An Asymmetric Modulation Strategy for Three-Phase Buck Rectifier with Low Input Current Total Harmonic Distortion
Cao Haibin, Xu Jianping, Xie Fei, Zhou Chenli, Chen Xin
Key Laboratory of Magnetic Suspension Technology and Maglev Vehicle Ministry of Education School of Electrical Engineering Southwest Jiaotong University Chengdu 610031 China
Abstract：The input current total harmonic distortion (THD) is an important performance indicator of a three-phase Buck rectifier, closely related to the modulation strategy. When calculating the dwell time of the vector in the previous research on space vector pulse-width modulation (SVPWM), the ripple of the DC-link current was ignored, i.e., it was considered a constant value. However, in practical applications, to improve dynamic performance and power density, the output inductor of the three-phase Buck rectifier takes a small value, which leads to a large DC-link current ripple. In this case, the impact of the DC-link current ripple on the input current becomes obvious. Secondly, the input current of the three-phase Buck rectifier with the conventional SVPWM modulation strategies has severe distortion during the sector transition. Therefore, this paper proposes a low input current THD modulation strategy, which considers the influence of DC-link current ripple on the input current quality and eliminates the input current distortion during the sector transition. Therefore, the input current THD is effectively reduced. The impact of the DC-link current ripple on the input current is analyzed in this paper. Firstly, based on the operating states of the three-phase Buck rectifier in sector 1, the relationship between the input current of the three-phase bridge arm and the DC-link current is obtained. Secondly, the relationship between the input average equivalent current amplitude of the three-phase bridge legs is analyzed for different modulation strategies. Then, the amplitude relation between the input voltage and the corresponding input current is obtained. When the ripple of the DC-link current is large, the results show that the ratio of the three-phase input current to the corresponding input voltage is not equal to the conventional asymmetric modulation, which leads to a high harmonic content in the input current. For the conventional symmetric modulation, the ratio of the three-phase input current to the corresponding input voltage is equal, which means that the harmonic content of the input current is lower than the conventional asymmetric modulation. In addition, the paper also analyzes the reason for the input current distortion during the sector transition of the conventional SVPWM modulation strategies. The relation between the three-phase input current and dc-link inductor current is given, the input inductor current of each vector is deduced, and the transition process of drive signals with different modulation strategies during the sector transition is given. The analysis shows that the input current of the conventional SVPWM modulation strategies has distortion during the sector transition. A low input current THD modulation strategy is proposed. In this modulation scheme, the influence of the output inductor current ripple on the input current is considered, and the input current distortion during the sector transition is eliminated. The simulation results show that among the three modulations, the input current THD of the conventional asymmetric modulation is the highest (4.9%), and the input current THD of the conventional symmetric modulation is lower (2.3%) than that of the conventional asymmetric modulation. The input current THD of the proposed low input current THD modulation strategy is only 1.7% at half load and 1.2% at full load, respectively, which is the lowest among the three modulation strategies.
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2024, Vol. 39 
