Study on Pollution Flashover Performance of Short Samples of Composite Insulators Intended for ±800 kv UHV DC

Transcription

1 1192 Study on Pollution Flashover Performance of Short Samples of Composite Insulators Intended for ±800 kv UHV DC Xingliang Jiang, Jihe Yuan, Zhijin Zhang, Jianlin Hu and Lichun Shu Chongqing University The Key Laboratory of High Voltage and Electrical New Technology of Ministry of Education College of Electrical Engineering Shapingbazhengjie 174 Shapingba District, Chongqing, , P. R. China ABSTRACT Based on the artificial pollution tests, the effects of pollution and high altitude on the flashover performance of short samples of five kinds of UHV/EHV dc composite long rod insulators are analyzed. The exponent characterizing the influence of salt deposit density on the flashover voltage is related with the profile and the material of the insulator shed. The values of the samples exponents vary between 0.24 and 0.30, which are smaller than those of porcelain or glass cap-and-pin insulators, namely, the influence of the pollution on the composite long rod insulators is less, relatively. Thus, the composite insulators have certain advantages in severe pollution regions. The best ratio of the leakage distance to the arcing distance is about The exponent characterizing the influence of air pressure on the flashover voltage is related with the profile and the material of the insulator shed and the pollution severity, the values of the samples exponents vary between 0.6 and 0.8, which are larger than those of porcelain or glass cap-and-pin insulators. Therefore, the dc composite insulator used in high altitude regions should have enough arcing distance. Based on the test results, if insulator sample is selected for the ±800 kv UHV dc transmission lines, the basic arcing distance should be no less than 8.16 m and the basic leakage distance no less than 30.2 m. Index Terms - UHV DC transmission lines, composite insulators, pollution, high altitude, flashover. 1 INTRODUCTION THERE are several ±800 kv Ultra-High-Voltage (UHV) dc transmission lines to be constructed in China. The external insulation of these lines will be polluted by industrial contaminant, coastal fog, natural dust, bird feces and etc. Because of special geographical conditions in China, the lines will also pass through the high altitude regions, for example, the partial lines of Yun-Guang ±800 kv project will be situated in the areas with the altitude of 1000 m to 2700 m. Therefore, the insulators of the lines will be affected by pollution and high altitude. Owing to the favorable anti-pollution capability, the composite insulators have been one of the main selections for ±800 kv UHV dc transmission lines. There has been much investigation on ac and dc pollution flashover performance of various types of insulators. Because of the invariable electric field of dc voltage, there are more Manuscript received on 7 September 2006, in final form 15 April pollutant will accumulate on dc insulators which is times as high as those on ac insulators under the same air environment [1,2]. [3] shows that the ratio of the pollutant which accumulated on dc insulators to those on ac insulators is 1-3 and the ratio of the pollutant on the top surface to those on the bottom surface of dc insulators is 1/3-1/20. Under the same pollution and wetting conditions, the dc pollution flashover voltages are 20%-30% lower than those of ac because of the effect of the steady dc arc without zero point, moreover, the dc pollution flashover voltages will decrease more than ac voltages with the increase of the pollution degree [4-6]. Based on the pollution flashover tests on various types of dc insulators, the flashover voltages of composite insulators are still 30% higher than those of glass insulators even though the composite insulators have lost their hydrophobicity [2,7]. At present, there is very little information on design and construction, and there is no operational experience of the /07/$ IEEE

2 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 5; October UHV dc transmission lines in the world. Therefore there is almost no reference data for the design of the external insulation. So as to meet the demands of the construction of ±800 kv UHV dc transmission lines, the artificial pollution flashover performance of the short samples of five kinds of UHV/EHV dc silicone rubber (SIR) composite long rod insulators are investigated in this paper. 2 TEST FACILITIES AND PROCEDURES 2.1 TEST FACILITIES The artificial pollution tests have been carried out in the large multi-function artificial climate chamber with the diameter of 7.8 m and the height of 11.6 m, in which the air pressure can be controlled as low as 30 kpa and the air conditions at an altitude as high as 7000 m can be simulated [8]. The power supply is leaded in through a 330 kv wall bushing. 2.2 TEST POWER SUPPLY A dc high voltage of up to ±600 kv is supplied by a cascade rectifying circuit controlled by a thyristor voltagecurrent feedback system, which ensures a dynamic voltage drop of less than 5% when the load current is 0.5 A. The test power supply satisfies with the requirements commended in [9]. 2.3 SAMPLES The samples are the short samples of four kinds of FXBW- 500/160 dc SIR composite long rod insulators and one kind of FXBW-±800/400 dc SIR composite long rod insulator, which are denominated by, B, C, D and E. The profiles and technical parameters of the samples are shown in Figure 1 and Table 1, in which D 1 is the diameter of larger shed, D 2 is the diameter of smaller shed, h is the arcing distance, L is the leakage distance, d is the diameter of rod, S is the ratio of the leakage distance to the arcing distance, N 1 is the number of larger sheds and N 2 is the number of smaller sheds =38* Figure 1. Profiles of the tested short samples of composite insulators.

