Abstract: In the large grounding current system, grounding fault is one of the most common fault forms. The transformers in this system should be equipped with grounding protection (i.e. zero sequence protection), which is used to reflect the grounding fault of the high voltage side winding and lead-out line of the transformer, and serves as the backup protection for the main protection of the transformer and the grounding protection of adjacent buses and lines. This paper introduces an incident analysis of main transformer tripping caused by faulty wiring in voltage transformer transformation, and the preventive measures against such incidents.