Inner control of Modular Multilevel Converters - An approach using open-loop estimation of stored energy

The inner control of a Modular Multilevel Converter (M2C) must be designed so that the submodule capacitor voltages are equalized and stable independent of the loading conditions. It has been shown that a submodule selection mechanism, included in the modulator, can provide voltage sharing inside the converter arm. Several procedures for controlling the total stored energy in each converter arm exist. A new approach is described in this paper. It is based on estimation of the arm energy from measured ac output current and dc voltage. No feedback controllers are used. Experimental verification on a 3-phase 10 kVA prototype is presented along with the description of the new procedure.