Effect of the DC Electric Field Polarity and Amplitude on Partial Discharge of Oil–Paper Insulated Under Wedge-Shaped Electrodes

As an important piece of equipment of ultrahigh-voltage direct current, the valve side of a converter transformer contains a large dc component prone to partial discharge (PD) under the action of a slightly uneven electric field, which can be simulated using a wedge electrode. To study the PD characteristics of oil–paper insulation under a wedge-shaped electrode, a 2-D axisymmetric simulation model was established based on the bipolar-charge transport theory and the fluid dynamics drift-diffusion theory. The microscopic discharge images at nanosecond scale under different dc voltage polarities were calculated, and the change process of the space charge and electric field intensity distribution was obtained. The effects of the voltage amplitude, polarity, and dielectric constant on PD characteristics are discussed. The simulation results show that the discharge pattern and velocity exhibit a notable polarity effect. This polarity effect is related to the adsorption of the cellulose hydroxyl group to negative charge, which leads to the amplitude of the homopolar charge accumulated in negative polarity being greater than that accumulated in positive polarity under the same applied voltage intensity so that the homopolar charge has a greater barrier to the discharge. This type of adsorption is positively correlated with the difference of dielectric constant between the paperboard and insulating oil. The results of this study are of great theoretical significance for elucidating the dc discharge mechanism under wedge electrodes.