News Flash

Waukesha Service Crew Performs Successful Untanking in the Field

Core and coils being removed at customer’s facility from a gassing transformer.

June 10, 2010 – A Waukesha Service crew, consisting of Michael Cheney, Brian Johnson and Jesse Figueroa, reported to a Power Generation facility in New York earlier this year to repair a General Electric 25MVA auxiliary transformer. The unit had been taken out of service due to voltage irregularities and overheating. Advanced diagnostic testing revealed that insufficient contact pressure on the center phase de-energized tap changer (DETC) was causing the identified issues as well as the evolution of combustible gasses. A de-energized tap changer is typically mounted on the cleat and lead assembly attached to the core and coil package inside the transformer. Since real-world electrical systems rarely operate at identified voltages due to losses, the DETC is used to match the transformer’s primary voltage to the actual transmission line voltage and adjust the turns to match design core flux density.

The transformer was drained and inspected by the Waukesha Service team, and a determination was made that the core and coil assembly needed to be removed from the tank to enable the repair due to limited clearances inside. At the equipment owner’s request, our team developed a detailed plan and repair procedure to fix the unit in the field.

Upon acceptance of the plan, the transformer was pressurized with dry air and relocated into the facility’s maintenance bay. Inside the building, the transformer was completely disassembled, the coils covered with a canvas tarp, the tank filled with oil to prevent damage to the insulation from errant sparks and the remaining dry air in the tank purged. Once the cover was cut and removed, the coils were craned out of the tank and placed in a temporary “clean room” erected specially to house them and facilitate repairs to the DETC.

Waukesha Service crew member prepares transformer tank for cover welding after completing on-site DETC repairs.

With the core and coil assembly accessible, the DETC was repaired and properly aligned. The service team pressure-tested the tank to confirm it was still leak-free, returned the coil package to the tank, reconnected the DETC and secured the cover after properly protecting the coils from the welding process. The transformer underwent rigorous dryout procedures and vacuum processing due to the coils’ brief exposure to the atmosphere before being reassembled, vacuum-filled, tested and
successfully returned to service.

This entire process, which would normally have required a return to the factory for repair, provided considerable savings to the customer because the team was able to accomplish all necessary service in the field.




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