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The commutation failure fault usually occurs in the line-commutated-converter-based high-voltage direct current transmission system. When commutation failure fault occurs, the voltage of sending alternate current (ac) system changes rapidly, and the connected doubly fed induction generator (DFIG)-based wind turbine may be tripped. Thus, the fault ride through (FRT) control strategy of DFIG should be investigated for enhancing the stability of the sending ac system. However, the voltage of the sending ac system during commutation failure is not changed in rectangular in shape, besides, the voltage presents the “first reduce then rise” characteristic, which is not considered in the existing FRT control strategies. In order to realize the continuous FRT of DFIG during commutation failure, the stator flux and electromotive force when the stator voltage changes continuously have been analyzed for the first time in this article. Furthermore, based on the analysis results, an improved continuous FRT control strategy is proposed. The simulation and experiment results validate the effectiveness of the proposed method. The proposed control strategy is not only suitable for the commutation failure condition, but also for the scenario with continuous voltage variation during grid fault, which indicates that the proposed method is general.

With the increasing penetration of the wind power, the stability issues of the weak AC grid-connected doubly fed induction generator (DFIG)-based wind turbines during low-voltage ride through (LVRT) cannot be neglected. In order to explore the instability mechanism of DFIG system during weak grid fault, the small signal state-space model is established in this paper. The results of the modal analysis show that the dominant unstable poles are mainly impacted by the phase-locked loop (PLL), rotor current control loop, and terminal voltage during the fault, where the PLL is the dominant factor. Furthermore, the impact of each factor on the system dynamic stability is comprehensively evaluated, which indicates that the controller bandwidth under normal grid condition is no longer applicable to the fault condition due to the interaction between the controller and grid. Then, the optimal current proportion which can significantly improve the system stability is deduced. Finally, based on the analysis, this paper proposes the improved LVRT control schemes from two aspects of either injecting active current or decreasing PLL bandwidth to enhance the small signal stability of the system. The effectiveness of the proposed LVRT control strategies is validated by the simulation and experiments.

A non-communication-based coordinated control strategy for a hybrid wind farm with doubly fed induction generator (DFIG)-based and direct-driven permanent magnet synchronous generator (PMSG)-based wind farms under severe asymmetrical grid faults is proposed in this paper. Firstly, the in-depth research of the severe asymmetrical fault and its impact on the operation characteristics of the DFIG and PMSG systems are investigated. Secondly, based on the operation characteristics analysis, the control objectives and priorities of the hybrid DFIG and PMSG systems are described first time during severe asymmetrical fault, respectively. In addition, the current allocation principles of each control unit in the DFIG and PMSG systems are investigated in detail according to the converter capacity and the system operation conditions. Furthermore, a coordinated control strategy for the hybrid wind farm is proposed. This strategy make full use of each wind farm's current capability, both the operation performance of the entire hybrid wind farm and the voltage quality of the power grid was greatly improved collectively. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy for the hybrid wind farm with DFIG and PMSG are validated by the simulation and experimental results.

The synchronization characteristics of phase-locked loop (PLL) based renewable energy generators (REG) are considerably sensitive to the grid condition and the inner sequential switching actions of their control system, especially suffered from grid faults. In this article, the general output characteristics of REG systems are investigated considering the nonlinear behaviour of PLL. In addition, general sequential switching control schemes for the entire grid fault process are introduced. Then, to physically determine and theoretically analyze the transient synchronization stability of REG systems, the synchronization model of a REG system for different fault stages is built and presented in the form of rotor swing equations, which are similar to those of a synchronous generator (SG). The proposed model was able to deduce the characteristics of virtual torque, virtual inertia, and virtual damping coefficient of the REG system, and identify the coupling relationship between the angular frequency/magnitude states of the terminal voltage of the REG system during the synchronization process. Thus, the synchronization stability criteria were proposed based on those of the SG, in which the transient instability phenomenon and mechanism for different fault stages is physically explained by the deduced equal area criteria. Finally, the analysis was verified by simulations and experiments.

When the mono-polar blocking fault occurs at the line-commutated converter (LCC)-based high voltage direct current (HVDC) transmission system, due to the long switch time of the installed mechanical switching type VAR compensations, the reactive power of the sending alternating current (AC) system will be surplus. Consequently, the voltage of sending AC system will be sharply increased during the switch process of VAR compensations, which may trip off the connected doubly fed induction generator (DFIG)-based wind farm. For ensuring the sending system can operate in stability and security, the power flow and the overvoltage mechanism of sending AC system under mono polar blocking have been investigated firstly. Furthermore, the power controllable operation area of the DFIG system as well as the transient response of the synchronous compensator (SC) and static synchronous compensator (STATCOM) have been researched. Then, a fault ride through control strategy by coordinating the VAR compensations, DFIG-based wind farm, HVDC system converter and SC is designed. The simulation results show that the proposed scheme can significantly restrain the sending AC system’s overvoltage during the switch process of the installed VAR compensators under mono-polar blocking fault, thus, the transient stability of the HVDC sending system can be effectively enhanced.