Analysis and Weakening of Sequence Impedance Coupling in Grid-Forming Converters

Couplings exist between power-electronic converters' positive and negative impedances. While such couplings have been considered in many works for stability analysis of grid-forming converters, many other studies have accurately predicted the converter-grid stability with couplings ignored. This paper provides a detailed comparative analysis of the sequence impedance coupling in droop control-based converter (DBC) and virtual synchronous machine (VSM). The impacts of droop gains, virtual inertia, and voltage controllers are mainly outlined. It is shown that the VSM has notably less impedance coupling than the typical DBC, basically due to its virtual exciter's integrator. Additionally, this paper introduces the VSM's impedance coupling saturation phenomenon. The improper design of the VSM's voltage exciter leads to impedance coupling as high as that of the typical DBC, regardless of the virtual inertia. Based on the analysis and the impedance-coupling saturation phenomenon, a simple yet effective control-design-based mechanism is proposed to weaken the impedance coupling in VSM and in a modified DBC with an integrator in its voltage loop. The proposed impedance weakening method eases the converter-grid interactions and stability analysis by deploying the normal Nyquist stability criterion. Detailed time-domain and real-time simulations are carried out under different grid strengths to justify the theoretical analysis.