Review of transient control strategies for grid-forming converters under grid voltage sag conditions

As a large amount of renewable energy is integrated into the grid via power electronic converters，the power system is faced with challenges，such as reduced inertia，diminished disturbance resistance，and declining transient support capabilities. The stability of the renewable energy interconnected with the grid and the capability of supporting the grid are enhanced by the autonomous synchronization and active support capacity of grid-forming converters. The grid-connected control strategy of grid-forming converters is closely related to the stability of the device and system under transient voltage disturbances. This paper focuses on disturbances caused by grid voltage sags and reviews the transient control strategies of grid-forming converters. Firstly，current synchronization control strategies of grid-forming converters are summarized. Next，the mechanism of overcurrent generation in grid-forming converters is revealed，and the overcurrent behavior and support characteristics of converters during transient periods are analyzed. Then，transient control strategies are classified into two major types，and the principles，research status，advantages and disadvantages of these two types of control strategies are comparatively discussed. Finally，future research directions for grid-connected issues of grid-forming converters in high-penetration renewable energy power systems are prospected.