Research And Application Of Transformer DC Bias In Metro Power Supply System

The growing population and expanding scale of construction has put heavier burden on urban transportation.Subway,which has the advantages of fast-speed、safe and high-punctual rate,has been the key measure to solve traffic congestion.Meanwhile,as the transformer of DC bias caused by railway running is increasing,DC bias has become a significant threat to transformer stability.This thesis do a detailed theoretically research about DC bias caused by peripheral AC power grid transformer in subway power supply system.It proposals a model of multilocomotive stray current distribution,improves J-A formula in bias magnetic field,builds a numerical model that combines metro traction system with transformer DC bias,and suppression techniques about transformer DC bias by subway are analyzed and discussed.The details are as follows:Firstly,a multi-train stray current distribution model is proposed,which solves the shortcomings of the existing model with a single structure and a large difference from the actual situation,and establishes a stray current simulation modeling method that is closer to the actual line characteristics.The distribution model of the geodesic electrical quantity is obtained by solving the set of equations by using the micro-element equivalence circuit to derive the non-chiral differential equation set under the action of a single injection source.Then the whole line is divided,and the individual injection sources on each segment are superimposed to obtain a distribution model of the electrical quantity at each layer of the ground network.The simulation results can be used as a reference for preconstruction and post-construction protection against stray currents in the subway.Secondly,the J-A model of the transformer in the state of magnetic bias is improved to solve the problem of incomplete description of the hysteresis curve in the state of magnetic bias by the existing J-A theory,and the numerical precision of the excitation current in the state of magnetic bias is effectively improved.This paper deduces and modifies the formula of J-A under normal excitation,on this basis,improves the model of J-A under magnetic bias by introducing the DC component,and modifies its coefficient k,then puts forward the iterative model of DC magnetic bias of transformer based on the improved formula of J-A.An example transformer is used to verify the accuracy of the improved model for DC bias.Then,on the MATLAB/Simulink platform,a comprehensive system simulation model combining the metro traction system and the DC bias magnetic field of the transformer was established to further study the interaction mechanism between the two.The traction power supply system and traction drive system are modeled respectively,and then the train model is built according to the actual parameters of chengdu metro locomotive.In order to verify the correctness of the model,the amount of the DC current invading the neutral line of the transformer is simulated by changing the stray current value of the subway.Finally,based on the specificity of the DC bias phenomenon caused by subway,the DC bias suppression technology of transformers caused by subway power supply system is analyzed and discussed.The combination of two suppression techniques,weakening stray currents and installing DC bias suppression devices,is used for comprehensive management to ensure the safe and stable operation of grounding transformers in the AC grid around the metro.This article has a total of 99 graphics,27 tables,and 2 attached tables.