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PART A

POWER SYTSEM PROTECTION

CHAPTER

1

BASIC PRINCIPLES

1.1 INTRODUCTION TO PROTECTIVE RELAYING

Protective Relaying or Protection is the term that defines the branch ofelectric power engineering that is concerned with the detection anddisconnection of short-circuits (faults) and other abnormal conditions onthe power system.

There are three aspects of the design and operation of a power systemthat are important in considering the role of protective relaying:

Normal operation Prevention of electrical failure Mitigation of the effects of electrical failure.The term normal operation assumes no failures of equipment, no

mistakes of personnel, nor acts of God. It involves the minimumrequirements for supplying the existing customer load and a certain amountof anticipated future load. Design of the power system for normal operationinvolves major expense for equipment and includes consideration of:

Choice between hydro, steam, or other sources of power Location of generating stations Transmission of power to the load Study of the load characteristics and planning for its future

growth Metering Voltage and frequency regulation System operation

4 POWER SYSTEM PROTECTION AND COMMUNICATIONS

Maintenance requirements The consequences of equipment or plant failure.Protection systems must not interfere with or limit the normal

operation of the system but must continuously monitor the system to detectelectrical failure or abnormal electrical conditions.

Further important aspects in the design of the power system are: Incorporation of features aimed at preventing failures, and Provisions for mitigating the effects of failure when it occurs.Modern power system design employs both recourse as dictated by

the economics of any particular situation. Notable advances continue tobe made toward greater reliability. However also, increasingly greaterreliance is being placed on electric power. Consequently, even though theprobability of failure is decreased, the tolerance of the possible harm to theservice is also decreased.

The type of electrical failure that causes greatest concern is theshort-circuit, or fault as it is usually called, but there are other abnormaloperating conditions peculiar to certain elements of the system that alsorequire attention. Some of the features of design and operation aimed atpreventing electrical failure are:

Provision of adequate insulation Coordination of insulation strength with the capabilities of

lightning surge arresters Use of overhead ground wires and low tower-footing resistance Design for mechanical strength to reduce exposure, and to

minimise the likelihood of failure caused by animals, birds,insects, dirt, sleet, bushfires, etc.

Proper operation and maintenance practice.Some of the features of design and operation for mitigating the effects

of failure are: Features that mitigate the immediate effects of an electrical failure

1. Design to limit the magnitude of short-circuit current(a) By avoiding too large concentrations of generating

capacity(b) By using current-limiting impedance

2. Design to withstand mechanical stresses and heating owingto short-circuit currents

3. Time-delay undervoltage devices on circuit breakers toprevent dropping loads during momentary voltage dips

4. Ground-fault neutralisers.

BASIC PRINCIPLES 5

Features for promptly disconnecting the faulty element1. Protective relaying2. Circuit breakers with sufficient interrupting capacity3. Fuses.

Features that mitigate the loss of the faulty element1. Alternate circuits2. Reserve generator and transformer capacity3. Automatic reclosing.

Features that operate throughout the period from the inception of thefault until after its removal, to maintain voltage and stability

1. Automatic voltage regulation2. Stability characteristics of generators.

Means for observing the effectiveness of the foregoing features1. Automatic oscillographs2. Efficient human observation and record keeping.

Frequent surveys as system changes or additions are made, to be surethat the foregoing features are still adequate.

Thus, protective relaying is one of several features of system designconcerned with minimising damage to equipment and interruptions toservice when electrical failures occur. WHEN WE SAY THAT RELAYSPROTECT, WE MEAN THAT, TOGETHER WITH OTHER EQUIPMENT,THE RELAYS HELP TO MINIMISE DAMAGE AND IMPROVE SERVICEAT A MINIMUM COST. It will be evident that all the mitigation featuresare dependent on one another for successfully minimising the effects offailure.

1.2 POWER SYSTEM PLANT AND LAYOUT

The following section gives an overview of plant and the electricalcharacteristic that are relevant to design of the protection system. Theyalso outline some of the various switching arrangements that are likely tobe encountered on the power system.

