Enhanced multilevel modular converter with reduced number of cells and harmonic content

This paper presents an alternative implementation Several derivatives of modular multilevel converters that of a modular multilevel converter (MMC) that generates a large number of voltage levels per phase with high resolution voltage steps from a reduced number of cells per arm. The presented MMC employs a half-bridge chain-link of medium-voltage cells and a full-bridge chain-link of low-voltage cells in each of its arms. The total blocking voltage of the full-bridge chain-link is equivalent to half that of the medium-voltage half-bridge cell. The use of half and full-bridge cells with two distinct rated voltages in each arm permits full exploitation of the full-bridge cells to generate high resolution multilevel voltage waveforms with fine stepped transitions between major voltage steps of the medium-voltage half-bridge cells. In this manner, errors in the synthesis of the common-mode voltages of the three phase legs due to switching of the cell capacitors in and out the power path are reduced. The nested multilevel operation of the proposed MMC results in a number of voltage levels which is related to the product, rather than the sum, of the numbers of half and full-bridge cells. Detailed comparisons with existing MMC implementations show that the proposed MMC implementation offers the best design trade-offs (superior AC and DC waveforms with reduced control and power circuit complexity). The validity of the proposed MMC implementation is confirmed using simulations and experimentally.