Fig.1. Single-line diagram of the faulty

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1 Analysis of broken conductor with ground contact faults in medium voltage power network Dumitru TOADER, Ştefan HĂRĂGUŞ, onstantin BLAJ Politehnica University Timişoara P-ţa Victoriei nr., Romania; Abstract. Broken conductor with ground contact faults make a relatively important figure (3 to 10 %) in the total faults statistics. The design of protective devices to selectively detect such faults should be based on the analysis of fault conditions. This paper analyses how the fault resistance, the consumer s power and structure, the total capacitive current of the network, the capacitive current of the faulty line behind the fault, influences the zero-sequence voltage at the substations busbars and the grounding system current. Networks with non-grounded neutral point, grounded via a reactor respectively a resistor are considered in the analysis. The results are compared with measurements made in real medium voltage networks. 1 Introduction A relatively frequent fault that occurs in medium voltage power network is that of a broken conductor with ground contact either on the generator s side, or on the consumer s side. In this paper an analysis of the second case is performed. Figure 1 shows the single-line diagram of the analysed network where Tr is the 110/0 kv transformer, L 1, L,...L n are the 0 kv lines, T..I. is the internal services transformer also used to create the neutral point, Zn is the neutral point grounding impedance,.i. are the internal services and R t is the fault resistance. For nongrounded networks, respectively for network with the neutral point grounded via a reactor, the zero-sequence voltage at the 0 kv busbar will be computed, and also the resistor current for a network with the neutral point grounded via a resistor. Fig.1. ingle-line diagram of the faulty Fig. Equivalent network for analysis network Various parameters, like the fault resistance R t, the capacitive current of the line behind the fault I c, the total capacitive current of the network I c, the apparent power of the consumer fed by the faulty line c, and the consumer s negative-sequence impedance Z ci do influence the zero-sequence voltage and the neutral point grounding current in a way that will be analysed. 1

2 The Mathematical Model The type of fault analysed can be seen as two single faults: a single-line interruption and a singlephase shortcircuit. The faulty network can be partitioned into three symmetrical subnetworks, denoted with A, B and in Fig., linked by two nonsymmetrical impedances which implement the fault; A and B refers to the network which stays on the generator s, respectively consumer s side. is the equivalent symmetrical network of the ground [1,, 3] and [Z a1a ] and [Z bb3 ] are the sequence-matrices of the linking impedances. At nodes a 1, a and b the following equation can be written at [4, 1,, 5]: [ U ea1 ] [ U ] [ ] + [ ] + [ ] [ ] [ ] ea Z a1a Z a1a 1 Z aa Z ab I a1a =, (1) [ U eb ] [ U eb ] [ Z a b ] [ Z b b ] + [ Z b b ] [ I b b ] where [Uex]s is the sequence column matrix of the electromotive voltage in node x (x=a 1, a, b, b 3 ), [Z xx ]s is the sequence matrix of the impedances as seen from node x and [I a1a ]s, [I bb3 ]s are the sequence matrices of the currents shown in Fig.. After some transformations eq.() becomes [ U ] [ ] [ ] [ ] [ ] [ e Z + Z + Z ] 1 Z I [] [ ] [ ] [ ] [] =, () 0 Z Z + Z I where [Z s ]s=[z a1a1 ]s, [Z c ]s=[z aa ]s, [Z a1a ]s=[z 1 ]s, [Z ]s=[z bb3 ]s, [I ]s=[i a1a ]s, [I]s=[I bb3 ]s, [U e ]s=[u ea1 ]s. The matrix is of the form Z h Z Z [ Z ] = Z Z d Z. (3) Z Z Z i olving eq.() for the currents I h and I h assuming Z cd, after some transformations, it follows b5 ( U cd U ch ) b6 ( U ci U ch ) I h = I h =, (4) b b b b 1 3 where coefficients b k (k=1,5) are expressed in terms of the matrices [Z s ], [Z c ] and [Z ] [4]. In general, the positive- and negative-sequence reactances of electrical drives are different from each other [6, 7, 8]. As it is hard to evaluate how many electrical drives are fed by the faulty line, the negative sequence reactance of the consumer is expressed as a fraction of the positivesequence reactance, Xci=kXcd (k=1 stands for a purely static consumer). Finally, the current through the neutral point grounding system I n and the voltage U hb can be expressed in terms of Ih, jx jx ( Z hti + 3Z n ) I n = 3 I h, U hb = I h, (5) Z + 3Z jx Z + 3Z jx hti n where Z hti is the zero-sequence impedance of the neutral point reactor, X c the total capacitive current, and Z n is the neutral point grounding system impedance; the zero-sequence current I h is given by eq.(4). 3 Dependence of U hb and I n on the fault conditions The parameter values needed for computing U hb and I n (eq.5) depend on the fault conditions, i.e. the fault resistance Rt, the capacitive current of the faulty line behind the fault location Ic, the total capacitive current of the network Ic, the apparent power of the consumer fed by the faulty line c, and the negative-sequence impedance of the customer Zci. In the following the fault 4 hti n

