Abstract: An accident of generator trip caused by over temperature of excitation transformer which run through the UNITROL 5000 excitation system had been in-troduced in this paper.According to the protection action and the recorded data, through the comprehensive inspec-tion, it is concluded that the fundamental cause of the acci-dent is the sampling distortion of the temperature measure-ment element in the excitation transformer, and the indirect reason is that the temperature protection configuration of the excitation transformer is not reasonable.

Key words: excitation transformer, over temperature, temperature protection