How to Conduct the Lightning Impulse Withstand Test of. Three Gorges Right Bank Substation 550kV GIS

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1 How to Conduct the Lightning Impulse Withstand Test of Three Gorges Right Bank Substation 550kV GIS Hu Wei, Chen Yong, Wang Qifa, et al HIMALAYAL - SHANGHAI - CHINA Abstract: With the rapid development of power grid in China, an increasing number of GIS are adopted in new substations. The lightning impulse test is recommended after the installation of GIS in order to check its insulation performance and reduce the risk of equipment failure. However, owing to the design and production, common lightning impulse generator with large capacitance load especially GIS cannot generate lightning impulse waveform, which meets the requirement. Therefore, based on lightning impulse withstand test of Three Gorges Right Bank 550kV GIS, models of lightning impulse generator and field equipment are established, and simulation calculation is done. According to the result, a method of installing the inductor in the impulse circuit is put forward to generate oscillating lightning impulse which meets the requirement of the test. This method is successfully applied in the test and results are good. Valuable experience has been accumulated for the lightning impulse test on GIS of 750kV, 1000kV substations in China. Key words: Lightning impulse, GIS, wave front time, withstand voltage, field test, inductor 1. Introduction Owing to many advantages, such as small volume, small floor area, easy to install, high reliability and less maintenance, GIS is recommended by a growing number of people. The impulse test is needed after the installation of GIS on site to check whether the equipment is well installed and reduce the risk of equipment failure, and finally to ensure that GIS can operate in safe and reliable mode. Entrance capacitance of GIS is far greater than voltage test with capacitive load is more difficult. In most cases, load capability of lightning impulse generator is thousands pf. While entrance capacitance of GIS can even reach 10 thousands, ten times higher than generator. Large capacitive load can lengthen wave front time of lightning impulse, reduce impulse peak, and cannot generate lightning impulse waveform, which conforms to the standard, in the field test. That imposes a new challenge for field test. In order to make common lightning impulse generator applicable to GIS other equipment, so lightning impulse lightning impulse test, this paper info@himalayal.com Page:1 All right reserved.

2 comes up with a method of adding the inductor to impulse circuit to generate oscillating lightning impulse which meets the requirements of the test by means of theoretical calculation and model and simulation. This method is successfully applied in the Three Gorges Right Bank 550kV substation GIS field test and the results are satisfied. 2. Equipment Parameter 2.1 Impulse voltage generator 12-stage bilateral impulse voltage generator with high performance is adopted in the test. The body belongs to triangle type. Due to large size, the influence of stray inductance and capacitance on impulse waveform cannot be neglected. Structure of impulse generator circuit is shown in Fig.1. Main parameters are as follows: 2 sets of 100kV main capacitors C s at each stage and bipolar charge; serial discharge stage voltage - 200kV and 12 stages in all; the capacitance of each main capacitor - 1.5μF, the inductance - 1μH, wave front resistance per stage r f - 20Ω, residual inductance - 1μH; wave tail resistance r t - 100Ω; the length of the lead - 2m, estimate via 1μH/m, the inductance per stage - 5μH, stay inductance of discharge circuit - 80uH or so; arc lead and circuit contact resistance - 10Ω; lead inductance of external circuit is estimated at 20μH. The capacitive voltage divider is composed of 4 impulse capacitors in series. The total capacitance of high-voltage arm C d is 500pF. Fig.1 Lightning impulse generator circuit 2.2 GIS The Three Georges Station consists two parts: Left Bank station and Right Bank station. The 550kV switch station adopts 3/2 connection method while unit connection is applied in the generator and transformer. Two sets of generator and transformer comprise alliance unit connected into 550kV. GIS is adopted ranging from main transformer high-voltage bushing to outlet disconnecting switch. After the installation of GIS, it is necessary to carry out on-site AC withstand voltage test and lightning impulse test. Owing to a large number of switches, time limitation, 550kV GIS field AC withstand voltage test of Three Gorges Right Bank station is conducted according to five test intervals. Among them, the capacitance of longest internal one is about 23nF. The lightning impulse voltage test is conducted for each test interval. According to standards, in the GIS lightning impulse test, front wave time of lightning impulse wave 8μs while wave front time of oscillating lightning impulse waveform 15μs. 3. Theoretical Calculation info@himalayal.com Page:2 All right reserved.

