Demystifying inertial specifications: supporting the inclusion of grid-followers

Inertia provision from converters is often separated into two categories according to the control approach. Inertia from grid-followers (GFLs) is deemed to be "synthetic" due to a slow response. In contrast, grid-forming (GFM) inertia is deemed to be "true", and more useful for frequency stability, due its faster provision. This paper analyses the distinctions between GFM and GFL inertia by carrying out parametric sweeps of each approach at different operating conditions. The analysis aims to assist the ongoing efforts to quantify grid stabilising phenomena, particularly the recent adaptation of the GB grid code to incorporate GFM converters. The optimal tuning configurations are identified, showing that the GFL can achieve fast inertial provision that can contain the grid frequency as effectively as GFM inertia on strong grids, despite the opposing consensus in the literature. The simulations also highlight the importance of voltage-source behaviours in determining the initial evolution of grid frequency, that these features should be considered more explicitly by system operators, and that GFLs should not be excluded so readily. Neglecting GFL control could limit the assets available to support the grid and inhibit the rate that the net zero transition can occur.