Measures of restraining DC current through transformer neutral point: A comprehensive survey

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. In an attempt to prevent or minimize these harmful effects, more efforts were put on the research activities on how to restrain DC current through transformer neutral point and a number of blocking devices have been studied in the past and some even installed in the neutrals of transformers during the past three years in China. Despite much progress made both numerically and experimentally, it cannot be stated that a full understanding of restraining DC current through transformer neutrals problem was achieved. Therefore, it will remain as one of the most extensive research topics of AC/DC transmission system in the coming years. In this paper, the background and introduction of DC bias is presented, which is followed by an overview of the research and progress concerning methods to evaluate the degrees of dc currents and measures of restraining DC current through transformer neutral point, including injecting reverse direct current, inserting in series with the neutral point an appropriate resistance and installing capacitors to the main transformer neutral point. Conclusions and recommendation for future directions are addressed in the end.