This paper presents a case study of a root cause failure analysis of a 373 MW (nominal) steam turbine generator that exhibited both “reversible” and “irreversible” thermal sensitivity vibratory behavior, as well as reduced output, resulting from a unique rotor winding insulation failure mode. An analysis of the historical vibration behavior of the generator is presented along with the techniques applied to diagnose the source of vibration, which included flux-probe testing, full-speed varied-load vibration diagnostics, and an inspection of the rotor field winding followed by a full-field rewind. These techniques allowed for an in-situ root cause analysis, which proved effective in reducing the generating unit’s disassembly, repair and reassembly time, and assuring the steam turbine’s availability for the peak seasonal electric period.

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