Research On The Influence Of Subway Load Characteristics On The DC Bias Of Power Transformers

As of February 2022,there are a total of 272 cumulative rail transit lines and 8,819 km of operating mileage nationwide.With the widespread use of metro,the metro has an increasingly wide range of impact on the surrounding equipment,known as the metro stray current caused by oil and gas pipeline corrosion,the surrounding power transformer bias current increases leading to power transformers DC bias and other problems.Therefore,it is of great significance to investigate the mechanism of DC bias of power transformers caused by Metro and to manage the DC bias characteristics of power transformers.This paper first analyze the mechanism of stray current and induced current inside the power transformer,and investigate the operating schedule of a metro in Guangzhou,the metro power supply system,train operation period,train traction current and other parameters,and determine the simulation model length and simulation period by the distribution law.According to the possible types of currents in the neutral point of the power transformer,the measured sampling frequency is determined to be 4K(Hz),and the field measurement method is designed to measure the ground potential difference and induced voltage distribution around the Metro.On this basis,the time domain waveforms of stray currents and induced currents intruding into the power transformer are analyzed by CDEGS and PSCAD simulations,and the frequency domain analysis of the waveforms by FFT algorithm reveals that the stray currents intruding into the power transformer are mainly DC components and the induced currents are mainly low frequency components.The distribution characteristics of the bias current in the grid circuit are analyzed,and the reasons for DC bias magnetization in power substations located far from the Metro lines are given.The results of the frequency domain analysis of the measured data show that the DC component has the highest amplitude in the ground potential difference,and the induced voltage in the laid wire is dominated by the 0.01 Hz component,and the correlation with the simulation results is high.Based on the results of frequency domain analysis,the component analysis method of bias current of power transformer is proposed.Taking the measured data as an example,it is calculated that 78.24% of the bias current of this power transformer is caused by stray currents and 21.76% by induced currents.It is shown that stray currents still act as the main cause of DC bias in power transformers,but induction currents cannot be ignored.Finally,the article establishes a numerical calculation model based on electromagnetic field theory according to the low-frequency characteristics of stray currents and induced currents,and uses this model to calculate the potential difference between power transformers and the line induced voltage,so that the bias current intruding into the power transformer can be calculated.This paper describes the characteristic frequencies of stray current and induced current waveforms intruding into power transformers,the proposed analysis method of transformer bias current components,and the numerical calculation model of stray current and induced current,which can provide the theoretical basis for suppressing the DC bias phenomenon of power transformers caused by Metros.