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Protection Relay Static Character Emulation Program Based on MATLAB@

Liu Shiming Li Xiaob inYanTai Electronic Information Industry Co LTD, China)

Abstract: This paper recommends a program-PRESCmade by author. The program can be used to em ulate thestatic character of distance relay or static character ofsome elements of distance relay. It is programmed withMATLAB language and works under MATLABenvironment. Adopted the benefits of MATLABenvironment, PRESC has an open structure. That meansusing PRESC, users can expand relay functions toemulate the static character of any distance relay freely.The paper illustrates several usage of PRESC, whichinclude analyzing distance relay character, simulatingfault-phase choosing element and simulating afault-location formula using only local data. The resultdiagrams of these samples are also demonstrated in thepaper.

Key words: MATLAB ; distance relay; staticcharacter; simulation

1 The Design Thought of PRESCIt is a hard work to thoroughly analyze characters ofdistance relays, because there is many factors that canaffect the characters. For example, the change of systemimpedance, the differences of transmission angle, thedifferences of fault resistance etc., must all be concerned.Upon that, there are also many methods to analyzedistance relay characters. The most basic one is showingrelay characters in impedance plane. This method candisplay impedance track clearly and easily, hut it isdifficult to take the system parameter change into account.Another way is to analyze in voltage phasor plane. This

method can analyze the effects of system parameter, faultlocation and fault resistance in detail. But it needs toomuch acknowledge of plane geometry, so' is hard to hemastered by relay operators.There is another way to analyze distance relays -Impedance Relay Analysis Program (IR AP) '. It candisplay the distance relay character in resistance-distanceplane. From the plane, we can clearly see the faultresistance endurance of a simulated relay while faultshappen at different point along the protected line. Thispaper introduces a simulation program based on theanalyze method. With some improvem ent and expanding,it has more functions in simulating distance relaycharacters. The author calls it PRES C - ProtectiveRelay Em ulator of Static Character.PRESC is programmed with MATLAB language.Because MATLAB is an interpreted language, PRES Cmust run in M T L B^ environment. MATLABcalculation is based on com plex number. So it is very easyto express and calculate electrical phasor values in it.Owing to the powerful graphic display capability ofMATLAB' language, the PRESC simulation results areclear at a glance. PRESC has a friendly CUI with detailinstruction manual with it. Thanks to the merit ofMATLAB language and environment, PRE SC is an openplate. Users can program any relay functions withMA TLAB @ anguage and simulate these functions usingPRESC.

2 Mathematical Modal of PRESCFor analyzing relay comprehensively, PRESC uses apower system modal with two sources, parallel circuit

2004 The Institution of Electrical Engineers.Printed ardpublished by the IEE, Michael Faraday House, Six Hills Way, Stevenage,SGI 2AY

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Fig1 The diagram ofbasic system of PFESC

lines. The diagram is as Fig 1.While Es and Er are power sources potential, Zs and Zrare power sources inner impedances. ZI is the protectedline impedance, and ZcO is the mutual impedance.PRESC uses a parallel circuit, it can consider the effectof parallel line fault to the relay. As it is a simplesimulator, it doesnt concern cross-line faults. As a staticstate simulator, it also doesnt take the distributedcapacitance into account. For study the relay overreachcharacter and backward faults distinguish capability,PRESC can add a backward line and a forward line tothe protected line. Whose imped ances are represented asZm and Zn in Fig 1. Zm and Zn can he set equal to ZI ordifferent to Z1. And line lengths can be set separately. Ifbackward line and or forward line are not needed, thenthe length(s) can be set to zero.PRESC can simulate 9 types simple faults, including 3types of single-phase ground faults, 3 types of doublephase ground faults and 3 types of phase-phase faults.As the symmetrical fault steady state is very easy to heanalyzed, it neednt concerning of mutual inductance,fault resistance or transmission angle, etc. PRESCdoesnt sim ulate three-phase-fault. PRESC is a simplesimulator; all parameters and results are phasor values.The results include normal state voltage and loadcurrent, fault state positive, negative and zero sequencevoltage and current at relay installation point. Allphasors are under system frequency. That means, nofrequency change be considered. And all phasors havethe same reference phase.While simulating, at each step, PRES first figure out thesequence currents at fault point. .Then the currents atrelay installation point is equal to fault point currentsmultiply dividing coefficients. Sequence voltages can hegot by multiplying sequence currents and impedancesfrom relay installation point ,t o the power sourcebackward the fault point. These sequence cnrrent and

voltage phasors plus normal state phasors are theafter-fault phasors. Then PRESC calls relay function todetermine the relay response. If the relay hips, then therelay function return 1, if not, then return 0. At eachpoint along the line, PRESC will search from zero faultresistance to the presetted maximum fault resistance tofind the interface between tripping and no-tripping.After all the points along the line have been calculated,PRESC will show the result in distance-resistance plane.All electrical phasor calculations are camed out byPRESC inner functions, while relay function isprogrammed by user. System parameters can also be setby user. So PRESC is an open simulation plate. Allfaults are calculated .using standard fault-calculationequations. Here just explains the calculation of parallelline faults.If a ground fault happened in either line of the parallelcircuit, the system sequence circuit can be illustratedlike Fig 2.Then the system integrated resistance viewed at faultpoint is:

