A PWM Strategy Based on State Transition for Cascaded H-Bridge Inverter under Unbalanced DC Sources

 ABSTRACT:

 Cascaded H-bridge converter has been widely used and researched in industry, since it is suitable for the operation under both normal and fault conditions. This paper proposes a novel PWM strategy based on state transition for cascaded H-Bridge inverter with unbalanced DC sources to achieve high quality line-to-line output voltages and maximize the linear modulation range. In this modulation strategy, the duration time of each switching state will be modified directly through the correction value. Ranges of correction value are derived by analyzing the modulation index limitation. Then, proper correction value is added into duration times to transform the switching states and extend modulation index to the maximum value. Meanwhile, balanced AC currents can be obtained under unbalanced DC sources condition, even under larger unbalanced coefficients. Furthermore, a three-phase power control algorithm (PCA) is introduced to achieve the balanced distribution of three-phase power. Compared with the traditional zero-sequence voltage injection method, the proposed strategy is more convenient and effective theoretically, and it can be applied to the higher-level cascaded H-bridge converter. The advantage and effectiveness of the proposed strategy are verified by simulation and experiment results.

KEYWORDS:

1.      State transition

2.      Linear modulation range

3.      Unbalanced DC sources

4.      Power control algorithm

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Circuit of Cascaded H-Bridge Converter.

 EXPECTED SIMULATION RESULTS

Fig. 2. The simulation results with ten cells per phase under m =0.9 (a) line-to-line voltages uXX of simplified SVM and ST-PWM(b) output AC currents iX of simplified SVM and ST-PWM

 CONCLUSION:

 

A novel PWM strategy based on state transition for CHBI with unbalanced DC sources has been proposed in this paper. Compared with ZSVIM and NVM, the duration times of each switch states can be modified directly by correction value and the gate signals can be acquired easily through ST-PWM. To acquire the maximum linear modulation index, the reason of the modulation index limitation and the novel modulation strategy based on the state transition are studied. The proposed strategy can achieve high quality line-to-line output voltages and extend the modulation range as high as possible. Besides, the three-phase power control algorithm is introduced to acquire balanced power distribution. The effectiveness has been verified by simulation and experiment results.

In our current work, we incorporate PCA into the ST-PWM strategy, which is a prototype of multi-objective control. Since both modulation index extension and power control are achieved by adjusting ΔT, there is a conflict on the control objectives. That is to say, the control ability of PCA will decrease when the modulation index is extended. However, we have not yet found a strict mathematical relationship between them due to time constraints. And we will do a further research on multi-objective optimal PWM strategy and multi-objective control boundaries under unbalanced dc sources in the future.

REFERENCES:

 

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