A Systematic Approach via IIR Filters for Enhancing the Robustness of LCL-Type Shunt Active Power Filters to Grid Impedance

A dual-loop current control method (grid current loop and inverter-side current loop) is promising for digitally controlled LCL-type shunt active power filters (APFs), due to its inherent active damping effect for LCL resonance. However, the damping region is only up to one-sixth of the sampling frequency (f
_s
/6), which leads to limited system robustness against grid impedance variation at the point of common coupling (PCC). Hence, in this study, the robustness of such control is discussed through mathematic derivations and diagram analysis, which reveals that both the system delays and PCC voltage feedforward affect the system robustness. On the basis of theoretical analysis, a systematic approach via infinite impulse response (IIR) filters has been proposed for enhancing the robustness of grid impedance. Thereof, a second-order IIR filter-based inverter-side current feedback is proposed to widen the damping region up to almost the Nyquist frequency (f
_s
/2), while a band-stop IIR filter-based PCC voltage feedforward is proposed to achieve the superior stability margins under a large grid impedance in comparison with unit feedforward. Finally, experimental results on a 30-kVA APF prototype have verified the effectiveness of the proposed approach.