Colorado Springs Takes Transmission Underground
Last September, Colorado Springs Utilities (CSU), Colorado Springs, Colorado, U.S., completed the installation of a 13.5-mile (21.7-km), three-phase, 115-kV cable in underground vaults and conduit. To date, the maintenance-free system developed by NK Cables, a Nokia subsidiary, is the second longest, next to the solid-dielectric cable that runs 40 miles (64 km) in duct in Chicago, Illinois, U.S.
The CSU project resulted from a 1994 city mandate that all new high-voltage cables be buried. The four-year project cost US$15 million, and the utility saved US$1.25 million by tapping into the skills of an in-house nine-person installation team.
The project's success can be attributed in large part to an extensive amount of proactive coordination between CSU, Black & Veatch (BV), the city of Colorado Springs, other local utilities, the U.S. Corps of Engineers, the Colorado Department of Transportation and several area developers.
“Because the city had required all new transmission lines and those requiring rebuilding to be buried within city limits, CSU asked BV to perform a conceptual study for the new line,” stated an article in Electric Energy magazine by CSU's Steve Arndt and BV's Andy Rawlins. Approximately 90% of the preliminary route followed existing public right-of-ways (R/Ws). At the completion of the studies, CSU and BV had identified a preliminary route and selected the high-voltage extruded dielectric system.
The new line extends from CSU's Martin Drake Power Plant Substation in the city's central downtown to the new Rampart Substation, built as part of the project in northwest Colorado Springs, and then to the Cottonwood Substation in the northeast. The route generally follows city streets (Fig. 1). The majority of the run is in concrete-encased duct bank, although several special crossings (Fig. 2) were made with small directional drills, jack and bore casings, a bridge underhang and large horizontal drills (for two 9000-ft [2.7-km] crossings).
Quality Control
Early in the development, CSU Electric Department officials determined that the project would be cost-prohibitive if they contracted out all the work. However, CSU developed some fairly progressive alternatives.
“This was a significant project, and we wanted to be able to control quality and maintain high standards, while at the same time using the most cost-effective methods possible,” says Arndt, a senior transmission engineer for CSU.
Supervisors from the electric department handpicked nine employees to comprise the in-house project crew. Hiring two local contractors for the civil engineering construction saved money and retained quality control.
After careful surveying, CSU engineers mapped a route along which the independent contractors built duct banks. The contractors installed 49 7-ft-high (2-m-high) underground concrete vaults precast in two sections, each measuring 6 ft by 20 ft (2 m by 6 m) and weighing 30 tons (27.2 t) (Fig. 3). They then connected the vaults with four lines of 6-inch (15-cm) concrete-encased polyvinyl chloride (PVC) pipe. In addition, the system includes a fifth 4-inch (10-cm) conduit for future fiber-optic lines. Each of the two local contractors, Sun Construction and ICG Electric, handled about half of the 13.5-mile (27-km) route.
“It was the first time either contractor had done a project like this, and it was very successful,” says Dave Coil, CSU's project construction manager.
Coil attributes that success to the fact that CSU supervised, inspected and approved the civil work with staff inspectors.
“Whatever hours their crews worked, we had an inspector there — and it shows,” Coil says. “We made sure it was all proof tested before we accepted it from the contractors, and there were only a few places it had to be redone. The end result is 13.5 miles (27 km) of a duct system that's problem free.”
Challenging Construction
CSU encountered nearly every imaginable obstacle along the route, which made the vault-construction stage extremely challenging.
Though the bulk of the duct system runs under the streets, CSU needed to obtain R/Ws in some areas. The utility acquired 17 easements, five of which it obtained at no cost through negotiation with developers.
Another challenge was that the 213,840 ft (65 km) of transmission line extends beneath two creeks (one of which is habitat to the endangered Preble's Meadow Jumping Mouse). The line also runs underneath Colorado's main north-south interstate, a busy railroad line and a major inner-city corridor. CSU opted to bore underneath the creeks, interstate and major roadways instead of excavating (Fig. 4). The Laney Co. of Texas performed the directional boring with computer-controlled drills, using drill heads weighing as much as 6000 lb (2721 kg).
“We looked at excavating, but we'd have had to replace a lot of trees, disrupt traffic and disturb the Preble's Mouse habitat, so we went below Monument Creek and below Academy Boulevard (a major thoroughfare),” Coil explains. “Deep directional drilling also meant we didn't have to get a U.S. Corps of Engineers permit.”
Crews installed the Academy Boulevard/Woodman Road intersection crossing 30 ft (9 m) below the road surface to avoid a proposed underpass at the same location.
They started the crossing with a 16-inch (41-cm) bit, increasing the bit size in stages up to 48 inches (122 cm) (Fig. 5). The casing was high-density polyethylene (HDPE), 36 inches (91 cm) in diameter, with a 3.5-inch-thick (8.9-cm-thick) wall (Fig. 6). Crews welded the casing sections together into the full 900-ft (274-m) length and then pulled them into the hole along with the inner four 6-inch (15-cm) conduits and one 4-inch (10-cm) conduit. The pulling tension was about 700,000 lb (317,514 kg) (Fig. 7). Crews then installed a bentonite slurry inside the casings to help dissipate heat from the cables.
Active railroad tracks also entered into the equation, so Coil worked with the Union Pacific Railroad to pull up the tracks for two days so that contractors could lay conduit.
