Space Charge Measurement Under Different Electrical Field Intensity And Temperature Gradient In Oil-paper Insulation

In HVDC converter transformer plays an important role,its main insulation is mainly composed of transformer oil and insulating pressboard.The main insulation will be withstand DC,AC / DC and polarity reversal voltage effect during operation.The difference of resistivity and relative dielectric constant between transformer oil and insulating pressboard will lead to the space charge accumulation at the interface of oil and pressboard under DC voltage which will have an important influence on the breakdown characteristics of oil paper insulation.For this reason,the study of the mechanism of space charge injection and accumulation in oil paper insulation is helpful to reveal the relationship between the space charge accumulation and its electrical properties,to improve the performance of transformer oil paper insulation and to improve the performance of transformer oil paper insulation,to reduce the occurrence of transformer insulation accident.The research contents and conclusions in this paper are as follows:According to the space charge testing technology at home and abroad,a space charge measurement system with temperature gradient was designed,which was used the principle of the Pulsed Electro-acoustic method.The space charge measurement system can realize the space charge measurement in oil paper insulation under different temperature gradient.On this basis,the space charge accumulation and dissipation characteristics in monolayer and double layer oil paper insulation was measured under different electric field strength and in different temperature gradient in the laboratory.At the same time,the influence of space charge at the polarity reversal on electric field was also measured.The results show that with the increase of applied electric field strength,the field distortion rate increases obviously.The space charge was formed at the interface of the double pressboard insulation,which was opposite to the polarity of the adjacent electrode.Due to the accumulation of space charge at the interface of double board,the distortion rate of field intensity was obviously higher than that of single board.The temperature gradient caused serious distortion of the electric field,with the increase of the temperature gradient,the internal field distortion first added then reduced.When the voltage was removed,the larger electric field applied,the more space charge accumulate.At the beginning of the power off time,the bigger temperature gradient the larger amount of charge remaining.After one hour times voltage off,the bigger temperature gradient the less space charge remaining.Because of the presence of space charge,the voltage applied on the board was distorted when the polarity was reversed.