Performance Analysis of Converter Transformer Differential Protection under Valve-Side Winding Single-Phase Grounding and Improvement Scheme

This paper studies firstly the fault analysis method of valve-side winding single-phase grounding of converter transformer in line-commutated-converter high-voltage direct-current system. One power frequency cycle after fault is divided into 12 time periods according to the valve state, then the 12 time periods of high-voltage bridge faults are classified into 6 types based on the common characteristics, and the 12 time periods of low-voltage (LV) bridge faults are classified into 3 types. For each type of time period, the current flow path is analyzed, the corresponding circuit equation is written. By combining multiple time periods, the expressions of valve-side current and grid-side current are solved. On this basis, the differential current of converter transformer differential protection is derived and the performance of differential protection under valve-side winding single-phase grounding is studied. The research results show that when a single-phase grounding fault occurs in the valve-side winding of the LV bridge converter transformer, the second-harmonic ratio in the differential current is bigger than 15%, so the differential protection refuses to act due to mis-locking. In order to prevent differential protection from failing to act, an improved scheme of converter transformer differential protection is proposed. Through the logic coordination between the zero-sequence current unit and differential protection, the improved scheme can not only reflect various internal faults of converter transformer, but also ensure no action under the inrush current. The effectiveness of the proposed scheme is verified by simulation.