Traditional fixed frequency-droop control strategy cannot be adapted to available reserve capacity of doubly-fed induction generators with the change of wind speed. If the droop coefficient is too small

the generators will respond excessively

a double fall will occur to the system frequency and the rotor speed cannot be restored. On the contrary

if the droop coefficient is set too large

it will not make full use of the frequency response capability of wind turbines. In order to solve this problem

a variable droop control strategy is proposed. At low or middle wind speeds

the sum of available mechanical load shedding capacity and kinetic energy is taken as the available reserve capacity of wind turbines to adjust the droop coefficients

while at high wind speeds

the actual available mechanical capacity is taken to adjust the droop coefficients. The effectiveness of the proposed variable droop control strategy in avoiding the over responding of wind turbine and enhancing the frequency response ability is verified by simulation results. This work is supported by National Natural Science Foundation of China (No. 51377047) and Fundamental Research Funds for the Central Universities (No. 2013B27614).