The number of latent faults for converter transformers gradually increased after a long-term service. In an annual maintenance for converter transformers, the abnormal increase of the DC resistance for the valve-side windings with bushings of the transformers were observed firstly in the world. Based on various experiments and visual inspections, it was found that the contact fingers on the tail plug of valve-side bushings were overheating and blackened, resulting in the abnormality of DC resistance. The problem-solving process in this report will provide rich experience for similar works in future. Moreover, the overheating mechanism is still controversial and deserves further research.

Converter transformers;DC resistance;valve-side bushing;contact fingers;overheating