Abstract
This paper is focused on a dual-loop current control for digitally-controlled LCL-type shunt active power filters (APFs), which has an inherent active damping effect for LCL resonance. However, due to the system delays, the damping region is only up to one sixth of the sampling frequency $(f_{\mathrm{s}}/6)$, leading to a limited robustness against grid impedance variation. To resolve this problem, an enhanced active damping method is proposed with proportional-resonant-differential inverter-side current feedback, where the proportional-differential term serves as a delay-compensation link. Then, two modified digital implementation schemes of proportional-differential term are given to overcome the gain amplification problem at Nyquist frequency. Theoretical analysis proves that the proposed active damping method can successfully extend the damping region to approximately $(0,f_{\mathrm{s}}/4)$ and improve the system robustness. Finally, experimental results on a 30 kVA APF prototype have validated the effectiveness of the proposed method.
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