2 Z m k . m1 ZaIZbZ z =

z ZZ = ZI zm -. ZI -. Zl :Zd z dZ* Zmd Z dh = z r Z -. z r - . zr

The dividing coefficient of current though ZI divided bywhole.fault current is:

The dividing coefficient of current though Zd dividedby whole fault current is:

Fig2 The diagram of couple-line fault circuit

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With upper equations, any faults within the parallelcircuit can be carried out correctly. It must be m entionedthat for the zero sequence circuit, Zm and Zn mustinclude zero sequence mutual inductance. PRESC hastaken this into account.

3. PRESC Simulation DemosThe usage of PRESC is explained clearly in theintroduction document in PRESC in detail, includinghow to use the PRESC menu, how to make functionfiles, how to read the result figures etc. In this paper, afew examples are given to show what can PRESC do.Examples are just to sh ow methods of simulation ratherthan illustrating the performance of particularprinciples.

3.1 Distance Relay Simu lation

A mho relay with memory function is simulated in thissection. The system is configured as duel power,paralleled transmission line with a backward line and aforward line. The result figures are showed below. Fig 3. is the result of mho relay with compensation ofparalleled line 10, and fig 4. is that withoutcompensation.Comparing two figures it was showed that, paralleledline IO com pensation can m ake the trip zone o f mhorelay exactly equal to the setting value (in thissimulation, the setting zone of distance relay is 80 ofthe protected line), but the compensation can alsoinduce unwanted trip while theres ground fault somewhere on the parallel line, as shadow in the undersidehalf plane shows. Whats more, it also induce overreachwhile theres ground fault in certain area on the forwardline, as shadow between 100%-120% shows. So, if theparalleled line IO comp ensation princ iple is used in mhorelay, some m easurements must he taken to avoid thesemalfunctions and overreaches.

If mho relay has no such compensation, then there areno unwanted hips when ground fault happened on theparallel line and no overreaches, but this brings littleunder-reach. In this simulation, the setting zone of therelay is SO%, while Fig 4shows that the relay can onlyreach about 75 of the line.

Fig 3 The character 1 of mho relay withmemory function

a o c i i l a ~ ~ ~ < ~ . c I l l a - : ~4 S t b g j i J t ~ & : B ~ S i ~ - t m , W A . PFig 4 The character 2 of mho relay withmemory function

3.2 Fault Phase Selective ElementSimulation

This section takes a phase selective element usingsteady state currents for instance. The phase selectiveelement compares the negative and zero sequencecurrents in each phase and making use of the fact that inthe faulty phase, for a phase to earth fault, the negativeand zero sequence current are in phase, whereas in thetwo healthy phases the current vectors are 120 out ofphase. So the complex plane can be divided into threesections named A, B and C. If the phase angle betweennegative and zero sequence current is located in thesection A then a single-phase-ground of phase A fault

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or two-phase-ground of phase B and C happened. Buthow to divide the sections? Here are the simulationresults. Fig 5. shows the scheme of section A area from-60 -60 '(scheme one). Fig 6. shows the scheme ofsection A area from -30 -60 '(scheme two). Theshadows in Figs means two-phase-ground of phase Aand B will he judged as phases BC or phase faultwhere a mistake happened.The effects of two section division schemes can becompared between Fig5 and Fig6. Under the simulationsystem parameters, the chance of wrong-selection ismuch bigger in scheme one than that in scheme two. So,if the scheme one is used in distance relay, some othermeasures must b e taken to avoid such wrong-selection.

Fig5 Characterof phase-identify function

-Fig

3.3 Fault Location Principle Simulation

This version o f PRESC can'only calculate the electricalvalues at the line end where relay installed. So it canonly check fault location principles ;sing only localvalues. As an example, this section simulate theprinciple discussed in refer [2] The calculation of linereactance is as follow:

XIU. zos -us zoc)

zos I C . ctgpl - Is)- zoc Is ctgpl +ICWhile in the equation, subscript c mean s real part ofthe corresponding phasor, subscript s meansimaginary part. s bushar voltage phasor, is linecurrent (with self line IO compensation), 10 is self linezero-sequence current, q is line positive-sequencereactance angle.In the simu lation, if the error of calculated fault locationis under 5 , then the result is assumed correct. Fig 7.shows the simulation result for the power sending end o fthe line, while Fig 8. shows the result for the powerreceiving end. They all simulated under the same systemparameters.The simulation results show the effect of this principleis different for using at sending end and at receiving end.The validity of fault location result at sending end isbetter then that at receiving end. While refemng to thesimulation result figure, it must be noticed that the linepercentage axis is different between Fig 7 and Fig 8 . Forexample, the point of 90 in Fig 7.(the sending end ofthe line) is equal to the point of 10 in Fig %(thereceiving end of the line), and vice versa.

j;g RWFA[dwalb. Pt?

Fig7 Fault-locate result a t send-end

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