Comparison between open phase fault of arc suppression coil and single phase to earth fault in coal

Availableonlineatwww.sciencedirect.comJOURNAL OF CHINA UNIVERSITY OFDirectMININGTECHNOLOGYELSEVIERJ China Univ Mining Technol 18(2008)0368-0372www.elsevier.corm/locate/jcumtComparison between open phase fault of arc suppressioncoil and single phase to earth fault in coalmine distribution networkLI Xiao-bo, WANG Chong-linSchool of Information Electrical Engineering, China University of Mining& Technology, Xuzhou, Jiangsu 221008, ChinaAbstract: When, in a coal mine distribution network whose neutral point is grounded by an arc suppression coil (ASC), a faultoccurs in the ASC, compensation cannot be properly realized. Furthermore, it can damage the safe and reliable run of the network.We first introduce a three-phase five-column are suppression coil (TPFCASC)and discuss its autotracking compensation theoryThen we compare the single phase to ground fault of the coal mine distribution network with an open phase fault at the TPFCAScusing the Thevenin theory, the symmetrical-component method and the complex sequence network respectively. The results showthat, in both types of faults, zero-sequence voltage of the network will appear and the maximum magnitude of this zero-sequencevoltage is different in both faults. Based on this situation, a protection for the open phase fault at the TPFCASC should be estab-lishedKey words: open phase; protection; single phase to ground fault; three-phase five-column arc suppression coil (TPFCASC)1 Introductionand reliable operation. However, when a fault occursin the ASC itself, compensation cannot be realizedIn coal mines, the safety of production is to some properly. It can actually damage the safe and reliableextent determined by the reliability of power supply run of the distribution network. One problem is theand the electrical equipment. A considerable amount overvoltage in the distribution network, another is theexperts-3. A main branch of this field is the problem lected and it is quite necessary to do some researchof research has been carried out on this issue by some false ground fault. These problems are usuallyof the neutral point of the distribution networkthemWith the development of a distribution network incoal mines, much attention has been paid to the issue 2 Basic structure of TPFCASC and itsof the neural point. One problem is its increasinglyautomatic compensation theorylarger capacitive current. Such a large capacitive current can do considerable harm to the network and2.1 Basic structure and operation principle ofresult in accidents such as gas outbursts, coal dustexplosions and cable bursts, etc, when a single phaseFig. I shows the principle of TPFCASC applied toto earth fault occurs. The best method to reduce thetypical coal mine distribution network and Fig. 2capacitive current is to use an arc suppression coils Its structure(ASC) 46In this new asc which is also a transformer, threeThe newest ASC can now autotrack the variation middle cores are surrounded by three primary windin capacitive current of the coal mine distribution ings A, B and C which are connected to the distribu-network and realize full compensation", which tion network and the earth respectively. The threemeans that the ASC can provide an inductive current secondary windings are open-delta connections. Thewhose phase is contrary to that of the capacitive cur- two auxiliary cores, located at both sides, are used asrent when a single phase to ground fault occurs, the zero-sequence magnetic path. The four air gaps,making the entire phase to ground current of the coal betyYH中国煤化工d the three middlemine distribution network small and ensuring its safe coresalize the rough ad-CNMHGReceived 10 October 2007: accepted 14 April 2008CorrespondingauthorTel+86-516-83885605;e-Mailaddressxbli_cumt@126.comLI Xiao-bo et alComparison betweease fault ofjustment in inductance of the ASc4-sI. The magnetic distribution network to ground, I. is the resistiveresistance of the three middle limbs is very low, i.e.current of the entire distribution network to groundthe positive-sequence impedance of the TPFCASC is (In a neutral point to ground via ASC system, it alsovery high and implies that its positive-sequence curincludes the current of the damping resistor. For exrent is very small. It approximately equals the no-load ample it includes the current of R, in Fig. 1),I, is theurrent of the transformer. In contrast. the