Quantitative Effect of the Inertia Emulation Block of Grid-Forming Inverters on Frequency Stability

The short-term frequency stability of power systems has decreased with the replacement of synchronous generators (SGs) with inverter-based resources. Grid-forming inverters (GFMIs), developed to maintain power system stability, are also effective for frequency stability. Virtual synchronous generators (VSGs) and droop controls (Droops) are major algorithms of GFMIs. VSGs are considered to outperform Droops in terms of frequency stability because of the existence of the inertia emulation block in the active power controller. However, few studies have identified the quantitative effect of the inertia emulation block of GFMIs on frequency stability. This study analyzes the effect by deriving the relationship between the inertial response and the frequency based on the swing equation of SGs. The method of the analysis is verified by the simulation by the PSCAD/EMTDC. As indicated in this study, after a disturbance, the inertia emulation block immediately increases the load share of the VSG and decreases that of the SG, resulting in improved frequency stability. Moreover, it is implied that the Droop, i.e., the GFMI without inertia emulation block, can improve frequency stability as the VSG by increasing the droop gain, although the Droop requires larger energy provision from the direct-current side than the VSG. This study clarifies the better performance of the VSG in frequency stability by quantifying short-term load sharing among GFMIs and SGs that are significantly related to the short-term frequency stability.