The aim is to identify, for the reader, some of the underlying issuesthat need to be considered in the design of protection.

1.2.1 Power System PlantThe system for generation, transmission and distribution of electricity ismade up of generators, lines, transformers, reactive plant (capacitors andstatic compensators) etc. connected in a network to provide reliabletransport of electrical energy from the generation source to the customer.The parameters of the plant (size and electrical specifications) and its


associated auxiliary equipment, together with the arrangement of thenetwork, have a significant influence on the design of the protection system.

Plant impedances and earthing arrangements will determine themagnitude and path of fault currents. Number and location of current andvoltage transformers will determine the arrangement of protection zoneswhich, in turn affects the reliability of the whole power system.Consequently, the protection engineer must have a sound knowledge ofthe design of both the electrical plant and the power system in order toinfluence the design.

The following paragraphs briefly outline some of the plant andsystem design considerations.

1.2.2 GeneratorsGenerators appear in a number of sizes ranging from less than 1 MW(typically in a cogeneration plant) to 600 MW or more in a large fossilfuelled station. Generated voltages are generally constrained in the rangeof 6.6 kV to 33 kV due to design limitations in the generator insulationsystems. This means that step-up transformers are generally needed toconnect the generator to the transmission system. Important parametersin the design of protection for the system and the generator are the generatorimpedances.

A distinction is made for two conditions, namely the direct andquadrature axis which cover the positions when the axis of the rotor polesare in phase with the machine poles, or 90 electrical degrees out of phase.Fault currents (resulting from a short-circuit on the power system) aremainly reactive and as they cause drops in the direct axis voltage, we usethe direct axis impedances for fault calculations.

The impedance of the generator varies with time following inceptionof a fault, due to the inductive nature of the generator electrical circuit.The value depends on the time that has elapsed from the inception of ashort-circuit. Impedances in three time zones are specified for calculationof currents and voltages:

Subtransient impedance (Xd)determines the level forshort-circuit current during the first 1 to 3 cycles after short-circuitinception.

Transient impedance (Xd)determines the level of current thata particular generator will contribute to a short-circuit duringthe transient period between 3 to 20 cycles.

Synchronous impedance (Xd)determines the steady state valueof short-circuit current after the transient period.

BASIC PRINCIPLES 7

The time constant that determines the duration of the subtransientand transient periods and related offset of the short-circuit current isdetermined by the inductance and resistance of the generator. It is oftenreferred to as the X/R ratio of the generator. In a multi-generator systemthe X/R ratio is highest near the generation source and reduces as linesand transformers are interposed between the generation and the load. TheX/R ratio is important in the determination of required current transformerperformance, as you will see in Chapter 4.

For protection calculations, we assume that the nominal terminalvoltage of all machines is acting behind the machine impedance i.e., allmachines are unloaded, and their voltages are all in phase. Someorganisations use the subtransient impedance Xd for fault calculationsand apply a decrement to reduce current with time, depending on themeasuring and operating time of the protection relays. This can beappropriate if accurate high speed measurement is required, however, themajority use the transient impedance Xd and assume that the current doesnot change during the protection relay operating period. This is adequatefor most applications and these sections are based on the use of the transientimpedance Xd.

Also of importance in the design of the protection system is themethod of earthing, which determines the paths for earth fault currents inthe system. Generator neutrals are generally earthed through a highimpedance to limit the flow of earth fault currents in the generator windingsand eliminate the damage that this would cause. The path for earth faultcurrents on the external power system is established through earthing oftransformer neutral connections.

1.2.3 Transformers

Power transformers of various sizes are located throughout the powersystem. Step-up transformers convert the generator voltage to levelssuitable for the transmission system which transmits bulk power to theload centres. Depending on the size of the system, transmission voltageswill range from 132 kV to 500 kV. Step-down transformers reduce thevoltages at the bulk load centres to typically 66 kV or 33 kV for distributionthrough a subtransmission network which supplies the high voltagedistribution system. The distributions system is typically 33, 22 or 11 kVand supplies distribution substations that transform the voltage to thecustomer level.


