Risk Analysis Of DC Bias Of Urban Main Transformer Caused By Subway Stray Current

In recent years,as an important part of the urban rail transit system,the subway developed extremely fast.However,subway rails could not fully insulate from the ground,caused the stray current produced a ground potential gradient,DC bias of the main transformer of the urban power grid,which threatened the safety operation of the power system.Researched the distribution of the geoelectric field generated by subway operation,and the risk of DC bias in the urban main transformer could provide a useful reference for the prevention of transformer faults.Therefore,under the subway stray current,considered various locomotive operating conditions,a dynamic model of the geoelectric field was built.Based on this,a risk assessment method for DC bias of urban main transformers was proposed.The main work of the thesis was as follows:Firstly,based on the micro-equivalent model of subway stray current,a dynamic model of the geoelectric field was established which considered the stray current under various operating conditions of the locomotive.The distribution rules and characteristics of the stray current were studied.The effect of factors such as the traction current of the train,the rail-to-ground transition resistance,the power supply distance and the drain network were analyzed.Secondly,under the ANSYS software platform,a three-dimensional dynamic simulation model of the stray current geoelectric field was built.A dynamic model considered the stray geoelectric field during operation of multiple locomotives was established.Considered the longitudinal resistance of the rail,the traction current of the train,the rail-to-ground transition resistance,the power supply distance,and whether the drainage network discharges the influencing factors,the variation law of the ground electric field was studied.Took the Urumqi subway in Xinjiang as an example,the stray current induced geoelectric field was calculated and simulated.Compared with the measured data of DC bias in urban substations,the model was verified to be correct.Finally,based on the above calculation results,a risk assessment method for DC bias of urban main transformers considered the effect of metro stray current was proposed.Under the structure and parameters of Urumqi subway,urban main network,built a risk assessment framework.Analyzed and compared the influence of different factors on the main neutral point bias DC in the urban area to determine the key factors.Then gave suggestions on preventing and controlling the DC bias risk of urban main transformers.