High penetration of wind power in power grid would seriously affect the frequency safety. Synchronous generator can adjust system power and stabilize grid voltage and grid frequency. The inverter can be equivalently regarded to be synchronous generator with the help of super capacitor. Not only the wind turbines can participate in the frequency modulation and voltage regulation, but also the inertia of the system will be improved. On the basis of the analysis of virtual synchronous generator

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... G), a method of calculating the capacity of energy storage system (ESS) is proposed and a control strategy of the bi-directional converter is researched to improve the virtual inertia of wind turbines and suppress the fluctuation of power and frequency when load or wind speed is changed. This method makes the wind farm behave like a virtual synchronous generator, and effectively enhance the grid-connected performance of wind turbines. The simulation results based on Simulink show the feasibility of the control strategy.