| Pollution flashover of external insulation is the main threat to the safe operation of power system, with the implementation of the UHV DC transmission lines, it will improve continuously the selection and design requirements for external insulation. The composite insulator is widely used by power utilities in many countries for its excellent anti-contaminated characteristic. So far, there has a large number of researches about the DC electrical characteristics of uniformly polluted composite insulators, but the study on the influence of non-uniform contamination on flashover voltage is very few, and now there has no way to correct the influence of it for DC composite insulator. In this paper, laboratory investigations on the DC flashover performances of several polluted short samples of EHV/UHV DC composite insulators were carried out in the climate chamber, the main work and conclusion for this paper is as follow:①Take two short samples of±800kV composite insulators with same shed configuration and different structure height, study the relation between 50% DC flashover voltage (U50) and arc distance (h) under uniformly polluted and with Hydrophilic contamination. The results show that there has a linear relationship between U50 and h when h≤3.435m.②Testing results show that both soak period and pH of solution have significant effect on decrease of hydrophobicity for silicon rubber. And testing results indicate that the hydrophobicity of silicon rubber before contaminated also has influence on hydrophobicity transfer property after contaminated.③The DC pollution flashover performance of composite insulator is related to its shed configuration, too low distance of adjacent sheds will reduce the effectiveness of creepage distance because of arc-bridging phenomenon between sheds or ribs. The ideal specific value of creepage distance and arc distance is between 3.2 and 3.4.④Use FXBZ-±800/530-1800 as tested insulator, study the flashover performance under condition of non-uniform contamination based on average salt deposit density (SDD). The results show that if the ratio of salt deposit density of top surface and bottom surface (T/B) keep constant, the U50 will decrease with the increasing of SDD by the relationship of power function. The characteristic exponent (b) characterizing the influence of SDD on U50 is basically not affected by T/B.⑤When SDD keep constant, the U50 will increase with the decreasing of T/B. It indicates that the expression for correction factor (K) correcting the influence of non-uniform contamination on flashover voltage suitable for composite insulator can be expressed as of engineering application by experimental verification.⑥By using high-speed camera, the DC pollution flashover process can be described as four phases: corona discharge, developing of partial discharge, developing and elongating continuously of partial arc, flashover of insulator. Differ from uniformly polluted composite insulator, the initial stage of discharge mainly occurs on the top surface of sheds under non-uniform contamination condition, and the partial arc is not easy to hike up from the surface of insulator under this condition.⑦Establish a circuit model of DC pollution flashover for composite insulator, and analyzes the flashover mechanism and factors which affect the critical flashover voltage by this model.
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