A Microprocessor Based Protection System for Generator-transformer Units

8/3/2019 A Microprocessor Based Protection System for Generator-transformer Units

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A ~IICROPROCESSOR ASED PRIYI'ECTIW SYSTEN FOR GENERATOR-TRANSFORMER UNITS

I. Korbasieuicz, PI. Korbasiewicz and W. 'ilinlcler

Silesian Technical University of Gliwice, Poland

INrRODUCTIONThe last decade has brought a large numberof works in the field of digital relays andprotective schemes, However, they ueremainly concerned with individual relays orschemes, lilce distance and/or differentialprotective devices of power lines and/ortransformers (1) ( 2 ) (3) or substations (4).Less attention was turned to the implemcn-tation of the digital technique in protectimsystems of generators and generator-transfor-mer units. Just lately some of papers havebeen published in the latter field (5) (6)(7)This paper describes a general concept ofa digital protective system for large gene-rator-transformer units, The main principleof this system is presented and some of theprotective functions are discussed. Specialattention is given to the overall impedanceprotective scheme which integrates all thetasks realised in the conventional techniqueseparately by individual relays, i.e. back--up distance, loss of excitation and loss ofsynchronism (pole-slipping) protection.CONCEIT OF THE MULTIPROCESSOR PROTECTIVESYSTEMHardware outlineConsidering that the protective system mustfulfil the requirements of high reliability,dependability and redundancy, all protectiverelays used so far for large generator--transformer units have been divided intothree main groups shown in Table 1.A n important role in such a groupage playsalso the required computation capabilityand speed.Fig. 1 shows the structure of the protectivesystem, where the individual units serve the

f l l ow i ng purposes- Three independent units based on 16-bitmicroprocessors (CWI-CPU3) are devotedfor the realisation of tasks within thoframework of the given groups. Each pro-cessor cooperates with the local memory,local channel of analog-to-digital con-version and the local system of two-stateinputs.processor (CPU 4) is dedicatod for thedistribution of tripping and signalisationimpulses as well as the self-monitoringof measuring and control circuits andinput quantities.- A system memory enables the exchange ofinformation between the above main units,since all up-to-date results of measure-ments, protective data (state vectors) andalgorithm settings are stored there.- The protective system is connected by acdmmon mu1 mas r bus. The ind ividualmicroprocessors obtain the access to thisbus by an interrupt routine execution.A non-conflict cooperation is assured bythe bus arbiter.

- A distribution system based on a 16-bit

There is a possibility to introduce a hostcomputer for such tasks like: the modifi-cation of protective algorithm in conjuctionwith the power system protection in emer-gency and failure conditions, the visuali-zation of states of the protected unit aswell as the recording of selected signalsin fault conditions.Preparation and processine; of measurandsThe analog current and voltage input signalsreceived from the main C.T.'s and V.T.'sof the protected generator-transformer unitare subsequently processed in the analog-to-

TABLE 1 - Arrangement of the generator-transformer protective relays into three main groupsGroup 1 Group 2 Group 3

Generator-transformer differ. Back-up distance protection Generator differentialHV-r es d ear h-f ault Loss of excitation Unit-transf ormer differentialStator earth-fault-100 $ Pole-slipping Stator earth-fault - 95 $Negative phase-sequence Motoring Rotor earth-faultBack-up overcurrent Under-f requencyRotor overload Overf uxingStator overload 0ver ol ge


