Effect of Temperature on the Internal Electric Field Distribution and Discharge Mechanism of Converter Transformer Under AC–DC Composite Voltage

To investigate the effect of temperature on the internal electric field distribution and oil streamer discharge development under ac-dc composite voltage, the dielectric parameters of insulating oil and insulating pressboard at different temperatures are measured experimentally. The nonlinearity of dielectric parameters with temperature is compared and analyzed in terms of its effect on electric field distribution. Additionally, based on the bipolar carrier transport model and drift-diffusion model, the influence of temperature on the discharge development process is simulated and studied. The results indicate that considering the nonlinear temperature dependence of dielectric parameters, the electric field strength in oil is positively correlated with the ratio of dielectric constants of insulating pressboard to that of insulating oil and negatively correlated with the ratio of conductivity. There is an opposite law for insulating pressboard. Increasing temperature exponentially enhances charge mobility by altering conductivity, which exacerbates discharge development process. The attraction of insulating pressboard to streamer discharge is positively correlated with the difference in dielectric constants between oil and pressboard, and the ionization rate of the oil is also intensified, shortening the discharge time at each stage, which indirectly affects the time and morphology of the discharge.