Comparison between Different InstallationLocations of Surge Arresters at Transmission Line Using EMTP-RV

Transcription

1 No. E-13-HVS-2308 Comparison between Different InstallationLocations of Surge Arresters at Transmission Line Using EMT-RV Soheil Derafshi Beigvand, Mohammad Morady Electrical Engineering Department, Engineering Faculty Razi University Kermanshah, Iran Abstract Line surge arresters used to improve transmission line performance when a lightning strike hits the shield wire or the top of a tower. In these studies, surge arresters location is very important. So, simulate these positions and predict the overvoltage caused by lightning current is essential. Therefore, should be choose some positions in order to obtain optimal performance. This paper done above study for all positions on a part of a high voltage double-circuit overhead transmission line to compare and evaluate the optimal positions. Therefore, the modified IEEE recommended dynamic model for surge arresters is chosen. In order to do this study, EMT-RV program is selected and the results are compared. Based on the simulations, recommendations for transmission line performance improvement are made. Keywords EMT-RV; lightning; surge arrester optimal location; transmission line; zinc oxide; I. INTRODUCTION To prevent overhead line faults, one must either prevent lightning from striking the line, or prevent the voltage from exceeding the insulation level. The first idea is one of the strategies open to utilities for lines that are particularly susceptible to lightning strikes is to shield the line by installing a grounded neutral wire over the phase wires. This can help, but will not necessarily prevent line flashovers because of the possibility of backflashovers [1]. Because, when a lightning strike hits top of a tower or the ground wire, the potential at the top of the tower rises in an important way and can exceed the dielectric strength of insulators string [2-4]. The second idea is becoming more popular in recent years with improving surge arrester designs. Surge arrester devices protect equipment from transient overvoltages by limiting the maximum voltage. Most arresters manufactured today use a MOV (Metal - Oxide Varistor) as the main voltage-limiting element. The chief ingredient of a MOV is zinc oxide (ZnO). The polymeric ZnO surge arresters have been developed quickly and have been put into operations on transmission lines to limit the overvoltages based on excellent characteristics. Today, these devices are operational and demand for their use at different voltage levels, increased [5, 6]. So, in addition to removing the over voltage caused by lightning, reliability increased, faults and equipments destroy caused by lightning strike, reduces. The objective of this paper is to simulate and compare between surge arrester install locations in a part of a HV double-circuit overhead transmission line when a lightning strike hits on ground wire or top of the towers by using EMT-RV (ElectroMagnetic Tra nsient rogram Restructured Version) program. To take into account the dynamic modelling of surge arrester, the modified IEEE recommended model proposed in [7, 8] is chosen. II. DIGITAL SIMULATION BY USING EMT-RV EMT-RV is a high performance computational engine for advanced transient analysis of various phenomena in different areas of power system operation and protection [9]. Using this program, we can represent in the time domain all the essential elements of an overhead line that are relevant to lightning studies [10]. It allows us to simulate an overhead line with all essential elements and show results in shapes. A typical geometry for modelling of HV double-circuit shieldedoverhead line is shown in Fig. 1. III. CHARACTERISTICS OF THETRANSMISSION LINE The studied line is a HV double-circuit overhead line that has two 288mm 2 AAAC conductors per-phase with 250mm distance bundle and has an 116mm 2 ground wire. Metallic towers type in this study is T2-30 steel towers. IV. COMONENT OF AN EMT-RV MODEL A part of the transmission line under study include frequency-dependent model of phase conductors and shield wire, steel towers, insulators and footing resistor are represented in the digital model. Also models of lightning and frequency-dependent arrester will be simulated.

