Research On The Modulation Of Cascade Multilevel Inverter And Its Application In MMC

Multi-level inverter has many advantages that includes excellent output performance,low harmonic and small voltage stress, which has been widespread concern in the power electronics industry. The topology of cascaded multi-level inverter is researched in this paper,especially modulation strategy has been deep researched. Two new modulation strategy are studied and applied to modular multilevel inverters.Firstly, combining two traditional multi-carrier sinusoidal pulse width modulation(SPWM) technology, a hybrid carrier SPWM technology is introduced. The basic principle of two conventional multi-carrier SPWM technology are analyzed, and the operating rules that include CPS-SPWM and PD-SPWM are analyzed. Mathematical analysis has been done in two modulation techniques, meanwhile the features of two modulation techniques are summarized in cascaded multilevel inverter. Combined with the advantages and disadvantages of the two modulation techniques, we propose a hybrid carrier SPWM technology, which not only ensures the cascade multi-level inverter for each module unit output power balance, but also improves the output performance of the line voltage. Finally,through a large number of simulation experiments and comparative analysis of the proposed method, the correctness and the effectiveness of the strategy are verified.Secondly, an improved SPWM strategy is introduced. The switch status of each unit module of a cascaded multi-level inverter are analyzed. It is revealed that the working condition of the switch of a chopper arm may cause unnecessary switching under the conventional unipolar sinusoidal pulse width modulation(SPWM). With an increase in the number of cascaded multilevel inverters, the superposition of unnecessary switching gradually occurs. In this issue, an improved SPWM strategy is proposed to reduce switching in cascaded multilevel inverters. The switching times are greatly reduced by improved SPWM method without changing the output performance of inverters. Finally, a prototype is built in the laboratory. The correctness of the introduced modulation strategy is verified by experimental results.Thirdly,Research on carrier hybrid SPWM technology in a modular multi-level inverters.The carrier hybrid SPWM technology is applied to modular multilevel inverter, which instead of the commony modulation strategy of modular multi-level inverter. Firstly, introducing the working principle of the modular multilevel inverter and the operating rules carrier hybrid SPWM in MMC, the three-loop control method is applied to the system, and the capacitance voltage of module unit is controlled. Finally, a three-phase modular multilevel simulationmodel is built. The effectiveness of the introduced method and the stability of the system are verified.