8/12/2019 00608218 Ojo 5

1/7

331

ADAPTIVE DIGITAL RELAY FOR COMPREHENSIVE DISTANCEPROTECTION O F TRACTION OVERHEAD EQUIPMENT

R.P .Maheshwar iA M U Aligarh (U P), IndiaProtect ion of the overhead equipment OHE ) ofs ingle-phase 25 k V t i action system is a special caseof dis tanc e protection. In addi t ion to the ear th faul ts ,wro ng pha se coupl ing faul ts take place on h e OHE.In th i s paper , a n adap t ive p ro tect ion s cheme su i t ab lefor t he s ing le-phase 25kV 50Hz t ract ion sys tem usedby he Ind ian a nd s evera l o ther Ra ilways i s p r esen ted .T he proposed relay character is t ic i s a combinat ion oftwo quadr i l a t e r a l s in RL-p lane , one fo r the ea r thfau l t s an d o ther fo r wrong-phase-couplingfaults. h erelay automat ical ly selects the appropr iate relayingalgor i th m on detect ing the nature of faul t . Secondly,i t autom at ical ly adjus ts i t s reach to sui t the prev ai l ingfeed ing cond i t ion .Perform ance of the relay has been ev aluated both bys imula t ion on a personal computer an d by real t imeimplementa t ion on a p ro to type . The p ro to type i sdes ign ed around a 16-bit mi croprocessor, Intel 808 6.For the evaluation propose the fault dat a was generatedus ing the s imula tion package EMTP/A TP deve lopedby Salford Univers i ty . The resul ts of s imu lat ion andreal - t im e tes t prove that the relay is capa ble ofdiscr im inat ing between internal faul ts , external faul tsan d heavy loads , and detects internal faul ts in abouta quar ter cycle .INTRODUCTIONProblems in providing protect ion to the t ract ionoverh ead equipment (OHE) are di f ferent f rom thosefaced in protect ing an ordinary transmiss ion l in e or adis t r ibut ion sys tem of the sanie vol tage level . Thefac to r s tha t make i t dif ferent and di f f icul t are thesys tem conf igurat ion. change in the length of the l inedu r ing opera t ion and the amount and na tu re o f theload ing .

H.K;. Verma

University of Roo rkee, IndiaDue: to continuous movement of locomotives load isnot located at the far thes t point of the l ine. I t mayhappen at t imes that several locomotives ruin in thesame section of the OHE, ead ing to a l a rge increasein load . Th i s s itua t ion i s fu r th er aggrava tedd ue to theuse of DC series motors in ele ctr ic locom otives, whichdraw l a rge cur r en t on s t a t ing . As a resul t , theimpedance seen by the dis tan ce relay on such heavyloads may be even smal ler th an tha t on dis tant ear thfaullt . Therefore, best discrimination between loadsand ear th faul t (EF) can be prov ided by a quadirilateralcharacter is t ic dis tance relayThe: load current drawn by locomotives is r ich inh a r m o n i c s . T h u s a n y d i s t a n c e r e l a y m e a s u r i n gimpedance ( R a n d X ) muslt use h ighe r order f i l ter tohave good accura cy of mea uure me nt. Th is fi ll ter willcause a long de lay in r e l ay opera t ion . A betteraltet rnative is to measure res is tance and inductancewhich is affected neither Iby presence of harmonicsnor transients .Th e power supplies from aldjacent substatio ns have aphase difference of eith er l: OOor6 0 .The y are isolatedfroim each other by a neut:ral section during normalfeeding and by an insulated over lap du r ing emergencyfeeding, when due to the fai lure of one subs tat ion.supply to i ts feeding section is dr aw n from the adjacentsubs tation by clos ing br idg, ing inter rupter ; Fi g . I A ninadver tent coupl ing between these suppl ies leads toa wrong- phase-coup l ing ( W P C ) f au l t . A s theimpedance seen by the relay at laggin g subs tat ion willl ie in the second quadran t of R-L lane, another RL-m e a s u r in g d i s t a n c e r e l a y w i t h a q u a d r i l a t e r a lcharacteristic i n that quadrant can be very e,ffective.D u r i n g e m e r ge n c y f e e d i n g c o n d i t i o n s u pp l y I Sextended upto next feeding sect ion, thus tlhe relayshould provide protection upto the end of next section

Developments in Power System Protection 25-27th March 1997Conference Publication No. 434 EE, 1997

8/12/2019 00608218 Ojo 5

2/7

332

OHEImpedance

i m ) in>0 040 2 0 . 8 L 7 28 0 4 1 . 6 L72'

rw* &-2SIIY T R AN W O R M U ? BR EAKER

Arc Faul tRes is tance Res is tance

Q

8/12/2019 00608218 Ojo 5

3/7

333

Imuedance seen bv relav o n wrong phasecoupl ing a).

