits) but often the conditions for achieving these two ideal states are mutuallyincompatible.

4. Reliability

In the event of a fault in a zone, the protection of that zone should initiate the tripping of thenecessary circuit breakers to isolate that zone, and only that zone, from all live supplies. If itdoes not do so, the protection is said to maloperate and it should be noted that, for statisticalpurposes, the operation is either perfectly correct, or wrong. Reliability covers the correctdesign, installation and maintenance of all C.T.s, V.T.s (voltage transformers), relays, a.c.and d.c. wiring; also the accidental tripping of relays due to mistakes by personnel (this is amaloperation). For the whole of the electricity supply system protection is at least 95 %reliable.

Reliability is generally understood to measure the degree of certainty that a piece ofequipment will perform as intended. The relays have two alternative ways in which they canbe unreliable: they may fail to operate when they are expected to, or, they may operatewhen they are not expected to. Therefore, a reliable relaying system must be dependableand secure. Therefore, the reliability of the protective relay system has two aspects:dependability of operation and security from false operation.

Dependability means that each relay sends a trip signal when a fault is present in its zone.It is defined as the measure of the certainty that the relays will operate correctly for all thefaults for which they are designed to operate

Security means that no relay sends a trip signal if no fault is present in its zone. It is themeasure of the certainty that the relays will not operate incorrectly for any fault.

Since no human invention is perfect, and the protective relay system is no exception,compromise between dependability and security are inevitable. Lack of dependability meansthat faults are not cleared, unless backup protection is active (which usually involves aconsiderable time delay to allow coordination between backup and primary relays). Lack ofsecurity means that false trips may occur, leading to unnecessary customer outages.

As the relaying system becomes dependable, its tendency to become less secure increases.Thus, in the present day, there is a tendency to design relays that are more dependable atthe expense of some degree of security. Much of the art of protective relaying arisesbecause of the tension between dependability and security. A typical problem is to choose


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between two available protection schemes, the one having better dependability and worsesecurity, the other having better security and worse dependability.

Primary and backup protection

It should be obvious that some form of backup protection is needed, especially fortransmission and generation levels of the system. Since protective relays do fail and sincecompromises in protection are sometimes required, backup relays will be necessary for anyimportant line, transformer, or generator.

Main protection is the system, which is normally expected to operate in the event of aninternal fault.

Back-up protection is a second (often cheaper, slower) protective system whichsupplements the main protection should the latter fail to operate. The trip contacts of therelays are in parallel. The failure to clear the fault could be due to some component common

to both systems (e.g. the circuit breaker), so most schemes provide overall back-up to clearthe fault at another circuit breaker.

The primary protection is the first line of defense at which primary relays clear faults in theprotected zone as fast as possible. As 100% reliability not only of the protective scheme butalso of the associated C.T.s, P.T.s nad C.B.s cannot be guaranteed, some form of backupprotection must be provided.

The backup relay operates if the primary relay fails. Usually back up relays cover widersections and have time delay long enough to allow the primary relay to operate. This can beaccomplished by:

1. Duplication principle which the important protective devices (relays, C.B.,auxiliaries, ..etc.) are duplicated. This method of back up protection can be classified as:

a. Relay backup. b. Breaker backup. c. Remote backup.

2. Backup protection by time grading principle at which the tripping time at different sectionsare graded such that the tripping time is shorter close to the fault and longer in the sectionsthat follows.


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5. Simplicity

Simplicity of construction and good quality of the relay, correctness of design and

installation, simplicity of maintenance are the main factors which influence protectivereliability. As a rule, the simple the protective scheme and the lesser the number of relays,circuits and contacts, the greater will be its reliability.

6. Economy and adequateness

Although protective relay systems are not expensive compared to the costs of majorapparatus such as transformers, switchgear, lines, generators, etc., their cost is an issue.Protective relays require considerable engineering and technical support, including propersettings and routine maintenance by skilled personnel. Protection system costs must beacknowledged, but should be traded off against the cost of service interruptions, equipment

damage, and system disturbances that invariably result from inadequate protection systems.

Too much protection is as bad as too little. Good engineering design compromise betweenpractical situation and cost.The designer should consider the following:a. Rating of the system (or element to be protected).b. Location of the protected element.c. Probability of abnormal conditions.d. Cost and importance of the protected element.e. Continuity of the supply as affected by failure of this element.

Types of Relays

Relays may be classified in several ways, but here we look at their logical performance.In other words, the fundamental type of relay is determined by its function. One functionalclassification system is given in the table below:

1. Magnitude Relays2. Directional Relays3. Ratio Relays4. Differential Relays

5. Pilot Relays

Note that these types are not always mutually exclusive; for example, a relay may be a ratiorelay and also be a directional relay. Despite this, these terms are commonly used andshould be understood.

Magnitude relays respond to the magnitude of a current or voltage. They may trip on low orhigh values, such as an overcurrent relay (trips when the current it senses is above itspickup setting), or an under-frequency relay (trips when the frequency is below its setting). Acurrent magnitude relay may be combined with a directional relay to make a directionalovercurrent relay.


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Differential relays respond to the difference between two quantities. For example, inproviding short circuit protection for a power transformer, a differential overcurrent relay willtrip if the current on the primary does not balance the current on the secondary (taking intoaccount the turns ratio), since this indicates an internal fault. An external fault will notproduce a trip, since the primary and secondary currents balance regardless of the

magnitude of the current.

Directional relays respond to phase angle differences. Since phase angles must bemeasured with respect to some reference, these relays require a polarizing quantity.

Common Relay Terms

Rated value

It is the value of the energizing quantity, marked on the rating plate, on which theperformance of the relay is based. In the case of a current-operated relay, its rated current

will normally be the rated secondary current of the C.T. to be used with the relay (i.e. 1 A or5 A).

Setting valueIt is the nominal value(s) (usually as a percentage of rated value), marked on the settingplug (or dials) of the relay, at which the relay is designed to operate (e.g. 40% of 5 A). Sincea protective relay and its C.T. cannot be considered separately, the setting of a protectivesystem is often quoted as a percentage of the, rated primary current of the C.T.

Pick-up or operating level: the operation of a relay is called pick-up. Pick up level oroperating level is the threshold value above which the relay operates and closes its contacts.

Dropout or reset time: dropout is the value below which the relay resets and opens itscontacts to return to its normal position.

Drop-out / pick-up ratio is called Reset Ratio or Holding RatioAC 90 95 %DC 60 65 %

Flag: is a device usually operates to indicate the relay pick-up.

Static relays: are electronic circuits capable of performing the control functions in a mannersimilar to the regular relays without using moving parts.

SCADA: Supervisory Control and Data Acquisition is a computer system whichperforms measurements, data acquisition, data transmission, operating and controlfunctions.

Differential protection: is a protective system, which responds to vector difference inphase or magnitude between two similar electrical quantities.

Distance protection: is a protective system which depends on the ratio of the voltage tocurrent (V/I) at relaying point which gives a measure of distance between relay location and

fault location.


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Directional protection: in which operation occurs when the applied current assumes aspecific phase displacement with respect to the applied voltage. The relay operates andresponds to fault flow in particular direction.

