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Distance Protection and Voltage Stability

SUB 1SUB 2

PAPERNUMBER PC-A7-26

E lect r o Solid-state[ YO] Numerical [%]mechanical [%I65.5 16.5 18.0

72.5 27.5

Mattias Jonsson Jaap Daaldermatti as.j onsson@l tekni k.chalmers.se j aap.daal der@el tekni k.chal mers. se

Departmentof Elechic Power EngineeringChalmers University ofTechnologyGothenburg, S-41296 Sweden,wwwel teknik.cha1mers. seAbstract: In this paper the influence of zone3distance reaysin caseof voltage instability is examned. Different typesof reay character-istics are analysed.An adaptive agorithm to prevent undesirablezone3operationsduringvoltage instability is proposed.Studies are based onsimulationsin a fifteen bus systemdeveopedby the authors. Data onthe mxtureof eectromechanica, solid-stateandnumerica distance reaysin the Swedish transmssion systemarepresented togetherwith statisticsof zone3operations fromthe startof year 1985until the end of 1998.The investigation shows that undesirable zone3distance reay opera-tionsduring voltage instability can bethedifference between a totablackout and a recovering system Simulations have shown that theadaptive agorithmmay save thesystemfrom acollapse.Keywords: Voltage stability, zone3distance protection reays, cur-rent limters, adaptive reaying,fieldstatistics.

I. INTRODUCTIONUndesirable zone 3 distance relay operations have contributedto blackouts world wide. The most well knownincident is per-haps the July 2, 1996 outage in the WSCC system[13wherezone3mal-trips occurred which gave rise to a voltage col-lapse. Another example s given by Taylor [2].Zone 3distance relays are mainly used to provide remoteback-up protection for adjacent sections of transmssion cir-cuits. In the ideal case the entire length of all adjacent circuitsare back-up protected. However long lines, infeed and loadencroachment may cause difficulties to obtain that result.Zone 3 distance relays are also applicable in breaker failureprotection schemes and in some pilot relaying applications.It is the opinion of the authors that the disconnection of powerlines should be avoided during voltage instability. For instancedistance relays should not trip transmission ines prior to thedisconnection of generators by their own protection equip-ment asthispossibly will cause cascading outages when all ofthe remaining generation capacity s further used. Thus wheretransmssion lines are provided with a duplicate main protec-tion and/or where difficulties with settings exist there may bereasons for not applying zone3distance elements at all, as thezone3protection will offer little improvement in fault clear-ance but may be a considerable threat to system security.

Numerical relays account for an increasing share of all relaysin operation and hence solutions necessary for complexsystem configurationsand system conditions will be easier torealise. Often software is implemented where the user cancreate individual functions based on mathematical logicblocks. This in combination with the fast development ofinformation technology in power system applications makesadaptive relaying a powerful tool.The type of events examnedin this paper are rare occasions.However when they occur usually a major part of the systemload is affected. Hence these disturbances may be extremelycostly and can cause much social inconvenience. It is notunlikely that the frequency of these occasions will increase inthe future as power systems are operated closer to their stabil-ity limts due to environmental constraints and deregulation.11 DISTANCE PROTECTION IN THESWEDISH TRANS-MISSION GRIDThe Swedish grid consists of twovoltage levels: 400kV and220kV, and has two independent redundant protection sys-tems for each line calledSUB1 andSUB2. The main reasonfor this is to be able to do maintenance work on protectionequipment when the line is loaded and still have the line satis-factorily protected.About 700 distance protection devices are installed and thecomposition based on the type of relay is shownin table 1.

I Total 1 37.5 I 40.5 I 22.0 ISUB 2 was introduced in the middle of the 1970s. Thus noelectromechanical relays were used since solid-state relays

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already had entered the market.The zones of operation for all electromechanical relays havethe shape of a (mho)circle. Solid-state and numerical relayshave a quadrilateral shape or a combination of a circle and aquadrangle. Table 1 shows that electromechanical relays stillconstitute a large part of the line protection and probably willbe used for a long time into the future.111ZONE 3DISTANCE RELAYS MAY CONTRIBUTE TOVOLTAGE INSTABILITYVoltage instability isaphase symmetrical phenomenon whereno zero sequence current is present.Thusthe apparent imped-ancezfas seen by a distance relay during voltage instabilitymay be writtenas in equation1.U is the line to line voltage andP andQ are the injected active and reactive power at the locationof the relay.