3 3.1 Lightning impulse generator circuit In order to facilitate the production, the lightning impulse discharge circuit in Fig.1 can be simplified to equivalent circuit in Fig.2. According to Fig.2, RLC s circuit differential equation is listed below: C1C 2 tf = 3.24 (r+rf) C C 1 2 (1) t t = (r+r f )(C 1 +C 2 ) (2) Where: tf wave front time tt half peak time C1 = C8 /n main capacitance of impulse circuit n stage of impulse circuit C2 = Cd + Cg load capacitance effect on wave tail. Given circuit stray inductance L, equivalent circuit of wave front time calculation is shown in Fig.3. In order to prevent output waveform from oscillating, circuit resistance R should meet the requirements of formula (3). If the oscillation never occurs to the line, wave front time can be obtained via formula (4). R 2 L C (3) tf = 2.33RC = 4.66 LC (4) Where: R total resistance of the circuit L total inductance of the circuit C total capacitance of the circuit C =C 1 C 2 / (C 1 +C 2 ) r arc lead and circuit contact resistance Rf = nrf wave front resistance Rt = nrt wave tail resistance Fig.2 Equivalent circuit of lightning impulse generator The stray inductance in the discharge circuit will impose certain effect on impulse waveform, even cause circuit oscillation. Test results indicate that circuit inductance has a significant effect on wave front time but little Fig.3 Equivalent circuit of wave front time According to the formula (4), as load capacitance increases, wave front time lengthens. If main capacitance and load capacitance are fixed, wave front time can be altered by means of reducing the circuit resistance. As for impulse voltage generator without consideration of large capacitive test object, R value is taken little in order to restrict wave front time within international standards. However, little R value may cause impulse info@himalayal.com Page:3 All right reserved.

4 waveform oscillation and overshoot exceeding, and the equipment cannot output the waveform which meets the requirement. 3.2 Features of impulse voltage output with large capacitive load The charge voltage of each stage capacitance should be less than 100kV. Based on equipment parameters provided by manufacturers, entrance capacitance of GIS reaches 23nF. By means of PSCAD/EMTDC, simulation calculation is done for lightning impulse waveform. The result is shown in Fig.4. Fig.4 Lightning impulse waveform with GIS From the Fig.4, if impulse source is directly connected into GIS, due to insufficient load capability of impulse generator with large capacitance, output lightning waveform deforms seriously. Its wave front time is 14μs, which is higher than standard lightning impulse wave front time - 8μs. The output efficiency of impulse voltage is 59%, which cannot meet the test requirement. inductance L is connected between impulse voltage generator body and load in series, the voltage U L of energy storage element L is proportional to current change ratio of circuit charge, which can be represented by U L = L (di L /dt). During the lightning impulse process, U L is attenuated oscillation wave of voltage polarity alternating transformation. Therefore, U L can generate oscillating lightning wave in the test. At the moment of discharge, U L changes all of a sudden and the inductor starts to store the energy; at the first peak of discharge current, the inductance energy begins to release. In other words, main capacitance and inductance in series continue to charge load capacitance, generating impulse overshoot. The maximum peak overshoot appears after the first charge. After the oscillation inductance is connected in the test circuit, higher impulse voltage will be generated around impulse peak due to the release of storage energy but large oscillation inductance will cause longer wave front time. Hence, before the test, suitable inductor should be selected. According to test condition, three 2mH inductors are selected and connected in series to test circuit in order to generate oscillation lightning wave. The lightning impulse waveform is shown in Fig Influence of inductance on impulse voltage According to standards, oscillation lightning impulse wave can be adopted in the GIS test. After the info@himalayal.com Page:4 All right reserved.