Over voltage, over current, and over power relays: in which the relays operate when V,

P or I rise above a preset value.

Under voltage, under current and under power relays: in which the relays operate whenV, P or I fall under a preset value.

Over-reach: (of distance protection), operation of a relay for a fault beyond its set protectiondistance. For other types, the relay is over-reach when it operates at a current which islower than its setting.

Under-reach: is the failure of a distance relay to operate for a fault within its set protecteddistance.

Electromechanical Relays

The classic relay technology is electromechanical, usually in the form of magnetic forces ortorques exerted on movable parts carrying contacts. The construction may be hingedarmature, plunger, induction disk, induction cup, etc.

The operation of such relays is based upon the following effect of electrical current:

1. Electromagnetic attraction (a.c. or d.c. actuation)2. Electromagnetic induction (a.c. actuation only3. Thermal effect (I2rt) heat generated.

Some other electromechanical relays depend on the gas pressure generated due to the archeat (ex. Buchholz Relay).


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CHAPTER 2Types of Relays According to Theory of Operation

1. Attraction Type Relays

These are the simplest types of relays. These relays have a coil or an electromagnetenergized by a coil. The coil is energized by the operating quantity which may beproportional to circuit current or voltage.

These relays respond to both A.C. and D.C. currents. When the coil is energized by acurrent I, and saturation phenomena are neglected, the energy stored in the magnetic fieldis given by:

E =1/2 L I

2(1)

Where L is the inductance of the coil and I is the current flowing in the coil.

The force that tries to pull the plunger inside the coil is given by:

F = dE = K1 I2 (2)

dxwhere K1 is a constant depending upon the constants of the electromagnetic circuit such asthe number of turns, the plunger diameter, the air gap and the dimensions of the ironcore.

Since the relay has a restraining force ( K2 ) through a restraining spring, the force equation

can be written as:

F = K1 I2

K2 (3)

When the relay is on the verge of operation, F is zero, then:

Ip = kk 12 (4)

This idea is used in two types of relays Plunger Type Relay& Hinged Armature Relay.

The attraction armature relay can be designed to respond to over/under current andover/under voltage for both A.C. and D.C.


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Plunger-type relay.

Balance Beam Relay

This type consists of a horizontal beam pivoted centrally with an armature attached to bothsides and a coil on each side. The beam remains in the horizontal position till operatingforce becomes more than the restraining force.

The net torque is given by :T = K1I1

2 K2I2

2+ Ks (5)

Neglecting the spring effect, the net torque equals zero at the verge of operation, i.e.

I1/I2 = kk 12 = constant (6)

This relay is fast and can be used to compare two currents. If one of the coils is actuated byvoltage and the other by current then V/I = constant is used in impedance relay.


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2. Induction Relays

Induction-disk type of relay

Induction type relays are based upon the principle of operation of a single phase A.C. motor.Therefore, they cannot be used for D.C. currents. Two variants of these are fairly standard;one with an induction disc and the other with an induction cup. In both cases, the movingelement (disc or cup) is equivalent to the rotor of the induction motor.Induction type relays require two sources of alternating magnetic flux in which the moving

element may turn. The two fluxes must have a phase difference between them; otherwiseno operating torque will be produced.

Torque is produced in these relays when one alternating flux reacts with eddy currentsinduced in a rotor by another alternating flux of the same frequency but displaced in timeand space.

let: 1 = 1m sint

2 = 2m sin(t+)then the eddy currents (i1 and i2)produced in the disc are

i11m cost

i22m cos(t+)

Each of the rotor currents interacts with the


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flux produced by the other coil produce a force. The two forces are in opposite direction withrespect to the other, and the net force or, the net torque is given by:

F = F2 F1 2 i 1 - 1 i 2

F 12 [ sin(t+) cost - sint cos(t+) ]

F = K1m2m sin

Also we can see that the net torque is

T = K1 I1mI2m sin

One of the most common types of the induction relay is the shaded pole structure.

The air gap flux produced by the current flowing in the coil is split into two out of phasecomponents by a copper shading ring which encircles part of the pole face of each pole.The two rings have current induced in them by the alternating flux of the electromagnet andthe magnetic fields set up by these induced currents cause the flux in the portions of the ironsurrounded by the rings to lag by 40

oto 50

othe flux in the unshaded portions of the poles.

The torque in this case will be proportional to I12

and sine the phase difference, i.e.

T = K I12sin

Since sin depends on the design then

T = K1 I12


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Induction Relay

(a) Watt-hour-meter (or double wound) type (b) Shaded pole type

Watt-hour-Meter type Induction RelayThis structure gets its name from the fact that it is used for watt-hour meters. As shown in

the figure, this structure contains two electromagnets. The upper electromagnet carries twowindings: the primary and the secondary. The primary carries the relay current I1 whichinduces emf in the secondary and so circulates a current I2 in it. With this arrangement, theleakage flux entering the disc from the upper magnet and the leakage flux entering the discfrom the lower magnet are displaced sufficiently in phase and thus the two fluxes producethe driving torque in the disc. This torque is given by:

T = 12 sin

Where 1 & 2 are the leakage fluxes of the two coils and is the phase differencebetween these fluxes.


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3- Permanent Magnet Moving Coil Relay

In this relay the coil is free to rotate in the magnetic field of the permanent magnet. A torqueis produced due to the interaction between the field of the magnet and the field of the coil.This torque is given by:

T = 2r NHILWhere

R is the coil radiusN is number of turnsI is coil currentL is coil lengthH is Magnetic field

strength in the air gab

Note: This relay responds only to D.C. However it can be used with A.C. by using rectifiercircuits.

Plug Setting and Time Setting of Relays

The operation of the relay requires a certain flux and ampere turns. The current settings ofthe relay are chosen by altering (by means of a plug setting P.S) the number of turns of the

exciting coil in use: e.g. for a 5 A (rated current) earth-fault relay, 1 A in the whole coil or 2 Ain half of the coil or 4 A in a quarter of the coil will create (with good design) the sameampere turns, flux, torque and operating time. Thus only one time calibration curve is givenand is applicable to all settings.

Most over-current relays have a range of adjustments to make them adaptable to as widerange of applications as possible. Hence, various relays are available, each having adifferent range of adjustment. The adjustment of plunger type relays is carried out throughthe restraining spring tension. The adjustment of current-actuated relays (induction type) isusually carried out by coil taps. The operation of the relay requires a certain flux and ampereturns. The current settings of the relay are chosen by altering the number of turns of the

exciting coil in use. The plug setting (P.S.) can either be given directly in amperes orindirectly, as percentages of rated current. Typical settings for an earth-fault relay are 20 to80% in steps of 10%, and for an over-current relay (phase-to-phase fault relay) are 50 to200% in steps of 25%.

Time setting is generally in the form of an adjustable back-step, which decides the arc-length through which the disc travels. By reducing the length of travel, the time is reduced.The time multiplier setting is marked from 0.1 to 1, with major divisions marked in between.If a relay takes a certain time ( t ) seconds with time setting ( time multiplier setting ) = 1 , thesame relay take time equals xt seconds when the time setting = x .

