The existing control strategies are difficult to realize the coordinated control between the constant power control and the inertia and damping support for grid-connected virtual synchronous generators(VSGs)when the power grid frequency fluctuates. To solve this problem

a PQ control mode of VSGs is proposed to achieve the steady-state constant power operation in the grid-connected state and provide the inertia support for power systems in dynamic process. Based on the comparative analysis on the power loop model of VSG and the phase-locked loop(PLL)model based on virtual power

the power loop model is designed to be a second-order system with Zero to implement the PQ control mode of VSGs under frequency disturbance of power grids while keeping synchronizing mechanism. The smooth switchover is achieved between constant active power control and frequency droop control by the parameter design and the switch between proportional-integral(PI)link and inertia link. The constant active power control mode and the frequency droop control mode are compared and analyzed from the two aspects of input and disturbance responses. Finally

the stable operation and smooth switchover performance of two modes are simulated and experimentally verified. The results show that the steady-state performance of the PQ control mode is better than that of the frequency droop mode

while the dynamic performance is weaker than that of the frequency droop mode.