23 Oct 2013

The Pros and Cons of Materials for Overhead Transmission Line Structures

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When it comes to electrical overhead transmission structures, there are four main materials commonly used to construct them: wood, steel concrete and fibre reinforced polymers. Historically, wood, steel and concrete have been the most frequently used materials for transmission structures. Recently, advances in materials technology have provided the opportunity for companies to use high tech engineered materials such as fibre reinforced polymers (FRP). In this post we are going to run through the pros and cons of using each of these materials.

Wood

Wood is a versatile material for structural purposes and has been used for a very long time throughout human history. Wood continues to be a popular material for transmission construction since it’s upfront cost is relatively inexpensive when compared with metal or FRP, partly because the cost of wood production is quite low.

Once wood has been selected as a material for the transmission structure, it must be regularly inspected and treated to maintain reliability requirements. The timber must be treated because wood is susceptible to attack by natural enemies such as woodpeckers, termites, fire and rotting. So, to increase longevity, wooden utility structures are treated with preservatives, including chemicals such as Pentachlorophenol (PCP) and Creosote. It’s useful to note that PCP has been banned for any other application but utility wood structures because it is a biocidal agent, and there is considerable concern about adverse ecosystem effects in areas of PCP contamination. According to the United States Environmental Protection Agency, coal tar creosote is a probable human carcinogen based on both human and animal studies.

With the continued popularity of wood for transmission structures, due to several reasons, the availability of large quantities of good quality wood is becoming increasingly limited and it can be difficult to source adequate supplies. Due to issues finding enough high quality wood, some utilities find that newly installed wood structures are performing less effectively than the structures they replaced.

While wood still remains a choice for transmission line structures, some alternatives are needed.

Steel

Steel can be designed to the shape required, is lightweight and does not biodegrade. However, corrosion caused by aggressive environments, chemicals and pollution can significantly decrease its lifespan.

Steel transmission structures have good track records and some of them have been around for more than 100 years. That being said, at the beginning of the 20th century, metallurgical processes were different and produced more resilient steel than what become available later. Also, due to quality control issues, many older structures ended up with a much thicker galvanized zinc corrosion coating leading to much longer lifespans than originally expected. However, in the 1970s and 1980s, quality control improved and the galvanized coating became thinner, making it less corrosion-resistant. This means that transmission structures built during those times will not last as long as the earlier built lines. Because of this, regular corrosion inspection and and remedial work is required to maintain steel structures with extensive labour demand and associated costs.

During the refurbishment process of some transmission structures, many of the wood components are replaced with steel structural elements, such as cross arms and cross bracings. It’s important to point out that steel is electrically conductive and induction can also create currents. In live energised environments, electrical conductivity can be hazardous for the installing personnel making live, energised structural work more dangerous.

Implementing steel transmission structures requires some planning ahead; lead time procuring, manufacturing and delivering steel transmission line structures can be quite long, making post disaster recovery efforts and building new lines a slow process.

Although steel has great properties there is still room for improvement.

Concrete

Concrete is mainly used in above ground utility structures in the form of poles. If well made, concrete can be quite durable and resilient, but the lifespan of concrete transmission structures varies based on the quality of the concrete and environmental conditions. Concrete works very well for certain applications in transportation, building and pavement, but overhead transmission line structures are not an application where concrete is widely used for many reasons including logistics, energy and strength requirements.

When it comes to logistics, concrete is quite heavy and requires heavy machinery to install. Therefore, for overhead transmission structures in remote locations, concrete can be a very expensive structural option. Also, when concrete structures are damaged, they can be difficult to repair for these same reasons.

The production of concrete is very energy intensive. Creating cement requires high temperatures and grinding, which is a long process. Mixing and transportation is also quite energy intensive given the heavy weight of the components concrete is made of. Some structures that use autoclaving are even more energy intensive due to heating requirements. Overall, from a Green House Gas (GHG) emission point of view, concrete is not a good option.

Concrete itself has very low tensile strength and requires some reinforcement, which is usually steel. When moisture penetrates into the concrete, for example through cracks, and concrete cannot protect the rebar anymore, it leads to rebar corrosion. Corroded steel (rust) has a much higher volume than steel and creates an internal splitting force in the concrete leading to splitting and spalling of the concrete surface. This further exposes steel leading to rapid deterioration of the concrete structure. This mechanism is particularly dominant in salty environments, like from a nearby sea or road salting.

Concrete has its place in construction, but new high tech materials such as Fibre Reinforced Polymer Composites can provide better options for overhead transmission line structures.

Fibre Reinforced Polymer (FRP)

Fibre Reinforced Polymer transmission line structures are a more recent addition to the market with the lowest life cycle costs and longest expected lifespan of up to 80 years. The properties of FRP allow it to be designed to a variety of shapes and forms such as transmission poles, beams and cross braces. As the most lightweight option, it can be easily installed and transported to difficult-to-access areas, saving on installation costs with fewer crews and without the need for heavy machinery.

FRP utility structures are essentially maintenance-free and are quite resilient to large overloads. This comes in handy when large wind forces act on transmission lines. Even when they are damaged, they fail with very large deflection, holding up loads very well. An additional benefit of FRP is that it is electrically non-conductive making live line energized work much safer for utility personnel.

When it comes to costs, at the moment FRP is slightly more expensive than wood, upfront competitive with steel and less expensive than concrete. However, overall life cycle cost analysis comparing FRP to other materials shows significant cost savings for FRP users.

On the logistical side, lead time procuring, manufacturing and delivery of FRP structural components is faster less than wood, steel or concrete structures. This can make recovery efforts after a storm much faster and easier.

Environmental friendliness is also on the side of FRP. FRP requires less energy to manufacture, ship and install than steel or concrete and does not release any chemicals into the environment like wood or steel. This makes FRP a good choice for all environments, including those that are environmentally sensitive. As well, the disposal of FRP does not require special consideration—it could be placed in regular landfills or preferably reused or recycled. Recent technology developments are making FRP biodegradable, recyclable and reusable in controlled industrial processes.

FRP is relatively new technology, which makes potential customers a little hesitant to commit or try, but many utilities have been using this material for over 10 years with good experiences and outcomes. The major concern for utilities is UV protection which can be addressed by ensuring the FRP product has an integrated UV protection system

Overall, there are many benefits of using Fibre Reinforced Polymer composite transmission structures making it a good potential option for the private sector and electrical utilities.

If you would like to learn more about transmission structure options and how they apply to your situation, contact one of our representatives today to discuss: 778-285-8447.

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