Plug setting refers to the magnitude of current at which the relay starts to operate. Thismeans that if a current = 100 A is injected in a relay coil which has a plug setting = 5, ( relay


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current setting = 5 ), then the plug setting multiplier = 100 /5 = 20 . i.e. the current passing inthe relay coil is 20 times the plug setting.

The relay should start to operate at a current equals to the plug setting. However, due tofriction, dust, .. etc. , so as to make operation of relays reliable, their actuating quantity ismade at least 1.5 times the plug setting.

It is important to note here that the plug setting equals the pick up current if there is no C.T.is used. If we use a C.T. ,

The plug setting = Pick up current x C.T. ratioAs mentioned before, the current setting multiplier or (the plug setting multiplier) indicatesthe number of times the relay is in excess of the current setting (plug setting).

Plug Setting Multiplier (P.S.M.)

The actual r.m.s. current in a relay, expressed as a multiple of the setting current is calledthe plug setting multiplier (P.S.M.): e.g. if a 5 A (rated current) over-current relay is set at200% = 10 A and if the relay current is 150 A then the plug setting multiplier = 15, and if theC. T. is rated at 400 /5 A, then the fault current is 12 kA.

Time Setting (Time Multiplier T.M.)

The time setting of the relay marked 0 to 1 and called the time multiplier (T.M.), adjusts theposition of the movable back-stop. With the time multiplier set at 1 the back-stop is as farback as it can go while with the time multiplier set at 0 the back-stop is so positioned that therelay contacts are almost closed. The time multiplier is very nearly proportional to the angleof travel of the relay contacts, but is calibrated to allow for the initial acceleration and the

resetting spring. The minimum practical setting is 0.1. With any lesser setting the relaycontacts might close accidentally due to vibration etc.

i.e. PSM = Relay CurrentRelay setting current

Or = Primary Current x C.T. RatioRelay setting current

Or = Primary CurrentPrimary Setting Current

B.S. 142 gives, for the standard I.D.M. T .L. relay, the relation between the plug settingmultiplier and the relay operating time for a time multiplier of 1. The data is given in Fig.2 .The relation is an inverse one except that the time tends to become constant betweenP.S.M. 10 and 20 (the so-called definite minimum time). A P.S.M. = 1 means that the relaycurrent equals its setting current. Clearly the relay must not operate for a current less thanits setting. The manufacturer is allowed a tolerance of 30% on the pick-up current so that therelay must operate (pick-up) between P.S.M. = 1 and P.S.M. = 1.3, and take about 30seconds to operate. To avoid working in this uncertain region, it is usual to arrange that, forminimum fault current the P .S.M. > 2. In the case of over-current relays it is necessary tocheck that for maximum load currents the P .S.M. < 1 by a reasonable margin of safety,

since protection is not intended to deal with temporary overloading of the system.


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I.D.M.T.L. relay characteristic

For each relay, a characteristic is plotted with multiples of plug settings (P.S.M.) as anabscissa (x axis) (log scale) and time in seconds (log scale) as an ordinate (y-axis).

Relay Characteristics

Time Margin Between Relays

To assure proper coordination between two relays a time margin between their operations isusually set. This margin consists of : circuit breaker total time, 0.12 second , relay errors ,0.1 second each , one fast and the other slow 0.2 second, safety margin , 0.1 second .

Total 0.42 second ( 0.5 second)

4. Thermal Relays

These relays operate due to the thermal effect of the electric current. They can be made ofbi-metal type or thermocouples type.The bi metal type consists of two metal strips having different coefficient of thermalexpansion joined together. When the combined strip is heated, one expands more than theother causing the strip to bend. This process closes the relay contacts.

The other type is employing thermocouples. A thermocouple consists of a junction of twoselected materials. The junction touches a coil in which the circuit current is passing. Thedifference in temperature between the two materials ( as their time coefficients are different )induces e.m.f. which is a function of the coil temperature. This e.m.f. can operate a staticrelay or a sensitive moving coil.


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5. Buchholz Relay

It is a gas-actuated relay used for protecting oil immersed transformers against all types ofinternal faults and makes use of the fact that during faults, oil decomposes and hence

generates gases.

Buchholz relay consists of a cast housing containing a hinged hollow float inside of whichthere is a mercury switch. This float is located in the upper part of the housing.

When a fault occurs inside the transformer, bubbles of gas are evolved by the heatgenerated and rise up to the top of the housing causing the oil level to fall and the mercuryfloat to tilt. The mercury therefore contacts the relay switch. An alarm is actuated and thecoil of the circuit breaker trips according to the design.

ON OFF


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CHAPTER 3Types of Relays according to their function

1. Directional Relays

1.1 The role of directional protection equipment

Protection equipment has the basic role of detecting an electrical fault and disconnectingthat part of the network in which the fault occurs limiting the size of the disconnected sectionas far as possible.Directional protection enables better discrimination of the faulty part of the network than withovercurrent protection. It is necessary to use it in the following conditions:

9 in a system with several sources,9 in closed loop or parallel-cabled systems,

9 in isolated neutral systems for the feedback of capacitive currents,9 to detect an abnormal direction of flow of active or reactive power (generators).

Figure 1 illustrates a situation in which both power sources would be tripped if overcurrentprotections were used.

Directional current protection equipment is capable of only tripping the faulty incomer. Thedirection in which the fault occurs is detected by measuring the direction of current flow, orin other words the phase displacement between the current and voltage.

Power protection equipment measures either the active or the reactive power flowingthrough the connection in which the current sensors are placed. The protection equipmentoperates if the power is greater than a set threshold and if it is flowing in a given direction.Directional power and current protection requires the current and the voltage to bemeasured.

Fig. 1: Illustration of an application of directional protection equipment.


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CHAPTER 4Protection of Power System Elements

1. Feeder Protection

The various protection schemes as applied to feeder protection can be classified to:a. Time graded protection b. Differential protection.c. Distance protection. d. Carrier current protection.

a. Time graded protection

i) Radial feeder:Protection on radial feeder is obtained by employing fuse shunted trip coils as shown in the

figure 1.

If an earth fault occurs on feeder F4, thefault current will pass through theprimary windings of all the CTs and soas this current will appear in each ofsecondary windings of CTs. If themagnitude of the fusing currents of the trip coil fuses are arranged in the decreasing order,from the power station to the remote substation correct discrimination (taken into accountthat the magnitude of the fault current is not so heavy as to operate all the fusessimultaneously) will be obtained and fuse No. 4 will be the first to blow off. The current,which would have so far been passing through the fuse due to its low impedance, will nowpass through the trip coil to open the circuit breaker on feeder F4 and thus clear the fault.

This also can be done by inverse definite time relays as shown in fig. 2 are also is set sothat the minimum time of operation decreases from the power station to the remote sub-station.

ii) Parallel feeder:

Figure 3 shows a system where three feeders areconnected in parallel between a power station and remotesupply point.

