Frequency-domain network modelling for the identification of converter-grid resonance during power system restoration

The dynamics of the electric power system are prone to significant change due to the increasing share of renewable energy generation. Despite their reduced number, conventional generators remain critical to day-to-day operations, particularly in the provision of ancillary services, due to their reliable on-demand availability. Inverter-based resources (IBRs) are under-utilised in these services, specifically in the power system restoration (PSR) process, due to their weather-dependent operation. However, as more reliable IBRs, such as batteries, become available, PSR could benefit from their inclusion. This paper focuses on modelling a network comprising synchronous generators, transmission lines, cables, and a PV farm to assess small-signal stability in the high frequency range. Using the sequence immittance-based method, these elements are analytically modelled and validated via frequency scans in Matlab/Simulink. A critical frequency range is then identified, where the Bode criterion is applied to assess converter-grid stability. The phase margin defined for this frequency interval is used to quantitatively describe resonances during various network operating states. Results indicate possible oscillations that can be mitigated by adjusting converter control delays or filter parameters. Moreover, the study provides insight into the characteristics of such resonances and demonstrates that under appropriate conditions, IBRs can support PSR.