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TCSC impact on communication-aided distance-protection schemes and its mitigation

T.S. Sidhu and M. Khederzadeh

Abstract: The paper analyses the impact of the thyristor-controlled series capacitor (TCSC) on theperformance of conventional communication-aided distance-protection schemes and proposes newschemes for its mitigation. The associated TCSC control actions introduce rapid changes thatcreate certain problems in the primary-system parameters such as line impedances and loadcurrents, causing the apparent impedance seen by the distance relay to be affected during the faultperiod; hence the positive-sequence impedance measured by the traditional stand-alone distancerelays is no longer an indicator of the distance to a fault. It is shown that communication-aideddistance-protection schemes that perform successfully in lines with xed series capacitors haveproblems in lines with TCSC. This impact is observed not only on the relays of the compensatedline with TCSC, but also on the relays of adjacent lines. Mitigation of this problem is proposed byusing new communication-aided schemes. The proposed schemes use the information available atthe substation to inhibit relay malfunctions. The performance of the techniques is studied fordifferent TCSC locations in the transmission line. Real-time digital simulation and commercial

relays are used to perform the analysis. The results indicate the effectiveness of the proposedmethods to be applied in the power systems equipped with TCSC.

1 Introduction

Transmission lines can be compensated by thyristor-controlled series capacitors (TCSC) to increase power-transfer capability; limit short-circuit currents; mitigatesubsynchronous resonance (SSR); damp power oscillations;and enhance transient stability. However, the employmentof TCSC can affect the performance of associated protectiverelays using conventional techniques, mainly because of therapid changes in line impedances and load currents.

TCSC introduces new power-system dynamics that mustbe considered for the power-system-protection behaviour.In the limited research reported in the literature, thetransient process of TCSC is not modelled and the TCSC issimply considered as a linear component with nonlinearlimits. In [1], TCSC is being operated with a 10% verniersetting prior to the fault, and the thyristors are operated inthe bypass mode during the fault. The study did notencompass the relay performance during TCSC modula-tion. In [2], only the open-loop-impedance control mode is

embedded in the TCSC control system and, as a result,ring angle could be taken as one of the inputs to the ANN-based relay. In [3], the equations to determine the lineimpedance to the fault are derived based on the TCSC-bypass-mode assumption during the fault. In [4], the

inuence of sampling frequency on the distance-protectionperformance is the main topic considered.

In normal operation, the capacitive reactance of TCSC isnot xed and depends on the operating point and controlstrategy. When a fault occurs, different modes of operationare implemented by the TCSC control system to suppressthe overvoltage, which would change the impedance of theline signicantly. Meanwhile, the transition from normaloperation to other possible modes does not occur instantly.The transition time is considerable in the time frame of theprotection of transmission lines. For a comprehensiveanalysis of the impact of TCSC on protection of the lines,it is necessary to undertake the dynamics of TCSC and thepower system without ignoring their linkage.

The objective of this paper is to analyse and investigatethe impact of TCSC on the performance of conventionalcommunication-aided distance-protection schemes andpropose new solutions to mitigate the problem by usingthe available facilities in the power stations, avoidingessential changes and investments. For an accurate and

reliable analysis, the tests are done by real-time digitalsimulator (RTDS) [5] and sophisticated commercial relays.TCSC is modelled with detailed characteristics such asring-angle control and overvoltage protection and thepower system is designed with traveling-wave transmission-line models. In most of the work done up to now, theoperation of a single relay is considered; while in this papersimultaneous behaviour of four commercial relays locatedat the ends of the main compensated line and its adjacentline is analysed.

The results indicate that TCSC will affect the distanceprotection on both the main compensated line and adjacentlines. Some problem areas are forward overreach of theprotection of the main line and the adjacent lines; reverseoverreach of the adjacent-line protection; deciency of conventional communication-aided distance schemes; anddirectional integrity of a fault. To mitigate the problem,

T.S. Sidhu is with the Department of Electrical and Computer Engineering,University of Western Ontario, London, ON, Canada 5B9 N6A

M. Khederzadeh is with the Electrical Engineering Department, Power & WaterInstitute of Technology, Tehran, Iran. He is currently on leave at the Universityof Western Ontario

E-mail: [email protected]

r IEE, 2005

IEE Proceedings online no. 20045261doi:10.1049/ip-gtd:20045261Paper rst received 29th November 2004 and in nal revised form 27th April2005

714 IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 5, September 2005
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different methods based on communication-aided schemesare proposed. The proposed techniques use new logics fortripping and sending permissive/blocking signals using extradata normally available at the stations. The test results showthe effectiveness of the methods in overcoming theimportant issues of overreaching and directional integrity.

