prote atuom.pdf

8/12/2019 prote atuom.pdf

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N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e

as an incident and only those that are cleared by thetripping of the correct circuit breakers are classed as'correct'. The percentage of correct clearances can thenbe determined.

This principle of assessment gives an accurate evaluationof the protection of the system as a whole, but it issevere in its judgement of relay performance. Many

relays are called into operation for each system fault,and all must behave correctly for a correct clearance tobe recorded.

Complete reliability is unlikely ever to be achieved byfurther improvements in construction. If the level of reliability achieved by a single device is not acceptable,improvement can be achieved through redundancy, e.g.duplication of equipment. Two complete, independent,main protection systems are provided, and arranged sothat either by itself can carry out the required function.If the probability of each equipment failing is x/unit, the

resultant probability of both equipments failingsimultaneously, allowing for redundancy, is x2 . Where xis small the resultant risk (x2 ) may be negligible.

Where multiple protection systems are used, the trippingsignal can be provided in a number of different ways.The two most common methods are:

a. all protection systems must operate for a trippingoperation to occur (e.g. two-out-of-twoarrangement)

b. only one protection system need operate to cause

a trip (e.g. one-out-of two arrangement)The former method guards against maloperation whilethe latter guards against failure to operate due to anunrevealed fault in a protection system. Rarely, threemain protection systems are provided, configured in atwo-out-of three tripping arrangement, to provide bothreliability of tripping, and security against unwantedtripping.

It has long been the practice to apply duplicateprotection systems to busbars, both being required tooperate to complete a tripping operation. Loss of abusbar may cause widespread loss of supply, which isclearly undesirable. In other cases, important circuits areprovided with duplicate main protection systems, eitherbeing able to trip independently. On critical circuits, usemay also be made of a digital fault simulator to modelthe relevant section of the power system and check theperformance of the relays used.

2 .5 SELECTIVITY

When a fault occurs, the protection scheme is requiredto trip only those circuit breakers whose operation isrequired to isolate the fault. This property of selectivetripping is also called 'discrimination' and is achieved bytwo general methods.

2.5.1 Time Grading

Protection systems in successive zones are arranged tooperate in times that are graded through the sequence of equipments so that upon the occurrence of a fault,although a number of protection equipments respond,only those relevant to the faulty zone complete thetripping function. The others make incomplete

operations and then reset. The speed of response willoften depend on the severity of the fault, and willgenerally be slower than for a unit system.

2.5.2 Unit Systems

It is possible to design protection systems that respondonly to fault conditions occurring within a clearlydefined zone. This type of protection system is known as'unit protection'. Certain types of unit protection areknown by specific names, e.g. restricted earth fault and

differential protection. Unit protection can be appliedthroughout a power system and, since it does not involvetime grading, is relatively fast in operation. The speed of response is substantially independent of fault severity.

Unit protection usually involves comparison of quantitiesat the boundaries of the protected zone as defined by thelocations of the current transformers. This comparisonmay be achieved by direct hard-wired connections ormay be achieved via a communications link. Howevercertain protection systems derive their 'restricted'property from the configuration of the power system and

may be classed as unit protection, e.g. earth faultprotection applied to the high voltage delta winding of apower transformer. Whichever method is used, it mustbe kept in mind that selectivity is not merely a matter of relay design. It also depends on the correct co-ordination of current transformers and relays with asuitable choice of relay settings, taking into account thepossible range of such variables as fault currents,maximum load current, system impedances and otherrelated factors, where appropriate.

2 .6 STABILITY

The term stability is usually associated with unitprotection schemes and refers to the ability of theprotection system to remain unaffected by conditionsexternal to the protected zone, for example through loadcurrent and external fault conditions.

2 .7 SPEED

The function of protection systems is to isolate faults onthe power system as rapidly as possible. The mainobjective is to safeguard continuity of supply byremoving each disturbance before it leads to widespreadloss of synchronism and consequent collapse of thepower system.

