Angular stability of grid forming converters subjected to large disturbances

Grid-forming control strategies are increasingly being considered for Inverter-based Resources (IBRs) to ensure stable operation under weak-grid or islanding conditions. Unlike Grid-following IBRs (GFLs), which implement power regulation through fast current control, Grid-forming IBRs (GFMs) independently regulate the voltage magnitude and frequency, subject to the IBR limits. Virtual Synchronous Generator (VSG) control is a typical GFM strategy, which emulates a synchronous generator with tunable “inertia” and “damping” parameters. However, its large disturbance behaviour can be quite different from a synchronous generator (SG) due to current and frequency limiters present in the control system. The angular stability of GFMs when subjected to faults is of particular interest. This paper investigates four simple configurations with two sources (GFM-infinite bus, GFM-GFM, GFM-synchronous machine and GFM-GFL) in order to gain insight into their large disturbance behaviour. It is shown that susceptibility to loss of synchronism is significantly affected by the implementation of limiters in the controller.