This paper is focused on a dual-loop current control method (grid current loop and fundamental current loop) for digitally-controlled LCL-type shunt active power filters (APFs). Normally, a proportional-resonant (PR) unit is used as the fundamental current controller, where the proportional part of PR unit actually serves as the proportional feedback of inverter-side current and provides a certain degree of damping for LCL resonance. However, its valid damping region is only up to one-sixth of the sampling frequency (fs/6), which leads to less robustness to the grid impedance. To address this issue, a delay-compensation control link is proposed to replace the proportional part of PR fundamental current controller and widen the effective damping region. Consequently, the system robustness is improved without adding extra sensors or introducing an extra active damping loop. Theoretical analysis proves that it can obtain a wider damping region of (0, fs/4). Moreover, a systematic controller parameter design criterion is studied. In particular, a numerical optimization algorithm is developed to optimize damping property, and a design method based on root locus plot and Bode diagram is presented to enhance harmonic compensation accuracy. Finally, experimental results have validated the feasibility of the proposed current controller and parameter design criterion.

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.

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 (fs/6), which leads to a 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 to 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 (fs/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.

A dual-loop current control is discussed for LCL -type shunt active power filters (APFs), where inverter-side current feedback and point of common coupling (PCC) voltage feedforward both provide active damping (AD) effects for LCL resonance. However, two ADs are equivalently applied to different elements of LCL filter with narrow damping boundaries, which causes less robustness against grid impedance variation. Hence, this paper has presented a comprehensive approach to strengthen the system robustness against grid impedance variation. On the basis of phase compensated resonant unit, a delay-compensation scheme is firstly proposed to extend the damping boundary of inverter-side current feedback up to almost Nyquist frequency ( $f_{s}/2$ ). Theory analysis considering PCC voltage feedforward indicates that the control system can keep robust to grid impedance when the initial LCL resonance frequency falls between $f_{s}/3$ and the extended damping boundary. Moreover, a typical weighted proportional-resonant feedforward scheme is suggested to prevent high-frequency resonant units from inducing system instability under a large grid impedance. We finally validate the effectiveness of the proposed approach through experiments.

Due to the unmatched feeder impedance, the conventional voltage and frequency droop control method is unable to accurately allocate the output reactive power of DG units in proportion to their power ratings. To solve this issue, this paper has proposed a feeder impedance identification based droop control method. Firstly, the relationship among the reactive power sharing, reactive power droop slope and feeder impedance in islanded microgrids is illustrated. An online feeder impedance identification method is subsequently adopted to modify the reactive power droop slope of DG unit. In this way, the effect of unmatched feeder impedance on reactive power sharing is eliminated without communication among DG units. Experimental results implemented on an islanded microgrid apparatus have validated the feasibility of the proposed droop control method.