Cascaded multilevel converter for medium-voltage motor drive capable of regenerating with part of cells

A new regenerative cascaded multilevel converter and its control strategy based on V/f control has been presented. The converter employs insulated gate bipolar transistor-based rectifiers as active front end (AFE) only in part of cells and diode-based rectifiers in the rest cells. Those cells with AFE enable the converter to deliver the power generated by the motor to the grid. In the course of the motor´s deceleration, the converter controls the output phase difference of the two types of cells to feed the motor generated electrical energy back to the grid through the cells with AFE and keep other cells´ DC-link voltages stable. Simulation results of a 6-kV six-cell cascaded converter with one regenerative cell per phase and an experiment conducted on a laboratory cascaded converter verify that the control strategy is feasible and the drive system´s speed regulation performance is improved. Compared with the converter with AFE in each cell, this converter has lower complexity and lower cost.