MMC-HVDC三相解耦二次谐波环流抑制算法

Three Phases Decoupled Second Harmonic Circulation Current Suppression Algorithm for MMC-HVDC

随着模块化多电平换流器(MMC)高压直流输电工程电压等级和输送容量的大幅提升,其拓扑结构中的桥臂串联子模块数急剧增加,使得整个换流阀的控制系统更加复杂,子模块电容电压均衡及二次谐波环流抑制等技术问题将更加突出。如果采用传统的最近电平逼近调制(NLC)策略对换流器进行控制,MMC内部存在明显的二次谐波环流。为了抑制二次谐波环流,假设环流中只含有直流分量,根据换流器交、直流侧瞬时功率平衡,推导出直流环流电流的计算公式,进而分别求得上、下桥臂子模块电容电压的参考值,该参考值在一个工频周期内围绕某一恒定值呈周期性变化,可使得各子模块电容电压的不均衡程度明显降低,二次谐波环流得到有效抑制。与传统的NLC策略相比,文中算法可以降低子模块电容电压的波动幅度,改善交流侧输出电压、电流的波形。当电容器容值存在差异时,文中算法还可以减小容值较小的电容器投入运行的时间,延长其寿命。最后,仿真结果验证了文中算法的有效性。

With voltage levels and transmission capacity increased dramatically in modular multilevel converter（MMC） based high voltage direct current（MMC-HVDC） projects,the number of sub-modules in each arm increase dramatically,making its control system more complicated.The issues of sub-module capacitor voltage balancing and second harmonic circulation current suppression technology will become more prominent.If the traditional nearest level control（NLC） strategy is used,second harmonic circulation current obviously exists in MMC.In order to suppress second harmonic circulation current,assume that circulation current only contains DC component,DC circulation current formula can be derived based on instantaneous power balance between AC and DC side of the converter.And then each sub-module capacitor voltage reference value in upper and lower arms is deduced.The reference value periodically varies around a certain constant value in a power frequency cycle period,reducing the degree of sub-module capacitor voltage unbalance,and the second harmonic circulation current is validly suppressed.Compared with traditional NLC,the proposed algorithm can reduce the sub-module capacitor voltage fluctuations and improve the output voltage and current waveforms of AC side.When differences exist among the capacitor values,compared with traditional NLC,the proposed algorithm can also reduce the running time of low-capacity capacitors,and prolong their useful life.At last,an eleven-level single end MMC converter simulation model is built to verify the effectiveness of the proposed algorithm.