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F u n d a m e n t a l s o f P r o t e c t i o n P r a c t i c e

1 0


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N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e 1 1

As the loading on a power system increases, the phaseshift between voltages at different busbars on thesystem also increases, and therefore so does theprobability that synchronism will be lost when thesystem is disturbed by a fault. The shorter the time afault is allowed to remain in the system, the greater canbe the loading of the system. Figure 2.8 shows typicalrelations between system loading and fault clearancetimes for various types of fault. It will be noted thatphase faults have a more marked effect on the stabilityof the system than a simple earth fault and thereforerequire faster clearance.

Figure 2.8

System stability is not, however, the only consideration.Rapid operation of protection ensures that fault damageis minimised, as energy liberated during a fault is

proportional to the square of the fault current times theduration of the fault. Protection must thus operate asquickly as possible but speed of operation must beweighed against economy. Distribution circuits, whichdo not normally require a fast fault clearance, are usuallyprotected by time-graded systems. Generating plant andEHV systems require protection gear of the highestattainable speed; the only limiting factor will be thenecessity for correct operation, and therefore unitsystems are normal practice.

2 .8 SENSITIVITY

Sensitivity is a term frequently used when referring tothe minimum operating level (current, voltage, poweretc.) of relays or complete protection schemes. The relayor scheme is said to be sensitive if the primary operatingparameter(s) is low.

With older electromechanical relays, sensitivity wasconsidered in terms of the sensitivity of the measuringmovement and was measured in terms of its volt-ampereconsumption to cause operation. With modern digitaland numerical relays the achievable sensitivity is seldomlimited by the device design but by its application andCT/VT parameters.

2 .9 PRIMARY AND BACK-UP PROTECTION

The reliability of a power system has been discussedearlier, including the use of more than one primary (ormain) protection system operating in parallel. In theevent of failure or non-availability of the primaryprotection some other means of ensuring that the faultis isolated must be provided. These secondary systems

are referred to as back-up protection.Back-up protection may be considered as either beinglocal or remote. Local back-up protection is achievedby protection which detects an un-cleared primarysystem fault at its own location and which then trips itsown circuit breakers, e.g. time graded overcurrent relays.Remote back-up protection is provided by protectionthat detects an un-cleared primary system fault at aremote location and then issues a local trip command,e.g. the second or third zones of a distance relay. In bothcases the main and back-up protection systems detect a

fault simultaneously, operation of the back-upprotection being delayed to ensure that the primaryprotection clears the fault if possible. Normally beingunit protection, operation of the primary protection willbe fast and will result in the minimum amount of thepower system being disconnected. Operation of theback-up protection will be, of necessity, slower and willresult in a greater proportion of the primary systembeing lost.

The extent and type of back-up protection applied willnaturally be related to the failure risks and relative

economic importance of the system. For distributionsystems where fault clearance times are not critical, timedelayed remote back-up protection may be adequate.For EHV systems, where system stability is at risk unlessa fault is cleared quickly, multiple primary protectionsystems, operating in parallel and possibly of differenttypes (e.g. distance and unit protection), will be used toensure fast and reliable tripping. Back-up overcurrentprotection may then optionally be applied to ensure thattwo separate protection systems are available duringmaintenance of one of the primary protection systems.

Back-up protection systems should, ideally, becompletely separate from the primary systems. Forexample a circuit protected by a current differential relaymay also have time graded overcurrent and earth faultrelays added to provide circuit breaker tripping in theevent of failure of the main primary unit protection. Tomaintain complete separation and thus integrity, currenttransformers, voltage transformers, relays, circuit breakertrip coils and d.c. supplies would be duplicated. Thisideal is rarely attained in practice. The followingcompromises are typical:

a. separate current transformers (cores and secondarywindings only) are provided. This involves little extracost or accommodation compared with the use of