Large transformers in the generating stations or transmission stationsmay be made up of three single phase units or a single three phase unit.Physical size and transport limitations can frequently determine the choicethat has to be made. Single phase units, as well as taking up more space,have more complex connection arrangements, particularly with theconnection of the delta or tertiary winding. The external delta connectionsare more exposed to faults and failure can result in high short-circuitcurrents, which can be disastrous mechanically for the delta windings.

Inter-winding impedances, winding connections (i.e., delta, star,interstar) and earthing arrangements are important for the protectionengineer. These factors determine the magnitude and path of fault currentsand consequently the ability for protection systems to selectively detectand clear faults from the system.

It is usually sufficient to use the inductive component of thetransformer impedance in protection calculations and this will usually beexpressed as a per cent or per unit at rating i.e., per cent impedance is thepercentage voltage drop across the transformer at rated voltage and current.With a short-circuit on the terminals the current will be:

IZ

Rated 100%

1.2.4 LinesImpedances, for calculation of fault currents, are the most important lineparameter for protection purposes. These are usually calculated in resistiveand reactive ohms at system frequency and are expressed in the form R + jxor ZQ. The R term is the resistance per phase and the jx term is obtainedfrom the basic equation of the type

jx = k log10 Separation of conductorsk Radius of conductor1

FHG

IKJ

Factors that influence the impedance include the presence ofoverhead earth wire and mutual coupling with parallel lines.

1.3 SWITCHING ARRANGEMENTS

Switching arrangements used in a particular power system or individualstations within the system are influenced by a number of factors and thereis no clear right or wrong arrangement. Factors that need to be consideredare:

BASIC PRINCIPLES 9

Economic and business investment criteria, History of development of the individual power system i.e.,

decisions made in the past can be uneconomic to change becauseof widespread changes that may be required,

Ease and safety of operation and maintenance, Security, reliability and quality of supply to the customer, Flexibility for future development.There are many switching arrangements used on the power system,

all of which influence the design of the protection system.A major consideration for the Protection Engineer is the ability to

establish appropriate protection zones that will selectively isolate faultyitems of plant. In this respect the number and location of current and voltagetransformers is a major consideration. The preference would be to locatecurrent transformers on each side of the circuit breaker, transformer andgenerator so that independent overlapping zones of protection can beestablished for each plant item. This practice can result in significant costs,either in the cost of the plant item itself (e.g., if the CTs are mounted withinthe CB structure) or in the cost of additional space and structures to mountfree standing CTs in the switchyard. A frequent compromise is to provideCTs on one side of the plant. With this arrangement it is possible to achieveoverlapping zones of protection but it can result in blind spots or deadzones which requires special measures. For example, with CTs locatedon the line side of a circuit breaker, a fault between the CB and the CT postwill be detected by the busbar protection zone but is outside the lineprotection zone. The bus protection will operate to trip the local circuitbreakers but the protection at the other end of the line must detect andclear the fault from that end.

1.3.1 Single Switching

Each item of plant has its own CB. This arrangement: (see Fig. 1.1) is economic in terms of plant requirements, is straight forward and safe to operate and maintain, has few complications from a protection viewpoint, apart from

selecting the location for CTs and VTs.The major disadvantage is the inflexibility in programming

maintenance. For example, an outage of a CB will result in the loss of theassociated plant item to the system (possible a major generator ortransformer).


FIGURE 1.1 Single switching

1.3.2 Double SwitchingEach plant item has two circuit breakers to provide the ability to switch toeither of two bus-bars.

This is a very flexible arrangement (Fig. 1.2) and has the majoradvantage that any item of plant can be transferred from bus to bus withoutinterrupting the circuit that it feeds. Again there are no particular designproblems from a protection viewpoint. It is relatively easy to establishselective zones for protection of each plant item, the bus-bars and theincoming and outgoing circuits.

