Abstract
Geomagnetically induced currents (GICs) and the converter modulation effect are the two main causes for dc magnetisation in power transformers. It is well known that a small dc-bias can saturate a large transformer and thereby generate high unbalanced magnetising currents, imposing a serious risk of excessive power losses and local overheating. Magnetising currents due to GIC phenomena have been studied extensively, whereas studies on converter related dc-bias are few. In particular, a discussion on loss characteristics related to converter modulation lacks. In this study, the dc-bias of common mode and differential mode in a three-phase, three-limb transformer is investigated experimentally. Additionally, to interpret the physical phenomena, the system was modelled using the finite element method. The results revealed that the power losses are significantly influenced by the dc current direction, arrangement of the structural parts, and the method of winding connection.