Combination Analysis and Switching Method of a Cascaded H-Bridge Multilevel Inverter Based on Transformers with the Different Turns Ratio for Increasing the Voltage Level
This paper analyzes the combination in a cascaded H-bridge multilevel inverter (CHBI) based on transformers with the different turn ratios for increasing the voltage level and proposes the switching method for achieving the output voltage distribution among H-bridge cells (HBCs). The transformers used in this paper are connected to the output of the respective HBCs, and the secondary sides of all the transformers are connected in series for generating the final output voltage. Only one of the transformers, in particular, has a different turn ratio for increasing the output voltage level. In this paper, the possible turn ratio of the special transformer with a different turn ratio is discussed in detail, and a switching method based on the level-shifted switching method for the topology used in this paper is proposed. To verify the effectiveness of the proposed method, a three-phase 21-level CHBI is experimentally tested.
Manuscript received December 6, 2016; revised March 29, 2017, June 23, 2017, August 24, 2017, and October 9, 2017; accepted October 16, 2017. Date of publication November 13, 2017; date of current version February 13, 2018. This work was supported in part by a grant from R&D Program of the Korea Railroad Research Institute, Republic of Korea, and in part by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016R1A2B4010636). (Corresponding author: Kyo-Beum Lee.) J.-S. Lee is with the Railroad Safety Research Division, Korea Railroad Research Institute, Uiwang 16105, South Korea (e-mail: ljs@krri.re.kr).
