Optimal Pulsewidth Modulation of Medium-Voltage Modular Multilevel Converter

Abstract

Modular multilevel converter (MMC) is now the state-of-the-art converter topology for high-voltage dc transmission (HVDC) systems. Another potential application for MMC is medium-voltage (MV) high-power industrial ac drives. In high-power applications, thermal constraints of power semiconductor devices limit device switching frequency to a few hundred hertz. However, there exists a tradeoff between device switching frequency and harmonic distortion of converter output currents. Synchronous optimal pulsewidth (SOP) modulation is an emerging low-device switching-frequency-modulation technique for high-power converters, which maintains the quality of converter output currents. SOP technique has been successfully demonstrated for classical multilevel topologies and it has been proved that maximum-device switching frequency can be limited to rated fundamental frequency for seven or higher level inverters without compromising on the quality of output currents. However, implementation of SOP technique for MMC topology is still pending. One of the main challenges for control of MMC is to maintain floating-capacitor voltages around their nominal value. The goal of our study is to propose, analyze, and implement enhanced SOP technique for MMCs to achieve low-device switching-frequency operation, better quality of converter output currents, and maintain capacitor voltages around their nominal value. The proposed technique has been validated using low-power prototype of five-level MMC feeding an 1.5-kW induction motor.