Influence Of Moisture And DC Component On The Creepage Discharge Characteristics In Oil-Pressboard Insulation

In China,the mismatch between energy resources and load center distribution necessitates the development of ultra-high voltage direct current(UHVDC)transmission technology.Converter transformer is an important electrical equipment in the UHVDC transmission system,and its stable operation is related to the safety of the whole system.Oil-pressboard insulation,as the main internal insulation structure of the converter transformer,would inevitably have defects due to dampness,aging,and other conditions in the long-term operation process,which lead to excessive concentration of field intensity on the pressboard surface and generates creepage discharge.Besides,it is difficult to repair and replace the damaged pressboard,so how to detect and prevent the creepage discharge in oil-pressboard insulation has become a research hotspot.Because of the potential threat of creepage discharge in oil-pressboard insulation,and based on the characteristics of voltage bearing and moisture susceptibility of oil-pressboard insulation,the influence of DC component and moisture content on the characteristics of creepage discharge in oil-pressboard insulation and deterioration degree of pressboard was researched in this paper.Firstly,a creepage discharge test platform for oil-pressboard insulation was built to carry out the creepage discharge test in oil-pressboard insulation under different working conditions to obtain discharge parameters and image information.Secondly,a polymerization degree test platform was built,and qualitative and quantitative analysis was made on the pressboard microstructure after the test by combining with microscopic imaging technology.Moreover,a surface potential test platform was built to explore the characteristics of charge transport on the pressboard surface under different working conditions,to analyze the mechanism of creepage discharge from the perspective of charge transport.Finally,the finite element multiphysics simulation software COMSOL Multiphysics was used to analyze the characteristics of electric field distribution of needle-plate creepage discharge model through the quantitative calculation of the electric field under the applied voltage.(1)The results of the research on the influence of moisture content on the characteristics of creepage discharge show that,with the increase of moisture content in the pressboard,the initial discharge voltage of creepage discharge is greatly reduced.However,the influence on the flashover voltage is relatively small.Moreover,the development process and characteristics of the creepage discharge under oil-impregnated pressboard with different moisture content are similar,and the development process of the creepage discharge has a one-to-one corresponding relationship with the development process of discharge parameters,PRPD patterns,and statistical parameters.Besides,the higher the moisture content in the oil-impregnated pressboard,the more serious the deterioration of the pressboard by creepage discharge.Through the simulation analysis,it can be seen that the generation of white marks makes the electric field distortion more serious.Under the same voltage,the maximum value of electric field intensity increases by 513.5%,which is an important reason for the damage of the oil-pressboard insulation system.(2)The results of the research on the influence of the DC component on the characteristics of creepage discharge show that the discharge stage division under AC-DC combined voltage can be divided into four stages,which is similar to that under AC voltage.Compared with the discharge path under the pure AC voltage,the positive DC component causes the creepage discharge to develop along the pressboard surface and increases the deterioration of the pressboard.However,the negative DC component increases the probability of discharge in the oil,which leads to the development path of creepage discharge from the pressboard surface to the oil,and reduces the deterioration degree of the pressboard.According to the simulation analysis,when the applied voltage is AC voltage and DC voltage,the electric field is concentrated in the oil with a lower dielectric constant and the pressboard with higher resistivity separately.When the applied voltage is AC-DC combined voltage,the electric field distribution is between the AC voltage and DC voltage applied separately.The above research results are helpful to understand the development process and characteristics of creepage discharge in oil-pressboard insulation.And they can not only provide theoretical support and experimental basis for the online monitoring of creepage discharge and insulation status evaluation of converter transformer,but also provide a reference for the miniaturization and lightweight of oil-pressboard insulation equipment.