Inductance-Emulating-Based Frequency Coupling Suppression Control for Three-Phase Grid-Tied VSC Under Unbalanced Grids

For three-phase grid-tied voltage-source converters (VSCs), the presence of unbalanced grids (unbalanced grid voltages, asymmetric line impedances, etc.) tends to excite the frequency coupling effect (FCE), posing a challenge to system modeling and stability analysis. In this article, a frequency coupling suppression control strategy is developed for three-phase grid-tied VSCs to deal with FCEs caused by unbalanced grids. The proposed control strategy consists of a dual second-order generalized integrator-based symmetric phase-locked loop (DSOGI-SPLL) and an inductance-emulating control loop. The DSOGI-SPLL suppresses the FCE induced by unbalanced grid voltages, while the inductance-emulating control loop is responsible for addressing the FCE arising from asymmetric line impedances. The presented control strategy allows the derivation of a concise single-input single-output complex vector model, facilitating a stability-oriented design. Furthermore, a phase-locked loop (PLL) correction method is introduced to enhance system stability under weak grid conditions. The effectiveness of the proposed control strategies is verified through simulation and experiments.