Capacitor Voltage Balancing Of Cascaded H-Bridge Rectifier In Power Electronic Transformer Locomotive

Power electronic transformer(PET) is an effective solution to achieve the miniaturization and lightweight of on-board electric equipment in high speed railway. The combination structure with single-phase cascaded H-bridge rectifiers(CHBR) and medium frequency DC-DC converters is a typical topology of PET. Therefore, it is significant in theory and engineering application that this paper analyzes and discusses the single-phase CHBR in PET solution. Actually, this paper focuses on the unbalance mechanism of dc-link capacitor voltage and compensation schemes of the adopted single-phase CHBR. The main contents of this paper are summarized as follows:Firstly, the topology of single-phase CHBR is analyzed, and the equivalent switching function is defined. On the basis of this, the corresponding three operation states of four switching modes are analyzed to deduce the mathematical model of the adopted CHBR. Several conventional control and modulation methods applied in single-phase one-cell H-bridge rectifier are discussed, and whose advantages and weaknesses in the adopted CHBR application are analyzed.Secondly, quantitative analysis is adopted to discuss the impact factors of dc-link capacitor voltage unbalance such as:modulation schemes, capacitance value, the equivalent series resistor, and the equivalent load resistor. Analysis results show that carrier phase-shift pulse width modulation method has effect on the harmonics distribution of the line current and the charging and discharging current of DC capacitor, which results in the unbalance dc-link capacitor voltages. The larger dc-link capacitance value is, the smaller dc-link voltage ripple is. The steady-state values of DC-link voltages keep constant. The larger the equivalent series resistance value is, the larger dc-link voltage ripple is. The ratio of capacitor voltages is the same with the ratio of equivalent load resistance. On the basis of the above analysis, a voltage balancing control and modulation algorithm is proposed based on the maximum voltage error module and verified in simulation using MATLAB/Simulink tool. Compared with the mentioned two traditional control schemes, the proposed voltage balancing algorithm can simplify the control system.Finally, a single-phase 7-level(three cells) CHBR experimental prototype in low power grade is designed with DSP+FPGA double-core controller framework. Experiment results verify the correctness of theoretical analysis, and the effectiveness of the adopted voltage balancing algorithm.