Effects on Transformer due to UHVDC Mono-polar Operation and Calculation of Ground Electrode Current

Recently years, UHVDC transmission system is paid more attention to in the field of China´s power system. It takes key part in the tactic of China electrical power development. But, many problems caused by UHVDC system, such as DC bias, corrosion of metal underground and so on. DC flows through the neutral point and causes DC bias of transformer, as well as some other problems, such as vibrations, overheating, or noise, which will harm the operation of transformer. In this paper, the influences of DC grounding current on transformer, caused by UHVDC Mono-polar operation, are introduced. And, some suggestions of DC bias solution were provided.