Resistor-Capacitor Combined DC Bias Protection of AC Power Grid of Jiuquan-Hunan ±800 kV Transmission Lines

DC bias was observed in transformers of the ac system surrounding Jiuquan-Hunan ±800 ultra-high voltage direct current (UHVDC) transmission lines in China, which were operating in the simulation of the unipolar-earth ground mode. Applying conventional dc bias protection methods in this case that showed certain disadvantages including large time and calculation resource demand, this paper proposed to use an RC combined method to control the dc bias in this ac system. By determining appropriate capacities of resistor and capacitor based on the requirement of insulation and zero sequence current, it is able to stabilize a system under a single-phase ground fault with a dc suppression device installed in the neutral point. In addition, after analyzing the regularity of the dc distribution in the ac system with different substation connections, we also proposed an implementation strategy of dc suppression in power grid. The protection method was further tested in a numerical dc power grid model and verified by field experiments. The result showed that in the most serious case of the model system, when a current of 5000 A flows into the ground, the dc level of the neutral point of substation under protection by the proposed method is well controlled. This indicates that the proposed method is applicable to the dc bias protection of the dc-ac system operating at the highest voltage ever worked.