Monticello Transformer Relocation

Location: Monticello, Minnesota

Areas of Expertise: Substation

Relocating a massive transformer is no small feat, but that’s exactly the challenge Hooper’s Electric Power Division tackled in Minnesota. The team collaborated with Xcel Energy to replace an aging transformer at the Monticello Nuclear Generating Plant. The project involved disassembling, transporting, and reassembling a 345kV transformer from the Hazel Creek Substation in Granite Falls to the Monticello Substation.

To carry out the replacement, Hooper crews first disassembled a spare transformer at the Hazel Creek Substation. This powerful 336 MVA, 345kV/230kV/13.8kV unit needed to be carefully prepared for transport. The team spent multiple weeks dismantling the unit reducing its weight to 254,410 pounds from the original assembled weight of 440,477 pounds. Hooper set up work groups for the move. Barnhart Crane & Rigging provided a 550-ton crane to lift the transformer onto a 175-foot trailer. With help from Pilot trucks, two Minnesota State Patrol escorts, high-load bucket trucks, and Xcel Energy linemen who raised power lines along the route, the transformer was successfully transported to its new location. The journey covered 125 miles and took approximately 12 hours, a stark contrast to the usual two-hour drive by car.

Once it reached Monticello, the team used a mobile crane and protective road plates to safely position the transformer on a temporary pad within the substation for reassembly. The transformer was carefully reassembled, filled with oil, and underwent thorough testing. The existing TR6 transformer will be decommissioned in the fall of 2025, allowing the new unit to be connected to the plant’s 345kV and 230kV buses. Additionally, the plant’s 13.8kV tertiary structure and cables will be replaced with new cables in concrete encased conduits for added durability and safety.

The project also includes the installation of a new 345kV MOD (Motor Operated Disconnect) and dead-end structure as part of the plant’s refueling outage. This upgrade ties into the Sherburne County 345kV line, further strengthening the facility’s infrastructure. The project is expected to be completed by late 2025 or early 2026, marking a significant milestone in the plant’s modernization.