Recent endwinding vibration problems in air-cooled turbine generators

SUMMARY The endwinding region of large turbine generator stator windings is one of the most complex parts of a generator to design and fabricate. During normal operation, the endwindings are subject to high mechanical forces at twice power frequency due to the currents in the stator bars, as well as mechanical forces transmitted via the core and bearings at rotational speed. During power system transients, the forces in the endwinding can be 100 times higher. Due to the presence of high magnetic and electric fields, metallic components to restrain the movement of the stator bars caused by these forces are normally avoided. This constraint has resulted in a wide variety of endwinding support structures from the various manufacturers. If a component of the endwinding or the endwinding basket as a whole has a natural frequency close to the forcing frequencies, the vibration response will be in a resonance condition and the result can be catastrophic. Off-line impact testing has long been a tool to identify these natural frequencies and help determine if a resonant condition may exist. Not only can this testing be used to assess the condition of a stator endwinding, but it can also be used to identify the locations that are most likely to vibrate. To avoid premature failure, excessive motion in the endwinding during operation can be monitored. To effectively do so, not only should the sensors be installed at locations most likely to vibrate, but they should cover a wide enough range to capture all of the vibrating frequency components. There is little guidance on acceptable vibration levels, so an increasing trend is of concern. Although it is clear that the endwinding support system of even the largest generators can be designed to achieve 30 or more years service without excessive loosening, in the past decade a large number of in-service faults have occurred due to endwinding vibration. Even more machines have been discovered during visual inspections to have premature deterioration of the endwindings due to looseness. These problems have been primarily associated with air-cooled machines typically installed in gas turbine or combined cycle plants. To greater or lesser degrees, most large generator manufacturers have been affected. It is suspected that competitive issues (and in particular cost) may be forcing manufacturers to compromise on proven design and manufacturing methods.