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-digital (A/ D) conversioii circuits uhich CO-sists of the following elements:- Separating units, i n c l u d a shieldedauxiliary transf m e r s , I/V - and V/ V -converters.- Overvoltage restricting devices.- Two-pole Butterworth filters with a cutofffrequency accommodate to the samplingfrequency.- Sample and hold circuits.- llult iplexer.- 16-bit A/D - converter.A sampling frequency of 800 HZ has bcenchosen, thus an indispensable time intervalfor fast protective algorithms of 1.25 mili-seconds is ensured.EXAMPLES OF S O F T Y I ~ R EARCHITECTURE ANDi1LGORITIII.lSIntegrated system of protections based onimpedance measurementIn conventional analog protection systemsthere is a strict separation of functionsrealised by the individual relays evenwhen the same electrical quantity is used inall cases for fault discrimination. Typicalexamples for such a solution are the fol-lowing protective schemes based on the con-trol of impedance seen from the terminalsof synchronous generators under varioustypes of faults:- Distance protection as back-up to diffe-- Loss of excitation protection.- Pole-slipping protection.A11 these protective functions can be inte-grated in one coherent system, taking ad-vantage of the benefits given by the digitaltechique. This concept is based on thecontinuous tracing of the impedance vectorlocus on the complex impedance plane inrelation to the selected operating charac-teristic which is shown in Fig. 2.Two groups of measuring input signals areused for the fault detection, i.e. the cur-rent ILV and voltage VLV at the generatorterminals as well as the current IIN andvoltage VI^ at the IIV-terminals of the step--up transformer. These signals are afteranalog filtering processed according to theflow chart shown in Fig. 3. First the realand imaginary parts of the fundamentalharmonics of the above measuring signals arecomputated in Block 2 using a DIT-spectralestimator. Subsequently the resistance andreactances are determined, thus the impe-dances ZLV and Z ~ I Vseen from the generatorterminals and HV - outputs of the step-uptransformer respectively are obtained.The first decision block (Block 3 in Fig.?),which determines the successive operations,is destinated for the detection of phase-to--phase faults within the generator-transf r-mer unit and partly in the external powersystem. There are two conditions which mustbe satisfied before a trip command is sent.The first one is fulfilled when thc rate ofchange of 11 and X crosses the sctted thres-holds d w i w powar swings. This stato,determined by the inequalities A Il)bil1&yand/or

rential protective schemes.

6 X ) A A & is stored for a given time

taz, which should be largcr than theoperatix times of thc main protcctivescheues, like tho dirf rential Generator--transformer prot c ion, 1I'J- bus a r pro c-tion and the first zoiics of transmissionlino distance protection. a l e sccond trip-ping condition will bc satisfied, when theimpedance locus ZL V enters area I in Fie. 2and remains therc f o r a tine tIi or tie.The determination, which of these time res -trictions have to be checked, depends uponthe inpedance vector Z11v seen from the HV --terminals of the step-up transfornor (Linca in Fig.2). This part of the altyorithm isrepeated for each ncw set of samples.The accomplishment of the succossivc part of%healgorithm in Uloclis 4,5 and 9 is distri-buted in time in such a way that thc systemelaborates a new decision four tines pcr onecycle based on the first harmonic wavefoxm.This principlc rosults from the range ofposs ible posit ions on the impedancc diagramof impedance vector loci with time underpole-slipping and/or loss of field condi-tions. Thanks to the introduction into thealgorithm the procedure of impedance planerotation (phase %le CL in Fig.2) in Clock 6 ,the verification of the impedance locus ismade easier, since all lines inclined to thecoordinate axes can be replaced by prependi-cular and/or parallel lines towards the newcoordinate system.The identification of loss-of-field condi-tions requires the verification of the impe-dance locus ZLV in relation to area I1 inFig. 2. Tripping conditions exist when thevector impedance enters into this area andstays there for a fixed time tII.Correct pole-slipping detcction requires thetra-cing and storage of information aboutthe sequence during the entry of the impe-dance locus into the individual areas of theresulting operating characteristic as wellas the respective timc intervals and sequenoecounting. This task is realized thanlcs theimplementation of logical variables whichare continously updated.After checking all possible conditions, astate vector is being created, which is acoded result of the algorithm operation,comprehensible to the distribution system(Block 8 and 10).Differential protective schemesFor reliability reasons three differentialprotective schemes have been proposed(Table I), ll based on the same concept butdepending upon the properties of the indi-vidual object realised in different ways.Identical shapas of biased relay characte-ristics are chosen in all cases. Iloreover,the same additional criterion for thc dis-tinction between extcrnal and internalfaults under transient current transformersaturations is used. This criterion is basedon the fact that under out-zone faults theoperating (i.e. difference) current waveromappears with a certain Lime delay, uhcreasLhe restraint current occurs simultaneouslyHiti1 the fault inccption ( 8 ) .Considerinc that no magnctiziw transientphenomen can occur in the generator--transformer-unit, since the stcp-up tran-d o r m e r rcnains co mec ed to the Generatorand is energized Eradually as the laiLcr isrun up to speed and excited, the unit pro-


