Test and Analysis on Restraining Transformer DC Bias by Changing Electric Potential of Grounding Grid

The transformers working in a power system are often affected by the DC bias. Based on fundamental of transformer DC bias when the HVDC transmission system operates in ground return mode, this paper gives a study of the influence of the HVDC (High Voltage Direct Current transmission) ground electrode current on neutral grounding transformer and analyze several measures restraining DC current through transformers neutrals which are connected to earth near the converter station of HVDC transmission system. They include putting a resistor in series between transformer neutral, putting a capacitor in series on AC transmission line and putting a capacitor in series between transformer neutral and ground. The advantages and disadvantages of these three measures is point out respectively. At the same time, a new measure named as the way of DC electric potential compensation (relate to positive electric potential compensation or negative electric potential compensation) to restrain transformer DC bias is proposed and several conclusions have been confirmed by relevant experiments. The tests show it is resultful and feasible because a majority of DC current through transformers neutrals can be counteracted in a certain extent, and there is higher utilization ratio of compensation current in positive electric potential compensation test than in negative electric potential compensation test, but latter can play a cathode protection role to grounding grid. Furthermore, the collocation of compensation device and its impact on peripheral facilities also are analyzed and demonstrated. A conclusion can be obtained that the way of DC electric potential compensation is advisable and exercisable in practice.