This paper presents a control strategy to improve the low-voltage ride-through capability of a doubly fed induction generator (DFIG); since the stator of a DFIG is directly connected to a grid, this sort of machine is very sensitive to grid disturbance. Grid voltage sag causes overcurrents and overvoltages in rotor windings, which can damage the rotor-side converter (RSC). In order to protect the RSC, a classical solution based on installation of the so-called crowbar is adopted; however, as the DFIG absorbs reactive power from the grid, this type of solution deteriorates grid voltage sags and cannot meet the requirements of a new grid code. An improved control strategy which uses virtual resistance to limit rotor side overcurrents is proposed in this paper, which can make a crowbar inactive and supply reactive power to fulfill the latest grid code requirement during voltage sags. In order to validate the proposed strategy, simulations and experiments have been carried out, and the results demonstrate the effectiveness of the proposed strategy.

When LCL-type converter is attached to weak grid, its current control and phase-locked loop (PLL) will interact with each other, via the point of common coupling voltage. Unfortunately, the conventional PLL controller design methods are mainly for PLL independent systems, regardless of the aforesaid interaction. As a consequence, PLL dynamic might deteriorate the grid current control and even leads to instability problem. For this issue, this article proposes an improved design of PLL controller parameters to mitigate the negative effect of PLL on the current control. First, the small-signal model of the current control system considering PLL effect is established, and the system stability with the conventional PLL controller design method is analyzed. Then, an improved design of PLL controller parameters is proposed, and the design guideline is given in detail. With the method, not only the dynamic and static response performance of PLL independent system can be maintained, but also the negative influence of PLL dynamic on the current control can be effectively reduced in weak grid. Moreover, the grid current control has a strong robustness to the grid impedance variation. Finally, the proposed method is validated by the simulation and experiment.

For three-phase LCL-type grid-connected converter, when it is attached to weak grid, current control interacts with phase-locked loop (PLL) via point of common coupling (PCC) voltage. Consequently, PLL dynamic might deteriorate grid current control and even result in system instability. However, the conventional design method of current controller neglects the impact of PLL, and therefore, it is hard for the current controller to mitigate the negative effect of PLL. In this paper, an improved design of current controller, i.e., PI controller and capacitor current feedback active damping, is proposed to reduce the negative effect of PLL on current control. First, a small-signal impedance model is developed to analyze the impact of PLL on current control. Then, the effect of current controller parameters on the converter output impedance is analyzed, and a design guideline to improve the current controller parameters is presented. With the improved parameters, in precondition of satisfying system stability margin under both stiff and weak grid, the negative effect of PLL on current control can be effectively mitigated without employing additional control strategies. Furthermore, the current control has a strong robustness against wide-range variation of grid impedance. Finally, the experiment demonstrates the effectiveness of the proposed design method.

For doubly fed induction generator (DFIG)-based wind turbines, the rotor side of DFIG is prone to suffering from overcurrent during grid faults, due to large electromotive force (EMF) induced in the rotor circuit. To solve this problem, this paper proposes an inductance-emulating control strategy for DFIG-based wind turbine to suppress the postfault rotor current, thereby enhancing its low-voltage ride through capability. Under the proposed control strategy, once the grid fault is detected, the rotor side converter (RSC) is controlled to emulate an inductance. Furthermore, with proper inductance value, both the required rotor voltage and postfault rotor current can be reduced within the permissible ranges of RSC, thus the controllability of control system can be maintained during transient process. Moreover, the oscillation of electromagnetic torque can be effectively suppressed during transient state of both grid fault and fault recovery. Finally, the simulation and experimental results are presented to demonstrate the effectiveness of the proposed method.

Virtual synchronous generator (VSG) control-based grid-connected converter (GCC) is an attractive solution to improve the stability of more renewable energy-integrated power system. Unfortunately, the inherent power coupling (i.e., the interaction between active and reactive power loop) defect of VSG control severely restricts the power delivery capacity and grid support capability of GCC. The virtual inductor is commonly used to reduce coupling, but its decoupling capability is very limited. And, the power coupling mechanism and its limiting factors are not clear. For this issue, the nature of power coupling in VSG is investigated firstly. The decoupling capability of virtual inductor is studied, and the reason for decoupling effectiveness is revealed. It indicates that the effectiveness of decoupling results from the proper voltage compensation, but this kind of positive effect is limited by the d-axis voltage drop across virtual inductor. Then, a q-axis voltage drop based power decoupling control (QVPDC) is proposed to further reduce power coupling, which does not consider d-axis voltage drop when applying virtual inductor. Compared with the virtual inductor method, the decoupling performance of QVPDC is better, and the computation burden is reduced by half. Finally, the analysis and proposed method are validated by simulation and experiment.

For doubly fed induction generator (DFIG)-based wind turbines, when severe grid faults occur, it is necessary to inject transient and negative-sequence components into the rotor current reference to achieve controllable low-voltage ride through (LVRT) capability. However, the classical proportional-integral (PI) controller cannot provide precise control of the transient and negative-sequence rotor current, because such current references in synchronous reference frame (SRF) are AC components at grid frequency and double grid frequency, respectively. To solve this problem, this paper proposes a feedforward current references control (FCRC) method for rotor side converter (RSC) to improve the system transient control performance. Under the proposed method, the additional feedforward of rotor current references is introduced into the current loop to improve its tracking capability of the aforementioned AC current references, thereby achieving the transient control targets precisely. Moreover, the proposed method will not affect the stability of the original current loop. Furthermore, the method has strong robustness against DFIG parameters deviation and grid frequency variations. In addition, the method is highly simple to implement. Finally, the validity of the proposed FCRC method is demonstrated by simulation and experiment.