I~

arth fault Pro tections already explained, protect ion on ear th faul ts on3HE can be best provided by a RL-meascr ing relayNith quadrilateral characteristic. In order to ensureelective protection in emergency as well as normal'eeding condition, the relay in provided with twonductance reach sett ings L, n d L,. T h e s e tt in g i sidaptively changed by m oni tor in g the pos i t ion of the,r idging interrupter.The relay charac teristic on RX- p l a n e is shown in Fig.. In the f igure, Vector OF A is the l ine impedance,3F i s the impedance of l ine for norm al reach of 40 kmind OA is that for the extended reach of 80km.nargin ofabout 15 can be g iven i n the va lues so thathe relay does protect the ent i re len gth inapi te of aninderreach. Thus the reach s et t in gs alon g the l inempedance vector are OF a n d ON Vectors OC andU3 represent the arc plus grou nd res is tance for theaul t occurr ing at subs tat ion and far thes t point of: a t e n a r y u n d e r e m e r g e n c y f e e d c o n d i t i o n s ,.espectively. From table 1, OC = 5 . 1 3 0 a n d AB =1 250 h e a n gl e p i s t ak e n a s loo a n d ct is found to,e 70'. The correspo nding slope s ar e p = 0 .176 and1= 2.75. The des i red operat ing zon es of relay underiormal and emergency feed cond i t ion s are taken ashe q uadrilaterals O ED'X,,and OEB'X L2 respectively.The correspon ding reach sett ing s a lo ng X-axis areCL,,nd XL2.An allowance of 1 5 % has been made heren the of fset a long R-axis , l ike tha t for the reach along

8/12/2019 00608218 Ojo 5

4/7

The operat ing condi t ion of the relay for ear th faul tprotect ion is given by

L < LEAND L > Q * ( R - R , )AND R > OAND L > P * R (1)Where LE s e q u a l t o L, r L, as th e case may be , and& i s the intercept on R -ax is .WPC rotectionUsually, protection for WPC faul t o n t ract ion OHE isprovided by an off se t mho r e l ay a nd tha t on ear thfault by an pla in mho r e l ay . Bu t due t o an over l ap oft he two charac te r i st i cs , some t imes the ea r th f au l trelay operates on WPC f au l t t oo . Th e impedance s eenby the relay on WPC faul t a t the subs tat ion wi thlaggingvo l tage always l ies in secon d quadrant ofRX-plane and tha t fo r ea r th f au l t i n the f i r s t quadran t .Therefo re , t hese two f au l t s c an be discr iminated byhaving the two relay chara cter is t ics wi thout anyover lap. The ear th faul t re lay character is t ic shouldl i e exc lus ive ly in the f i r s t quadran t ( as a l r eadyproposed) and tha t o f W P C relay in the secondquadrant .

T h e W P C r e l a y c h a r a c t e r i s t i c s h o u l d in c l u d eimpedances cor r esponding to bo th the va lues o f theangle between subs tat ions , i .e . 120 and 60 . Fig. 4s h o w s th e r e l a y c h a r a c t e r i s t i c m e e t i n g t h e s erequirements on RX plane . W, and W, are impedancesseen when angle between so urces is 60 ,under normaland emergency feed condi t ions respect ively. Thepoints W, and W, cor resp on ds to the 120 anglebe tween sources (Tab le 2), STR, is obtained bydrawing a l i ne para l l e l t o the one jo in ing W , and W,and a l lowing a marg in o f approx imate ly 15%. For thepresent case the value of R,,, t hus ob ta ined is 1ORThe s lope of l i ne ST R , is q, = 1. Th is s lope whentransformed on the RL. plane i s Q = -3 .183 1 . Forthe ease in tnicroprocessor implementat ion. Q , istaken as - 3 Giving an a l lowance of about 15 . t h ereach set t ings for normal operat ion and emergencyfeed ing a r e t aken a s X,,, = 3 7 R a n d X,,, = 7 2 . 5Cor responding reach s e t t ings on R L plane will beL W l = 1 1 8 i i i H a n d L W 2 - 2 3 1 m H .

334

Th e condi t ion for detect ion of WPC fau l t thus can begiven by

L > OAND R< 0AND L < L,W h e r e L, i n adap t ive ly chosen be tween LW1 andLW2 depend ing upon f eed ing cond it ion .AND L > Q, (R- \) ( 2 )

r i p . 1.1.1 ch.r.u.ri.tie cor m pmccctionon U-pl. .

