Enhanced Modular Multilevel Converter With Reduced Number of Cells and Harmonic Content

Abstract

This paper presents an alternative implementation 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 (HB) chain link of medium-voltage cells and a full-bridge (FB) chain link of low-voltage cells in each of its arms. The total blocking voltage of the FB chain link is equivalent to half that of the medium-voltage HB cell. The use of HB and FB cells with two distinct rated voltages in each arm permits full exploitation of the FB cells to generate high-resolution multilevel voltage waveforms with fine stepped transitions between major voltage steps of the medium-voltage HB 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 HB and FB cells. Detailed comparisons with existing MMC implementations show that the proposed MMC implementation offers the best design tradeoffs (superior ac and dc waveforms with reduced control and power circuit complexity). The validity of the proposed MMC implementation is confirmed using simulations and experiments.