Characterization of Grain-Oriented Electrical Steels Under High DC Biased Conditions

Power transformers are designed to operate under pure ac excitation; however, they may be subjected to superimposed dc voltages and consequential offset currents, for example, from geomagnetically induced currents, geomagnetically induced quasi-dc currents, arising from solar activity. To determine the effects of dc offsets, a single-sheet tester was used with a digital excitation signal created with an ac waveform summed with a dc voltage addition and measured with a digital feedback control algorithm to ensure an ideal flux density waveform. Using this method, results with 300% additional dc contribution were able to be measured for results up to B
_ac
+ B
_dc
= 1.9 T. The measurements of grain-oriented 3% Si-Fe show increasing power losses and peak applied magnetic field for peak-to-peak flux densities above 3 T, with a 33% increase in power loss and 500% increase in applied magnetic field for an offset of 25%.