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| The Voltage Balance Scheme of Three Level-Dual Active Bridge Based on Cycle Combination Sequence Modulation |
| Li Linzhe, Zou Minyang, Wu Jingchi, Fei Yue, Shu Zeliang |
| School of Electrical Engineering Southwest Jiaotong University Chengdu 611756 China |
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Abstract The bipolar DC microgrid (BDC-MGs) operates without considering phase synchronization and compensation issues, which has the advantages of high flexibility, high reliability, and easy access to multiple load applications. However, when the three level-dual active bridge (TL-DAB) converter is widely used in BDC-MGs, the system is unstable due to unbalanced bipolar DC bus voltages on the output port when matching multiple types of unbalanced loads. Therefore, this paper proposes a cycle combination sequence modulation scheme based on the power transmission and voltage balance cycles, which can effectively reduce the unbalance of positive and negative bus voltages.
Firstly, the voltage imbalance of the upper and lower sub-capacitors on the output side of the TL-DAB converter in unbalanced load conditions is analyzed. Second, a cycle combination sequence modulation scheme based on the power transmission and voltage balance cycles is proposed without adding additional hardware circuits such as switches. The power transmission cycle adopts the triple phase shift (TPS) modulation scheme based on the minimum inductor current RMS value, while the voltage balance cycle adopts asymmetric modulation to change the pulse width ratio between high and low levels of the primary and secondary voltage during one switching cycle. Thus, the differential power distribution of the sub-capacitor is realized, and the multiple power trans cycles and one voltage balance cycle form a cycle combination sequence modulation scheme to balance voltage. Finally, a Matlab/Simulink simulation model and a low-power SiC prototype based on an FPGA controller are built to verify the proposed voltage balance scheme.
The simulation and experimental results show that the inductor current of the TL-DAB converter is DC-biased, and the voltage of the output-side sub-capacitor of the converter is unbalanced in unbalanced load conditions. By adjusting the duty cycle value of the voltage balance cycle, the differential power distribution of sub-capacitor is achieved. Therefore, the proposed voltage equalization strategy can solve the DC bias problem of the converter inductor current and achieve voltage equalization in various load conditions. In the dynamic load-cutting experiment, the voltage imbalance on the output side of the TL-DAB converter decreases from 12 % to less than 1 % after using the proposed voltage balance scheme, which indicates that the voltage balance scheme is effective.
The feasibility of the proposed voltage balance scheme is verified by simulation and experimental results, and the following conclusions are obtained. (1) After adopting the voltage balance scheme proposed in this paper, the voltage imbalance on the output side is decreased from 12 % to less than 1 %, and the voltage balance is effectively realized. (2) The proposed voltage balance scheme does not require additional hardware circuits and has no topology change, which has more cost and volume advantages than the traditional one.
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Received: 18 October 2022
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2024, Vol. 39 