Let an earth fault develop on feeder 2 as shown in the figure. It will be seen that this fault isfed via three routes.

a. directly along feeder 2 from the power source.b. from feeder 1 via the receiving end busbars, andc. from feeder 3 via the receiving end busbars.

Now to clear this fault, only circuit breakers 3 and 4should open. This is achieved by employing non-directional relays on the supply end and directional relays

operating only when fault power is feeding in the directionof the arrow on the receiving end.


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CHAPTER 5Generator Protection

There are a number of abnormal conditions that may occur with rotating equipment such as

generators. These include:

1. Faults in the windings.2. Loss of excitation.3. Motoring of generators.4. Overload.5. Overheating.6. Over speed.7. Unbalanced operation.8. Out of synchronization.9. External faults.

Several of these conditions require prompt tripping and can cause complete failure of thegenerator or serious damage.

1. Faults in the windings

Figure 1 shows some types of faults that mayoccur in the insulation system of a generatorswinding. These faults are identified as:a. Inter-phase short circuit.b. Inter-turn fault.

c. Stator earth fault.d. Rotor earth fault.e. Inter-turn fault in rotor.

a. Inter-phase short circuitA short circuit between parts of different phases of the winding such as fault #1 , results in asever fault current within the machine. A consequence of this a distinct difference betweenthe currents at the neutral and terminal ends of the particular winding which can be detectedby a differential protection system. We usually use Biased differential protection or Merz-Price protection .

Under normal operating conditions, the secondary outputs of the line current transformersare equal to the current transformer at the neutral end. Thus there is a balanced circulatingcurrent in the phase pilot wires and the relay restraining windings. Current does not flow inthe operating coils or in the common return pilot. Under fault condition this balance is upsetand current flows in the operating coils of these phase elements corresponding to primaryphases on which the fault has occurred. If this current reaches the preset magnitude, therelay operates.

Differential protection gives complete protection to generator windings against phase to

phase faults. It is arranged to trip the main circuit breaker and to suppress the field.


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Desirable features of generator differential protection are:

- High speed operation ( about 15 ms ).- Low setting.- Full stability on external faults.

b. Inter turn fault:

The incidence of turn-to-turn fault in generators is rare. It can take place between turns ofthe same phase or between parallel coils of the same phase. In large generators we protectthe system by using residual voltage method. A voltage transformer is connected betweeneach phase terminal and the neutral of the winding. The secondary terminals are connectedin an open delta to a polarized voltage relay as shown in figure 4. In the event of an interturn fault, a voltage appears at the terminal of the open delta causes the relay to trip.

During normal conditions, the residual voltage is zero, i.e.

VRes = VRY + VYB + VBR = 0

Any short circuit between turns gives residual voltage of fundamental frequency, which

should operate the relay. Note that VRes is essentially a zero sequence voltage.

Fi ure 3 Connection for a delta enerator


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The relay should not operate for earth fault. Earth fault also causes zero sequence voltagesof third harmonic. A low pass filter is used to offer a low resistance path to power frequencyand high resistance path to 3

rdharmonic currents.

c. Stator earth fault

The protection against earth faults by differential protection is influenced by the groundingscheme. When the generator winding is earthed through impedance, a separate additionalearth fault protection is necessary in addition to differential protection. The differentialprotection provides earth fault protection to about 85% of generator windings. Figure 5shows two types of earth protection which is usually used for generators above 1 MW. It canalso be used as a backup for external phase to phase faults. However it does not givesatisfactory protection against internal faults.

The resistance Rg is used to limit the earth fault current. If Rg is too small (solid earthing),earth fault current is very high. Hence this method is not used for large machines (> 1 MW).Medium resistance limits the fault current to 200 A is used for generators up to 60 MW while

large resistance that limits the fault current to 10 A is used for larger generators.


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As stated before, when we use differential protection only about 85% of generator windingis protected. This is a function of the impedance between neutral and ground ( Rg ), line toneutral voltage (V) and the minimum relay operating current ( Io ) in the primary of the CTwhich is usually between 10 20 % ( relay setting ).

The %age of windings unprotected = Rg Io x 100V

d. Rotor windings faults

These are caused by mechanical and temperature stresses. The field system is notconnected to earth so that a single earth fault does not give rise to any fault current. Asecond earth fault will short circuit part of the winding and may produce unsymmetrical fieldsystem giving unbalance force on the rotor. Such a force will cause excess pressure onbearing and shaft distortion if not cleared quickly.Rotor earth fault protection is provided for large generators. Rotor temperature indicates or

alarm is employed to indicate the level of temperature. If the temperature level is higher thana preset value, an alarm will signal.

Figure 7 presents a schematic diagram of rotor earth fault protection. A high resistance isconnected across the rotor circuit. The centre point of this resistance is connected to earththrough a sensitive relay. The relay detects the earth fault for most of the rotor circuit exceptthe center point of the rotor.

The preferred type of protective relaying equipment is shown in figure 8. Either d.c. or a.c.voltage may be impressed between the field circuit and the ground through an over voltage

relay. A single earth fault in the rotor circuit completes the circuit the fault is then sensed bythe voltage relay.


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It may be necessary to provide a brush on therotor shaft that will effectively ground the rotor.One should not rely on the path of the bearing oilto ground for two reasons:

1. The resistance of this path to ground may behigh enough so that the relay would notoperate at grounded fault.

2. Even very small current flowing through thebearing may pit the surface and destroy thebearings.

2. Loss of excitation

When a synchronous generator losses excitation and if it is connected in parallel with otherunits, it can draw the magnetizing current from the bus bars and continue to run as aninduction generator. It will then run above synchronous speed. Some systems depend onthe rotor design (with salient poles and damper windings) can run for long time without anyproblem. Some systems cannot tolerate the continued operation without excitation.Automatic quick-acting protective system should be used. Undercurrent relays connected inthe field circuit have been used quite extensively.

The most sensitive type of loss of excitation protection is a distance relay operated from thea.c current and voltage of the main generator terminals. This application is based on thebehavior of the system impedance as seen from the generator terminals for various under-excited conditions. This will not be explained in this course.

3. Motoring of generators

Motoring protection is for the benefit of the prime mover and not the generator. When theinput to the turbine is topped, the generator continues to run as a synchronous motor. Asensitive power directional relaying is widely used for such protection. This is called reveredpower protection.

4. Overload

Overload protection is used to provide backup protection for bus or feeder faults than toprotect the machine directly. We dont use an over current relay since the generatorssynchronous impedance limits the fault currents of a sustained faults to obtain the same asthe maximum rated load current . A thermal relay may be used to monitor the stator windingtemperature and signal an alarm if a certain limit is exceeded.

5. Overheating

Bearing overheating or loss of prime mover cooling system or lubricating oil may damage

the mechanical system and cause serious problems. Temperature detection is then


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important. Thermocouple system is usually inserted in the bearings and different otherlocations in the system to monitor the temperature and signal an alarm if necessary.

6. Over speed

It is essential to incorporate safety device in turbine governing system to prevent overspeeding, which may cause over voltages and increase frequency.

Over speeding can occur due to sudden loss of the load before disconnecting the primemover. To maintain the speed of the generator, the prime mover shaft is equipped by acentrifugal device or electronic sensors.

