Avoiding Frequency Second Dip in Power Unreserved Control during Wind Power Rotational Speed Recovery

With the incremental quantities of variable speed wind turbines (VSWT) integrated into microgrid, the frequency fluctuation and the generation efficiency decrease are caused. To solve the problem, power unreserved control has been well studied, which emulates inertia and stimulates the VSWTs to transiently release the kinetic energy stored in the rotating masses, thus providing response to the frequency excursions promptly and timely. Meanwhile, inertia control and temporary over-production (TOP) control, as two main strategies of power unreserved control, have been put forward to improve the effectiveness of frequency regulation. Nevertheless, a frequency second dip will occur during the rotational speed restoration process in both methods. Therefore, a modified control method is proposed in this paper to strike an appropriate balance between frequency second dip prevention and rotational speed recovery. Our strategy is elaborated through theoretical demonstration and it can adapt to different wind power penetration. Through explicit simulation analysis, the performance of the proposed control strategy is compared with conventional implementations of the inertia control and the TOP control. The results indicate that the modified strategy can effectively offset the frequency second dip while guaranteeing a satisfactory rotational speed restoration to maintain the frequency within an appropriate range in an islanded microgrid.