Research On The Breakdown And Flashover Characteristic Of Oil-paper Insulation System Under Hybrid Electric Field

Converter transformer plays an important role in high voltage direct current transmission system, its reliability directly affect to the safe operation of the system.?The operating converter transformer? not only withstands superimposed AC-DC voltage, but also subjected to various operative-over voltage,lightning impulse voltage and polarity reversal voltage effects, these will bring serious challenges to valve side winding insulation. For this purpose, the breakdown and surface flashover problems of valve side oil-paper structure is researched in this paper.The thesis is divided into two parts: breakdown and surface flashover characteristics research of oil-paper insulation system. Firstly, we built the breakdown simulation model of oil-paper insulation structure, and simulating the electric field distribution under DC, AC and superimposed AC-DC voltage. Two basic theories were introduced:? Secondary Electron Emission Avalanche (SEEA) theory and Electron Triggered Polarization Relaxation(ETPR) theory,summarizing the impact factors that affect the insulating material surface flashover and suppression measures. The results show that increasing the roughness of materials, changing the shape of insulators and other pre-discharge treatment can effectively improve the surface flashover voltage.Secondly, the three-electrode system of breakdown experiment was introduced. producing the surface flashover electrode system and carrying out simulation research, the result shows that the produced electrode system meets the test requirements. The pretreatment process of experimental pressboard and transformer oil is introduced, and we also built the experimental pretreatment system and detection system.Finally, we carried out the experimental research. The result shows that AC breakdown voltage of transformer oil has the little affected by temperature, but it can't be ignored for DC breakdown voltage. The breakdown voltage of oil-immersed cardboard also has the little affected by temperature, it's clearly negative exponential decay with the increasing of AC content, and we derived the negative exponential decay formula. The temperature's influence of the oil-paper composite insulation breakdown voltage is small as it is the AC and DC voltage, but the temperature has the biggest influence when the ac content is fifty percent. Superimposed AC-DC breakdown voltage (AC content of 50%) is biggest, following by DC voltage, and AC voltage is minimum.?With the increasing of distance, the voltage's increase is not more and more obviously, showing a "saturation" phenomenon.?The leakage current of parallel component electrode systems is very small at the beginning of applied voltage,? it is rapidly increase in the eve of the flashover and immediately occur flashover,?normal components electrode system can occur once or several times as it is flashover. The flashover voltage of normal component electrode system is lower than parallel component electrode system and it is more obviously when the voltage is AC situation. The study can be referred to the insulation design of converter transformer and manufacture of high-voltage DC devices.