Abstract: Abstract:when the No. 1 grounding transformer of a 110kV substation is grounded with arc suppression coil, a phase groun ding fault occurs in the 10kV Shannan line, and the zero sequence protection of No. 1 grounding transformer and 10kV Shannan li ne is started. When the 10kV small resistance is not put into operation, the protection of zero sequence section I and zero sequence section II at the high voltage side of No. 1 grounding transformer acts, the 10kV section 010 circuit breaker, the outlet of 011 circ uit breaker at the low voltage side of No. 1 Main Transformer trips, the 10kV section I bus loses voltage and many 10kV lines in t he urban area are shut down. This paper aims at the zero sequence protection action of No. 1 grounding transformer and 10kV Sha nnan line; This paper analyzes the capacity selection and reliability of capacity regulating arc suppression coil, and puts forward i mprovement measures and suggestions on how to prevent similar accidents, so as to improve the reliability of primary equipment operation.