Not long ago, the Nordic country of Denmark became the first place in the world where the ubiquitous lattice transmission tower is no longer accepted for any power line construction. While relatively small and with a population of only some 5.6 million, what made this development noteworthy on the international stage is that Denmark is the frontrunner in a trend that is impacting a growing list of countries – public opposition to new overhead lines. Fortunately, resistance to added power infrastructure by affected communities across the globe seems to relate more to appearance than function. Rightly or wrongly, the lattice tower has become a symbol of the blemish that overhead lines sometimes impose on natural landscapes. That suggests that if power companies can find alternative, more aesthetic structures, there is reason to expect that the public will object less, if at all. Of course, leaving behind the steadfast lattice tower is something that would probably never be driven only by economics: time has demonstrated that these towers are cost effective and offer outstanding performance and service life. Indeed, that explains why they have remained in use, basically unchanged, for decades. In 2014, INMR visited one of the world’s most recent alternatives to lattice towers and met with the industrial designers who developed the concept as well as the transmission system operator who made use of it. Apart from valuable information on how to make new transmission lines more accepted by an increasingly wary public, this article also offered insight into the specification and application of insulators for these types of projects.
The origins of the new double circuit 400 kV Kassø-Tejle line that runs northward along the Jutland Peninsula lie in the longstanding goal of strengthening the interconnection between Germany and Denmark, and from there to Norway and Sweden. However, realizing this goal by means of another overhead line required overcoming strong objections by those who lived in this flat region of mostly small farms and historic towns.
Energinet, the Danish grid operator, first began planning the line back in March 2009 with an environmental impact assessment, as required by regional planning authorities. This document proposed that the initial route for the new line would be aligned as much as possible with an existing transmission line between the two points. The first public planning then took place in June that year and Energinet was asked to study different routes in terms of their expected effect on people, buildings as well as local wildlife and vegetation. Within 3 months, these alternatives were set out, each with a 400 meter wide planning zone.
A second pubic hearing for the project took place between March and May of 2010 during which the environmental impact assessment was published and the public was given a time frame to express their views as well as any proposals for adjustments. By the autumn of that year, the final environmental impact assessment and detailed route planning were both reviewed and within 6 months negotiations began in regard to compensating affected landowners. Construction of the line began around the start of 2013, with the project scheduled to be complete by November 2014.
The firm who developed the design of the stylistic structures used along this line – dubbed Eagle Pylons – is a Copenhagen-based firm of industrial designers whose background lay more in public works than energy. However, back in 2001, the founder, Erik Bystrup, was intrigued by a competition aimed at finding new tower designs for another sensitive transmission line in Jutland. Until then, like most of the public, he found himself driving along motorways looking out at what he refers to as “the vast amount of gear and steel elements that comprise modern power lines” and wondering why these were not yet being replaced by more aesthetic designs. Says Bystrup, “it seemed clear to me that these structures were not being designed by people who cared all that much about the sky. Then, in 2001, when we were awarded not only 1st but also 2nd and 3rd prizes in the competition in Jutland, it made me realize that applying our design ideas to the power sector might prove an attractive proposition.”
Bystrup talks about some of the initial concepts developed to make power structures better blend into their environments. For example, one of his earliest proposals, called the Sky Pylon, involved polished stainless steel monoliths that reflect light in every direction and therefore appear almost invisible. However, this design ran into concerns that motorists might be blinded by reflected headlights and therefore never actively pursued.
The Eagle Pylon selected for the Kassø-Tejle project was a subsequent design concept that Bystrup says was expressly developed to convey a “calm elegance” and help a major 400 kV transmission line blendinto a scenic rural landscape. He explains that the final tower selected was based on a hot-galvanized cylindrical steel shaft and rhombus shaped cross-arms and represents a departure from what he originally proposed – namely weathering steel – an alloy that among other ingredients contains a very small percentage of copper.
