Current Control of LCL-Type Shunt APFs: Damping Characteristics, Stability Analysis, and Robust Design Against Grid Impedance Variation

This paper is focused on a dual-loop current control (grid current loop and inverter-side current loop) with the point of common coupling (PCC) voltage feedforward control for digitally-controlled LCL-type shunt active power filters (APFs). However, both inverter-side current feedback and PCC voltage feedforward introduce active damping (AD) effects on different LCL elements, leading to uncertain system damping characteristics and robustness against grid impedance variation. To address this issue, the accurate system damping characteristics are evaluated through open-loop pole analysis, and the benchmarking of system stability and robustness is revealed with respect to various control and system parameters. Moreover, the system instability may occur under a large grid impedance due to high-frequency resonant units of current controllers, and an improved PCC voltage feedforward control is presented to avoid this problem. Based on these findings, a robust design procedure is proposed to obtain proper control parameters and enhance system stability and robustness. Experimental results on a 30 kVA APF prototype have finally validated the correctness and effectiveness of the proposed analysis and method.