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| A Control Strategy Based on Virtual Impedance for Three-Phase Buck Rectifier under Unbalanced Phase-Voltages |
| Xie Fei, Xu Jianping, Guo Xia, Cao Haibin, Chen Xin |
| Key Laboratory of Magnetic Suspension Technology and Maglev Vehicle Ministry of Education School of Electrical Engineering Southwest Jiaotong University Chengdu 610031 China |
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Abstract When the three-phase buck rectifier is used in electric aircraft applications, ac input voltages are often unbalanced by asymmetric transmission impedance, grid failure, and single-phase load. Accordingly, the three-phase buck rectifier with conventional control strategies exhibits double-line frequency fluctuations in output voltages and distorted input currents, seriously affecting rectifier performance. This paper constructs an equivalent circuit model and a power transfer mathematical model for the three-phase buck rectifier in the two-phase stationary frame.
The generation of double-line frequency fluctuations is analyzed for three-phase buck rectifiers with unbalanced ac input voltages. According to the phase relationship between ac input currents and ac input voltages, an equivalent circuit model is established using the conventional control strategy in a two-phase stationary frame. Subsequently, double line frequency fluctuations in active power are analyzed, which causes double line frequency fluctuations in output voltages and input current distortions.
An input unbalance control strategy is proposed based on virtual impedance under unbalanced ac input voltages. Firstly, an improved equivalent circuit model is proposed. The virtual impedance is added to the equivalent circuit model. Adjusting the amplitude and phase of the virtual impedance can eliminate the ac component of the active power. Thus, constant active power is achieved, i.e., double line frequency fluctuation of output voltage is eliminated, and sinusoidal input currents are obtained. Secondly, the method to construct input current references is given. Input voltages are applied to construct input current references. The impacts of the peak and zero-crossing detection on the dynamic performance of three-phase buck rectifiers is analyzed. Thirdly, the space vector pulse width modulation (SVPWM) strategy is analyzed. The switching loss-optimized SVPWM strategy is applied. Its sector division and pulse width modulation sequences are given.
Simulation and a 1 kW experimental prototype verify the theoretical analysis and the proposed control strategy. The results show that the proposed control strategy eliminates double-line frequency fluctuations in output voltages and input current distortions. Thus, low ripples of output voltages and sinusoidal input currents with low total harmonic distortion are obtained. In addition, when the ac input frequency or input voltage amplitude varies, the proposed control strategy exhibits low output voltage fluctuations and short response time. When the load power varies, no distortion exists in input currents.
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Received: 11 July 2023
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2024, Vol. 39 
