An overlapping multi-hexagon space vector modulation scheme for modular multilevel cascade converters

This paper presents an adapted space vector modulation scheme for modular multilevel converters cascaded to higher levels. Instead of extending a single hexagon to the regions corresponding to the number of levels, this scheme uses multiple overlapping hexagons which are phase displaced relative to each other. The approach simplifies the modulation algorithm and brings flexibility in shaping the output voltage waveforms for different applications. The method is analyzed in simulation for modular multilevel cascaded flying capacitor converters in single star/single delta configurations, and is validated using an experimental two-tier cascaded converter. Output performance, including harmonic spectra, and the capacitor voltage balancing, compares well with those of other control schemes such as conventional multilevel SVM. A key feature is ease of extension to higher cascading levels without forfeiting overall performance.