ABSTRACT:ased 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 3.35. 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 Type E 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 pollutant will accumulate on dc insulators which is 1.2-1.5 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 theUHV 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.
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