In Washington State's Chelan County, Glacier-Clad Mountains Make for a Scenic View. Since 1936, a public utility district has been serving the power needs of the county, which is located in central Washington. Today, Chelan County Public Utility District (PUD) operates the second-largest nonfederal, publicly owned hydroelectric generating system in the nation, including responsibility for two of the 11 dams on the U.S. portion of the Columbia River.
The Rocky Reach and Rock Island dams along with nearby Lake Chelan form a three-pronged hydroelectric generating system for Chelan County PUD, with a combined capacity of more than 2000 MW of clean, renewable energy. The annual generating capacity of the system is about 9 million MWh of power.
The region has become an attractive destination for families, retirees and vacationers. System demands caused by this growth prompted Chelan County PUD to increase reliability and capacity along its extensive network of transmission lines. In preparation for this, the PUD contracted with HDR Inc. (Omaha, Nebraska, U.S.) to provide design engineering, material and construction specifications, structure staking and right-of-way support for the first of three phases of system expansion, a 9-mile (14.5-km) extension of its transmission lines.
NOT AN EASY PATH
The new transmission line would carry electricity from the switchyard at Rocky Reach Dam, just north of Wenatchee, to the Andrew York/Monitor switchyard on Easy Street, about 6 miles (10 km) northwest of the city. The PUD selected this route after completing five years of analysis and study to determine the most cost-effective, environmentally friendly route for the new line.
Due to the remote location and lack of existing roads, new temporary access roads were designed to reduce the construction cost. A majority of the new transmission line falls on public land and the public agencies were extremely concerned with minimizing environmental impact. The agencies required all access roads be restored to their original state upon completion of construction.
The PUD completed a comprehensive study of different routing corridors between the Rocky Reach and Andrew York switchyards. A transmission corridor across Burch Mountain provided the optimum route as a function of environmental impact, operational and maintenance issues, and cost. Of primary importance to the PUD in determining the route was to minimize the visual impact of the new transmission line. The selected route takes advantage of the natural terrain to stay well hidden as it traverses Burch Mountain. The transmission line crosses mostly public property, including lands managed by the Washington State Department of Natural Resources, the Washington State Department of Fish & Wildlife and the Bureau of Land Management.
To gain the permitting from the agencies, the PUD had to adhere to strict guidelines. Construction activities had to occur during months when wildlife and related recreation would be least affected. All the temporary access roads had to be recontoured to their original state and planted with approved native vegetation. The transmission wires in certain areas had to have bird flight diverters installed to make the wires more visible to migrating birds.
The decision to install what ended up to be 8.5 miles (13.7 km) of new 230-kV transmission lines from the dam to the switchyard resulted in a challenging design project. The design team developed a solution that would allow the lines to pass over Burch Mountain, which rises nearly 4600 ft (1400 m) above sea level just west of Wenatchee.
Burch Mountain is a popular destination as local hikers looking for a challenge can traverse the Billy Goat Pass Trail, which gains 1800 ft (550 m) in elevation in just 3 miles (4.8 km). Providing beautiful views of the Cascade Mountains and the Columbia River, Burch Mountain presented significant challenges for the design engineers. The transmission line would have to rise and descend a total of 3000 ft (915 m) in elevation over its course.
CHALLENGING SPANS AND HEIGHTS
The first step was to survey and develop routing for the transmission lines. David C. Smith & Associates Inc. (Portland, Oregon, U.S.) provided the photogrammetric aerial survey including color digital orthophotography for a corridor based on the PUD's initial study. The design team prepared a preliminary design using PLS-CADD from Power Line Systems Inc. (Madison, Wisconsin, U.S.), and then went to the field to adjust the angle points to provide for the most cost-effective and easily constructible route. Once the final route was selected, the team used PLS-CADD to do the final design and placement of the structures.
The final design called for installation of 44 steel structures distributed along the path over the mountain. Weathered rust-colored steel was used to blend into the natural terrain. Valmont Industries provided the tubular steel for the poles, which were manufactured in the company's Tulsa, Oklahoma, U.S., plant.
