Under a large disturbance

similar to synchronous generators(SGs)

the transient instability may occur on the voltage-source-type droop-controlled inverters. Moreover

the inverter will turn into a current source since its current saturates

resulting in a more complicated synchronous stability problem. This paper studies on the single-inverter infinite-bus system. The virtual power angle is defined to obtain the virtual power angle characteristics with and without current saturation

which is used to analyze the synchronous stability mechanism of the inverter as well as the instability process. The influences of the fault-clearing time on the virtual power angle stability are also analyzed. And this method is extended to the transient stability analysis of the virtual synchronous machine. Finally

the rationality of the instability analysis method is verified via simulations under MATLAB/Simulink. This work is supported by National Natural Science Foundation of China(No. 51577168)and National High Technology Research and Development Program of China(863 Program)(No. 2015AA050202).