2 TCSC operation and fault handling

Figure 1 shows a TCSC module with different protectiveelements [6] in the middle of a simple power system.Basically, it comprises a series capacitor C in parallel with athyristor-controlled reactor (TCR), Ls. A metal-oxidevaristor MOV is connected across the series capacitor toprevent the occurrence of high capacitor overvoltages. Acircuit breaker is also installed across the TCSC module tobypass it if a severe fault or equipment malfunction occurs.A current-limiting inductor Ld is incorporated in the circuitto restrict both the magnitude and the frequency of thecapacitor current during the capacitor-bypass operation.

There are different modes of operation of TCSC in thenormal and fault conditions [7]. During a fault, anovervoltage appears across the TCSC due to the fault

current. For a conventional series capacitor, MOV andbypass switches are used for its protection, while for the

TCSC, co-operative operation of MOV, thyristors andbypass switches are applied for a reliable protection [8]. Thisis the main difference in the behaviour of the distance relaysin the two compensation methods. The apparent impedanceseen by the distance relay depends largely on the TCSCmode of operation. The TCSC mode of operation during afault is not unique and it may transit from a mode to otherssequentially; meanwhile the distance relay experiencesvarious apparent impedances in the fault period. In normaloperating mode, called capacitive-boost mode or verniermode, the thyristors are ring properly and the bypassswitch is open. From the system point of view, this mode

inserts capacitors into the line up to nearly three times thexed capacitor.

circuit breaker

R 1 R 2

BA

E 2

Z s2

E 1

Zs1 0.5Z line 0.5Z lineMOV

Ls

Ld

F1

C

Fig. 1 Simple power system compensated by TCSC

Z seen

MOV+ verniermode (low I fault )

X

vernier mode

circuit-breaker bypass modebypass mode

R F

Z actual

blocked mode

Z tcsc

MOV+ verniermode (high I fault )

MOV+ blockedmode (high I fault )

busbar B

R

busbar A

MOV+ blockedmode (low I fault )

Fig. 2 Impedances seen by R 1 due to TCSC protection modes for a fault in the middle of AB

IEE Proc.-Gener. Transm. Distrib., Vol. 152, No. 5, September 2005 715
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Figure 1 can be used to estimate the apparent impedanceseen by different relays. For simplicity, only three phasefaults are considered. From this Figure, relay R 1 measuresthe impedance beyond the TCSC as

Z seen Z actual R F Z tcsc 1where Z seen is the impedance measured by the relay, Z actual is the line impedance between the relay and fault location,RF is the fault resistance, and Z tcsc is the TCSC impedance.

Figure 2 shows the behaviour of relay R1 in Fig. 1 basedon different TCSC protection modes. As can be deducedfrom Fig. 2, in contrast to series-compensated lines withxed capacitors, the protective relays under- and over-estimate the distance to a fault in a sequential manner.Sometimes, the underestimation may lead to loss of directional integrity. According to Fig. 2, different possibleTCSC modes of operation during a fault and the behaviourof the relays can be summarised as follows:

(i) Capacitive-boost mode without MOV conduction : For alow fault current, the protection function of the TCSCdevice does not work; therefore the TCSC remainsconstantly in its vernier mode of normal operation; i.e.the TCR branch is triggered by its prespecied ring angle.In this case a signicant compensation exists, so theconventional distance relay overreaches considerably; ormay even lose its directional integrity.(ii) Capacitive-boost mode with MOV conduction : For a highfault current, MOV operates for decreasing the voltageacross the capacitor. The MOV is fast enough to conductand reset within a half cycle. The MOV would not short-circuit out the capacitor as the circuit breaker would. Thiscondition is usually very short but may be repeated severaltimes during the fault period. During MOV conduction,Z tcsc is the impedance of the parallel combination of theTCR, capacitor and the MOV in a lower resistance mode.In considering the equivalent MOV circuit proposed in [9],the combination characteristic can be considered as a

resistance in series with a boost capacitor (xed capacitor

and the TCR branch). At this moment, the relay over-reaches but less than is the case without MOV conduction.(iii) Blocking mode : When the thyristors are not triggeredand are kept in nonconducting state, the TCSC is operatingin blocking mode. In this mode, the TCSC performs like axed series capacitor. When the TCSC detects an over-voltage by the MOV current, the TCR branch stops itsring sequence by a protection function. The process iseffective for avoiding the overcurrent of the thyristors orcapacitor caused by uctuation of the ring angle under thecondition that the voltage phase of the capacitors changessuddenly. Herein, the equivalent MOV circuit will be thecombination of a resistance in series with the xed series. Atthis moment the relay overreaches less than is the case withthe thyristor-ring mode. By the process, if the overvoltageand overcurrent are cleared, the system returns to thenormal mode, otherwise the energy absorbed in the MOVexceeds its limitation and the TCSC transits from theblocking mode to the bypass mode to protect the MOV andcapacitor.(iv) Bypass mode : In this mode, the thyristors are triggered