3 1194 Table 1. Technical Parameters of the Test Short Samples of Composite Insulators. 1 /D 2 (mm) h(mm) L(mm) d(mm) S=L/h N 1 /N 2 Color Materials A 175/ /50 brown SIR 1 B 164/ /27 brown SIR 1 C 178/ /21 brown SIR 1 D 175/ /52 red SIR 2 E 190/ /78 brown SIR TEST PROCEDURES Referred to the test standards [9-11], the test procedures in the paper were as follows PREPARATION Before the tests, all samples were carefully cleaned so that all traces of dirt and grease were removed and dried naturally. The surfaces of the samples were coated by a very thin layer of dry kieselguhr to destroy the hydrophobicity which would be at the degree of WC4 or WC5. Because the layer of kieselguhr was very thin, the effect of the kieselguhr on non-soluble deposit density N (in mg/cm 2 ) could be neglected ARTIFICIAL POLLUTING In 1 hour after preparation, the solid layer method was used to apply to the pollution layer on the samples where sodium chloride and kieselguhr were electric and inert materials, respectively [12]. The ratio of Salt Deposit Density (in mg/cm 2 ) to Non- Soluble Deposit Density N was 1:6 and the values of /N were 0.03/0.18, 0.05/0.30, 0.08/0.48, 0.15/0.90 mg/cm 2, respectively ARRANGEMENT The tested insulators were equipped by two corona rings on the ends and hung up vertical in the chamber. The minimum clearances between any part of the samples and any earthed object were larger than 3.5 m, which satisfied with the requirements in [9] WETTING After 24 hours natural drying, the pollution layer on the insulators was wetted by the steam fog, the inputting intensity of fog was 0.05±0.01 kg/h m 3. The hydrophobicity would gain a certain recovery and transfer after 24 hours natural drying of the polluted samples, which would be different from the tests after only 1 hour drying. The pollution layer on the insulators could be wetted completely after 7-15 minutes time. In the whole wetting process, the temperature in the chamber was controlled lower than FLASHOVER TESTS After the pollution layer was wetted completely, dc voltage was applied to the samples. A negative polarity applied voltage generally result in lower flashover voltage than does positive polarity for suspension (cap-and-pin) type insulators, but no substantial polarity differences have been experienced with long-rod insulators in tests [11], so negative dc voltages were introduced in this paper. Only one sample of, B, C and D, 5 times polluting were carried out at one salt deposit density. For every time polluting, 5-7 times flashover tests were carried out, in which 3-4 flashover voltages, deviating from their average lower than 10%, were valid values. The flashover voltage U f (in kv) was the average of 19 valid values at the salt deposit density and the standard error σ (in %) was calculated with the 19 valid values. For, there were 3 samples, only 3 times polluting were carried out for every sample at one salt deposit density. 4-5 times flashover tests were carried out for every time polluting, in which 3 flashover voltages, deviating from their average lower than 10%, were valid values. The flashover voltage U f was the average of 27 valid values and the standard error σ was calculated with the 27 valid values SIMULATION OF HIGH ALTITUDE The low air pressure was used to simulate the high altitude. The air pressure P (in kpa) is the synthetical reflection of the air temperature t (in ), the relative density δ d of dry air and absolute humidity h a (in g/cm 3 ) of air [13], thus, P can be used to character the influence of the air parameters on the discharge voltage. [14] shows that the relationship between the air pressure and the altitude H (in km) is as following: P H = 45.1 (1 ( ) ) (1) where P 0 is the standard air pressure. In this paper, the altitudes of 232 m and 0 m were simulated in the chamber, whose corresponding air pressures were 98.6 kpa and 74.6 kpa, respectively. For the artificial pollution tests under the high altitude conditions, the air in the artificial climate chamber was taken out so that the air pressure were 5-7 kpa lower than calculated value corresponding the pre-concerted altitude, then the steam fog was used to wet the sample. When the sample had been wetted completely and the air pressure in the chamber had reached the calculated value, the flashover test was carried through. In the whole test process, the air pressure in the chamber should be kept invariably. 3 TEST RESULTS AND ANALYSIS 3.1 TEST RESULTS The dc pollution flashover performance of, B, C, D and E samples have been investigated based on the above artificial pollution test methods in this paper. Based on the test results, the pollution flashover voltages U f of five kinds P 0

