I. Introduction
The frequency stability of the modern power system is affected significantly by a high penetration of wind power generation. To ensure that the system frequency is within the allowed range, the synchronous generators (SGs) are required to have frequency control capability. For instance, SGs can increase the output power by releasing part of the kinetic energy (KE) stored in the rotor when the system frequency drops owing to generation tripping. As a result, the frequency nadir (FN) is not lower than the predefined threshold and the load-shedding relays are not activated. However, traditional variable-speed wind turbines (VSWTs) lack the frequency regulation capability. The VSWTs have high efficiency because they perform maximum power point tracking (MPPT). However, the MPPT function causes VSWTs to decouple from the grid. When the grid frequency decreases, VSWTs cannot increase the output power to improve the frequency stability. Thus, in some countries, wind farms (WFs) consisting of VSWTs are required to provide the frequency response during a system contingency [1]–[3].