Transmission towers and distribution poles have become such a part of the landscape they are barely noticed anymore, but that does not mean wood, steel and concrete structure technologies are a dead end in today's high-tech world. Technology has been quietly at work changing the transmission and distribution structures every bit as much as it has the rest of the grid. Like the rest of the smart grid, this is a world of powerful computers, sophisticated software programs and cutting-edge materials science.
Without using the overworked “smart” label on poles and towers, consider how contemporary technology is being applied. Advancements in insulators and conductors have long been reported as radical, revolutionary and ultramodern, but what about the supporting structures?
The technology of overhead power line structures is a fascinating subject and one that is still developing. It is being shaped by the same forces that are driving the rest of the 21st century grid. The trend of bigger, better and more complex — while reducing costs, labor and maintenance — is alive and well with structures, too.
In the early days, the transmission engineer had three choices: wood, steel or concrete. Over the years, however, the designer's bag of tricks has grown to include some pretty complex engineered materials. Fabricators are providing custom designs blending both old and new materials to solve structural problems. McWane Inc., which has moved from pipes to poles, uses the time-tested technology of cast iron. With it, McWane produces modern, lightweight, corrosion-resistant utility poles and substation structures (T&D World, February 2011) that are gaining a lot of interest.
Valmont has taken an interesting approach, as well. It has developed the MusclePole, a small-diameter steel pole filled with concrete, which makes it stronger. It also has a smaller footprint for a tight-fit solution. There appears to be no limit to the resourcefulness of the manufacturers.
Wood is Still on Top
The transmission and distribution industry's love affair with wood began almost 170 years, which is a long time for a technology to remain in favor, but it has. That is a testimony to the basic soundness of the material.
Consider in 1844, Samuel Morse wanted to put his newfangled telegraph wires underground, but the technology was not there yet. After 7 miles (11.3 km) of trouble, he gave up and went overhead. There were trees, lots of trees, so he supported the conductors on wood poles; end of problems.
Today, wood is still the most common material for transmission and distribution structures worldwide. Some estimates place the number of wooden utility poles at around 150 million in the United States alone, with more being added every day. They have intrinsic values. They are from a renewable source, which is a big plus in today's sustainable green world. They are resistant to oxidation, corrosion and, to some extent, fatigue, but there are problems.
Wooden utility poles originally came from old-growth timber. The trees were abundant, dense, straight and resilient hardwoods. Old-growth timber was so much in demand it became scarce and pretty much unavailable. That opened the door to some technological forest tinkering.
The solution was fairly simple. If the old-growth timber was disappearing, then new-growth trees were the answer, and rapid-growth technology was developed. They were still wood, of course, and they were still cheaper than the alternatives, but they were not the quality of those old timbers.
By rushing the growth, the trees were less robust than the old-growth trees. The poles made from them were less durable, had reduced stiffness, had decreased longevity and were more susceptible to insect infestation, fungi, wood rot and woodpeckers. Once again, technology was used to address these shortcomings.
Enhancements, such as preservatives and pesticides, were developed for the timbers. The most popular enhancement method was to pressure treat poles with chemicals such as pentachlorophenol (penta), creosote, and chromated copper arsenate (CCA).
Unfortunately, many of these chemicals have been discovered to have unwanted side effects. It now appears there are carcinogenic characteristics associated with the penta and creosote processes. In addition, chromium is a heavy metal and arsenic is a poison; this has led to handling and disposal problems at the end of a pole's life. Governments have stepped up environmental regulations and restrictions.
Many jurisdictions now consider treated wood poles a hazardous waste. Disposal has to be in specially permitted landfills, with many jurisdictions restricting creosote-treated products. The wood preservation and timber industries are researching better ways of dealing with treated timber waste, and it is a good bet technology will once again provide an answer.
Another old technology that has been blended with wood is the science of lamination. Genghis Khan's warriors carried composite bows of wood, bone and sinew, improving the wooden bow's characteristics. Like that composite bow, the composite wood pole does the same thing. It takes strips of wood and bonds them together with synthetic resins to form polygonal shapes.
Composite poles can be engineered to provide the exact characteristics needed by the utility. They are manufactured from various wood sources such as recycled poles, low-grade trees, small-diameter trees, short logs and crooked logs, proving the axiom that the whole is better than its parts.
Laminated Wood Systems Inc. has been supplying laminated wood structures for more than 20 years, but electric utilities have been using laminated wood poles since 1963. Laminated Wood Systems reports it can supply direct-embedded laminated poles, from 25 ft to 150 ft (8 m to 46 m) in length, for just about any application including substation structures. These structures have design life of more than 60 years, which exceeds a typical wood pole's life expectancy of 30 to 40 years.
Concrete is another engineered material that makes great poles. Amazingly, concrete poles had their origin in the telegraph era, too. The earliest references date back to 1856 in Germany, where they were used for carrying telegraph wires. Even more amazing is that spun-cast concrete poles were first produced in 1907, also in Germany. Utilities in the United States did not begin using concrete poles until the 1930s, but they were available. Prestressed concrete poles were developed in the mid-1930s in Algeria, but it took until the mid-1950s to blend technologies to produce spun-cast prestressed concrete poles.
