Five-Level Standalone Neutral Point Clamped Rectifier with Time Division Multiplexing-based Voltage Balancing Control and Active PFC

The biggest drawback of Neutral Point Clamped (NPC) converters lies in their inability to maintain voltage balance in the dc-link capacitors for some operating conditions when the number of voltage levels (N) exceeds beyond three. To overcome this limitation, NPC inverters are usually combined with active dc-link voltage generation by utilizing multi-winding transformers and controlled rectifiers, or by using a second NPC converter as an active frontend, thus resulting in a complex system. Moreover, previously proposed Multilevel Rectifier (MLR) solutions have relied on additional auxiliary circuitry for handling the dc-link voltage balancing. In this article, a Five-level NPC-based rectifier with a Time Division Multiplexing-based voltage balancing control is revealed and verified by digital simulations and controller plus hardware in loop real-time simulations. The proposed PWM-based control scheme allows the NPC to be operated as either a standalone MLR with an unbalanced resistive load or as an active frontend to drive a second NPC-based inverter, in which dc-link voltage balancing is carried out only from the rectifier side, while also maintaining the ability to control the power factor and the amount of reactive power exchanged with the ac grid.