4 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 5; October of short samples and their standard errors σ are shown in Table 2. Table 2. Flashover Voltages of Tested Insulator Samples B C D E U f (kv) σ U f (kv) σ U f (kv) σ U f (kv) σ P=98.7kPa, H=232 m U f (kv) σ B C D E U f (kv) σ U f (kv) σ U f (kv) σ U f (kv) σ P=76.4kPa, H=0 m U f (kv) σ For the standard errors σ of the flashover voltages, the maximum is 7.3% and the minimum is 2.7%, all σ are less than 7.5%. Because there is a tiny difference in the polluting, the destroying hydrophobicity and the wetting of insulators in every test, σ has certain dispersity although there were enough tests for every flashover voltage. 3.2 RELATIONSHIP BETWEEN FLASHOVER VOLTAGES AND SALT DEPOSIT DENSITY The relationships between pollution flashover voltages and salt deposit density are shown in Figure 2. For various air pressures, the pollution flashover voltages of the short samples reduce with the increase of salt deposit density and there is a power function between U f and, which can be expressed as [15, 16]: U a f = A (2) where A is a constant related with the profile of insulators and the air pressure, a is the characteristic exponent characterizing the influence of the salt deposit density on the flashover voltages. The value of a is dependent on the conditions of partial arc burning, thus the environmental conditions, the test methods, the pollution materials and the materials of insulators will influence on it. Analyzed the test results, the values of A and a of five kinds of short samples are obtained and shown in Table 3. Flashover voltage, U f (kv) Flashover voltage, U f (kv) Flashover voltage, U f (kv) Flashover voltage, U f (kv) Flashover voltage, U f (kv) Figure 2. Pollution flashover voltages U f vs. salt deposit density.

5 1196 Table 3. Values of A and a P (kpa) a A a A B C D E Table 6. Values of E l and E h for P=74.6 kpa B C D E E L (kv/cm) E h (kv/cm) E L (kv/cm) E h (kv/cm) Based on Figure 2 and Table 3, the exponent a is related with the profile and material of the insulator shed and the air pressure. For and D have the same shed profile and different materials, the a of them are different, thus the materials of the shed have effect on the pollution performance of composite insulators. At the higher altitude, the value of a is smaller, which means that the lower the air pressure is, the lesser the influence of the pollution on the electrical performance of insulators is, for example, the values of a vary between 0.27 and 0.30 at the air pressure of 98.6 kpa and between 0.24 and 0.27 at the pressure of 74.6 kpa. In this paper, the values of a are higher than those of composite long rod insulators in [17] and are smaller than those of porcelain or glass cap-and-pin insulators [18], as shown in Table 4. Therefore, the composite long rod insulators have certain advantage of porcelain or glass capand-pin insulators in sever pollution regions. Table 4. Comparison of a Insulator This paper [17] [18] a POLLUTION FLASHOVER VOLTAGE GRADIENTS OF SAMPLES In this paper, E L (in kv/cm) is the flashover voltage gradient along the leakage distance L and E h (in kv/cm) is the flashover voltage gradient along the arcing distance h of the insulators, namely, E L = U f / L (3) E h = U f / h (4) Based on the test results in Table 2, E L and E h of five kinds of samples are shown in Table 5 and Table 6. Table 5 and Table 6 can be plotted visually in Figure 3 and Figure Table 5. Values of E l and E h for P=98.6 kpa B C D E E L (kv/cm) E h (kv/cm) E L (kv/cm) E h (kv/cm) E L (kv/cm) E h (kv/cm) E L (kv/cm) E h (kv/cm) Flashover voltage gradient, E h (kv/cm) Flashover voltage gradient, E h (kv/cm) E L (kv/cm) E h (kv/cm) E L (kv/cm) E h (kv/cm) (a) Air pressure of 98.6 kpa (b) Air pressure of 74.6 kpa Figure 3. Flashover voltages gradient E h vs. salt deposit density.

