Simulation Of Carrier Migration Process In Oil Paper Insulation Under High Electric Field

High Voltage Direct Current(HVDC)transmission technology is related to our country’s energy strategy and national security strategy.Converter transformer is one of the core equipment of HVDC transmission system.Its valve-side winding withstands DC voltage during operation,which causes the accumulation of space charge in the oil-paper insulation system,thus leading to local electric field distortion and insulation deterioration,which affects the safe and stable operation of the system.Therefore,the simulation analysis of charge distribution and electric field distribution of oil-paper insulation can provide guidance for the design of oil-paper composite insulation in converter transformers,and has important significance for the safe and stable operation of HVDC system.In this paper,two common oil-paper insulation structures,such as insulating pressboard+oil gap and double-layer insulating paper,are taken as the research object.Considering the effect of strong electric field in different media generated by electric field distribution in oil paper insulation,and the effect of electrode limitation on charge migration,impurity ionization and interface barrier limitation under strong electric field,the simulation model suitable for solid-liquid structure and solid-solid structure is built by combining the bipolar carrier model and ion current model.The validity of the simulation model is verified by the electro-acoustic pulse method.Firstly,in terms of interfacial charge accumulation,the ionization process of impurities is integrated into the bipolar carrier model.Based on the conversion relationship between conductance current and Poole-Frankel effect at the interface,an interfacial barrier limiting current model is constructed to provide a mechanism explanation for the source of interfacial charge accumulation.Secondly,in order to fit the characteristics of the thick paperboard-oil gap structure under DC voltage,such as fewer traps in the oil,low electric field,and less charge,the ion current model which is more suitable for the physical reality is used to simulate the carrier transport mechanism in the oil.Combined with the interface barrier limiting current model,the space charge simulation model for the solid-liquid structure is obtained.The reliability of the model is proved through experiments.Finally,in order to simulate the characteristics of the space charge distribution in the double-layer insulated paper,a simulation model for the double-layer identical media was constructed by combining the bipolar carrier model with the rapid charge migration process.Based on the model,the influence of the microscopic migration process of carriers on the space charge accumulation and electric field distribution mechanism of the double-layer insulated paper structure was analyzed.The work of this paper is helpful to assist the experimental measurement of space charge distribution in oil paper insulation rule for further explanation,and then take corresponding measures,the study for the optimization of converter transformer oilpaper insulation design provides guidance.