Study on Autotransformer’s Low Voltage Side Over-current Protection with Circulating Current

500kV autotransformer has split type and integrated type. The current transformer (CT) position of low voltage side is different for these two types of autotransformer. This paper analyzes circulating current in autotransformer’s delta winding when single-phase grounding fault takes place on high voltage side. Protection setting method considering circulating current in delta winding is proposed for split type autotransformer. Simulation results verify the position of two types of autotransformer’s CT influences the circulation current in low voltage side protection. Protection setting for an autotransformer used in Shandong electric power grid tests the correctness of the over-current protection setting with circulating current proposed in this paper.