Voltage drift mechanism in modular multilevel converter

The capacitor voltage drift problem in Modular Multilevel Converter (MMC) system has been widely discussed in the past. It is well known that sometimes the capacitor voltages are balanced, without any active controls. However, due to the uncertainty and lack of theoretical explanations, active capacitor voltage controls are typically preferred for real applications, and the voltage balancing/drift mechanism of MMC topology did not draw much attention in the past. This paper covers this gap: it is explored that the cell voltage drift effect is caused by the sideband overlap effect. Moreover, the requirements of voltage drift effect are discussed systematically. The conclusion is that for high-power applications with limited switching frequencies, low but non-integer carrier ratios are highly preferred to avoid the intrinsic voltage drift between MMC cells. By doing this, active voltage balancing algorithms are only needed as backup.