A Robust Singular Perturbation Method for Decoupling of Power and Voltage Control of Grid-Forming Converters

Grid-forming converters (GFMCs), which provide superior grid formulation and supportive services, have been identified as the enabler of more-electronics power systems. Control of GFMCs mainly comprises outer-power and inner-voltage loops. Although existing literature assumes that the two loops can be independently analyzed and designed, we reveal that the converter may be unstable even with two individually stabilized control loops, thereby implying their strong coupling effect. To realize the decoupling design of power and voltage control loops, we propose a robust singular perturbation method, which comprises a marriage of robust control and singular perturbation theory. Specifically, we construct the converter model as an interconnection of slow and fast subsystems, and derive two quantitative criteria regarding system stability and performance, respectively. Through the proposed method, we successfully quantify the coupling effects between the two loops at each frequency point, enabling the identification of the maximum coupling and the critical coupling frequency. Accordingly, we provide insightful guidelines for simplified controller design. The correctness of the theoretical analysis and the effectiveness of the proposed decoupling method are validated by simulation and experimental results.