Impact of DC Dynamics of Grid-Forming Converters on Transmission System Stability

In this paper, the impact of the DC-side dynamics on the stability of grid-forming converters for transmission system applications is assessed. First, a reduced order model for the DC-side dynamics of a two-stage, grid-forming converter is adapted within a small-signal stability analysis framework. This reduced model captures the essential dynamics of the DC-side, achieving sufficient accuracy but without excessive computation cost for system-level studies. Then, the DC-side model is interfaced with a standard AC-side, small-signal model of a droop-controlled grid-forming converter. The combined linear model of the converter is deployed for assessing relevant parameter stability limits in a case study based on a variation of the well-known WSCC transmission system. The study shows that neglecting the DC-side dynamics and considering an ideal DC voltage source, leads to overly optimistic stability limits for the AC-side parameters. The validity of the proposed linear model and of the calculated parameter stability limits are supported by non-linear EMT simulations performed in Matlab/Simulink environment.