General

Wrapping up a key repair

A problem with collector rings discovered the week leading up to Labor Day 2022 could have spelled real trouble at OPPD’s Nebraska City Station (NCS).

But through ingenuity and smart work, a team of about 30 employees from across the utility found a temporary solution. That solution ensured unit 2 at OPPD’s largest generating station would run as needed though the tough Nebraska winter.

And this spring, during a recently completed scheduled maintenance outage, employees replaced the temporary solution with a more permanent one.

Maintenance outage

The turbine unit was completely taken apart during the outage, for only the second time in the unit’s history, said Greg Schroeder, supervisor of Maintenance Services  at OPPD.

The scheduled outage was a long one for unit 2: 67 days. It wrapped up on June 11.

One of the higher priority projects was replacing the collector rings in the unit generator after they were found to be too worn back in September.

Collector rings are crucial to the operation of the generator rotor. Stationary collector brushes transfer field current to the collector rings through surface contact. The rings were worn beyond the original equipment manufacturer’s recommended minimum dimensions.

Ordering new collector rings and waiting for their arrival would mean NCS unit 2 could be offline for at least six months. That wasn’t a realistic option, so employees had to find a way to ensure the unit could run during the cold winter months, which over the past few years have seen much higher peak energy load.

Finding replacements

The group working on the problem decided to machine, or straighten, the thinned collector rings and use them to keep the unit running until a replacement was ready.

Meanwhile, OPPD found replacements for unit 2’s rings in Texas, and Mark Stevenson, Rigging Coordinator at OPPD, flew to J.K. Spruce Power Plant in San Antonio, Texas, to inspect them.

Stevenson developed a plan to cut the ring portion of the rotor from the used generator rotor and ship just a small portion to Nebraska City.  The 7-year-old used collector ring assembly was first shipped to Wisconsin so its manufacturer, Toshiba, could extricate the two rings and refurbish them.

OPPD used the refurbished rings to replace the unit’s 12-year-old collector rings during the recently completed maintenance outage.

Ready for summer

Employees also tackled a number of “critical path” projects during the maintenance outage,  said Kyle Brinkerhoff, manager of Maintenance Services at OPPD.

Among those projects were:

• Spray dryer absorber (SDA) cone coating. The SDA cone collects the fly ash as it falls out of the air stream and sprays a mixture of lime slurry to capture the mercury from the air.
• Air heater basket and catalyst work.
• Cryogenic cleaning.

“In general the collector ring removal went pretty smooth,” said Schroeder.  “A lot of hours and a lot of hard work was done on this and the other aspects of this outage, and we feel we are ready to handle the hot months ahead of us.”