SELECTION OF DISTANCE RELAYING SCHEMESWHEN PROTECTING DUAL CIRCUIT LINES

Darren Spoor* and Joe Zhu**

*Transmission DevelopmentTransGrid

** School of Electrical EngineeringUniversity of Technology, Sydney

ABSTRACTThe difficulties associated with the distance protection of dual circuit transmission lines is wellknown and appropriate schemes have been implemented worldwide that enable reliable protectionof these assets. However, the application of these schemes usually fail to consider the moreextraordinary fault occurrences that can plague transmission systems.

This paper considers the application of various faults to a simulated dual circuit transmission line.The observations have shown that inter-circuit faults may be undetectable in the instantaneouszone of protection when lines incorporate some of the presently used distance schemes. Thesefaults can present impedances to both line terminals that are larger than those required for Zone 1operation. Such an event has the potential to lead to a loss of major loads, mal-operation of singlepole tripping schemes and even system instabilities based on the critical clearance requirements.

1. INTRODUCTIONMutual coupling and the possibility of multi-circuitfaults are the main difficulties when protecting dualcircuit lines.

Coupling can produce severe underreaching andovereaching errors for distance relays, whereby thenetwork topology, and zero sequence sourceimpedances are both important factors in determiningthe magnitude of such errors. In some cases, distancerelays may see less than 50% or far more than 100%of the line, depending on the infeed and couplingconditions experienced. [4]

The operation of the transmission network can resultin large variations in these errors. The status of theparallel circuit produces the most noticeable effect ona simple dual circuit line, as having the circuit inservice will cause overreach. However, underreachwill be experienced if the line is out of service andearthed at either end. [4]

Consequently, the relay zone reaches are affected, asthe distance elements must observe all faults on thelines irrespective of the network configuration.

The inherent line arrangements also make them proneto multi-circuit faults, of which the earthed cross-country fault is the most common. However,

unearthed inter-circuit faults create unusual problemsfor the protection engineer as zero sequence currentsare present in the circuits themselves, but these do notextend beyond the busbars. [1]

Inter-circuit faults are known to result in relayunderreach for the phase elements [I]. This occurs asthe faults change their appearance from double phaseto earth to single phase faults as the position along theline is varied. This transition can push past theboundaries of the impedance characteristics requiredfor successful operation of many protection schemes.[7]It is also known that there can be a loss of phaseselectivity for single-pole tripping schemes underthese conditions [2]. This can be a serious concern onimportant dual circuit lines such as networkinterconnectors.

The probability of experiencing an Inter-circuit fault issignificantly high [I], as they can result from bushfireactivity, line galloping or broken conductors. It isgenerally understood that some faults require differentsolutions for the protection engineer, but whendesigning dual circuit protection schemes, theconsequences of inter-circuit faults are often notconsidered in conventional design philosophies

This paper presents an analysis of different faults on asimulated dual circuit transmission line with assumed

zone distance reaches, and outlines the need for thedesign philosophy to include an analysis of complexfaults such as inter-circuit faults.

2. DISTANCE PROTECTION

2.1 Line Impedance ParametersFor a short dual circuit line, the transverse voltagesand line currents present can be defined by thetransmission line impedance matrix.

I ~ ' I

I I L L 11/.1

I I I 7 B I

I I I L L I[\, I" I, Z\I I,

,I, I, I, I 7" I, C !J I,

11/ ' I I, z i'. II /.

Equation I - Transmission line impedance matrix

As is the case for a single circuit under balancedconditions, the matrix is diagonally symmetrical (allthe self-impedances and coupling impedances areidentical). Similarly, for an unbalanced circuit theimpedance matrix will remain diagonally symmetricalalthough the self and mutual impedance terms differ.

This matrix can be divided into four sub-matrices. asshown above. A and D contain the self and mutualimpedance terms for the two circuits. However, theparameters within C and D describe the inter-circuitcoupling between the lines.

