| As the core equipment of DC power network, the converter transformer is under a role of complex AC, DC and polarity reversal electric field. When voltage level is under400kV, the design of converter transformer has larger margin, Contrarily Speaking, When voltage level comes to800kV and above,the range of areas where distribute higher field in ultra high voltage converter transformer extended,more precise design parameters are needed as input quantity to lean engineering of UHV converter transformers, for instance electric conductivity. The inner-insulation of converter transformer is oil-paper insulation system. The DC electric field distribution in oil-paper insulation is inversely proportional to material’s electrical conductivity, with the change of temperature and other factors, the oil-paper conductivity varies nonlinearly, which raises the difficulty to test and get more accurate condunctivity. Due to lack of calculation parameters in engineering application, the simulation analysis of converter transformer insulation margin usually will be taken on normal condition, while the effect of temperature on the conductivity of oil-paper insulation are always ignored. Hence, it has a great engineering significance to lean engineering of converter transformer to obtain the nonlinear variation of the conductivity of the paper-oil insulation materials under different temperature and electric field strength, by the way of simulating real operation conditions and precisely designing electrical conductivity test platform of oil-paper insulation material.A set of oil-paper insulation materials processing and electrical conductivity characteristic measuring platform has been built, which consists of transformer oil treatment system, insulating paperboard drying and vacuum-immersion system and oil-paper insulation properties test platform. Through precise control of the transformer oil and oil-immersed paperboard’s dielectric loss, particle size, moisture content, temperature, and other indicators, the conductivity of the paper-oil insulation materials has been online measured.As the first time in domestic, it has been online tested that the conductivity of oil-paper insulating materials varied exponentially with the change of temperature and electric field strength, which involves the temperature-conductivity curve and electric field intensity-conductivity curve of routine transformer oil, DC dedicated oil and oil-immersed paperboard. Empirical formulae has been deduced utilizing online test results.Based on the outlet device model of the converter valve side, the phenomenon that local electric field concentrates in the outlet device and potential gradient increases at high potential areas has been found in accordance with different oil-paper conductivity ratio. That change of oil-paper conductivity ratio will affect the potential distribution and DC filed in the converter transformer has been proved.A simulation model based on a set of±400kV valve side outlet device in engineering application has been established, of which the electric field distribution under AC, DC and polarity reversal electric field in oil-paper insulation system has been calculated and analyzed at the temperature of20℃and65℃. Insulating margin at the valve side outlet parts has been focused under DC electric field. The results of simulation analysis indicates that the internal and surface electric field distribution in oil-paper insulation system will be effectively improved with the temperature rises when the thermal-conductivity of oil-paper insulation materials considered to be nonlinear. Consequently, the concentration extent of insulation surface field strength will be eased at a higher temperature so that the design margin of valve side of the outlet device can be greater. |
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