The total equivalent rotational inertia of power system decreases with increasing wind power penetration. It is because the mechanical parts of wind turbines based on doubly-fed induction generator (DFIG) are decoupled from their electrical parts

which will cause pressure to the main system frequency control. On the basis of operation and control characteristic of DFIG-based wind turbines

a calculation method of equivalent virtual inertia time constants is presented

and the variable parameter virtual inertia control strategy based on the DFIG-based wind turbines effective energy storage is proposed. By releasing or absorbing effective kinetic energy and providing dynamic frequency support to the grid

the proposed strategy can control wind turbine output power according to the frequency deviation and operating state. The time domain simulation is carried out on the basis of theoretical analysis

simulation results show that the proposed control strategy can provide effective frequency support to the grid under all kinds of operating conditions

and the frequency stability of power system with large wind power integration is improved. In the mean time the stability of DFIG-based wind turbines is also maintained in the process of frequency regulation. This work is supported by National Basic Research Program of China (973 Program) (No. 2012CB215105).