5 Fig.5 Lightning impulse waveform with oscillating inductor Compared with the result shown in Fig.4, the amplitude of output waveform improves a lot after the connection of the inductor but wave front time increases as well. In order to make wave front time meet the international standards, R f should be 20 Ω in the test. Both the amplitude and wave front time of impulse voltage meet the requirement of the test, which is shown in Fig.6. Fig.6 Oscillating lightning impulse waveform via best proposal By this connection method, wave front time of output oscillation lightning impulse wave is 14μs and output efficiency reaches 112%, which meets the requirement. Fig.7 Layout of GIS lightning impulse test equipment According to test requirement, it is necessary to conduct lightning impulse test for many times. One positive and negative polarity test of lightning impulse with 620kV, 990kV and 1120kV peak; three positive polarity tests and five negative polarity tests for 1240kV lightning impulse. The updated equipment fully meets the requirement. Test waveform and simulation waveform of 1240kV lightning oscillation impulse is shown in Fig Field Test Based on simulation result of oscillation lightning impulse waveform described above, only partial transformation to available equipment can meet the requirement. As for Three Gorges 550kV GIS substation lightning impulse withstand test, the inductor is installed in the impulse voltage circuit to generate oscillation lightning wave. Layout of test equipment is shown in Fig.7. Fig.8 Comparison of test waveform and simulation waveform (dot lime--simulation waveform info@himalayal.com Page:5 All right reserved.

6 solid line--test waveform) The peak of lightning oscillation impulse waveform generated by the equipment in the test is 1240kV and wave front time is 14μs, which meets the requirement. In the Fig.8, the simulation waveform conforms to test waveform; oscillating frequency of simulation waveform is close to test result; oscillating depth is slightly lower than test result. Hence, impulse generator and GIS model established in the paper is accurate. In the impulse tests with different peaks, simulation result conforms to test result. The breakdown discharge never occurs among many lightning impulse tests of 550kV GIS, which indicates that GIS insulation meets the requirement of the design. 5. Conclusions Some 12-stage bilateral-charge lightning impulse generator with high performance is analyzed. Based on equivalent circuit, output performance of this generator under large capacitive load is discussed. Conclusions are as follows: 1) When lightning impulse voltage test is carried out for test objects with large capacitance, such as GIS or high-voltage cable, wave front time of lightning impulse wave will increase, and impulse peak will decrease because load capacitance exceeds the maximum value bore by impulse voltage generator. 2) The inductor can be installed in the lightning impulse circuit. The inductor s property of storing energy can be used to generate lightning oscillation impulse wave which conforms to international standards, and detect the insulation level of GIS. 3) Simulation result shows that the inductor can be made use of to generate lightning oscillation impulse waveform with different amplitude and frequency by means of basing on common lightning impulse generator. 4) The proposal of installing three 2mH inductors in series is put forward in this paper according to simulation result and withstand test of Three Gorges Right Bank 550kV GIS substation. This proposal is successfully applied in the field test and results are good. Moreover, valuable experience has been accumulated for the lightning impulse test on GIS of 750kV, 1000kV substations in China. 5) Simulation result and field test result indicate that installing the inductor is an effective method for generating oscillating lightning impulse, and can meet the special requirement. REFERENCES [1] Meppelink J, Diederich K J, Feser K, et al. Very fast transients in GIS[J]. IEEE Transactions on Power Delivery, 1989, 4(1): [2] Martinez H, Rebollar G V, Madero V, et al. Design and simulation of a grounding grid for GIS substation [J]. IEEE Latin America Transactions, 2008, 6(2): info@himalayal.com Page:6 All right reserved.

