The design included one-piece polymer insulators as opposed to a typical series of 18 to 20 porcelain “bells” found on other 345-kV circuits. These insulators and associated hardware assemblies, packaged and supplied by Hubbell Corp., produced a lower initial installed cost and, ultimately, is expected to reduce maintenance costs.

In some cases, the design required custom structures to fulfill special needs. This included single-circuit, single-shaft structures; double-circuit, single-shaft structures with four different voltage levels for the second circuit; double-circuit, single-shaft structures with 46-kV underbuild; two-pole, heavy angle deadend structures; and single- and double-circuit H-frame structures. All steel structures were supplied by Thomas & Betts.

Project designers chose to construct the line using weathering steel poles (with the exception of two galvanized steel poles and one metallized steel pole required by the National Park Service (NPS) at the Namekagon River Crossing) to carry the wires in vertical configurations on single concrete pier foundations. For aesthetics and future maintenance cost savings, ATC specified a weathering finish, which does not require a maintenance cycle like painted poles, on the more than 50 million lbs (22,680 metric tons) of steel used for the single-pole structures.

NAMEKAGON RIVER CROSSING

One of the more complex designs was a double-circuit line section that crossed the Namekagon River, which is the northern tributary of the St. Croix River and is protected by the NPS as a Wild and Scenic Riverway for canoeing and camping. This river crossing is under the jurisdiction of the NPS, which considered several options for the crossing, including both overhead and underground alternatives. The NPS selected an overhead crossing as the preferred alternate mandating that the visual impact of the structures be mitigated by a vegetation screen at the river bank, so the supporting structures would not be easily seen from a canoe in the river. NPS also limited the number of conductors to six instead of the typical 11, in an effort to minimize the line's visibility and to maintain the aesthetic viewscape.

The team used a single-conductor-per-phase design on the 345-kV circuit and removed the two shield wires from three spans — the crossing span and one adjacent span on each side of the river. One of the shield wires was an optical ground wire, so it had to be buried under the river using a directional drill bore to maintain continuity of the fiber signal. The team used surge arresters at the two structures on either side to limit voltages due to lightning strikes or equipment faults, preventing damage to equipment and disruption of service.

To mitigate visual impacts, the NPS required that the crossing span be about 1500 ft (457 m), around double the average span length, and that the structures on either side of the river be less than 125 ft (38 m) tall. To accommodate this, the design team used a special conductor for each of the 345-kV and 161-kV circuits crossing the river. They ordered specialized heavy conductor from Japan, supplied by Sumitomo Cable/J-Power Systems Corp., which was much larger than typical wire and had a rated breaking strength of 200,000 lbs (90,718 kgs) for the 345-kV conductor and 140,000 lbs (63,503 kgs) for the 161-kV conductor. The conductors were deadended to the crossarms to avoid the added height of installing suspension insulator assemblies. All of this required yet another set of special-structure designs, which were handled using custom-designed tubular-steel poles.

ENERGIZED AHEAD OF SCHEDULE

In January 2008, the Arrowhead-Weston line was energized four months ahead of schedule and within the approved budget levels. After 10 years and countless challenges, one of the largest transmission line projects built in the United States in decades was providing a much-needed improvement to the reliability of the regional grid. With the line in place, system operators are able to transfer additional power from western markets through Minnesota into Wisconsin.

The design for this major project was no small feat. The line spans two states, eight counties and 44 towns. At a cost of $435 million, the line can carry 800 MW, providing enough electricity to power 250,000 homes. Although there were many factors to the project's success, much of it can be attributed to the flexibility allowed by the decision to construct the line using single-pole steel structures.

It was also successful because the team was able to adjust to change. When the team unexpectedly hit karst rock while drilling the foundations early in the project, the need for a fast yet durable solution was apparent. The design team was undeterred. They quickly devised a foundation that consisted of four drilled piers tied together with a pile cap and secured to the rock via post-tensioned rods. The on-the-fly design was the epitome of ingenuity and representative of the team's attitude throughout the project: Expect the unexpected.

Pete Holtz is a general manager at American Transmission Co. He has managed several projects in his tenure at ATC since its formation in 2001. Since 2002, he has been the overall manager of the Arrowhead-Weston transmission project. Prior to joining ATC, he spent 26 years at WE Energies managing customer service activities. He holds a bachelor's degree in business administration from the University of Wisconsin-Madison. pholtz@atcllc.com

Ron Gullicks is a supervising engineer at Minnesota Power with 29 years of experience in the design and construction of electric generation, substation and transmission line facilities. Gullicks is a registered professional engineer in Minnesota, Wisconsin and North Dakota. He holds a bachelor's degree in civil engineering and a master's degree in sanitary engineering from the University of North Dakota. rgullicks@mnpower.com

Dave Valine is an operations manager for Minnesota Energy, a subsidiary of Integrys Energy Group and a sister company to Wisconsin Public Service. In his 19 years at Wisconsin Public Service, Valine worked primarily in transmission licensing, engineering and construction. He holds bachelor's degrees in mining engineering and electrical engineering from Michigan Tech University and is a registered professional engineer in Wisconsin. dlvaline@minnesotaenergyresources.com

David W. Wedell is a senior project manager at POWER Engineers. He has more than 30 years of experience in the design of high-voltage and extra-high-voltage transmission lines. He has managed transmission line design projects in the United States, Saudi Arabia and Thailand. Wedell holds a bachelor's degree in civil engineering from Southern Illinois University. dwedell@powereng.com