Changes are being implemented in the electrical power grid to accommodate the increased penetration of renewable energy sources interfaced with grid-connected inverters. The grid-forming (GFM) control paradigm of inverters in active power grids has emerged as a technique through which to tackle the effects of the diminishing dominance of synchronous generators (SGs) and is preferred to the grid-following (GFL) control for providing system control and stability in converter-dominated grids. Therefore, the development of the GFM control is important as the grid advances towards 100% inverter-based grids. In this paper, therefore, we aim to review the changing grid scenario; the behaviour of grid-connected inverter control paradigms and major GFM inverter controls, including their modifications to tackle low inertia, reduced power quality, fault-ride through capability, and reduced stability; and the state-of-the-art GFM models that are pushing the universality of GFM inverter control.

Virtual synchronous generators (VSGs) incorporate virtual resistance (VR) in inverter control as a damping mechanism for oscillations that appear around the synchronous frequency due to interactions between the inverter impedance and the line impedance while operating in low inertia grids. Damping is critical for synchronization stability in the small-signal and large-signal disturbance scenarios to suppress oscillations that threaten system stability. While VR enhances stability in the steady state, it is shown that its conventional application affects the transient stability of VSGs by reducing the angle overshoot range in the dynamic behaviour of the power angle δ, illustrated using the power-angle ( P − δ ) and frequency-angle ( δ ̇− δ ) curves. In this article, a novel damping mechanism to improve the transient stability of VSGs using VR to enhance the power angle dynamics is proposed. By using the concept of virtual point of common coupling, the virtual power of the internal VSG is introduced in the active power feedback during a voltage sag, providing an equivalent damping effect proportional to the VR applied. In this way, the negative effects of VR on transient stability are averted while VR is used to improve the power angle dynamics of conventional VSG. Simulations of different transient stability scenarios are carried out in MATLAB/Simulink to verify the proposed method.