Preventing maloperation scheme for saturation protection of converter transformers based on waveform prediction of magnetizing inrush current

Abstract

In high-voltage direct current transmission systems, the unloaded energizations of converter transformers may cause magnetizing inrush currents with high amplitude and slow decaying, as well as the zero-sequence inrush current in the neutral line. This may cause the maloperation of saturation protection based on the amplitude of the neutral line current. Aiming at this problem, a scheme for preventing the maloperation of converter transformer saturation protection based on the waveform prediction of magnetizing inrush current is proposed. The proposed scheme utilizes the differences between the mathematical models of pure magnetizing inrush and magnetizing inrush superimposed with DC bias. Based on the mathematical analytical expression of magnetizing inrush current during the transformer unloaded energization, the complete waveform of the second cycle of phase current is predicted by using the actual measured values of the phase current within 1¼ cycles. The deviation between the predicted and measured peak values of the phase current of the second cycle is calculated. On this base, a modification coefficient is constructed to modify the measured current in the neutral line, which eliminates the influence of magnetizing inrush and prevents the maloperation of saturation protection. Simulation results indicate that the proposed scheme can separate the influences of magnetizing inrush and DC bias on the saturation protection, effectively preventing the maloperation of saturation protection caused by magnetizing inrush.