The Geotechnical Engineer Had Good News. He had taken the soil boring for the proposed structure and encountered no rock at the design depths, just sand and clay soil to depths of 60 ft (18 m). It was music to the ears of the design team, who wanted to get off to a fast start in Wisconsin with concrete pier foundation designs for more than 1500 structures along the 220-mile (354-km) Arrowhead-Weston 345-kV line, the biggest transmission line project in the state's history.
Imagine their surprise, then, when they later found karst rock below groundwater, while drilling the foundation for one of the most heavily loaded structures near Weston Substation. Karst consists of outcroppings that create unpredictable vertical seams, a designer's nightmare. It makes it difficult to install any type of foundation, let alone the concrete pier foundation planned for this structure. Although the soil borings had been drilled properly, the geotech had hit a seam and missed detecting the karst rock; a larger drill revealed otherwise. Still, the design team was ready. After all, they would have been crazy to expect everything to unfold exactly as planned; they knew to expect the unexpected.
DEMAND JUSTIFIES NEED
When the U.S. Department of Energy declared northwestern Wisconsin one of the four most-constrained transmission line areas in the country in the late 1990s, the need for a new, reliable line became clear. Because of limited power-transfer capabilities between Minnesota and Wisconsin, certain load conditions could cause blackouts in Wisconsin and surrounding areas. On top of this, the demand for electricity in the region was growing at the rate of 2% to 3% per year. The existing transmission grid was not equipped to reliably accommodate this growth. At the time of construction, Wisconsin only had four interstate high-voltage lines, compared to dozens in neighboring states. This was the impetus for the Arrowhead-Weston 345-kV transmission project.
Originally proposed and initiated by Wisconsin Public Service Corp. (WPSC; Green Bay, Wisconsin, U.S.) and Minnesota Power (Duluth, Minnesota, U.S.) in 1998, American Transmission Co. (ATC; Waukesha, Wisconsin), at the time a newly formed regional transmission operator, took over the project from the two utilities in 2002.
ATC immediately ordered a review of the original US$165 million cost estimate that had been previously approved by the Public Service Commission of Wisconsin (PSCW). In just five years, the project's estimated cost had ballooned in the face of rising material and construction prices, and growing opposition from environmentalists and concerned landowners. To account for more robust environmental impact mitigation, environmental inspection, farm disease mitigation, increased real-estate rights-of-way costs and increased public outreach efforts, ATC increased the project budget to $420 million, which was reapproved by the PSCW.
A BALANCED DESIGN
Because of the critical nature of the project, the design and construction team — comprised of ATC, WPSC, Minnesota Power, POWER Engineers (POWER; Hailey, Idaho, U.S.), M.J. Electric (Iron Mountain, Michigan, U.S.), Tri-State Drilling and others — set out to build a new line that would not only be highly reliable, but also highly flexible to meet each of the project's stringent requirements. The design was predicated on the idea that lines like this didn't get built every day. To do the project justice, it would have to be designed to the highest-possible standards.
The overarching goal was to find a design that balanced all of the project's elements, including environmental and landowner concerns, aesthetics, land use, extreme meteorological conditions, widely varying subsurface conditions, durability, constructability and more. The numerous stakeholders involved further complicated matters. Because the PSCW mandated that the new line use existing rights-of-way as much as possible, it became necessary to replace portions of 12 different existing lines with new double-circuit structures. The design required close coordination with various utilities to account for each of their unique design standards.
One of the early steps was to identify a basic structure design. The existing structures were primarily wood-pole H-frames that required large rights-of-way corridors. Adding additional wood structures was unrealistic, given the constraints placed on the design. Wood also had insufficient strength to carry more than one circuit. Because many of the structures would have to be double-circuit, wood was quickly ruled out. Although durable and able to accommodate several different tower configurations, lattice towers were deemed unsuitable because they, too, would create footprints that would exacerbate landowner and environmental impacts. That left single-pole tubular-steel structures, which best accommodated the host of line-design constraints. Steel poles offered engineers and designers the flexibility to create the most efficient designs with high levels of reliability.
