Research On The Influence Of Rail Transit DC Stray Current On The Operation Of The Main Transformer Of The Substation

As a green and convenient mode of transportation,urban rail transit has made great contributions to alleviating ground traffic congestion and promoting high-quality urban development.However,the DC(Direct Current)stray current generated during the operation of urban rail train can not only cause serious electrochemical corrosion of nearby metal structures but also generate DC bias at the neutral point grounding transformer of the adjacent substations,which can harm the safe and stable operation of transformers and AC(Alternating Current)power grid.At present,most of the related research focuses on the distribution of urban rail stray current and the phenomenon of transformer DC magnetic bias caused by the grounding pole of HVDC(High Voltage Direct Current),and the research of transformer DC bias caused by urban rail DC stray current may be insufficient.Therefore,it is necessary to study the influence of rail transit DC stray current on the operation of the main transformer in the urban AC power grid.Firstly,the calculation of traction current of urban rail power supply is expounded,and the method of establishing the stray current model of urban rail is summarized.Furthermore,the principle of transformer DC bias and the suppression device of transformer DC bias are introduced.The method of series resistor or capacitor at the neutral point of grounding transformer has been used for engineering examples,and it has good universality.In this way,the basic functions of CDEGS and PSCAD/EMTDC software are introduced briefly.Secondly,the DC stray current distribution of the four-layer ground grid model is calculated by the analytical method.Furthermore,a multi-layer zoned soil model is established based on the measured soil data.Considering the areas heavily affected by the biased magnetic current in the AC power grid,a coupled simulation model for the stray current of subway and AC power grid in a karst mountain area in southwest China is established by using CDEGS software.The train operating conditions of urban rail trains would be considered,and the validity of the proposed coupled model is verified by comparing the simulation values with the measured data.Meanwhile,the influence of rail-to-ground resistance on the DC stray current distribution is analyzed.Thirdly,according to the actual electrical parameters,the external DC power input is set by the simulation results for the neutral point DC of the grounding transformer in the coupled model.The simulation model of stray current distribution in the power grid in a certain area of a karst mountain area in southwest China could be further obtained with the help of PSCAD/EMTDC software.The distribution of DC stray current in the transmission line,lightning line,ground resistance,and transformer field winding is analyzed.Meanwhile,the influence of rail-to-ground resistance and transformer leakage reactance on stray current distribution in the AC power grid are analyzed.Finally,there is an analysis about the influence of the urban rail DC stray current on the operation of the main transformer based on the measured data of neutral point DC,vibration velocity,and noise intensity.Furthermore,the DC blocking device of neutral series resistors and capacitors is adopted to put forward the treatment schemes of biased magnetic current in the main transformer of the power grid in a certain area.The related discussion and conclusion have certain reference value to the protection of urban rail DC stray current and the formulation of main transformer DC bias suppression measures.