Modeling and Analysis of Transformer DC Bias Current Caused by Metro Stray Current

Abstract

The metro stray current leaked during the subway operation causes the nearby earth surface potential to change, so that the various neutral‐grounded transformers to be at different earth surface potentials, resulting transformer DC bias. In order to analyze the influence of metro stray current on transformer DC bias, the polarization potential monitoring data in metro traction power supply system and neutral point current of nearby transformer were compared, and the positive correlation characteristics between the two were pointed out. By studying the stray current leakage reason in the metro traction power supply system and the circulation path causing transformer DC bias, the mechanism of the transformer DC bias under the subway operating environment was revealed. Based on the discrete model of the stray current distribution, considering the mechanism of stray current on DC bias, the equivalent resistance model of the transformer DC bias caused by metro stray current was built with double‐end power supply environment. Then the calculation formula of transformer neutral point current changing with subway operating conditions was derived, the numerical calculation of DC bias current of transformer caused by metro stray current was realized. The validity of the proposed model was verified by current distribution, electromagnetic field, grounding and soil structure analysis, the simulation results showed that the proposed model can calculate the stray current distribution and the nearby transformer DC bias current with high accuracy, which provided a quantitative analysis method for studying the correlation between metro stray current and transformer DC bias phenomenon. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Mitigation Strategy of Neutral-Point DC for Transformer Caused by Metro Stray Currents

“…Using the proposed method, with thresholds of indicators set as 0, through 20,000 samplings of dynamic metro operation scenarios, the installation probabilities of optimized placement results are shown in Figure 11. Substations 2,5,6,12,13,22,23,61,62,64,67,70, and 71 need to have BDs installed, and the BD installation probability exceeds 90 % with the train operations. Among them, the probability percentage of substations 7, 26, 72, and 73 is less than 20%.…”

“…Since the direct current (DC) metro rail is not completely insulated from the soil, the DC current can leak from the rail and form metro stray currents [1][2][3]. The metro stray current can flow into the transformer's neutral points through the soil, ground wire and cable armoring, as shown in Figure 1, which can result in the transformer working in the DC bias state [4][5][6]. Because the metro network is widely distributed, stray currents may cause a large number of transformers to operate in the DC bias state [7].…”

“…Therefore, after the transformer neutral DC exceeds the threshold, the installation of BD can completely block neutral DC and mitigate the DC bias. However, when DCs simultaneously flow into a large number of transformers, the blocked DC by BD can flow to other neutralpoint-grounded transformers, because transformers are interconnected by transmission lines and cables [6,7,13]. Thus, just installing BDs in series at the neutral points of the transformers that have high neutral DCs cannot eliminate the transformer DC bias risk of the whole urban power system.…”

Uncertainty of Standardized Track Insulation Measurement Methods for Stray Current Assessment

“…All electrified transportation systems (ETSs) of the guideway type are affected by return current leakage from the guiding track into the soil, coupling to structures nearby. Examples of victims are sleepers and the track bed [1][2][3], platform screen doors and other metallic parts at platforms [4], viaducts, bridges and building foundations [5,6], pipelines and reservoirs [7][8][9], etc., as well as corrosion of power earthing systems and saturation of transformers [10][11][12].…”

Calculation and Analysis of DC Magnetic Bias Current of Urban Main Transformer under the Action of Stray Current

“…ere are three main causes of DC bias in a transformer: HVDC system operating in monopole mode [1][2][3], geomagnetic induction current (GIC) [4][5][6], and stray current generated by subway operation [7][8][9][10]. e safe operation of a transformer is the basis of power grid safe power supply, and the above three reasons will cause different degrees of DC bias of the transformer.…”