This paper investigates the phenomenon of reverse active power in grid-forming voltage source converters during grid voltage recovery events. The study demonstrates that this issue originates from two primary aspects: the operating point during the fault steady state and the control dynamics. The operating point analysis reveals that maintaining a certain level of active power transmission during the fault steady state is necessary to keep the operating point within the positive-power region; otherwise, the operating point may shift into the negativepower region. Furthermore, the control dynamics cause the phase angle of the modulation voltage vector to rotate clockwise, which reduces the power angle and exacerbates the power reversal issue. Theoretical findings are validated through both simulation and experimental results.

Active damping is crucial for the stable operation and dynamic performance of grid-forming (GFM) voltage source converters (VSCs). This paper presents a comparative analysis of active damping methods used in the synchronization control of GFM-VSCs. These methods are classified into frequency-difference-based, active-power-based, and q-axis-voltage-based damping approaches. The comparison is based on three key metrics: reference tracking performance, inertial response capability, and stability impact. The advantages and drawbacks of each approach are analyzed, and practical design guidelines are proposed, including hybrid configurations that integrate power and voltage feedforward strategies. The effectiveness of the comparative evaluation is validated through both simulation and experimental results.

This article proposes a simplified discrete space vector modulation (DSVM)-based model predictive direct speed control (MPDSC) with an improved load disturbance observer for permanent magnet synchronous motor (PMSM) drives. First, a simplified DSVM method is used to improve the steady-state performance of MPDSC. In this DSVM method, a novel geometric method relying only on three auxiliary lines in each sector is designed to simplify the algorithm’s complexity. In this way, the set of candidate vectors is quickly determined. Then, the current pulsation and speed of MPDSC are suppressed, and the computational burden of the DSVM execution process is reduced. Second, the reasons that affect the dynamic performance of the conventional linear extended state observer (ESO)-based mechanical disturbance observer are analyzed, and the observed error of the observer is derived. Based on the observer error, an improved mechanical disturbance observer is proposed to accelerate the convergence process. The Lyapunov theory proves the stability of the proposed observer. Finally, the feasibility of the proposed method is verified by experiments.

This article proposes an improved nonlinear extended state observer-based model-free predictive current control (MFPCC) with an extended control set. First, the nonsmoothness problem of conventional nonlinear extended state observer model-free solutions is explained. By constructing a high-smoothness polynomial series at the transition points of the piecewise nonlinear function, an improved nonlinear extended state observer is designed to enhance the performance of the MFPCC, and relevant stability proofs are derived by the Routh criterion. Furthermore, a newly designed extended control set based on the voltage vectors (VVs) at the center of triangular subregions (TSs) of the modulation area is proposed to improve the output voltage accuracy. Based on the geometrical characteristics of the subregion center VVs, a fast-layering selection method is proposed to reduce the complexity of determining the optimal VV. Finally, experimental results are presented to verify the proposed method's feasibility and effectiveness.

Dual three-phase PMSM (DTP-PMSM) has gained significant attention benefiting from their exceptional reliability and high torque density. However, DTP-PMSM exhibits poor steady-state performance and large calculation amount with the conventional finite-control-set predictive current control. To address these concerns, an asymmetric modulated predictive current control (AM-PCC) for DTP-PMSM is presented. Therein, the three-phase decomposition and deadbeat control scheme are combined to eliminate the weighting factor and optimize the execution efficiency. An asymmetric modulation strategy is presented and integrated into PCC to reduce the current harmonics and switching frequency. Extensive simulation tests are conducted to demonstrate the effectiveness of the proposed AM-PCC scheme.

Continuous control set predictive speed control (CCS-PSC) predicts and optimizes the speed trajectory of a drive system. The prediction horizon is commonly short to avoid a high computational burden. However, stability issues may arise with the short prediction horizon. In this article, we propose an efficient strategy that enables CCS-PSC with a long prediction horizon, where Laguerre functions are adopted. Differ from existing methods, the equivalent speed tracking error is a controlled variable, which consists of the speed tracking error and its derivative. In this manner, speed overshoot can be eliminated. The dynamics of the equivalent speed tracking error and the direct-axis current are naturally decoupled, where the linearization of the surface-mounted permanent magnet synchronous motor (SPMSM) model is not required. The performance of the proposed strategy is evaluated comprehensively. The simulation and experimental results show that the proposed strategy improves the system stability and the steady-state performance, while maintaining high dynamic performance.

Four-switch three-phase inverter (FSTPI) has been considered as a promising fault-tolerant topology for six-switch three-phase inverter (SSTPI) with an open-phase fault. However, the maximum linear voltage of FSTPI is reduced by half compared to SSTPI due to the reduced power switches, which limits the high-speed operation capability. In this article, a flux-weakening scheme for the FSTPI-fed induction motor (IM) using optimized overmodulation strategy-based modulated predictive torque control (M-PTC) is proposed. Therein, a voltage closed-loop flux-weakening control method considering the dc-link capacitor voltage offset (CVO) is developed to suppress the ripple in stator flux. An M-PTC strategy is proposed to handle the nonlinearities and constraints in IM drives, together with a two-long-voltage-vector-based pre-excitation scheme designed for maintaining the minimum CVO during the initial startup process. To increase the utilization rate of the dc-link voltage and the maximum output torque in flux-weakening region, an optimized overmodulation strategy is proposed to extend the available voltage from inscribed circle to quadrilateral region. Extensive experimental studies are carried out to verify the effectiveness of the proposed method.

This paper explores the limitations of the fault current contribution of grid-forming modular multilevel converters (GFM-MMCs) under asymmetrical grid faults. Differing from the voltage source converters with a centralized dc-link capacitor, the arm current of MMCs comprises a circulating current for internal energy balancing, in addition to the dc-link current and ac output current. Under asymmetrical grid faults, the interaction between the positive- and negative-sequence components results in a power imbalance among the phases (or legs) of the MMC. This requires the presence of the dc circulating currents in each phase of MMC to eliminate this imbalance. The presence of this dc circulating current not only complicates the design of current limiting strategies but also constrains the fault current contribution of GFM-MMC. To address this issue, we propose a current limiting strategy ensuring that GFM-MMC can provide its allowable maximum fault current during asymmetrical grid faults.

Model predictive control (MPC) usually suffers from high computational complexity when it comes to modular multilevel converters (MMCs). Some researchers have attempted to use a modulated approach to reduce the computational burden and improve the control performance. But these methods do not consider the actual physical limitations of the control system, and therefore the control performance degrades at high modulation indices or transients. To solve this problem, a modulated MPC with bound-constrained quadratic programming has been proposed. With this method, the optimal solution of the control problem can be obtained, ensuring a better control performance under high modulation index conditions or in transients. Finally, a comparative experiment with the conventional modulated MPC methods has been carried out. The experimental results validate that the proposed method can achieve superior performance when the MMC operates at high modulation index, transients, and low frequencies.