NEW ADAPTIVE ALGORITHM FOR DETECTING LOW- AND HIGH OHMIC FAULTS IN MESHED NETWORKS

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1 EW ADAPTIVE ALGORITHM FOR DETECTIG LOW- AD HIGH OHMIC FAULTS I MESHED ETWORKS Gernot DRUML Rüdiger-W. KLEI Olf SEIFERT A.Eberle GmbH&CoKG Germny DB Energie GmbH Germny Siemens AG - Germny g.druml@ieee.org ruediger-willy.klein@bhn.de olf.seifert@siemens.com ABSTRACT Sttistics show tht the mjor portion of grid fults re erth fults. Conventionl relys re designed only for low ohmic erth fults under sttionry conditions nd for non meshed systems. They cn hndle neither high ohmic erth fults, which occur especilly in rurl networks with overhed lines, nor intermittent erth fults in compensted cble networks. Additionlly ll lgorithms re very sensitive to rel nd virtul crosstlks from the lod current to the zero-sequence-system, which occur in every meshed system. As consequence, very often the erth fult is not recognized or the wrong feeder is selected to be helthy. In the pper the resons for the crosstlk from the lod current to the zero-sequence-system, the influence even to trnsient relys nd the benefits of the new dptive lgorithm re explined in detil. The theoreticl explntion is completed with the presenttion of rel field results from very lrge meshed distribution network. Therefore trnsient relys with this new dptive lgorithm cn be used successfully in meshed networks with non negligible crosstlk. The lgorithm cn lso be used for the detection of restriking erth fults in compensted cble networks nd intermittent erth fults in isolted networks. BASICS OF THE EARTHFAULT To explin the behvior of single line erth fult, two different processes cn be superposed [3], [6], [9]. The following two processes re strting t the sme time, but with different durtion: dischrge of the fulty line over the erth chrging of the two helthy lines over the erth The two processes re ending in the sttionry stte of the erth fult. The explntion of the two processes will be mde by using network with three feeders (A, B nd C) nd n erth fult in line 1 of feeder A ccording to Fig. 1. ITRODUCTIO In mny countries of the EC the "resonnt grounding" is one of the most importnt options in electricl network design to obtin the optiml power supply qulity. The first min dvntge of this tretment of the neutrl point is the fct, tht in most cses the system is self-heling, s the rc distinguishes without ny intervention of the protection system[1]. The second min dvntge is the possibility of continuing the network opertion during sustined erth fult. As consequence, the number of interruptions of the power supply for the customer is reduced. But with this improved power qulity problems rise for the selective detection of erth fults. The conventionl relys re designed only for non-intermittend low ohmic erth fults nd for non meshed grids. With the new dptive lgorithm directionl erth fult detection from low ohmic up to some kohm is possible in non-meshed nd meshed networks. Up to now the preferred fields of ppliction for trnsient relys in the medium voltge were lrge unmeshed cble networks. With the new dptive lgorithm the rely is lso pplicble for rurl networks, where the probbility of high ohmic erth fult is much higher. This new dptive lgorithm is now lso ble to suppress crosstlk from the lod current to the zero-sequence-system. Fig. 1: Dischrge of the fulty line over the erth Dischrge of the fulty line over the erth The lines cn be considered s distributed lttice network, consisting of complex seril impednce Z LXX nd line-toground cpcitnce C XX.The gretest probbility for the first ignition is ner the mximum of the line-to-ground voltge u 1G. At this time the line hs bout the mximum chrge. The dischrge of the lttice network of line 1 will