continuously and the TCR branch conducts in the whole

R B2R A2

TCSC

R D1 R C2F1

a

D A B C

R B1R A1

TCSCTCSC

F2

b

A CD

R B2R A2R D1 RC2R B1RA1

B

Fig. 3 Sample test systems

commercial protective relays

R A2R D1

b

real-time digital simulator (RTDS)

communication link

CB

TCSC

RTDS analogue output

TCSC

D A

RTDS digital input

commercial protective relaysa

RTDS analogue output

AD

communication linkR A1R D1 R A2

R A1R D1 R A2

real-time digital simulator (RTDS)

TCSC

CB

R B2

R B2

RTDS digital input

Fig. 4 Test-system arrangement

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Table 1: Behaviour of relays using conventional schemes for the network in Fig. 3 a with load ow from bus D to C

Fault type Fault location Line DA protection

RD1 trip status R A2 trip status

POTT DCUB DCB POTT DCUB DCB

3-phase 50% of AB Zone 1 Zone 1 Zone 1 Zone 1 Zone 1 Zone 1

Phase-to-phase 50% of AB Zone 2 Zone 2 ZCOM Zone 3 Zone 3

Phase-to-neutral 50% of AB Zone 2 Zone 2 ZCOM Zone 3 Zone 3

3-Phase 60% of AB ZCOM ZCOM Zone 1 Zone 1 Zone 3

Phase-to-phase 60% of AB Zone 2 Zone 2 ZCOM Zone 3 Zone 3 Phase-to-neutral 60% of AB Zone 2 Zone 2 ZCOM Zone 3 Zone 3

3-Phase 70% of AB

Phase-to-phase 70% of AB Zone 2 Zone 2 ZCOM

Phase-to-neutral 70% of AB Zone 2 Zone 2 Zone 2

3-Phase 80% of AB



3-Phase 90% of AB

Phase-to-phase 90% of AB Zone 2 Zone 2 Zone 2


3-Phase 100% of AB

Phase-to-phase 100% of AB Zone 2 Zone 2


3-Phase 10% of BC

Phase-to-phase 10% of BC

Phase-to-neutral 10% of BC Zone 2 Zone 2

3-Phase 20% of BC


Phase-to-neutral 20% of BC

Fault type Fault location Line AB protection

RA1 trip status R B2 trip status


3-phase 50% of AB ZCOM ZCOM Zone 1 ZCOM ZCOM Zone 1

Phase-to-phase 50% of AB ZCOM ZCOM Zone 1 ZCOM ZCOM Zone 1

Phase-to-neutral 50% of AB ZCOM ZCOM Zone 1 ZCOM ZCOM Zone 1

3-Phase 60% of AB ZCOM ZCOM Zone 1 ZCOM ZCOM Zone 1














Phase-to-neutral 100% of AB ZCOM ZCOM ZCOM ZCOM ZCOM Zone 1

3-Phase 10% of BC Zone 1 Zone 1 Zone 1

Phase-to-phase 10% of BC Zone 1 Zone 1 Zone 1

Phase-to-neutral 10% of BC Zone 1 Zone 1 ZCOM

3-Phase 20% of BC Zone 1 Zone 1 Zone 1


Phase-to-neutral 20% of BC Zone 1 Zone 1 Zone 1 Zone 1 stands for unconditional direct trip, ZCOM stands for trip by communication-aided schemes and stands for no trip


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Table 2: Behaviour of relays using conventional schemes for the network in Fig. 3 a with load ow from bus C to D




3-phase 50% of AB Zone 1 Zone 1 Zone 2 Zone 3 Zone 3


Phase-to-neutral 50% of AB

3-Phase 60% of AB Zone 3 Zone 3 Zone 3

Phase-to-phase 60% of AB Phase-to-neutral 60% of AB

3-Phase 70% of AB

Phase-to-phase 70% of AB


3-Phase 80% of AB



3-Phase 90% of AB



3-Phase 100% of AB



3-Phase 10% of BC


Phase-to-neutral 10% of BC Zone 2

3-Phase 20% of BC






3-phase 50% of AB ZCOM ZCOM Zone 1 ZCOM ZCOM Zone 1


















3-Phase 10% of BC ZCOM ZCOM Zone 1

Phase-to-phase 10% of BC Zone 2 Zone 2 ZCOM

Phase-to-neutral 10% of BC Zone 1 Zone 1 ZCOM

3-Phase 20% of BC ZCOM ZCOM Zone 2


Phase-to-neutral 20% of BC Zone 1 Zone 1 Zone 2 Zone 3Zone 1 stands for unconditional direct trip, ZCOM stands for trip by communication-aided schemes and stands for no trip


