The synchronization stability and instability risk assessment of multi-paralleled wind farms during asymmetrical grid faults have received little attention to date. In this article, a simplified equivalent model of multi-paralleled wind farms is established based on the symmetrical components method to deal with the problem of dual-sequence synchronization stability during asymmetrical grid faults. Based on the model, the effects of multiple coupling characteristics such as sequence coupling and mutual coupling of wind farms on the stability of dual-sequence synchronization are studied in detail. Then, an assessment method is proposed to evaluate the dual-sequence synchronization stability of multi-paralleled wind farms under various asymmetrical grid faults. This assessment method can judge whether there are positive-sequence (PS) and negative-sequence (NS) steady-state equilibrium points for each wind farm in the system, and identify the wind farms that are instability due to the lack of a PS/NS equilibrium point. In addition, a current control strategy is derived to maximize the stability margin of dual-sequence synchronization. This strategy can ensure that each wind farm in the system has equilibrium point. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed method are verified by simulation.

The stability of a voltage source converters (VSC) system based on phase-locked loop (PLL) is very important issue during asymmetric grid faults. This paper establishes a transient synchronous stability model of a dual-sequence PLL-based VSC system during low voltage ride-through by referring to the equivalent rotor swing equation of synchronous generators. Based on the model, the synchronization characteristics of the VSC system under asymmetric grid faults are described, and the interaction mechanisms, as well as the transient instability phenomena of positive and negative sequence PLL during asymmetric faults are explained. Using the equal area criterion, the influences of sequence control switching action, detection delay, and interaction between the positive and negative sequence PLL on the transient synchronous stability of the VSC system are analyzed, respectively. In addition, a transient stability assessment criterion based on the critical fault clearance angle and time and an enhancement control strategy based on the improved positive and negative sequence PLL are proposed. Finally, the analytical results are validated through simulation and experiments.

随着风电场规模的不断增大,电网不对称短路故障期间,风电场与电网之间的耦合作用不断增强,电压失稳风险增大.针对现有风电场接入电力系统技术规定,在电网发生不对称短路故障持续阶段,研究辐射状风电场的静态电压稳定性分析方法.建立了考虑多风电场与连接线路的阻抗模型,根据不对称故障时的动态无功支撑要求,对多风电场的电压稳定性进行分析,获得了风电场和电网正负序阻抗之间的电压相互作用关系.分析结果表明,多风电场满足现有风电并网导则时,所提方法可准确判断电网不对称短路故障持续阶段的静态电压是否稳定.最后,通过仿真验证了理论分析的正确性和所提方法的有效性.

In this paper, the transient stability for dc-link voltage of doubly-fed induction generator (DFIG)-based wind turbine (WT) is studied in detail during low voltage ride-through (LVRT). Firstly, referring to the rotor swing equation of synchronous generator (SG), the nonlinear large-signal model of dc-link voltage is established. In addition, the damping power, static slip power, and dynamic slip power are derived. Consequently, the transient performance of dc-link voltage under the different operation conditions of DFIG is revealed during LVRT. Furthermore, the instability form of dc-link voltage with the moving process of equivalent power angle is analyzed by the energy function of dc-link voltage. In addition, the impacts of dc voltage control loop's parameter, the active current of the stator, and the slip on the stabilization process of dc-link voltage are studied. Analysis result indicates that the insufficient damping and large unbalanced power would deteriorate the transient behavior and steady-state level of dc-link voltage. Therefore, an additional damping and slip power feedforward control strategy is proposed, which can make the transition process of dc-link voltage smoother and significantly improve the voltage steady-state level. Finally, simulation and experimental results validate the effectiveness of theoretical analysis.

新能源并网变换器在电网短路故障期间与电网之间的交互作用显著增强,增加了暂态失稳风险.该文首先建立可再生能源并网换流器(renewable energy grid-connected converter,REGC)在低电压穿越(low-voltage ride-through,LVRT)期间的简化等效转子摇摆方程,刻画并分析其同步特征属性.然后,借鉴传统同步发电机的同步稳定理论,推导等效整步转矩系数、等效等面积准则以及阻尼比3个同步特征指数,物理性地揭示不平衡虚拟转矩驱动REGC等效功角运动,甚至引发暂态失步的内在机理,并同时量化衡量REGC的暂态同步稳定性及其在低电压穿越期间的准静态小干扰同步稳定性.最后,提出一种基于自动虚拟变阻器的改进锁相环(phase-locked loop,PLL)架构,使REGC能够自适应地抵消/补偿线路电阻的压降效应,不仅具备自主平衡能力,而且同时显著增强REGC的暂态同步稳定性及其准静态小干扰同步稳定性.仿真和实验结果验证了理论分析的正确性和所提控制策略的有效性.

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.

Under grid fault, the grid-connected voltage source converters (VSCs) have the risk of transient synchronization instability during low voltage ride through. Based on the equivalent rotor swing equation of VSC, this letter proposes an improved transient synchronization stability control strategy, which improves the existence of equilibrium points and transient synchronization behaviors of the VSC system. Thus, the transient synchronization stability of grid-connected VSC can be significantly enhanced. The simulation and experimental results demonstrate the validity of the proposed strategy.

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.

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.

The high voltage direct current (HVDC) transmission technology based on modular multilevel converter (MMC) is widely used in the offshore grid-connected wind power generation projects, and the small signal stability problems of the grid-connected system have been attracted great attention. This paper focused on the influence of the zero-sequence circulating current (ZSCC) controller on the stability of the offshore wind power generation MMC-HVDC grid-connected system. The closed-loop frequency domain model was derived by means of complex vectors and harmonic state space firstly. Then the paper developed the single-input-single-output (SISO) impedance model of MMC which considering the ZSCC controller. In addition, the impedance model of the offshore wind farm and the distribution network were established. Based on the impedance model, the influence law and mechanism of the ZSCC controller on the stability of the offshore wind power generation MMC-HVDC grid-connected system were explored. The analysis results show that when the output power of wind is too large, the system is prone to oscillation. The application of the ZSCC controller equivalently increases the bridge arm resistance and enhancing the system damping, which effectively suppressing the resonant peaks of MMC impedance in island control mode. Finally, from the point of view of enhancing the damping of the system, the parameter optimization scheme for ZSCC controller was designed, and the correctness of the theory and the feasibility of the scheme were verified by simulations.