Comprehensive ProtectionSince bo th EF a n d W C rotect ion are based onmeasurement o f R and L and many ca lcu la t ions a r ec o m m o n to them, they can be combined in a s ingler e la y . T h e re l a y a f t e r c o m p l e t in g t h e c o m m o ncalculat ions , wi l l adapt ively select the approp r iatepro tec t ion a lgor i thm.Th e com posi te character is t ic i s the comb inat ion ofthe individ ual quatr i la teral character is t ics wi th onemodif icat ion. Persusal of the operat ing condi t ion ofthe two relay s , given by 1) a n d ( 2 ) br ings ou t the f ac tthat the one ch eck namely R = 0 is common to them.One m ore check namely , L > PR, can be made commonwithout jeopa rdizing the select ivi ty of the W P Crelay. I ts implica tion is to change the bottom bo undaryof the W PC relay cha racter is t ic f rom a hor izontal l ineto a slant ing l ine.Com plete process f lowchar t of the com posi te relay isgiven i n Fig. 5 By moni tor ing the br idging inter rupterthe relay sele cts i t s reach, L E or ear th faul t and L,,q or

8/12/2019 00608218 Ojo 5

5/7

335

WPC protection. As condit ions L>PR. s common toearth fau l t and WPC protection, it is tested first . Ifth i s condi t ion i s t rue then depen ding upon the s ig n ofres i s tance e i ther the ear th fau l t p ro tec t ion or t heWPC protec t ion a lgori thm is se lec ted . di

dtDK =

Fig. 5 Operation flow chart of compositerelay f o r OHE protection.

CALCULATION OF RL From equation 3) and 4) ,alues of R and L can be writtenas;

Let the samples of current and voltage signa ls to the relay kvk.] k-1 k-at instants k-1 and k be I- , , i ~, k-land v L. Now system R =differential equation, neglecting line ca pacitanc e, at these k L-1 I; -1 kinstants can be written as

VL., = LD,, + R I ( 3 )

8/12/2019 00608218 Ojo 5

6/7

336

Cault at substationr h o u g h 5 i resistanceFault at Substationthrough ze ro resistanceCault at 'F' through

R resistance( Fig. 2 )Fault at ' A throughR resistanceOperation On4 rated loadFig. 2 )

U . A . Not applicable

The values of the derivative of current, D and D t, reobtained by Newtons 5 point backward dfferen ce formula.Accordtngly, the expression for derivative at instant k sg v e n byD,= ( 2.O83ik-4 i L-, 3 i k-2- 1.333 i k - 3 + 0.25 i k4]/h (7)In this expression coefficient 2.083 in approximated to 2to achieve multiplication by binary siuft and thus, savingso computational time. lus 5- point formula besidesbeing computationally simple gives fairly accurate results.PERFORMAN- EVALUATIONRrformance oft he relay was evaluated both by simulationon apersonal computer aswell by real -time implementationon a prototypedeveloped arou nd 16-bit microprocessor, Intel8086. Samp ling frequency was taken as 800 Hz Thesimulation program was written in FORTRAN and real-time testing program was written in assembly language of8086.For the dyn amic tests on the relay, the traction supply systemwas s imula ted wi th the he lp o f EMTP /ATPElectroMagnetic Transient Package / Alternative TransientProgram). Faults were created at various locations on theOHE line ). In order to study the w orst case, fault werecreated at the voltag e zero. Relay operating time for variousearth fault conditions are tabulated in Table 3 ,For WPC faults the relay detected the fault wt hi n 6 . 25 msof the fault inception, both for normal as well as extendedreach. In order to avoid false relay operation due tooscillation in the calculated values of R and L, trip signal isissued only when fault is detected on two consecutivesampling instants.

Steady state characteristic of composite relay with normalreach setting in shown in Fig. 6 .CONCLUSIONA digital distance relay for a co mprehensive protection oftraction OHE on both earth fault an d wrong phase -coupling faults, is presented. The relay characteristic is acombination of two suitablly designed quadrilaterals in RL-

plane. In order to provide immunity to relay from error dueto transients and high frequency noise, R and L instead ofR and X re measured. For tlus purpose New ton's five pointbackward dlfference formula with some approximation isused. The relay is provided adaptive feature in terms ofreach setting and selection of appropriate protectionalgorithm for earth fault or wrong phase coup ling fault asthe case may be.For the testing of the realy test signals were obtaiced bymod elling the traction OHE on simulation programe EMTP/ ATP of Salford University.Th e relay is found to successfbly &scrim inate between earthfault an d wrong-phase-coupling faults. The relay IS able todetect boundary earth faults acurately. It also operates on afault close to the substation.The ea rth fault relay is capableof clearing even the boundary faults within a quater cyclebesides remaining stable at four times the n ormal load. Thewrong-phase-coupling protection is capable of issuing tripsingal within 6.25 ms of fault incep tion.TABLE 3 Relay W ra ti ng time for earth fault on Traction

Relay operating time ( ms)Normal reack2 5

2.5

3.75

N. A

N o operatior

Extended react2.5

2.5

N . A

3 . 7 5

N o operation

8/12/2019 00608218 Ojo 5

7/7

337

Fig. 6 Steady state characteristicof composite relay withnormal setting.