Demystifying inertial specifications: supporting the inclusion of grid-followers

Inertial response from grid-followers (GFLs) is deemed to be “synthetic” due to a slow response. In contrast, grid-forming (GFM) inertial response is deemed to be faster and therefore “true” and more useful for frequency stability. This paper explores the differences and similarities between an established example of a GFM and a GFL inertial controller by carrying out parametric sweeps at different operating conditions. The analysis aims to assist the ongoing efforts to quantify grid stabilising phenomena, particularly the recent adaptation of the British grid code to incorporate GFM converters. The optimal tuning configurations are identified, showing that some configurations of the GFL can achieve fast inertial provision on strong grids. These configurations are shown to contain the grid frequency as effectively as the GFM, despite the opposing consensus in the literature. The results also highlight the importance of voltage-source behaviours in determining the initial evolution of grid frequency. Although a blanket inclusion of all GFL inertial configurations is not appropriate, equally, the existing blanket disqualification could limit the assets available to support GFMs (who will certainly be required to stabilise the grid in a fundamental sense) and could inhibit the rate that the net zero transition can occur.