Temporal And Spatial Distribution Of Surface Charge And Evolution Process Of Surface Discharge Under High Frequency Electrical Stress

As a novel type of power transmission and transformation equipment,high frequency power transformer,which can be utilized to realize flexible transformation of AC and DC voltage,has broad application prospects in the field of gird integration of renewable energy.The gas-solid insulation in high frequency power transformer bears the high frequency voltage whose waveform is characterized by short rise time and large amplitude.Its operation condition is much worse than that of traditional power transformer.The current research shows that the surface discharge is one of the important reasons for the deterioration and failure of the gas-solid insulation.Therefore,it is necessary to carry out the research on the surface discharge according to the actual working condition of the high frequency power transformer,so as to investigate the evolution process of the patterns and the characteristic parameters for the surface discharge.Furthermore,it is significance to study the evolution mechanism of high frequency surface discharge,by exploring the characteristics of surface charge accumulation and its influence on the development of surface discharge.In this paper,a surface discharge experimental platform with needle-bar electrode was set up.Based on that platform,evolution process of the surface discharge under 20 kHz and 30 kHz high frequency sinusoidal voltage polyimide was tested.The surface discharge signal was collected by pulse current method,and the original discharge data was denoised by wavelet decomposition.Furthermore,the discharge signal was accurately extracted by windowing processing.Analysis results about the characteristic parameters of the surface discharge shows that the maximum discharge amplitude and the average discharge times increase with the development of discharge,which can reflect the development degree of the surface discharge.Based on the development regularity of surface discharge patterns and characteristic parameters,the surface discharge evolution process can be divided into the surface corona discharge stage,the surface streamer development stage and the near surface flashover stage.Based on the capacitive probe method,a two-dimensional surface charge measurement platform for thin film materials was established.The surface charge distribution at different stages under the high frequency sinusoidal voltage is measured.It is found that the negative charge accumulates on the surface of the material,and the accumulation quantities increases with the development of the surface discharge.The physicochemical properties of the surface discharge area on polyimide film were tested by SEM and FTIR for analyzing the accumulation mechanism of the surface charge.There are many physical and chemical defects on the surface of the material,which is beneficial to the continuous accumulation of surface charge.Based on the fluid dynamics model,the surface charge accumulation dynamic model of needle-rod electrode surface discharge under high frequency sinusoidal was established,which takes into account the process of ionization,recombination,migration and diffusion of charged particles in the air.The simulation results show that the dynamic distribution of surface charge in a period and after multiple cycles is consistent with the experimental results.The drift effect of charged particles in the air and the migration of charge on the material surface are the main accumulation ways of the surface charge.The different distortion effect of negative surface charge on the surface electric field in positive and negative half wave is the important reason for the polarity effect and the evolution of surface discharge.