2 strt t the fult loction nd will propgte in both directions to the ends of line 1. A reflection of the wves occurs t the end of the line respectively t every chnge of the imge impednce of the line. These reflections cn be detected in form of oscilltions t high frequency in the zero-sequence current nd in the zero-sequence voltge. The oscilltion frequency essentilly depends on the seril impednce nd the line-to-ground cpcity which re, in first pproximtion, proportionl to the length of the line. The frequency is higher for smll networks nd it is lower for lrge networks. Chrge of the two helthy lines over the erth As result of the dischrge of the fulty line the tringle of the line-to-ground voltges is destroyed. As the supplytrnsformer is still delivering symmetricl three phse system, the two helthy lines will be chrged to the line-toline voltge. This chrging process for the network with three feeders is shown in detil in Fig. 2. Feeder C isc Feeder B isb ZL2C ZL3C ZL1C ZL2B C1C C3C ic3c C2C ic2c C u is ZL2 ZL3 ZL1 C1 C3 C2 Fig. 3: Chrge of one helthy feeder 1 t u () t = u ( t) + S ( ) C i τ t eq dτ (1) q () () ( ) S t u t = u t + Ceq (2) Strting the integrtion t point where u (t ) = results in: q () () S t u t = Ceq (3) Drwing digrm of this reltion, with q on the ordinte nd the zero-sequence voltge u on the bsciss results in stright line with the grdient C eq, which is the equivlent zero-sequence cpcitnce of the feeder. Subsequently, this digrm shown in Fig. 4 will be referred to s qu-digrm. ic3 ic2 ZL3B ZL1B C1B C3B C2B B ZTr2 ZTr3 ZTr1 Feeder A isa ZL2A ZL3A ZL1A ic2a ic3a ic3b ic2b A qo /As qoa qob qoc ug u21 u31 u1g ZF C1A C3A C2A -.1 LP 1 GD ip Fig. 2: Chrge of the helthy lines over the erth The distribution trnsformers respectively the lods re comprtively high ohmic nd cn be neglected in the first pproximtion. The influence of the Petersen-Coil cn lso be ignored, s the impednce of the Petersen-Coil is much higher thn the lekge inductnce of the trnsformer. QU-ALGORITHM The following considertions re bsed on the trnsient definition of the zero-sequence-system ccording to the spce-vector-theory [5]. For exmple, for the helthy feeder B or C, s shown in Fig. 3, of our smple-network the chrging cn be described with eqution (1). if ic3a ic2c ic3c ic2b ic3b ic2a uo /% Fig. 4: qu-digrm of the three feeders in cse of low ohmic fult in feeder A In the cse of fulty feeder this reltion is no more vlid. The sum of the chrging currents of ll helthy feeders flows out from the fulty feeder, it strts with negtive grdient nd in compensted networks it is not stright line. The lst sttement cn be used s dditionl informtion for the erthfult detection. The dvntges of the qu-lgorithm re: Simple evlution o high speed smple rte is necessry. A smple rte of bout 2 khz is sufficient Influence of the dischrge is reduced due to the integrtion of i The integrtion nd evlution cn be done over hlf period The integrtion of i over lrger rnge before the trigger-point enbles the detection lso of high ohmic erth fults up to some kohm

3 on-line versions of the lest squres lgorithm [8] nd pttern recognition lgorithms (e.g. [4] ) improve the computtionl efficiency The disdvntges of the stndrd qu-lgorithm re: Sensitive to nlog-digitl-converter sturtions, s the stright line is modified to curve Sensitive to not negligible phse-splitting Sensitive to crosstlk from prllel systems The first disdvntge is only relevnt in cse of integrtion over the complete hlf period of u nd in combintion with testing of the feture "stright line" of helthy lines. The sensitivity ginst phse-splitting is generl problem of ll relys, especilly for cos(phi) relys. Using Fig. 5 the reson of phse splitting in helthy meshed network will be explined. The symmetry of line my be cused for exmple by the kind of lying the cbles, s shown in Fig. 6 (for further detils the reder is referred to [5],[1],[11]). If the cbles re lid in tringle (see Fig. 6 b) the mutul coupling of the three phses nd therefore the seril impednce is obviously the sme. 1 Z M12 Z M13 Z 2 M23 3 2r ) b) Fig. 6: ) Single conductor cbles in prllel b) Single conductor cbles in tringle A similr sitution cn be found for overhed lines where n improvement cn be mde, by trnsposing the phses. Z M Z M13 Z M23 3 Phse-splitting I W ISA 16.6 A I W 16.6 A ISC ZL2 C ZL3 C ZL1C =>.5 Ω => A ZL2 B1Ω 5 A ZL3 B1Ω 5 A ZL1 B1Ω 5 A One possible wy to compenste this influence in loops is to mesure the currents in ll feeders nd to dd the currents of the feeders, which re switched to loop. But this version needs very high number