It must be recognized that in practical situations when many generators are connected, overspeed can not occur unless the unit is disconnected from the system.

7. Unbalanced load

The unbalanced 3 phase stator currents causes negative sequence currents ( 100 Hz ) andinduces double frequency current in the rotor which tends to flow on the rotor surface and inthe non-magnetic rotor wedges and rings. The resulting I2R loss quickly raises thetemperature and damage the rotor surface.

The time for which the machine can be allowed to operate for various amounts of relativeasymmetries depends on the machine design. Usually the time that a generator may beexpected to operate with unbalanced stator currents without damage can be expressed inthe form:

T0 (i2

)2 dt = K

Where i2 is the instantaneous negative sequence per unit component of the stator currentbased on the generator rating and K is a constant = 30 for steam turbines and 40 forhydraulic turbines.

If we let I22

be the average value of i22

over the time interval then the foregoing equation canbe expressed in the form

I22 T = K

The recommended type of relaying equipment is an inverse time over current relayoperating from the output of a negative phase sequence current filter that is energized fromthe generator CTs.

The relay time current characteristics are of the form I2

T = K , so that with the pick up andtime delay adjustment, that are provided, the relay characteristic can be chosen to matchclosely any machine characteristic.


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8. Out of synchronization similar to loss of excitation & over speed.

9. External faults similar to overload.

Example:A 6600 V , 3 phase turbo-alternator has a maximum continuous rating of 2000 kW at 0.8 p.f.and its reactance is 12.5 %. It is equipped with Merz-Prize circulating differential relay whichis set to operate at fault current not less than 200 A. Find the value of the neutral earthingresistance leaving 10% of the windings unprotected.Solution:Let the earth resistance be Rg

IFL = 2000 x 103 = 219 A

0.8 x3 x 6600

the reactance per phase of the alternator = X

where 12.5 = 3 X x 219 x 100

6600

x = 2.19 reactance of 10% of the winding = 0.219

voltage induced in NA = 6600 x 0.1 = 381 V

3the protection operates at a current = 200 A

200 = 381 Rg = 1.89 Rg

2+ (0.219)

2


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CHAPTER 6

Busbar Protection

Busbars are important and need special attention due to:

Fault level at busbars is very high. The faults at busbars cause disconnection of power to large portions of the system.

Busbars faults should be cleared in very short time ( 50 ms ) to avoid damage toinsulation.

Stability of the system is affected by busbars faults.

Its desirable to include the following in busbar protection:

1. High speed.

2. Stability for external faults.3. Freedom from unwanted operation.4. No operation due to CT saturation.5. Interlock with generators for over current.6.Use main and check protection to assure disconnection only when desirable.

Methods of busbar protection

1. Overcurrent relays of connected circuits.2. Directional interlock.3. Differential protection.

4. Frame leakage earth fault.

1. Overcurrent Relays

The fault in bus A can be sensed by R5 and R4. In this type of protection, all the circuitbreakers of the busbar zone are disconnected. These are C1, C2, C3, C4 & C5. R6 & R7 willact as backup protection if C4 and C5 do not clear the fault.

For fault in bus B, R8 will sense the fault and C6, C7 and C8 will open.

This type of protection is slow and evolves complicated control system to discriminate faultswithin the zone. Also, the zone of the busbar is not clearly identified.


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2. Directional Interlock

It uses directional relays in source circuits and over current relays in load circuits. It makesthe discrimination between internal and external faults possible.

The contacts of the relays are interlocked in such a way that if power flows from the busbaris sufficiently low, all the circuit breakers on the source side and the load side are tripped.

3. Differential Protection

For normal conditions, the vector sum of currents entering the bus zone is equal to thevector sum of currents leaving the bus zone. i.e.

Ii = zero

During internal faults, the vector sum of currents in the circuits connected to bus bar is equalto fault current, i.e.

Ii = If

Disadvantages:

1. Large number of circuits having different current levels.2. Saturation of CT cores due to d.c. component in s.c. current.3. Sectionalizing of bus makes the circuit complicated.


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4. Frame leakage Earth Fault

One of the famous connections for busbars: 2 out of 3

Only 2 of the 3 circuit breakers can operate at the same time. The circuit breakers on thetransformers are normally closed but the circuit breaker in the centre of the busbar isnormally open.


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CHAPTER 7Transformer Protection

A number of fault conditions can arise within a power transformer. These include:

1. Earth fault on a transformer winding.2. Core faults due to insulation breakdown which allow sufficient eddy current to flow

causing over heating.3. Inter-turn faults occur due to winding flashover caused by line surges.4. Phase to phase faults. This are rare in occurrence but will result in a substantial currents

of magnitude similar to earth faults.5. Tank faults due to loss of oil which produces abnormal temperature rises.6. External abnormal conditions such as overloading, over-voltages due to transient core

losses and corresponding temperature rise.

Transformer Size

Fuses usually protect transformer with capacity less than 500 KVA in industry and 2500KVA in residential areas. With ratings up to 5000 KVA in residential areas, instantaneousand time delay over current relays may be more desirable. For industrial loads greater than1500 KVA and for transformers that are part of the bulk power system it is recommended touse differential protection or harmonic restraint percentage differential relays. Also, thehigher the voltage, the more sophisticated and costly the protective device.

1. Differential Protection

The protection of transformers is usually performed by differential protection. The differentialprotection responds to the vector difference between two similar quantities. The C.T.connected on the transformer windings should be arranged so that the same current isflowing between the two sides.

A General Rule Is To Connect The CTs On Any Star Windings In Delta and In Any DeltaWindings Connect CTs in Star

Two basic requirements that the relay connection must satisfy are:1. The relay must not operate for loads or external faults.

2. The relay must operate for internal faults.

Fig. 1 represents differential protection of Delta-Star transformer


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Fig. 2 shows a star-star transformer to which circulating current protection has been applied.Here it will be noted that the current transformers on both sides are connected in delta.

Fig. 3 (a) is included to show how had the current transformers been connected in star,operation of the protective relay would occur on a fault outside the protected zone which wewish to avoid while Fig 3 (b) shows how this can be avoided by connecting the currenttransformer secondaries in delta.


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Problems arising in differential protection applied to transformers

Simple differential protection system is inadequate because the following difficulties arise:

1. Difference in length of pilot wires on either sides of relays. The difficulty is overcome by

connecting adjustable resistors to pilot wires. These are adjusted on site to getequipotential points on pilot wires. Taps can be provided on operating coil andrestraining coil of relay for adjusting the balance.

2. Difference in CT ratios due to error difference at high values of short circuit currents.Because of this difference relay operates for external faults. This difficulty is over comeby using biased (percentage) differential relay. In such a relay a restraining coil isconnected to pilot wires. The current flowing through the restraining coil can be taken as(I1 + I2 ) /2 . With increase in current the restraining torque increases too and the currentdue to the CT inaccuracy is not enough to casue the relay operation.

