Distance Protection scheme

Introduction 12.1

Zone 1 extension scheme 12.2

Transfer trip schemes 12.3

Blocking scheme 12.4

Directional comparison unblocking scheme 12.5

Comparison of transfer tripand blocking relaying schemes 12.6

• 1 2 • D i s t a n c e P r o t e c t i o nS c h e m e s

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 9 3 •

12.1 INTRODUCTION

Conventional time-stepped distance protection isillustrated in Figure 12.1. One of the main disadvantagesof this scheme is that the instantaneous Zone 1protection at each end of the protected line cannot beset to cover the whole of the feeder length and is usuallyset to about 80%. This leaves two 'end zones', eachbeing about 20% of the protected feeder length. Faultsin these zones are cleared in Zone 1 time by theprotection at one end of the feeder and in Zone 2 time(typically 0.25 to 0.4 seconds) by the protection at theother end of the feeder.

This situation cannot be tolerated in some applications,for two main reasons:

a. faults remaining on the feeder for Zone 2 time maycause the system to become unstable

b. where high-speed auto-reclosing is used, the non-simultaneous opening of the circuit breakers atboth ends of the faulted section results in no 'deadtime' during the auto-reclose cycle for the fault tobe extinguished and for ionised gases to clear. Thisresults in the possibility that a transient fault willcause permanent lockout of the circuit breakers ateach end of the line section

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Figure 12.1: Conventional distance scheme

Relay Aend zoneRelay ARelay A

Relay Bend zoneRelay Relay

Z3B

Z2T

Z3 0

0

Z3

Z2

Z1

B

Z1B

Z2A

Z3G

Z1A

Tim

e

(a) Stepped time/distance characteristics

Trip

(b) Trip circuit (solid state logic)

0F B CA

≥1

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

Even where instability does not occur, the increasedduration of the disturbance may give rise to powerquality problems, and may result in increased plantdamage.

Unit schemes of protection that compare the conditions atthe two ends of the feeder simultaneously positivelyidentify whether the fault is internal or external to theprotected section and provide high-speed protection forthe whole feeder length. This advantage is balanced by thefact that the unit scheme does not provide the back upprotection for adjacent feeders given by a distance scheme.

The most desirable scheme is obviously a combination ofthe best features of both arrangements, that is,instantaneous tripping over the whole feeder length plusback-up protection to adjacent feeders. This can beachieved by interconnecting the distance protectionrelays at each end of the protected feeder by acommunications channel. Communication techniquesare described in detail in Chapter 8.

The purpose of the communications channel is totransmit information about the system conditions fromone end of the protected line to the other, includingrequests to initiate or prevent tripping of the remotecircuit breaker. The former arrangement is generallyknown as a 'transfer tripping scheme' while the latter isgenerally known as a 'blocking scheme'. However, theterminology of the various schemes varies widely,according to local custom and practice.

12.2 ZONE 1 EXTENSION SCHEME (Z1X SCHEME)

This scheme is intended for use with an auto-reclosefacility, or where no communications channel isavailable, or the channel has failed. Thus it may be usedon radial distribution feeders, or on interconnected linesas a fallback when no communications channel isavailable, e.g. due to maintenance or temporary fault.The scheme is shown in Figure 12.2.

The Zone 1 elements of the distance relay have twosettings. One is set to cover 80% of the protected linelength as in the basic distance scheme. The other, knownas 'Extended Zone 1'or ‘Z1X’, is set to overreach theprotected line, a setting of 120% of the protected linebeing common. The Zone 1 reach is normally controlledby the Z1X setting and is reset to the basic Zone 1 settingwhen a command from the auto-reclose relay is received.

On occurrence of a fault at any point within the Z1Xreach, the relay operates in Zone 1 time, trips the circuitbreaker and initiates auto-reclosure. The Zone 1 reach ofthe distance relay is also reset to the basic value of 80%,prior to the auto-reclose closing pulse being applied tothe breaker. This should also occur when the auto-reclose facility is out of service. Reversion to the Z1Xreach setting occurs only at the end of the reclaim time.For interconnected lines, the Z1X scheme is established(automatically or manually) upon loss of thecommunications channel by selection of the appropriaterelay setting (setting group in a numerical relay). If thefault is transient, the tripped circuit breakers will reclosesuccessfully, but otherwise further tripping during thereclaim time is subject to the discrimination obtainedwith normal Zone 1 and Zone 2 settings.