of current mesurements nd, in ddition, lwys the ctul informtion, which feeders re switched to loop. The requirements to such SCADA System would be enormous. UG ZTr2 U2 U3 ZTr3 ZTr1 U1 U2G U3G U1G A => 16.6 A => A ZL2 A 1Ω 5 A ZL3 A 1Ω 5 A ZL1 A 1Ω 5 A ISTr ISA A => 16.6 A 1 A 1 A 1 A Crosstlk from prllel systems Systems switched to loop re lso very sensitive to the mgnetic coupling of glvnic isolted prllel systems. GD Fig. 5: Phse-splitting in the helthy network In symmetricl sitution the lod current in ech phse will be splitted symmetricl to the feeders A nd B. In this sitution the mesured sum-current ( I S = 3 I ) t the substtion will be zero. Due to unblnces in the seril impednces of the lines the distribution of the lod current will chnge. In the exmple the current in phse 2 is chnging from 5 A to 33.3 A respectively to 66.6 A. UG prllel System ZTr2 U2G U2 U3 ZTr3 U3G ZTr1 U1G ISTr U1 ISA ZL2 ZL3 ZL1 ISC ZL2 B ZL3 B ZL1 B ZL2 A ZL3 A ZL1 A mgnetic coupling ow the sum-currents of the feeders in the substtion re no longer zero. In our exmple the sum-current increses up to 16.6 A nd the directions in the two feeders re opposite. We cn find this behvior in ny loop, in prllel lines nd in meshed networks. The size of the phse-splitting current depends on: lod current point of lod on the loop physicl rrngement of the symmetry of the seril impednces [2] number of meshes GD Fig. 7: Mgnetic coupling of prllel systems The influence of the prllel system my be negligible under norml opertion, due to the smll distnces between the three phses. But in cse of n erth fult in the prllel system, the currents through the three wires re no more symmetricl, lso in helthy feeders (see Fig. 2). The symmetric loding currents cn be in the rnge of 1 A. Therefore the mgnetic coupling must be tken into ccount. A worse sitution rises in cse of cross-country-fult in the prllel system.

4 Also phse-splitting in the prllel system cn be the reson for crosstlk. QU2-ALGORITHM AS A SOLUTIO With the ssumption of no chnge of the crosstlk from neighbor system nd no chnge of lod in the own system during the few periods of interest, lineriztion round the working point combined with nonliner filtering solves the mjor problem. For the explntion rel mesurements of n erthfult in the 16.7 Hz 11 kv network of the rilwy will be used. The network hs bout 12 A cpcitive current nd is compensted with severl distributed Petersen-Coils. In the following figures the first chnnel shows the zerosequence voltge in the substtion. The next two chnnels re the sum-currents of two prllel lines with high current due to phse splitting. These two currents re more or less opposite. These currents include lso the smller zero-sequence currents due to the nturl symmetry of the network. The lst two chnnels show two independent feeders. As long s the nturl symmetry of the net, represented by u, is very smll, lineriztion round the working point E1: qu1 E1: qu E1: qu2 E1: qu Fig. 9: Stndrd qu-digrm for the four feeders without correct identifiction of the fulty feeder results in reltive smll inccurcy of the equivlent model. Fig. 1 shows the result of the lineriztion. The first periods re zero nd the following periods up to the erthfult hve only smll vritions due to vritions in the lod. 1 FultRec_8_E1_361.csv E1: uo(t), ie(t) 1 FultRec_8_E1_361.csv E1: uo(t), ie(t) Fig. 8: Zero-sequence-voltge nd -currents of four feeders with phse-splitting in feeder 1 nd 2 Fig. 8 shows, tht the chnge of currents in the feeders 1 nd 2 due to the erthfult is very smll. In Fig. 9 the corresponding stndrd qu-digrms re shown. Compring to Fig. 4 or [3], cler decision, which feeder is the fulty feeder nd which re the helthy feeders is not esy, respectively impossible Fig. 1: Currents fter lineriztion t working-point with strong reduction of the phse-splitting in feeder 1 nd feeder 2 With n dptive nonliner filtering from the first period up to the erthfult, lso these vlues cn be reduced. The prmeter of the dptive filter hs n effect on the mximl detectble high ohmic erthfult. On the other side, the sturtion of nlog-digitl-converter nd currents of cross-country fults re lso modifying the filter-prmeters.

