HOW SYMMETRICAL COMPONENTS MAY HELP TO SUPPRESS VOLTAGE SENSORS IN DIRECTIONAL RELAYS FOR DISTRIBUTION NETWORKS

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1 C E D 17 th nternational Conference on Electricity Distribution Barcelona, May 2003 HOW SYMMETCAL COMPONENTS MAY HELP TO SUPPESS VOLTAGE SENSOS N DECTONAL ELAYS FO DSTBUTON NETWOKS Xavier LE PVET Patrick BASTAD sabelle GAL Supelec France Supelec France Schneier Electric - France Xavier.lepivert@supelec.fr Patrick.bastar@supelec.fr isabelle_gal@mail.schneier.fr 1. NTODUCTON Z n 1918, Fortescue evelope a theory to stuy polyphase network through original components of the various electric quantities, known as symmetrical ones [1]. This theory has been mainly applie to three-phase networks. t has been wiely use in various protection relays using negative or zero sequence current an/or voltage. t is also the base of some location algorithms [2 ][3][4]. Most of the time, symmetrical components are use to increase the efficiency of tripping algorithms. Our aim in this paper is to show how these components can also be processe to avoi the use of voltage sensors in irectional relays esigne for istribution networks. Z o (c) o V o 2. THE PHASE-TO-EATH FAULT Let s consier a phase-to-earth fault on a balance network. t s quite usual to stuy the fault by replacing it with a 3- phases loa as shown in figure 1. Fig. 2 : Symmetrical scheme of a balance network c) zero sequence where : - Z. -. i V o - Z o. o (3a) (3b) (3c) a b c ef V a V b V c Fig.1 : groun fault moel By using the equations of the fault, we obtain : E i Z + Zi+ Zo+ 3. ef So the fault can be represente by a current source which epens on the network state an configuration an also on the value of its resistance. o (4) s equations are : b c 0 V a ef. a (1a) (1b) o i a /3 (2a) V o ef. (2b) On the other han, the balance part of the network can be moelise by its Thevenin representation in each sequence : ESE_LePivert_A1 Session 3 Paper No

2 C E D 17 th nternational Conference on Electricity Distribution Barcelona, May 2003 Z i i i Fig. 4 : Symmetrical scheme of network + fault Z o V o o For every kin of fault it is possible to o the same an to replace the faulte section by current sources. Nevertheless it can be more or less complicate, especially in case of a series fault (broken conuctor). (c) Fig. 3 : Symmetrical scheme of network + fault c) zero sequence The fault is consiere as a current source in each sequence. Positive, negative an zero sequence sources are the same. These currents can flow all over the network, epening on the impeance of the various lines an transformers. Accoring to the kin of grouning, rules an relations between symmetrical components can lea to a useful criteria for protection an location. 3. PHASE-TO-PHASE FAULT 4. EXAMPLE OF THE DECTONAL POTECTON UPSTEAM A BUSBA WTH AN MPEDANCE EATHED NEUTAL n this part, let us etail an example to explain how to chose the right relay tripping criteria. A MV busbar is fe by two transformers. A phase to earth fault occurs on one of the two upstream feeers. The relays (cf figure 5) must etermine where the fault is. Transformer 1 Busbar Protection Loas The phase b-to-phase c fault can be stuie in the same way. esults are : n o o E i (5) Z + Zi The network an the fault can also be moelise as : HV network Transformer 2 n o Fig. 5 : locate upstream busbar ESE_LePivert_A1 Session 3 Paper No

3 C E D 17 th nternational Conference on Electricity Distribution Barcelona, May 2003 Principle an propose algorithm As we can see in figure 5 a threshol on zero sequence current is not efficient to locate the fault. A conventional irectional protection analyses currents an voltages an particularly the phase ifference between the zero sequences voltage an current. What is propose here is to use the phase angle between the negative sequence current (prouce by the fault) an the positive sequence current before the fault The two iagrams which can be use to calculate these two currents are shown in figures 6 an 7 : Z 1 Connecte loas Fig. 8 : Z ch in impeance iagram is between 0 an π/6. 2 f few loas are really connecte an if the overall length of cables is significant, Zch will be then very capacitive an it woul be avisable to take near to 0.Except this particular case, π/6 is satisfactory. Fig. 6 : Positive sequence network before fault Now that we roughly know where the ratio (0)/ (0) is in the complex iagram, we are going to a two other ratios : 1 i / (0) an 2 i / (0). Let s call fault the fault current an Z A //Z B Z A.Z B /(Z A +Z B ). transfo 1 1 i Connecte loas C 0/2 C 0/2 fault 3. V Z+ Zi+ Zo+ 3. On the other han : éf 3. V éf (6) 2 i fault 2i 1i 0,5. i 0,5. (7) 3 Fig. 7 : Negative sequence network uring fault Let us consier the positive iagram before fault. We can assume that 1 # 2, so that : 1 (0) 2 (0) 0,5. (0)/ Z ch where Z ch is the impeance of all the loas connecte to the substation. This total loa correspons mostly to the sum of an unergroun cable or an overhea line (or a mix) an a supplie loa (with a power factor greater than 0.8, ie approximately ϕ<35 ). On the impeance iagram, this impeance is roughly in the quarter-plan characterize by an angle from the real axis : So : 0,5. V 2i an éf 0,5. V 1i éf (8) (9) ESE_LePivert_A1 Session 3 Paper No