The disadvantage of the Zone 1 extension scheme is thatexternal faults within the Z1X reach of the relay result intripping of circuit breakers external to the faultedsection, increasing the amount of breaker maintenanceneeded and needless transient loss of supply to someconsumers. This is illustrated in Figure 12.3(a) for asingle circuit line where three circuit breakers operateand in Figure 12.3(b) for a double circuit line, where fivecircuit breakers operate.

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Figure 12.2: Zone 1 extension scheme

Z1A

Z1B

Z3B

Z2B

Z3T

Z2T O

O

CBA

Z1extA

Z1extB

Z2A

Z3A

(a) Distance/time characteristics

Trip

(b) Simplified logic

Zone 1ext

Zone 1

Zone 2

Zone 3

Auto-recloseReset Zone 1ext

≥1

≥1

&


12.3 TRANSFER TRIPPING SCHEMES

A number of these schemes are available, as describedbelow. Selection of an appropriate scheme depends onthe requirements of the system being protected.

12.3.1 Direct Under-reach Transfer Tripping Scheme

The simplest way of reducing the fault clearance time atthe terminal that clears an end zone fault in Zone 2 timeis to adopt a direct transfer trip or intertrip technique, thelogic of which is shown in Figure 12.4.

A contact operated by the Zone 1 relay element isarranged to send a signal to the remote relay requesting atrip. The scheme may be called a 'direct under-reachtransfer tripping scheme’, ‘transfer trip under-reachingscheme', or ‘intertripping under-reach distance protectionscheme’, as the Zone 1 relay elements do not cover thewhole of the line.

A fault F in the end zone at end B in Figure 12.1(a)results in operation of the Zone 1 relay and tripping ofthe circuit breaker at end B. A request to trip is also sentto the relay at end A. The receipt of a signal at Ainitiates tripping immediately because the receive relaycontact is connected directly to the trip relay. Thedisadvantage of this scheme is the possibility ofundesired tripping by accidental operation ormaloperation of signalling equipment, or interference onthe communications channel. As a result, it is notcommonly used.

12.3.2 Permissive Under-reach Transfer Tripping(PUP) Scheme

The direct under-reach transfer tripping scheme describedabove is made more secure by supervising the receivedsignal with the operation of the Zone 2 relay element beforeallowing an instantaneous trip, as shown in Figure 12.5. Thescheme is then known as a 'permissive under-reach transfertripping scheme' (sometimes abbreviated as PUP Z2scheme) or ‘permissive under-reach distance protection’, asboth relays must detect a fault before the remote end relayis permitted to trip in Zone 1 time.

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Figure 12.4: Logic for direct under-reachtransfer tripping scheme

Z1

Z2

Trip

Signal receive

Z3

Signal send

Z2T

Z3T O

O

≥1

Figure 12.3: Performance of Zone 1 extensionscheme in conjunction with auto-reclose relays

(a) Fault within Zone 1 extension reach of distance relays(single circuit lines)

Z1extA

Z1extC Z1C

Z1extB2Z1extB1 Z1B2Z1B1

Z1A

Z1extA

Z1extD

Z1extP

Z1extN

Z1extMZ1extL

Z1extC

Z1D

Z1C

Z1extB Z1B

Z1P

Z1N

Z1MZ1L

Z1A

A

AB C

N M

D

P L

B C

Breakersmarked thusauto-reclose

(b) Fault within Zone 1 extension reach of distance relays(double circuit lines)

Figure 12.5: Permissive under-reachtransfer tripping scheme

Z1

Z2

Trip

Signal receive

Z3

Signal send

Z2T 0

Z3T 0

0 T

≥1

Signalsend

Signalreceive

Signalsend

Signalreceive

Sendcircuit

(f1)

Receivecircuit

(f1)

Sendcircuit

(f1)

Receivecircuit

(f1)Signalling equipment

-End ASignalling equipment

-End B

(b) Signalling arrangement

Dist

ance

rela

y

Dist

ance

rela

y

(a) Signal logic

&


A variant of this scheme, found on some relays, allowstripping by Zone 3 element operation as well as Zone 2,provided the fault is in the forward direction. This issometimes called the PUP-Fwd scheme.