2 Comparison between Different Installation Locations of Surge Arresters at Transmission Line Using EMT-RV 28 th ower System Conference Tehran, Iran 3h,G 30 km 300 m 300 m 300 m 300 m 300 m 300 m 30 km 3h,G 3h,G 3h,G Tower Tower 5 Tower 6 Tower 7 Tower 8 Tower 9 tower Fig. 1. EMT-RV model of transmission line A. Transmission Line A frequency-dependent transmission line exist in the EMT-RV library to provide transmission line modeling when a range of frequencies involved. Geometric data gathered from Fig. 2. The overhead line between towers is 300 meters and the overhead line between sources and towers is 30 kilometers. B. Steel Tower Model A multi-story tower model proposed in [11] and illustrated in Fig. 3 is used to represent steel towers. The multi-story tower model is composed of four sections divided at the upper, middle and lower phase cross-arm position. Each section consists of a lossless lint (to consider the surge impedance) in series with a parallel RL circuit, included for the travelling waves attenuation. This model can easily be interfaced with EMT-RV, and is suitable for multiconductor analysis [10]. Model parameters can be calculated as follows : H h 1 h 2 h 3 h 4 1 r r 1 h 1 r 2 h 2 r 3 h 3 2H 2 Z t = ln(h/r) 1 3 R i = 2Z t h i ln ( h 1 h 2 h 3 i = R 4 = 2 Z t ln 5 L i = R i H / V t i = Fig. 3. Multi-story tower model Whrere : H: Tower height (meter), Z t : Surge impedances of sections (ohm), R i : Resistances of sections in (ohm), L i : Inductances of sections in (H), : Damping coefficient, : Attenuation coefficient, V t : ropagation velocity, h1, h2, h3, h4, r1, r2, r3 defined in Table I. For T2-30 tower, the parameters determined in Table II. For this study, R f is tower footing resistance that constant and has value equal to ohms. TABLE I. TOWER DATA h 1 (m) h 2, h 3 (m) h 4 (m) r 1 (m) r 2 (m) r 3 (m) TABLE II. TOWER ARAMETERS R 1 (ohm) R 2 (ohm) R 3 (ohm) R 4 (ohm) Z t (ohm) Fig. 2. Geometric data L 1 ( H) L 2 ( H) L 3 ( H) L 4 ( H)

3 Comparison between Different Installation Locations of Surge Arresters at Transmission Line Using EMT-RV 28 th ower System Conference Tehran, Iran C. Insulator Flashover The insulator flashover will start when the actual terminal voltage of the insulator exceeds the flashover critical voltage [2, 3, 12]. Backflashover occurs when a lightning strike hits on the ground wire or top of the towers. Lightning current travel across the shield wire and steel tower. So, voltage increase in the insulators. If this voltage equal or exceed the critical flashover voltage, flashover is formed. Insulators themselves represent capacitances with only very moderate influence on the occurrence of overvoltage. The decisive parameter for the behaviour of overhead line insulation subjected to lightning overvoltages is its corresponding flashover voltage, which depends on the voltage level due to different insulation clearances. Flashover occurs when the following integral becomes greater or equal to DE [13]: Where: ( ( ) 0) 7 U( ): Voltage applied at time t to the terminals of the air gap in (volt), U0: is a minimum voltage to be exceeded before any breakdown process can start or continue, T0: time from which U( )>U0. U0, k, DE are determined by using the voltage- time curve and basic impulse insulation level (BIL) of 1050kV. These parameters are: U0=958kV, k=1, DE= The insulators flashover mechanism can be represented by Airgap block and other controlled functions, available in EMT-RV (Fig. 4). C3 c p3 v(t) p4 v(t) C4 c Air U D. Surge Arrester Dynamic Model Many researchers have represented ZnO surge arresters with the exponential non-linear resistive model available in the EMT [10]. In this work, we use a frequency-dependent model that is shown in Fig. 5 and proposed by the authors in [4] which is a modified version of the IEEE recommended model is used. By using the Genetic Algorithm optimization techniques proposed in [7, 8,10] have been determined the parameters for a Siemens 189kV arrester in Table III.Voltage-Current characteristic shown in Fig. 6. E. Lightning Characteristics The lightning strike is represented by a current source and a channel surge impedance. These values are 120kA, 1/ sec and 400 ohms, respectively. V. SIMULATIOS AND RESULTS For all simulations, the time-domain solution has been selected and power frequency value is 50 Hz. Also, the striked point was considered to be in the vicinity of the tower no. 8. Reference [10] shown that overvoltage increase when the amplitude of lightning current increase or faster lightning current decrease. Also, overvoltage increase when footing resistance increases. This is due to the fact that the lightning current meets an increasingly large resistance. Also, form [10], it can be concluded that the farther away from the tower no. 8, the maximum recorded overvoltages decreases and rate of the maximum overvoltages change, is approximately uniform. A. Influence of Surge Arrester Install surge arresters in a transmission line, will increase the lightning performance of overhead lines. TABLE III. TECHNICAL DATA FOR 189KV ARRESTER R 0 (ohm) R 1 (ohm) L 0 ( H) L 1 ( H) C 0 (pf) C 1 (pf) p 0 p 1 q 0 q 1 V ref0 (kv) V ref1 (kv) D cmp3 Compare 2 1 cmp4 Compare 2 1 Gain3-1 Gain4-1 sum2 U R L0 0.67uH C nF ZnO ZnO0 R L1 4.8uH nF ZnO ZnO1 D Fig. 4. Insulator flshover mechanism Fig. 5. Modified IEEE recommended dynamic model 3

