This paper presents a detail research on a vibration high fault after overhaul occurred on a generator. During the overhaul, the generator rotor is returned to factory for processing and replaces section across the line of the generator excitation end. The gas turbine unit vibrates high in the idle 3000 r·min−1 and full load. The large vibration of the generator is mainly caused by the following two factors:the imbalance in the overhanging end of the generator rotor and the electrical fault in the generator itself. The unbalanced amount of generator overhang was successfully eliminated by dynamic balance test. The electrical fault of generator itself needs to be returned to the factory to handle.

The finite element model of a 1000MW unit was set up and vibrations of the three-bearing structure were studied. The results show that the generator supporting characteristics have great effects on vibrations of the three-bearing structure. Many factors are needed to be considered when field balancing. The generator supporting characteristics should be considered when judging the type and axial position of imbalances. Repeated balancing are invalid and even the vibrations diverge if just following simply the harmonic component method to determine directly the imbalance types. The conclusion reliabilities were further verified with the tests.

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Poor heat treatment of the generator may cause cracks during operation. The presence ofcracks on the rotor causes the rotor to vibrate. The paper establishes a finite element modeling of aunit. The model is used to study the vibration response characteristics of the shafting system when thesupport characteristics of the generator are similar and the difference is significant. When thedifference in support characteristics between the two ends of the generator is large, the first-orderunbalance of the generator not only causes a large in-phase vibration at the first-order critical speed ofthe generator, but also causes a large reverse component. Finally, the reliability of the conclusion isfurther verified by combining the abnormal vibration diagnosis and processing cases of a generator.The innovation of the paper is to diagnose the possible cracks in the generator through the vibration ofthe generator, and then look at the original data in the generator manufacturing process to find that theheat treatment process of the generator rotor is poor.