Analysis of multi-converter network impedance using MIMO stability criterion for multi-loop systems

This work presents a novel method of simplifying multi-converter systems containing complex grid-forming and grid-following controllers for stability analyses. Analysis is conducted via the equivalent converter output impedance. Important points for impedance-based stability analysis of larger networks are considered, including impedance rotations, proper combination of sources and correctly obtaining a minimum system realisation with appropriate tolerances. The proposed method is used to conduct a sensitivity analysis of different grid-forming and grid-following converter control parameters to determine tuning recommendations for the modern electricity network. To achieve this, complex control structures including power/voltage control, negative sequence regulation and synchronous machine emulation are included. Network stability is found to be sensitive to current control, active power control and frequency-based components such as PLLs. However, voltage control exhibits a smaller effect on stability for a range of short circuit ratios. Some recommendations are provided on when the simplified method can be utilised and how to ensure correct application.