2


Figure 2.8: Typical power/time relationship for various fault types

Time

Loadpower

Phase-earth

Phase-phase

Three-phasePhase-phase-earth


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common current transformers that would have to belarger because of the combined burden. This practiceis becoming less common when digital or numericalrelays are used, because of the extremely low inputburden of these relay types

b. voltage transformers are not duplicated because of cost and space considerations. Each protection relay

supply is separately protected (fuse or MCB) andcontinuously supervised to ensure security of the VToutput. An alarm is given on failure of the supply and,where appropriate, prevent an unwanted operation of the protection

c. trip supplies to the two protections should beseparately protected (fuse or MCB). Duplication of tripping batteries and of circuit breaker tripping coilsmay be provided. Trip circuits should be continuouslysupervised

d. it is desirable that the main and back-up protections (orduplicate main protections) should operate on differentprinciples, so that unusual events that may causefailure of the one will be less likely to affect the other

Digital and numerical relays may incorporate suitableback-up protection functions (e.g. a distance relay mayalso incorporate time-delayed overcurrent protectionelements as well). A reduction in the hardware required toprovide back-up protection is obtained, but at the risk thata common relay element failure (e.g. the power supply)will result in simultaneous loss of both main and back-upprotection. The acceptability of this situation must beevaluated on a case-by-case basis.

2.10 RELAY OUTPUT DEVICES

In order to perform their intended function, relays must befitted with some means of providing the various outputsignals required. Contacts of various types usually fulfilthis function.

2.10.1 Contact Systems

Relays may be fitted with a variety of contact systemsfor providing electrical outputs for tripping and remoteindication purposes. The most common typesencountered are as follows:

a. Self-reset The contacts remain in the operated condition onlywhile the controlling quantity is applied, returningto their original condition when it is removed

b. Hand or electrical reset These contacts remain in the operated conditionafter the controlling quantity is removed. They canbe reset either by hand or by an auxiliaryelectromagnetic element

The majority of protection relay elements have self-resetcontact systems, which, if so desired, can be modified toprovide hand reset output contacts by the use of auxiliary elements. Hand or electrically reset relays areused when it is necessary to maintain a signal or lockoutcondition. Contacts are shown on diagrams in theposition corresponding to the un-operated or de-energised condition, regardless of the continuous servicecondition of the equipment. For example, anundervoltage relay, which is continually energised innormal circumstances, would still be shown in the de-energised condition.

A 'make' contact is one that closes when the relay picksup, whereas a 'break' contact is one that is closed whenthe relay is de-energised and opens when the relay picksup. Examples of these conventions and variations areshown in Figure 2.9.

A protection relay is usually required to trip a circuitbreaker, the tripping mechanism of which may be asolenoid with a plunger acting directly on themechanism latch or an electrically operated valve. Thepower required by the trip coil of the circuit breaker mayrange from up to 50 watts for a small 'distribution'circuit breaker, to 3000 watts for a large, extra-high-voltage circuit breaker.

The relay may therefore energise the tripping coildirectly, or, according to the coil rating and the numberof circuits to be energised, may do so through theagency of another multi-contact auxiliary relay.

The basic trip circuit is simple, being made up of a hand-trip control switch and the contacts of the protectionrelays in parallel to energise the trip coil from a battery,through a normally open auxiliary switch operated bythe circuit breaker. This auxiliary switch is needed toopen the trip circuit when the circuit breaker openssince the protection relay contacts will usually be quiteincapable of performing the interrupting duty. Theauxiliary switch will be adjusted to close as early aspossible in the closing stroke, to make the protectioneffective in case the breaker is being closed on to a fault.

2


1 2

Figure 2.9: Contact types

Self reset

Hand reset

`make' contacts(normally open)

`break' contacts(normally open)

Time delay onpick up

Time delay ondrop-off


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Where multiple output contacts, or contacts withappreciable current-carrying capacity are required,interposing, contactor type elements will normally be used.

In general, static and microprocessor relays have discretemeasuring and tripping circuits, or modules. Thefunctioning of the measuring modules is independent of operation of the tripping modules. Such a relay is

equivalent to a sensitive electromechanical relay with atripping contactor, so that the number or rating of outputs has no more significance than the fact that theyhave been provided.

