A new algorithm to identify magnetizing inrush of converter transformers in HVDC transmission project

This paper proposes an algorithm to detect the magnetizing inrush of two double-winding converter transformers in bipolar high voltage direct current (HVDC) transmission project by analyzing the waveform characteristics of the resultant current in the grid side. Delivery test of transformer may remain the different polarity of remanent flux in each converter transformer, which causes the inrush directions different when the two double-winding converter transformers are energized at the first time. So that the resultant current at the grid side is close to sinusoid and this leads to the mal-operation of differential protection. In this paper, the generalized nonsaturation zone of inrush current is proposed and the virtual current in the generalized nonsaturation zone is calculated by subtracting the DC offset in the inrush. A normal sinusoidal waveform is structured according to the virtual current, and the waveform coefficient between the structured current and actual current is calculated. The waveform coefficients are used to distinguish magnetizing inrush from fault current. The extreme cases simulated by RTDS (Real Time Digital Simulator) and the field data in China are analyzed and tested. The test results indicate that the proposed algorithm successfully detects magnetizing inrush during the transformers energized at the first time and other times. It is ensured that the differential protection is reliably blocked during inrush and quickly enabled during the short-circuit fault and even a small turn-to-turn fault.