Boundary Dam achieves significant milestone in Unit 51 overhaul with new rotor installation

At Boundary Dam, all eyes have been on Unit 51, one of six generators at the hydroelectric plant that together provide 28.6% of City Light’s power generation portfolio. Unit 51 is being refreshed and upgraded to provide carbon-free, reliable power for the next 40-plus years. Last week, the Boundary project team reached a significant milestone during this overhaul – installing a 772,000-pound rotor, enabling the generator to operate at its optimum, increasing its output from 158 megawatts to 180.

​​​​​​​So, what exactly is a rotor, and how does it help produce electricity? Simply put, the rotor is the part of the generator that spins. As water comes down the penstock and past the turbine, it causes the turbine and rotor to turn. The rotor has electromagnets (which look like red poles) that alternate between positive and negative magnets. As the magnetic field created by the electromagnets spin in the stator (the stationary part of the generator), the mechanical power from the falling water is converted to electric power.

The new rotor inside the machine hall at the Boundary Hydroelectric Project.

Though the installation took about 2 ½ hours, the entire process was two-plus years of planning and preparation, including design, modeling, fabrication, assembly, mechanics, logistics, safety protocols and COVID delays. Unit 51 was initially taken out of service in August 2019. Its new-and-improved debut is slated for May 12.

​​​​​​​“The rotor was rebuilt with a new design to improve airflow and cooling in the generator,” explained Capital Project Manager Josh Jackson. “The old rotor had 10 straight arms, but this new design has 15 oblique arms that will push more air through the stator. Many of our employees were stationed around the rotor to make sure it didn’t rub during the installation (there is less than ¾ inch of space between the rotor and stator), so it was a Boundary team effort. In total, about 22 people were involved in the installation – machinists, electrical constructors, operators, hydro maintenance workers, painters, and warehousers.”

Old rotor inside the machine hall at the Boundary Hydroelectric Project.
Old rotor
New rotor during assembly inside the machine hall at the Boundary Hydroelectric Project.
New rotor during assembly

“This process has been all-hands-on-deck, and because of our talented workforce and support from Seattle trades, this extremely delicate part of the project was completed safely and efficiently,” said Senior Operations Manager Janet Hart. “Many thanks to all involved!

“It was also awesome to see our new machine hall bridge crane in action,” she continued. “It has amazing features that allowed us to store coordinates when we removed the original rotor. Once the rotor was rigged and lifted, the crane operators recalled the coordinates and moved the new rotor over the exact point necessary to drop it down into the stator frame. We obtained exact weights on the pick: The lifting beam with the rotor was 412 tons (824,000 pounds)!”

Though installation was successful, the work doesn’t stop for the folks at Boundary. “The next step is to complete the final assembly of the generator, followed by the entire testing and commission of Unit 51,” shared Engineering and Technology Operations Director Faz Kasraie. “This is a critical three-week effort that’s necessary before releasing the machine for commercial operation. We expect to complete testing and return the unit to service around June 1. Stay tuned for more exciting news in the coming weeks.”

​​​​​​​Congratulations to all our employees who were involved in such a tremendous endeavor. Your hard work and dedication are shaping the utility of the future, ensuring clean and reliable power for generations to come!