3. Tap changing alters the ratio of voltage ( and currents ) between HV side and LV side.Differential protection should be provided with bias (restrain) which exceeds the effect of

variation in secondary current due to tap changing.4. Magnetizing current inrush:

When power transformers are switched on, initially there is no induced e.m.f., the conditionsis similar to switching an inductive circuit. Since the resistance of the coil is low, a largeinrush of magnetizing current takes place. The magnitude of which depend on circuitconditions and the voltage at the instant of switching. Maximum values of 6 to 8 times therated current can flow in the winding. Usually this high current decays after few cycles to thenormal current but in some cases it may take 2 4 seconds.

Formerly, the relay was provided with time lag of 0.2 1 second. By this time, the inrushcurrent would vanish and the relay does not trip unnecessary. However for many faults, therelay time lag might cause substantial damage to the transformer.

Next development was the use of kick of fuses to shunt the relay coils as shown in fig. 2.These fuses are of the time limit type that do not operate in the time of switching. Undersustained fault conditions, the fuses operate and the current then passes through the relaycoil and trip the C.B. This also is a slow protection and may cause some problems. It alsodepends on the fuse.

The next development was to desensitizing the relay for a short period of 0.1 to1 sec during

switching. After this time the shunt across the relay coil is removed. This method can lead toswitching on a transformer for long period during faults. The latest method adopted isharmonic current restraint.

Since inrush current has very high contents of 2nd

and 3rd

harmonic currents, which mayreach 65% and 25% of the fundamental respectively, the restraining differential relaysenses only the fundamental component. Because the harmonic component of the shortcircuit current is negligible, this relay operates at faults but not sensitive to switching current.The operating coil in these relays will receive only the fundamental component of thedifferential current. The harmonics are usually separated and fed back into the restrainingcoil.


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The overload fuses shown in fig.2 provide a form of back up protection. In the event ofsustained through fault, damage may be caused to the transformer. One or more of theoverload fuses will operate; leaving the relay to be fed from one of CTs and thus causingrelay operation.

2. Frame leakage protection

3. Restricted earth fault protection ( differential protection )

Earth faults on secondary side are not reflected on primary side when the primary winding isdelta connected or has unearthed star point. In such cases, an earth fault relay connected inresidual circuit of 3 CTs on primary side operates on internal faults in primary windings only.Because earth faults on secondary side do not produce zero sequence currents on primaryside, restricted earth fault protection may then be used for high speed tripping for faults onstar connected earthed secondary winding of power transformers.

Figure 5 shows the connections of the earth fault relays connected in the residual circuit ofthe line CTs. Figure 6 shows the connection of the restricted earth fault protection relay inthe secondary side and earth fault protection in the primary side.


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If the fault F1 is beyond the transformer windings, I1 and I2 will flow so that the current in theearth fault relay is negligible. For earth fault within the transformer star connected windings,I2 flows and I1 is negligible. Hence I2 causes the relay to trip the circuit.

When fault occurs very near the neutral point of the transformer, the voltage available for

driving the earth fault current is small and the fault current would be low. If the relay isadjusted to sense such small currents, it may operate under normal unbalance conditions. Itis common to set the relay to pick up at about 15% of the rated current. Such setting leavesa portion of the windings unprotected. Therefore it is called restricted.

4. Bucholoz Protection: it is frequently used in transformers.

Example 1: Describe with the help of a neat diagram the connections of differentialprotection of a transformer. A 3-phase 33/6.6 kV star/delta connected transformer isprotected by Differential system. The CTs on LT side have a ratio of 300/5. Show that theCTs on HT side will have a ratio 60 : 5/3

Solution: CTs on delta side are star connected. Hence the secondary phase currents areequal to currents in pilot wires. CTs on star connected side are delta connected hencecurrent in secondary is equal to current in pilot wires divided by 3.

Assume 300 A is flowing in the lines on LT side3 x 6.6 x 300 =3 x 33 x I

I = 60 A ( current in HT lines )

which is primary current of CT on HT side.Current in pilot wires: On the delta side of transformers the CT secondaries are star

connected. Their secondary current is 5 Amp. Hence current fed in pilot wires from LT sideis 5 Amperes. Same current is fed from CT connections on HT side which are deltaconnected.

Hence secondary current of CTs on HT side is 5 /3 Amp.

Hence CT ratio on HT side is 60 : 5/3

Example 2:A 30 MVA, 11.5 kV/ kV, star-delta power transformer to be protected bydifferential protection. The high voltage side phase lags behind low voltage side by 30

o.

Formulate the complete differential protection for the transformer by selecting CT ratios, CTconnections. The continuous current carrying capacity of restraining coils of the differential


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relay should not exceed 5 Amp. CT ratio is 3000/5 on 11.5 kV side. Determine CT ratio on69 kV side.

Solution: Draw work sheet for connection of differential relays showing the maintransformer, CTs, operating and restraining coils of CTs (Fig. 7). Connect the pilot wires

with operating coils and restraining coils as described in the earlier section.

Calculate full load current for a 30 MVA, 11.5 start/69 delta power transformer.On 11.5 kV side

Ip = 30000 = 1505 A3 x 11.5

CT ratio = 3000 / 5 = 600 ( given )

Is = 1505 = 2.51 A600

since 11.5 kV side is star connected, CT secondaries will be delta connected. Hence currentfed into pilot wires from 11.5 kV side CT secondaries is

3 x 2.51 = 4.35 A

On 69 kV side

Ip = 30000 = 251 A3 x 69

Current in secondary CTs = current in pilot wires. Since 69 kV side CT secondaries areconnected in star = 4.35 A

hence CT ratio = 251 / 4.35 = 57.7select CT ratio = 60

secondary current = 5 Aprimary current = 60 x 5 = 300ratio on 69 kV side = 300/5


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Example 3: Consider a delta/star connected, 15 MVA, 33/11 kV transformer with differentialprotection applied, for the current transformer ratios shown in figure 8. Calculate the relaycurrents on full load. Find the minimum relay current setting to allow 125 percent overload.

Solution:The HV line current is given by

Ip = 15x106 = 262.43 A

3 x 33x103

The LV line current is

Is = 15x106 = 787.30 A

3 x 11x103

The CT current on the HV side is thus

ip = 262.43 ( 300/5) = 4.37 A

The CT current in the LV side is

is = 787.30 (5/2000)3 = 3.41

Note that we multiply by 3 to obtain the values on the line side of the delta connected CTs.

The relay current at normal load is therefore

ir= ip is = 4.37 3.41 = 0.9648 A

with 1.25 overload ratio, the relay setting should be

Ir= 1.25 (0.948) = 1.206 APlug Setting = 1.206/5 = 24.1 %


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CHAPTER 8STATIC & DIGITAL RELAYS

For many years, utilities have used electromechanical relays for power system protection.

The result of using electromechanical relays has been an extensive maintenance anddesign practice. Both the maintenance and design of protection schemes using theserelays is expensive and time-consuming.

Over the last ten years, static and microprocessor-based relays have come of age. Staticand microprocessor-based relays offer many advantages over electromechanical relays.This unit of the course compares a typical transmission line protection scheme in terms ofcost, engineering design, and maintenance. The information presented in this unit of thecourse shows that microprocessor-based relays offer significant savings in cost,engineering design, and maintenance.