For larger switchgear installations the tripping powerrequirement of each circuit breaker is considerable, andfurther, two or more breakers may have to be tripped byone protection system. There may also be remotesignalling requirements, interlocking with otherfunctions (for example auto-reclosing arrangements),and other control functions to be performed. These

various operations may then be carried out by multi-contact tripping relays, which are energised by theprotection relays and provide the necessary number of adequately rated output contacts.

2.10.2 Operation Indicators

Protection systems are invariably provided withindicating devices, called 'flags', or 'targets', as a guidefor operations personnel. Not every relay will have one,as indicators are arranged to operate only if a trip

operation is initiated. Indicators, with very fewexceptions, are bi-stable devices, and may be eithermechanical or electrical. A mechanical indicator consistsof a small shutter that is released by the protection relaymovement to expose the indicator pattern.

Electrical indicators may be simple attracted armatureelements, where operation of the armature releases ashutter to expose an indicator as above, or indicatorlights (usually light emitting diodes). For the latter, somekind of memory circuit is provided to ensure that theindicator remains lit after the initiating event has passed.

With the advent of digital and numerical relays, theoperation indicator has almost become redundant.Relays will be provided with one or two simple indicatorsthat indicate that the relay is powered up and whetheran operation has occurred. The remainder of theinformation previously presented via indicators isavailable by interrogating the relay locally via a man-machine interface (e.g. a keypad and liquid crystaldisplay screen), or remotely via a communication system.

2.11 TRIPPING CIRCUITS

There are three main circuits in use for circuit breakertripping:

a. series sealing

b. shunt reinforcing

c. shunt reinforcement with sealing

These are illustrated in Figure 2.10.

For electromechanical relays, electrically operatedindicators, actuated after the main contacts have closed,avoid imposing an additional friction load on themeasuring element, which would be a serious handicapfor certain types. Care must be taken with directlyoperated indicators to line up their operation with theclosure of the main contacts. The indicator must haveoperated by the time the contacts make, but must nothave done so more than marginally earlier. This is to stopindication occurring when the tripping operation has notbeen completed.

With modern digital and numerical relays, the use of

various alternative methods of providing trip circuitfunctions is largely obsolete. Auxiliary miniaturecontactors are provided within the relay to provideoutput contact functions and the operation of thesecontactors is independent of the measuring system, asmentioned previously. The making current of the relayoutput contacts and the need to avoid these contactsbreaking the trip coil current largely dictates circuitbreaker trip coil arrangements. Comments on thevarious means of providing tripping arrangements are,however, included below as a historical reference

applicable to earlier electromechanical relay designs.

2


Figure 2.10: Typical relay tripping circuits

(a) Series sealing

PR TC

PR TC

PR TC

52a

(b) Shunt reinforcing

52a

(c) Shunt reinforcing with series sealing

52a


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2.11.1 Series sealing

The coil of the series contactor carries the trip currentinitiated by the protection relay, and the contactor closesa contact in parallel with the protection relay contact.This closure relieves the protection relay contact of furtherduty and keeps the tripping circuit securely closed, even if chatter occurs at the main contact. The total tripping time

is not affected, and the indicator does not operate untilcurrent is actually flowing through the trip coil.

The main disadvantage of this method is that such serieselements must have their coils matched with the tripcircuit with which they are associated.

The coil of these contacts must be of low impedance,with about 5% of the trip supply voltage being droppedacross them.

When used in association with high-speed trip relays,which usually interrupt their own coil current, the

auxiliary elements must be fast enough to operate andrelease the flag before their coil current is cut off. Thismay pose a problem in design if a variable number of auxiliary elements (for different phases and so on) maybe required to operate in parallel to energise a commontripping relay.

2.11.2 Shunt reinforcing

Here the sensitive contacts are arranged to trip thecircuit breaker and simultaneously to energise theauxiliary unit, which then reinforces the contact that isenergising the trip coil.

