A regenerative cascaded multilevel converter with reduced active front ends

This paper presents a new topology of cascaded multilevel converter which substitutes active front ends for the diode-based front ends only in part of cells making the converter regenerative. Its control strategy based on V/f control is developed. During the induction motor's deceleration, by controlling the phase difference of the output fundamental voltages of the two types of cells, the converter makes the electrical energy generated by the motor wholly collected by regenerative cells and thus other cells' DC-link voltages remain stable. A six-cell cascaded converter with three regenerative cells per phase is analyzed as an example. Simulation results show that the new converter can deliver the power generated by the motor back to the grid and the power quality is not substantially affected. An experiment on a laboratory cascaded converter verifies that the control strategy is feasible. Compared with the converter in which each cell has an active front end, the new converter has lower complexity and lower cost.