3 resistance is taken as the independent variable, in the range [0 to ] Ω. The zero-sequence voltage is determined at the secondary winding of the zero-sequence filter. The numerical values needed for the computing and are the following [9, 10, 11]: pre-fault line voltage at the fault location 0kV; source impedance Z = (0,l + j,1) Ω (110/0 kv, 5 MVA transformer); line impedance up to the fault location Z Ld = Z Li = (1,16 + j0,76) Ω (0kV overhead line, km), Z Lh = (1,46 + j,8) Ω; neutral point grounding reactor impedance Z n = (0,5 + j105,8) Ω; zero-sequence impedance of internal services transformer (T..I.) Z hti = (,78 + j8,35) Ω; consumer s power factor cosϕ = 0,98; zero-sequence voltage at the substation s medium voltage busbar prior fault occurrence U eh = 153 V, negative-sequence voltage assumed zero; resistance of neutral point grounding resistor 38,5 Ω; apparent power of the consumer fed by the faulty line c = 1076 kva. Figure 3 and 4 show the zero-sequence voltage as function of the fault resistance for a network with the neutral point grounded via a reactor at resonance, respectively at 14,5 % overcompensation. The structure of the consumer highly affects the zero-sequence voltage. Fig.3 Fig.4 3

4 Fig.5 Fig.6 For the non-grounded, respectively the resistor-grounded network the consumer s structure is of less importance, as concerning the zero-sequence voltage and current (Fig.5 and 6). At those networks the fault resistance has a much more importance. 4 Experimental results The computed results were compared with measurements taken in two different medium voltage power networks. The first network has the neutral point grounded via a reactor and a measured total capacitive current of 100 A. Two reactors, each having the current adjustable in the range A were used to compensate this capacitive current. The fault was provoked on a 0 kv line with a capacitive current of 3A, at 485m from the substation s medium voltage busbars. The measured quantities are shown in table 1, for Zsd = Zsi =0,1+j,1 Ω, ZhTI =,8 + j8,39 Ω, ZLd = ZLi =,94+j1,8 Ω, ZLh = 3,18+j5,4 Ω, and Ic =,53A. It can be seen that the zero-sequence voltage is lower than the sensitivity level of the ground contact sensing relay RPP so that this relay did not detected the provoked fault. Tab. no. 1 Neutral point grounded via a reactor c R t U R U U T U hb [V] Operating regime [kva] [Ω] [V] [V] [V] U hb [V] calc. U hb [V] meas. ε [%] 50 55, 79, ,8 10,3 Resonance , 35 34,7 0,8 Overcomp. 14,5% ,1 63, ,1 15,5 9 Overcomp. 14,5% (sand) , ,8 14,5 4,8 Overcomp. 14,5% (sand) ,8 69,4 46,3 0,8 1,6 3,7 Overcomp. 14,5% (snow) 4

5 Measurements made in different conditions clearly show that the RPP relay do not detect ground contacts in a medium voltage network with neutral point grounded via but in very particular conditions. Nonselective ground contacts in medium voltage network is realized by zero-sequence voltage protective relays connected at the medium voltage substation s busbar. The minimum-voltage relay of this system is usually set at 15 V, and for the in use RPP s there is no possibility to lower this level. For a proper ground contact fault detection it would be necessary a minimal level of (5-10)V. This condition is satisfied by the Digital Protective Block BHT-10a which has a (5...0) V level setting range [1]. The second network has the neutral point grounded via a resistor, and a measured total capacitive current of 103,4A. The results are shown in table. Tab. no. Neutral point grounded via a resistor R t I c I n [A] [Ω] [kva] [A] meas. calc. ε [%],6 63 6,98 1,6 1,71 6, ,77 0,83 0,9 10, ,77 0,71 0,78 9, ,77 1,8 1,34 4, ,46 0,5 0,53 1,9 The measurements revealed that the RPP detected high fault resistance faults. Therefore, resistor grounded networks are better protected against broken conductor with ground contact faults than reactor grounded networks. The computed and the measured results are in good agreement with each other, for the assumed precision level of the system s parameters. 5 onclusion The analysis shows that the zero-sequence voltage of the medium voltage busbar and the neutral point grounding current, if a broken conductor with ground contact on the consumer s side fault occurs, are affected as follows: - the fault resistance do significantly affect the zero-sequence voltage for non-grounded networks, respectively with the neutral point grounded via a resistor - for a network with the neutral point grounded via a reactor, at resonance, the apparent power of the consumer fed by the faulty line has little effect on the zero-sequence voltage for Z ci =Z cd. The effect is significantly greater if Z ci is les then Z cd. - for a non-grounded network, by increasing the consumed power the zero-sequence voltage also increases. - for a network with the neutral point grounded via a resistor, at low consumed power the structure of the consumer has practically no influence on the grounding resistor current. For higher consumed power, lower values for Z ci lead to lower values for the resistor current. - the capacitive current of the line behind the fault, I c, also influences the zero-sequence voltage of the medium voltage busbars. Thus, for a neutral point grounded via a reactor, at resonance, by reducing I c the zero-sequence voltage also reduces. - the total capacitive current of the medium voltage network I c has a less pronounced effect on the zero-sequence voltage. Thus, reducing I c from 100A to 50A the zero-sequence voltage reduces by 5,5% if Z ci = Z cd, by 1,9% if Z ci = 0,5Z cd, and by 1,4% if Z ci = 0,3Z cd. For most 110/0 kv transformer substations with the neutral point of the medium voltage network grounded via a reactor, nonselective detections of groundings on the medium 5

