Abstract: When the power grid fails,the fault current is large and the power distribution among three phases is not equal,which results in the unbalanced voltage of three phases and affects the stability of the system. The application of static synchronous compensator（STATCOM）can emit or absorb the active and reactive current which meet the requirements of the system and improve the operation safety of the power grid.Firstly, the characteristics of the sequence components of voltage and current are analyzed, and the operating characteristics of the system under symmetric and asymmetric faults are discussed. Secondly, through simulation, a distribution network is set to have a single-phase grounding fault,a two-phase short-circuit fault and a three-phase grounding failure respectively,and the operation of the system is observed. During the failure period,the positive sequence current is controlled by adjusting the positive sequence voltage of STATCOM,so that STATCOM can provide certain active and reactive power support,keep the system voltage stable,and maintain the safe operation of the system.