FIGURE 1.2 Double switching

BASIC PRINCIPLES 11

The major disadvantage is the high cost of providing circuit breakerand their associated auxiliary equipment and space requirements. Thisadditional expenditure has to be weighed against the gain in revenue orconvenience of keeping generation and other plant in service duringoutages of circuit breakers or bus-bars for maintenance or as a result ofplant failure.

Some savings can be achieved by using a combination of single anddouble switching. For example, it can be argued that the generator couldbe single switched and any maintenance requirements on the CB wouldbe to coordinate with generator maintenance. In this case the generatorsshown in the double switched arrangement, two CBs could be eliminatedby single switching the generators to alternate bus-bars.

1.3.3 Mesh LayoutThis scheme (Fig. 1.3) has most of the advantages of a double bus layout,in that all plant can be kept in service for the outage of any one CB. But, itrequires only one CB for each item of plant in its simplest form comparedwith the two CBs for each item of plant in the double switchedarrangement.

The limit on the number of items of plant in a mesh layout is usuallyaround six in order not to prejudice the system in the event of outages e.g.,if CB A is open for maintenance and a fault occurs on Feeder 1, the system

G1

Feeder 1

G2

A

FIGURE 1.3 Six circuit breaker mesh

is left with generator 2 disconnected. The mesh layout is flexible and usesless CBs than the double switched arrangement. There are no particular


design problems from a protection viewpoint. It is relatively easy toestablish selective zones for protection of each plant item and the incomingand outgoing circuits provided current transformers are provided witheach circuit breaker and plant item and, depending on the protectionselected, voltage transformers are provided in the outgoing lines.

1.3.4 1 CB SwitchingA more elaborate system than the mesh system is the 1 CB arrangement(Fig. 1.4).

FIGURE 1.4 1 Circuit breaker

This arrangement uses more CBs than the mesh arrangement butgives better reliability for faults in the transmission lines or generationplant. Again, provided current transformers and voltage transformers arecarefully located the protection arrangements is straight forward.

1.3.5 Transfer Bus ArrangementThis arrangement (Fig. 1.5) is applicable to stations where there are a largenumber of feeders. It permits more flexibility than the single switchedarrangement as any feeder may be kept in service while its CB is out ofservice, by using the transfer bus and connecting the feeder either in parallelwith another feeder or to a spare CB.

The system is more complex to operate and can require switching ofcurrent transformers and protection circuits through auxiliary switcheson the transfer isolators, to maintain adequate protection on the feeders.

Problems can also arise with the operation of earth fault protectionwhen feeders are operated in parallel due to the unbalance in load currents

BASIC PRINCIPLES 13

giving rise to artificial earth fault current in the relay circuits. Specialoperating procedures may be required to overcome this problem.

FIGURE 1.5 Transfer bus

1.4 THE FUNCTION OF PROTECTIVE RELAYING

The function of protective relaying is to cause the prompt removal fromservice of any element of a power system when it suffers a short-circuit, orwhen it starts to operate in any abnormal manner that might cause damageor otherwise interfere with the effective operation of the rest of the system.It achieves this through relays and protection schemes that measure powersystem quantities, detect a fault or abnormal condition and open (trip)appropriate circuit breakers.

Circuit breakers are generally located so that each generator,transformer, bus, transmission line, etc., can be completely disconnectedfrom the rest of the system. These circuit breakers must have sufficientcapacity so that they can carry momentarily the maximum short-circuitcurrent that can flow through them, and then interrupt this current; theymust also withstand closing in on such a short-circuit and then interruptingit according to certain prescribed standards.

Fusing is employed where protective relays and circuit breakers arenot economically justifiable.

A secondary function of protective relaying is to provide indicationof the location and type of failure. Such data not only assists in expeditingrepair but also, by comparison with human observation and automaticoscillograph records, they provide means for analysing the effectivenessof the fault-prevention and mitigation features including the protectiverelaying itself.