Crossing the mountain required developing some unique designs to accommodate the structures. The height above ground varied from 70 ft (21 m) to a maximum of 140 ft (42 m), depending on the terrain. Creating supportable spans was perhaps the biggest challenge, with the final range from a short span of 153 ft (47 m) to a single span of 2665 ft (810 m) to cross the Rocky Reach Reservoir. In addition, several deep canyons had to be traversed, which required special designs for spans up to 2600 ft (792 m).
The engineers selected direct-embedded H-frame tubular steel poles for most of the transmission line structures. Direct-embedded structures were used because of the remote location; transporting and placing concrete would not have been cost effective. Of the 44 steel structures, five were in locations not accessible by roads, so the construction crew dug the holes by hand and the structures were set using a Chinook helicopter. The large lifting capacity of the Chinook was required due to structure section weights up to 21,000 lbs (9525 kg).
Conductor stringing was performed by using a Hughes MD500E helicopter that placed the sockline into the travelers on the structures. The tricky part of this operation was placing the center sockline on the H-frame structures. The helicopter pilot had to use a specially made hook apparatus that he could hook on one side of the structure, release the apparatus, fly to the other side of the structure and grab the apparatus again and place the sockline into the traveler. This method of stringing installation was selected because of the severe terrain and the benefits of minimum manpower required and reduced installation time. Wilson Construction (Canby, Oregon) was awarded the contract for construction of the 230-kV transmission line.
Outside both switchyards, self-supporting structures on concrete drilled piers were used to reduce the visual impacts as well as reduce the footprint of the structures. GeoEngineers (Spokane, Washington) provided the geotechnical data for foundation design and data for determining the embedment depth of the steel poles. Additional angle and deadend structures were direct-embedded guyed tubular steel poles.
CROSSING THE RIVER AND BEYOND
The transmission line began at Rocky Reach Dam, which required a 2665-ft (812-m) span over Rocky Reach Reservoir. This span required a special design for three 1272-kcmil 54/19 ACSR conductors, one 0.44-inch (11.11-mm) extra-high-strength steel static and one fiber-optic ground wire with 96 fibers.
The transmission conductor selected for the remainder of the line was 1272-kcmil 45/7 ACSR with one 0.375-inch (9.5-mm) extra-high-strength static and one fiber-optic ground wire with 96 fibers. The larger conductor over the reservoir was required to produce a balance of acceptable sag/tension as well as structure height and to maintain ground clearance over the reservoir, which is used extensively for recreational use. Using this conductor, the tension in the crossing span is 26,000 lbs (11,800 kg) and produces 165 ft (50 m) of sag under the heavy ice case for which it was designed.
The ground line elevation of the 3-pole deadend structure on the east bank is 34 ft (10 m) above the water line of the reservoir and rises 140 ft (43 m) above the ground. The 3-pole deadend on the west bank sits 272 ft (83 m) above the reservoir and rises 120 ft (37 m) above the ground.
Bundled insulator assemblies were used on the crossing, as well as two additional spans of 2373 ft and 2571 ft (724 m and 784 m), and designed such that a failure of one insulator would not cause a failure of the line. Polymer insulators were used for both suspension/deadend and post structures as the PUD uses polymer-type insulators almost exclusively throughout the rest of its system.
The overall transmission-line design included detailed design, material specifications, construction specifications, steel-pole specifications and right-of-way drawings. HDR design engineers also designed and prepared drawings for construction access roads. Construction of the new transmission line was completed in September 2006. The new line can carry enough power to serve about 125,000 homes and is one of the largest in the PUD system. The transmission project is the first of three phases of a US$15 million upgrade to Chelan County's system.
Randall B. Kono is an electrical engineer for Chelan County PUD with more than 11 years of utility experience in transmission and distribution. He holds a BSEE degree from the University of Alaska-Fairbanks and is a professional engineer.
Wayne K. Bauer is a senior engineer in HDR's Billings, Montana, office. He has more than 29 years of experience in power-system-related work with HDR, specializing in design and project management of transmission and distribution lines and distribution substations. He holds a BSEE degree from Montana State University in Bozeman and is a professional engineer.
Jason F. Brunner is a project engineer in HDR's Billings, Montana, office. He has more than seven years of experience in transmission and distribution design. He joined HDR in 2003 when SSR Engineers joined HDR. He holds a BSEE degree from Montana State University in Bozeman and is a professional engineer.