Analysis of the Influence of DC Magnetic Bias Induced by Rail Transit Stray Current on Harmonic Characteristics of Transformer Neutral Current

In order to reveal the influence of DC magnetic bias induced by urban rail transit stray current on the harmonic characteristics of transformer neutral current, this paper first analyzes the measured data of shenzhen power grid neutral current, and summarizes the relationship between the measured neutral current characteristics, harmonic content of neutral current, noise level and low frequency component. The transient variation process of DC component of flux linkage under DC magnetic bias is analyzed with flux voltage integral formula. Based on the transformer parameters of a 220kV transformer substation in Shenzhen, a simulation model was established in PSCAD/EMTDC, and the influence of stray current on transformer harmonic characteristics was verified through the simulation of low-frequency sinusoidal current input at the neutral point. The results show that when the positive and negative alternating low frequency stray current acts, the reverse stray current plays a demagnetization role, which reduces the level of dc magnetic bias. The influence of amplitude and frequency factors on the degree of DC magnetic bias caused by positive and negative alternating low frequency stray current is illustrated. The research results reveal the source and influence factors of neutral-point current harmonics in field operation, and provide a basis for studying the influence of harmonics caused by stray current in rail transit on relay protection.