Research On Novel Topology And Control Strategy Of Cascaded Multilevel Inverters

Multilevel power conversion has the ability to make lower voltage semiconductors be stably used to medium-voltage and large-capacity applications while reducing the high order harmonics produced by PWM control efficiently. But its topology and control is rather complicated. Due to expanding to medium-voltage fields easily, cascaded multilevel power inverters have been utilized in medium-voltage motor drives and reactive power compensations of power system. This dissertation is devoted to research the topological structure, PWM control method and application in motor drives of cascaded multilevel inverters, so that some effective methods for optimizing topology and improving control performance could be proposed. First of all, based on further analysis of the already presented cascaded multilevel inverters, this paper focuses on researching the hybrid seven-level inverter. The multi-connected rectifier topology and 3~10kV hybrid multilevel adjustable speed drive topology is proposed, and it optimizes the traditional cascaded converter. The mathematic model of three-level H-bridge cascaded inverters reflecting relationship between output voltage and driving signal is established. This model is also suitable for the hybrid cascaded converter. In the next part, through the further investigation of various multilevel PWM control methods and their output voltage harmonic characters, hybrid frequency carrier-based PWM control method suitable for hybrid seven-level cascaded inverters is presented. Furthermore, a space vector PWM control method applied in equal voltage three-level H-bridge cascaded inverters is proposed. Through comparing the harmonic characters of various multilevel PWM methods, a multilevel converter output filter design method is presented. At last, through combining space vector modulation with direct torque control in cascaded inverters, the direct torque control strategy based on power cell space vector phase-shifting synthesis principle for cascaded multilevel inverters is proposed, and its speed control performance is analyzed. This method can be easily realized in industry, and improve the control performance of medium-voltage motor drives. It offers an effective way to transplant high performance control strategy of two-level inverters into multilevel inverters. A large number of simulation waves are accompanied with the principle analysis. To prove the accuracy and validity of the proposed control methods, the experiments of various control principles are finished in the three-cell cascaded motor drive laboratory system.