1.5 PRINCIPLES OF PROTECTIVE RELAYING

The protection system can be divided into two main groups: primary relaying back up relaying.Primary relaying is the first line of defence, whereas back up relaying

provides for failure of the primary protection to clear the fault orabnormality, either through failure of protection equipment or primaryplant.

1.5.1 Primary RelayingFig. 1.6 illustrates primary relaying.

Generator

Circuitbreaker

Low voltageswitchgear

High voltageswitchgear

High voltageswitchgear

Transmission linePower transformer

FIGURE 1.6 Single line diagram of a portion of an electric

power system showing primary relaying

Observation: Circuit breakers are located in close proximity to each power

system element. This provision makes it possible to disconnectonly a faulty element. Occasionally, a breaker between twoadjacent elements may be omitted, in which event both elementsmust be disconnected for a failure in either one.

A separate zone of protection is established around each systemelement. The significance of this is that any failure occurringwithin a given zone will cause the tripping (i.e., opening) of allcircuit breakers within that zone, and only those breakers.It will become evident that, for failures within the region wheretwo adjacent protective zones overlap, more breakers will betripped than the minimum necessary to disconnect the faultyelement. However, if there were no overlap, a failure in a regionbetween zones would not lie in either zone, and therefore no

BASIC PRINCIPLES 15

breakers would be tripped. The overlap is the lesser of the twoevils. The extent of the overlap is relatively small, and theprobability of failure in this region is low; consequently, thetripping of too many breakers will be quite infrequent.

Adjacent protective zones of Fig. 1.6 overlap around a circuitbreaker. This is the preferred practice because, for failuresanywhere except in the overlap region, the minimum numbersof circuit breakers need to be tripped. When it becomes desirablefor economic or space-saving reasons to overlap on one side of abreaker, as is frequently true in metal-clad switchgear, the relayingequipment of the zone that overlaps the breaker must be arrangedto trip not only the breakers within its zone but also one or morebreakers of the adjacent zone, in order to completely disconnectcertain faults.

1.5.2 Back up RelayingBack up relaying is intended to operate when a system fault is not clearedin due time because of failure or inability of the main protection or theassociated protection to operate.

A clear understanding of the possible causes of primary-relayingfailure is necessary for a better appreciation of the practices involved inback up relaying. When primary relaying fail several things may happento prevent primary relaying from causing the disconnection of a powersystem fault. Primary relaying may fail because of failure in any of thefollowing:

Current or voltage supply to the relays DC tripping-voltage supply Protective relays Tripping circuit or breaker mechanism Circuit breaker.It is highly desirable that back up relaying be arranged so that

anything that might cause primary relaying to fail will not also cause failureof back up relaying. Two principles are applied:

Remote back up Local back up.With remote back up the back up relays are located so that they do

not employ or control anything in common with the primary relays thatare to be backed up. So far as possible, the practice is to locate the back uprelays at a different station. Consider, for example, the back up relayingfor the transmission line section EF of Fig. 1.7. The back up relays for this


line section is normally arranged to trip breakers A, B, I, and J. Shouldbreaker E fail to trip for a fault on the line section EF, breakers A and B aretripped; breakers A and B and their associated back up relaying equipment,being physically apart from the equipment that has failed, are not likely tobe simultaneously affected as might be the case if breakers C and D werechosen instead.

A C

B D

E F

G I

H J

Station K

FIGURE 1.7 Illustration for back up protection

The back up relays at locations A, B, and F provide back up protectionif bus faults occur at station K. Also, the back up relays at A and F providesback up protection for faults in the line DB. In other words, the zone ofprotection of back up relaying extends in one direction from the locationof any back up relay and at least overlaps each adjacent system element.Where adjacent line sections are of different length, the back up relaysmust overreach some line sections more than others in order to provideback up protection for the longest line.