Generally the line sequence impedances can beobtained from the self and coupling parameters of thecircuit. However as most lines are not symmetrical,the actual impedances observed at a particular locationwill also depend marginally on the combination offaulted conductors and the geometry of the line.

z; = z, +2Z"

Z, = Z, = Z, -Z"

z . ~~(Z" +Z" +Z,)3

Equation 2 - Symmetrical Impedances

2.2 Phase-to-Phase Fault DetectionTo measure the distance to all faults involving morethan one phase, a simple distance relay compares thevoltage between the two faulted phases with thedifference between the phase currents.

ZR' =(vR-v,·)/(JR-I,)z>Ii = (v, - VB )j(I r -1/1 )

ZER =(VH -VR)/(Ifj -IR)

Equation 3 - Phase element operation

Therefore it is often necessary to implement a numberof phase elements to correctly measure the fault type.Other techniques exist, such as the use of a polyphasedistance element. but these can have seriousdeficiencies under certain system conditions. [6]

2.3 Phase-to-Earth Fault DetectionIt is also necessary to measure the positive sequenceline impedance between the relay and any earth faults.Consequently. relays also contain earth elementswhich incorporate the faulted phase voltage andcurrent.

However. earth faults incorporate zero and positivesequence currents. As the positive and zero sequenceimpedance is usually different in overhead lines, somecalculations are needed to equate the observed phaseimpedance to that of the positive sequence lineimpedance. This is achieved with the ResidualCompensation Factor Ko.

ZR =VR/(IR + IIIK,,)z, =Vw/(Iw +JoK,,)

Zfj = VB/(IiI + JIIK,,)

KII = (Zo/Z, -1)

Equation 4 - Earth element operation

2.4 Relay Impedance CharacteristicsOver the years many different characteristics havebeen proposed by universities and utilities around theworld. Some of the most commonly utilisedimpedance 'shapes' include the mho, offset mho,quadrilateral, peanut and lens. Selection of thesecharacteristics often depends on the requiredsensitivity to load, resistance and source impedance.

Today most manufacturers offer a choice of quad ormho characteristics. Although others are still availablefor situations where high load currents areexperienced.

Generally mho characteristics provide very reliableand adequate responses when used in most protectionapplications. However, quadrilateral characteristicscan provide increased resistive reach in situationswhere load currents will not constrain thecharacteristic. These benefits are limited as theresistive reach is restricted to approximately 3.5 timesthe reactance value for zones set to 80% of the lineimpedance. Thus quadrilaterals are mostly employedfor the earth elements on short lines without earthwires; non-effectively earthed systems and feederswith high footing resistance. Conversely, phaseelements should incorporate a mho characteristic as

fault asymmetry can further increase an angulardisplacement between a relay and the fault current. [3]

2.5 Protective Zones of OperationThe three-zone mho relay has been developed to allowadequate discrimination when protecting transmissionlines with distance protection. Each element is used inconjunction with timers dividing the system intodifferent zones with different tripping times.

The first zone is instantaneous and extends from therelaying point to a location just short of the remotebusbar. This is commonly set to 800/e to allow fortransducer. relay and line parameter errors.

Zone 2 is graded to provide remote backup protectionfor the next zone in the power system, which iscommonly the remote busbar. Hence a reach of 120%of the line impedance is usually applied with a timersetting close to 0.4 seconds for transmission circuits.

The third zone is configured for backup protection ofequipment further embedded in the system.

rceec-r. ..etrroeoenc.e

.~,,,

,

--j

Figure I - Protection zone grading of mho distance elements

3. DISTANCE PROTECTIONSCHEMES

Without a suitable protection signalling scheme. faultsnear one end of a circuit will result in a Zone 2operation at one of the relaying locations.Consequently, a total clearance time of around 27cycles is possible for some faults. However this isoften unacceptable due to system stability and loadsensitivity constraints.

Distance schemes incorporate communicationbetween the relaying locations, which enables areduction in the total tripping time for the fault. Someof the more common schemes include:

3.1 Permissive Underreach (PUR)Generally, these schemes will trip a lineinstantaneously if the fault is seen:

• By both ends in Zone I

• By one end in Zone and the other end inZone 2

Where hath relays see a Zone 2 fault, the Zone 2 timer(typically 400ms) will delay breaker operation.Consequently, to protect the line. all faults must beobserved by at least one relay within the Zone Iimpedance characteristic.

