Varying and unequal carrier frequency PWM techniques for modular multilevel converters

Carrier-based pulse-width modulation (PWM) strategies are commonly applied to modular multilevel converters (MMC). This paper proposes a modified PWM technique with unequal and varying carrier frequencies between consecutive levels. The variation between two consecutive carriers is calculated in order to maintain a constant average switching frequency regardless of modulation index and number of levels in the output. The proposed method effectively shifts the major harmonic content of output waveforms to higher frequencies at the cost of a slight increase in low order harmonics. Details on the derivation of the carrier frequencies as well as simulation and experimental results of the proposed method demonstrate the viability of the method as an alternative modulation technique for the MMC.