A Novel Voltage-Balancing Control for MMC with Flying Capacitor Submodules in Medium-Voltage Variable-speed Drives

The three-level flying capacitor submodule (3L-FC SM) is emerging as a substitute for the traditional half-bridge submodule in the modular multilevel converter (MMC) owing to the benefits of its smaller footprint size and lower circulating current. This paper introduces a method for controlling voltage balance in the MMC equipped with 3L-FC SMs, utilizing a carrier-based pulse-width modulation (PWM) scheme, which employs simple proportional-integral (PI) controllers for the averaging, circulating-current, and individual capacitor balancing controls. The output voltage commands are then matched against the triangular carriers to create control signals for SMs. As a result, the voltages across the outer and inner capacitors of 3L-FC SMs can be balanced well at their references. The validity of the suggested approach has been confirmed in motor drives in full-speed range, with a 4160-V/1-MW simulation model using a hardware-in-the-loop (HIL) based system. Additionally, the paper includes a comparison of total harmonic distortion (THD) between MMCs utilizing 3L-FC SMs and half-bridge SMs.