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cycle. TCSC behaves like a parallel connection of the series

capacitor with the inductor L s in the thyristor branch. Forpractical TCSCs with an X L /X C ratio between 0.1 and 0.3,the capacitor voltage at a given line current is much lower inbypass mode than in blocking mode. Therefore, the bypassmode is utilised as a means of reducing the capacitor stressduring faults. In this mode, Z tcsc is a pure inductance with asmall value; therefore, the distance relay underreachesslightly. If the capacitor overvoltage and thyristor over-current are suppressed, the state transits to blocking modeafter detecting the fault clearance. Otherwise, after a denitetime, the state transits to the circuit-breaker-bypass mode.(v) Circuit-breaker-bypass mode : In this mode, the circuitbreaker in Fig. 1 closes. Since the series reactor in thecircuit-breaker circuit is very small, Z tcsc D 0, and the relayexperiences the normal situation. Circuit-breaker-bypassmode is used only for back-up protection, because it isexpected that the fault would be cleared before the circuit

breaker operates; hence if the fault clearance cannot be

detected after primary protection time, the TCSC willtransit to this mode.

3 Results of the TCSC impact

Figure 3 shows single-line diagrams of the sample networksused for testing. [The data for these networks are presentedin the Appendix (Section 8)]. In Fig. 3 a, the TCSC isconsidered in the middle of the line and in Fig. 3 b the TCSCis split into two equal modules located at the line ends. Thebehaviour of the relays of the main line AB and adjacentlines DA and BC for faults comprising TCSC isinvestigated. The tests are performed by real-time digitalsimulator (RTDS) and commercial relays. Test arrange-ments are shown in Fig. 4. The voltage and current signalsare injected from the RTDS to the relays and the outputs of

Table 3: Behaviour of relays using conventional schemes for the network in Fig. 3 b with load ow from bus D to C




3-phase 10% of AB Zone 1 Zone 1 ZCOM ZCOM ZCOM ZCOM


Phase-to-neutral 10% of AB ZCOM ZCOM Zone 1

3-Phase 20% of AB Zone 1 Zone 1 Zone 1 ZCOM ZCOM Zone 1

Phase-to-phase 20% of AB ZCOM ZCOM ZCOM

Phase-to-neutral 20% of AB ZCOM ZCOM ZCOM

3-Phase 30% of AB ZCOM ZCOM ZCOM Zone 1 Zone 1 Zone 1



3-Phase 40% of AB ZCOM ZCOM ZCOM Zone 1 Zone 1 ZCOM


Phase-to-neutral 40% of AB ZCOM ZCOM Zone 2

3-Phase 50% of AB ZCOM ZCOM ZCOM Zone 1 Zone 1 ZCOM






3-phase 10% of AB Zone 1 Zone 1 Zone 1 ZCOM ZCOM ZCOM

Phase-to-phase 10% of AB Zone 1 Zone 1 Zone 1 ZCOM ZCOM ZCOM

Phase-to-neutral 10% of AB Zone 1 Zone 1 Zone 1 ZCOM ZCOM ZCOM

3-Phase 20% of AB Zone 1 Zone 1 Zone 1 ZCOM ZCOM ZCOM



3-Phase 30% of AB Zone 1 Zone 1 Zone 1 Zone 1 Zone 1 Zone 1

Phase-to-phase 30% of AB Zone 1 Zone 1 Zone 1 Zone 1 Zone 1 Zone 1

Phase-to-neutral 30% of AB Zone 1 Zone 1 Zone 1 Zone 1 Zone 1 ZCOM



Phase-to-neutral 40% of AB Zone 1 Zone 1 Zone 1 Zone 1 Zone 1 Zone 1




Zone 1 stands for instantaneous direct trip, Zone 2 for timed trip, ZCOM for trip by communication-aided schemes and stands for no trip


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the relays are used to open the circuit breakers. The

commercial distance relays used for testing are all the sameand have the feature of series-compensated line protectionby use of positive-sequence memory polarisation and longmemory duration for directional stability [10]. Figure 4 isused for testing the congurations in Fig. 3 for faults behindTCSC, as indicated by F 1 and F 2 in the Figures,respectively. In this case, there is no TCSC in the faultloop seen by relays R C2 and R B1 ; hence only the relays R D1 ,R A2 , R A1 and R B2 are considered.

