Solar Farm Underground Wiring is Long-lasting With High-density Polyethylene Conduit

South Burlington Solar Farm, the largest solar farm of its kind in Vermont, placed power and control cables in underground conduit made from high-density polyethylene (HDPE) to increase the life of the facility.

Oct 1st, 2012
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South Burlington Solar Farm, the largest solar farm of its kind in Vermont, placed power and control cables in underground conduit made from high-density polyethylene (HDPE) to increase the life of the facility. The 2.2 megawatt (MW) solar farm came online in July 2011, covering 25 acres and including 382 solar trackers. Cables convey power from the solar trackers to the electric grid. AllEarth Renewables Inc. of Williston, Vt. designed the solar farm and manufactured the AllSun Trackers Series 24 solar trackers.

"From a sustainability standpoint, HDPE pipe and solar power are a unique partnership," said Tony Radoszewski, executive director of the Plastics Pipe Institute Inc. (PPI), the major trade association representing all segments of the plastic pipe industry.

"Like solar, a plastic pipe system is a sustainable and environmentally responsible choice that will serve generations to come. The HDPE conduit is ecologically efficient during manufacturing. The conduit is also strong, durable, lightweight and flexible while providing excellent protection for the power and fiber cables inside, and requires significantly less energy to fabricate, transport and install than other pipe products. And, here in Vermont, HDPE conduit is the perfect choice to stand up to the extreme cold weather and frozen earth, plus it was the key to a successful trenchless run in one section," Radoszewski said.

Installation of the HDPE conduit was done using cut and cover, and horizontal directional drilling (HDD) under a forested area. All low voltage wiring used Underwriters Laboratories (UL) 1990 certified Cable In Conduit (CIC) provided by Blue Diamond Industries LLC, a member company of the PPI.

The AllSun Tracker conduit included some 70,000 feet of 3/4-inch CIC pipe with three 6AWG cables-black, green and white-and 5,000 feet of 2.5-inch diameter CIC with 4/0 wiring for the panel feeder lines. Blue Diamond supplied CIC on reels ranging from 2,000 to 10,000 feet. The company is UL listed for specification UL 1990, Nonmetallic Underground Conduit with Conductors, and its CIC is manufactured under American Society for Testing and Materials (ASTM) Draft 3485.

"Most solar farms use underground polyvinyl chloride (PVC) pipe," said Doug MacDonald, engineering technician for AllEarth Renewables. "Wire is pulled onsite after conduit installation. HDPE conduit saves in multiple ways that includes less labor to install. For this project, the HDPE conduit came preloaded with the wire, so it was really just a one step process after digging the trench. It also enabled us to use a smaller conduit size, reducing the cost of materials. In addition, less handling meant more savings."

"Typically when other types of conduit are used, it can turn into a very labor-intensive project," Radoszewski said. "This is because a trench must be dug, the conduit installed, and then a pull rope is blown through the conduit so each wire can be pulled back through. Because the HDPE conduit can have the wire installed when the pipe is being extruded, it comes to the job site ready to go. This saves considerable time.

"Custom lengths can be ordered, and sometimes a single reel will hold thousands of feet, which puts more conduit at the site to further expedite installation.

"Installing the cable in the conduit at the factory ensures a clean and damage-free cable is delivered to the field. CIC also eliminates the added expense of pulling the cable onsite under muddy and dusty field conditions, which aren't the best for a good, clean installation," Radoszewski said.

The AllSun Trackers are pole-mounted, dual-axis trackers that follow the sun throughout the day, guided with a global positioning system (GPS). Each of the 382 units move from east to west and up and down, tracking the sun to maximize efficiency as it transforms solar energy into electrical energy. Total power produced by the South Burlington Solar Farm is estimated to be some 3,000,000 kilowatt hours (kWh) a year-45 percent more than the electricity that could be produced by a fixed roof-top photovoltaic system of the same size. The project is owned and operated by Chittenden County Solar Partners LLC. The value of the electricity generated by the farm has been $924,000 a year and is enough to power more than 400 homes.

While most of the installation was done using cut and cover, the HDPE conduit also provided the solution to go under a heavily forested section of private property and a wetland section.

"We directionally drilled two, 400-foot runs of 5-inch diameter HDPE pipe used for the main power feed and communication cables," said Aaryn French, project manager for Engineers Construction Inc.

French agreed having CIC was a benefit. It took his four-person crew one month to install the low and medium voltage conduit at an average depth of 3 feet.

"The trenching, which was the majority of the work, went smoothly as well," French said. "It was nice because we'd start pulling the wire out and leave the proper length at the end and lay it in the ground. This was more efficient than using sticks of PVC-probably five to 10 times faster.

"And, because we already had the cable in the conduit, it saved us a lot of time. We were killing two birds with one stone. Installing the pipe and wire at the same time-that was a huge benefit.

"This is probably one of the biggest applications of using CIC in this specific manner. In this application it was cost efficient, time efficient and it worked well," French said.

"We're seeing more power and fiber optic cables being placed underground," Radoszewski said. "This is especially true of new housing and building construction. CIC is also popular for installing Federal Aviation Administration (FAA) lighting for airport applications. In addition, there is a strong move for communities to bury overhead lines to eliminate wire blight and provide safer, more secure power and telecom systems, which also are protected from severe weather."

About PPI: The Plastics Pipe Institute Inc. (PPI) is the major trade association representing all segments of the plastic pipe industry and is dedicated to promoting plastics as the material of choice for pipe applications. PPI is a technical, engineering and industry knowledge resource-publishing data for use in development and design of plastic pipe systems. In addition, PPI collaborates with industry organizations that set standards for manufacturing practices and installation method.

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