Research On Evaluation Method Of Influence Of Urban Rail Transit On Transformer Bias

Urban rail transit adopts DC power supply.As part of the power supply loop of subway rail,some current leaks to the ground,that is,stray current.Stray currents will affect transformers of 220kV and above around the track,causing them to produce DC bias.With the development of the city and the continuous expansion of the subway scale,this influence is increasing day by day.Assessing the impact of rail transit on the DC bias of hub substations has become an issue that needs to be studied urgently.This paper takes Shanghai Metro and Shanghai Power Grid as the object,comprehensively considers the simultaneous operation of multiple trains and the same power station being affected by multiple subway lines at the same time,studies the time and space characteristics of the stray current generated by Shanghai subway lines,and evaluates the DC bias level of 220kV transformers.The main research contents and results are as follows:(1)The flow mechanism and path of stray current are analyzed,and a complete calculation model of stray current value is established.Combined with the train traction power supply strategy,the stray current generated by a single train is calculated as a function of time under normal subway operation,and the stray current is correlated with time to obtain the stray current when a single train is running.Conclusions such as the magnitude of the value.(2)The related theory of ground potential is used to study the temporal and spatial distribution of stray current under the combined action of multiple trains.By collecting the ground resistivity data of the urban power grid and the near area of rail transit,taking into account the influence of urban construction and other factors,a calculation model of stray current ground potential along the rail transit line based on the equivalent ground resistivity model is established;multiple trains are proposed.The processing method of the stray current injection source during the common operation,and the finite element simulation software is used to calculate the geoelectricity and its influence of the Shanghai subway on the near track area and the surrounding hub substations;the time and space characteristics of the urban rail transit stray current are proved.The results show that the geometry of the subway track and the number of subway lines around the substation will affect the relationship between the size of the ground potential and the time.In the extreme case of multiple trains starting together,the ground potential will rise to 80V.(3)Introduced the grid DC bias equivalent model,and studied the evaluation method of the grid side bias current level.Based on the grid structure parameters provided by Shanghai Power Grid,a DC equivalent model of Shanghai Power Grid was established,and the neutral point bias current calculation equation generated by stray current under dynamic ground potential was derived;the program was written to complete the calculation of the bias current of the hub substation,And compared with the measured data to verify the correctness of the method.The calculation results show that under the common influence of multiple lines,transformers in 220kV stations near the track will be affected by a bias current with an amplitude greater than 4A for more than 70%of the time,which will affect the safety of the power system;finally,the risk of impact is relatively low.The high 220kV station has carried out theoretical calculations and predicted the waveform of its transformer bias current.The calculation method proposed in this paper is of great significance for the evaluation of stray current caused by urban rail transit.At the same time,it systematically explores the time-space relationship of subway stray current in cities with densely distributed subways and large-scale power grids,and solves the problem of transformers.Bias current evaluation problem.The research results can provide references for subway line planning and power grid development planning,and provide theoretical support for the monitoring and control of stray currents.