Taking use of the equipment act information, fault record data and setting values of protection equipment, the reason for the override trip of the transformer's backup protection is found. The paper has described the accident process. Because of damaged line protection and extortionate bus voltage, the compound voltage locking unit of the backup protection of the main transformer low voltage side refused to act, and the override trip of the backup protection of the main transformer high voltage side took place. In order to prevent accidents like this, improvements of the existing main transformer backup protection were proposed, that is revising action logic and adding time-based low voltage unit whose action equations had been given in the paper.