Frequency analysis and calculation of transformer neutral DC caused by metro stray currents

Metro stray currents flowing into transformers can lead to high-level transformer neutral direct current (TNDC). Since the TNDC is not constant and varies with time randomly, the frequency characteristics of TNDC are confusing and unclear, which will lead to complex transformer electromagnetic characteristics and increase the difficulty of transformer protection and DC bias mitigation. Therefore, it is important to understand the frequency characteristics of TNDC caused by stray currents. To address this issue, this paper performs extensive tests on TNDC. Through the detailed frequency analysis of the test data, the relationship between the main frequencies of neutral DC and metro train time headway is found. To further explain this relationship, a frequency calculation modeling method of TNDC is proposed, in which the traction current along the metro line is innovatively equivalent to impulse and shock frequency signals. Then, comparing the calculation result of the main frequency with the test data, the proposed model is verified. Moreover, the relationship between the main frequency and train time headway is explained and clarified based on the model. Furthermore, the effect of train time headway and dwell time is analyzed. The results show that with the train time headway decreasing and dwell time increasing, the complexity of the neutral DC frequency distribution increases to some extent. • TNDCs are tested and the frequency is mainly concentrated in 0-0.05 Hz range. • A frequency calculation model of TNDC is proposed innovatively. • The TNDC main frequency is related to the metro train time headway. • The metro line number, train time headway and dwell time affect The TNDC frequency.