Novel multilevel PWM wave control method using series connected full bridge inverters

This paper proposes a novel multilevel PWM inverter and its control method. Increasing output voltage level is effective to reduce harmonic distortion of the output voltage and the output filter size. The inverter output voltage level can be increased by series connection of several inverters. However, isolated DC power sources or transformers are required at the inverter output side. When the output voltage level is increased without using isolated DC power sources and transformers, lots of switching devices are required. In general, 2(M-1) switching devices are necessary in an M-level inverter. The proposed system is composed of a 9-level inverter and series-connected full bridge inverters without the DC power sources. Only electrolytic capacitors are implemented across the DC buses of the full-bridge inverters. The DC bus voltage of the electrolytic capacitors is kept at 1/8 of that of the 9-level inverter by charge and discharge process. Also, a 17-level output voltage is obtained by superimposing the 1/8 of the DC bus voltage on the 9-level output. The proposed system achieved the 17-level voltage waveform without any DC power sources and transformers, using only 20 switching devices. Furthermore, another multilevel inverter system is proposed in this paper, of which output voltage level is 33. The proposed method also can be employed in n-series connected inverters. In this paper, effectiveness of the proposed method is verified by experimental results of the two systems.