Oil-paper capacitive transformer bushings are widely used. However, partial discharge (PD) is a serious threat to safe and stable operation of power system. In order to study the variation law of PD process with condenser foil layer damage and clarify the PD development stage, this paper establishes a defect bushing PD model to analyze the development law of parameters. Based on fluid drift diffusion theory and solid bipolar charge transfer theory, a simulation model for the surface PD process of the end screen defect needle plate is established, which combines the changes in voltage, charge morphology, PD duration and the charge density distribution at the oil-paper interface. Firstly, build a test sample bushing with process defect. According to a set of 220 kV test apparatus to test the relationship between the voltage level and PD parameters, capacitance, and discharge carbonization trace. Then based on the hydrodynamic drift-diffusion model and the bipolar charge transport model, a needle-plane surface PD simulation model was built to simulate the process and space charge distribution of PD. Finally, with the experimental phenomenon and simulation results, the 3 stages PD process and space charge distribution at different applied voltage are provided. Simulation and experiment results show that the bushing in the AC electric field causes the capacitor core plate to generate static electricity and causes electric field distortion, which results in charge migration and forming streamer. Tip defective foil layer is at extremely inhomogeneous electric field, the needle tip part of the high voltage positive pole, collision ionization will occur first at the tip, collision formation of positive ion density increases to form streamer, negative ions at the tip of the compound. Through experimental verification in needle-plane surface discharge simulation, it can be assumed that the process of PD is as follows: (1) At the initial stage, corona discharge at the tip of the foil layer can be observed. At this time the electric field strength near the tip of the foil layer is the largest, which releases of free electrons and molecular ionization. But the collision energy did not reach the electron avalanche. (2) At the development stage, the collisional ionization at the tip acquires sufficient energy to move and develop continuously under the action of the polarization swimming dynamics. Due to the local high temperature and high field strength, the moisture and gas within the insulating oil paper form bubbles and adheres to the oil paper interface, and the gas in its products also adheres to the interface and forms small bridges, leading to the streamer towards to the paper surface and breakdown along the surface. (3) At the damage stage, surface PD affect the space charge distribution within the oil-paper and start to breakdown discharge phenomenon on paper surface. High temperature and high field strength from discharge lead to paper insulation failure, resulting in carbonization traces. The following conclusions can be drawn from the simulation analysis: (1) By the process of physical parameter changes in PD experiments, from corona PD to the development of surface discharge, the amount of PD increases with the increasing voltage. At 60 kV, oil paper bubbles precipitate, causing a continuous increase in PD. The insulation paper cellulose breaks and forms carbonization channels, increasing conductivity and causing a further decrease in PD capacity. (2) Based on diffusion drift charge transfer theory and bipolar charge transfer theory to simulate surface PD, the tip PD process formed by the damaged edge of the bushing condenser foil layer is obtained. There are mainly three stages. (3) Based on carbonization traces and the simulation process of PD, the voltage level will cause changes in the PD process, morphology, and duration. When the voltage level is less than 40 kV, the PD is in the first and second stages, and when the voltage level is higher than 40 kV, it goes through three stages. The larger the voltage amplitude, the more branches of the electrical tree, the shorter the discharge time, and the more charges absorbed by the oil-paper interface. © 2024 China Machine Press. All rights reserved.
