Contents I. Introduction
To reduce the carbon footprint of the power & energy sector, many countries throughout the world introduced various measures encouraging the integration of renewable energy sources (RES). The most popular energy sources for renewable power generation are the photo-voltaic (PV) plants and the wind power plants (WPPs): in 2016, installed PV capacity was 291 GW and installed WPP capacity was 467 GW in the world [1] and will continue to rise. Variable-speed wind turbine generators (VSWTGs) fall under converter-connected generation and due to the intermittent and stochastic nature of wind they utilize variable-speed drives to maximize energy capture over a wide range of wind speeds. Consequently, they utilize frequency converters to ensure power generation at grid frequency. On the other hand, they effectively decouple the electrical frequency of the grid and the mechanical frequency of the rotor (DFIG) or completely decouple the generator from the grid (full converter wind generators), thus eliminating the inertial response of the WPPs, although there is a significant amount of kinetic energy stored in the wind turbine rotor due to its large mass [2], [3]. As this converter-connected renewable power generation replaces conventional units, the total grid inertia is reduced which negatively impacts the frequency stability of the power system: the grid becomes weaker and reduces the capability of the system to remain stable after the occurrence of faults or disturbances [4], [5]. Therefore, a significant amount of recent research has focused on utilizing the controllability of VSWTGs to provide an inertial response (so-called virtual or synthetic inertia) and/or primary frequency response which can be found in an excellent state-of-the-art overview [6]. Furthermore, even some system operators have started requiring active power support from WPPs [7].
