Abstract：Ningdong ± 660 kV high voltage direct current transmission projects under way will enable grounding transformer DC magnetic biasing phenomenon very near.DC bias is a type of transformer's abnormal operating state, which can endanger the safe operation of power grid.The research and development of technology of in-line capacitance method for direct current isolation transformer neutral point grounding device applied of ZnO valve block and large capacity fast-switched capacitor dual protection, which effectively blocking DC current of transformer neutral point inflows, avoiding the occurrence of transformer DC magnetic biasing appeared, ensuring stable operation of power system security.

Keyword：± 660 kV;HVDC;transformer;DC bias;