模块化多电平换流器子模块电容电压波动与内部环流分析

Analysis of Sub-module Capacitor Voltage Ripples and Circulating Currents in Modular Multilevel Converters

为了分析模块化多电平换流器(modular multilevel converter,MMC)子模块电容电压波动和内部环流的基本成分,以开关函数为出发点,对电容电压波动和环流进行了数学分析。分析结果表明:基频波动和2倍频波动是电容电压波动的基本成分,并得出其解析表达式;内部环流仅包含偶次谐波成分,且以2次谐波环流为主,并给出其幅值的解析表达式。针对环流对MMC运行的影响,分析了MMC交流侧相电压中的谐波成分,结果表明环流导致MMC输出相电压中的奇次谐波含量增大,尤以3次谐波最为明显,且给出了3次谐波幅值的计算表达式。仿真结果表明上述理论分析是准确的,为子模块电容值和桥臂电感的选取提供了理论基础。

In order to identify the basic components of sub-module （SM） capacitor voltage ripple and circulating currents in modular multilevel converters （MMC）, analysis was made through a mathematical method based on the switch function. The analysis implies that ripples with fundamental frequency and double line-frequency are basic components in the capacitor voltage ripple. In addition, their analytical expressions were derived. The analysis also indicates that circulating current consists of even harmonics, in which the 2nd harmonic is the major component. The analytical expression of the 2nd harmonic was also derived. For investigating the effect of circulating currents on the output voltage of MMC, the harmonics in output voltages were analyzed. The analysis shows that the circulating current increases the harmonic content in output voltages, especially the 3rd harmonic, with its analytical expression derived. Simulation results demonstrate the analysis above is precise, and it provides a theoretical proof to determine the size of SM capacitances and ann inductances.