由于风力资源与用电负荷呈逆向分布，我国风电一般远距离大规模接入电网，导致其处于弱联运行状态。弱联主要体现在风电接入电网的并网点短路比低和系统的等效惯量小。弱联运行时，风电系统注入电网的电流将影响并网点电压，加剧风电系统与电网之间的交互作用，导致锁相环（Phase-locked-loop，PLL）频率偏离电网频率，引发同步失稳。同时，由于风电系统的常规控制不响应电网频率变化，风电大规模接入将加剧电网频率波动，威胁电网频率的稳定。弱联风电系统应能稳定并网，并在此基础上向电网提供频率支撑。本文针对弱联风电场的同步稳定及其对电网频率的支撑控制问题展开研究，主要贡献为：（1）建立了聚焦PLL低频动态的弱联风电场简化模型，深入分析了大扰动条件下弱联风电场的同步稳定性，从系统静态与动态两方面揭示了同步稳定机理；提出了自适应电流注入方法以提高并网风电场的同步稳定性，并应用于双馈型和直驱型风电场；通过仿真与实验验证了所提方法的有效性。（2）提出了一种新型的基于曲线偏移的电力系统频率支撑控制统一设计框架，分析了曲线偏移机制削弱风机原控制环与频率控制环之间耦合抵消效应的机理，并分别设计了提供暂时性惯量支撑和长期性一次调频支撑的频率控制器。所提曲线偏移方法控制结构简单，针对一次调频控制，该方法能同时改善电网频率的动态和静态指标。（3）提出了基于聚类的多风电机组协同调频控制方法。该方法考虑了风电场内各风机之间的尾流效应，通过求解优化问题从而最大化风电场对电网的频率支撑能力。针对大规模风电场风机台数多，优化变量维度高，优化问题求解困难的问题，所提方法首先将风特性相似的风机划分为同一机群，并分配相同控制指令，进而将每个机群聚合成为一台功率倍乘后的单机。聚合后，优化变量减少，优化问题的解空间维度大大降低。
Weak connection between wind farm and power grid incurs two problems. One is that the short circuit ratio of the connecting point of wind farm would decrease, which will further lead to a fluctuant voltage of the connecting point. The other one is that the equivalent inertia and equivalent primary frequency regulating factor would decrease because the regular control of wind turbine doesn’t react to grid frequency variation.Since voltage of the connecting point is the input of Phase-locked-loop(PLL) of the wind generating system, the interaction between PLL and the grid may lead to loss of synchronization. Meanwhile, the reduction of system equivalent inertia and equivalent primary frequency regulating factor may lead frequency stability problem. Wind generating system should keep synchronizing with the power grid. Furthermore, it should offer frequency support to mitigate the frequency deviation. To deal with these problems, the contribution of this paper are listed as follow:(1) This paper establishes a simplified model of grid-connected wind farm focusing on the low frequency dynamic of PLL. Based on the model, this paper proposes the mechanism of grid-synchronization from the perspective of both steady state operating point and dynamic process. To improve the grid-synchronization stability of grid-connected wind farm, an adaptive current injection method is proposed, which is further elaborated for doubly fed induction generator and permanent magnet synchronous generator. Both simulation and experimental results verify the proposed method.(2) To support system frequency, this paper proposed a novel operating curve shifting based frequency support control frame. The reduction of the conflict between frequency control loop and the original control loop of wind turbine is analyzed. Controllers for temporary inertia emulation and permanent primary control are designed. The proposed method can improve both dynamic and steady-state performance of system frequency response.(3) This paper proposed a clustering based coordinated control for wind farm frequency support. The clustering based coordinated control method takes wake effect of wind farm into consideration and three operating states of wind farm is defined for frequency support. By solving optimization problems the wind farm could work at the proposed operating states. In large scale wind farm with large number of wind turbines, the proposed method firstly clusters the wind farm into several groups according to the wind profile of each wind turbine. Then by dispatching the same control commands to wind turbines in the same group, each group can be further aggregated into a re-scaled single wind turbine. As a result, the number of variables of the optimization reduces significantly.