4 Comparison between Different Installation Locations of Surge Arresters at Transmission Line Using EMT-RV 28 th ower System Conference Tehran, Iran Voltage (pu) Current (ka) Voltages (V) Fig. 6. V-I curve of ZnOsurge arresters By using this strategy, backflashover will be prevented. When a lightning strike hits on the top of the tower, due to footing resistance, tower top voltage increase and flashover occurs at several insulators. In other hand, when the applied voltage at insulator exceeds the corona inception voltage and remains high enough, streamers propagate and cross the gap after a certain time [10]. When ZnO arresters installed in parallel with insulators, the overvoltages limited ( Fig. 7). In other hand, they have an advantage over without a protection transmission line in that the voltage is not reduced below the conduction level when they being to conduct the surge current. Fig. 8 shows the voltages of three-phase in towers no. 8, 9 (strickedtower and adjacent tower), in the case of without protection (Fig. 8a) and with protection in all l phases (Fig. 8b). From Fig. 8, it can be concluded that always upper phase is more important. Install surge arrester in parallel with insulators, will result in a further reduction in voltages of adjacent tower. B. Different Installation osisions of Surge Arresters Install location of surge arresters, plays an important role in the transmission line performance improvement. For this purpose, all possible installation location according to Table IV, are simulated. osition 1 is without protection. In this work, we don't use any alternative, because e this alternative is not always useful if the lightning hits a tower which is not equipped whit surge arrester [10]. So, an optimization should be done. Voltages (V) Time ( s) a. Without surge arresters Times ( s) b. With surge arresters Fig. 8. Tower no. 8, 9 phase voltages TABLE IV. SURGE ARRESTER OSITION Overvoltage (V) Time ( s) Fig. 7. Tower no. 8 phases overvoltage whit surge arresters in all phases

5 Comparison between Different Installation Locations of Surge Arresters at Transmission Line Using EMT-RV 28 th ower System Conference Tehran, Iran Due to different footing resistance and lightning current, these values considered ohms and 120kA, 1/20 sec, respectively. Simulation results for all phase of tower no. 8, shown in Fig. 9. In fact, Fig. 9 shows the maximum recorded voltage for all possible installation locations for circuit 1, 2 of tower no. 8. These results show that surge arrester installation in some phases not always give the best results. Voltage in all phases shows that in some phases without protection, flashover may be occurring. For example, position 9, 14, 8 for upper, middle and lower phase in circuit 2, are maximum recorded voltage respectively. These are the same phases that unprotected. Maximum recorded voltage is obtained for position 9 in tower no. 8 upper phase-circuit 2. Upper hase (kv) The adjacent tower maximum phase voltages are shown in Fig. 10 for all positions. It can be seen, the maximum recorded voltage on the tower no. 9, are lower than those recorded on the striked tower. In some position, the insulators by their successive backflashover, participate in the limitation of the propagated overvoltages alone the line.fig. 10 also shows this result that upper phase is more important. Also, due to overvoltage remaining very high in other towers and lightning may be occurring at substations, to improvement of transmission line performance, secondary protection at substation are essential. Because, transmission line performance does not guarantee alone with transmission line protection. Economic issues also must be considered and an optimization should be done.some positions such as 2, 5, 7, 18 and 19 have the same behaviour in all phases. So, the best results are summarized in Table V.Also, the final result shows that 7, 27 and 29 are the best positions Middle phase (kv) p Upper hase (kv) 3500 Middle hase (kv) Lower hase (kv) Lower hase (kv) Fig. 9. Voltages of tower no. 8 phases Fig. 10. Voltages of tower no. 9 phases 5