Tables 14 present the results obtained for differentconventional schemes, fault types, fault locations, load-owdirections and system congurations. In Tables 14, ZCOMstands for trip by communication-aided schemes, and itsrelevant time is near to zone 1. Zone 1 means directtripping, without the aid of communication. Tables 1 and 2show the results for the conguration of Fig. 3 a andTables 3 and 4 show the same results for the con-

guration of Fig. 3 b. Note that zone 1 (direct tripping)

and ZCOM (communication-aided tripping) are com-bined by an OR logic in the control equations of therelays, so that each of them can individually issue the tripcommand.

As Table 1 indicates, R D1 and R A2 trip erroneously forthe faults behind TCSC and on the line AB to disconnectline DA, which is due to deciency in security anddirectional integrity, respectively. In Table 2, the problemis only an overreaching effect of R D1 , which indicates therole of load ow in affecting the TCSC impacts. When thefault is on the line BC, relay R A1 trips in zone 1 or ZCOMincorrectly.

The same results are presented in Tables 3 and 4 for theconguration of Fig. 3 b. As can be deduced from Table 4,the overreaching and directionality are less severe than forthe reverse load ow. According to Tables 14, conven-tional communication-aided schemes cannot mitigate the

Table 4: Behaviour of relays using conventional schemes for the network in Fig. 3 b with load ow from bus C to D




3-phase 10% of AB Zone 1 Zone 1 Zone 1 ZCOM ZCOM Zone 1



3-Phase 20% of AB ZCOM ZCOM Zone 1 Zone 1 Zone 1 Zone 1

Phase-to-phase 20% of AB ZCOM ZCOM Zone 2


3-Phase 30% of AB ZCOM ZCOM Zone 1 Zone 1

Phase-to-phase 30% of AB ZCOM ZCOM Zone 2











3-phase 10% of AB Zone 1 Zone 1 Zone 1 ZCOM ZCOM ZCOM



3-Phase 20% of AB Zone 1 Zone 1 Zone 1 ZCOM ZCOM ZCOM





Phase-to-neutral 30% of AB Zone 1 Zone 1 Zone 1 Zone 1 Zone 1 ZCOM







Zone 1 stands for instantaneous direct trip, Zone 2 for timed trip, ZCOM for trip by communication-aided schemes and stands for no trip

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impacts of TCSC on the security and directional integrity of the main and adjacent relays.

As can be deduced from these Tables, relays R D1 and R A1have signicant overreach for forward faults behindTCSC. For example, R D1 sees the faults occurring from50% up to 70% of the line AB in Fig. 3 a in Zone 1. Thesame situation exists in Fig. 3 b for faults behind TCSC

up to 40% of the line AB. The results for forward faultsindicate that the load ow in the tripping direction createsmore vulnerability to overreaching and security reduction.R A1 trips in zone 1 for all fault types in the entire length of AB and also up to 20% of BC. This means that zone 1 of R A1 has been extended beyond its setting at 80% of theline AB.

The results also show that, for some cases, relay R A2 seesa reverse fault erroneously in its forward direction. This isthe case for R A2 in Fig. 3 b with three-phase faults up to30% of line AB and load ow in the nontripping direction.In these cases R A1 , R B2 , R D1 and R A2 trip simultaneously inzone 1. It means that the adjacent line to AB, i.e. line DA, isdisconnected from both sides without any fault on it. Theresults also indicate that, for reverse faults when the loadow is in the nontripping direction, the relay loses more of its directional integrity.

4 Proposed methods to mitigate TCSC impact

As the results in Section 3 imply, application of communica-tion-aided protection and also features used for conventionalseries-compensated lines such as memory polarisation are notadequate here. The problems are mainly overreaching anddirectional integrity in the protection of the main line and

adjacent lines. In this study, the enhancement of directionalcomparison systems [11] is considered for mitigation. Thesesystems can be classied as:

(a) directional comparison blocking (DCB);(b) permissive overreaching transfer trip (POTT); and(c) directional comparison unblocking (DCUB).

The schemes, which incorporate direct trip, such as directtransfer trip (DTT), cannot be applied here, becauseunconditional zone 1 trips the relay instantaneously, soovertripping due to the overreaching or directional integritycould not be prevented.

4.1 Mitigation based on DCB This method requires two distance zones: a fast starter zonewhich sends the blocking signal to the remote end when the

compensated line

RB2 R B1RA2 RA1RD1 R C2

D B

ZF RA1

ZF RB2ZR RA1 ZR RB2

&

&signal send

TA

ZF

ZR

trip

signal receive

CA

a

&

zone 1 operationof R A1 (R B2 )

signal send

TA

ZF

ZR

trip&

signal receive

OR

&

b

Fig. 5 Proposed mitigation method based on DCB Z R reverse zone (blocking signal-sending zone)Z F forward zone (trip zone inhibits sending signal)T A co-ordination timea Blocking procedure for relays R D1 , R C2 , R A1 and R B2b Blocking procedure for adjacent lines by relays R A2 and R B1

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fault is in the reverse direction; and a directional over-reaching zone in the forward direction, which inhibits theblocking signal during faults in the forward direction, andinitiates tripping if no blocking signal from the remote endis present.

