Research On Traction Transformer Protection Based On Parameter Identification

Traction transformers are important devices in electric railways system, and secure operation has significant influence on the stability of electric railways system. At present, differential protection is often used as the main protection for traction transformers. However, the existing of certain factors, such as magnetizing inrush, over-excitation, CT saturation and mismatching of CT ratio, will result in that it is hard to distinguish state current and fault current, and differential protection misoperation or maloperation, Then secure operation of electric railways system will be threatened. In view of this fact, a novel method based on identification of winding parameter is presented, which need not to distinguish magnetizing inrush. Simulation results by using EMTP and MATLAB demonstrate the feasibility and availability.The main achievements of the thesis are as follows:(1) Kinds of techniques to detect magnetizing inrush of traction transformers are thoroughly investigated in this paper, and their limitation are indicated and corresponding improvement techniques are proposed.(2) In this paper, flux linkage equilibriun equations are used to build "ash-box" models of some traction transformers based on system identification, such as Two-winding Single-phase traction transformer, impedance-matching equilibriun traction transformer and SCOTT traction transformer. In the models, the iron core nonlinearly of the traction transformer and the magnetic hysteresis effect without the B-H curve are considered. Having tested by digital simulation, the winding parameter has not changed in the state of external fault or magnetizing inrush etc. but the winding parameter has obviously changed in state of inter fault or inter-turn fault.(3) The rules three phases converted to two phases of traction transformer in electric railways are developed in this paper. Using the conversion rules, the complexity of external connection of traction transformer is evidently reduced.(4) Based on system identification, the hardware-software system of traction transformer is designed in this paper, in which hardware structure of protection unit and action-criterion of protection are exploringly studied.