Revisiting frequency coupling effects in grid-tied converter: A phase shifting transformer based circuit modeling framework

Abstract

Due to asymmetric control algorithms and topology, the frequency coupling effect (FCE) extensively exists in the grid-tied converter (GTC), increasing the order of traditional impedance models and sacrificing their inherent physical insight. The two problems can be solved using the recently reported order reduction techniques. However, FCE is roughly regarded as a coupling phenomenon in the existing studies, which is not rigorous because a GTC system will present various degrees of FCE in different situations. Therefore, this paper revisits the FCE from the viewpoint of the complex circuit and categorizes the GTC systems into three types for analysis, i.e., strongly, weakly coupled, and decoupled systems. Firstly, the plausibility of using the complex-valued vectors to depict FCE is illustrated, and thus FCE is clarified as resulting from the rotation of complex-valued vectors and the admittance mismatching. Based on this new clarification, the phase-shifting transformer-based circuit modeling framework is proposed. Then GTC system can be intuitively presented by the proposed circuit model, whose mathematical model is still order-reduced. More importantly, coupling paths of strongly, weakly coupled, and decoupled systems can be flexibly modeled by adjusting the ratio of the phase-shifting transformer. Finally, simulations and experiments prove the analysis and proposed modeling framework.