3.2 Permissive Overreach (POR)Similarly permissive overreach schemes will tripinstantaneously if the fault is observed:

• By both ends in Zone I

• By one relay in Zone 1 and the remote relayin Zone 2

• By both relays in Zone 2

Permissive Overreach schemes will trip for faultswhere the apparent impedance at both relays is quitelarge. However, the Zone 2 setting must be configuredto grade over all the possihle impedance valuesobserved at the relays, produced by faults on the line.

3.3 Blocking Schemes (B)Blocking schemes are slightly different in their Zonearrangements and protection signalling logic.However. they will also trip a line instantly when afault is seen:

• By hath ends in Zone I

• By one relay in Zonerelay observes theimpedance regioncharacteristic.

I, and where the otherfault in the highof the Zone 2

• By hath ends in Zone 2, as long as theapparent impedance is greater than thatobserved by the smaller Zone 3 forwardreach.

With correctly set zone reaches (especially Zone 3),blocking schemes are similar to permissive overreachschemes in terms of the Zone 2 setting requirements.Blocking schemes are often applied in conjunctionwith overreaching schemes in situations where the

reliability of the protection signalling channel cannotbe assured.

3.4 Current Differential (CD)Current differential schemes do not rely on distancetechniques but are often used in conjunction withdistance schemes. These incorporate the measurementof current at each end of a feeder, on a per-phasebasis. This information is then transferred between therelays, creating a trip signal if the difference in currentis adequately large.

4. COMPARATIVE FAULT ANALYSISTo assess the adequacy of such protection schemes ona dual circuit line. a base scenario was chosen. Themodel included a 330kY source voltage, as well asequal positive and zero sequence source impedancesof 4 1.9L82.4°Q and 23.4L83.4°Q.

• 4 • ("

• c • D

• • 4

Figure 2 - Simple dual circuit line topology with lowreactance phasing

The dual transmission circuits themselves were 172kmin length, assuming a low reactance construction. Thisalso applied to the geometry of the conductors andsizes, resulting in positive sequence line impedancesof 58.9L84.9°Q. Similarly the zero sequence lineimpedance obtained was l65.5L74.8°n. resulting in aresidual compensation value of O.735L-15.6°.

This scenario was then analysed using the AlternateTransients Program using a distributed parameter linemodel for the transmission lines, and a sequencecomponent equivalent of the source impedances.

The asymmetrical nature of the lines resulted in asmall disagreement between the actual impedancesobserved by each relaying element for a bolted fault atthe remote busbar. This means that a fault on onebusbar will result in observed impedances that varybetween 55.8Q and M.4Q. where the two verticalphases closest to the earth obtain the lowest loopimpedance. The upper phase conductors have slightlyincreased impedances. while the lower and upperconductors have elevated impedance values due to thereduced mutual coupling between the phases.

RelayElement

Observed Impedance

R-BR-WW-B

6·U8L852'Q5587 L8-l.8'n5659L8-l7'n

Table I - Phase impedances for the line

4.1 Selection of Zone SettingsGenerally, the zone boundaries of the protective relaysare chosen so that any credible fault on the line for allsystem configurations will be observable. Thisgenerally means that the Zone I and Zone 2 reachesare reduced and increased respectively in accordancewith a set of system studies. This guarantees thecorrect operation of the relay under all systemoperation conditions.

An alternative approach IS to use a residualcompensation value that eliminates the mutualcoupling effect for any setting, if the parallel line isgrounded and there is no infeed from the remote end[2]. Although this eliminates the overreaching errors.there is a corresponding increase in the underreachingerrors. However, a system study approach has beenadopted in this analysis.

In this situation. a solution was obtained using anasymmetrical line model. This approach required theuse of the original ATP distributed parameter model todetermine the impedances seen by each individualelement for a fault on the remote bus bar. Here eachcombination of phase. earth and double phase to earthfault was applied while noting the impedancesobserved by each relay element.