For the main-line protection, i.e. line AB, this method isapplicable, since for faults on the main line the blocking

signal will not be sent by both R A1 and R B2 ; hence atripping command is issued after the tripping delay time T A .This is the same for both congurations, i.e. TCSC in themiddle or at the ends of the line. When the fault is on theline BC, R A1 overreaches but does not trip because itreceives the blocking signal from R B2 . The same situationapplies for the faults on line DA.

The adjacent lines cannot be protected by this methodsince, for some cases when the fault is on the main line, thedirectional-integrity properties of R A2 causes the fault to beseen in zone 1 by this relay so no blocking signal will be sentto the remote relay R D1 , which has already seen the fault inzone 1 due to its overreaching effect. In this case, bothrelays of the adjacent line DA or BC will trip erroneously inzone 1. To solve this problem, the solution in Fig. 5 isproposed. As can be seen from Fig. 5, nonoperation of R A1in zone 1 is a prerequisite for operation of R A2 in the forward

reach, and its operation also triggers sending of the blockingsignal to R D1 , as does the operation of R A2 in zone 3.

The signicant advantage of the blocking procedure isthat no signal needs to be transferred during faults on theprotected line. The disadvantage of the method is the triptime delay T A for internal faults. In case of channel failure,overtripping occurs on adjacent lines for faults on the main

line, and vice versa, so this method tends more towardsdependability than security. Note that the blockingprocedure for the adjacent lines can also be used for themain line. In this way, there is a unique solution based onDCB for both types of line. In conventional systems, theDCB is used for zone 2 acceleration, so the tripping-timedelay T A is equal to T send zone + T channel T trip zone +securitymargin. In this case, there is no unconditional zone; hence,the tripping-time delay would be T channel +security margin.

4.2 Mitigation based on POTT In the classical sense, this method only achieves fast trippingwhen the relays at both ends of the line detect a fault in theoverreaching zone and send each other a release signal.Usually, zone 1, with 80% to 90% coverage of the line, tripsunconditionally, while zone 2, which extends beyond theremote end, trips instantaneously on receipt of permissive-

ZFRB2

compensated line

RB2 R B1RA2 R A1R D1 R C2

D A CZFRA1Z1RB2

Z1RA1

OR

&

signal sendZF

Z1 limited reach

trip

signal receive

B

a

Z1 = zone1

ZF = forward zone

&

&

zone 1 operationof R A1 (R B2)

ZF

trip

signal receive

signal send

b

Fig. 6 Proposed mitigation method based on POTT a Blocking procedure for relays R D1 , R C2 , R A1 and R B2b Blocking procedure for adjacent lines by relays R A2 and R B1


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trip signal. POTT with unconditional zone-1 trip cannotbe used here, so the only possibility is releasing an over-reaching distance zone by the received signal. In thismethod, sending of the signal is triggered by the forwardoverreaching zone.

For the main line, POTT can be used successfully. Forthe faults on the main line, both end-line relays see the faultat their overreaching zone and send transfer-trip signals toeach other, so the only delay is channel time. For faults

beyond TCSC and on the adjacent line, the overreaching of R A1 or R B2 can be compensated for by the remote relayrefraining to send a trip signal.

POTT cannot inhibit adjacent line relays fromfalse tripping when the directional integrity of R A2 or R B1is lost. In this case, the modied scheme in Fig. 6 isproposed. As Fig. 6 shows, permissive trip-signal transfer isdependent on the nonoperation of the main-line relays R A1and R B2 , so when relays R A2 or R B1 incorrectly see thefault in the forward direction, false tripping is avoided.The proposed scheme can be used for both the main lineand the adjacent lines. The dependency of the permissivetrip on the condition of the other relays is not requiredfor the main line. The salient advantage of this methodis fast tripping of internal faults, because no intentionaldelay is required and, on receiving the permissive signal,the trip command is issued. Transfer-trip systems tend

toward higher security rather than higher dependability.A failure to receive the channel signal results in a failure totrip for internal faults. Hence, channel failure delays thetripping by the time of the zone-2 unconditional trip. Thisscheme is therefore proposed for cases with high channelreliability.

