Study on backup protection for traction transformer

At present, gas protection and current differential protection are generally used as the main protection for traction transformer in China high-speed railway. Overcurrent protection with busbar low-voltage start-up is used as the backup protection installed at HV side and LV side of transformer. This paper introduces the principle of overcurrent protection with busbar low-voltage start-up, and then analyses its advantages and disadvantages. According to the equivalent circuit model of the traction power supply system, this paper discusses the applicable condition of overcurrent protection with busbar low-voltage start-up, and analyses the misoperation possibility of the protection when there are remote end faults or large fault resistance. Based on the requirements of backup protection for traction power supply system, a backup protection scheme is introduced in this paper. In the scheme, a two-stage overcurrent protective relay is configured as the HV side backup protection for transformer, and a distance protective relay is used as the LV side backup protection.