A given set of back up relays will provide incidental back upprotection for faults in the circuit whose breaker the back up relays control.For example, the back up relays that trip breaker A of Fig. 1.7 may also actas back up for faults in the line section AB. However, this duplication ofprotection is only an incidental benefit and is not to be relied on to theexclusion of a conventional back up arrangement when such arrangementis possible; to differentiate between the two, this type might be calledduplicate primary relaying.

A second function of back up relaying is often to provide primaryprotection when the primary-relaying equipment is out of service formaintenance or repair.

It is perhaps evident that, when back up relaying functions, a largerpart of the system is disconnected than when primary relaying operatescorrectly. This is inevitable if back up relaying is to be made independentof those factors that might cause primary relaying to fail. However, itemphasises the importance of the second requirement of back up relaying,that it must operate with sufficient time delay so that primary relayingwill be given enough time to function if it is able to. In other words, whena short-circuit occurs, both primary relaying and back up relaying will

BASIC PRINCIPLES 17

normally start to operate, but primary relaying is expected to trip thenecessary breakers to remove the short-circuited element from the system,and back up relaying will then reset without having had time to completeits function. When a given set of relays provides back up protection forseveral adjacent system elements, the slowest primary relaying of any ofthose adjacent elements will determine the necessary time delay of thegiven back up relays.

Local back up provides for the initiation of the required action atthe same location as that at which the main protection is situated. Localback up usually involves the provision of two completely independent(duplicate) protection systems including relays, current transformers,circuit breaker trip coils, etc.

For many applications, it is impossible to abide by the principle ofcomplete segregation of the back up relays. Then one tries to supply theback up relays from sources other than those that supply the primary relaysof the system element in question, and to trip other breakers. This canusually be accomplished; however, the same tripping battery may beemployed in common, to save money and because it is considered only aminor risk.

1.6 UNIT AND NON-UNIT SCHEMES

The purpose of an electrical power generation system is to distribute energyto a multiplicity of points for diverse applications. The system should bedesigned and managed to deliver this energy to the utilisation points withboth reliability and economy. As there is a natural conflict between thesetwo requirements, some compromise is necessary. Reliability in systemdesign is very important and although it is possible to achieve very highreliability, the economics of doing so due to the excess plant required areprohibitive. Several ways of improving security of supply without addingtoo much to the costs are by:

improving plant design increasing the spare capacity arranging alternative circuits to supply loads.Also such division of the system into zones, each controlled by its

own switchgear in association with protective gear, provides flexibilityduring normal operation and ensures a minimum of dislocation followinga breakdown.

In practical power systems any fault condition, especially ashort-circuit, is a potential threat to a secure supply as such a conditioncannot only disrupt supply to consumers but can also cause irreparable


damage to very expensive equipment. The importance of removing suchabnormal conditions as rapidly as possible, is therefore, quite obvious. Thisis where the protective gear plays its part.

It is the function of protective gear to detect and initiate action toremove disturbances, as soon as it is practicable. Protection is thereforeapplied in overlapping zones to cover the system completely, leaving nopart unprotected. Another important requirement of the protectiveequipment is that only the faulted section should be disconnected andprotective devices must therefore be selective i.e., when a fault occurs theprotection is required to select and trip only the nearest circuit breakers.This property of selective tripping is also called discrimination and isachieved by two general methods.

1. Non Unit SchemesThese are invariably time-graded systems that utilise information

(voltages and currents) derived from a particular point on the system.Protection systems in successive zones as shown in Fig. 1.8 are arranged tooperate in times that are graded through the sequence of equipments tothat upon occurrence of a fault, although a number of protective equipmentsrespond, only those relevant to the faulted zone complete the trippingfunction. The others make incomplete operations and reset. Distanceprotection and time graded overcurrent devices are prime examples ofnon-unit protection.

