Study On AC Pollution Flashover Performance And Discharge Process Of Transmission Line Insulators

Pollution flashover is one of the key issues affecting the external insulation design and safety operation of transmission lines, and there have been many studies of flashover performance and mechanism of polluted insulators in China as well as other countries. However, because of the complexity of flashover mechanism and the diversity of weather, geography and pollutants in various areas, the pollution flashover accidents still occurred continually and the study need to be putted up more. Based on the AC artificial pollution tests, this thesis aims to study the combined effect of salt deposit density and non-soluble deposit density on AC pollution flashover voltages and analyze the difference of AC pollution flashover performances and discharge processes for various types of transmission line insulators, as well as, analyze the AC flashover performances of artificial polluted insulators at high altitude sites. At the same time, based on the discharge process of insulators,the AC pollution discharge model is discussed. The studies in the thesis will make to understand the AC pollution flashover performance and mechanism, as well as, will contribute to the West-to-East Power Transmission, South-and-North Transaction and Nationwide Electricity Interconnection and will be of significant referenced values to the constructions and operations of EHV and UHV transmission lines.Based on the existing studies in China as well as other countries, a mass of AC artificial pollution tests of some types of transmission line insulators have been carried out in the laboratory and at the high altitude sites. The AC pollution flashover performance and mechanism is analyzed through the flashover processes gained by a high speed frame camera and the combined effects of multi factors on the pollution flashover voltage. The main conclusions in the thesis are as following:Under the same pollution grade, the pollution flashover voltage gradients of the outer-rib type insulators are lower while the effectiveness of leakage distances of the insulators with the long ridges are higher. The flashover voltages of composite insulators are higher because of the hydrophobicity on its material surfaces. Comparing two types of non-soluble materials, kieselguhr and kaolin, the pollution flashover voltage of insulators polluted with kaolin is lower than that of insulators with kieselguhr because that kaolin on the insulators surfaces will absorb more water and the water is not easily run off in the wetting process. There is a power function relationship between the pollution flashover voltage and equivalent salt deposit density or non-soluble deposit density. By using the least square method to analyze the test data, the calculational formula of pollution flashover voltages at the combined effect of equivalent salt deposit density and non-soluble deposit density is gained.The flashover voltages in high altitude areas are combined affected by the pollution grades and air conditions. If the surface property is hydrophilic, the exponent characterizing the effect of air pressure varies between 0.507 and 0.587 for various insulators, which is not immediately related with the materials and configurations of insulators, and the exponent characterizing the effect of ambient temperature is about 0.21 because the conductivity of the wetting pollution layer is affected by the ambient temperature.The propagation of the partial arc on the polluted insulator surface mainly depends on the balance of electrostatic force and thermal buoyand force, the effect of latter is more obvious for the bigger leakage current, which results in the arc-floating. The electrostatic force and thermal buoyand force will change with the changing of the air pressure, and the lower the air pressure is, the more obvious the effect of the thermal buoyand force is. The serious arc-floating is one of the main reasons for the reducing of the pollution flashover voltages. The comparison and analysis of the pollution discharge processes of the various insulator strings in the plains show that the partial arc develops along the surfaces of the standard insulators and their leakage distances are used well, the leakage distances of the outer-rib type insulators are not used well because of the arc-floating between their sheds, and the effectiveness of leakage distances of the anti-fog insulators because their long ridges retrain the development of the partial arc. The differences of the discharge processes verify the artificial pollution test results.In the AC pollution discharge process of insulators, the partial arc includes surface arcs and air-gap arcs begot by arc-floating. The surface arc and air-gap arc can be characterized by a combined arc, so a discharge model of polluted insulators is introduced. The air-gap arc can be neglected for AC pollution flashover in the plains while its effect on the flashover voltages must be considered under the high altitude conditions. The statistical arc constants of the combined arc are gained by the tested results.