The Research Of Non-Power Frequency Traction Transformer Conversion System Based LLC Resonant Conversion

With the soft switching technology has been more and more attention, the locomotive traction system without the transformer based on resonant converter begins in the research. Resonant converter can take use of the leakage inductance of the transformer, solve the leakage inductance voltage spike, reduce the switching losses on the primary side through ZVS over the entire load range and ZCS on the secondary side diode rectifier, which transmits power with high efficiency and power density. This thesis studies non-power frequency traction transformer conversion system based LLC resonant conversion. The conversion system is mainly used to replace the AC/DC conversion aspects of traditional AC/DC/AC traction system, to achieve the cancellation of the power frequency transformer. An input end of the cascade 4QC, output in parallel bi-directional half-bridge LLC resonant converter conversion system has been discussed in this paper. For this system, the main research content in the paper is as follows:1. The main circuit of non-power frequency traction transformer conversion system based LLC resonant conversion has been in research. Working principle, working process of 4QC and the implications based carrier phase shift technique are analyzed. Be sure that bi-directional half-bridge LLC resonant converter could work over the entire load range in traction condition, which just provides voltage insulation and voltage adaptation as DC transformer (DCT). The topology has been selected. And its working principle and DC gain characteristic are analyzed.2. The parameters’calculation formula of the main circuit about the conversion system have deduced. Such as the selection of the number of cascad module, the establishment of swtich device and the working frequency of transformer, the range of input inductor, the output capacitor in 4QC which get good tracking, the optimization of resonant parameters. And the output capacitor of bi-directional half-bridge LLC resonant converter has been given.3. With establishing the conversion system main circuit equivalent space model, the voltage and current double close loop control strategy with the average voltage method has deduced. The factors which cause the unbalance of output current in bi-directional half-bridge LLC resonant have been analysed, and the error tolerance has been disscused. A bidirectional power conversion control strategy of bidirectional half-bridge LLC resonant converter has been given.4. An AC/DC conversion system has designed to simulate the CRH5 as the simulation object, the simulation method combination with MATLAB/SIMULINK has been used. The close loop control with CPS-SPWM of the system have been simulated,which includes the system’s traction and braking condition, the power grid voltage fluctuations, load mutation, the voltage instruction trace, and equalizing voltage conditions.Simulation results show:the conversion system can get a power factor of 1 in power grid, a stable DC side output voltage and the bidirectional energy transfer. Bi-directional half-bridge LLC resonant converter can get a constant gain over the entire load range with a fixed switching frequency whose resonant parameters has been optimized with the research on magnetizing inductance and dead time. The conversion system can realized with the double close loop control based CPS-SPWM, multi-resonant PR and FMA. And the cascade system can realize same input terminal voltage. The simulation results demonstrate the correctness of this paper about the conversion system analysis.