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In this paper, the coupling mechanism of phase-locked loop (PLL)-synchronized multiparalleled wind farms under grid faults is analyzed in detail. First, the coupling effects of the currents and the equivalent power angles (EPAs) among wind farms are identified by analyzing the voltage characteristics during low voltage ride-through (LVRT). Subsequently, a transient stability assessment method is proposed for multiparalleled wind farms to evaluate the effect of output currents of multiple wind farms on the EPA distribution characteristics. This method can be applied to evaluate whether each wind farm in the system has equilibrium points as well as evaluate the transient instability risk of the system during LVRT. In addition, the dominant wind farms for the system transient instability can be identified. Due to the coupling effects, any inappropriate current in the wind farm may deteriorate the EPA distribution characteristics and even cause loss of synchronism. Consequently, the current distribution method to minimize transient instability risk can ensure that each wind farm in the system has equilibrium points and effectively improve the multiparalleled wind farms stability during LVRT. Finally, the simulations results validate the effectiveness of the theoretical analysis and proposed method.

In this article, the frequency coupling of weak grid-connected doubly fed induction generator (DFIG) based wind turbines (WT) during asymmetric faults have received little attention. To analyze the frequency coupling phenomenon of the system during asymmetric faults, this article established the complex transfer function model in αβ frame based on the transformation relationship between complex transfer function and transfer matrix. According to the complex transfer function, the frequency coupling mechanism of the DFIG system during asymmetric faults is revealed, and the transfer evolution law of coupling frequencies is described in detail. Finally, the correction of theoretical analysis and the validity of proposed complex transfer models are verified by the simulation and experiment.

In this article, the transient synchronization process of the grid-connected voltage source converters (VSC) is studied detailly. Firstly, the phase-locked loop (PLL)-synchronized VSC is modeled according to the rotor motion equation of synchronous generator (SG). Furthermore, the VSC's damping ratio is derived, and the effects of the VSC's control parameters and the operation status on the equivalent power angle (EPA) is investigated. Moreover, the influence of the PLL's parameters and the voltage dips degree on the VSC's transient frequency behavior are analyzed. Analysis result reveals that the VSC's damping ratio decreases once the grid voltage drops, so that the EPA and frequency of the VSC may appear large overshoot and oscillation, which may trigger the frequency protection and deteriorate the VSC's transient stability. In order to ensure that the VSC can smoothly operate to new equilibrium points, an improved PLL is proposed, which can adaptively adjust the VSC's damping ratio for different voltage sags. The proposed method can not only reduce the overshoot of the EPA but also restrain the VSC's frequency dips degree. Finally, the simulations and experimental tests validate the effectiveness of the theoretical analysis.

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.

The voltage–frequency coupled transient stability and risk assessment issues of weak grid-connected large-scale renewable energy plants (REPs) during low voltage ride through (LVRT) have received little attention to date. The shape and mechanism of voltage–frequency coupled transient instability during LVRT are investigated in detail by the voltage-vector-triangle graphic (VTG) method utilized in this article, which avoids complex mathematical calculations. In addition, the margin of the transmission line's voltage angle (MTVA) is defined in this article and is then used to quantitatively assess the transient instability risk of the large-scale REPs during LVRT. Furthermore, the expressions of the MTVA are deduced under different conditions and the major influencing factors, which resulted in instability risk, are determined correspondingly. On this basis, the transient instability risk assessment standard of the large-scale REPs is built and the instability risk minimized scheduling strategy is proposed in this article, which does not change the internal control structure and parameters of the REPs during LVRT. Finally, both the correctness of the VTG method and the risk assessment standard, and the effectiveness of the scheduling strategy proposed in this article are validated by experiments and simulations.

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.

The instability issues of grid-connected voltage source converters (VSC) may easily occur during low voltage ride-through (LVRT), especially when connected to a weak ac grid. In this study, a small-signal model of the grid-connected VSC system was developed to deal with the stability problems during deep voltage sags. Based on the model, the interaction between the phase-locked loop (PLL) and current control has been illustrated. In addition, the eigenvalue and modal analysis method was employed to investigate the influencing factors of the VSC system stability, which include the bandwidths of the PLL and current control loop, grid strength, and voltage sags. Furthermore, on the basis of the interaction between PLL and current control loop, a novel additional damping controller that is placed in the active current control loop was proposed and designed. Finally, experiments were conducted to verify the theoretical analysis and proposed control strategy for enhancing VSC system stability during LVRT with a high impedance grid connection.

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.

This paper establishes a small-signal impedance model of the doubly fed induction generator (DFIG) system under weak grid faults taking the dynamic of controller and impedance controller into account. Based on the impedance model, the instability mechanism of the DFIG system during LVRT is analyzed using the generalized Nyquist criterion (GNC). The analysis result shows that the small-signal instability of the DFIG system is due to the DFIG-based wind turbines (WTs) in account of the control showing the negative damping characteristics during grid fault. The small-signal stability of the DFIG system can be improved by changing the bandwidth of the phase-locked loop (PLL). The Simulation results verify the correctness of the analysis.

In} this paper, the small signal state space model of the DFIG system taking into account the phase-locked loop (PLL) under the weak grid fault is established. Based on the small signal model, the dynamic behavior of the DFIG-based wind turbines during LVRT is investigated. Moreover, the impacts of voltage dips degree, operating conditions, bandwidth of the PLL and control parameters of the RSC etc. on the dynamic behavior of the DFIG-based wind turbines under weak grid symmetrical faults are revealed in detail. Finally, the simulation for the DFIG-based wind turbine system is carried out using Matlab/Simulink, and the simulation results are presented to verify the correctness of the theoretical analysis.

A new spiropyran-based stimuli-responsive delivery system is fabricated. It encapsulates and then releases an extraneous compound in response to elevated levels of Zn

Expression of AQP1 and AQP5. qPCR (2-Î Ct) results normalised to reference gene PMM1. (PDF 95 kb)