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.

This article is focused on the current harmonic suppression of a permanent magnet machine with the enhanced extended state observers (ESOs). For traditional ESO, there are three defects: coupling issue between fundamental current regulation (FCR) and disturbance attenuation (DA), limited periodic disturbance attenuation (PDA) property, and coupling issue between aperiodic disturbance attenuation (APDA) and PDA. Three kinds of enhanced ESOs are then proposed. That is decoupling-type ESO (DESO), resonance-type DESO (RDESO), and notch-type DESO (NDESO). For DESO, the structure of traditional ESO is redesigned. Decoupling between FCR and DA is, thus, achieved but the other two defects still exist. For RDESO, an additional link is inserted into DESO. Foregoing decoupling property and enhanced PDA are thus achieved simultaneously. Whereas, decoupling between APDA and PDA still cannot be reached. For NDESO, the added link in RDESO is redesigned. Foregoing three defects are thus all solved. Experiments on a test rig have verified the effectiveness of proposed methods.

This paper focuses on the dc voltage control based on capacitance energy ( $E_{C}$ ) for a flywheel energy storage system (FESS) with the permanent magnet synchronous machine (PMSM) as the energy transformation device. For $E_{C}$ -based PI controller, performance degradation occurs due to the rapid speed change, potential dc-side load step and parameter mismatch. To address this, system properties containing stability, tracking performance and robustness against the aforementioned factors are revealed for $E_{C}$ -based dc voltage control methods. It is derived through illustrations of dominant poles and maximal magnitudes with respect to speed, capacitance and coefficients. Moreover, a linear-tracking-differentiator-based energy controller (LTDEC) is presented to achieve fast tracking performance and foregoing robustness of dc voltage. It includes proportional control for direct feedback of $E_{C}$ and feedforward of speed and indirectly calculated total consumed power. A selection region for coefficients of LTDEC is designed to enhance stability, tracking performance and robustness. Experiments have verified the effectiveness of the proposed control method and designed coefficient selection region.

High-speed Permanent-Magnet Synchronous Motors/Generators (PMSMs/Gs) in a Flywheel Energy Storage System (FESS) are faced with high cross-coupling voltages and low switching-to-fundamental frequency ratios. High cross-coupling voltages between d-q axis current loops lead to transient current errors, which is more serious at lower switching-to-fundamental-frequency ratios. If the delays are not properly considered during the current controller design in a digital control system, the low switching-to-fundamental-frequency ratios may result in oscillatory or unstable responses. In this study, an accurate discrete current controller for high-speed PMSMs/Gs is proposed based on an accurate discrete model that takes the phase and magnitude errors generated during the sampling period into consideration, and an Extended State Observer (ESO) is applied to estimate and compensate the back EMF error. The cross-coupling problem is well settled, and the current loop dynamic at lower switching-to-fundamental frequency ratios is improved. Finally, the proposed discrete controller is validated on a 12,000 rpm PMSM/G prototype.

Finite control set model predictive control (FCSMPC) based on virtual vectors for dual three-phase permanent magnet synchronous machines inherently lacks control capability in the x–y subspace, resulting in an inability to suppress unmodeled disturbances decoupled in this subspace and leading to severe current distortion. Existing studies have incorporated x–y current prediction to compensate for disturbances in a closedloop manner but require prior knowledge of the stator leakage inductance. Mismatches in this parameter degrade harmonic suppression performance. In this work, we introduce a modelfree solution that relies solely on measured input/output data, eliminating dependence on x–y subspace model parameters. To enable the linear mapping between the input/output data to describe the rich periodic disturbances in the x–y subspace, sine and cosine feature excitations are added to the regressor. The enhanced data model better approximates the physical model for predicting x–y current dynamics. A low-complexity normalized least mean square (NLMS) algorithm is employed to optimize the regression coefficients, allowing the proposed method to be implemented on a digital signal processor with 10kHz sampling frequency. Experimental results validate the superiority of the proposed strategy over other existing methods.

On account of limited current controller bandwidth for small film capacitor based compressor drive, which seriously deteriorates the reference tracking and disturbance attenuation of the inverter power loop. A non-inverse internal plant model (IPM) based cascade loop control structure is proposed, the multi loop power-current control characteristics is fully investigated. The plant model is reconstructed and an independent controller is induced, which decouples the disturbance issue from the reference tracking and extra freedom degree is provided for disturbance attenuation. The inherent drawbacks of conventional internal model controller such as feasibility of the control system, high frequency noise rejection and robustness for model mismatches can be improved. In addition, the essence of the system instability is revealed when compressor operating within high frequency range. The phase lag is precisely derived and improved with the phase compensated resonant type power controller. Experimental results considering grid voltage sag, background harmonics and model mismatch are presented. The enhanced inverter power control performance and grid side power quality can be ensured.