Low-speed rated-torque operation of a motor drive using a five-level diode-clamped PWM inverter with a front-end diode rectifier

This paper focuses on a medium-voltage motor drive requiring no regenerative braking. It is operated not only in a high-speed region but also in a low-speed region with the rated torque, intended for an application to drilling rigs. The power conversion system consists of a front-end diode rectifier, a five-level diode-clamped PWM inverter, and a dc-voltage-balancing circuit including a single coupled inductor. The five-level diode-clamped inverter is characterized by injecting a common ninth-harmonic zero sequence voltage on each of the three-phase reference volt ages for mitigating magnetic-flux fluctuation of the coupled inductor of the dc-voltage-balancing circuit. Moreover, the voltage injection can make the power-loss distribution of eight insulated gate bipolar transistors (IGBTs) per leg close to an even profile. Experimental results obtained from a 200-V 5.5-kW downscaled model confirm the viability and effectiveness of the power conversion system, indicating that the ninth-harmonic zero-sequence voltage injection mitigates the magnetic-flux fluctuation.