Static relay is a relay in which the comparison or measurement of electrical quantities isdone by stationary network, which gives a tripping signal when the threshold condition ispassed, (threshold means 'on the verge of', 'on the border of'). In simple language staticrelay is one, which has no moving parts except in the slave device. The static relay includesdevices the output circuit of which may be electric, semiconductor or even electromagnetic.But the output device does not perform relay measurement; it is essentially a trippingdevice. The slave relay in output circuit may be electromagnetic type, or the trip coil maybe connected directly in the output circuit.

Fig.1, Block of diagram of a static relay-simplified.

Fig. 1 illustrates the essential components in the static relays. The output of CT's or PT's isrectified in a Rectifier. The rectified output is fed to the measuring unit. The measuring unitcomprises comparators, level detectors, filters, logic circuits. The output is initiated wheninput reaches the threshold value. The output of measuring unit is amplified by Amplifier.

The amplified output is given to the output unit which initiates the trip coil only when relayoperates.


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In conventional electromagnetic the measurement is carried out by comparing operatingtorque/force with restraining torque/force. The conventional relay operates when operatingtorque/force exceeds the restraining torque/force. The pick-up of relay is obtained by motionof moving element of the relay. Whereas static relay have static circuits which perform the

measurement.

A simplified block diagram of a static relay is given in Fig. 1. The figure is rather a generalfigure. In individual relays there is a wide variation. The entity voltage, current, etc. isrectified and measured. When the quantity to be measured reaches certain well definedvalue, the output device is triggered. Thereby current flows in the trip circuit of the circuit-breaker.

With the inventions of semi-conductor devices like diode, transistor, thyristor, Zener diodeetc. there has been a tremendous leap in the field of static relay. The development ofintegrated circuits has made an impact on static relays. Integrated circuits are more reliableand more compact. Furthermore, the digital computers are being increasingly used in powersystem protection.

The static relays and static protection has grown into a special branch in its own right. Thischapter covers principles and applications of static relays and static protection systems inbrief.

Static Vs. Electromagnetic Relays

(a) Advantages of Static Relays

The static relays compared to the corresponding electromagnetic relay have manyadvantages and a few limitations.

(i) Low Power Consumption. Static relays provide fewer burdens on CT's and PT's ascompared to conventional relays. In other words: the power consumption in the measuringcircuits of static relays is generally much lower than for their electromechanical equivalents.The consumption of 1 milli-watt is quite common in static over-current relay. Whereas, anequivalent electromechanical relay can have consumption of about 2 watts. Reducedconsumption has the following merits.

CT's and PT's of less VA rating

The accuracy of CT's and PT's is increased

Problems arising out of CT saturation are avoided

Overall reduction in cost of CT's and PT's.

(ii) No Moving Contacts. Solid state devices do not have moving contacts. Therefore, thereare no problems of contact bounce arcing, contact erosion, etc. in the static relay circuits.There is no effect of gravity on static relays.

(iii) Operating Times and Various Characteristics. As the levels increase, rapid faultclearing becomes a must. The static relays do not have moving parts in their measuring

circuits, hence relay times of low values can be achieved (1 cycle, cycle, etc.). Such


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low relay times are impossible with conventional electromechanical relay systems whichcomprise measuring relays plus auxiliary relays.

A variety of characteristics can be obtained with static relays. Thereby selectivity, stabilityand adequateness can be achieved. Measurement of several quantities such as negative

phase sequence component, frequency, harmonics, temperature, impedance, etc. can beperformed by static measuring circuits

(v) Remote Back up and Monitoring. Static relays resisted by power line carrier can beused for remote back-up and network monitoring.

(vi) Hardware and Space Requirements Static relays are compact. Furthermore, with useof integrated circuits, complex protection schemes can be installed on a single panel.

A typical three-zone step time distance scheme consists of instantaneous trippingelements, two levels of time-delayed tripping elements for phase faults and an

instantaneous tripping element, and time overcurrent element for ground faults. For thisexample, we shall assume that the step time distance scheme uses phase distance anddirectional ground overcurrent elements. Phase faults are detected using three zones ofphase distance relays. Ground faults are detected using a directional ground overcurrentrelay, which includes a time-overcurrent element and an instantaneous overcurrentelement. The protection scheme also includes a single-shot recloser for automatic linerestoration after a fault has been cleared.

The electromechanical relay scheme uses three-phase distance relays. These relays maycover all fault types on a per-zone basis or all three zones on a faulted phase pair basis.This depends upon the manufacturer of the distance relays. However, in either case, threedistance relays are required. A timer is also required for the time-delayed backup elements.Typically, the time delay is provided from separate timers, so if one timer fails, the entirestep time distance scheme is not lost. A single directional ground overcurrent relay shall beused for ground fault detection. A single-shot reclosing relay shall also be provided forrestoring the line. A non-directional overcurrent relay shall be used to supervise thedistance relays.

The electromechanical relay scheme panel layout is shown in Figure 2. Note that theelectromechanical scheme requires nearly all of the space contained in an 84-inch by 19-inch panel.

The microprocessor-based scheme shall consist of a multifunction relay that provides threezones of step time distance protection, three levels of instantaneous or definite timedirectional ground overcurrent protection, a directional ground time-overcurrent function,and three-shot recloser. The microprocessor-based scheme shall also include a single-zone microprocessor-based relay as a backup in case of failure of the primary multi-zonerelay.

Figure 3 shows the panel layout for the microprocessor-based relay scheme. The spacerequirement for the microprocessor-based relay scheme is much less than theelectromechanical relay scheme.


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Figure 2 Figure 3

Given that the cost of all the relays for the electromechanical scheme is 1 per unit (p.u.),

the cost of the microprocessor-based relay scheme is 0.35 p.u.

(vii) Static Relays can 'think'. Complex protection schemes employ logic circuits. ['Logic'means the process of reasoning, induction or deduction.] Suppose, several conditions areimposed on a protective system such that for certain conditions, the relay should operate,and for some other conditions, the relay should remain stable; in such cases, logic gatescan he adopted.

(viii) Repeated Operations Possible. Static relays can be designed for repeated operationsif necessary.

(ix) Effect of Vibrations and Shocks. Most of the components in static relays, including theauxiliary relays in the output stage are relatively indifferent to vibrations and shocks. The riskof unwanted tripping is, therefore, less with static relays as compared to theelectromechanical relays. This aspect makes the static relays uniquely suitable forearthquake prone areas, ships, vehicles locomotives, airplanes, etc.

(x) Transducers. Several non-electrical quantities can be converted into electrical quantitiesand then fed to static relays. Amplifiers are used wherever necessary.

(XI) Easy Testing


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Installation Tests

Installation tests are used to verify that the relays are set correctly and that the scheme isdesigned correctly for the intended application. Routine tests are performed to ensure thatthe relays are functioning within established specifications.

