Study on the Network Suppressing Measures of the DC Bias Current in AC Power System

Large current injected into the earth from grounding pole of HYDC systems leads to the change in the earth surface potential and the DC currents in ac systems. The usually suppressing measures are just proposed based on single substation, which may cause increase of DC current in other substations in dense power grid area. In this paper, according to the accurate calculation of the DC currents of Shanghai area power grid, several suppressing measures of the DC bias current in ac power system are proposed and accessed. For the current topology of Shanghai power grid, the DC current is less serious when grounding current is reduced by using metal return mode or limiting system load. Besides, the operation mode, the location of new substations and position of grounding poles can be designed to reduce the DC currents. Connecting grounding poles in parallel is also proved a reliable way to reduce DC bias current.