6 voltage side are performed by sensing the zero-sequence voltage on the secondary winding of zero-sequence filter connected at the medium voltage busbar. The same is true for nongrounded medium voltage networks. The minimal voltage level for this protective device is 15V, and therefore a broken conductor with ground contact can be detected in very few situations (namely if I c 50A, I c 10A, c 30kVA, the network is at resonance and Z ci = Z cd ). For selective groundings detection relays of RPP type are in use. Experimental measurements showed that very few broken conductor with ground contact on the consumer s side faults have been detected by these protective devices. We conclude therefore that such faults cannot be proper selectively detected in medium voltage networks with neutral point grounded via a reactor. The situation is even worst if the neutral is non-grounded, for the zero-sequence voltage is less than the minimal sensing level of 15 V. Networks with the neutral point grounded via a resistor are provided with protective devices which can selectively detect a fault that causes a minimal 3A zero-sequence current. Therefore broken conductor with ground contact on the consumer s side can be selectively detected in most situations. To avoid holding a broken conductor on ground contact in a medium voltage network with non-grounded neutral, or grounded via a reactor, for a long period of time, and thus minimizing the risk of electroshocks, it is necessary to design high sensitivity protective devices which are able to detect single line-to ground faults. Protective devices with minimal-voltage relays with low voltage level of about (5-10)V, like the Digital protective block BHT-10a make possible proper detection of broken conductor with ground contact faults in a larger variety of situations. References 1. M. Bercovici, A. Arie, and M. Tudose. Aspecte privind aplicarea teoriei componentelor sistemice în analiza regimurilor nesimetrice a reţelelor electrice, Bul.şt.şi tehn. al I.P. Bucureşti, (XXIX), nr.4, 1967, p M. Bercovici and A. Arie. Aplicarea unor operatori de ortogonalitate pentru determinarea defectelor nesimetrice, multiple în reţele electrice, tudii şi cercetări de energetică şi electrotehnică, nr., 1968, p A. B. ernin. curtcircuite în cazul regimurilor cu număr incomplet de faze ale sistemelor electrice, Ed. Tehnică, Bucureşti, D. Toader. ontribuţii la studiul defectelor de tip conductor întrerupt şi căzut la pământ în reţele electrice de medie tensiune, teză de doctorat, Institutul Politehnic Timişoara, D. Toader. Analysis of Multiple Faults in Three-Phase Networks, Revue Roumaine des ciences Techniques, erie Electrotechnique et Energetique, Tome 48, L.A. Danilievici, V.V. Dombrovski and K.B. Kazovski. Parametrii maşinilor de curent alternativ, traducere din lb.rusă, Ed.Tehnica., Bucureşti, T. Dordea. Maşini electrice, Ed. didactică şi pedagogică, Bucureşti, P. Gheju. ontribuţii la studiul influenţei consumatorilor formaţi din motoare asincrone asupra curenţilor de scurtcircuit din reţeaua lor de alimentare, a comportării lor în timpul scurtcircuitelor şi a determinării parametrilor motoarelor, teză de doctorat, I.P. "Traian Vuia" Timişoara, Şt. Hărăguş, D. Toader and V. Toaxen. Fault transients simulation in distribution networks with improved neutral-point grounding, Proceendings of the IATED International onference, Power and Energy ystems, eptember 19-, 000, Marbella, pain, p A... Marched, G. A. Tench and P. Kundar. Accurate calculation of asymetrical fault currents in complex power systems, I.E.E.E. Transaction on P.A.. nr.8, 1981, p

7 11. D. Toader, and Şt. Hărăguş. Numerical simulation of transient phenomena triggered by singlegrounding faults, Acta Universitatis ibiniensis, eries Electrical Engineering and Electronics, 1999, p D.Toader, Şt.Hărăguş, I. Haţegan, BHT-10A: A zero-sequence voltage digital protective block for medium-voltage networks with isolated neutral-point, Proceedings of Power ystems onference, Timişoara, 003 7