Influence Of Dc Bias On AC-DC Hybrid System Operation And Power System Active Response Countermeasure Research

In recent years, domestic and international experience shows that increasingly linked AC and DC hybrid power system also face new threats. The threat comes from the DC bias phenomenon. When DC transmission system is under monopolar-ground operation mode, huge DC current flow into the transformer windings by AC and DC coupling path. It affects the normal operation of the transformer and is extremely detrimental to the safe operation of power system. At present, domestic and foreign research focus on the affection of DC bias on transformer and suppression measures from the point of view of transformers. This paper prospectively studied the effects of DC bias on the AC-DC hybrid system, and proposed DC bias suppression measures from the point of view of power system operation.This paper analyzes how DC bias influence the operation of AC and DC hybrid power systems.The influence of DC bias on the transformer noumenon is described, issues related to occupied reactive capacity are discussed. Moreover, mechanism of the increasing risk of commutation failure cuaused by DC bias is analysised and verificated by PSACD simulation.Based on the concept of self- resistance and mutual-resistance of ground electrode and AC substation grounding grid, AC and DC coupling network equivalent model is built. The PSCAD / EMTDC model of Yichang area hybrid system is established according to the actual parameters. The neutral current of focused substations in Yichang area is simulated and compared to the operation data of Hubei power grid.A new DC bias suppression measure from the point of view of system coordinated operation is proposed. By transferring the transmission power of one unipole- earth operation DC system to the other DC line, making it in the unbalanced operation, the transformer neutral current throughout the region is reduced. Taken Yichang area as an example, amplitude of transformer neutral point current before and after power transfer is simulated based on PSCAD / EMTDC model of Yichang area hybrid system. And PSASP model of Yichang area is established. Optimization of generator output after transfering DC power deliver is calculated using Matlab programming. Last but not the least, node voltage, line overloading and transient stability of the grid after changing the DC system operating mode has been checked to verify the effectiveness of the proposed measure.Another suppression measure of changing the neutral grounding configuration have been proposed. The two transformers is made to be simultaneously grounded in the station of which neutral bias current exceed to reduce the neutral current for each transformer. Also, taken Yichang area as an example, amplitude of transformer neutral point current after optimizing neutral grounding configuration is calculated and checked based on PSCAD / EMTDC model of Yichang area hybrid system. Finally, the setting values and sensitivity of zero sequence current protection before and after the change of transformer grounding mode have been calculated and compared.