MMC modules with control circuit powered from module capacitor voltage

A Modular Multilevel Converter (MMC) is one of the latest multilevel converters used for high and medium voltage power conversion. It is based on cascade connection of multiple identical modules using IGBTs as switching devices. In each MMC module, the control circuit consisting of gate drivers is powered from a dc supply derived from the local capacitor. The module capacitors need to be pre-charged, to power the control circuit. If a fly-back converter is used to generate the power supply for driving the control circuit, the module capacitor voltages are becoming unstable during pre-charging. This paper proposes a method for stabilizing the same. By using an open loop forward converter with a linear regulator at its output, the module capacitor voltages can be maintained stable during the pre-charging of MMC. This method allows simultaneous pre-charging of module capacitors directly from the dc bus, without using any auxiliary supply. This solution is verified experimentally in the laboratory.