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.

I. Introduction

Over the past decades, voltage-source converters (VSCs) have attracted much attention in the field of new energy grid-connected power generation due to their high flexibility, controllability, and sustainability [1], [2]. However, with the increasing penetration of VSCs in power systems, the system inertia gradually decreases compared with legacy power grids, leading to instability oscillations phenomena across a wide frequency range [3], [4]. Therefore, there is a growing interest in studying the stability of VSC-grid interactive systems.