Study on Development Characteristics of Partial Discharge in Oil-Pressboard Insulation under Constant DC Voltage

The converter transformer is the core equipment of HVDC transmission system, the valve-side winding of which needs to withstand DC voltage. Partial discharge is one of the main threats to the safe operation of converter transformer, yet the characteristics of partial discharge development of the oil-pressboard insulations under constant DC voltage are insufficiently understood. In order to better understand the partial discharge characteristics of the oil-pressboard insulation under DC voltage and provide deeper theoretical support for insulation diagnosis of converter transformers, development characteristics including the time-varying tendency of discharge magnitude and repetition rate of partial discharge in oil-pressboard insulation under constant positive and negative DC voltage were studied. The results indicate that the development of partial discharge in a needle-plane oil-pressboard insulation model under constant DC voltage has three stages: the intensive discharging stage, the silent-burst stage, and the breakdown stage. Throughout all stages, the partial discharge magnitude and repetition rate first decrease and increase afterwards. At the silent-burst stage, the partial discharge appears in the form of a "cluster" with very large magnitude and repetition rate. Each cluster exists for tens of seconds but with at a very long interval with each other. Further analysis shows that the repeated accumulation and dissipation of free charges on the surface of the pressboard cause the above phenomena. Negative charges are easy to accumulate and difficult to dissipate under the same voltage amplitude compared to positive charges, leading to a weaker actual electric field at the needle tip and thus partial discharges under negative DC voltage with a lower magnitude and longer interval.