Impact on transient stability of self-synchronisation control strategies in grid-forming VSC-based generators

Grid-forming voltage source converters (GFM-VSCs) are emerging as a solution for integrating renewable energy resources (RERs) into power systems. GFM-VSCs need a self-synchronisation strategy to ensure that all converters and generators in the power system are in synchronism and they reach the same frequency in steady state. The self-synchronisation strategy in GFM-VSCs that has received most attention in previous research is virtual synchronous machine (VSM) control. However, no systematic study of the effects on transient stability of different variants of this strategy has been carried out in previous work. This paper analyses and compares transient stability of four self-synchronisation strategies for GFM-VSCs: VSM without phase-locked loop (PLL), VSM with PLL, VSM without PLL using wash-out filter and integral-proportional (IP) controller. The paper also analyses two different methods that can \color{black} be applied to GFM-VSC self-synchronisation strategies to improve transient stability: the concept of virtual unsaturated active-power controller (VAPC), proposed in previous work, and an algorithm for frequency limitation in the GFM-VSC (FLC), which is proposed in this paper.