A Novel DC-Link Capacitor Voltage Balance Control Based on Coordination of Rectifier–Inverter Stage in a Modified Unidirectional Five-Level Converter for AC Motor Drives

Aiming at the difficulty of voltage balancing control of four dc-link split capacitors in a recently proposed transformerless unidirectional five-level (5-L) converter for ac motor drives, a novel dc-link capacitor voltage control based on the coordination of rectifier–inverter stage is proposed. To facilitate the independent voltage control of four dc-link split capacitors, in the rectifier stage, two additional diodes are added to each single-phase rectifier. Compared with those similar control for the same purpose such as those requiring adding additional bulky storge components or those by utilizing nonnearest vectors and virtual medium vectors to achieve pulsewidth modulation (PWM) control, which will cause higher harmonics in the output and larger switching loss, this proposed coordinated control can achieve the dc-link capacitor voltage balancing control under full operating range with simple implementation, reduced calculation, and better performance due to the coordination. In this article, the basic operating principle of the modified 5-L rectifier is comprehensively analyzed, and the voltage imbalance mechanism of four dc-link split capacitors and the balancing principle of the proposed coordinated control are discussed in detail. Simulations and a downscaled (2.2 kW) prototype experimental results are provided, which validate the feasibility and advancement of the proposed topology and the corresponding coordinated control strategy.