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Stator Bar Failure Analysis

In-depth assessment and failure analysis by rotating machines specialists

Project objective

Kinectrics' client requested destructive examination of a stator bar that had failed while in service, to determine if the root cause of the failure could be established.

Failure analysis

Inspection of the bar showed a puncture site located approximately 36” from the interface between the semicon and stress grading coatings on the bar. A section of the bar, including the puncture site, was subsequently cut from the bar for dissection.  The bar segment was approximately 12” long with the failure located near the centre line of the section. 

An observation was made that the ground wall build at the corners of the bar exhibited significant variation.  Measurements made with a micrometer showed that the insulation thickness at the corners varied from a minimum of 0.105” to a maximum of 0.165”. The puncture site was located on the side of the bar with the thinner insulation build. 
The observation made that the thinner and thicker builds were diametrically opposite pointed to some issues with the processing of the bar during manufacture.  On removal of the first surface layer of insulation, it became evident that the semicon layer was composed of a varnish rather than a tape. 
Further examination indicated that there were some relatively deep wrinkles in the ground wall insulation.  Despite the wrinkling, the tape layers were found to be well-bonded, demonstrating that the insulation had been properly impregnated, consolidated and cured.
Closer to the copper stack, the quality of the bonding between tape layers had deteriorated. This degradation of adhesion between layers was due to the resin component being subject to Partial Discharge (PD) and insulation damage was observed.
Dissection of the failed bar revealed a number of anomalies in the stator bar. Issues noted included:
·         Significant local variations in ground wall build
·         Formation of sharp edges
·         Tape wrinkling
Evidence of PD-related degradation of the resin was also found in the insulating layers close to the copper conductor stack in the vicinity of the corners.  This region is the most highly electrically stressed and the observation of PD damage is not completely unexpected, especially on older, service-aged machines.
Assessment results
Dissection of the stator bar showed:
  • Variation in the ground wall build, especially in the vicinity of the puncture location
  • Sharp edges on two of the bar corners (due to lack of control of the tension of the tape during manufacture)
  • ignificant wrinkling of the ground wall insulation—another indication of loss of control of tape tension. Tignificant wrinkling of the ground wall insulation—another indication of loss of control of tape tension. The failure site itself was bisected by one such wrinkle
  • Ground wall insulation had been properly impregnated, consolidated and cured
  • Evidence of PD-related degradation of the resin component of the ground wall insulation was noted at the corners of the bar
  • Minimal damage to the copper stack had been incurred due to the in-service failure despite the unit operating at near full load at the time of failure 
The defects noted were contributing causes to the failure, although it should be noted that the machine had operated for approximately 30 years. The noted defects cannot be construed as being severe life-limiting. Otherwise premature failure would have occurred many years earlier.
The in-service failure may have resulted from long-term aging of the ground wall insulation, exacerbated by the noted manufacturing anomalies. However, a transient event, in conjunction with the bar dissection observations, may have resulted in the stator bar failure.