Time delayed resetting of the 'signal received' element isrequired to ensure that the relays at both ends of asingle-end fed faulted line of a parallel feeder circuithave time to trip when the fault is close to one end.Consider a fault F in a double circuit line, as shown inFigure 12.6. The fault is close to end A, so there isnegligible infeed from end B when the fault at F occurs.The protection at B detects a Zone 2 fault only after thebreaker at end A has tripped. It is possible for the Zone 1element at A to reset, thus removing the permissivesignal to B and causing the 'signal received' element atB to reset before the Zone 2 unit at end B operates. It istherefore necessary to delay the resetting of the 'signalreceived' element to ensure high speed tripping at end B.

The PUP schemes require only a single communicationschannel for two-way signalling between the line ends, as thechannel is keyed by the under-reaching Zone 1 elements.

When the circuit breaker at one end is open, or there isa weak infeed such that the relevant relay element doesnot operate, instantaneous clearance cannot be achievedfor end-zone faults near the 'breaker open' terminalunless special features are included, as detailed insection 12.3.5.

12.3.3 Permissive Under-reaching Acceleration Scheme

This scheme is applicable only to zone switched distance

relays that share the same measuring elements for bothZone 1 and Zone 2. In these relays, the reach of themeasuring elements is extended from Zone 1 to Zone 2by means of a range change signal immediately, insteadof after Zone 2 time. It is also called an ‘acceleratedunderreach distance protection scheme’.

The under-reaching Zone 1 unit is arranged to send asignal to the remote end of the feeder in addition totripping the local circuit breaker. The receive relaycontact is arranged to extend the reach of the measuringelement from Zone 1 to Zone 2. This accelerates thefault clearance at the remote end for faults that lie in theregion between the Zone 1 and Zone 2 reaches. Thescheme is shown in Figure 12.7. Modern distance relaysdo not employ switched measuring elements, so thescheme is likely to fall into disuse.

12.3.4 Permissive Over-Reach Transfer Tripping(POP) Scheme

In this scheme, a distance relay element set to reachbeyond the remote end of the protected line is used tosend an intertripping signal to the remote end. However,it is essential that the receive relay contact is monitoredby a directional relay contact to ensure that trippingdoes not take place unless the fault is within theprotected section; see Figure 12.8. The instantaneouscontacts of the Zone 2 unit are arranged to send thesignal, and the received signal, supervised by Zone 2operation, is used to energise the trip circuit. Thescheme is then known as a 'permissive over-reachtransfer tripping scheme' (sometimes abbreviated to‘POP’), 'directional comparison scheme', or ‘permissiveoverreach distance protection scheme’.

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Figure 12.6: PUP scheme: Single-end fedclose-up fault on double circuit line

BF

A

Open

(b) End A relay clears fault and currentstarts feeding from end B

(a) Fault occurs-bus bar voltage low sonegligible fault current via end B

AF

B

Figure 12.7: Permissive under-reachingacceleration scheme


Z2AZ1A

Z1BZ2B

Z3B

Z3T O

OZ2T

Z1

Z3

Z2

Z3A

A B C

Trip

Signal receive

Range change signal

Signal send

≥1

≥1

(b) Signal logic

&

&


Since the signalling channel is keyed by over-reaching Zone2 elements, the scheme requires duplex communicationchannels - one frequency for each direction of signalling.

If distance relays with mho characteristics are used, thescheme may be more advantageous than the permissiveunder-reaching scheme for protecting short lines,because the resistive coverage of the Zone 2 unit may begreater than that of Zone 1.

To prevent operation under current reversal conditions ina parallel feeder circuit, it is necessary to use a currentreversal guard timer to inhibit the tripping of the forwardZone 2 elements. Otherwise maloperation of the schememay occur under current reversal conditions, see Section11.9.9 for more details. It is necessary only when theZone 2 reach is set greater than 150% of the protectedline impedance.