When is withm the area of one of the pre-defined zones ofoperation the relay will operate. Low system voltages and highpower flows characterize voltage instability events. It followsfrom equation1that these events may cause distance relays tomal-trip.Thisbehaviour is undesirable since it most likely willaggravate the power system stateinanalready severe situation.IV. DIFFERENT OPERATING CHARACTERISTICS

Roughlyt wo types of operating characteristics are used today:circle and quadrilateral characteristics. During normal operationconditions or at the onsetof a voltage instability the amount ofactive power in a transmission circuit is considerably higherthanthe reactive power.Thusthe power factor is rather high andthe apparent impedance is dominated by the resistance. Hencethe zones of operation will be approached close to the resistanceaxis in the RX-diagram. The design ofstartrelays, and in case ofnumerical relays thesocalled general startcriteria are mainlybased onthisphenomenon of load encroachment and thereforeoftenhave the possibility to limt the area closeto the resistanceaxis. Still in some relay applications the zone3element itself isused as astartreay.Relays with opt-hting zones formedas an ellipseas well as mhorelays usually offer a limted number of adjustments. Still theyare very reliable when protecting long lines since they have anatural defence to the phenomenon described above dependingon how much the circle leans in the =diagram. Quadnlat-era1 relays usually offer more possibilities of settings comparedto themhorelays.

V. AN ADAPTIVE ALGORITHMTO PREVENT MAL-TRIPS DUE TO VOLTAGE INSTABILITYIn figure 1 an adaptive algorithm is proposed basedonsimplemathematical logic blocks where the dependability s decreasedand the security increased as compared to traditional distancerelaying. The algorithm avoids mal-trips duetovoltage instabil-ity. The idea is not only to consider the relation between theapparent impedance and the zones of operation but also the dif-ferent events which happeninthe system when the zone3 isentered.When a short circuit fault occurs in a power system the voltagechanges are high in the area close to the fault. Incase of voltageinstability the voltage changes are mostly low to moderateunti lclose to the collapse. Hence the derivative of the voltage canbeused to distinguish between short circuit faults and voltagechanges due to stressed system conditions.However it is very important that the relay will operate asplanned when a voltage instability is present and a short circuitfault occurs at the same time. Hence the derivative of the voltageis analysed during the time when the apparent impedance iswithin zone3 so that a fault can be properly detected andcleared.

0 hlt+1l YES

Fig. 1. Anadaptive algorithmfordistance protection to avoid mal-tr ips due tolow system voltage and high power flows.The algorithm operatesas follows:

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Block 1:Block 2:

Block 3:

Block4:

Block 5:

Checks if the measured impedance is within zone3.Decides if a short circuit fault has happened.When a fault occurs AVlAt will have a negativevalue with a high magnitude.A-g / a 1 d 1 1 The fault has been cleared.At - At m i n i m u m

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the nuclear power units have a fvted active output. The loads inthe Middle and South areas dominate the total consumption inthe system. Thus a large amount of power will be transportedfromNorthto Middle and aso from Middle towards South. Thetransmssion lines between bus 2 and bus 3 are equipped withsene capacitors giving a line compensation of 50%. The loadmodels used in the system are based on the experimental workof le DousetaI[3] and all the other models are identical withthe ones applied in the Nordic32 system [4].When the reach of the zone 3 element is determned infeed isconsidered and the idea is that the whole length of al adjacentlines should be protected. The zone 3 element itself is assumedto be the starthction of the relay.VII. SIMULATIONS IN THE 15BUS SYSTEM

The simulations have been performed in SIMPOW [SI . Theinfluence ofmho and quadrilateral zone 3 relays on voltageinstability is illustrated, and the improvement due to the adap-tive algorithm proposed above is demonstrated.At the pre-fault state the system is heavily oadedinthe Southarea. The power transfer from bus 3 towards bus 34 is about2200 MW and frombus 34 to bus 4 1200 MW. The initial dis-turbance is a permanent 3 phase short circuit fault having zerofault resistance and located in the middle of one of the trans-mssion lines between bus 34 and bus 4. The fault is clearedby primary protection. When the adaptive algorithm is usedone additional fault occurs. This fault is a busbar fault at bus342 and occurs when the apparent impedance is alreadyFigure 3 shows the voltages at bus 3 and bus 34 before andafter the faults. When zone 3 is in operation with settingsbasedon the criteria mentioned earlier the system will col-lapse approximately 83 seconds after the initial fault inde-pendent of theshape of the operating zone used. Inthe caseofmho relays the collapse will be initiated by the zone 3 ele-ments at bus 3 protecting the lines between bus 3 and bus 34.When quadrilateral relays are used the collapse will be initi-ated by the zone 3 element located at bus 34 protecting theremaining line between bus 34 and bus 4.This can be compared with the situation that either the reachof the relays is decreased or the adaptive algorithm is applied.In these cases the system will recover to a stable operationstate. Without the adaptive algorithm the reach of the mhorelays at bus 3 protecting the lines between bus 3 and bus34must be decreased to avoid a voltage collapse. If this approachis applied only 60% or less of the adjacent lines will beremote back-up protected. When quadrilateral relays are inoperation their reach at bus 34 must be decreased in the resis-tive direction from 105R to about 85R to avoid a collapse.

the zone 3.

