Frequency drift of sub-synchronous oscillations caused by the DFIG wind farms when the wind speed varies

This paper examines the frequency drift of sub-synchronous oscillations (SSOs) caused by the DFIG wind farms when the wind speed varies. The examination is carried out by applying the analysis about the open-loop modal coupling, a newly appeared concept to investigate power system oscillations. Analysis is presented in the paper to reveal that when the wind speed varies, the open-loop electromechanical oscillation mode of the DFIG moves on the complex plane. When it moves close to an open-loop SSO mode of the rest of power system, the open-loop modal coupling occurs to lead to the damping degradation of the SSOs caused by the DFIG. Thus, the variation of wind speed may possibly cause the open-loop modal coupling between the DFIG's electromechanical oscillation mode and different open-loop SSO modes of the power system. Consequently, poorly or even negatively damped SSOs happen at different SSO frequency. Therefore, the paper provides a better understanding on and deeper insight into the mechanism about how the change of wind speed causes the frequency drift of the SSOs from the standpoint of the open-loop modal coupling.