Two generator stator cores failed in a six-month time span: one failed on-line creating a forced outage situation, and the other had a 180-milliamp indication on an ELCID test during a routine test and inspection. The stators were similar in design and size; however, the ages of the units were considerably different.
The generator was disassembled after the unit suffered an on-line failure by tripping on a stator winding ground fault. There was evidence of core iron burning, so the winding was removed immediately. Once the core flange was removed there was evidence of heating/melting around a thru-bolt (Figure 1).
As disassembly progressed, the end plates were removed and damage became more evident. Molten core iron could be seen on the laminations coming out around the thru-bolt hole. A small hole was observed in the slot bottom. Grinding around the hole was ineffective and seemed to be leading to a larger cavity below the surface. Un-stacking continued and the uncovered damage continued to worsen.
The large cavity appeared under the hole in the slot bottom. Iron was melted and fused to the building bolts as well as the thru-bolts. Other thru-bolts showed distress on the insulation-like mechanical damage due to vibration (Figure 2).
Seven packages were un-stacked to remove all the damaged iron and welding to the building bolts. Approximately 30 percent of the removed iron had to be fabricated while the balance was recoated for re-use (Figure 3). The core was re-stacked using new Nitronic thru-bolts. Maintenance records did not indicate any core tightening performed on the unit since installation, not unusual for a unit of this core length and only 10 years of age.
During a regularly scheduled maintenance outage an ELCID test was performed on the stator core. The test uncovered one area that had an indication of 180 milliamps and the type of peak indicated a deep-seated fault. Also, inspection of the area of concern did not reveal any surface damage to any core iron teeth.
Using a borescope to more closely examine the slot, a steel chip was found lodged in the air duct between the air vent backing plate and the core laminations. Removing the chip required removing several top bars and a bottom bar to access the affected area.
Once the bars were moved out of the way, the chip fell out with very little prodding. Subsequent ELCID testing still reflected the indication, which led us to believe the chip was not a contributing factor.
No visible damage was seen in the slot bottom and there was no evidence of excessive heating. As the slot was narrow and deep, any type of polishing would be difficult to perform.
The next step was to find another area in the vicinity that would be suspect. The through bolt in that area was only 3" away from where the chip was found. The thru-bolt was removed and a burning indication was seen in the insulation.
The thru-bolt insulation also had core iron impressions down its length. The bolt was apparently loose enough to vibrate and hit the core iron hard enough to cut small grooves into the insulation (Figure 4). This mechanical damage was seen on mot of the thru-bolts once they were removed; however, none of the other bolts showed any signs of overheating. Maintenance records did not indicate if or when the thru-bolts had been tightened in the 40-year history.