“By making arrangements beforehand with the railroad inspector, we saved a lot of money by not having to bore beneath two railroad tracks,” Coil says.
Flexibility became an important part of installing the system. Crews worked with the city of Colorado Springs to suspend conduit beneath the Colorado Avenue bridge during its construction. CSU had to coordinate the cable installation parallel to the interstate with the Colorado Department of Transportation, which had combined the installation of a CSU water pipeline project with its interstate expansion project. To avoid traffic disruptions and save more money, crews excavated and trenched through busy intersections at night.
Other city and utilities departments offered assistance as well. City traffic engineers handled street logistics during the civil construction phase. Water Resources provided truck drivers to pull cable. Staff from a power plant let crews set up a site to store cable reels and machinery.
“The project was so complex it required a lot of coordination, but we had tremendous cooperation, which made it easier,” Coil says.
High-Quality Cable Requirements
As the system took shape, two CSU in-house crews began pulling three lines of cable through the conduit.
“There's a lot more involved in pulling this cable than regular cable,” says Jeff Wilson, a CSU line crew supervisor. “This is an advanced product that has to be pulled one line at a time. The high voltage and complexity of the system makes it critical that we be perfect.”
Bruce McCormick, manager for electric transmission engineering, says a US$100,000 cable-pulling trailer, designed specifically to meet the needs of this project, is one reason the pulling went so smoothly. CSU fabricated a special trailer bull wheel in its metal shop with NK Cables' specification.
The hydraulically operated trailer minimized the pulling tension by spinning the 20,000-lb (9072-kg) reels as crews pulled the cable through the conduit. With a maximum pulling tension of 5500 lb (2495 kg), damage to the conductor became a major concern. CSU crews were able to pull three runs per day. On one 10-hr day, they pulled six runs.
“The part that weighed heaviest on my mind was measuring from vault to vault so the cable could be cut to match the premeasured distances. We had to make sure we were correct because we'd waste a lot of money if we miscalculated,” Coil says. Moreover, a crew wouldn't know that a line was too short until it had been pulled the entire length of a conduit between vaults.
The cable has a minimum temperature restriction of 23°F (-2.22°C). So during the winter, crews prewarmed cable reels by storing them inside a power plant. This permitted them to pull cable all winter. NK Cables offered a per-foot discount because of the volume of cable CSU ordered.
Overseas Training
After four years of laying the system's groundwork, CSU reached the stage of splicing and hand welding the conductors in the underground vaults, after which the utility tested and completed the system. Unfortunately, local contractors were not familiar with the unique splicing and welding technique the cable required, so the in-house CSU crew traveled to NK Cables' headquarters in Delft, Netherlands, for two weeks of training.
The crew learned the company's custom-designed and patented connection methods of splicing and welding the cable, which is 1750-kcmil (887mm
“The training was intensive and at first difficult to learn,” says Rick Fluegel, CSU line crew supervisor. He cites the lead wiping as the most challenging. “It requires some artistic skill to take the lead and mold it from the lead sheath to the tinned-copper sleeve that goes over the splice. But the splices are more user friendly, and there's less taping than with other cable we've used.”
The splices were NK Click Fit, a brand that saves time by using prepared end plugs welded to the cable ends. To speed the splicing process, each reel of cable was shipped with one special end and one normal factory-sealed end. During splicing, the two prepared ends (one from each cable) are pulled into the splice body. The splice body has spring-loaded bayonets that lock the cable into place using the special ends, eliminating the possibility of the cable separating. After this, a lead shroud is connected across the lead sheaths and shrink-wrap material is installed over the metal shroud. The lead sheaths of the cable are connected to link boxes, which are connected to ground rods in each vault.
In the adjacent splicing vaults, the two cable ends that do not contain the special end plug are carefully measured, and the cable is cut to the exact length required for the splice. After cuttings, the end plugs are installed, the insulation is again sanded and the cable ends are inserted into the splice body. A set of three splices containing the factory-prepared ends required about 35 hr to complete. Splices made without the pre-installed special ends require about 50 hr.
Coil estimates that CSU saved a minimum of US$250,000 by using the in-house crew for both the cable pulling and connection termination work.
Although developed a decade ago and widely used in Europe, this cable technology is just being introduced to American utilities. Soon, they too will begin to realize the many savings and benefits this technology offers.
Note: This article contains information from Black & Veatch that was added by the editors.
Debbie Warhola is a free-lance writer based in Woodland Park, Colorado. She has written several articles about CSU over the past 10 years. She has the BA degree in English and communications from Colorado University, Colorado Springs.
George Dushan is a senior communications specialist with CSU, which he joined in 1978. He has the BS degree in mass communications from the University of Southern Colorado. He manages media relations for CSU and is a member of the Public Relations Society of America.
Major Utilities Study the Colorado Springs Utility Project
Darrell Sabatka, principal design engineer with New Century Energies in Denver, Colorado, U.S., said it was useful to study the required accuracy, scope and complexity of the CSU project.
“We have three to four underground projects going on ourselves, but nothing of this significance,” Sabatka says. “It was an opportunity for us to trade experience and design information. And it was pretty interesting to observe the routing, installation methodology, construction management and directional bores.”
In addition, New Century is benefiting from the project. The utility was able to piggyback a small order of cable in with the larger CSU order, saving about US$5 a foot. Sabatka says New Century also signed a 50% ownership agreement to share a spare reel of cable with CSU in case of emergency repairs.
“That'll be another significant savings for us,” he says.
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