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 the PWM inverter. Parallel connection of power semiconductor devices permits increasing the output current, while 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 (NPC) 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 are considered that behave as five-level units. Synchronous optimal pulsewidth modulation is applied for their control. This permits reducing the switching frequency to the 100-Hz region without sacrificing on harmonic distortion.