Energy and Power Engineering, 2017, 9, 625-634 http://www.cirp.org/journal/epe ISSN Online: 1947-3818 ISSN Print: 1949-243X Study on Short Circuit Current Calculation of Power Sytem with UHVDC and New Energy Source Zaihua Li 1, Huan Yan 2, Yunting Song 1, Jincheng Guo 2 1 China Electric Power Reearch Intitute, Beijing, China 2 State Grid Shaanxi Power Corporation Economic Reearch Intitute, Xi an, China How to cite thi paper: Li, Z.H., Yan, H., Song, Y.T. and Guo, J.C. (2017) Study on Short Circuit Current Calculation of Power Sytem with UHVDC and New Energy Source. Energy and Power Engineering, 9, 625-634. http://doi.org/10.4236/epe.2017.94b068 Received: February 16, 2017 Accepted: March 30, 2017 Publihed: April 6, 2017 Abtract With the development of power ytem, the level of hort circuit current will increae accordingly. In general, the influence of the HVDC ytem and the new energy ource i not conidered in the calculation of the hort circuit current. For the power grid that hort circuit current level cloe to the interrupting capacity of circuit breaker, it neceary to fully conider all kind of influence factor, careful checking, o a to obtain more accurate calculation reult of hort circuit current. In 2018, two ±800 kv high-power Ultra High Voltage Direct Current (UHVDC) tranmiion project will be connected with Shaanxi power grid, accompanied by a lot of concomitant foil-fuel generating plant a power reource, and alo a large number of new energy ource, include wind power generation and photovoltaic power generation. Around one of the UHVDC converter tation, hort circuit current may exceed the withtand limit of ome certain circuit breaker. In order to get more accurate hort circuit current calculation reult, three meaure are ued: 1) contrative calculation and analyi by algorithm baed on cheme and algorithm baed on power flow; 2) analyi the influence of UHVDC by electromagnetic tranient and electromechanical tranient hybrid imulation; 3) conidered a detailed model of Doubly Fed Induction Generator (DFIG) with low voltage ride through characteritic. The calculation reult how that: in the typical operation mode of Shaanxi power grid of 2018, the original calculation reult by conventional calculation method are coincident with the reult by conidering the influence of algorithm, UHVDC and DFIG in large, in which: the reult of the algorithm baed on power flow are maller than that of the algorithm baed on cheme about 2-8 ka; the teady value of the hort circuit current provided by UHVDC convertertation (include rectifier and moothing capacitor) are about 0-3 ka; the teady value of the DOI: 10.4236/epe.2017.94B068 April 6, 2017
hort circuit current provided by DFIG are about 0-5 ka. The calculation reult can provide reference for the election of the circuit breaker, and it can be verified by fault recording data in the future. Keyword Short Circuit Current, UHVDC, New Energy Source, Hybrid Simulation 1. Introduction With the development of power ytem, the level of hort circuit current will increae accordingly. In general, the influence of the HVDC ytem and the new energy ource i not conidered in the calculation of the hort circuit current. However, for the power grid that hort circuit current level cloe to the interrupting capacity of circuit breaker, it neceary to fully conider all kind of influence factor, careful checking, o a to obtain more accurate calculation reult of the hort circuit current. In the year of 2018, two ±800 kv high-power Ultra High Voltage Direct Current (UHVDC) tranmiion project will be connected with Shaanxi power grid, accompanied by a lot of foil-fuel generating plant a power reource, and alo a large number of new energy ource, include wind power generation and photovoltaic power generation. Around one of the UHVDC converter tation, hort circuit current may exceed the withtand limit of ome certain circuit breaker. In fact, the hort circuit current calculation reult by mean of conventional calculation method may be omewhat conervative. In order to get more accurate hort circuit current calculation reult, three meaure are ued in the paper: 1) contrative calculation and analyi by algorithm baed on cheme and algorithm baed on power flow; 2) analyi the influence of UHVDC by electromagnetic tranient and electromechanical tranient hybrid imulation; 3) conidered a detailed model of DFIG with low voltage ride through characteritic [1]. 2. Algorithm of Short Circuit Current Calculation 2.1. Algorithm Baed on Scheme In the calculation of hort circuit current, the principle formula include: I k Uk = (1) 3* Z where U k i the open circuit voltage of hort circuit point, in BPA it often fixed to 1.05 U n, Z k i equivalent impedance of hort circuit point, other hypothee and condition include whether or not conider tandard ratio of tranformer, charging power of line, high voltage impedance, low voltage reactive power compenation, actual tranformation ratio, tatic load, induction motor load and etc. k 626
