Abstract: Taking an abnormal damage of the protection device of a transformer caused by improper selfhold design of the protection outlet trip circuit as example, the reasons of the damage were analyzed, and the design defects of the same selfhold circuits were treated thoroughly. The correctness of the judge, the analysis and the problem treatment were verified by field analysis and simulation test. The results in this paper can be used as reference in maintenance of protection devices of similar generators and transformers put into operation earlier.

Key words: protection trip circuit, outlet trip, hidden defect, self-hold, periodic inspection