The wide adoption of a new type of electric trains results in multiple frequency-scale instability problems, such as low-frequency oscillation (LFO), harmonic resonance, and harmonic instability. Among them, LFO has been commonly reported at electric railway depots with multiple-energized electric trains around the whole world, thus attracting widespread research attention. LFOs cause adverse effects on the system stability and protection infrastructure of both the traction network and electric trains. This article presents a comprehensive review of LFOs that consider both academic theoretical analysis and industrial engineering requirements. A unified mathematical model of the traction network and electric train is derived with a view to revealing the mechanism and factors that induce LFOs when these two systems interact. The article also draws deep insights from results of experimental measurements and industrial mitigation schemes already implemented in Chinese railway systems. A variety of mitigation techniques and recommendations based on theoretical principles and practical constraints are presented.

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