Because of the critical importance of this new line, ATC decided to design for high reliability. Although most lines are designed for a 50-year or 100-year return period event, ATC determined that this line needed to be able to withstand a 400-year return period weather event. (Designing for a 400-year return period means that one must design for a weather event that has a 1 in 400 probability of occurring in any given year.)
The decision proved complicated and required the design team to use two distinct loading criteria, one for the northernmost 20 miles (32 km) of the line (to accommodate the local weather effects from Lake Superior) and another for the remaining 200 miles (322 km) of the line (to accommodate the weather of north central Wisconsin).
The microclimate found on the northern portions of the line typically provides more severe icing conditions because of its proximity to Lake Superior. On this first section, the design team assumed that the conductor, supplied by Alcan Cable, could accumulate up to 1.85 inches (47 mm) of radial ice. In contrast, designs along the Wisconsin portion of the line took into account 0.925 inches (23.5 mm) of radial ice, about one-third as much when measured by volume. The more severe ice loadings near Lake Superior required heavier conductor, heavier structures and larger foundations. In both regions, the line was designed for an extreme wind of 108 mph (174 kmph).
Because the PSCW required that the line share corridors with as many existing utility easements as possible, cross-utility communication and coordination was essential. Seventy-five percent of the line was located within or along existing transmission lines, pipelines and railroad rights-of-way. This mandate required that double-circuit lines be constructed to replace portions of 12 existing transmission lines owned by four different utilities. So the design team had to simultaneously meet four different sets of design and material standards on the double-circuit lines. The existing lines were demolished and double-circuited with the new structures.
Steel poles were vital for narrower rights-of-way in both Minnesota and Wisconsin. Rights-of-way width was a key element in reducing the impact to the environment and agricultural land. It offered a smaller footprint and allowed designers to come up with custom designs ideal for multiple special-structure configurations required for entering and exiting double-circuit line sections. Drilled pier foundations provided flexibility to accommodate the different subsurface and geologic conditions along the route. The majority of the 1564 supporting structures were single-shaft steel poles with drilled pier foundations, which allowed construction crews to build lines along narrow rights-of-way, specifically 100 ft (30 m) in Minnesota and 120 ft (37 m) in Wisconsin. The average span length was about 750 ft (229 m). Most of the drilled pier foundations ranged in diameter from 6 ft to 8 ft (1.8 m to 2.4 m), although a few of the heavier angle and deadend structures required foundation diameters up to 12 ft (3.7 m).
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. email@example.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. firstname.lastname@example.org
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. email@example.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. firstname.lastname@example.org
|Northern 20 miles||Remainder of line (excluding Namekagon River Crossing)||Namekagon River Crossing|
|Voltage||345 kV||345 kV||345 kV|
|Conductor||1272 kcmil||954 kcmil||JP-KTACSR|
|45/7 ACSR-Bittern||54/7 ACSR-Cardinal||UGS-1360|
|Two per phase||Two per phase||One per phase|
|Diameter||2 × 1.345 inches||2 × 1.196 inches||2.114 inches|
|Weight||2 × 1.434 = 2.868 lbs/ft||2 × 1.229 = 2.458 lbs/ft||4.4 lbs/ft|
|Rated breaking strength||2 × 34,100 = 68,200 lbs||2 × 33,800 = 67,600 lbs||200,077 lbs|
|Max. design tension||2 × 24,000 = 48,000 lbs/phase||2 × 14,500 = 29,000 lbs/phase||64,200 lbs|
|At 2770 A||212°F||212°F||222.6°F|
|*All conductor ratings based on 90°F ambient; wind speed 4.4 ft/sec; 44 degrees latitude; emissivity 0.5; absorptivity 0.5, June 30 at 12 p.m.|