对由双馈风电机组组成的海上风电场采用混合高压直流输电技术并网时风电场内部的电压和频率控制进行了研究。混合高压直流输电系统由双桥十二脉波不控整流换流器(DBC)、模块化多电平换流器和高压直流输电线路组成。首先，通过深入的理论分析阐明当由双馈风电机组组成的海上风电场采用混合高压直流输电技术并网时，风电场内部的电压可以自动维持在一个合适的范围内并随双馈风电机组输出有功功率的变化而变化。在此基础上，设计了双馈风电机组转子侧换流器的控制器以实现对风电场内部交流系统频率的控制，同时实现了双馈风电机组输出有功功率的最大功率点跟踪。为防止岸上公共连接点发生三相接地短路故障时基于 DBC 的高压直流输电系统发生过电压，设计了故障时双馈风电机组的控制策略。最后，对建立的采用混合高压直流输电技术并网的海上风电场模型进行了数字仿真，仿真结果验证了理论分析的正确性和所提出控制策略的有效性。%A novel control strategy is designed for the offshore doubly fed induction generator (DFIG) based wind farm connected to the onshore system through a hybrid high voltage direct current (HVDC) transmission topology.The hybrid HVDC is composed of a modular multilevel converter and a diode rectifier.Detailed analysis is made to prove that the offshore AC-grid voltage can be maintained in a proper range during normal operation.Based on this precondition,a control strategy for the offshore AC-grid frequency and maximum power point tracking (MPPT)of the DFIG is proposed.In order to ensure that the voltage of the HVDC link is lower than its upper limit during the onshore point of common coupling (PCC) short-circuit, the fault ride-through control of the DFIG is also designed.Finally,a digital simulation system of the offshore DFIG-based wind farm and the hybrid HVDC link is established with the effectiveness of the proposed control strategy verified by simulation results.
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