6 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 5; October Flashover voltage gradient, E L (kv/cm) Flashover voltage gradient, E h (kv/cm) 1.30 : :0.05 : : Ratio of the leakage distance to the arcing distance, S (a) Air pressure of 98.6 kpa 1.20 Flashover voltage gradient, E L (kv/cm) (a) Air pressure of 98.6 kpa Flashover voltage gradient, E h (kv/cm) Ratio of the leakage distance to the arcing distance, S (b) Air pressure of 74.6 kpa :0.03 :0.05 :0.08 :0.15 Figure 5. Flashover voltages gradient E h vs. ratio of leakage distance to arcing distance S (b) Air pressure of 74.6 kpa Figure 4. Flashover voltages gradient E L vs. salt deposit density. From the tables and figures, the profiles and the leakage distances of composite insulators have effects on their U f at various salt deposit density and various air pressures. E L and E h of various samples have certain differentia, so an appropriate ratios of the leakage distance to the arcing distance S of composite long rod insulator is needed. Based on the test results, the relationships between the flashover voltage gradients E h and S are shown in Figure 5. From Figure 5, E h is related with S, for example, at of 0.03 mg/cm 2, 1.31 and 1.38 for and A, respectively. However, the longer L does not always mean the better flashover gradient, for example, E h of is lower than that of whose S is smaller than that of. In all samples, the leakage distance of with the highest S has not completely been used, while has the best effectiveness of leakage distance. It can be concluded from Figure 5 that the best S is about RELATIONSHIP BETWEEN POLLUTION FLASHOVER VOLTAGES AND ALTITUDES With the increase of the altitude namely the decrease of the air pressure, the pollution flashover voltages of all samples diminish. For example, when the altitude increases from 232 m to 0 m, U f of the short sample of ±800 kv composite insulator named diminish 16.7% at of 0.08 mg/cm 2, which means that the pollution flashover voltage of will diminish 7.2% when the altitude increases 1 km.

7 1198 The change of constants of the partial arc and the arc floating phenomenon are the main reasons for the decrease of the pollution flashover voltage at low air pressure [19]. Consulted [20,21], the relationship between the pollution flashover voltages of insulators and the air pressure P can be expressed as: n U f = U 0 ( P / P0 ) (5) where U 0 is the pollution flashover voltage of insulators at the standard air pressure P 0 (101.3 kpa), n is the characteristic exponent characterizing the influence of the air pressure on U f and varies between 0 and 1. The values of U 0 and n of five kinds of short samples are obtained from the test results in Table 2 and are shown in Table 7. Table 7. Values of U 0 and n for Various Type A B C D E U 0 (kv) n U 0 (kv) n U 0 (kv) n U 0 (kv) n U 0 (kv) n According to the table, the exponent n is respectively 0.71, 0.70, 0.71 and 0.68 at of 0.05 mg/cm 2 for, B, C and E with dissimilar profiles, the values of n are different but not obvious, which means that the profiles of samples have certain influence on n. There are differences between and with the same profile and dissimilar material, thus, the materials of insulator sheds will influence n, but the reason is unclear. Overall, the profile of and the material of may be more adapt to the high altitude regions. n is also related with the pollution severity. The lighter the pollution is, the larger the value of n is, for example, the value of n of is 0.72 at of 0.03 mg/cm 2 while 0.61 at of 0.15 mg/cm 2. The reason probably is that the arc floating is prone to occur at the lower salt deposit density. The values of n of dc composite long rod insulators in this paper is similar to ac composite long rod insulators [13] and are higher than those of porcelain cap-and-pin insulators [22], as shown in Table 8. Table 8. Comparison of n Insulator This paper [13] [22] n Analysis of a great number of flashover and withstand tests of polluted insulators show that there is a standard error σ between the 50% withstand voltage gradient along the arcing distance U 50 (in kv/m) and the flashover voltage gradient, namely, U 50 = Eh (1 σ ) (6) If the standard error σ is 7%, U 50 is 100 kv/m at of 0.05 mg/cm 2 and the altitude of 1000 m, which is almost accordant with the test result (CFO=104.0 kv/m) of EPRI [23]. The pollutant quantities on the top surface and the bottom surface of the natural polluted insulators are different, so a non-uniformity factor is introduced and expressed as [24]: K = lg( T / B) (7) where K is the non-uniformity factor, T/B is the top to bottom ratio of the pollutant on the insulator surface. T/B is 1:3 at of 0.03 mg/cm 2 while 1:5 at of 0.05 mg/cm 2. The highest run voltage is 1.02 times as the rated voltage U N (in kv) of ±800 kv transmission lines, so the basic arcing distance is 8.16 m in the regions with the altitude of 1000 m and below and /N of 0.05/0.30 mg/cm 2 by using equation (8). 