模块化多电平换流器PR环流抑制器优化设计

Optimal Design of PR Circulating Current Suppressing Controllers for Modular Multilevel Converters

模块化多电平换流器(modular multilevel converter,MMC)适用于高压大容量柔性直流输电,已经有部分工程采用MMC作为柔性直流输电系统换流阀。MMC换流阀内部桥臂环流直接影响其工作特性,而通过比例谐振控制器(proportional resonant controller,PR controller)可以有效抑制环流,但系统稳定性有待提高。该文对PR环流抑制器进行优化设计,通过在PR环流抑制器的基础上增加桥臂电流比例负反馈环节,提高系统的稳定性与环流快速抑制能力。该控制实现简单,同时可以增强上下桥臂子模块电容电压的自平衡能力。通过建立系统闭环传递函数对系统稳定性进行分析;同时,通过建立李雅普诺夫能量函数对附加控制的促进桥臂电容电压自平衡能力进行验证。最后,在PSCAD/EMTDC搭建的仿真模型上进行了仿真验证,仿真结果证明了优化PR环流抑制器的有效性,提高了控制器的稳定性,同时有效增强了上下桥臂子模块电容电压的自平衡能力。

The modular multilevel converter （MMC） is a suitable topology for voltage source converter high-voltage direct-current （VSC-HVDC） power transmission system. The MMC topology has already been used in some VSC-HVDC projects. The performance of MMC is affected directly by the circulating current which is flowing in arms of the valve. The circulating current can be suppressed by the proportional resonant controller （PR controller）. However, the stability needs to be enhanced. This article proposed an optimization design of PR circulating current suppressing controllers. An arm current proportional negative feedback was added in PR circulating current suppressing controllers to enhance the stability of the system and the fast circulating current suppressing ability. The additional negative feedback control is simple in realization. The self-balanced ability of the capacitor voltage in upper and lower arms can also be improved by this supplementary control strategy. The system stability was analyzed through the establishment of the close-loop transfer function. Meanwhile, the promotion of the capacitors self-balanced ability was validated through the establishment of Lyapunov function. A simulation model was established in PSCAD/EMTDC simulation environment. The simulation .result verified the effectiveness of this supplementary strategy. The stability of the system is improved and the self-balanced ability of the capacitor voltage in upper and lower arms is enhanced.