6 Comparison between Different Installation Locations of Surge Arresters at Transmission Line Using EMT-RV 28 th ower System Conference Tehran, Iran TABLE V. Number of Surge Arresters VI. THE BEST RESULTS The Best ositions , , DISCUSSION AND CONCLUSION This paper study on improvement of the transmission line performance with installation location of surge arresters. For this purpose, a HV double-circuit shielded overhead line in all possible positions for surge arrester installation, considered. Simulation results show that when a lightning strike hits the top of the tower, in case of without protection, a backflashover may be occurring and phase voltages drop under the acceptable voltage limitation; but in case of with protection, it did not happen. Different positions of surge arresters show that some positions do not always give the best performance and an optimization should be done (such as 7,27 and 29). Furthermore, transmission line protection alone is not sufficient and secondary protection at substations is necessary. REFERENCES [1] R. C. Dugan, M. F. McGranaghan, S. Santoso, H. W. Beaty, Electrical ower Systems Quality, 2 nd ed., NewYork: McGraw-Hill, 2004, pp [2] CIGRE 33.01, Guide to rocedures for Estimating the Lightning erformance of Transmission lines, October, [3] J. G. Anderson, Lightning erformance of Transmissions Lines, Transmission line reference book 345kV and above, 1982, pp [4] IEEE WG on LTL, Estimating Lightning erformance of Transmission Line II Updates to Analytical Models, IEEE Trans. on ower Delivery, vol. 8, pp , July, [5] F. M. Gatta, F. Iliceto, S. Lauria, Lightning erformance of HV Transmission Lines with Grounded or Insulated Shield Wires, 26 th ICL, Cracow, oland, p. 6b. 5, [6] T. Ito, T. Ueda, H. Watanabe, T. Funabashi, A. Ametani, Lightning Flashovers on 77-kV Systems: Observed Voltage Bias Effects and Analysis, IEEE Trans. On WRD, vol. 18,pp , [7] A. Bayadi, K. Zehar, S. Semcheddine, R. Kadri, A arameter Identification Technique for a Metal-Oxide Surge Arrester Model based on Genetic Algorithm, WSEAS transactions on Circuits and Systems, issue 4, vol. 5, pp , April, [8] A. Bayadi, arameter Identification of ZnO Surge Arrester Models based on Genetic Algorithms, Electr. ower Syst. Res., issue 7, vol. 78, pp , July, [9] S. Yang, G. A. Franklin, Switching Transient Overvoltage Study Simulation Comparison Using SCAD/EMTDC and EMT-RV, roc. of IEEE Southeatcon, pp. 1-5, March, [10] S. Bedoui, A. Bayadi, A. M. Haddad, Analysis of Lightning rotection with Transmission Line Arrester Using AT/EMT: Case of an HV Double Circuit Line, 45th UEC, pp. 1-6, September, [11] M. Ishii, T. Kawamura, T. Kouno, O. Eiichi, K. Shiokawa, K. Murotani, T. Higuchi, Multistory Transmission Tower Model for Lightning Surge Analysis, IEEE Trans. on ower delivery, pp , August, [12] TF Working Group, Modeling Guidelines for Fast Front Transients, IEEE Trans. on ower Delivery, vol. 11, pp , April, [13] I. Uglesic, A. Xemard, V. Milardic, B. Milesevic, B. Filipovic-Greie, I. Ivankovic, Reduction of Flashover on Double-Circiuts Line, International Con. on ower Systems Transient, Kyoto, Japan, pp. 1-6,