Modular Multilevel Converter Design for VSC HVDC Applications

The modular multilevel converter (MMC), originally proposed by Professor Marquardt, has made it practical to realize converters with ratings up to 1000 MW by using standard components developed for variable speed drives. With the power electronics packed into individual submodules, realizing a converter where the ac and dc voltages are under direct control and have very little distortion appears to be ideal; although there have been several papers published covering the design considerations required to ensure these converters operate correctly from an academic perspective, the method specifying the design of a fully rated MMC, where several hundred submodules may be required for each valve to meet all conditions in service has not been discussed. This paper outlines a procedure for calculating the values of the transformer turns ratio, the transformer reactance, and the valve reactance before considering how the ripple that appears on the submodule voltage affects the operating performance of the converter. This ripple limits the submodules from being able to fully use their capability, so the principal operators that affect the ripple voltage are considered. The main independent variables available are the submodule capacitor value, the number of submodules per valve, and the average capacitor voltage setting, which is set as a servo demand within the control. All these inputs have an effect on all aspects of the converter performance and so must be set together while considering other system variables such as the variation in the ac voltage.