A

Z3AEnd Zone

Z2AZ1A

BXY

Z1BZ2B

Z3BEnd Zone

Tim

e

Zone 1 = 80-90% of protected lineZone 2 = Protected line + 50% of shortest lineZone 1 = Protected line + longest second line + 25% of third line

X = Circuit breaker operating timeY = Discriminating time

FIGURE 1.8 Protective systems arranged in successive zones

BASIC PRINCIPLES 19

2. Unit Protection

These are schemes that respond to fault conditions lying within aclearly defined zone. They utilise information from two or occasionallymore points in a system. In most cases a unit protection system involvesthe measurement of quantities at each end of the zone, and the transmissionof information between the equipment at zone boundaries. Examples ofunit protection are differential current relays where the current entering azone is compared with that which leaves it. Also phase comparison carrierprotection is another example.

1.7 ZONES OF PROTECTION

The protected zone is that part of a power system guarded by a certainprotection and usually contains one or at the most two elements of thepower system. For a non-unit scheme, the zone lies between the currenttransformers and the point or points on the protected circuit beyond whichthe system is unable to detect the presence of a fault (Figs. 1.9 & 1.10). Fora unit scheme, the zone lies between the two or several sets of currenttransformers and the point or points which together with the relaysconstitute the protective system (Fig. 1.11).

A B

Protected zone

FIGURE 1.9 Protected and back up zones of a non-unit system of protection

A

Protectedzone

R R

C

Y

Back upzone

FIGURE 1.10 Application of a non-unit scheme of protection (i.e. distance

protection with its associated VTs on the line side of the

isolator) and the standby protection zone of the normally

shorted standby protection


X Y

Line

Voltagetransformer

X = Main protection relayY = Standby protection relay

Standby protectionzone

FIGURE 1.11 Protected zone of a unit system protection

1.8 COMMON TERMINOLOGIES

A list of Recommended Terminology is included at the beginning of thebook. Some of the terms that are important for understanding the basicprinciples of the protection system are:

Stability This term refers to the ability of the system to remaininoperative to all load conditions and faults external tothe relevant zone. This quality is present in unit system,as they remain inoperative under all conditions, withfaults outside their own zone. However, non-unitsystems can respond to faults anywhere on the powersystem.

Selectivity Protection is arranged in zones so as to assure no part isleft unprotected. When a fault occurs the protection isrequired to select and trip the nearest circuit breakersonly. Also known widely as Discrimination. In the non-unit systems the discrimination is not absolute, but it isdependant on responses of a number of similar systems,all of which respond to a given abnormal condition.However, for the unit systems, the discrimination isabsolute and it is able to detect and respond to abnormalcondition occurring within the zone of protection.

Sensitivity This term is frequently used when referring to theminimum operating current of a complete protectivesystem. Hence protective system is sensitive, if theprimary current is low. The requirements of all relaysshould be quite sensitive for reliable operation. This termis usually expressed in amperes referred to the primary

BASIC PRINCIPLES 21

circuit or as a percentage of the rated current of thecurrent transformers.

Reliability Power system represents a large capital investment andin order to get maximum return it must be loaded to itsmaximum. The purpose of power system is not only tosupply energy but also to keep the system in fulloperation, in order to give the best service to theconsumers and earn revenue for the supply authority.Failure is not confined to the protective gear but mayalso be due to the failure of the circuit breaker. Henceevery component involved in fault clearance can beregarded as a source of failure.Failures can be reduced by: reliable designs regular maintenance site testing.

Speed The objective of speed is to safeguard continuity ofsupply. Hence if fault can be isolated in the shortest time,the greater the system can be loaded. Fig. 1.12 showstypical values of power that can be transmitted as afunction of fault clearing times for various types of faults.It can be seen that the fault involving phases has markedeffect on stability compared with the line-to-earth faults.The other advantage of having fast clearance times isthat unnecessary changes can occur in the system dueto: high fault arc burn copper conductors machine or transformer lamination weld.

Phase-earthPhase-phase

Two phase-earthThree phase

Time

Load

pow

er

FIGURE 1.12 Typical values of power that can be transmitted as

a function of fault clearance time

Fault currents can cause irreparable damage if allowedto continue for more than a few seconds. Hence fastfault clearance is imperative.

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