Impacts on Frequency Stability Studies of Modeling DC-DC Converters of Two-Stage Photovoltaic Power Plants in Grid-Forming Mode

The growing interest in grid-forming mode (GFM) and the increasing participation of photovoltaic (PV) technology in the grid have highlighted the necessity to investigate the frequency stability of power systems with high levels of PV power plants operating in grid-forming mode (PVGFM). However, frequency stability studies with GFM generators mainly consider the converter connected to the grid and do not model the PV source and the DC-DC converter. In this work, we model and study the frequency stability in a power system with high participation of two-stage PV-GFM, where the DC-DC converter is represented. The study cases compare the dynamic frequency performance of PV-GFM plants and CIGGFM generators without PV representation. The objective is to investigate the impacts of modeling PV source and DC-DC converter from a frequency stability perspective. The results reveal that oscillatory instabilities are observed for operating points in networks with the participation of two or more PVGFM plants while their CIG-GFM counterparts are stable. Accordingly, the modeling of PV-GFM, especially the DC-DC converter, should be adequately represented in the frequency stability study to avoid misleading conclusions.