With the increasing penetration of wind power

using virtual inertial control technology to address the frequency security problem under frequency accident conditions is an inevitable demand of power grids in the future. The newly-presented topic of the dynamic frequency characteristic incorporating virtual inertial control technology of wind turbines will be confronted by the power system. The quantitative representation of the equivalent virtual inertial time (HeqWF) constant of the doubly fed induction generator (DFIG) based wind farm is researched. HeqWF is defined and its physical significance is clarified while the relationship between solving HeqWF and studying the dynamic frequency characteristics of power grid is analyzed. A simplified model of DFIG and a wind farm model incorporating the virtual inertial control are developed and the analytical solutions of HeqWF in the frequency and time domains are solved. The correctness and accuracy of HeqWF are verified by comparing calculated and simulated values

the time-varying characteristic of HeqWF is explored and the resulting mechanism is investigated. This work is supported by National Natural Science Foundation of China (No. 51190104).