According to Bystrup, there is rapid formation of an outer ‘skin’ on such masts that quickly stabilizes against further oxidation, thereby giving the structure an indefinite lifespan. Cost-wise, he estimates the alternatives of hot galvanizing versus copper steel alloy without galvanization are similar. However, from a sustainability point of view, he suggests that the copper steel solution is preferable since zinc from the galvanized layer will leech down over the decades and contaminate the soil. Still, in this particular case Bystrup reports that local farmers did not want something on their land that looked rusty.
Another point of departure from the original concept was the use of two cross-arms instead of a single longer one to support all the conductors. The decision here was also made by resident landowners and based largely on how far away structures could be sited from existing homes. Says Bystrup, “going with two crossarms in place of one increased the amount of steel and cost of each structure by about a third but, in the end, the decision was not financial but based on satisfying local tastes. My preference would have been the single cross-arm which would also have allowed us to reduce the height of the mast.”
Bjarke Jensen, a transmission engineer who is now with the substations side at Energinet but who was closely involved in planning the Kassø-Tejle line, explains that the policy adopted in Denmark since 2007 is that all transmission lines less than 400 kV must be placed underground. Moreover, while overhead 400 kV lines are still permitted, these must now be based only on single pole structures. While he estimates from local experience that the cost premium of burying lines is some 3 to 4 times in Denmark, Bystrup and colleague Henrik Skouboe believe that this factor is probably closer to between 6 and 12 times, based on data from the National Grid in the U.K.
At the same time, Bystrup claims that transitioning from overhead to buried lines can often prove as much a challenge technically as economically. “We are now firmly committed to the business of designing aesthetic power structures,” he argues, “because experience tells us that replacing all overhead transmission lines by cable is just not yet there. Nor is it likely to appear anytime in the near future. The biggest problem is that it cannot be done effectively over long distances due to issues of induction and capacitance that require expensive reactors.” As example, he points to other power companies in Europe that have chosen to place even 90 kV lines above ground due to technical challenges linked with certain cable installations. He also emphasizes the high cost of dealing quickly with service interruptions whenever power lines are buried.
Notwithstanding Bystrup’s views on the comparative future of cables versus aesthetic overhead lines, the new Kassø-Tejle line has been placed underground at three environmentally sensitive points along its 166 km route. This same approach has been taken elsewhere in Denmark and also in the Netherlands, where the first 380 kV Wintrack line features a 10 km section of cable near the historic city of Delft (see INMR Q1, 2013).
Bystrup takes the interesting position that such transitions from overhead line to cable are actually more disruptive, both environmentally and aesthetically, versus simply continuing the overhead line without interruption. “Whenever you go underground,” he remarks, “you need a transition station filled with structures and equipment that can cover an area the size of a football field. The same occurs at the other end, while in-between is for example a beautiful town or valley. But from a purely aesthetic point of view, my opinion is that there would be much less environmental impact from simply letting the line run uninterrupted. In fact, if anything, such changes only add disorder and a messy overall appearance by breaking the continuity.”
This same reasoning also applies to deviations in a line around socalled ‘environmentally safe’ zones. Any such route deviations, claims Bystrup, cause a line to zigzag unnecessarily and destroy the alignment of structures that he says is as important as are the aesthetics of individual towers. “To create maximum calmness when adding a transmission line to an environment,” he stresses, “you need to organize all the structures, not just create beautiful structures.”
One of the interesting features of the new line has been the selection of only composite type insulators for all of its more than 500 structures. This decision represented quite a departure for Energinet, which until this point has relied almost exclusively on glass insulator strings. “We are a conservative utility,” says Jensen, “and have always used glass at 400 kV. However, while glass strings could have worked on suspension towers, the flying angle type of tower design could not succeed well with glass because it combines both tension and compression loading – something that is difficult to achieve with glass – especially for insulators angled at 45°.”
For example, flying angle structures for route adjustments of from 4° to 10° along the line employ a matching pair of silicone insulators with 120 mm core rods so as to handle all the tensile and compressive forces. Says Jensen, “this is actually nice thinking when it comes to aesthetic line design because the conductor is fixed in place with exactly the same configuration of insulators in every case, including on angle towers. Our typical past angle tower without such an insulator arrangement would have had to be moved out of alignment to compensate for the outswing of insulators which must align with the route. By contrast, these new towers can be sited in perfect alignment because the conductor position is held fixed by the V-string.”