Notice that the collapse plotted in figure 3 isvalid for the caseofmho relays.

I.r@1 4

1 3

12- .1gb1 13aI

09

0.8

0 7 50 100 15 0 200 250 300Tune S

Fig.3.Voltages atbus3andbus 34. In caseofnormal zone3 settings collapseoccurs at indication 7 whereas usingtheadaptive algonthmor a decrease inelement reach the systemsurvives.The sequence following the 3 phase fault isas follows:1 Pre-fault state.1-2: At t=5 sa 3 phase fault occurs on one of the linesbetween bus 34 and bus 4.2-3: At H.09 sthe fault is cleared.3-4: At e5.5sthe shunt capacitor at bus 32 is connected andthe reactors at bus 1 and bus 2 are disconnected.4-5: At t-5.8sauto reclosing.5-6: At H.89 sthe faulted line is permanently disconnected.6-8: The impedanceas seen by the relaysisslowlydecreasing due to decreasing voltages and increasingpower flows.

At e87.2 sthe zone 3 of themhorelays located at bus 3protecting the lines between bus 3 and bus 34 areentered. About the same time zone3 of the quadrilateralrelay located at bus34protecting the remaining linebetween bus 34 and bus4 is also entered.

7:

8-9: At e250sa bus fault occurs at bus 342.9-10: At e250.09sthe bus fault is cleared.11 The systemstartsto recover and will end up in a stableoperation state.

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-OM50 002 00 4 006 008 01 012 014 016 018R [PUI

Fig.4.The impedance as seenbyamh o and quadrilateral relay located at bus34 protecting the non faulted transmission h e between bus 34 and bus4.

Total number of operations

TheRX diagram for the relay located at bus34indicates that themho relay has a natural defence against load encroachmentwhen the impedanceas seen by the relay is located close to theresistive axis. However if the injected reactive power increases ahigher value of the reactance will be seen by the relay and henceit ismore likely that a mho relay will mal trip incomparisonwith a quadnlatem reay.Intl us simulation the injected reactive power atbus3 towardsbus34 increases with about 100%during the fmt83 secondsafter the3phase fault. Theother power flows inthe systemareapproximately the same. Whenmho relays are used it is not therelay located at the bus with the lowest voltage level which oper-ates fmt but the relays that meet the largest increase in reactivepower flows.The procedure for the algorithm includedinthe distance relaysat bus3 protecting the lines between bus3 and bus34 is exam-ined below.In f i gure 5weseethat thereisa considerable differ-ence in magnitude among the negative peaks of the derivative ofthe voltage which fall in between indications6and 8 caused bytap-changer operations and current limter actionsas comparedwith the peaks caused by the short circuit faults.The negative and positive peaks of the derivative which arisewhen the initial fault occurs and is cleared (i.e between indica-tion 1to6)make that the proposed algorithm et the relays workas traditional back-up distance relays.When the zone3 elements are entered due to the decreasingvoltages and increasing power flows block 2 in figure 1 will nottreatths as a fault since no high negative peak of the derivativeof the voltage occurs.Thuswhen the impedanceas seen by therelay further moves into zone3 the algorithm will make therelay to continue to work as normal since the algorithm willalternate between block 1.2 and6.

5 0

When the bus fault at indication 8 occurs the impedanceas seenby the relay will be within zone3 at the same timeas the relaywill apprehend a high negative peak. Hence the algorithm willstartto alternate between block4 and5and wait for a high posi-tive peak to arise; i.e the fault to be cleared.Thisprocess maycontinue until the time delayof zone3 expires and a trip signalwill be sent to the circuit breaker. However in the case investi-gated the fault is cleared by primary protection. Thus the algo-rithm will reset and continue to altemate between block 1,2 and6. The line will not be tripped and the system may start torecover.

I50 100 150 200 250 30030' Time[s]

Fig. 5.The derivative of the voltage at bus 3.

V I E ZONE 3DISTANCE RELAY OPERATIONS IN THESWEDISHTRANSMISSIONGRID

The system regulatorof the Swedish transmssion systemmain-tains a database where information about all disturbances in thesystem is stored. From the start of year 1985 until the end of1998 about 1000 distance relay operations at the 220 kV leveland 1200 at the400kV level were recorded.Table2.Zone3operations in the Swedish transmission system from thestartof year 1985 until the end of 1998.