ELEMENT

B-R R-W W·B R·E WoE B-E

3 Ph 59m 55.4Q 55.8Q 55.4Q 57.6Q 57.8Q

R-B 59.6QR-W 55.3QW-B 56.0nR-E 61.3nWoE 67.0QB-E 66.1!1

R-W-E 54.m 59.0Q 64.4Q

R-B-E 60.4Q 61.0n 65.9QB-W-E 55.9Q 65.5Q 61.6Q

Table 2 - Both lines in service

B-R R-W W-B R-E WoE B-E

3 Ph 64.5Q 56.0f! 56.2Q 58.2Q 55.9Q 620QR-B 64.4QR-W 55.9QW-B 56.6QR-E 52.8f!WoE 55.4Q

B-E 59.2QR-W-E 55.5Q 54.0n 55.2QR-B-E 64.911 53.1!1 62.6QB-W-E 56.4Q 57.6f! 56.8f!

Table 3 - One line out of service

B-R R-W W-B R-E WoE B-E3 Ph 64.1!1 55.8Q 56.1Q 57.0Q 56.8f! 61.5QR-B 64.011R-W 55.mW-B 56.5QR-E 47.5f!WoE 48.6QB-E 51.2Q

R-W-E 55.5n 52.3Q 47.4QR-B-E 64.4f! 47.0Q 57.112B-W-E 56.3Q 54.1Q 49.1Q

Table 4 - One line out of service and earthed

The Zone 1 reach should observe as much of thecircuit as practically possible whilst never reaching. theremote busbar, Applying an 80% Zone I reach to theworst-case minimum line impedance results in a ZoneI setting of 37.9Q: or 7307c of the positive sequenceimpedance.

Conversely, the Zone 2 reach must always observe thefull line impedanc«, even in the worst case. Hence, asetting of 80.40. or 136% of Ihe positive sequenceline impedance, is required when a margin of 120St isemployed using the ATP loop impedances. However,no fault on the circuit should result in an observedimpedance within this 120(:"~margin

4.2 Detection ·01 FaultsThe analysis incorporated the application of faults Onthe transmission line at lYlo of the line length. Thislocation is significant as it may require Zone 2operation by one terminal relay.

Applying a low impedance three-phase, phase-to-earth, phase-to-phase, a double-phase'w-earth.or anearthed cross-country fault; results in apparentimpedances which will be observed in Zones I and 2at the respective relaying locations. This results in thecorrect operation for all the protective schemesdescribed previously.

However. care must be taken when implementingsingle pole tripping schemes as both the phase andearth elements may pick up for cross-country faults,resulting in a three-pole operation on both feederswhere only single pole tripping is required. This isalso a concern for phase to earth faults dose to abusbar as the phase elements can observe animpedance that is within their operating characteristic.

4.2.1 Observation of Inter-Circuit FaultsAlternative! y. unearthed inter-eircui t faults appear tobe a serious concern for dual circuit lines. In thisanalysis an observed impedance of l2Y7c of the linewas detected, and the observation of the fault wouldonly have been possible as a result of the 120%margin previously applied to the Zone 2 settings.

The resulting phase and earth element impedances ateither end of the line can be seen in Figure 3.However, Figure 4 indicates the effects of increasingthe busbar source impedance ratio toa value of 2:1.

The effects on the phase element impedance can beobserved by varying the fault position along the line.Here the maximum under-reach is obtained for faultsoccurring 60%·80ckof the line length.

-- }~ii<T-;..~~_:>:/~~~L::~~;~-_~~r::r~:<L~;"{(:<::;~~':~':r: ph:::.·~~~d\.~<E~:~=~~::;~:·t~:·,j~h~:·i'"t~:T:'njr::~':_:~.}:'~ S~R.; ,..

Figure 5 - Impedances observed by phase clement s for twosource impeo,lncc ratios

4.2.2 Selection 01 Protection SchemeDetection of these faults using a permISSIveunderreaching scheme would require both relayimpedance plots to exist below the Zone I limit of73% at.all locations.