A scheme designed with electromechanical relays requires a large number of tests duringinstallation to ensure that the overall scheme is functioning properly. Each discrete relaymust be tested and calibrated. For the step time distance scheme example, at least sevendiscrete relays must be tested. The testing of each relay requires that the relay beconnected to the test equipment, the various setting adjustments are made, and the relay istested per an established test routine. If the relay test results are outside establishedguidelines, the relay must be calibrated. The calibration routine can be very time-consuming.

After each relay has been tested, the scheme must be "trip-checked" to ensure that all of thewiring and trip circuits are correct. Many times, trip-checking an electromechanical schemeis a simple matter of manually closing an output contact. Therefore, the trip-checks can bevery simple. However, due to the many discrete devices used in the scheme, the trip-checkscan be very time-consuming and, in case of an incorrect design or wiring error, require manyhours of trouble shooting when searching for problems.

A static or a microprocessor-based relay scheme is very simple to test and verify .Amicroprocessor-based relay operates using software programming. The operation of thevarious functions and logic has been fully verified and tested by the relay manufacturer. Inmany cases, the utility has also tested the relay to ensure that the relay conforms to thespecifications stated by the manufacturer. Once the relay has been fully tested, the software

that defines the operating characteristics of the relay has been verified. Therefore, it is notrequired to fully test each relay given that the relays are of the same type and softwareversion.

The installation tests for a static or a microprocessor-based relay should be designed toverify that the relay settings have been entered correctly. The test series should bedesigned to check the relay pick-up at critical points. For example, the distance elementshould be tested at the angle of maximum torque and 30 degrees off the angle of maximumtorque. These test points verify the distance element settings. Overcurrent elements shouldalso be tested using a very simple test routine.

Trip checks using a static or a microprocessor-based relay are very simple due to the factthat there are fewer contacts to check and less wiring to verify .In many cases, a softwarecommand may be used to close specific output contacts. Using a software command toclose relay outputs is simpler than connecting voltage and current test sources to the relayto perform fault simulations.

Routine Tests

Routine tests must be performed on electromechanical relays to verify that they areoperating within specified guidelines. These tests may be at one to three year intervals fordistance relays based upon the specific utility's practice. The routine tests performed on an

electromechanical relay are very similar to those done during the installation process. Therelays must be thoroughly tested to verify that all of the 6 internal components are operating


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within specified tolerances. Routine tests also confirm that all contacts and external circuitsare functioning properly.

Most static and microprocessor-based relays perform routine self-checks to ensure that thecritical circuitry in the relay is functioning properly. Microprocessor-based relays

continuously run the same software routines. Therefore, if the relay is functioning properly,the relay algorithms shall operate correctly. Routine maintenance in a microprocessor-basedrelay consists of verifying that the inputs, outputs, and data acquisition system arefunctioning properly. If the relay is properly measuring the analog currents and voltages andthe self-check status show that the relay is healthy, the relay shall function correctly. Theonly other checks necessary are to verify that the output contacts and logic inputs areoperating correctly. Given the microprocessor-based relay includes sufficient self-checkingand a common data acquisition system is used for relaying as well as metering, routinemaintenance can be significantly reduced. Many utilities have extended the routinemaintenance cycle of microprocessor-based relays from one and one-half to three times thatused on electromechanical relays.

Other Features of Static and Microprocessor-Based Relays

Static and Microprocessor-based relays offer many other features that electromechanicalrelays do not offer such as fault locating, event reporting, advanced metering functions andcontrol capability. Fault locating has become a standard feature in nearly all microprocessor-based relays. The fault locating information reduces patrol time on permanently faultedlines. The fault locating information can also be used to evaluate problem areas ontransmission lines.

The event record provides data on the internal relay element operation and the currents andvoltage waveforms at the time of operation. This is similar to having a fault recorder onevery breaker where a microprocessor-based relay is installed. The event data is aninvaluable tool in evaluating relay and system performance.

The microprocessor-based relay also provides analog metering quantities such as three-phase currents, voltages, megawatts, and megavars. In many cases, analog transducersare not required. The data can also be directly interfaced digitally to the SCADA RTU .Youcan also send the fault locator information to the system control center for dispatching apatrol crew.

(b) Limitations of Static Relays

Static relays have certain limitations as compared to their equivalent electromechanicalrelays. During last seventy five years electromechanical relay technology has beendeveloped to a satisfactory extent. Reliable and economic electromechanical protectivesystems are being manufactured and used in almost all countries in the world. The usershave enough experience about choice, installation, maintenance, testing, etc. of suchrelays. Whereas static relays have been developed only during past twenty-years. Theirmanufacture and use has increased substantially during last twenty years. The cost of staticrelays is tending to become favorable, especially with the use of integrated circuits which

are now used as building blocks in static relay.The disadvantages and limitations of static relays are the following:


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(i) Auxiliary Voltage Requirement. This disadvantage is not of any importance as auxiliaryvoltage can he obtained from station battery supply and conveniently stepped down to suitlocal requirements.

(ii) Voltage Transients. The static relays are sensitive to voltage spikes or voltagetransients. Such voltage transients caused by operation of breaker and isolator in theprimary circuit of CT's and PTs. Serious over voltage are also caused by breaking of controlcircuit, relay contacts, etc. Such voltage spikes of small duration can damage thesemiconductor components and can also cause maloperation of relays. Several relayfailures were recorded during 1960's due to the above mentioned cause. Themeasurements showed that the voltage spikes in secondary circuits can attain an amplitudeof 20 kV in rare cases and generally 12 kV. Special measures are taken in static relays toovercome this difficulty. These include, use of filter circuits in relays, screening the cableconnected to the relays.

(iii) Temperature Dependence of Static Relays. . The characteristics of semiconductorsare influenced by ambient temperature. For example the amplification factor of a transistor,the forward voltage drop of a diode, etc., change with temperature variation. This was aserious limitation of static relays in the beginning. Accurate measurement of relay should notbe affected by temperature variation. Relays should be accurate over wide rangetemperatures(-10

oCto+50

oC). This difficulty is overcome by the following measures:

Individual component in circuits are used in such a way that change in characteristicof components does not affect the characteristic of the complete relay.

Temperature compensation is provided by means of thermistor circuits, digitalmeasuring techniques, etc. Thus, modern static relays are designed to suit widetemperatures (-10

oC to +50

oC).

(vi) Price. The price of static relays is higher than the equivalent electromechanical types. Inadvanced countries, the difference is gradually reducing now.

(v) In electromagnetic relays, the pick-up of relay or reset of relays does not affect the relaycharacteristic since the operation is based on the comparison between operating torques.However, the static relay characteristic is likely to be affected by the operation of output

device.

Reliability of Static Relays

Reliability of protective relaying is very important. Electromechanical relays have highreliability, due to (1) precision, manufacture (2) few, reliable components in theirconstruction, (3) experience gained in, designing, manufacturing, testing, and maintenance.Relays are in early stage and have to prove their reliability. As the static relays have severaldiscrete components such as resistors, capacitors, semi-conductors in their construction,reliability depends on reliability of these components and reliability of the total assembly.


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Static Relays circuits

The static relay unit comprises several functional circuits such as :

input circuit with main CT's, Auxiliary CT's

rectifiers, smoothing circuits, filters comparator

level detector

amplifiers

time

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