Interface Charge Overshoot in Vegetable Oil–Paper Insulation Under DC Voltage

Oil-paper insulation is the primary insulation material utilized in transformers, including converter transformer. As an environmentally friendly liquid dielectric, vegetable insulating oil is considered a good substitute for mineral insulating oil, but it still lacks application in converter transformers. Converter transformers mostly use multiple layers of insulating paper and insulating oil, leading to the existence of oil-paper interfaces. In the vegetable oil-paper sample, there is an overshoot phenomenon of charge accumulation at the oil-paper interface under the applied dc voltages. Owing to discrepancy in the mobility of injected charges and ionization charges, the initial equilibrium value of the interface charge is 12%–36% higher than the final equilibrium value. Due to the existence of the interface barrier, the interface charges remain at the high initial equilibrium value for a long time before decreasing to the final equilibrium value, which may potentially lead to the risk of partial discharge. Mitigating the fiber defects in the insulating paper aids in reducing the interface barrier and enabling charges that migrate to the vicinity of the oil-paper interface to cross the interface easily and recombine with the heteropolar charge on the other side, which not only facilitates the faster attainment of the final equilibrium value of interface charge under dc voltage but also enables the rapid dissipation of the accumulated charge shortly after the removal of the applied voltage. This characteristic is beneficial to the operation stability of converter transformers, especially in the condition of polarity inversion.