Abstract: The setting calculation of the overexcitation protection for generatortransformer unit must be based on the tolerance of the generator and the transformer to the overexcitation. The settings must be matched up with U*-f* limit curve of the excitation system; it must be adjusted in accordance with the backup protection of U*-f* limit component of generator field regulator. According to the analyses of overexcitation curve of the 680MW generator and transformer in Hubei Xisaishan Power Generation Corporation Limited and the U*-f* limit curve of ABB excitation system, the responding value of overexcitation protection and the actuation time were analyzed and calculated concretely.

Key words: over-excitation curve, U*-f* limit curve, match up, analysis