Power converse system (PCS) has weak over-current capability under fault. The virtual impedance strategy can limit the fault current

but it will deeply affect the transient synchronization instability of PCS under charge discharge mode. To solve this problem

this paper first reveals the power angle instability mechanism of PCS in the charge and discharge mode. It is concluded that the acceleration instability pattern similar to the traditional synchronous machine will appear in the PCS discharge mode

but the deceleration instability pattern will appear in the charge mode. Then

it is found that the virtual impedance ratio has the opposite effect on the two types of power angle instability pattern

which indicates that the fixed virtual impedance ratio can not ensure that PCS has good transient stability margin under both operating modes. Therefore

a virtual impedance strategy with variable impedance ratio is proposed. According to the change of angular velocity under PCS fault

the virtual impedance ratio is adaptively selected to enhance the transient stability margin of PCS in charge discharge mode. Finally

simulation and experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed strategy.