| The severe atmospheric environment of low atmospheric pressure at high altitude with pollution and icing has an important effect on the external insulation performance of transmission lines and threatens the reliability of a power system. But up to now, the study on DC transmission line' external insulation in the severe atmospheric environment could not meet the requirement of electric power industry development. With the large-scale exploitation of hydroelectric resource in the West, the effects of severely cold, high humility and high altitude on the external insulation performance of overhead transmission line insulator will more and more server. Therefore, a systematic study of discharge performances of several kinds of insulators for the complex atmospheric environment has both a very important academic meaning and an engineering value. Upon existent theoretical and experimental results and based on a great of laboratory investigations carried out in Chongqing University, for the first time this paper studies systematically the insulators DC discharge performance and flashover mechanism of iced insulators in the complex atmospheric environment, which will contribute to the understanding of flashover phenomena observed on ice surface at low air pressure and provide the test and theoretical reference for HV and EHV transmission lines design and safety service. The main work and conclusion for this paper is as following:According to the flashover test results of several kinds of iced insulators under positive voltage and negative voltage, this paper obtained: for the single porcelain insulator, the single glass insulator and the composite insulator bridged completely by icicle, the metal cathode's strong ability of emission electron is the primary reason that results in the lower negative flashover voltage; the position difference of the high resistance district in method anode side results in that porcelain insulator cluster and glass insulator cluster have an evident effect of polarity; when less ice and no icicle at the brim of the sheds, due to numerous non-polar arcs on the composite surface ,the effects of polarity of composite insulator was lost Using the test method of the average flashover voltage in the freezing period, DC flashover performances were investigated of several insulators with some typical structures and different material. Test results indicated: with the hoist of altitude, the increase of ice amount and the rise of pollutant, the average flashover voltage reduced. The character exponent generally depends on the insulator profile, ice amount, ice state and pollution severity etc. By means of a high-speed camera, a data acquisition system and high voltage test facilities, a series of the flashover processes on ice surfaces were record. The experimental results form this study and the subsequent theoretical analyses suggested: the thermal ionization of the air in front of an arc root resulted in arc movement; the electrostatic force had an auxiliary effect of impelling arc propagation; the electricalbreakdown only occurred during the final jump period of the flashover. During flashover process, related parameters were recorded. These parameters include the applied voltage, the leakage current, the freezing water conductive, the flashover time, the arc length and the arc root radius. Arc speeds in various periods of arc development, such as the arc-starting period, the arc developing period, and the finial flashover period, were measured and the critical arc length decreased with an increase in freezing water conductivity. An analyses of arc propagation process suggested an arc could propagate in two different ways: inside or outside the ice. This resulted from the different manner of Na atom entering the arc body. Based on above test results and theoretical analyses, this paper put forward both a calculation expression for correcting insulators flashover voltage and a method for choosing insulator in the complex atmospheric environment.
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