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tective scheme has only to be equipped withthe fifth harmonic restraint against over-saturation. IIonever, the differential protec-tive scheme for the unit transformer shouldbe fitted with a second harmonic restraintcircuit if a circuit breaker exists on theHV-side of this transformer. Fig. 4showsan algorithm of a differential protectionfor such a transf rmer.Stator - earth fault protectionTwo types of stator earth fault protectiveschemes are proposed, the first based on thedetection of the zero-sequence voltagc offundamental frequency (95 $ - stator windingprotected), the second ono covoring 100 $of tho winding, based on the natural thirdharmonic e.w.f. produced by tho protectedsynchronous generator. Here only the latterprotection will be described.Fig. 5shows thc algorithm of the proposed100 $ - protcctive scheme. It combines twodiscrimination methods proposod by Plartilla(9) and K h a n and Cory ( 6 ) . As input quanti-ties the generator neutral voltage U, andgenerator terminal voltage U,(residualvoltage) are selected (3lock I ) , which allowthe calculation of the e.m.f. After filte-ring by DFT-techique the first harmonicsUOIH and UAIH as well as the third harmonicsU~313 nd E ~ Hre being computed (Block$?"kubs equently the comparison be tu een thepresent measured magnitudes with setted trip-ping values TR is done (Block 8). Thus, about95 $ of the stator winding can be protectedby an optimal choice of TR. The remaining5 $ of the winding is being protected usingadditional criterions checked in the leftpart of the algorithm. If E ~ Hs relativelysmal1,so the measuring and filtration errorsmay have a significant influence on the faultdetection. Therefore, two inequalities shownin Blocks 5 and 6 must be checked. T o avoidmaloperations due to some fault transientconditions, the rate of change of the ratioU O ~ H / U ~ ~ Hs checked in Block 7. The designa-tions TI and T3 correspond to logic varia-bles, e.g. TI=l means fault existence,whereas T1=0 is an indication for non faultconditions. If one of the conditions eitherTI=l or T3=l is fulfilled, the trip commandis send to the distribution system (Fig.1)after a selected time lag.CONCLUSIONSIt has been shown that thanks the digitalteclmique further improvement in reliableand flexible fault detection and discrimi-nation within generator-transformer unitscan be ensured. This can be achieved by theoptimum choice of operating characteristics,criterions and a1gorithms.A typical exampleis the overall impedance protective schemewhich integrates several functions, realisedso far separately by individual analogrelays.REFERENCES

Sachdev, 5I.S. and Baribeau, M.A. , 1979,"A new algorithm for digital impedancerelays1!,IEEE Trans. on PAS,E, 232-Phadke, A,G. and Thorp, J.S., 1983,"A new computer-based f l u x restrainedcurrent d iff rent al relay for pow er

-2240,

transformer protection", ~ E E ET&S. on-AS,102, 2624-3629,

3 . hlisznieuslci, A. , 1937, "Digital algorithnsfor differential Protection of powertransformers", Proc. of the 9th-PSCC1Cascais, 725-731.

4. Bornard, P. , 1988, "Power systemprotection and substation control: trends,opportunities and problems1' ElectricalPower & Energy Systems, E, 101-109.5. Sekine, Y., Hatata, PI. and Yoshida, T.,1984, "Recent advances in digital protec-t on" EXec rical Ponor & Energy Systems- , 181-191.6, K a h n , K.R. and Cory, B.J., 1985, "Developments in digital generator protection",IEE Conference Publication, No. a,223-226.7. Fromm, U. , Franc, 2. , Ilulendik, B. andSteiner, Ch., 1987, I'Generatorschutz mit

dem digitalen Schutasystem MODURES 216",Brown Boveri Tochnik,2, 693-700.G. Ilar PI. , 198 , "Neue Different alrelaisfilr Transformatoren und Leitungenl',Brown Boveri Mitt., 68, 0-78.9. Martilla, R. J. , 1986, tlDesign rinciplesof a new generator stator ground relayfor 100 7 coverage of the stator winding,IEEE Trans. on Power Delivery, 1,41-51.


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MEASUREMENT , DIGITIZING AND PRE-PR OCESS ING OFMEASURING SIGNALS ILV ULV Im,U, p

Figure 1 Hardware outline of a digital protective systen for generator-transformer units

measuring signals

. -Figure 2 Operating characteristics of theoverall impedance protection

I FILTERING OF MEASURING SIGNALS AND COMPUTATIONOF IMPEDANCES ZLv .Zm dFAULT INCEPTION 3'

LOSS-OF-EXCITATION ?1 I P O L E - S L P r I N G ? l e ]I

COMPUTATION OF STATE VECTORFOR DISTRIBUTION SYSTEM

Figure ;I Flow chart of' th e overall impedanceprotection


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1

COMPUTATION OF Iop& I, 1I

I - YESIop >MAXCOMPUTATION OF I= & IL B

Ia k , I ,I1I OMPUTATION OF I,II II

Figure 4 Algorithm of the unit transf ormer differential proteotion

T

Figure 5 AlgorZthm of th e stator earth fault proteotion

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A Microprocessor Based Protection System for Generator-transformer Units