A d-axis current error compensation method based on coupling mechanism for Grid-Connected inverters under weak grid

Abstract

Three-phase grid-connected inverters (GCI) are widely used in wind farms as they serve as the interface between the renewable energy system and the grid. However, the sub-synchronous resonance (SSR) issues have been observed between GCI and weak grid, which seriously threatens the stable operation of grid-connected system. In this paper, the coupling mechanism between the GCI and the weak grid is analyzed from the perspective of power angle relationship. Revealing that there is a small signal coupling between d-axis current and power angle due to the grid impedance introduced by weak grid, which further destabilizes the studied system through PLL feedforward paths. It provides an explanation for the limitations of short circuit ratio (SCR) and phase-locked loop (PLL) bandwidth. Then, a d-axis current error compensation method employed in current control is proposed to damp SSR. The method introduces an extra parallel branch to compensate the PLL feedforward effect and increases the qd-axis impedance amplitude of GCI to decrease the coupling degree between the GCI and weak grid simultaneously. By employing the proposed compensation method, the SSR issues of the GCI under weak grid can be effectively solved under grid impedance fluctuations and high PLL bandwidth. Finally, the experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed compensation method.© 2017 Elsevier Inc. All rights reserved.