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.
- Cascaded H-bridge inverter (CHBI)
- Cascaded multilevel
- Level-shifted switching method
- Multilevel inverter
Fig. 1. Transformer-based CHBI topology used in this paper.
EXPECTED SIMULATION RESULTS
Fig. 2. Simulation results of the proposed switching method with the
balanced voltage distribution at Mi = 1.
Fig. 3. Simulation results of the proposed switching method atMi =1. (a) With and (b) without the balanced voltage and power distributions.
Transformer-based CHBI topology was introduced in this paper. The comparison analysis between topologies was shown in Table XI. The theoretical analysis regarding the selection of the turns ratio of the subtransformer was presented. In addition, a switching method based on the level-shifted switching method with the balanced voltage and power distributions was proposed for the transformer-based CHBI topology. Several requirements related to the decision of switching devices and the design of transformer were suggested.A21-level CHBIwas used to determine the feasibility and effectiveness of the proposed switching method.
When using the proposed switching method, two issues are to be noted: 1) to use the proposed switching method, the configuration of transformer-based CHBI topology should follow that of Table II. It guarantees that the minimum variation (dVx ) of the voltage level is always the same. 2) If the system operates in the wide voltage range (wideMi ), Table VII should be changed as that explained in this paper to guarantee a balanced voltage distribution for Mi range required. Table VI guarantees a balanced voltage distribution for 0.8<Mi < 1. Consequently, this configuration can be applied for the main power supply system generating the ac voltage in grid.
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