Analysis of Power Transformer Magnetizing Inrush and Overexcitation Conditions in Case of Overvoltages due to Line-to-Ground Fault

In this paper, simulation of power transformer operation during external phase-to-ground faults are presented, with special attention paid to the overvoltage, magnetizing inrush and overexcitation conditions due to such faults. The motivation for conducted studies were recorded cases of maloperation of transformer differential protection during external faults. The paper starts with initial analysis of the recorded cases. Then developed model of the network with protected transformer is described and verified by simulations with MATLAB/Simulink software. Theoretical and simulation investigations lead to observations related to the level of overvoltage and harmonic content in the currents during considered faults, as well as reasons of protection wrong operation. Discussion about the possibility of harmonic blocking application concludes the paper.