Abstract: This paper presents a case study of continuous overstep tripping of the medium-high voltage backup protection in a 110 kV main transformer caused by a 35 kV line fault and secondary AC-DC coupling faults of the transformer. Through fault dispatching, comprehensive substation equipment inspection, and root cause analysis, this paper identifies the reasons for the continuous overstep tripping of transformer protection. The study highlights issues in this case, including substandard quality of transformer live status indicators, non-standardized alarm information, flaws in secondary circuit design, and inadequate lightning protection measures on transmission lines. Corresponding improvement suggestions are proposed to prevent similar incidents, contributing to the safe and stable operation of the power grid.