2.2. Algorithm Baed on Power Flow In the calculation of hort circuit current of the algorithm, the principle formula i a ame a the above algorithm. Where U k i the open circuit voltage of hort circuit point, in BPA it come from the reult of power flow calculation. Z k i equivalent impedance of hort circuit point, other hypothee and condition are a ame a the above algorithm baed on cheme. 2.3. Brief of Relevant Standard At preent, the main relevant international tandard of calculation of hort circuit current include IEC60909 erie tandard and ANSI/IEEE/UL erie tandard. Furthermore, in order to guide the calculation of hort circuit current in DC auxiliary intallation in power plant and ubtation, IEC 61660 erie tandard are etup. Many countrie or enterprie developed their own national tandard, indutry tandard or enterprie tandard baed on thee erie tandard and combined with their actual ituation. In china, the national tandard, indutry tandard or enterprie tandard of hort circuit current calculation are no exception, all baed on the IEC60909 or IEC 61660 erie tandard, combined with the actual ituation of the pecific application area, i almot equivalent to utilize the two erie of tandard, but a light modification. The general approach i divided into two tep. At firt the hort circuit current of the individual ource are calculated eparately. Subequently, the individual current are uperpoed reulting in an overall hort circuit current of the ytem. The international imulation oftware of calculation of hort circuit current include BPA of Bonneville Power Adminitration, PSS/E and NETOMAC of Siemen, ARENA of EDF, PSAPAC/ LISP of EPRI, NEPLAN of ABB, EXTAB of TEPCO. In china, the main oftware include PSD-BPA and PSASP. They all abide by thee above erie tandard by and large. In the paper, PSD-BPA i adopted, and all kind of hypothee and condition are baed on PSD-BPA [2] [3]. 3. Short Circuit Current from HVDC 3.1. Configuration of HVDC Converter HVDC can be ditinguihed into two type by converter: Current ource converter (CSC) and voltage ource converter (VSC). CSC are alo called linecommutated converter (LCC), becaue they rely on a ynchronou voltage for proper operation. The thyritor i the key element of a CSC, which allow very high tranmiion power. CSC-HVDC ytem with voltage of ±800 kv and power of more than 8000 MW are poible [4]. VSC are referred a elf-commutated converter becaue of the ability to operate independently from the power grid. The AC/DC-converion i accomplihed by inulated gate bipolar tranitor (IGBT) with turn-on and turn-off capability. Becaue of the high complexity of the control ytem, there are only a few realization by far now. In 627
thi paper, UHVDC baed on LCC i conidered. The Equivalent circuit of a ix-pule bridge at a hort circuit on the DC ide i hown a Figure 1. The ix-pule bridge i the baic element of a CSC-HVDC, which i hown in Figure 1 at a hort circuit on the DC ide. The larget hort circuit current of a ingle bridge can be expected, if a line-to-line hort circuit occur directly at the terminal (R DC = 0, L DC = 0). For thi particular cae the DC hort circuit current can be derived according to the loop I and II hown in Figure 1. In thi intant the diode D1, D3 and D2 are conducting. 3.2. Rectifier The reitance R DC and the inductance L DC of the DC ytem comprie the parameter of the rectifier, the moothing reactor and the line. The calculation of the characteritic parameter of the hort circuit current i baed on the correponding value of the AC and DC ytem. The quai-teady-tate hort circuit current of a rectifier in a three-phae bridge connection i I kd 3 2 c Un UrTLV = λd (2) π 3Z U where U N i the nominal voltage of the AC ytem, Z N the ytem impedance, U rtlv /U rthv the ratio of the rated voltage of the econdary and primary tranformer ide and λ D conider the influence of the DC ide reitance. N rthv The peak hort circuit current i pd i calculated uing: ipd = kd IkD (3) The factor k D depend on the ratio of the inductance L DC /L N and the parameter R N, R DC and X N. The calculation of the time contantτ 1D, τ 2D and the time to peak t pd i decribed in the reference [2]. 3.3. Capacitor The equivalent circuit diagram of the capacitor i depicted in Figure 2. The reitance R CBr and the inductance L CBr include the reitance and inductance of the capacitor and of the connection to the line. Figure 1. Equivalent circuit of a ix-pule bridge at a hort circuit on the DC ide. 628
