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 presents a DC-link voltage fast control strategy for high-speed Permanent Magnet Synchronous Motor/Generator (PMSM/G) of Flywheel Energy Storage System (FESS) to ensure fast dynamic performance within its wide operation range. Instead of the conventional strategy with cascaded outer DC-link voltage loop and inner current loop, the proposed strategy is a direct voltage control (DVC) strategy without an intermediate current loop. The nonlinear DC-link voltage loop model is globally linearized by treating the square of DC-link voltage as the state variable and lumping the nonlinear and uncertain terms in the power balance equation as the total disturbance. An Extended State Observer (ESO) is designed to observe the total disturbance, and a control law which compensates the speed variation and the total disturbance is proposed. The inner dynamic of the proposed ESO is analyzed, and the tracking performance and anti-disturbance capability of the proposed strategy is hereby derived. Finally, the proposed strategy is validated on a high-speed FESS test bench.

Fault-tolerant capability is one of the dominant advantage of multiphase drives. In case of open-phase fault occurrence, the control strategy needs to be modified in order to maintain smooth operation, resulting in larger amplitude of the phase current. Consequently, the winding losses are increased in comparison with healthy operation. Also, the torque operation range (TOR) is reduced inevitably. This article proposed a unified fault-tolerant strategy that can ensure the widest TOR while minimizing losses. Compared with state-of-the-art literature, the execution of the fault-tolerant algorithm does not require any online or offline global optimization process. Instead, simple closed-form expressions are adopted, which could be considered a feasible simplification. Furthermore, the proposed strategy can be adopted for symmetrical or asymmetrical multiphase machines with any number of phases. Experimental results validate the effectiveness of the proposed fault-tolerant method.

Conventional space vector pulse width modulation (SVPWM) would degrade the operating performance of the multiphase motor drives under open-phase fault. To resolve this issue, existing solutions require the reconfiguration of transformation matrices. However, for the variable positions of faulty phases, the structure of matrices needs to be redefined, which is a restraint to the industrial implementation of such technologies. This study examined the offset of the voltage vectors after fault occurrence. Accordingly, a universal fault-tolerant SVPWM strategy is proposed for the purpose of natural fault-tolerance, as well as a simplified fault detection scheme. Compared with conventional fault-tolerant SVPWM, it is highlighted with the characteristics of minimum reconfiguration, torque-ripple free operation, and applicability to handle various types of open-phase faults using a single transformation matrix. Besides, proposed strategy eliminates the redundant controllers and also reduces the degree of difficulty for fault diagnosis. Finally, the effectiveness of proposed approach is evaluated experimentally with a seven-phase induction machine, which shows that the torque ripple could be reduced by about 40% in the presence of multiple faults compared to the conventional SVPWM strategy.

This paper proposes a novel model predictive control scheme for dual three-phase permanent magnet synchronous machines to further suppress the x – y current harmonics caused by unmodeled disturbances such as inverter nonlinearity and back-EMF harmonics, in which the average deception effect is taken into account to avoid larger x – y current spikes. Instead of following the conventional virtual vectors to synthesize a zero-average voltage in the x – y subspace, the rotating non-zero voltage vector is dynamically modulated in a control period by scheduling the dwell times of the three adjacent large vectors and zero vectors to realize the closed-loop current control in the x – y subspace. In the implementation of synthetic vectors, to ensure that large vectors with a higher ratio of α–β/ x – y voltage are applied directly to the inverter, the compare registers A and B of the PWM module are configured to solve the state conflicts issue in the switching sequence. Since the x – y unmodeled disturbances have considerable periodic components, a resonant extended state observer (RESO) is employed to provide a more accurate estimate within a limited bandwidth, resulting in more targeted suppression of the 5th and 7th harmonics. Experimental results validate the superiority of the proposed strategy.

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.