Jensen points out, for example, that to achieve better appearance near a GIS switching station near the town of Vejen, he chose three small angle structures instead of one tension tower since such towers, with their jumpers, can prove “a little bit disturbing”. He also notes that while the tension insulator for each V configuration on flying angle towers could have had a smaller diameter rod due to less need for mechanical strength, Energinet wanted the two to be identical in order to achieve the best possible visual impact.
The arcing distance of 3.2 m and total creepage of 12 m selected for these insulators was chosen to be the same as normally specified for glass strings operating in an area of mostly low contamination. “We could have specified less specific creepage for insulators made with silicone,” notes Jensen, “but settled on specifying the same parameters we normally use for glass.”
Another issue when it came to insulator selection was reducing noise, which Jensen says also favored using composite types based on information he received from suppliers. Indeed, to keep audible noise levels along the route as low as possible, Energinet also chose a three-conductor bundle when a duplex arrangement would have been sufficient to handle the load. “Adding a third conductor represented an additional investment for us,” he notes, “but these types of choices were governed by making the line as acceptable as possible to local communities – not by finding ways to control costs.”
Construction of the new 400 kV line was assisted by the decision to use steel tube foundations, each equipped with a cement collar to protect the steel against water and passing farm machinery. According to Bystrup, this decision not only reduced typical foundation costs but also the time subsequently needed to erect each tower. For example, Jensen reports that such foundations can be hammered into the ground in only about 30 minutes. Then, following a standard procedure where all tower parts are laid out in advance in the field, the time needed to erect each structure averaged only one day per team.
The policy regarding prohibition of new lattice power structures adopted in Denmark in recent years has been a trigger for several related projects by Energinet, apart from the design of the aesthetic Kassø-Tejle line. For example, four 120 m high lattice towers that carried two 400 kV circuits from Jutland to the nearby island of Fyn since the 1970s have just been dismantled and replaced by a subsea cable. On the Jutland side of this new link, two new cable transition substations have recently been completed and are housed in a futuristic enclosure made of steel plate containing numerous holes. The cable terminations at these stations, like most normally used these days by Energinet at 400 kV, are silicone housed and oil-filled.
Bystrup, whose firm was not involved in the design of these cable station enclosures nor in a modernistic GIS switching station along the Kassø- Tejle line, acknowledges that these structures may have a high design content compared to typical structures of the past. At the same time, he wonders whether the aim in each case should be not to make them standout but instead be as hidden and unobtrusive as possible – a goal his team always strives for whenever creating aesthetic new towers.
Bystrup sees a number of lessons learned from the experience of this line that he feels apply to new such single pole lines going up anywhere in the world – including the award-winning 400 kV T-Pylon design his firm has developed for the grid operator in the U.K. One of these lessons is that the overall cost of each aesthetic tower is not nearly as prohibitive as some might imagine at the start. While Energinet’s Jensen estimates that each Eagle pylon came with a 30 to 40 percent cost premium versus the now-banned lattice steel tower, Bystrup argues that in fact it is only marginally higher if one looks at total installed cost. “Considering the large savings realized on foundations as well as the much faster erection times,” he claims, “this line’s cost would have been close to the same as traditional structures if the one storey design had been chosen.”
Another lesson is that the final design of aesthetic lines should not be dictated only by those who live nearby or who might own the land on which towers are sited. Says Bystrup, “we are committed to the idea that affected landowners should not be the only ones who decide how power lines will look since the landscape in fact belongs to us all.”
Bystrup’s firm is already working in co-operation with universities and manufacturers on next generation structures based on composite elements that will enable reduced tower heights while allowing conductors to be placed much closer together. He even foresees the day that these types of structures will not only look more beautiful and less obtrusive in any environment but may even help reduce classical transmission problems such as conductor galloping.