Center-Point Voltage Balancing of Hysteresis Current Controlled Three-Level PWM Rectifiers

The output center-point voltage of a hysteresis current-controlled three-level, unidirectional PWM rectifier is not naturally stable. This paper shows that the balancing of the center-point voltage can be realized by adding an offset to the input current-reference and that this offset results in a change of the relative on-times of the redundant switching states. The space-vector representation of the input rectifier voltage and current is used to help explain the effect of the current-offset injection on the output center-point voltage. The influence of the offset-current injection on the mains currents as well as the control and center-point characteristics are analyzed and verified by measurements. Finally, the design of the center-point controller is presented and its dynamic performance experimentally demonstrated.