Surface Discharge Characteristics Of Polyimide Films Subjected To High Frequency Electrical Stress

High Frequency Power Transformer(HFPT)is a new type of Power transmission and transformation equipment,which can realize flexible conversion of AC and DC voltage.It has obvious advantages in voltage conversion and energy transmission,and has broad application prospects in distributed energy grid connection and other fields.Gas-solid insulation is the most widely used high-frequency power transformer,which has the advantages of being able to operate under a variety of harsh conditions,compact structure,and having a large value,fast-rising high-frequency voltage waveform.According to the existing research,the key reason for the failure or deterioration of the gas-solid insulation system of high-frequency power transformers is the discharge along the surface,which should be based on the actual working conditions of high-frequency power transformers,to explore in depth the formation process and causes of the phenomenon of power generation along the surface,to study the general law of the change of the characteristic parameters of discharge during the process of power generation along the surface,considering that the phenomenon of power generation along the surface is largely caused by the accumulation of surface charge.It is necessary to accelerate the research on the formation mechanism and causes of the along-surface discharge phenomenon at high frequencies.In this paper,we apply high-frequency sinusoidal voltage to polyimide using a pin-plate electrode based high-frequency along-surface discharge experimental platform to analyze its along-surface discharge characteristics,aiming to analyze the along-surface discharge characteristics of gas-solid insulation surface subjected to high-frequency sinusoidal electrical stress.Then,based on the fixed-point windowing technique,the discharge signal is effectively extracted to obtain the discharge spectrum and the discharge characteristic parameters,so as to analyze the characteristics of the discharge along the surface of polyimide.In order to analyze the differences of polyimide along-surface discharge characteristics with different actual working conditions,this paper completes the simulation modeling of high-frequency voltage needle-plate electrode on the basis of hydrodynamic model,and adds the material body conduction model and surface charge migration model to the simulation model,which helps to analyze the influence of air charged particles diffusion,migration,loading and ionization process on the polyimide along-surface discharge characteristics.According to the simulation results,it is found that the experimental results and simulation results of surface charge discharge development process and charge dynamic distribution in single cycle and multi-cycle are consistent,and the surface charge accumulation paths include both material surface charge migration and drift of charged particles in air.In the process of simulating the change of along-surface electric field for the accumulation of polar charges on the material surface,it is found that the negative and positive half-wave along-surface field strengths are affected by the effect of negative polar surface charge distortion for two reasons:first,the along-surface discharge has evolved into a flashover;second,the polarity effect exists within the phase spectrum of the along-surface discharge.The experimental and simulation results show that the along-surface discharge process of polyimide film under the action of high-frequency sinusoidal voltage includes three stages,which are corona discharge stage,flow injection discharge stage and along-surface flashover stage,and the along-surface discharge pattern of polyimide film in each stage has certain differences.The average number of discharges and the maximum discharge amplitude are generally used as the evaluation index of the development degree of along-surface discharge.The discharge along the surface shows a strong polarity effect,and the number of positive half-cycles and the discharge amplitude are larger than the negative half-cycles.The damage to the insulation surface caused by the discharge process leads to electric field distortion,which has a certain influence on the development process of discharge along the surface,and the average number of discharges and discharge amplitude show a continuous increasing trend in the development process of discharge along the surface.In addition,we discuss whether the electric field distribution along the surface is affected by the accumulation of surface charge,and thus we can obtain the causes of the polarity effect of negative and positive half-wave formation.