模块化多电平换流器型直流输电内部环流机理分析

Mechanism Analysis on the Circulating Current in Modular Multilevel Converter Based HVDC

模块化多电平换流器型直流输电(MMC-HVDC)是新一代多电平电压源换流器直流输电拓扑,它利用多个子模块串联,并采用特定的触发方式使桥臂电压波形逼近正弦,但其三相间的内部环流使得流过桥臂的正弦电流产生畸变,增大了桥臂电流的峰值,从而提高了对开关器件电流容量的要求。为研究桥臂环流的计算方法,通过瞬时能量平衡关系,分析了MMC内部环流的产生机理,指出其为2倍基波频率,且为负序性质,并进一步推导了环流大小的解析表达式,为选择桥臂串联电抗器,抑制环流提供了依据。最后通过PSCAD/EMTDC搭建了21电平MMC-HVDC双端系统模型,并采用了子模块电压均衡控制方法,仿真结果验证了该模型的有效性和环流计算公式的准确性。

Modular multilevel converter based HVDC（MMC-HVDC） is a new generation of VSC-HVDC topology proposed by Siemens.Several sub modules and specific modulation are applied in a sine waveform of arm voltage.Due to the discrepancy of three-phase arm voltage,circulating current appears among the three-phase unit.The circulating current distorts the sine arm current and increases its peak value,so the requirements of current rating increase.Based on the conservation of energy,we analyzed the mechanism of circulating current,indicating that it is a negative sequenced,frequency-doubled current.Furthermore,the analytical expression of circulating current was derived.So the basis for selecting series reactor to damp the circulating current was provided.The model of MMC-HVDC with 21 voltage-levels was realized by PSCAD/EMTDC.Simulation results show a close agreement with the calculation results of circulating currents.