4 C E D 17 th nternational Conference on Electricity Distribution Barcelona, May 2003 The quarter-plans in which 1 i / (0) an 2 i / (0) are locate can thus be rawn: see figure 9. Protections Transformer1 Zn i / (0) faulty sie HV network Transformer2 i / (0) unfaulty sie Zn Loas Busbar Fig. 11 : stuie network Fig. 9 : Fresnel iagram for i/(0) Multiplying the two stuie quantities lea to i / (0). t is then possible to complete the complex iagram : see figure 10. The stuie network is a typical MV network connecte to the HV network by two transformers. t is mainly raial but there s also a close loop. The fault can be locate between a transformer an the busbar, or on a feeer ownstream the busbar. The aim of each protection is first to etermine where the fault is : upstream or ownstream. i / (0) faulty sie i / (0) unfaulty sie Three kin of irectional algorithms have been evelope an teste : - Upstream busbar protection ( n 1) - Close loop protection (n 2) With logic iscrimination on the loop - Feeer protection (n 3) Actually, there are six ifferent algorithms because two kin of grouning have been taken into account : resistive earthing an compensate earthing. They use the current in each phase, the nominal voltage of the network an the value of the neutral resistance. Concerning the irectional close loop protection, it s necessary to have an aitional information to efine the power flow irection before the fault. This can be a bip coming from the busbar when voltage or current in the feeer is maximum uring a perio, or a local ba quality voltage measurement (to have an iea of the voltage phase angle), Fig. 10 : Fresnel iagram for i / (0) Obviously, the criteria for a irectional protection is really simple : i f Arg [ π + θ ; 3. π 1 + θ1] then a fault is ( 0) 2 2 etecte. 4. APPLCATON OF SMLA ALGOTHMS TO POTECT A MV NETWOK Characteristics of the teste network : - Loas : 8 (as shown in figure 11) between 5 kva an 7.5 MVA - Total length of lines ownstream the busbar : up to 90 km - n : 1 Ω to 2400 Ω (in case of resistive grouning system) - ef : 1 Ω to 1500 Ω - Feeer length : 2 km, 5 km an up to 70 km (the last one aggregates all the other feeers) - Loop length : 13 km - Transformers : 2 x 40 MVA - Tuning : between 0.5 an 1.5 (in case of compensate grouning system) ESE_LePivert_A1 Session 3 Paper No

5 C E D 17 th nternational Conference on Electricity Distribution Barcelona, May esults : A test ata base has been simulate with the EMTP incluing thousans of various cases. [4] T.Balwin, F. enovich, L. Sauners an D. Lubkeman, 2001, locating in unergroune an high-resistance groune systems, EEE trans. On nustrial Applications, vol. 37, esistive earthing, phase to earth fault Algorithm 1 : some errors for n > 600 Ω + small loa + many unergroun cables (total > 70 km) + ef > 50 Ω Algorithm 2 : about 6% of errors Algorithm 3 : about 1% of errors Compensate earthing, phase to earth fault Algorithm 1 : no error Algorithm 2 : about 3% of errors Algorithm 3 : some errors for big loa an/or a high rate of unergroun cables Phase to phase fault All the algorithms work perfectly. 5. CONCLUSON This paper presents new algorithms for etermining the irection of faults. These algorithms are base on current symmetrical components. We have seen that the results are really goo, for resistive earthing as well as for compensate earthing. Moreover it s still possible to improve them by waiting some hunres of ms when a oubtful case is etecte in orer to let the right protection eliminate the fault or by using harmonic symmetrical components. The main avantage of these algorithms compare to usual irectional ones is the cost reuction for the relay as there s no voltage sensor. An other one is the settings simplification : inee the user only nees to specify the nominal voltage an the earth impeance. EFEENCES [1] C.L. Fortescue, 1918, Metho of symmetrical coorinates applie to the solution of polyphase network, Proceeings of the American institute of electrical engineers, vol. 37, [2] T.Balwin, F. enovich an L. Sauners, 2002, Directional groun fault inicator for high-resistance groune systems, Proceeings EEE nustrial an Commercial Power Systems Technical Conference,vol.1, [3] P.Bastar, L. Garcia-Santaner, X. Le Pivert an. Gal, 2002, A voltage-base fault location metho for raial istribution network, Proceeings EE fifth international conference on power system management an control, vol. 1, ESE_LePivert_A1 Session 3 Paper No