混合型MMC降压运行特征及控制策略

Voltage Reduction Operation Characteristics and Control Strategy for Hybrid Modular Multilevel Converter

针对由全桥子模块和半桥子模块构成的混合型换流器,重点分析了降压运行情况下可能出现的半桥子模块无法均压问题,提出了一种混合型换流器降压运行控制策略。首先建立了混合型换流器的桥臂电流数学模型并分析了其特征;针对混合型换流器在降压运行时可能出现的桥臂电流仅为正或仅为负的情况,利用柔性直流模块化多电平换流器(modular multilevel converter,MMC)无功控制灵活的特性提出了采用无功补偿的控制策略,实现了桥臂电流的有正有负。最后在PSCAD/EMTDC中建立了柔性直流混合型换流器的模型,仿真结果表明,所提控制策略在直流系统大幅降压运行情况下可保证子模块均压要求,避免直流降压运行过程中出现子模块无法均压的情况。

For a hybrid converter composed of full bridge sub-modules and half bridge sub-modules,the possible problem of half bridge sub-modules being unable to equalize voltage during voltage reduction operation is analyzed,and a control strategy for voltage reduction of the hybrid converter is proposed.Firstly,a mathematical model of the bridge arm current of the hybrid converter is established and its characteristics are analyzed.In response to the possibility that the bridge arm current of a hybrid converter may only be positive or negative during voltage reduction operation,a control strategy of reactive power compensation is proposed to achieve positive and negative bridge arm current by utilizing the flexible reactive power control characteristics of the modular multilevel converter(MMC).Finally,a hybrid converter model of voltage source converter-high voltage direct current(VSCHVDC) is established in PSCAD/EMTDC.The simulation results show that the proposed control strategy can meet the voltage equalization requirements of the sub-modules under the condition of significant voltage reduction operation in the DC system,avoiding the situation where the sub-modules cannot be equalized during the DC voltage reduction operation.