Figure 2. Equivalent circuit diagram of the capacitor to calculate the hort circuit current. The quai-teady-tate hort circuit current of the capacitor i zero: I = 0 (4) The peak hort circuit i calculated by uing i kc E C pc = kc (5) RCBr where k C depend on the eigen-frequency ω 0 and the decay coefficient δ. The correponding calculation of ω 0 repectively δ and the equation for τ 1C, τ 2C and t pc are pecified in the reference [2]. 3.4. Electromagnetic Tranient and Electromechanical Tranient Hybrid Simulation When a fault or an operation happen, electromagnetic tranient procedure and electromechanical tranient procedure occur in power ytem. Uually we analyze the two procedure individually, becaue uniform analyi i very complex and impractical. We ue electromechanical tranient imulation to analyze electromechanical tranient procedure and electromagnetic tranient imulation to analyze electromagnetic tranient procedure. Electromechanical tranient procedure varie lowly compared with electromagnetic tranient procedure, therefore the integration time tep of electromechanical tranient imulation i much larger than that of electromagnetic tranient imulation. Uually the integration time tep of electromechanical tranient imulation i 10 milliecond while that of electromagnetic tranient imulation i about 20-200 microecond. Electromechanical tranient imulation can imulate quite large power ytem, baed on three-equence phaor model with much larger time tep. While, Electromagnetic tranient imulation, baed on three-phae intant value model, can analyze electromagnetic tranient procedure and repreent thoe power electronic device very eaily, while the cale of the ytem imulated i very limited for it much larger computation amount. Hybrid imulation combine the advantage of the both. With hybrid imulation, we can analyze the electromagnetic tranient procedure appearing in a pecial mall network under the background of a large power ytem. In hybrid imulation, Norton equivalent circuit of electromechanical tranient network i connected to the electromagnetic tranient network. If there are ge- 629
nerator in electromechanical tranient network, Norton equivalent admittance matrix become aymmetric when tranformed from three equence to ABC three phae. PSD-BPAFDS (Full Dynamic Proce Simulation Software) provide the performance. 4. Short Circuit Current from Wind Farm To exploit and utilize new energy ource i the need of the time and hence it development i on the rie from lat couple of decade. Renewable energy ource i a major new energy ource. Wind power generation and photovoltaic generation are the mot popular two kind of renewable energy ource. The hort circuit current caued by wind power generation or photovoltaic generation i uually not conidered. DFIG i the mot widely ued limited variable peed generator. The four quadrant operation of converter allow DFIG provide decoupled control of active and reactive power. The ability to control reactive power i beneficial for grid voltage upport. Wherea the cae of DFIG employing an induction machine, the hort circuit current i governed by the machine characteritic. With the development of wind power generation, hort circuit current become more and more prominent, o it not appropriate to neglect hort circuit current from wind farm [5]. 4.1. Modeling of DFIG DFIG can be decribed a an induction machine model in pace vector notation. The pace vector model i ubjected to a reference frame tranformation to get decoupled d-q model. The reference frame i uually aligned with tator flux reference frame. Thi facilitate in decoupling the active and reactive power of the machine. The terminal rotor voltage vector which i a control input in cae of a DFIG will then be able to decide i dr and i qr independently. The detailed model of a DFIG i a fifth order model having two voltage equation for tator and rotor each. The typical tructure of a DFIG ytem with crowbar protection uing a wound rotor induction machine i hown a below Figure 3 [6]. Figure 3. DFIG with crowbar protection. 630
