Control of Modular Multilevel Converters Using an Overlapping Multi- Hexagon Space Vector Modulation Scheme

Adapting the conventional Space Vector modulation (SVM) scheme for modular multilevel cascaded converters is complicated as the number of switching vectors increases with the number of voltage levels. This paper introduces a novel SVM scheme that can be applied for the control of modular multilevel cascade converters (MMCC) with any number of levels. Instead of extending a single hexagon to the regions corresponding to the number of levels, the proposed method treats the three-phase MMCC as multiple inverters with a phase limb being a chain of basic three level H-bridge, five-level flying capacitor, or neutral point clamped inverters. Basic two or three level hexagons can be applied to determine the switch states and duty cycles separately within one tier of the converter and many such hexagons can be overlapped, with phase shift relative to each other, for the control of a complete MMCC. This approach simplifies the modulation algorithm and brings flexibility in shaping the output voltage waveforms for different applications. Simulation results confirm the good waveform performance of this scheme. An experimental 5-level MMCC, with a total of six H-bridges as the basic modules, is presented to validate the advantageous features of the method.