Studying on Measures of Restraining DC Current through Transformer Neutrals

Monopole ground return mode may cause DC current through the transformer neutrals which are connected to the ground and make the transformers DC biases. With the increase of HVDC transmission power, some transformers through which there will be more DC current going may be saturated, which brings some problems such as acutely shaking, increased noises and overheating when power system is in the normal operation. Not only the safety of transformer but also the normal operation of power system may be impacted in this case. There are three main measures to restrain DC current through transformer neutrals. They are: (1) putting a resistor in series between transformer neutral and ground, (2) putting a capacitor in series on AC transmission line and (3) putting a capacitor in series between transformer neutral and ground. Combining with the real instance of the transformers at Lingao and Dayawan nuclear power stations, these three measures to restrain the DC current caused by monopole ground return mode of HVDC transmission system are studied by building a module of power system with ground grid according to the DC current values measured at the transformer neutrals of Lingao and Dayawan nuclear power stations. The results of study show that the optimum measure to restrain DC current through transformer neutrals at Lingao and Dayawan nuclear power stations is to put a capacitor in series between transformer neutral and ground. This measure is studied further. Some main technology parameters are put forward according to the results of study