Optimal Control of a Dual Three-Level Inverter System for Medium-Voltage Drives

Actual developments in medium-voltage drives aim at increasing the power capability of pulsewidth modulated (PWM) inverters. Parallel connection of power semiconductor devices permits increasing the output current, whereas series-connected devices increase the output voltage. In either case are additional means required for balancing the current or voltage stress of the devices. The three-level neutral-point-clamped inverter topology offers an inherent means to ensure dynamic voltage balancing of a maximum of two series-connected devices. Higher operating voltages can be achieved by series connection of entire inverter topologies. Two circuit topologies that behave as five-level units are considered. Synchronous optimal PWM is applied for their control. This permits reducing the switching frequency to the 100-Hz region without sacrificing on harmonic distortion.