Abstract: Digital substations are characterized by the collection of analog signals at the process layer via the Sampled Value (SV) network and the transmission of switching signals through the GOOSE network. Electronic transformers are employed to achieve on-site digitization of analog signals, which are then transmitted to protection, measurement, and control devices through fiber optic circuits such as acquisition units and merging units. This paper analyzes a typical case of mis-operation of power frequency variation differential protection for a main transformer in a digital substation. A detailed technical investigation of the accident process and handling methods is conducted. Through theoretical analysis and simulation experiments, the accident waveform is reproduced, and it is determined that the root cause of the tripping accident is a cold solder joint at the shielded cable connection between the transformer and the acquisition unit. This defect leads to abnormal waveforms and data loss on the high-voltage side, resulting in the mis-operation of the main transformer's power frequency variation differential protection. Corresponding countermeasures are proposed to address this accident, aiming to provide a reference analysis method for resolving similar issues.