Similarly, neither of the curves could extend beyondthe Zone 2 boundary of 136% for permissiveoverreaching or blocking schemes to protect the lines.

Consequently, permissive underreachingschernes willnot observe inter-circuit faulls at any location.Permissive overreaching QI' blocking schemes wouldhave detected this event for a line with equal sourceimpedances. despite the acute loss of IheI20i'!c Zone 2margin below the forward reach. However, a sourceimpedance ratio above ItI can result inon!y Zone 3detection of such faults, making the scheme incapableof protectl fig the circuit.

Figure 6 - PUR blind section. and loss of 1209< margin forPOR and B schemes. with equal SIR.

Current differential schemes appear to be the onlyreliable approach to detecting inter-circuit faults whencompared to conventional mho based protectivetechniques. Nevertheless. it may be possible to detectinter-circuit faults when the Zone 2 reaches areincreased in permissive overreaching or blockingschemes. Similarly. setting the mho angle at 200

below the line angle, for certain source impedanceratios, can assist in detection of the earth elements.

A quadrilateral characteristic could also aid in faultdetection due to the extended resistive reach, enablingobservation of the earth element impedance at thelocation furthest from the fault. Although, theexpected load impedance locus should be consideredcarefully in such situations.

Shorter dual transmission circuits will extend theapparent earth fault impedance away from the originof the impedance plot. However, as quadrilateralcharacteristics are commonly used on short lines, theresistive reach of the quadrilateral characteristic maybe set to compensate.

Nevertheless. the use of earth elements for detectionof inter-circuit faults should only be considered as partof a system study that considers the impacts of inter-circuit faults in critical locations. It should be notedthat the required zone reaches depend heavily on thesource impedances and other system parameters.Consequently the impact of these faults shouldconsider these parameters in a sequence. ATP or othersimilar model.

5. CONCLUSIONConventional philosophies directing the use ofprotection schemes for dual circuit lines may notenable the detection of unearthed inter-circuit faults inthe instantaneous zone of operation. This is anessential requirement in many cases. includingnetwork interconnectors or lines carrying heavy orsensitive loads.

To permissively detect inter-circuit faults, at least oneoverreaching or blocking scheme is required with a

Zone 2 setting large enough to cover the apparentimpedances observed from each busbar.

In cases of single pole tripping, an inter-circuit faultwill trip the three poles from both circuits as a resultof the phase element impedances. This can beovercome by using at least one current differentialscheme in conjunction with logic that will trip thefaulted phases only. Otherwise, all six voltage andcurrent signals should be analysed by a single relay todetermine the fault condition.

As a result of this analysis, there is an identifiableneed to adopt different protection design philosophiesbased on the potential implications of inter-circuitfaults.

6. REFERENCES[I] Agrasar. M., et al "A useful methodology for

analysing distance relays performance duringsimple and inter-circuit faults in multi-circuitlines". IEEE Transactions on Power Delivery, Vol12, No 4. July 1997.

[2] Agrasar. M., et al "Evaluation of uncertainties indouble lines distance relaying. A global sight".IEEE Transactions on Power Delivery, Vol 12,No 4, July 1998

[3] Domzalski, M.I., Nickerson. K.P., Rosen, P.R."Application of Mho and Quadrilateral distancecharacteristics in power systems" Developmentsin Power system Protection. ConferencePublication N0479, lEE 200 I.

[4] Jongepier, A.G., et al. "Adaptive DistanceProtection of a Double-Circuit Line". IEEETransactions on Power Delivery, Vol 9, No 3,July 1994.

[5] Mcl.aren, P.G .. et al "Enhanced Double CircuitLine Protection". IEEE Transactions on PowerDelivery, Vol 12, No 3, July 1997.

[6] Martilla. R.I., et al "Effect of Transmission LineLoading on the performance characteristics ofpolyphase distance relay elements" IEEETransactions on Power Delivery, Vol 3, No 4, Oct1988

[7] Turner. S .. et al "An application in relay-to-relaylogic communications for single pole tripping"Proceedings of the 4th international conference onadvances in power system control, operation andmanagement, APSCOM-97. Hong Kong,November 1997.