Voltage level [ kV]Number of correct operations

Number of incorrect operations

From table2 we see that zone3 operations are rare occasions.The only incorrect operation was due to human activity duringmaintenance work. From the simulations and statements

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above it may seem remarkable that no zone 3 operations havecaused or aggravated power system instability events inSweden during this period. Of course operation conditionswhere these type of events occur are rare but other reasonsmght be that start relays and in case of numerical relaysso-called general start criteria which obstruct mal-trips due toload encroachment are widely used. Also a restrictive viewconcerning line loading is applied when the reach of the zone3 element is determined. A safety margin of at least 2 is usedbetween peak load and the load which makes the relay tooperate. However the consequence s that the whole length ofall adjacent lines are not always remote back-up protected bydistance protection. An additional reason for the absence ofzone3mal-trips might be that the settingsof the relays arecontinuously updated after changing system configurationsand/or new system operation conditions.One way to find out the necessity of zone 3 elements is thestudy of what could have happened if the elements had notbeen in use. In one case zone 3 has been operated as localback-up to achieve a satisfactory selectivity due to infeedfrom T-lines. Another zone 3 operation was due to a two phaseto ground fault. In this case the omission of a zone 3 distanceelement had not mattered since earth-fault protection hadcleared the fault in approximately the same time. The twofinal cases were remote back-up operations which very likelyhave saved the system from costly consequences. However itshould be observed that in these cases duplicated line protec-tion was not accessible.

IXCONCLUSIONSUndesirable zone 3 distance relay operations during voltageinstability can be the difference between a total blackout and arecovering system. The reasons for these mal-trips are often acombination of decreasing system voltages and increasingreactive power flows.In system analysis it is important to consider both relays withcircular and quadrilateral shapes since they make up (in Swe-den) about 50%each of the distance relays used today. Forline loading at a high power factormho relays have a naturaldefence against load encroachment. During voltage instabilitythe reactive power demand will most likely be increasing andthus quadrilateral relays are at an advantage. To increase thesecurity devices which restrict the area of coverage in theresistive direction may be usedincombination with the zone3relays.An adaptive algorithm which prevents mal-tripping due tovoltage instability has been proposed. The algorithmnot onlyconsiders the relation between the apparent impedance and thezone of operation but also uses the derivative of the voltage todistinguish short circuit faults from other events in the system.

Recordings from the Swedish transmission system show thatzone 3 operations are rare occasions. No mal-operations arereported but there are cases where zone 3 relays have workedas remote back-up protection and thus very likely saved thesystem from costly consequences. However in the Swedishsystem measures which prevent load encroachment have ahigh priority and relay settings are frequently checked. Theseare most likely the reasons for the absence of zone 3 misoper-ations.

X. ACKNOWLEDGEMENTSThe authors would like to thank Svenska Kraftnat for theirinterest in this research area and for their financial support.The authors also would like to thank ABB Power Systems forproviding Simpow.

XI. REFERENCES[11 Western Systems Coordinating Council DisturbanceReport For the Power System Outages that Occurredon the Western Interconnection on July2, 1996, 1424MAST and July 3, 1996, 1403 MAST, Approved Sep-tember 19, 1996, InternetURC http://www.wscc.com/distnews.htm.

Taylor C.W, Power System Voltage Stability, ISBN0-07-1 13708-4, McGraw-Hill, 1994, pp. 263.le Dous G, Daalder J , Karlsson D, Dynamcal LoadMeasurement in a 2000 MW System in order to Oper-ate Power Systems Closer to their Limits, CIGREReport 39-107, Paris, August 1998.CIG& TF 38-02-08, Long Term Dynamics Phase11, 1995.Fankhauser H.R, AnerosK, A-A Edris. and TorsengS,Advanced Simulation Techniques for the Analysis ofPower System Dynamics, IEEE Computer Applica-tions in Power, Vol. 3, No. 4, October 1990, pp. 31 -36.

[2]

[3]

[4][5]

X I . BIOGRAPHIESM attias J onsson received his M. Sc. from Chalmers University of Technol-ogy in 1998. His interest lies in power system stability and power systempro-tection. At present he works towards a Ph.D. thesis.J aap E. Daalder obtained his Ph.D in power engineering from the Eind-hoven University of Technology, The Netherlands. He worked there as anAssociate Professor until 1984 when he left for Norway to become a Directorof T echnology and Memberof the Boardofa subsidiary of the Brown BoveriCompany, nowadays ABB . In 1993 he was appointed as full Pr ofessor atChalmers UniversityofTechnology. Since 1997 he is heading the DepartmentofElectric Power Engineering.Hisareas of interestare power systems andenvironmental i ssues related to power engineering.

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