| The auxiliary converter system is an important part of electric locomotive, with the rapid development of the power electronics technology, the auxiliary converter system is also progressing towards to small size, light weight, low cost, high efficiency and high reliability. In present market the comparatively ideal main circuit structure of auxiliary converter system consists of two stages:firstly step-down dc-dc conversion and then dc-ac inversion. Howerver, the dc link voltage of electric locomotive traction converter is usually high, so that it requires the step-down dc-dc converter has the traits of high power density and high conversion ratio, which has brought a huge challenge for designing an appropriate step-down dc-dc converter. With this background, this thesis proposed a four-level step-down dc-dc converter that can be applied in electric locomotive auxiliary converter system.The four-level step-down dc-dc converter proposed in this thesis has a modular structure, mainly consists of positive cell, negative cell and dc bus filter capacitor, the positive and negative cell is easy to expand so that can achieve high conversion ratio;And it has characteristics of independent resonant loop, soft switching, and self-balancing of dc bus capacitor voltage. The working principle, the operating modal, the self-balancing mechanism of dc bus capacitor voltage, the feature of independent resonant loop and the feature of zero-current switching of the four-level step-down dc-dc converter all have been analyzed in detail. According to the application characteristic of the dc link power converter in the electric locomotive auxiliary converter system, this thesis has provided the parameter design method, the loss calculation method, the equivalent circuit model and the effects of parasitic parameter on the four-level step-down dc-dc converter. The main circuit parameters of the four-level step-down dc-dc converter also has been designed, and the accordingly simulation has been carried out. The results of simulation shows that the main circuit parameters designed by this thesis of the four-level step-down dc-dc converter is reasonable, and the converter can achieve self-balancing of dc bus capacitor voltage and zero-current switching under open-loop control, the control model is simple and it is with less loss and highly reliable. In the configuration of four-level step-down dc-dc converter, it is mainly composed of capacitors and switches except the adoption of little ac resonant inductor for the realization of zero-current switching, so it has the advantages of small volume, lightweight, high power density.Finally, this thesis has established a prototype and has conducted experiments.The experiment results correspond with the circuit principle analysis and simulation, which proving that the four-level step-down dc-dc converter have a good practical engineering value. |
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