Complex Circuit Modeling Methodology and its Application to Depict Multifrequency Coupling Effect of Asymmetric Grid-Following Converter

Due to the multifrequency coupling effect (m-FCE), precise modeling and effective stability assessment of an asymmetric grid-following converter (GFC)-based ac system are complex to achieve. Traditional small-signal modeling methods and recently reported order-reduction methods could establish multi-input and multioutput (MIMO) and equivalent single-input and single-output (SISO) models to characterize m-FCE effectively. However, revealing the physical implications of coupling components and the generation mechanism of m-FCE are still challenging. Meanwhile, although the amplitude and phase dynamics have been considered in the existing modeling, a concrete conclusion illustrating these dynamics' impact on m-FCE is still missing. To fill the gaps, this article thoroughly considers the impacts of amplitude and phase dynamics and finds that phase dynamics form additional positive feedback loops to enhance m-FCE. Based on this cognition, an accurate complex circuit modeling framework is proposed. It reveals that m-FCE is generated by the admittance mismatching between conjugated responses and 'chain reaction' among coupling components. The proposed method has two merits: 1) it is an SISO modeling approach and helps reduce analysis complexity and 2) its graphical form helps provide a deep physical insight into m-FCE. Simulations and experiments verified the correctness of the proposed method.