As the penetrated level of wind in power grids increases, the online system inertia becomes weak. Doubly-fed induction generator (DFIG)-based wind turbine generators (WTGs) are required to provide virtual inertia response to support system frequency. The present inertia control strategy with fixed control gain is not suitable and may cause stall of the DFIG-based WTG, as the virtual inertia response potential from the DFIG-based WTG is different with various wind speed conditions. This paper addresses a virtual inertia control method for the DFIG-based WTGs to improve the system frequency stability without causing stalling of the wind turbine for various wind speed conditions. The effectiveness of the proposed virtual inertia control method is investigated in a small system embedded with the DFIG-based WTG. Results demonstrate that the proposed virtual inertia strategy improves the frequency stability without causing the rotor speed security issue. Thus, the proposed control strategy can secure the dynamic system frequency security of power systems under the scenarios of full and partial loads, and, consequently, the proposed method provides a promising solution of ancillary services to power systems.

Doubly fed induction generators can participate in frequency support following a disturbance by releasing their rotational energies. However, when regaining the rotor speed, a secondary frequency dip (SFD) tends to produce a sudden output drop. This study suggests a dynamic power-based stepwise inertial control (IC) scheme of a wind power plant for minimizing the SFD while reducing the maximum frequency deviation (MFD), considering the non-negligible wake. To this end, the reference of the output power increases to the torque limit. Afterward, the power reference smoothly decays with the dynamically decreasing incremental power and then automatically switches to the maximum power point tracking operation. The performance of the temporary frequency support with the suggested stepwise IC strategy is investigated with various penetration levels of wind power. Test results demonstrate that the suggested stepwise IC strategy can minimize the SFD and reduce mechanical stresses on the wind turbine during the recovery of the rotor speed while reducing the MFD. Therefore, the suggested stepwise IC strategy secures the dynamic frequency stability of an electric power system dominated by wind power generations.