Re-synchronization Capability Analysis of Virtual Synchronous Generators in Microgrids

Since distributed energy resources (DERs) are widely connected to the system resulting in the reduced inertia of microgrid, converters are controlled as virtual synchronous generators (VSGs) to emulate the dynamic response of traditional synchronous generators. During transient period, like grounded faults, virtual power angle (VPA) of VSGs may go beyond the unstable equilibrium point (UEP), which probably leads to the reaction of the protection devices and cause the disconnection of VSGs. This would further deteriorate the system stability. However, even the VPA goes beyond the UEP, whether the system continues to operate under unstable condition is still unclear. In this paper, the re-synchronization phenomenon of VSGs is found and the physical mechanism is illustrated through P-δ curve. It is concluded that it is more likely for VSGs to return to stable state after fault clearance even the VPA goes beyond UEP, due to the large damping coefficient of VSGs and the short transmission line in microgrids. Moreover, reactive power control loop has an unnegligible effect on the re-synchronization process. These findings are validated by simulations in PSCAD/EMTDC.