Reactive power demand of transformers with DC bias

Transformers in power electronic circuits are inadvertently but frequently exposed to direct current (DC) bias due to circuit imperfections. Such undesirable bias currents occur, for example, in power system transformers near high voltage DC terminals or in the event of geomagnetically induced currents (GICs) or nuclear explosions. An experimental study of three-phase transformers with three legs explores the reactive power demand as a function of the imbalance of the quasi-direct fluxes within the three legs and the magnetic construction of the tank. Transformers of various ratings have been studied by measuring the voltages and currents, and computing the kVAr demand with an accurate and newly developed data acquisition network consisting of A/D converter and a personal computer. The reasons causing detrimental effects are analyzed from an electromagnetic theory point of view, thereafter, methods for suppression/cancellation and mitigation of the reactive power demand due to DC biases are proposed. Most of the methods were tried out in the laboratory. The results are discussed and highlighted with figures