Two contacts are required on the protection relay, sinceit is not permissible to energise the trip coil and thereinforcing contactor in parallel. If this were done, andmore than one protection relay were connected to tripthe same circuit breaker, all the auxiliary relays would beenergised in parallel for each relay operation and theindication would be confused.

The duplicate main contacts are frequently provided as a

three-point arrangement to reduce the number of contact fingers.

2.11.3 Shunt reinforcement with sealing

This is a development of the shunt reinforcing circuit tomake it applicable to situations where there is apossibility of contact bounce for any reason.

Using the shunt reinforcing system under thesecircumstances would result in chattering on the auxiliaryunit, and the possible burning out of the contacts, notonly of the sensitive element but also of the auxiliaryunit. The chattering would end only when the circuitbreaker had finally tripped. The effect of contact bounce

is countered by means of a further contact on theauxiliary unit connected as a retaining contact.

This means that provision must be made for releasing thesealing circuit when tripping is complete; this is adisadvantage, because it is sometimes inconvenient tofind a suitable contact to use for this purpose.

2 .12 TRIP CIRCUIT SUPERVISION

The trip circuit includes the protection relay and othercomponents, such as fuses, links, relay contacts, auxiliaryswitch contacts, etc., and in some cases through aconsiderable amount of circuit wiring with intermediateterminal boards. These interconnections, coupled withthe importance of the circuit, result in a requirement inmany cases to monitor the integrity of the circuit. Thisis known as trip circuit supervision. The simplestarrangement contains a healthy trip lamp, as shown in

Figure 2.11(a).The resistance in series with the lamp prevents thebreaker being tripped by an internal short circuit causedby failure of the lamp. This provides supervision whilethe circuit breaker is closed; a simple extension givespre-closing supervision.

Figure 2.11(b) shows how, the addition of a normallyclosed auxiliary switch and a resistance unit can providesupervision while the breaker is both open and closed.

2


1 4

Figure 2.11: Trip circuit supervision circuits.

PR TC 52a

PR TC

PR TC

52a

52b

(c) Supervision with circuit breaker open or closed with remote alarm (scheme H7)

52a

A

Alarm

52a

52b

TC

Circuit breakerTrip

Trip

(d) Implementation of H5 scheme in numerical relay

(a) Supervision while circuit breaker is closed (scheme H4)

(b) Supervision while circuit breaker is open or closed (scheme H5)

C

B


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In either case, the addition of a normally open push-button contact in series with the lamp will make thesupervision indication available only when required.

Schemes using a lamp to indicate continuity are suitablefor locally controlled installations, but when control isexercised from a distance it is necessary to use a relaysystem. Figure 2.11(c) illustrates such a scheme, which is

applicable wherever a remote signal is required.With the circuit healthy, either or both of relays A and Bare operated and energise relay C . Both A and B mustreset to allow C to drop-off. Relays A , B and C are timedelayed to prevent spurious alarms during tripping orclosing operations. The resistors are mounted separatelyfrom the relays and their values are chosen such that if any one component is inadvertently short-circuited,tripping will not take place.

The alarm supply should be independent of the trippingsupply so that indication will be obtained in case of failure of the tripping supply.

The above schemes are commonly known as the H4, H5and H7 schemes, arising from the diagram references of the Utility specification in which they originallyappeared. Figure 2.11(d) shows implementation of scheme H5 using the facilities of a modern numericalrelay. Remote indication is achieved through use of programmable logic and additional auxiliary outputsavailable in the protection relay.

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Introduction 3.1

Vector algebra 3.2

Manipulation of complex quantities 3.3

Circuit quantities and conventions 3.4

Impedance notation 3.5

Basic circuit laws, 3.6theorems and network reduction

References 3.7

3 F u n d a m e n t a l T h e o r y


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3 .1 INTRODUCTION

The Protection Engineer is concerned with limiting theeffects of disturbances in a power system. Thesedisturbances, if allowed to persist, may damage plantand interrupt the supply of electric energy. They aredescribed as faults (short and open circuits) or powerswings, and result from natural hazards (for instancelightning), plant failure or human error.