1.02U N h = (8) K (1 3σ ) U 50 where K is at of 0.05 mg/cm 2. For of 0.05 mg/cm 2, E L of is 0.31 kv/cm at the altitude of 232 m while 0.26 kv/cm at the altitude of 0 m. E L of is computed at 0.29 kv/cm at the altitude of 1000 m, then the 50% withstand voltage gradient along the leakage distance U 50 (in kv/m) is 27.0 kv/m at of 0.05 mg/cm 2 and the altitude of 1000 m based on equation (6). So the basic leakage distance of 30.2 m also can be obtained by the above equations (7) and (8). In the high altitude regions, the pollution severity is very light in general. For of 0.03 mg/cm 2, E h of is 1.31 kv/cm at the altitude of 232 m while 1.07 kv/cm at the altitude of 0 m. Then E h of is computed at 1.13 kv/cm at the altitude of 2000 m based on equations (1) and (5). So it is gained that U 50 is kv/m at of 0.03 mg/cm 2 and the altitude of 2000 m. By using equations (7) and (8), it can be obtained that the arcing distance is 8.41 m in the regions with the altitude of m and /N of 0.03/0.18 mg/cm 2. E L of is 0.36 kv/cm at the altitude of 232 m while 0.29 kv/cm at the altitude of 0 m. E L of is computed at 0.31 kv/cm at the altitude of 2000 m. So the leakage distance of 30.6 m also can be obtained by the above same method. 4 DISCUSSION For of 0.05 mg/cm 2, E h of ±800 kv composite insulator is 1.14 kv/cm at the altitude of 232 m while 0.94 kv/cm at the altitude of 0 m. E h of is computed at 1.08 kv/cm at the altitude of 1000 m based on equations (1) and (5). 5 CONCLUSION Based on the above test and analyzed results, the following conclusions could be obtained in this paper: (1) The characteristic exponent a characterizing the effect of salt deposit density on the flashover voltages of the short samples is related with the profile and the material of the

8 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 14, No. 5; October insulator shed and the air pressure. The value of a varies between 0.24 and 0.3, which is smaller than those of porcelain or glass cap-and-pin insulators, namely, the influence of the pollution on the flashover voltages of the composite long rod insulators is less, relatively. Thus, the composite long rod insulators have certain advantages in severe pollution regions. (2) The best ratio of the leakage distance to the arcing distance is about (3) The characteristic exponent n characterizing the influence of the air pressure on the pollution flashover voltages of the samples is related with the profile and the material of the insulator shed and the pollution degree. The value of n varies between 0.5 and 0.8, which is larger than those of porcelain cap-and-pin insulators. Therefore, the dc composite insulator used in high altitude regions should have enough arcing distance. (4) If insulator sample is selected for the ±800 kv UHV dc transmission lines, its basic arcing distance should be no less than 8.16m and the basic leakage distance no less than 30.2m in the regions with the light pollution and the altitude of 1000 m and below. (5) If is selected for the ±800 kv UHV dc transmission lines in high altitude regions, its arcing distance should be no less than 8.41 m and the leakage distance no less than 30.6 m in the regions with the very light pollution and the altitude of m. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grand ) and China Southern Power Grid Co., Ltd. The authors gratefully acknowledge the contributions of Bo Wang, Feng Mao, Yuan Du, Liyun Luo, Rong Xue and Yongji Zhang for their work on the test of this document. The authors also are grateful to Hui Long for her work on the original version of this document. REFERENCES [1] W. Lampe, T. Höglund, C. Nellis, P. Renner and R. 