END WINDING VIBRATION AND DEGRADATION

28/07/2016

END WINDING VIBRATION AND DEGRADATION

Case Study:
Repair of a Generator Stator with signs of
extensive end winding vibration and degradation.


ISSUE

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During a major inspection of a 55375 KVA Generator, signs of extreme winding vibration and resonance issues were apparent.

After completing a visual inspection of the end winding
support systems, the following issues were identified and documented:

  1. Extensive greasing and dusting on coil-to-coil ties, circuit ring to gunstock ties and circuit ring blocking
  2. Extensive greasing and dusting of stator bars where they exited out of the slot
  3. Broken and loose coil-to-coils ties
  4. Cracked and, in some cases, broken gunstocks
  5. Indications that circuit ring ties had cut into the insulation on the circuit rings


A bump test was performed to identify, measure and analyze any resonances that would be
readily excitable by the electromagnetic forcing frequency. The result of the bump test showed the end windings to be out of the required parameters and indicated corrective action was necessary.

As part of the original scope, a wedge tap test was performed, which indicated loose wedges
in a large portion of the unit. This would also contribute to the extensive vibration issues
the unit was experiencing.

All other contracted “as found” electrical testing was completed and found to be satisfactory.


RECOMMENDATIONS

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After discussions with the client, it was decided that a complete re-wedge and an end winding blocking modification should to be performed.

  1. All ties found to have greasing or dusting were removed.
  2. Damaged and broken gunstocks were removed.
  3. Circuit rings and stator bars were checked for abrasion and surface damage.
  4. All wedges and filler material were removed.
  5. An extensive clean up was made on the end windings and slot sections to remove
    any remaining dust and grease.

RESOLUTION
    1. New stator slot wedges and fillers were manufactured and installed.
    2. New gunstocks were manufactured, installed and secured.
    3. All coil-to-coil ties and circuit ring to gunstock ties were installed using industry-approved materials and methods.
    4. Circuit rings were re-insulated in areas that had suffered abrasion.
    5. New circuit ring blocking was installed in specific locations identified by our
      Engineering staff and braced using industry proven materials and methods.
    6. End winding stiffening was performed using specific resins and epoxies.
    7. Bump test was performed in order to confirm the repairs had improved the condition of the machine.
    8. Final acceptance electrical testing was completed.
    9. Painting of the unit and demobilization.

CONCLUSION

FinishedAlthough the condition of the end winding support systems were in a serious state of degradation, Sidewinders’ staff were able to steer the customer away from an imminent failure by performing a successful and very effective repair. This machine went back into service with its overall condition vastly improved and life significantly prolonged.


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