ze-ro-sequence magnetic resistance is very large because inductive current provided by the TPFCASC, Xc isof the presence of airspaces in the zero-sequence the capacity reactance of phase to ground of the dis-magnetic path. That means the zero-sequence current, tribution network and Xwhich has the same phase and magnitude in the three ductive reactance of phase to ground of thewindings, is very high. The secondary windings, fit-ted with the thyristors and reactors L2 and L,,areTPFCASC. When Ice >I,, it is referred to as unopen-delta connections. When the conduction angles decompensation, when Ics=l, as full compensaof the thyristors are changed, the inductive current of tion and whenCe 0%, the zero-sequence voltage is 5 Conclusionsless than 104. 4 per cent of the phase voltage. WhenV=-50%, the zero-sequence voltage is 500 percent An ASC is beneficial to the coal mine distributionof the phase voltage and when v=50%, the network when it is sound. However, when a fault oc-zero-sequence voltage is 24.7 percent. Fig 9 is the curs itself, it will damage the network. We havesimulated result of the secondary zero-sequence compared, in detail, the single open phase fault at thevoltage at a single openault by the TPFCASC with the single phase to ground fault in aPSCAD/EMTDC. The simulation conditions are distribution network. The following conclusions canV=0%, d=10% and the capacity current is 50 A. be drawn: in both types of faults a zero-sequenceIt is almost similar to the result of Fig. 8.voltage of the network will appear. In a single phaseto ground fault, the maximum magnitude of thezero-sequence voltage is about the same as the elec-trical source phase voltage. In a single open phasefault of an ASC, the maximum magnitude of thezero-sequence voltage may be very high. For examplewhen v=-50% and d=10%, the zero-sequencevoltage is about 500 per cent of the electrical sourcephase voltage, meaning that a severe overvoltage will0.8-040040.8appear in the distribution network. Meanwhile thevery high zero-sequence voltage can mislead the op-erator to diagnose a false ground fault. It is stronglyFig8 Curve between the out-of-resonance degree and the recommended to set an open phase protection for theatio of zero-sequence voltage to electrical source phaseoltage at single open phaseASCReferences100[1] Zhang J W, Zhu N H, Yang L, et al. A fault diagnosisroken rotorTechnology,2007,17(2):205-209[2] Sun Y J, Zhang S, Miao C X, et al. Improved BP neuralnetwork for transformer fault diagnosis. Journal of ChinaUniversity of Mining Technology, 2007, 17(1): 138-Fig 9 Secondary zero-sequence voltage at single open phasefault in tPFCASC when v=0%, d=10%,[3] Meng F R, Huo Y, Zhou Y. Network multifunctionalthe capacitive current is 50Asubstation with embedded system in coal mine. Journalof China University of Mining Technology, 2006, 16(2)From this presentation, it is clear that in some236-240[4] Wang C L. Methods of Neutral Point to earth and Arecases the zero-sequence voltage is very high. ThatSuppression Coil. Xuzhou: China University of Miningmeans a severe overvoltage will appear in the distriTechnology Press, 1999.(In Chinese)bution network of the mine. This overvoltage ca[5] Wang C L, Liang R, Liu J H, et al. A novel principle ofthreaten the network and its electrical equipment.arc suppression coil based on changing magnetic couMeanwhile this very high zero-sequence voltage canpling to adjust inductance. Journal of China University ofmislead the operator in diagnosing a ground faultMining Technology, 2005, 34(5): 613-616. (In Chi-which is often called a false ground fault. In fact there [6] Bjan G, Jan A w A case of abnormal overvoltages in aPeterson grounded 132-kV system caused by brokenearth fault line selection device cannot give an answerconductor. IEEE Transactions on Power Delivery, 2002about this so-called fault line and of course the op- [7] Hu T L. Leakage Protection of Mining Power Systeml8(1):195-200.erator cannot find the ground fault"line either,which can cause the operator to panic. It is thereforeBeijing: Coal Industry Press, 1987.(In Chineseessential to establish an open phase protection for theLiu W s. Power System Fault Analysis(2nd Edition)Beijing: China Electric Power Press, 1998.(In Chinese)ASC, which can trip off the ASC immediately when it [9] Xia D Z. Power System Analysis. Beijing: China Electricis in open phase fault and can guarantee the safety ofPower Press, 2004. (In Chinese)the distribution network中国煤化工CNMHG