Reported by Cliff Keys, Editorial Director & Editor-in-Chief, Power Systems Design
Reducing biomass trucking from power generation plant
This article describes a unique spacer cable system designed to power an industrial renewable energy plant pipeline.
When the L'Anse Warden Electric Company in Michigan's Upper Peninsula wanted to reduce road traffic associated with biomass trucked to its electric power generation station, it needed power for an above ground pipeline that would pump the material from an access point along a major highway to the plant. The distribution wiring solution required clearances considered safe enough to permit employees to conduct maintenance activities on the pipeline. L'Anse Warden found that a standard spacer cable system, coupled with underground residential distribution (URD) cable, gave the company the clearance they needed for worker safety while also providing the ability to use existing poles, thus saving significantly on overall costs. The system was supplied by Hendrix Aerial Cable & Systems, headquartered in Milford, NH.
Electric power generation station seeks power for pipeline
L'Anse Warden Electric Company, LLC, (LWEC) is a 20 megawatt biomass-fueled electric plant, located near Lake Superior in the Upper Peninsula region of Michigan. LWEC is a subsidiary of Traxys North America, LLC, which operates several electric power generation stations in the area and is a licensed alternative electric supplier (AES) to the state.
LWEC was converted from a coal, oil and natural gas-fired station in 2009, and burns forest products (woodchips, bark and fines), paper mill wood residues, railroad ties, and tire chips. This renewable energy feedstock was trucked from Highway 41 to downtown L'Anse, costing time and money as trucks wound their way through narrow local roads to the power plant. LWEC decided to install a facility adjacent to the highway where it could grind up the material and pump it directly into the plant.
To determine the best option for the overland pipeline and design the facility, Traxys enlisted the services of global consulting and engineering company Pöyry. The engineering firm designed a 20-inch diameter pipeline supported from H-frame structures approximately 30 feet above ground. The facility will utilize a dry blow line equipped with 700 horsepower pressure pumps to transport the material from the access drop off point to the power plant, a distance of 2700 feet.
Engineers considered a variety of alternatives for the distribution power needed to run the system. A key challenge was providing the 14 to 15 feet of clearance necessary to protect employees who might be working near the wire while maintaining the pipeline. The powerful blow line pump motor would limit the amount of material plugging the line, but periodic maintenance, cleaning, and servicing would still be necessary.
The route includes a 30-foot river crossing, so underground wiring was not deemed feasible. One alternative was using a 3-conductor metal clad (MC) cable on messenger wire. Another option was using a spacer cable system, manufactured by Hendrix Aerial Cable & Systems. Engineers from Pöyry's Appleton, WI office had experience with utilities that installed spacer cable to improve the reliability and power quality of primary distribution systems while making them more resistant to storm damage.
Spacer cable is a messenger supported three layer cable construction systems in a close triangular configuration. The high-strength Alumweld-Aluminum (AWA) messenger cable is the primary mechanical support member for the system and also can serve as both system neutral and lightning shield. Clipped to the messenger wire is a series of high density polyethylene spacers, which support, separate and clamp the phase conductors in a compact, triangular configuration. The phase conductor's high impulse strength dielectric covering allow reduced NESC phase spacing which provides a significantly reduced configuration when compared to standard, bare wire construction.
According to Ted Gardner, sales engineer for Hendrix Wire & Cable, LWEC's engineers originally decided to look at the spacer cable system because its required clearances were much lower, due to the dielectric covering on the spacer cable conductor.
However, as Gardner explains, "Spacer cable conductor has a small surface charge to it, so regulations require that we treat it like bare wire in terms of required clearances if the conductor is to be accessible by maintenance personnel."
LWEC's engineers then thought they would have to use aerial bundle cable (ABC), a relatively expensive cable in which all three (3) conductors are together in single, large cable housing. ABC cable is used primarily in areas where workers are very close to cable, for example in mining or tunnels.
"Instead, we suggested that they use our normal spacer cable construction, but couple it with underground residential distribution (URD) cable, which we also manufacture and distribute," said Gardner. "Normally, URD cable is used for in-ground direct-bury applications, but you can use it for overhead applications too. It has an outer covering that is grounded, so it is safer for technicians to be around. Using URD cable with standard spacer configuration is safer and significantly less expensive than the ABC option." LWEC was able to reduce costs by utilizing the existing 35 foot poles.
According to David Beard, senior electrical design specialist for Pöyry (Appleton) LLC, the decision was made to utilize the Hendrix spacer cable solution because it offered a fully engineered set of connectors and system for covered wire. "The price, available materials and methods for the Hendrix spacer cable solution was extremely attractive compared to engineering a system with a large 3-conductor MC on messenger wire," said Beard. He added, "By going with URD, we ended up with a system that was secure from the clearance point of view, and cost less than any other option considered."
Another benefit was that LWEC was also able to route the fiber optic communication line for process communications between the wood chip storage area and the power plant control planton the same pole. "Integrating the communication, power and blow line on one set of structures was a big plus for the project," said Beard.
Unique design posed installation challenges
Once the unique design was finalized, engineers worked with Hendrix to meet the installation challenges, which included significant elevation changes over the 2700-foot pipeline span, a river crossing, working underneath 69Kv transmission lines, and only a small budget for on-site engineering oversight.
Since LWEC did not have significant experience with distribution voltage, they contracted with the nearby Ontonagon County Cooperative Electric Association (REA) to install the required guy wires and termination of the cable next to the substation. Additionally, the REA had the proper equipment needed to pull the conductor the whole length of the circuit, which made a significant difference in the project cost savings. Hendrix also provided on-site project management support and expertise during the project installation. This support, along with that of the REA, allowed the installation of the spacer cable system to be completed in only four (4) days. Without that level of assistance, the project would have taken much longer and cost the client significantly more money and time. Soon after completion, the line was energized and fully tested by the electric contractor. The pipeline began operations in October 2011.
"Covered wire can be an excellent solution for large industrial or mining applications, because it significantly reduces clearances required to buildings and other structures," said Hendrix's Ted Gardner. He acknowledges that use of spacer cable structures with URD conductor is not something the firm usually promotes, but he is gratified that he was able to work closely with LWEC and its engineers to make it work. "It was a unique solution for a unique application, and it also turned out to be more cost effective than any other alternatives."