Analysis of Root Cause of Failure of a Turbo Generator Stator Winding

The important components that have direct bearing on the operational reliability and life of the machine are stator winding, stator core and rotor winding. The stator winding is the most important and expensive part of the generator where full power is generated. The life of the stator is affected by one or more combinations of various stresses like electrical, thermal, mechanical and environmental at any point in time during operation of the machine. In addition to these stresses, the stator insulation is also subjected to unforeseen stresses during transient over voltage conditions. Steep fronted over voltages generated during switching actions and system faults, propagate through the winding and have deleterious effects on the insulation. Since the mechanical stresses are relatively higher in turbo generators many of the stator winding failures are mechanically induced electrical failures. Some of the degradation processes include loosening of wedges and coils, occurrence of slot and end winding discharges, erosion stress grading and corona shielding coatings, de-lamination of insulation, de-bonding of copper from insulation and the like.In this paper the data obtained from the visual inspection and examination on the stator winding, and diagnostic tests conducted on the healthy phase of a 250 MW, 16.5 kV failed turbo generator have been analyzed to find the root cause of premature failure. The machine had completed a service life of about 10 years and had run for about 60,000 hours before it failed with a stator earth fault. Considering a notional design life of 35 years, it was vital for the asset managers to know the root cause of failure. Findings of the visual inspection and examination on the stator winding, and diagnostic tests conducted on the healthy phase have been discussed in detail. It is concluded that the root cause of failure is stator end winding discharges/electrical tracking. In particular, the causes of end winding discharges are contamination of the end winding due to condensed oil fumes and powder generated by insulation fretting. The insulation fretting is due to end winding vibration.