The timer is used to block the permissive trip and signalsend circuits as shown in Figure 12.9. The timer isenergised if a signal is received and there is no operationof Zone 2 elements. An adjustable time delay on pick-up(tp) is usually set to allow instantaneous tripping to takeplace for any internal faults, taking into account apossible slower operation of Zone 2. The timer will haveoperated and blocked the ‘permissive trip’ and ‘signalsend’ circuits by the time the current reversal takes place.

The timer is de-energised if the Zone 2 elements operateor the 'signal received' element resets. The reset timedelay (td) of the timer is set to cover any overlap in timecaused by Zone 2 elements operating and the signalresetting at the remote end, when the current in thehealthy feeder reverses. Using a timer in this mannermeans that no extra time delay is added in thepermissive trip circuit for an internal fault.

The above scheme using Zone 2 relay elements is oftenreferred to as a POP Z2 scheme. An alternative existsthat uses Zone 1 elements instead of Zone 2, and this isreferred to as the POP Z1 scheme.

12.3.5 Weak Infeed Conditions

In the standard permissive over-reach scheme, as withthe permissive under-reach scheme, instantaneousclearance cannot be achieved for end-zone faults underweak infeed or breaker open conditions. To overcomethis disadvantage, two possibilities exist.

The Weak Infeed Echo feature available in someprotection relays allows the remote relay to echo the tripsignal back to the sending relay even if the appropriateremote relay element has not operated. This caters forconditions of the remote end having a weak infeed orcircuit breaker open condition, so that the relevantremote relay element does not operate. Fast clearancefor these faults is now obtained at both ends of the line.The logic is shown in Figure 12.10. A time delay (T1) isrequired in the echo circuit to prevent tripping of theremote end breaker when the local breaker is tripped bythe busbar protection or breaker fail protectionassociated with other feeders connected to the busbar.The time delay ensures that the remote end Zone 2element will reset by the time the echoed signal isreceived at that end.

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Figure 12.10: Weak Infeed Echo logic circuit

Signalsend

Signalreceive

Breaker'open'

To 'POP' trip logic(Figure 12.8)

From 'POP' signalsend logic(Figure 12.8)

0T1 T2 0≥1& &

Figure 12.8: Permissive over-reach transfer tripping scheme

Signal receive

Z1

Z2T O

OZ3TZ3

Z2

Trip

Signal send

≥1

Signalsend

Signalreceive

Signalsend

Signalreceive

Sendcircuit

(f1)

Receivecircuit

(f2)

Sendcircuit

(f2)

f2f1

f2 f1

Receivecircuit

(f1)

Signalling equipment-End A

Signalling equipment-End B

(b) Signalling arrangement

Dist

ance

rela

y

Dist

ance

rela

y

(a) Signal logic

&

Figure 12.9: Current reversal guard logic – permissive over-reach scheme

Signal receive

Z3

Z2Trip

Signal send

Z1

Z2T O

OZ3T

tp td

≥1

&&

&


Signal transmission can take place even after the remoteend breaker has tripped. This gives rise to the possibilityof continuous signal transmission due to lock-up of bothsignals. Timer T2 is used to prevent this. After this timedelay, 'signal send' is blocked.

A variation on the Weak Infeed Echo feature is to allowtripping of the remote relay under the circumstancesdescribed above, providing that an undervoltagecondition exists, due to the fault. This is known as theWeak Infeed Trip feature and ensures that both ends aretripped if the conditions are satisfied.

12.4 BLOCKING OVER-REACHING SCHEMES

The arrangements described so far have used the signallingchannel(s) to transmit a tripping instruction. If thesignalling channel fails or there is no Weak Infeed featureprovided, end-zone faults may take longer to be cleared.

Blocking over-reaching schemes use an over-reachingdistance scheme and inverse logic. Signalling is initiatedonly for external faults and signalling transmission takesplace over healthy line sections. Fast fault clearanceoccurs when no signal is received and the over-reachingZone 2 distance measuring elements looking into the lineoperate. The signalling channel is keyed by reverse-looking distance elements (Z3 in the diagram, thoughwhich zone is used depends on the particular relay used).An ideal blocking scheme is shown in Figure 12.11.

The single frequency signalling channel operates bothlocal and remote receive relays when a block signal isinitiated at any end of the protected section.

