Failures of large turbo-generators on prolonged site storage - Case studies of indian power utility Gupta Hirdesh1, Senghani H.2, Chaturvedi D.K.3 1General Manager NTPC Ltd. 2Manager NTPC Ltd. 3Associate & GM (Retd.) NTPC Ltd Online published on 16 June, 2021. Abstract It has been seen that improper storage of critical capital intensive equipment such as turbo-generator stator, rotor, generator transformer, etc. can substantially reduce the equipment life and failures can happen during initial runs or within couple of months of operation although equipment are designed for atleast 25 years of plant life. First machine of 250 MW at one of the project site had failed after one year of operation while the second-unit had failed while synchronizing with the grid. In view of the failures in first two generators, third generator-which was due for erection was inspected thoroughly and extensive diagnostics were performed. This machine showed high ELCID (Electromagnetic Core Imperfection Detection) values and stator slot wedge looseness was observed. The flashover in second unit stator end winding region had occurred during Dielectric High Voltage test voltage-build-up process. The affected area was cleaned properly and re-insulated. High voltage test was conducted-after re-insulation on the generator stator winding, it was seen that the location, which was re-insulated could-sustain the voltage but visible corona observed from other locations in end winding region. In this paper, case studies related to failures of three large turbo-generators are discussed in detail. As the failures were noted in components of stator winding insulation system after prolonged storage, this paper aims to create awareness in respect of requirement for holistic review of stator winding insulation components aging rate particularly of braiding, which experience high electrical stress and become cause of early failures of the machine. The insulation in the winding overhang portion experiences voltages upto rated line to neutral voltage during machine operation and upto dielectric high voltage test level during dielectric one minute power frequency voltage withstand test. It becomes important that all elements used in end-winding insulation and support system are suitable to withstand electrical stress corresponding to rated voltage and dielectric test voltage level. Top Keywords Turbo-Generator, Dielectric, Braiding, End-Winding, Contamination, Insulation Resistance. Top |