7 [3] Zhang L, Ge D, Zhang C, et al. Study on a protection scheme for a 500kV GIS substation against direct lightning strokes [C]//2010 Asia-Pacific Symposium on Electromagnetic Compatibility, Beijing, China: CSEE, 2010: [4] Becker G, Koch H. Specification of GIS substations and equipment [C]. // 2007 IEEE Power Engineering Society General Meeting. Tampa, USA: IEEE, 2007: [5] Thasananutariya T, Spuntupong K, Chatratana S. Design of grounding system for GIS indoor substation [C] //2004 IEEE Region 10 Conference : Vol C (3). Bangalore, India: IEEE, 2004: [6] Messerer F, Boeck W. Gas insulated substation for HVDC [C]//2000 Annual Report Conference on Electrical Insulation and Dielectric Phenomena: Vol 2. Victoria, BC, Canada: CEIDP, 2000: [7] Bolin P, Koch H. Basic information on gas insulated substation (GIS)[C]//2008 IEEE Power Engineering Society General Meeting- Conversion an Delivery of Electrical Energy in the 21 st Century. Pittsburgh, USA: IEEE, 2008:1-4. [8] Liu Zhaolin. The application of GIS in east China grid and its prosperctive [J]. High Voltage Apparatus, 2005, 41(5): [9] He Hu, Han Shumo, Wang Yanhao, et al. Worksite installation management of 1100kV GIS for HUVAC southeast Shanxi substation [J]. Power System Technology, 2009, 33(4): [10] Chen Haiyan, Fan Yue, Chen Hongming. 1000kV GIS layout types for southern Anhui UHV substations [J]. East China Electric Power, 2008, 36(7): [11] Chen Liangjin, Li Sinan, Xie Peng, et al. Study on the lightning intruding over-voltage in 750kV GIS substation [J]. High Voltage Engineering, 2006, 32(8): [12] Li Yi, Zhou Lixing. 500kV protection against lightning impulse ingression to 500kV GIS and impulse analysis [J]. Insulators and Surge Arresters, 2008, 8(2): [13] Zhang Xi, He Zengke, Zhang Zunyan. Simulation and study on the lightning overvoltage for 330kV GIS substation [J]. Shaanxi Electric Power, 2007, 35(10): [14] Zang Xuyun, Zhao Gang, et al. Analysis of the capacitive load ability of 3600kV serial impulse generator [J]. High Voltage Engineering, 2002, 28(8): [15] Zhang Renyu, Chen Changchang, Chen Changyu. High voltage test techniques [M]. Beijing, China: Tsinghua University Press, [16] Zhu Shiquan. Estimation of inductor of impulse voltage generator and experiment circuit [J]. Transformer, 1994, 31(3): [17] Zhu Shiquan. Estimation of stray capacitance of impulse voltage generator [J]. Transformer, 1994, 31(10): [18] DLT Code for hand-over test of gas-insulated metal-enclosed switchgear on site [S]. info@himalayal.com Page:7 All right reserved.

8 Beijing, China: China Power Press, [19] Zhao Youbin, Li Zheng, Wang Jiansheng, et al. Analysis and improvement of load characteristics and its waveform to lightning impulse voltage generator [J]. High Voltage Apparatus, 1999, 35 (1): [20] Li Guangfan, Liao Weiming, Li Qingfu, et al. Voltage output performance of 7200kV/480kJ impulse voltage generator [J]. Proceedings of the CSEE, 2008, 28(25):1-7. [21] Zhang Yixiu. Study on the charging ununiformity of impulse voltage generators [D]. Shanghai, China: Shanghai Jiaotong University, [22] Wang Haoyang. Investigation of the load impact on the output capability and the synchronization of impulse voltage generators [D]. Shanghai, China: Shanghai Jiaotong University, [23] Zhu Xudong. 1050kV series oscillating voltage generator and its application [J]. High Voltage Engineering, 1995, 21(1): Page:8 All right reserved.