EFFECT OF POWER TRANSFORMER NEUTRAL GROUNDING ON HIGH VOLTAGE NETWORK SINGLE PHASE SHORT CIRCUIT LEVEL A. ABADLIA(1) M. ELLEUCH(2) (1) Société Tunisienne de l’Electricité et du Gaz (STEG) - Tunisia (2) University of Tunis El Manar, ENIT-L.S.E.-BP 37-1002 Tunis le Belvédère, Tunisia SUMMARY Authors present throughout this paper an approach for surveying single phase short-circuit level on high voltage transmission network. The work is intended to study and quantify the effect of grounding power transformers neutrals on High Voltage short-circuit currents level. Simulations are carried out on a five nodes network. Approach developed through this paper can be adapted to help solving single phase short circuit problem already observed in Tunisian 90kV transmission network. KEYWORDS Transformer- Neutral- grounding – Network – Transmission - Short circuit. (1) [email protected] 1. INTRODUCTION Oscillogram (Fig. 1) illustrates drained short circuit current generated by single phase earth fault regarding occurred on 200 MVA autotransformer 225KV/90KV. Short-circuit level has reached almost two times allowable value, generating consequently significant damage (Fig.2). High short circuit level is due to low value of zero sequence impedance, provided by grounding whole power transformers neutral. Zero sequence impedance of Wye-Delta transformer jumps to a huge larger value once its neutral is unearthed. This opportunity would be a technical solution to mitigate single phase short circuit current level [1]. 2. NETWORK MODELING AND FORMULATION Tunisian electrical transmission network is exclusively equipped with HV/MV power transformers; Wye-Delta connected. These latter will be however modeled, in zero sequence system, by correspondent short circuit impedance or an almost infinite one, respectively if neutral is grounded or not. HV network zero sequence impedance reaches its minimum level once all transformers neutrals are grounded. This is the case of Tunisian network providing highest presumed single phase short circuit level (1): (1) With: V : Pre-fault network phase to neutral voltage, Zd, Zi and Z0: denote respectively positive, negative, and zero sequence impedance. ZF: fault impedance Study is conducted on five-node network (Fig. 3), made of four power plants interconnected through cross linking transmission lines. A generic fault node will help to consider multiple fault location. 2.1 Modeling of network components Transmission line could be presented by following models: - Distributed parameter model; this is used for wave propagation studies and accurate fault location [2]. - Π or T model; generally used for load flow computation [3]. - Series impedance model; it’s restrained for short circuit studies [2]. We will use the latter one to simulate short circuit current computation. All HV/MV power transformers will be modeled by short circuit impedance for both positive and negative system. Phase 1 Phase 3 Phase 2 28 4 47 Neutral Short circuit current RMS (KA) 33 -30 0 30 60 90 120 Time (ms) Fig. 1. Short circuit current oscillogram, Fig. 2. 90kV damaged transformer bushing 2 1 3 . . 2 4 . . Icc 5 3ZF Fig. 3. Studied network single line diagram Fig. 4. Symmetrical component illustration With: Ucc: transformer short circuit voltage ; Un : transformer rating voltage ; Sn : transformer rating power Transformer zero sequence impedance, as obviously mentioned, depends on its vector group and neutral earthing: - First case, neutral grounded: - Second case, neutral ungrounded ( zero sequence current through transformer windings has a magnitude of magnetizing current, it will be neglected. Regarding generators, positive and negative impedances are expressed in terms of alternator power and voltage. ; Where: X’: transient direct reactance in %; U: generator rating voltage ; Sn : generator rating power; Xd”:subtransient reactance d-axe in % ; Xq”:subtransient reactance q-axe in % 2.2 Short circuit current formulation Faulted network can be studied by means of symmetrical components superposition [4] (Fig. 4). Nodes voltage and current are linked by equations’ system (2), (3) and (4): (2) (3) (4) This system could be presented by the following matrix representation: (5) (6) (7) Where: k=1,2, 3, 4 and F; Ek: pre-fault voltage at the terminals of kth generator. Vkd, Vki and Vk0: respectively positive, inverse and zeo sequence voltage at kth node. Where : k=1,2, 3, 4 and F; Ek: pre-fault voltage at the terminals of kth generator. 3 Vkd, Vki and Vk0: respectively positive, inverse and zeo sequence voltage at kth node. Zkd, Zki, Zk0: respectively positive, inverse and zero sequence impedance of kth generator associated to its step up transformer. Respectively positive, inverse and zero sequence current provided by kth generator to supply fault. ; ; ; The contribution of different plants, to short circuit current, depends on nodes voltage and connection admittance, according to (4), ( 5) and (6). (8) (9) (10) Short circuit current flow, through network connections, is governed by (11), (12) and (13): (11) (12) (13) With : Yknd , Ykni and Ykn0: respectively positive, negative and zero sequence admittance connecting nodes ‘k and n’ – ; . Short circuit Current at fault location is but the sum of current provided by nodes directly connected to faulty one according (14), ( 15) and (16). (14) (15) (16) This system could be presented by the following matrix representation: (17) (18) 4 (19) With: ; ; = = ; = Resolution of equation system (5), (6), (7), (11), (12), (13), (17), (18) and (19) leads to find out faulty nodes symmetrical voltage (20): (20) Where [M], 1, 1, 1, are detailed in Appendix. Fault current, denoted Icc, will be then deduced by (21): (21) 2.2 Effect of ungrounded neutral Expressions (4) and (10), regarding nodes zero sequence current, lead to the following equation system (22), (23), (24) and (25): (22) (23) (24) (25) Analysis of systems (22), (23), (24) and (25) shows that fault current zero sequence could be expressed as function of nodes voltage and admittance. (26): (26) Hence, we deduct that single phase short circuit current will be null once all nodes zero sequence admittances are null as well. However, disconnecting all transformer neutral from the earth will mitigate single phase short circuit level from its highest level to the lowest one. Consequently, managing power transformer neutral grounding could be a technical solution to survey fault current level. 3. SIMULATIONS AND INTERPRETATIONS A program was developed in Matlab space, where all network data are specified and short circuit current is simulated accordingly, as specified above and for different scenarios related to transformers neutral earthing. 3.1 Network features Considered network, object of this study, is made by four generators; each one is associated to a step up transformer, interconnected by looped transmission network made of six transmission lines. Generators, transformers and transmission lines features are recorded in TABLE I: 5 3.2 Scenarios and results A program was run for sixteen scenarios corresponding to whole possible combinations regarding grounding power transformers neutral. As pertinent result, it was found that single phase short circuit current can be limited to a maximum level depending on the scenario itself; as illustrated in TABLE II. Analysis of simulation results leads to the following conclusion: - Single phase short circuit current reaches its highest level at each node, once all power transformers neutral are grounded. - Fault current level is getting lower as transformers neutral are disconnected from the ground; it becomes null for boarder case; when all transformers neutral are unearthed. - Maximum fault current level is 12kA and it is recorded at Node ‘1’; however network should be designed to withstand this current effect; such as electrodynamics efforts, thermal constraint and circuit breaker rupturing capacity. TABLE I Node 1 Node 2 Node 3 Node 4 Line 1-2 1-3 1-4 2-3 2-4 3-4 NETWORK CHARACTERISTICS GENERATOR P(MW) X1(%) X2(%) 200 15 18 320 16 20 320 18 19 320 17 23 TRANSMISSION LINES Length (Km) 10,04 101,77 49,30 80,30 20,00 74,20 Rd (Ω/Km) 0,09 0,08 0,15 0,09 0,10 0,09 Xd (Ω/Km) 0,42 0,44 0,41 0,40 0,41 0,41 TRANSFORMER S(MVA) Ucc(%) 250 11,5 400 12,5 400 12,1 400 10,9 R0 (Ω/Km) 0,24 0,23 0,30 0,16 0,25 0,25 X0 (Ω/Km) 1,25 1,25 1,23 1,25 1,25 1,23 TABLE II SIMULATION RESULTS Transformer Node ICC (KA) short circuit current kA Max Min 1 2 3 4 1 2 3 4 X X X X X X X _ 11 12 9 11 2 10 11 9 8 12 11 9 8 3 X X _ X 10 12 5 11 12 5 4 5 X X _ _ 10 11 5 7 11 5 X X _ _ X X X _ 10 10 9 11 09 8 9 7 11 09 9 7 X _ _ X 10 9 5 11 11 5 X _ _ X _ X _ X 08 7 4 5 09 11 9 11 08 11 4 9 _ X X _ 08 10 9 8 10 8 X X _ _ X _ 09 11 5 11 12 _ _ 07 10 5 7 11 10 5 5 13 _ _ X X 07 8 9 10 10 7 14 15 _ _ X _ 05 5 8 5 08 5 _ _ _ _ _ _ X _ 06 7 4 10 00 00 0 00 10 00 4 0 Case 1 6 7 8 9 10 11 16 6 15 Icc1-max Icc1-cmin 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Fig. 5. Number of scenario versus short circuit current limitation - There is more alternatives to limit short circuit current at a certain desired level and keep it for all nodes within prefixed range (Fig.5). For instance; limitation of short circuit current level at 11, 10, 9 and 8kA could be achieved respectively by 14, 8, 4 and 3 alternatives (Fig. 5). This flexibility helps operators to manage fluently network and without constraints 4. CONCLUSION This work emphasizes the possibility of managing power transformer neutral grounding in trial to mitigate single phase short circuit current to a pre-preferred level. Tunisian network is well suited to the application of this solution as power transformers are standard insulation level type. So, neutral terminal could withstand all zero-sequence overvoltage generated during single phase earth fault Application of this solution to Tunisian network allows achieving two goals: to justify the extra-cost associated with the standard insulation option, in one hand and to solve a serious problem by the mean of the cheapest solution in the other hand. BIBLIOGRAPHY [1] International standard IEC 60076-8 first edition 1997-10 Power transformers- application guide [2] C.E.M. Pereira and L.C.Z. Jr " Analise Comparativa de Alguns Algoritmos de Localizçao Digital de Faltas em Linhas de Transmissaà " SBA Controle & Automaçao ,Vol.11,N°3, pp135-140, Dezembro de 2000. [3] P. KUNDUR, Power System Stability and Control, Electric Power Research Institute, California1993 [4] B. de Metz-Noblat, F. Dumas and G. Thomasset, Calcul des Courants de Court-circuit, Cahier technique n°158 SCHNEIDER, Octobre 2000. APPENDIX ; ; ; ; ; ); ; ; ); ); ; ); ); ; ); ) 7