5 The results of the dptive nonliner filtering of the lowohmic erthfult re shown in Fig FultRec_8_E1_361.csv E1: uo(t), ie(t) Fig. 11: Zero-sequence- voltge nd -currents fter dptive nonliner filtering. Influence of crosstlk, phse-splitting is reduced nd prefult vlues re set to zero. In Fig. 12 the results of this qu-lgorithm with dditionl nonliner dptive filtering re shown. ow it is no more problem to identify the feeder 2 s the fulty feeder E1: qu1 pfch E1: qu3 pfch E1: qu2 pfch E1: qu4 pfch Fig. 12: qu2-digrm with correct identifiction of the fulty feeder 2 COCLUSIO In this contribution we hve discussed the bsics of n erth-fult nd the effects of phse-splitting on the behvior of the trnsients in the zero-sequence system. To see the limits of conventionl trnsient relys nd the stndrd qu-lgorithm, we hve elborted different situtions, which my occur in every rel-world looped ore meshed network. According to these requirements we hve presented new lgorithm. Rel mesurements show the effectiveness of this new concept for trnsient relys, bsed on the qu2 lgorithm. REFERECES [1] Bergel J., Berthet L., Grob O., Bertrnd P., Lcroix B., 1993, "Single-phse fults on compensted neutrl medium voltge networks", Proceedings CIRED 1993, vol.2, [2] Druml G., Kugi A., Prr B., "Control of Petersen Coils", Proceedings XI. Interntionl Symposium on Theoreticl Electricl Engineering, 21, Linz [3] Druml G., Kugi A., Seifert O., "A new directionl trnsient rely for high ohmic erth fults", Proceedings CIRED 23, vol 3, pper 3.5 [4] Dud R.O., Hrt P.E., Stork D.G., 21, Pttern Clssifiction, John Wiley & Sons, ew York, USA. [5] Heinhold L., 1987, Kbel und Leitungen für Strkstrom, Siemens, Berlin-München, Germny, 4.Aufl. [6] Herold G., 22, Elektrische Energieversorgung III, J. Schlembch Fchverlg, Weil der Stdt, Germny. [7] Kovcs K.P., 1962, Symmetrische Komponenten in Wechselstrommschinen, Birkhäuser Verlg, Bsel und Stuttgrt, Switzerlnd. [8] Ljung L., 1999, System Identifiction: Theory for the User, Prentice Hll PTR, ew Jersey, USA [9] Pundt H., 1965, "Untersuchung der Ausgleichsvorgänge bei Erdschluß in Energieversorgungsnetzen", Energietechnik,15. Jg. Heft 1, [1] VDEW, 1997, Kbelhndbuch, VWEW-Verlg, Frnkfurt m Min, Germny [11] Weßnigk K., 1993, Krftwerkselektrotechnik, VDE Verlg, Berlin-Offenbch, Germny