To facilitate rapid removal of a disturbance from a powersystem, the system is divided into 'protection zones'.Relays monitor the system quantities (current, voltage)appearing in these zones; if a fault occurs inside a zone,the relays operate to isolate the zone from the remainderof the power system.

The operating characteristic of a relay depends on theenergizing quantities fed to it such as current or voltage,or various combinations of these two quantities, and onthe manner in which the relay is designed to respond tothis information. For example, a directional relaycharacteristic would be obtained by designing the relayto compare the phase angle between voltage and currentat the relaying point. An impedance-measuringcharacteristic, on the other hand, would be obtained bydesigning the relay to divide voltage by current. Manyother more complex relay characteristics may beobtained by supplying various combinations of currentand voltage to the relay. Relays may also be designed torespond to other system quantities such as frequency,power, etc.

In order to apply protection relays, it is usually necessaryto know the limiting values of current and voltage, andtheir relative phase displacement at the relay location,for various types of short circuit and their position in thesystem. This normally requires some system analysis forfaults occurring at various points in the system.

The main components that make up a power system aregenerating sources, transmission and distributionnetworks, and loads. Many transmission and distributioncircuits radiate from key points in the system and thesecircuits are controlled by circuit breakers. For thepurpose of analysis, the power system is treated as a

network of circuit elements contained in branchesradiating from nodes to form closed loops or meshes.The system variables are current and voltage, and in

3 Fundamental Theory


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3

F u n d a m e n t a l T h e o r y

1 8

steady state analysis, they are regarded as time varyingquantities at a single and constant frequency. Thenetwork parameters are impedance and admittance;these are assumed to be linear, bilateral (independent of current direction) and constant for a constant frequency.

3 .2 VECTOR ALGEBRA

A vector represents a quantity in both magnitude anddirection. In Figure 3.1 the vector OP has a magnitude|Z | at an angle with the reference axis OX.

Figure 3.1

It may be resolved into two components at right anglesto each other, in this case x and y . The magnitude orscalar value of vector Z is known as the modulus |Z |, andthe angle is the argument, and is written as arg. Z .The conventional method of expressing a vector Z

is to

write simply |Z | .This form completely specifies a vector for graphicalrepresentation or conversion into other forms.

For vectors to be useful, they must be expressedalgebraically. In Figure 3.1, the vector

Z is the resultant

of vectorially adding its components x and y ;algebraically this vector may be written as:

Z = x + jy Equation 3.1

where the operator j indicates that the component y isperpendicular to component x . In electricalnomenclature, the axis OC is the 'real' or 'in-phase' axis,and the vertical axis OY is called the 'imaginary' or'quadrature' axis. The operator j rotates a vector anti-clockwise through 90. If a vector is made to rotate anti-clockwise through 180, then the operator j hasperformed its function twice, and since the vector hasreversed its sense, then:

j x j or j 2 = -1whence j = -1

The representation of a vector quantity algebraically interms of its rectangular co-ordinates is called a 'complexquantity'. Therefore, x + jy is a complex quantity and isthe rectangular form of the vector | Z | where:

Equation 3.2

From Equations 3.1 and 3.2:Z = |Z | (cos + jsin ) Equation 3.3

and since cos and sin may be expressed inexponential form by the identities:

it follows thatZ may also be written as:

Z = |Z |e j Equation 3.4

Therefore, a vector quantity may also be representedtrigonometrically and exponentially.

