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As one of the most representative series active compensators, dynamic voltage restorer (DVR) can solve voltage-related power quality uses effectively. The in-phase voltage control and energy-optimised control are the two typical control strategies of DVR. However, the in-phase voltage control results in a large amount of active power exchange between the main grid and the dc link of DVR, and hence high capacity of storage system is needed in the dc link while the energy-optimised strategy requires high injected voltage, resulting in oversizing of the DVR inverter. Thus, in order to avoid the aforementioned drawbacks, this study proposes a modified DVR, which connects an external capacitor in series with the LC filter (LCC) of DVR (LCC-DVR). Based on the operational principles of the LCCDVR, an updated procedure of reference voltage is proposed to avoid overmodulation of DVR caused by load variations, ensuring the continuous operation capability of the LCC-DVR. The overall control scheme of the LCC-DVR and the comparative analysis among the proposed strategy and traditional strategies are also presented. The effectiveness of the proposed topology and method is clearly validated by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

For the grid-connected doubly-fed induction generator (DFIG)-based wind turbine, because of the stator connected to the grid directly, the stator flux easily suffers from the effects of grid voltage variations, such as grid disturbances and grid faults. Moreover, since the magnetic field is excited by the rotor current, stator flux is also affected by the rotor current. Therefore this study systematically studies the dynamic performances of stator flux under consecutive grid voltage variations and varying rotor currents, and its influence on the performances of the DFIG during grid faults. The analyses reveal that the stator flux can be accumulated by the consecutive variations of the stator voltage, and the instants of grid voltage variations can lead to different amplitudes of the stator flux. In addition, the conventional vector control strategy and the active damping strategy are compared with the behaviour of the stator flux. Furthermore, the simulation and experiment are carried out to validate the theoretical analyses, and the results have clearly confirmed the correctness and effectiveness of the analyses of the stator flux performances. [ABSTRACT FROM AUTHOR]

In this study, a duty cycle-based space-vector pulse-width modulation (SVPWM) with overmodulation and capacitor voltage balance control for three-phase three-level neutral-point clamped (NPC) inverters is presented. On the basis of simple linear calculations of three-phase reference voltages, the proposed SVPWM can be derived, which avoids the trigonometry operation required by the conventional SVPWM. The modulation region can also be smoothly extended from the linear region to the overmodulation region through re-synthesising the three-phase reference voltages. This avoids the conventional non-linear function and look-up table. The neutral-point voltage balancing control is achieved via changing the duty cycle of the redundant small vectors in each switching period, which can be easily embedded into the proposed duty cycle-based SVPWM. The dynamic and steady-state performances of the proposed SVPWM are eventually verified by both simulation and experimental results. [ABSTRACT FROM AUTHOR]