Theoretical Research And Realization Of A Cascaded Multilevel Converter Capable Of Regenerating With Part Of Cells

Traditional cascaded multilevel converter can not deliver the motor generated power back to the grid, and the regenerative cascaded converter with an active front end in each cell has a high cost and complex structure. To solve this problem, this paper proposes a regenerative cascaded converter capable of regenerating with part of cells. This converter replaces the diode-based rectifier with an IGBT-based rectifier only in part of cells, making the converter regenerative. It is a novel topology between the non-regenerative topology and the topology regenerating with all cells. A series of topologies can derive from this topology, so the research on this topology has important significance. This paper analyzes the configuration of these two types of cells in one phase. Comparing the differences among these three toplogies from the aspects of their component parameters and the input phase-shift transformer's structure, this paper points out that this new converter's increased cost for regenerating is lower than the converter regenerating with all cells, so this converter has significant economic value. Research on this converter can provide a new approach to those high-voltage induction motor drive systems requiring four-quardrant operation.According to the converter's topology characteristic and AC asynchronous motor's steady-state model, this paper proposes a control strategy based on V/f control. This control strategy can operate in open-loop mode and real-time power tracking mode. When the motor breaks and runs in generating mode, in open-loop mode, the controller estimates the motor generated power with a few motor parameters and the motor's load torque. In real-time power tracking mode, it directly detects the motor's active power. Then the controller uses the motor power and the detected RMS value of the motor current to calculate the power factor angle, and controls the regenerative cells and ordinary cells differently in order to make the motor generated power flow to the regenerative cells and then go to the grid through the regenerative cells'PWM rectifiers and the phase-shift transformer. Boundary conditions of this control strategy are pointed out, and the negative impact of the motor current's fluctuation on the control is analysed and a solution is discussed. Simulation results verify the feasibility of the proposed control strategy.For the regenerative cell's rectifier control, this paper analyses the impact of regenerative cells on the ordinary cells, and points out that in this converter extra filter inductors need to be installed before the regenerative cells'front ends. These filter inductors can not be replaced by the phase-shift transformer's leakage inductance. According to the converter's topology characteristic and control strategy, this paper compares currently used modulation methods for multilevel converter, and studies the selection of proper modulation methods under the condition of different proportions of these two types of cells. For the cases that each phase leg has only one regenerative cell and a few ordinary cells, the regenerative cell may adopt selective harmonic elimination method, and the ordinary cells use phase-shift PWM and compensate the low frequency harmonics produced by the regenerative cell. For the cases that in each phase leg the numbers of these two types of cells are both greater than1, it is better to use phase-shift PWM separately in these two groups of cells and the phases of their carriers are shifted too. This modulation scheme's effect of improving the quality of the voltage waveform is compared under the condition of different cell proportion.The paper modifies a cascaded converter and cascades one regenerative cell and four ordinary cells in each phase, and connects the converter with a380-V/7.5-kW induction motor to form an experimental platform. Some experiments in which the converter delivers the motor generated power back to the grid and all ordinary cells'DC-link voltages keep stable are conducted on this platform, verifying the feasibility of the open-loop and real-time power tracking control methods proposed by this paper. Finally the power loss in the experimental platform is explained by measuring and device power loss simulation.The simulation and experimental results show that converter's topology and its control strategy proposed in this paper can regenerate the motor's power to the grid. And this control method does not need elaborate motor parameters, thus is a universal method, and is easy to implement. For those induction motor drive systems which do not need high speed-regulation performance, the converter can meet their regeneration demand, and has a lower cost than the converter regenerating with all cells.