Study of the influence of dc currents in the ground on transformers and its suppression methods

When dc transmission system operates in monopole-ground mode, a huge dc current flows through the ground, which leads to dc magnetic bias in the transformers with ground neutrals near the dc ground electrode. A small resistor or a neutral blocking device (NBD) can be connected in series between the transformer neutral and the ground to suppress the ground dc currents flowing into the transformer. This paper introduces a type of reverse injecting device (RID). It does not change the transformer's original neutral ground way. It produces automatically a reverse dc current with the same magnitude to counteract the ground one. The separated current factor is important in the RID's parameter design. (5 pages)