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.
KEYWORDS:
1.
Cascaded
H-bridge inverter (CHBI)
2.
Cascaded
multilevel
3.
Level-shifted
switching method
4.
Multilevel
inverter
SOFTWARE: MATLAB/SIMULINK
CIRCUIT DIAGRAM:
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.
CONCLUSION:
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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