Harmonics and Reactive Power of Power Transformers with DC Bias

The online transformers are sometimes affected by dc bias caused by geomagnetically induced currents (GIC) or HVDC/UHVDC transmission system in monopolar grounding-return mode. With dc bias, the transformer is prone to saturate and rich harmonics are introduced, on the other hand, the reactive power consumption is more different from that without dc bias. Based on the piecewise model of iron core, the harmonics are deduced. It can be found that both odd and even harmonics are functions of the biased current. After mathematical models which both electrical and magnetic circuits are involved are built, theory analysis are given in this paper. The reactive power increases drastically as the dc current enlarges. Simulations are done to confirm the validity. It provides a guide for the filter mounting and voltage control strategy drawing.