4.3 Mitigation based on DCUB The blocking procedure has the disadvantage that, during

unfaulted system operation, no signal is transmitted. Thecommunication channel is therefore not monitored. Theunblocking technique does not have this disadvantage;moreover, this technique is also somewhat faster. Duringinternal faults, the signal is changed to a permissive signalsimilar to the POTT method. This implies that no trip delayis required to wait for an eventual block signal. Thisprocedure offers a good compromise of both highdependability (channel not required to trip) and highsecurity (blocking is continuous).

Figure 7 shows the proposed method. The proposedmethod can be used in both the main line and the adjacentlines. In this method, like the previous one, the dependencyof the unblocking-signal transfer on the condition of theother relays is not required for the main line. The logic is soimplemented that, 20ms after channel failure, it releases thezone ZF for a period of 100ms. If this happens during a

compensated line

R B2 R B1R A2 R A1 R C2R D1

D BZFRB2ZF RA1

&

&OR

&

&

&

&

CS

UB

0 20 ms

channel supervision

ZF

zone 1operationof R A1(RB2)

trip

0 100 ms

transmission channel disturbed

received UB

received CS

signal send

CA

Fig. 7 Proposed mitigation method based on DCUB Unblocking procedure for adjacent relays R A2 and R B1CS channel-supervision signalUB unblocking signal* Omit this part for unblocking procedure of relays R D1 , R C2 , R A1 and R B2


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Table 6: Behaviour of relays using proposed schemes for the network in Fig. 3 a with load ow from bus C to D




3-phase 50% of AB



3-Phase 60% of AB

Phase-to-phase 60% of AB Phase-to-neutral 60% of AB

3-Phase 70% of AB



3-Phase 80% of AB



3-Phase 90% of AB



3-Phase 100% of AB



3-Phase 10% of BC



3-Phase 20% of BC






3-phase 50% of AB ZCOM ZCOM ZCOM ZCOM ZCOM ZCOM

Phase-to-phase 50% of AB ZCOM ZCOM ZCOM ZCOM ZCOM ZCOM

Phase-to-neutral 50% of AB ZCOM ZCOM ZCOM ZCOM ZCOM ZCOM

3-Phase 60% of AB ZCOM ZCOM ZCOM ZCOM ZCOM ZCOM















3-Phase 10% of BC



3-Phase 20% of BC


Phase-to-neutral 20% of BC ZCOM stands for trip by communication-aided schemes and stands for no trip


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fault, then tripping will occur provided that the relay has

seen the fault in the forward direction correctly. If channelfailure occurs during normal system operation, no con-sequences arise because ZF is not picked up. 100ms later,the protection is again blocked for the duration of thechannel failure. This blocking is removed (reset time of 100ms) when a signal is again received.

5 Test results

Commercial relays have control equations for differenttasks such as conditions to assert the output contacts;elements and conditions to trigger event reports; elements totrip unconditionally; and elements to trip with communica-tions assistance. The methods proposed in the previousSection are implemented by control equations. Tables 56and 78 show the results of the study systems of Fig. 3 a and3b, respectively. Comparison of these tables with Tables 14

shows the signicant capability of the proposed schemes to

enhance the protection of the main line and adjacent lines.For simplicity, only the primary protection operation ismentioned in these Tables. The performance of the back-uprelays is as expected; but with some overreaching effects. Asit can be deduced from Tables 58, the TCSC impacts suchas forward overreach and directional integrity are solved.The results of extensive testing on study systems indicatethat all the proposed methods based on communication-aided schemes provide very high reliability and mitigate theTCSC impact on the protective relays.

5.1 Channel-failure effects The proposed method based on DCB needs a xedtime delay for receiving a blocking signal before releasingthe trip for internal faults. While this is a disadvantage for ahealthy channel, it would turn into an advantage forinternal faults when the channel fails, since no extra delay

Table 7: Behaviour of relays using proposed schemes for the network in Fig. 3 b with load ow from bus D to C




3-phase 10% of AB



3-Phase 20% of AB



3-Phase 30% of AB



3-Phase 40% of AB



3-Phase 50% of AB





















ZCOM stands for trip by communication-aided schemes and stands for no trip


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occurs for the release trip. Overtripping is a disadvantage of

this method for faults on adjacent lines. The method basedon POTT has the advantage of fast tripping of internalfaults on receipt of the permissive-trip signal, but it has along time delay for channel failure. Channel failure does notcause any overtripping for external faults. The proposedmethod based on DCUB seems more efcient, becausethe channel is supervised continuously, while it has theadvantage of tripping internal faults without any intentionaldelay.

5.2 Important remarks Ease of implementation : The salient feature of the proposedschemes is the possibility of using conventional numericalrelays and common communication channels that are usedin the stations, without the necessity of changing embeddedrelay algorithms.

