Unified Modeling and Analysis of Sequence Impedance of Grid-Forming Converters with Multi-loop Control

Abstract

The impedance of a converter is the basis for analyzing the interaction stability of the converter and the grid. Conventionally, the impedance models are built and investigated individually under different control schemes and parameters, which renders time-consuming but low efficient work. Devoted to revealing the common features of diverse grid-forming converters, this paper proposes a unified sequence impedance model that is applicable to various grid-forming converters with multi-loop control. The unified model indicates that the converter impedance is dominated by inner loops in a wide frequency range, and hence is further simplified for inner loop parameter analysis. Focusing on the complex interaction coupling among inner loop parameters, their coupling influences on the converter impedance are investigated comprehensively, and the corresponding rules are highly generalized, which provides guidance on loop parameter tuning for stability improvement. Experimental results verify the correctness of the theoretical analysis.