Mechanisms of Temperature and Electric Field Polarity Effects on Charge Dynamics in Oil-Paper Insulation

The converter transformer in the direct current transmission system is subjected to the combined effects of high temperature and electric fields of different polarities, and the space charge behavior under DC voltage stress can affect the dielectric performance of the oil-paper insulation in the transformer. This paper investigates the space charge characteristics of oil-impregnated paper under ±20 kV/mm and ±30 kV/mm conditions within the temperature range of 20~60 °C using the pulsed electro-acoustic (PEA) method. The conductivity currents of the samples are measured, and total charge, electric field distortion rate, and trap energy levels are calculated and analyzed based on the charge distribution. Additionally, a bipolar charge transport model is used to simulate the charge transport process and acoustic attenuation under varying temperatures. Experimental results indicate that the amount of injected charge and the degree of internal charge accumulation are closely related to temperature variations, the amplitude, and the polarity of the applied voltage. Temperature plays a dominant role in affecting local electric field distortion. High temperature increases the conductivity of the samples, accelerates the accumulation and dissipation of space charges, and significantly reduces the trap energy levels. The bipolar charge transport model, considering the disappearance of charge information at the upper electrode due to actual acoustic attenuation, demonstrates that the high temperature promotes heterocharge accumulation, leading to local electric field distortion, which aligns with the experimental trends. The established model can well simulate the space charge distribution of oil-paper insulation under different temperatures.