Xcel Energy's 69-kV transmission line crossing a national wildlife refuge on the northern reaches of the Mississippi River — near Nelson, Wisconsin, U.S., 70-miles (113-km) southeast of Minneapolis, Minnesota, U.S. — is just 2.7-miles (4.3-km) long. However, replacing this short segment of a more than 80-year-old line posed significant challenges for design, access, and construction.
Originally built in 1934, the line serves the border towns of Nelson, Wisconsin, and Wabasha, Minnesota, while providing an important grid connection between the two states. At that time, the line's wooden H-frame structures were set mostly on dry ground interspersed with streams and wetlands, as the line extended westward from Nelson toward the Mississippi River. A year later, in 1935, much of the route was submerged by 2 ft to 3 ft (0.6 m to 0.9 m) of water, after the completion of lock and dam No. 4, located approximately 7-miles (11-km) downstream on the Mississippi. Today, as part of the 240,000-acre (97,125-hectare) Upper Mississippi River National Wildlife and Fish Refuge, the area is an important environmental resource providing habitats for protected species as well as year-round recreation for hunters, anglers, and boaters.
Despite decades in which their bases either were submerged or had groundwater just below the soil surface, many of the existing wood pole structures performed well beyond their typical 50-year to 60-year life expectancy. Their service life was coming to an end in 2015 when three structures were temporarily stabilized and reinforced, with steel piles driven adjacent and secured to the existing wood poles. However, because of its age and condition, a 2.2-mile (3.5-km) long segment, beginning near the Nelson substation and extending westward to the last structure before the Mississippi River crossing, warranted a more permanent solution.
Faced with the challenges of designing a low-impact solution in an environmentally sensitive area with difficult access, Xcel Energy's team initially examined the possibility of rerouting the transmission line or simply retiring it altogether. The utility tabulated input from permitting, engineering, and construction staff to evaluate the various options and their corresponding costs, benefits, and feasibility.
You can read the full case study here.