In the procedure of calculation, crowbar reitance i often aumed to be connected to rotor winding throughout the duration of the fault. The aumption i valid becaue crowbar activation i fat and completed in few milliecond. It alo aume contant generator peed during fault condition. The above aumption can caue error in fault current prediction but i conidered for implification. The magnetic nonlinearitie are neglected in thi analyi. Conidering the effect of aturation on leakage reactance may increae the fault current level. The decaying tator natural flux will induce an emf in rotor of frequency, then the emf induced in rotor winding will caue a current to flow limited by crowbar reitance. Thi current will in tum produce a flux which will oppoe the tator flux. Thi will be the cae in a teady tate condition if tator natural flux perit. Thu DFIG fault current analyi i governed by crowbar reitance. Due to introduction of crowbar the AC component preent during the fault decay at much fater rate. Thi caue the tator fault current to be entirely DC component. The interruption of DC current may be a problem for AC circuit breaker. Thi could become even more hazardou a we move toward DFIG with higher MVA rating a they have a higher tator time contant. 4.2. Short Circuit Current Component The hort-circuit current of an idle running machine will be determined. Neglecting the mechanical loe, the machine rotate at the ynchronou rotational peed ω. The tator reitance can be neglected in teady tate. Before the occurrence of the hort circuit, the rotor current i zero: I r = 0. The tator current i: = Ve = Ve (6) jwt jwt jwt jx jwl Ie The maximum tator current of an induction machine in cae of a hort circuit at the tator terminal i: T T 2V 2T 2Tr i,max = e + (1 σ ) e (7) X The time contant for the damping of the dc component in tator and rotor are defined a below: T T L = (8) R L r r = (9) Rr Due to aymmetry of fault, the tator flux will have poitive and negative equence component. The magnitude and phae angle of thee component depend on the type of fault. The initial value of the current i determined under the aumption that the tator and rotor reitance can be neglected. When the bypa reitor are connected to protect the converter in cae of a fault, (6) become: 631
Ie jwt = jwt Ve jx + R cb (10) And the tranient time contant of the rotor become: T r = R r L r + R cb (11) The two exponential function inide the bracket in (7) are baed on the aumption that the rotor and tator flux are 180 out of phae after half a period, implying that the current reache it maximum value at t = T/2. Thi aumption i approximately valid for an induction machine, where the lip i mall and where the tator and rotor flux are approximately in phae with each other, at the moment the fault occur. A DFIG can however operate at a much larger lip. Thi implie that at the moment of the fault, the two flux vector are not in phae with each other. Taking into account thee difference between an induction machine and a DFIG, then (7) become: ΔT ΔT 2V T Tr i,max = e + (1 σ ) e 2 2 X + Rcb A larger bypa reitance will reult in a maller T r. At the ame moment, Δt decreae. A a reult, the term inide the bracket in (12) tay approximately (12) contant. A a rough approximation, the maximum tator current i then given a: 5. Comparion of Reult i,max 1.8V X + R 2 2 cb (13) Thi reearch ue the PSD-SCCPC hort circuit current calculation program (Verion 2.5.2) for the calculation of Shaanxi power grid 330 kv - 750 kv bu three-phae hort circuit and ingle phae hort circuit current, the following pecific principle are abided: 1) Conider the tatic load, and not conider the motor load. 2) Conider hunt capacitor compenation, inductance compenation and filter capacitor compenation in the poitive equence network. 3) Conider line charging power. 4) Conider bigget operation and completely connected mode, namely conidering the hort circuit current provided by the outage unit. 5) Conider the line, tranformer and generator impedance. 6) Conider the actual ratio a the ratio of tranformer. Under the above principle and algorithm, 750 kv bu hort circuit current in Shaanxi power grid in ummer peak operation mode of 2018 are hown partly a follow Table 1. Baed on the algorithm of power flow calculation, the hort circuit current of ome bue are hown partly a follow Table 2. 632
