Impedance-based method is an effective method to analyze the system stability. Based on the impedance stability theory, a doubly fed induction generator (DFIG) based wind power generation system can be decoupled into a positive- and a negative-sequence subsystem. Then, under small-signal operation, each subsystem is assumed to have linear characteristic in the frequency domain. Therefore, a single-in-single-out stability criterion can be applied for each subsystem to analyze the stability of the interconnected system consisted of the DFIG system and the grid. However, when there is frequency coupling in the DFIG system, this linear assumption may lead to wrong conclusion of the system stability analysis. The frequency coupling of DFIG can be caused by the PLL and the asymmetric rotor current control. In this paper, the frequency coupling characteristic of DFIG caused by these two causes will be studied, and an analytical model that describes the frequency coupling of DFIG will be developed and validated by simulations. Based on this derived model, the system stability and the influencing factors of frequency coupling will be analyzed.

This paper presents a flexible compensation strategy for voltage source converter (VSC) connected to an unbalanced and harmonic distorted grid, which can achieve flexible tradeoff between the two common control targets, i.e. compensating the voltage at the point of common coupling (PCC), and compensating the output current of the VSC. To achieve the control target, a hybrid virtual impedance method is proposed, which uses both the feedforward of the PCC voltage and the feedback of the output current to develop the harmonic (including the negative sequence) voltage references. Compared with the conventional virtual impedance methods using only feedforward or only feedback, the proposed hybrid method can control the harmonic output impedance (HOI) of the VSC from 0 to infinity, without the potential over-modulation or instability problems caused by the conventional methods when the desired virtual impedance varies in a large range. The effectiveness and superiority of the proposed strategy is theoretically analyzed and experimentally verified.

The output torque capability of a dual-three-phase permanent magnet synchronous machine (DTP-PMSM) can be increased by intentionally injecting proper harmonic currents, which can be beneficial in some applications that require high torque density. To achieve this, current references containing multiple harmonic components with time-varying frequencies must be accurately tracked by the current controllers, which can hardly be achieved by conventional control methods. Fortunately, to track these complex but predetermined current references, generalized predictive control (GPC) shows unique advantages for its characteristics of multistep prediction and receding horizon optimization. In this article, a current control strategy based on GPC is proposed for the DTP-PMSM, which enhances the output torque capability by tracking the harmonic current references with high accuracy in both phase and magnitude. Parameter tuning and model mismatch issues of the proposed GPC are discussed. The effectiveness and merits of the proposed control strategy are validated by a comparison study based on simulation and experiments.

In this article, an open-circuit fault diagnostic method is proposed for three-phase permanent magnet machines. By individually applying inverse coordination transformation to d - and q -axis currents, per-phase current can be decomposed into flux- and torque-producing currents. Their postfault behaviors are found to be highly related with the fault types and locations. Hence, the normalized flux-producing current can be used as a single fault index to detect and identify both open-switch and open-phase faults. Since the established fault index is close to zero, the proposed fault diagnostic method is naturally robust to load/speed variations. In addition, a low decision threshold can be set to reduce fault diagnostic time. The robustness and effectiveness of the proposed method are verified by the experiments on one prototype.

Generalized predictive control (GPC) is promising for permanent magnet synchronous machine drive systems, due to its outstanding dynamic performance and disturbance rejection capability. However, the speed measurement noise introduced by the encoder and the unmodelled disturbances introduced by the non‐ideal machine characteristics deteriorate the performance of GPC. The noise and disturbance issues are studied in detail, and an enhanced GPC method is proposed to address them. For the noise issue, the mathematical model of the speed measurement noise is derived, and the noise spectra at different speeds are analysed. Then, a novel GPC with internal low‐pass filter is proposed to reject the influence of the noise. For the disturbance issue, the influence of the unmodelled disturbances on the speed performance is assessed when applying different noise rejection methods, and a resonant controller‐based speed ripple mitigation method is proposed and integrated to the GPC. Experimental results show that the proposed methods can effectively deal with the noise and disturbance issues, as well as providing better dynamic performance than the conventional PI based speed control.

This paper proposes a full-order vector space decomposition (VSD)-based fault-tolerant control with harmonic current injection for surface-mounted dual three-phase permanent magnet synchronous machine with open-phase fault to explore the limits of copper loss reduction and available torque improvement. Different from the conventional methods based on maintaining spatial circular rotation of magnetic motive force (MMF), the proposed method can achieve disturbance-free operation during open-phase fault only when q-axis current undisturbed, rather than requiring both d- and q-axis currents to be undisturbed. Thus, the degree of control freedom on d-axis can be released, where the simple harmonic current injection method can be employed to further improve fault-tolerant performance. Both theoretical and experimental results are presented to validate the superiority and effectiveness of the proposed method.