METHOD OF MONITORING THE CONDITION OF ENERGY STORAGE DEVICE ASSEMBLIES OF A MODULAR MULTI-LEVEL CONVERTER (MMC)

Provided is a method of monitoring the condition of energy storage device assemblies of a modular multi-level converter (MMC) that includes series-connected submodules having a full-bridge or half-bridge topology Whereby each submodule include an energy storage device assembly with one or more individual energy storage devices electrically connected in parallel. The MMC so includes at least one AC terminal electrically connectable to an AC power source by means of a pre-charge circuit and during two or more pre-charge processes where the individual energy storage device assemblies are initially charged to a rated voltage by an AC pre-charge current supplied to the AC terminal, one or more measurements of DC voltage across each energy storage device assembly are obtained at one or more times between a start and an end of each pre-charge process and compared to determine the health of each energy storage device assembly.