12.4.1 Practical Blocking Schemes

A blocking instruction has to be sent by the reverse-looking relay elements to prevent instantaneous trippingof the remote relay for Zone 2 faults external to theprotected section. To achieve this, the reverse-lookingelements and the signalling channel must operate fasterthan the forward-looking elements. In practice, this isseldom the case and to ensure discrimination, a shorttime delay is generally introduced into the blockingmode trip circuit. Either the Zone 2 or Zone 1 elementcan be used as the forward-looking element, giving riseto two variants of the scheme.

12.4.1.1 Blocking over-reaching protection scheme usingZone 2 element

This scheme (sometimes abbreviated to BOP Z2) is basedon the ideal blocking scheme of Figure 12.11, but has thesignal logic illustrated in Figure 12.12. It is also knownas a ‘directional comparison blocking scheme’ or a‘blocking over-reach distance protection scheme’.

Operation of the scheme can be understood byconsidering the faults shown at F1, F2 and F3 in Figure12.11 along with the signal logic of Figure 12.12.

A fault at F1 is seen by the Zone 1 relay elements atboth ends A and B; as a result, the fault is clearedinstantaneously at both ends of the protected line.Signalling is controlled by the Z3 elements looking awayfrom the protected section, so no transmission takesplace, thus giving fast tripping via the forward-lookingZone 1 elements.

A fault at F2 is seen by the forward-looking Zone 2elements at ends A and B and by the Zone 1 elements at

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Figure 12.11: Ideal distance protectionblocking scheme

Z3AZ2A

Z2B

Z2T O

OZ3T

Z1B

Z3B

Z1A

F1

Z1

Z2

Z3

A B CF2


(b) Simplified logic

Signalsend

Sendcircuit

(f1)

Receivecircuit

(f1)

Sendcircuit

(f1)

Receivecircuit

(f1)Signalreceive

Signalsend

SignalreceiveDi

stan

ce re

lay

Dist

ance

rela

y

(c) Signalling arrangement

F3

Signal receive

Trip

Signal send

Signalling equipment-End B

Signalling equipment-End A

≥1

&

Figure 12.12: Signal logic for BOP Z2 scheme

Trip

Z2

Z3

Z1

Z2T

td

O

O

O

O

STL

Z3T

Signal send

Channel in service

Signal receive

≥1

&


end B. No signal transmission takes place, since thefault is internal and the fault is cleared in Zone 1 time atend B and after the short time lag (STL) at end A.

A fault at F3 is seen by the reverse-looking Z3 elementsat end B and the forward looking Zone 2 elements at endA. The Zone 1 relay elements at end B associated withline section B-C would normally clear the fault at F3. Toprevent the Z2 elements at end A from tripping, thereverse-looking Zone 3 elements at end B send ablocking signal to end A. If the fault is not clearedinstantaneously by the protection on line section B-C,the trip signal will be given at end B for section A-Bafter the Z3 time delay.

The setting of the reverse-looking Zone 3 elements mustbe greater than that of the Zone 2 elements at theremote end of the feeder, otherwise there is thepossibility of Zone 2 elements initiating tripping and thereverse looking Zone 3 elements failing to see anexternal fault. This would result in instantaneoustripping for an external fault. When the signallingchannel is used for a stabilising signal, as in the abovecase, transmission takes place over a healthy line sectionif power line carrier is used. The signalling channelshould then be more reliable when used in the blockingmode than in tripping mode.

It is essential that the operating times of the variousrelays be skilfully co-ordinated for all system conditions,so that sufficient time is always allowed for the receiptof a blocking signal from the remote end of the feeder.If this is not done accurately, the scheme may trip for anexternal fault or alternatively, the end zone trippingtimes may be delayed longer than is necessary.

If the signalling channel fails, the scheme must bearranged to revert to conventional basic distanceprotection. Normally, the blocking mode trip circuit issupervised by a 'channel-in-service' contact so that theblocking mode trip circuit is isolated when the channel isout of service, as shown in Figure 12.12.

In a practical application, the reverse-looking relayelements may be set with a forward offset characteristicto provide back-up protection for busbar faults after thezone time delay. It is then necessary to stop the blockingsignal being sent for internal faults. This is achieved bymaking the ‘signal send’ circuit conditional upon non-operation of the forward-looking Zone 2 elements, asshown in Figure 12.13.

