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HVDC power systems operating in ground-return mode produce DC voltages around the electrodes. During Monopolar operations of the HVDC electrode, DC biases of the transformers caused by the DC current through neutral points may damage the transformers and the whole network. The noises and the neutral point DC currents of the main transformers with earthed neutral points in Yihe substation and LN power plant were measured and recorded during the commissioning of the HVDC system from Three Gorges power station to Guangdong power network. Compared to injecting reverse direct current and inserting in series with the neutral point an appropriate resistance, the strategy of installing capacitors to the main transformer neutral point is more effective. According to the DC current values measured at the transformer neutrals of LN power station, this method was studied based on its corresponding module of power system with ground grid proposed. Simulations have been carried out and some main technology parameters were put forward according to the research results.