Change is Certain
Engineers, being engineers, always want to change things. It is in their nature as much as breathing. They are never satisfied to leave anything alone, because they know it can always be tweaked and made a little better.
Back in the 19th century, there were probably a couple of engineers taking a coffee break when one of them decided wood was good, but what about steel? So someone pulled out the Sears & Roebuck catalog and found windmills. A little modification to the lattice steel bases, and they had a four-legged lattice steel transmission structure. It proved to be a sound application, and many utilities used this simple approach.
Thankfully, today's engineers have more than a Sears catalog as a structure resource. They have access to a large selection of shapes and designs from a wide choice of fabricators like Dis-Tran, FWT Inc., Sabre Tubular Structures, Thomas & Betts, TransAmerican Power Products and Valmont to name a few. It is important to keep in mind that wood and steel structures are still the most common materials in use for overhead line structures throughout the world. The structures can be very simple or very complex, depending on the needs of the project.
Electric Vehicles Are Not the Only Hybrids
Sometimes there is a flash of inspiration and two good designs can be combined into a great design. Since there were steel poles and concrete poles, it was only a matter of time until they were combined. The folks at Valmont combined the two materials into one hybrid pole. This hybrid has a steel pole on top with a concrete pole base section. It gives the structure the strength of steel in the air and the impermeability of concrete in the ground.
Laminated Wood Systems developed a system that turns an existing wood pole and steel inserts into a sort of hybrid, called the PhaseRaiser lifting system. If a utility has a transmission line ground-clearance problem, it can use Laminated Wood Systems' system to raise, reinforce and reclassify existing wood structures through 345 kV. This is done using some specialized tools while the line remains energized. The linemen cut the wood pole, raise it hydraulically and then secure it permanently.
This process has been in use for many years, but it is getting some serious attention right now with the infamous North American Electric Reliability Corp. alert of Oct. 7, 2010 (T&D World, August 2011). Utilities have begun to verify 450,000 miles (724,205 km) of high-voltage (more than 100-kV) transmission lines and correct any clearance issues found. Many utilities have used light detection and ranging (LiDAR) to survey their lines already, and the results have been very promising.
So far, the clearance issues appear to be only a small percentage of the spans. However, considering there are 450,000 miles of lines and assuming only 5% to 10% of the spans have clearance issues, the remediation effort is going to be huge. Any technology that can expedite correcting the problem without pole replacement or outages is going to be popular.
Steel Steals the Attention
With voltage levels ranging from 15 kV to 765 kV, the steel designer needs a variety of tools to bring structures from the computer screen to the right-of-way. Structures include everything from single-shaft to dual-pole H-, K-, X- and Y-frame structures, to tri-pole (one-pole-per-phase) configurations.
As important as all the new materials and designs have been, Bill Sales, executive vice president of sales for FWT, reminds us, “With the advent of computer-controlled fabrication equipment, improved manufacturing processes and technology advancing the precision of tools, fabricators like FWT are able to drastically reduce their lead times in comparison to the fabricators of the past. Not only is the steel processing faster, but it is more exacting and efficient with a higher-quality finished product.”
Fabricators have developed their own proprietary software to optimize new designs supplying the most cost-effective structures possible. It also allows them to optimize structures, making them stronger, better able to withstand storms and possibly use less steel. In many cases, these designs are so much stronger than older designs that the number of structures used in the line can be reduced.
Jim Palmer, director of marketing for Thomas & Betts' steel structures division, points out, “What was true 20 years ago for structure design isn't true today. Tubular structures are surpassing lattice designs. Computer programs allow our engineers to analyze the designs in hours rather than the weeks it took back then. But equally important is being able to do full-scale testing on those designs. Thomas & Betts has the only full-scale vertical testing facility in North America. With so much riding on these designs, it just makes sense to test it and know it works.”
Other Factors Require Advancements
Utilities are under pressure to save money at the same time they are required to meet increased power demands. One way they have found is to increase the voltage levels for the transmission system. One 765-kV transmission line can carry roughly the same amount of electricity as five 500-kV transmission lines. Higher-voltage levels require conductors to go up higher to maintain those ever-present ground clearances.
As the industry pushes the limits, it becomes very important to take advantage of all technology has to offer. Lattice steel structures are the workhorse in the ultrahigh-voltage world. The structures are huge, but now there are 1,000-kV systems in service in China and 1,200-kV systems are in the planning stages. With each increase in voltage level, the challenges for the manufacturer and the transmission engineer increase, too.
The application of advanced technology has provided the industry with the next generation of wood and steel structures for transmission and distribution systems. Even though they have been in use for more than 100 years, they are still popular and relevant. As the choices expand, so must the industry's understanding to take full advantage of them.