Table 1. Bu hort circuit current by the algorithm baed on cheme (Unit: ka). No. of ubtation interrupting capacity Current of 750 kv bu Three-phae hort circuit ingle phae hort circuit interrupting capacity Current of 330 kv bu Three-phae hort circuit ingle phae hort circuit 1 50 49.067 46.542 63 46.805 52.415 2 50 40.849 41.431 - - - 3 63 39.406 34.691 63 40.363 45.49 4 50 38.007 33.501 63 52.975 56.535 5 50 37.706 32.601 63 38.329 45.513 6 50 37.695 32.586 - - - 7 50 35.676 36.331 - - - 8 50 35.092 32.699 63 43.011 47.475 9 50 31.062 26.722 63 35.293 38.942 10 50 29.569 23.669 63 25.078 26.91 Table 2. Comparion of by different hort circuit current algorithm (Unit: ka). No. of ubtation Current of Three-phae hort circuit Current of ingle phae hort circuit Algorithm 1 Algorithm 2 difference Algorithm 1 Algorithm 2 difference 1 49.067 41.336 7.731 46.542 40.666 5.876 2 46.805 40.231 6.574 52.415 46.228 6.187 3 38.007 34.499 3.508 33.501 30.934 2.567 4 52.975 49.167 3.808 56.535 52.79 3.745 5 32.681 30.102 2.579 27.938 26.026 1.912 6 43.464 40.525 2.939 40.371 37.902 2.469 7 38.319 32.746 5.573 45.497 38.846 6.651 8 41.697 38.789 2.908 38.3 35.932 2.368 According to the above table, the reult obtained by the algorithm baed on power flow are le than that by the algorithm baed on cheme with about 2-8 ka. According to the calculation, the maximum value of DC component of hort circuit current at the hort circuit point i about 4.5 ka, the teady value of the hort circuit current from UHVDC varie from about 0-3 ka. An analyi of hort circuit current of three different DFIG of rating l.5 MW, 4.5 MW and 7.5 MW how an increae in hort circuit current magnitude with increae in ize of machine. The increae in ize of DFIG alo may caue the DC component of hort circuit current to perit for longer duration [7]. According to the calculation, the teady value of the hort circuit current from wind farm varie from about 0-5 ka. So, the reult by conventional calculation method are coincident with the reult by combined conidering the influence of algorithm, UHVDC and DFIG. 633
6. Concluion The level of hort circuit current obtained by the algorithm baed on power flow i lower than that by the algorithm baed on cheme to ome certain extent. The crowbar reitance had ignificant impact on hort circuit current level and time contant for tator and rotor. With ue of crowbar reitance the AC component of hort circuit current decayed much fater leaving a ituation of pure DC current. The value of crowbar reitance might be more difficult to obtain. Therefore, it very neceary to invetigate how it can be determined. With combined conideration of the influence of algorithm, UHVDC and DFIG, the reult are coincident with the reult by conventional calculation method by and large. Reference [1] Huang, X.L., Liu, Z.R., Zhu, R.J., Yang, Z.H., Zhou, G., et al. (2010) Impact of Power Sytem Integrated with Large Capacity of Variable Speed Contant Frequency Wind Turbine, Tranaction of China Electrotechnical Society, 25, 142-149. [2] IEC 61660-1: 1997, Short Circuit Current Short Circuit Current in D.C. Auxiliary Intallation in Power Plant and Subtation Part 1: Calculation of Short Circuit Current, June 1998. [3] IEC 60909-0: 2001, Short-Circuit Current in Three-Phae A. C. Sytem Part 0: Calculation of Current, July2001. [4] Andrea, W., Benedikt, J. and Gerd, B. (2013) Contribution of HVDC converter to the DC Short Circuit Current. Proceeding of the 48thInternational Univeritie Power Engineering Conference, 1-6. [5] Muljadi, E., Samaan, N., Gevorgian, V. and Li, J. and Paupulati, S. (2010) Short Circuit Current Contribution for Different Wind Turbine Generator Type. Proceeding of IEEE Power and Energy Society General Meeting, 1-8. http://doi.org/10.1109/pes.2010.5589677 [6] Adio, O.S., Lin, X.N., Zhao, F. and Bo, Z.Q. (2013) Short Circuit Analyi for Integration of l0mw Windfarm in Nigeria at the PCC. Proceeding of IEEE Power & Energy Society General Meeting, 1-5. http://doi.org/10.1109/pesmg.2013.6672196 [7] Bhatia, R. and Bahirat, H.(2016)Short Circuit Current of DFIG baed Wind Turbine. Proceeding of IEEE 6th International Conference on Power Sytem (ICPS), 1-6. http://doi.org/10.1109/icpes.2016.7584193 634
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