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[11] CIGRE WG S-33.04, Artificial pollution testing of HVDC insulators: analysis of factors influencing performance, Electra, No. 140, pp , [12] Artificial pollution tests on high-voltage insulators to be used on a. c. systems, IEC 60507, [13] Xingliang Jiang, Jianlin Hu, Yu Liang, Lichun Shu and Shujiao Xie, Pollution flashover performance short sample for 750 kv composite insulators, IEEE Intern. Sympos. Electr. Insul., pp , [14] X. Jiang, L. Yu, J. Hu, Z. Zhang and C. Sun, Lightning impulse discharge performance and voltage correction of long air gaps at lower atmospheric pressure, Proc. Chinese Society for Electrical Engineering, Vol. 25, pp , 2005 (in Chinese). [15] X. Jiang, J. Xie, Q. Wang, Z. Liu and M. Chen, DC flashover performance and voltage correction of HV insulators at high altitude districts with icing, IEEE Intern. Conf. Properties and Application of Dielectric Materials, Nagoya, Japan, pp , [16] C. Sun, Y. Tian, X. Jiang, L. Shu and W. Sima, Flashover performance and voltage correction of iced and polluted HV insulator string at high altitude of 4000m and above, IEEE Intern. Conf. Properties and Application of Dielectric Materials, Nagoya, Japan, pp , [17] R. Matsuoka, H. Shinokubo, K. Kondo, Y. Mizuno, K. Naito, T. Fujimura and T. Terada, Assessment of basic contamination withstand voltage characteristics of polymer insulators, IEEE Trans. Electr. Insul., Vol. 11, pp , [18] G. Ramos N., M. T. Campillo R. and K. Naito, A study on the characteristics of various conductive contaminants accumulated on high voltage insulators, IEEE Trans. Electr. Insul., Vol. 8, pp , [19] Yu Li, Study of the Influence of Altitude on the Characteristics of the Electrical Arc on Polluted Ice Surface, Ph.D. dissertation, Université du Québec, [20] V. M. Rudakova and N. N. Tikhodeev, Influence of low air pressure on flashover voltages of polluted insulators: test data, generalization attempts and some recommendations, IEEE Trans. Electr. Insul., Vol. 4, pp , [21] H. P. Mercure, Insulator pollution performance at high altitude: major trends, IEEE Trans. Electr. Insul., Vol. 4, pp , [22] J. Xue, R. Zhang and X. Zhang, DC flashover characteristics of the contaminated insulators under low atmospheric pressure, IEEE Intern. Conf. Properties and Applications of Dielectric Materials, Beijing, China, pp , [23] R. J. Nigbor, DC Performance of non-uniformly contaminated insulators, Cigré, [24] EPRI, HVDC Transmission Line, Reference Book, Xingliang Jiang was born in Hunan province, China, on 31 July He graduated from Hunan University in 1982 and received the M.Sc. and Ph.D. degrees in Chongqing University in 1988 and 1997, respectively. His employments include the Shaoyang Glass Plant, Shaoyang, Hunan Province, Wuhan High Voltage Research Institute, Wuhan, Hubei province, and College of Electrical Engineering, Chongqing University, Chongqing China. His fields of interest include high voltage technology, external insulation, transmission line s icing and protection. Dr. Jiang published his first monograph Transmission Line s Icing and Protection in 2001, and has published over 80 papers on his professional work.

9 1200 Jihe Yuan was born in Hebei province, China, on 30 January He graduated from Chongqing University and obtained the B.Sc. and M.Sc. degrees in 2000 and 2004, respectively. He is working toward the Ph.D. degree in College of Electric Engineering, Chongqing University. He began work in Handan Power Supply, Hebei Electric Power Company as an Associate Engineer from 2000 to His main research interests include high voltage, external insulation and transmission line s icing. Zhijin Zhang was born in Fujian province, China, in July He graduate from Chongqing University and obtained the B.Sc. and M.Sc degrees in 1999 and 2002, respectively. He is now working toward the Ph.D. degree in the College of Electrical Engineering, Chongqing University. He has been a teacher in the College of Electrical Engineering, Chongqing University since His main research interests include high voltage, external insulation, numerical modeling and simulation. He is the author or co-author of several technical papers. Jianlin Hu was born in Hubei province, China, in January He received the B.Sc. and M.Sc. degrees from Chongqing University, Chongqing, China, in 2001 and 2003, respectively. He is now working toward the Ph.D. degree in the College of Electrical Engineering, Chongqing University. He has been a teacher in the College of Electrical Engineering, Chongqing University since His main research interests include high voltage and external insulation. Lichun Shu was born in Chongqing, China, in Februry He received the B.Sc., M.Sc. and Ph.D. degrees in engineering from Chongqing University, China in 1985, 1988 and 2002, respectively. In 1988, he began to work at Chongqing University as an Assistant Professor, then as a lecturer and Associate Professor from 1992 to He became a Professor in He joined the Research Group on Atmospheric Environment Engineering (GRIEA) of the Université du Québec à Chicoutimi as a Visiting Professor in Dr. Shu has worked mainly in the field of HV external insulation. He is author and co-author of several scientific publications.