Blocking schemes, like the permissive over-reachscheme, are also affected by the current reversal in thehealthy feeder due to a fault in a double circuit line. Ifcurrent reversal conditions occur, as described in section11.9.9, it may be possible for the maloperation of abreaker on the healthy line to occur. To avoid this, theresetting of the ‘signal received’ element provided in theblocking scheme is time delayed.

The timer with delayed resetting (td) is set to cover thetime difference between the maximum resetting time ofreverse-looking Zone 3 elements and the signallingchannel. So, if there is a momentary loss of the blockingsignal during the current reversal, the timer does nothave time to reset in the blocking mode trip circuit andno false tripping takes place.

12.4.1.2 Blocking over-reaching protection scheme usingZone 1 element

This is similar to the BOP Z2 scheme described above,except that an over-reaching Zone 1 element is used inthe logic, instead of the Zone 2 element. It may also beknown as the BOP Z1 scheme.

12.4.2 Weak Infeed Conditions

The protection at the strong infeed terminal will operatefor all internal faults, since a blocking signal is notreceived from the weak infeed terminal end. In the caseof external faults behind the weak infeed terminal, thereverse-looking elements at that end will see the faultcurrent fed from the strong infeed terminal and operate,initiating a block signal to the remote end. The relay atthe strong infeed end operates correctly without theneed for any additional circuits. The relay at the weakinfeed end cannot operate for internal faults, and sotripping of that breaker is possible only by means ofdirect intertripping from the strong source end.

12.5 DIRECTIONAL COMPARISONUNBLOCKING SCHEME

The permissive over-reach scheme described in Section12.3.4 can be arranged to operate on a directionalcomparison unblocking principle by providing additionalcircuitry in the signalling equipment. In this scheme(also called a ’deblocking overreach distance protection

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Figure 12.13: Blocking scheme using reverse-looking relays with offset

(b) Solid state logic of send circuit

Signal send


Z3G

Z3

Z2

Z2G

G H

Z1G

Z1H

Z2H

Z3H

&


scheme’), a continuous block (or guard) signal istransmitted. When the over-reaching distance elementsoperate, the frequency of the signal transmitted isshifted to an 'unblock' (trip) frequency. The receipt of theunblock frequency signal and the operation of over-reaching distance elements allow fast tripping to occurfor faults within the protected zone. In principle, thescheme is similar to the permissive over-reach scheme.

The scheme is made more dependable than the standardpermissive over-reach scheme by providing additionalcircuits in the receiver equipment. These allow trippingto take place for internal faults even if the transmittedunblock signal is short-circuited by the fault. This isachieved by allowing aided tripping for a short timeinterval, typically 100 to 150 milliseconds, after the lossof both the block and the unblock frequency signals.After this time interval, aided tripping is permitted onlyif the unblock frequency signal is received.

This arrangement gives the scheme improved security over ablocking scheme, since tripping for external faults is possibleonly if the fault occurs within the above time interval ofchannel failure. Weak Infeed terminal conditions can becatered for by the techniques detailed in Section 12.3.5.

In this way, the scheme has the dependability of ablocking scheme and the security of a permissive over-reach scheme. This scheme is generally preferred whenpower line carrier is used, except when continuoustransmission of signal is not acceptable.

12.6 COMPARISON OF TRANSFER TRIPAND BLOCKING RELAYING SCHEMES

On normal two-terminal lines the main deciding factors inthe choice of the type of scheme, apart from the reliabilityof the signalling channel previously discussed, areoperating speed and the method of operation of thesystem. Table 12.1 compares the important characteristicsof the various types of scheme.

Modern digital or numerical distance relays are providedwith a choice of several schemes in the same relay. Thusscheme selection is now largely independent of relayselection, and the user is assured that a relay is availablewith all the required features to cope with changingsystem conditions.

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Criterion Transfer tripping scheme Blocking scheme

Speed of operation Fast Not as fast

Speed with in-service testing Slower As fast

Suitable for auto-reclose Yes Yes

Security againstmaloperation due to:

Current reversal Special features required Special features required

Loss of communications Poor Good

Weak Infeed/Open CB Special features required Special features required

Table 12.1: Comparison of differentdistance protection schemes

Distance Protection scheme

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