The modications are applicable by the user through the

inputs, outputs and logic equations. The required data forthe proposed methods are available in the same station andit could be hard-wired or soft-wired by using the existinglocal-area network [12].TCSC out of service : When TCSC is out of service, there isno difculty in achieving normal operation of the relays byusing the proposed methods, because the imposed restric-tion always asserts in normal power-system operation.

6 Conclusions

The impact of TCSC on the protection of transmissionlines and remedial actions using existing relays are presentedin this paper. The detailed analysis is carried out by usinga real-time digital simulator (RTDS). For validation of the results, commercial relays with the capability of compensated-line protection are used. The results indicate

Table 8: Behaviour of relays using proposed schemes for the network in Fig. 3 b with load ow from bus C to D




3-phase 10% of AB



3-Phase 20% of AB



3-Phase 30% of AB



3-Phase 40% of AB



3-Phase 50% of AB





















ZCOM stands for trip by communication-aided schemes and stands for no trip

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that TCSC has signicant impacts on the power-systemprotection and can create serious problems such as forwardand reverse overreaching; directional integrity issues;and malfunction of common pilot distance schemes. Theseimpacts mainly lead to overtripping of the relays forfaults comprising TCSC. Mitigation of the problem bymodied communication-aided schemes is proposed. Theproposed methods are obtained by modication of theclassical DCB, POTT and DCUB schemes, which are wellproven for utilities and are available in the commercialrelays. The methods can be implemented by using inputs,outputs and trip-logic equations of the numerical relays.For verication, power systems simulated by RTDS andcommercial relays have been used. The results indicatethe effectiveness of the modied schemes for mitigating theeffects of TCSC on the performance of distance relays. Theproposed methods have a very desirable feature that theydo not require any new equipment or major modicationsto existing plant.

7 References

1 Adamiak, M., and Patterson, R.: Protection requirements for exibleAC transmission systems. CIGR

!

E Plenary Session, Paris, France,

1992, paper 34-2062 Song, Y.H., and Johns, A.T.: Flexible AC transmission systems(FACTS) (IEE, 1999), Chap. 12

3 Girgis, A., Sallam, A., and El-Din, A.K.: An adaptive protectionscheme for advanced series compensated (ASC) transmission lines,IEEE Trans. , 1998, PWRD-13 , pp. 414420

4 Weiguo, W. et al. : The impact of TCSC on distance protection relay.Proc. Int. Conf. Power System Technology, Powercon98, 1821Aug. 1998, Vol. 1, pp. 382388

5 Real time digital simulator, RSCAD, Ver. 1.177, 20036 Larsen, E.V., Clark, K., Miske, S.A. Jr., and Urbanek, J.:

Characteristics and rating considerations of thyristor controlledseries compensation, IEEE Trans. 1994 , 2002, PWRD-9 , (2), pp.9921000

7 IEEE 1534:2002: IEEE recommended practice for specifyingthyristor-controlled series capacitor (TCSC) (IEEE, New York, 2002)

8 Tanaka, Y., Taniguchi, H., Tanaka, M.Y., Taniguchi, H., Egawa, M.,Fujita, H., Watanabe, M., and Konishi, H.: Using a miniature modeland EMTP simulations to evaluate new methods to control andprotect a thyristor-controlled series compensator. Presented at IEEEWinter Meeting, 1999

9 Goldsworthy, D.L.: A linearized model for MOV-protected seriescapacitors, IEEE Trans. , 1987, PWRS-2 , (4), pp. 953958

10 Wedepohl, L.M.: The polarized mho distance relay, Proc. IEE , 1965,122 , pp. 525535

11 Ziegler, G.: Numerical distance protection: principles and applica-tions (Siemens, Publisis MCD, Erlangen, 1999)

12 IEC 61850:2003: Communication networks and systems in substa-tions (International Electrotechnical Commission, Geneva, 2003)

8 Appendix: System data

Lines (AB, BC, CD)Length: 100kmVoltage: 500kVPositive-sequence impedance: 0.0185+ j 0.3766 O/kmPositive-sequence parallel capacitive reactance: 0.22789M O kmZero-sequence impedance: 0.3618+ j 1.2277 O/kmZero-sequence parallel capacitive reactance: 0.34513M O km

Systems C and DPositive-sequence impedance: 1.43+ j 16.21 OZero-sequence impedance: 3.068+ j 28.746 OSystem frequency: 60 Hz

TCSC Main capacitor: 176 mFTCR inductance: 9.0mHLd in bypass breaker circuit: 0.2 mHMOV * reference current: 10 kAMOV reference voltage: 338kVMOV exponent: 24

* The V/I characteristics of MOV are commonly approxi-mated by the equation I I ref (V /V ref )n


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