Research On Topologies And Control Strategies Of Four-Quadrant Cascade Multilevel Inverter

Cascade multilevel inverter (CMLI) has been applied more widely in the variable frequency control field of high voltage motor because of the characters of using lower voltage form solve the drive problems of higher voltage field, excellent operating performance, extending voltage grade easily and obvious energy-saving effect. In the development process of high voltage inverter drive technology, limited by the factors of the development of relative subjects and itself technology, the traditional CMLI with the circuit of diode-based rectifiers cannot operate in the four-quadrant mode, which limits the application field of the CMLI. In addition, the presence of a diode rectifier with a capacitor filter leads to abundant input current harmonics, which leads serious pollution to power line. Therefore, the research of a novel high voltage motor variable frequency drive technology has important meaning of theory and utility value.To solve the former problems, a novel four-quadrant cascade multilevel inverter (FCMLI) topology is proposed in this dissertation, and then the application field of the CMLI is extended. The studied main points focus on the topology structure and control strategy of the FCMLI, as well as the multilevel PWM control method.Firstly, the generative principle of the regenerative power in AC motor, at the same time the operation characters of traditional CMLI are simulated to point out the drawbacks. Through combining the voltage PWM rectifier and traditional CMLI, a novel FCMLI topology is proposed and the mathematic model is given based on the mathematic model of four-quadrant power cell, which supplying the theoretical evidence to the control strategy. Through analyzing the power reverse transmission process of FCMLI, it is gotten that the allocation of regenerative power in various power cells has the characters of synchronism and equalization. Synchronously, the control strategy of using the secondary voltage as the oriented vector is confirmed, which can realize the higher efficiency operation of the system.The newer multilevel PWM technology is investigated deeply. Aimed at the drawbacks of CPS-SPWM and STS-SVPWM technology, a novel multilevel PWM technology, namely unit vector overlap time staggered SVPWM (OTS-SVPWM), which can reduce the occupation of the CPU source, is proposed. The work principle of OTS-SVPWM is expatiated. The relationship between the loss ratio of amplitude and sample period is derived. Through selecting the proper sample period, the finer multilevel ladder wave, which has equivalent result to STS-SVPWM, is gotten.The operating characters of FCMLI based on vector oriented control strategy are simulated to validate the proposed topology. In this dissertation, in order to obtain better operation character of the proposed FCMLI, the direct power control strategy based on improved virtual flux linkage observer is proposed. Through using the integer with amplitude limited feedback loop, the problem of selecting initial value is solved. And the AC voltage sensorless FCMLI are proposed to reduce the cost and increase the reliability of the system. The simulation results indicated that even in the case of no AC voltage sensors, the anticipative control effectiveness of the system is still realized.Finally, the lower voltage experimental three-unit FCMLI system is build up based on the main control circuit structure of DSP+FPGA. The proposed novel OTS-SVPWM technology is realized in this system, the detail realized method and main experimental results are also given. To verify the actuality of the proposed control strategy, the operation characters of the four-quadrant power cell are studied experimentally. The operation characters of the variable frequency control system of AC motor based on FCMLI are studied experimentally, and the experimental results verified the feasibility of FCMLI applied in this field. The proposed novel FCMLI extended the application field of cascade inverter.