3 .3 MANIPULATIONOF COMPLEX QUANTITIES

Complex quantities may be represented in any of thefour co-ordinate systems given below:

a. Polar Z b. Rectangular x + jy

c. Trigonometric |Z | (cos + jsin )

d. Exponential |Z |e j

The modulus |Z | and the argument are together knownas 'polar co-ordinates', and x and y are described as'cartesian co-ordinates'. Conversion between co-ordinate systems is easily achieved. As the operator j obeys the ordinary laws of algebra, complex quantities inrectangular form can be manipulated algebraically, ascan be seen by the following:

Z 1 +

Z 2 = ( x 1+ x 2) + j ( y 1+ y 2) Equation 3.5

Z 1 -

Z 2 = ( x 1- x 2) + j ( y 1- y 2) Equation 3.6

(see Figure 3.2)

cos

= e e j j

2

sin

= e e

j

j j

2

Z x y

y

x x Z

y Z

= +( )=

=

=

2 2

1

tan

cos

sin

Figure 3.1: Vector OP

0

Y

X

P

|Z| y

x

q


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Types of busbar protection systems . .15.4 . . . . . . . .235

Typical examples of timeand current grading,overcurrent relays . . . . . . . . . . . . .9.20 . . . . . . . .143

U

Unbalanced loading (negative sequence protection):- of generators . . . . . . . . . . . . . .17.12 . . . . . . . .293- of motors . . . . . . . . . . . . . . . . .19.7 . . . . . . . .346

Underfrequency protectionof generators . . . . . . . . . . . . . .17.14.2 . . . . . . . .295

Under-power protectionof generators . . . . . . . . . . . . . .17.11.1 . . . . . . . .293

Under-reach of a distance relay . . 11.10.3 . . . . . . . .187

Under-reach of distance relayon parallel lines . . . . . . . . . . . .13.2.2.2 . . . . . . . .204

Undervoltage (Power Quality) . . . . 23.3.9 . . . . . . . .415

Unit protection: . . . . . . . . . . 10.1-10.13 . . . .153-169- balanced voltage system . . . . . . .10.5 . . . . . . . .156- circulating current system . . . . . . 10.4 . . . . . . . .154- digital protection systems . . . . . . 10.8 . . . . . . . .158- electromechanical protection

systems . . . . . . . . . . . . . . . . . .10.7 . . . . . . . .156

- numerical protection systems . . . .

10.8 . . . . . . . .

158- static protection systems . . . . . . .10.7 . . . . . . . .156- summation arrangements . . . . . . 10.6 . . . . . . . .156

Unit protection schemes:- current differential . . . . . . 10.4, 10.10 . . . .154-160- examples of . . . . . . . . . . . . . . .10.12 . . . . . . . .167- multi-ended feeders . . . . . . . . . .13.3 . . . . . . . .207- parallel feeders . . . . . . . . . . . .13.2.1 . . . . . . . .204- phase comparison . . . . . . . . . . .10.11 . . . . . . . .162- signalling in . . . . . . . . . . . . . . . .8.2 . . . . . . . .113- Teed feeders . . . . . . . . .13.3.2-13.3.4 . . . .207-209

- Translay . . . . . . . . . . . .10.7.1, 10.7.2 . . . .156-157- using carrier techniques . . . . . . . .10.9 . . . . . . . .160

Unit transformer protection(for generator unittransformers) . . . . . . . . . . . . . . 17.6.2 . . . . . . . .286

Urban secondary distributionsystem automation . . . . . . . . . . . .25.4 . . . . . . . .447

V

Vacuum circuit breakers (VCBs) . . 18.5.5 . . . . . . . .322 Van Warrington formulafor arc resistance . . . . . . . . . . . . .11.7.3 . . . . . . . .177

Variation of residual quantities . . . . 4.6.3 . . . . . . . .42

Vector algebra . . . . . . . . . . . . . . . .3.2 . . . . . . . .18

Very inverse overcurrent relay . . . . . . 9.6 . . . . . . . .128

Vibration type test . . . . . . . . . . . .21.5.5 . . . . . . . .381

Voltage and phase reversalprotection . . . . . . . . . . . . . . . . .18.10 . . . . . . . .327

Voltage control using substationautomation equipment . . . . . . . . . .24.5 . . . . . . . .430

