In this paper

through analysis of two flashover faults on 10 kV bus equipment in certain 110 kV urban substation

the fault reason is found.If there are stronger coupling connections through circuit between two ineffective grounding system

when the single phase to ground fault takes place in a system

the produced zero-sequence voltage will cause a couple voltage

this voltage will influence another system which has an autotracking Petersen-coil compensated.The results of the influence are different because the modes of Petersen-coil compensated are different.Adopting the mode of excessive compensation can cause series resonance.And the measures to avoid the dangerous resonance over-voltage also presented in this paper.