Study on Restraining DC Neutral Current of Transformer During HVDC Monopolar Operation

HVDC transmission system operating in ground return mode often affects the neighboring ac power system seriously. This paper presented some field experimental results, including audible noise, voltage harmonics, and oscillation of the transformer while dc current passes through the neutral point. The relationship between the dc neutral current and those effects were discovered clearly by analyzing these data. Based on the analysis of the characteristics of the dc current distributed under ground and in the ac network, some useful measures to decrease or block the dc current through the neutral point of the transformer, such as the injection of reverse dc current, blocking capacitor, and series resistor are compared. And the series resistor method is chosen after carefully study of its feasibility in the ac power system. The parameters as well as the protection method of the resistor were selected by implementing several large current experiments. Finally, field experiments were carried out to observe the effect of reducing the neutral current with a series resistor device. Experimental results show that series resistor method is a simple structure, low cost, and of high reliability, which can restrain the neutral current effectively.