Low-Modulation-Index Operation of a Five-Level Diode-Clamped PWM Inverter With a DC-Voltage-Balancing Circuit for a Motor Drive

This paper presents a five-level diode-clamped pulse width-modulated (PWM) inverter intended for a medium-voltage motor drive with a constant-torque load but no regenerative braking. It is applicable to drilling rigs, extruders, and rubber mixers. The power conversion system consists of a three-phase six-pulse diode rectifier, a five-level diode-clamped PWM inverter, and a dc-voltage-balancing circuit including a single coupled inductor. The five-level inverter is characterized by injecting a common ninth-harmonic zero-sequence voltage on each of the three-phase reference voltages in a low-modulation-index region. This injection contributes not only to mitigating magnetic-flux fluctuation of the coupled inductor but also to making uniform the power losses produced by eight insulated-gate bipolar transistor modules per leg. In addition, the dc-voltage-balancing circuit introduces a control method capable of reducing an amplitude of magnetic flux of the coupled inductor. The combination of this control method and the ninth-harmonic zero-sequence voltage injection keeps the peak value of the magnetic flux within its designed value.