Characteristics of partial discharge in oil-paper insulated needle plate electrode model under composite AC-DC voltage

Converter transformers are the core device in ultrahigh voltage direct current (UHVDC) transmission systems, and the oil-paper insulation at its rectifying valve side deals with the composite AC-DC voltage. A detailed investigation of the partial discharge (PD) mechanism in the oil-paper insulation under the composite AC-DC voltage is, therefore, very important. The oilpaper insulated needle plate electrode simulation model for the electric field and charge distribution can be established using hydrodynamic drift-diffusion theory and bipolar charge carrier theory, and the simulation results can reflect the morphological changes of PD. The development characteristics of PD under composite AC-DC voltage are researched. In addition, the effects of simulation time, the AC-DC component proportion, and model parameters on PD are also explored. Results indicate that differences in applied voltage, interface charge, and geometry structure can cause changes in the electric field strength, which consequently influences the PD process in the simulation model. Oil-immersed paperboard is a type of liquid-immersed dielectric (LID). When the LID is arranged in perpendicular, only the positive streamers have narrow channels oriented toward the LID. When the LID is arranged in parallel, the PDs are in the form of creeping discharges and the negative streamers are more dispersed. The development characteristics of PD and its influencing factors reported here may provide theoretical and simulation support to explain the physical mechanism of PD at the rectifying valve side of the converter transformer.