No-load losses in transformer under overexcitation/inrush-current conditions: tests and a new model

Tests were conducted on several transformers rated at 100 kVA or less and on a power transformer rated 370 MVA in an effort to characterize the no-load losses and magnetizing resistance for transformers subjected to overexcitation and inrush current. Analysis of the results revealed that the magnetizing resistance changes as a function of the peak magnetization flux or the amplitude of the magnetic field. A new model of the instantaneous magnetizing resistance (IMR) as a function of the instantaneous flux has been developed and its dynamic use in the ElectroMagnetic Transients Program (EMTP) allows the form of the hysteresis cycle and the mean losses in overexcitation to be reproduced with a high degree of accuracy. The same model also accounts for losses due to the harmonics superimposed on the fundamental. The results showed that the IMR calculated under inrush current conditions is higher than that in overexcitation conditions.