| Compared with traditional AC traction chain, the locomotive traction system without the transformer which’s frequency is50Hz has canceled the bulky and inefficient power frequency traction transformer.This provides an effective way for the electric locomotive weight loss and efficiency improvements. In other countries, some of such conversion system has been launched and certain results have been achieved, which has got a wide of attentions. In China,the reports on such systems is still relatively rare, related theory is still in its infancy. So, the research for this kind of system is in favor of tracking foreign technology trends and technology developments, which has a great significance to the development of technology products with our own intellectual property.With a comparative analysis of several existing conversion system, an input end of the cascade cycloconverter,output in parallel full-bridge rectifier conversion system has been discussed in this paper. 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, to provide a stable DC voltage for traction inverter. Its main features are:multiple conversion module cascade, front-end frequency conversion and intermediate frequency transformers electrical isolation. For this system, the main research content in the paper is as follows:(1) the conversion system’s main circuit mode has been analyzed, transformer leakage inductance voltage spike problem during commutation has been solved, the system’s modulation technique and carrier phase-shifted technique have been shown,the system’s control characteristic has been discussed,every module’s front-end input voltage and output current with the variation of the main circuit parameters also have been analyzed, and corresponding simulations have been done to verify the analysis.(2)The parameters’calculation formula of the main circuit about the conversion system have deduced (such as the input inductor L, the secondary side of the LC filter, the output side of the capacitor), a few important parameters of the intermediate frequency transformer design requirements and restrictions have been given.(3)With the voltage and current double close loop control strategy, the control of the conversion system has been realized. On this basis, the average voltage and average current methods have been used to make every module has a same input terminal voltage and output side current.(4)Using the state space averaging method to establish the conversion system main circuit equivalent space model, the equilibrium point linearization method has been used to get a small-signal model of the system, the design of the voltage loop controller parameters have been done.(5)An AC/DC conversion system has designed to simulate the CRH5as the simulation object, the simulation method combination with MATLAB/SIMULINK and PSIM have been used. First the open loop control of the system has been simulated, and then the close loop control of the system have been done,which includes the system’s traction and braking condition, the power grid voltage fluctuations, load mutation, the voltage instruction trace, and equalizing voltage and current conditions.Simulation results show:the conversion system can get a stable DC side output voltage, an adjustable power factor in power grid and meet the relevant technical indicators. The cascade system can realize same input terminal voltage and output side current equalizing. In the commutation process, intermediate frequency transformer leakage inductance voltage spike has been elimination. The simulation results demonstrate the correctness of this paper about the conversion system analysis. |
PDF Full Text Request