Voltage controlled overcurrentprotection . . . . . . . . . . . . . . . .17.7.2.1 . . . . . . . .287

Voltage dependent overcurrentprotection . . . . . . . . . . . . . . . . .17.7.2 . . . . . . . .287

Voltage dips (Power Quality) . . . . 23.3.1 . . . . . . . .413

Voltage distribution due toa fault . . . . . . . . . . . . . . . . . . . .4.5.2 . . . . . . . .40

Voltage factors for voltagetransformers . . . . . . . . . . . . . . . .6.2.2 . . . . . . . . .81

Voltage fluctuations (Power Quality) . 23.3.6 . . . . . . . .415

Voltage limit for accurate reachpoint measurement . . . . . . . . . . . .11.5 . . . . . . . .174

Voltage restrained overcurrentprotection . . . . . . . . . . . . . . . .17.7.2.2 . . . . . . . .287

Voltage spikes (Power Quality) . . . 23.3.2 . . . . . . . .413 Voltage surges (Power Quality) . . 23.3.2 . . . . . . . .413

Voltage transformer: . . . . . . . . . 6.2-6.3 . . . . . .80-84- capacitor . . . . . . . . . . . . . . . . . .6.3 . . . . . . . .83- cascade . . . . . . . . . . . . . . . . . .6.2.8 . . . . . . . .82- construction . . . . . . . . . . . . . . .6.2.5 . . . . . . . . .81- errors . . . . . . . . . . . . . . . . . . .6.2.1 . . . . . . . .80- phasing check . . . . . . . . . . . .21.9.4.3 . . . . . . . .389- polarity check . . . . . . . . . . . .21.9.4.1 . . . . . . . .388- ratio check . . . . . . . . . . . . . .21.9.4.2 . . . . . . . .389- residually-connected . . . . . . . . .6.2.6 . . . . . . . . .81- secondary leads . . . . . . . . . . . . .6.2.3 . . . . . . . . .81- supervision in distance relays . .11.10.7 . . . . . . . .188- supervision in numerical relays . . .7.6.2 . . . . . . . .108- transient performance . . . . . . . .6.2.7 . . . . . . . .82- voltage factors . . . . . . . . . . . . .6.2.2 . . . . . . . . .81

Voltage unbalance(Power Quality) . . . . . . . . . . . . . 23.3.7 . . . . . . . .415

Voltage vector shift relay . . . . . .17.21.3 . . . . . . . .307

WWarrington, van, formulafor arc resistance . . . . . . . . . . . . .11.7.3 . . . . . . . .177

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11/11

Wattmetric protection, sensitive . .9.19.2 . . . . . . . .142

Wound primary currenttransformer . . . . . . . . . . . . . . . .6.4.5.1 . . . . . . . .87

ZZero sequence equivalent circuits:- auto-transformer . . . . . . . . . . .5.16.2 . . . . . . . .60- synchronous generator . . . . . . . .5.10 . . . . . . . .55- transformer . . . . . . . . . . . . . . . .5.15 . . . . . . . .57

Zero sequence network . . . . . . . . .4.3.3 . . . . . . . .35

Zero sequence quantities,effect of system earthing on . . . . . . .4.6 . . . . . . . . .41

Zero sequence reactance:- of cables . . . . . . . . . . . . . . . . . .5.24 . . . . . . . .69- of generator . . . . . . . . . . . . . . .5.10 . . . . . . . .55- of overhead lines . . . . . . . .5.21, 5.24 . . . . . . 66-69- of transformer . . . . . . . . . .5.15, 5.17 . . . . . . 57-60

Zone 1 extension scheme(distance protection) . . . . . . . . . . . 12.2 . . . . . . . .194

Zone 1 extension scheme inauto-reclose applications . . . . . . .14.8.2 . . . . . . . .226

Zones of protection . . . . . . . . . . . . .2.3 . . . . . . . . .8

Zones of protection,

distance relay . . . . . . . . . . . . . . . .

11.6 . . . . . . . .

174

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