Evolution Characteristics of Surface Discharge along Polyimide under High-Frequency Electric Stress

In the insulation design of solid-state transformers (SSTs), polyimide (PI) film has been widely used. The most significant reason for the insulation failure of SSTs is surface discharge. This paper proposes a two-dimensional fluid-kinetic plasma simulation model for a needle plate electrode experimental setup using COMSOL Multiphysics software to address this issue. We analyzed the surface discharge development process under high-frequency electric stress by coupling plasma chemical reaction, particle transport equation, and Poisson equation. We developed a simplified model that validates the space charge density from the fluid-kinetic plasma model. Firstly, at room temperature, applied a frequency of 30kHz and 10kV with a 200V/s short-term rapid boosting method. Record the discharge starting voltage and the flashover voltage along the surface of PI. The discharge signal was measured using the ETS-93686 high-frequency pulsed current sensor. The applied voltage and discharge signal data were collected using a Yokogawa DL6154 four-channel oscilloscope. The real-time collected data is sent to the computer for storage through the USB serial bus and LabVIEW's data acquisition storage system. The development of surface discharge is continuously photographed and recorded using a high-speed camera above the needle electrode. The experimental results show that the surface discharge carbonizes the dielectric surface and increases the PI surface conductivity. When the voltage frequency is 50Hz, the surface discharge is dendritic, and the discharges appear on both rising and falling edges during a positive half-cycle at this stage, the maximum discharge amplitude is 0.013V, and the number of discharges is 2 180. At high-frequency with the insulation's ageing, there is an increase in the discharge amplitude from 0.009V to 0.015V, and the total number of discharges is 3 610 recorded in the final stage. To reveal temporal-spatial parameters a fluid-kinetic plasma simulation model was used. This study constructed a fluid-kinetic numerical simulation model by coupling the fluid model with the kinetic model. In addition, the model solves the complex chemical reactions logically to tackle the numerical oscillation problem. The simplified model is predicated on the maintained current carried by charged carriers. Microscopic quantities such as space charge, electron density, reaction rates, and electron temperature are obtained by simulation. The PI surface discharge trajectory at high frequency is linear, and the electron density reaches up to 1.1×1019 m-3 at 176×10-6 s. The following conclusions can be drawn：(1)The development process of surface discharge can be divided into four stages. At each stage, the electron density exhibits a linearized characteristic. In this study, through phase resolved partial discharge (PRPD) analysis, it was found that the number of discharges and the discharge amplitude were larger in the positive half cycle. (2) The electron density and temperature increase with the applied voltage amplitude and frequency increase, and the distribution of positive and negative ion density and electron density is similar. (3) The space charge density distribution of surface discharge can be obtained using the simplified model, and the results are consistent with the self-consistent plasma model. (4) With the development of the surface discharge process, the electric field's influence gradually increases the reaction rate. In the final stage of the discharge, the total reaction rate can reach 1 200mol/(m3·s), and its distribution trend is similar to the electron distribution which indicates the correct chemical reaction in the background gas. © 2023 Chinese Machine Press. All rights reserved.