Overhead lines face severe environmental problems in the desert of the Gulf state of Oman, especially in the vicinity of coastal areas. Oman has desert areas bordering the Arabian Gulf and the Indian Ocean. Its wood poles with porcelain and glass insulators suffer from such environmental problems as vast temperature changes, sand abrasion, salt pollution (mainly calcium) from the limestone desert rock, an extremely dry atmosphere, high temperatures, coastal fog, overnight humidity changes (dew formation) and diesel (sulfur) fumes where the overhead lines are located in close proximity to desert tracks.
The problem with dry wood poles in Oman's high ambient temperatures is that fire often starts in the dead heartwood that surrounds the bolts supporting the crossarm. The surface tracking current comes from the overhead line conductors and across surface-polluted insulators. The presence of a properly bonded stay wire can divert the leakage current from going into the pole, but stays with poor bonding actually can lead to pole fires as surface tracking can start between the crossarm and stay wire.
Surface tracking reduces the overall impedance of the pole. This allows higher currents to flow in weather conditions such as high humidity or in other weather conditions that promote dew formation on insulators covered in salt and with damaged glaze. Several solutions are possible, but only two appeared to be realistic for Oman: low-polluting insulators and non-wood poles.
The Petroleum Development Oman Co. (PDO) spends more than US$2 million annually on washing polluted insulators in an attempt to minimize the effects of contamination, especially from marine salt fogs in the areas near Salahar. Pole fires often occur from May to September, after a series of extremely hot days, followed by overnight dew formation on the insulators. Dry-band arcing occurs as the insulators dry out at dawn, and this is increased significantly on polluted surfaces.
In these situations, loose connections at bolt positions on poles that have shrunk due to the heat can be a source of arcing and subsequent ignition of the long-dead dry heartwood. As the day progresses, the smoldering heartwood either extinguishes or continues to travel down the pole, at times burning the entire pole. The PDO's 33-kV distribution network supplies several thousand oil wells, so pole fires can lead to oil production ceasing.
RAY International LLC, an engineering consulting company for the oil and gas industry and distribution utilities, produced a new insulator specification for Omani desert conditions. RAY International identified ethylene vinyl acetate (EVA), a copolymer of ethylene material made by Tyco Electronics, as a material resistant to surface tracking in severe desert conditions. The satisfactory performance of EVA noted in Raychem's arresters and cable terminations in the Omani desert was the catalyst to the new insulator specification. The cable terminations were redesigned and 33-kV post insulators of EVA or high-quality silicone material were introduced. These insulators, supplied with a creepage distance of 40 mm/kV to 50 mm/kV (1.6 in/kV to 2.0 in/kV) are now standard components for all Omani projects.
The softwood Scots pine poles used in Oman exhibited several defects, including knots and poor creosote penetration, and even the new poles suffered deep splits due to rapid shrinkage in the desert heat. Electrical problems, namely pole surface tracking currents due to the extremely dry heartwood combined with reduced surface resistance because of salt pollution, make them very susceptible to pole fires. For these reasons, an alternative to wood poles was sought.
Given Oman's environment, it was preferable to opt for a man-made pole alternative — rather than a natural product — because a man-made pole's lifetime properties could be guaranteed. Hardwood, fiberglass, steel and concrete poles were considered as replacements for wood poles to support overhead line conductors. Hardwood was expensive and difficult to obtain. Fiberglass did not have the strength to deal with long spans and large conductors. Steel could become corroded and damaged from the perpetual sand abrasion. As a result, concrete poles offered the best alternative to wood poles.
Spun concrete poles, widely used in Germany and manufactured by Europoles GmbH & Co. KG, appeared to provide the best solution. These poles offered a range of positive qualities, from resistance to salt corrosion to long in-service lifetimes. Also, they could be designed to enable the use of 20% longer spans and used to replace wood H-pole structures with single concrete poles. The spun concrete poles provided a solution that would not only eliminate pole fires but also be cost-effective for 11-kV, 33-kV and 132-kV overhead line networks.
This solution also proved suitable for network supports in areas such as wadis (a dry riverbed that contains water only during times of heavy rain), where either long crossings or extremely strong supports are required, and wood pole or tower failures are relatively frequent.
Spun concrete poles offered the following advantages:
- Longer span lengths and single structures, reducing the overall cost by using less structures and pole installations
- Long maintenance-free lifetime
- Controlled quality and no variation caused by vagaries of nature as with wood poles
- Local production in Oman, thus reduced transport costs
- Flexible for line design
- Direct embedment of the pole in the ground with no risk of rotting
- No deterioration of strength during the lifetime
- Environmentally friendly with no leaking chemicals into the ground.
The initial trials — conducted by RAY International and Europoles in conjunction with the PDO, with concrete poles sourced from Germany — were successful. RAY International invested in a suitable pole-hole drilling machine, and further tests showed that plain augured holes drilled in the desert limestone rock provided the foundation required. However, the expense of sourcing poles from Europe restricted their wide-scale use. As the feasibility of using spun concrete poles became established, investigations were made into building a concrete pole manufacturing facility within Oman that could eventually serve the entire Middle East. An investigation was launched to determine whether locally sourced materials could be used.
Further design work was then conducted by Europoles on crossarms and the mounting of pole-top equipment. The successful conclusion to the investigations led to the formation of Europoles Middle East LLC, a joint venture of Europoles GmbH and RAY International. This was followed by the construction of a spun concrete pole manufacturing facility in Nizwa, Oman, funded with an investment of around $12 million. This plant has the capacity to manufacture 40,000 poles, from 5 m to 40 m (16 ft to 131 ft) in length, and produce and supply a range of spun concrete pole products for distribution and transmission networks and related services for the Gulf Cooperation Council region.
Europoles' spun concrete poles are made of high-strength concrete poured into steel molds and spun at 600 rpm around their longitudinal axis. This compresses the concrete with a force of around 20G. This produces an extremely dense surface, which has the highest possible resistance against external attack by water, chlorine, sulphate, seawater, ultraviolet radiation and carbonation. All international exposure classes are met. The compressive strength is up to international standard C100/115, making spun concrete one of the strongest concretes in the world.
To make the poles suitable for the Middle East conditions, a special concrete that has the highest resistance against environmental effects is used. It is more than 50% stronger than normal concrete, and the latest production and quality-control equipment aim to give the pole a lifetime of more than 50 years.
A reduction in the overall costs was achieved by using fewer concrete pole structures, thereby reducing crossarms, insulators and construction costs. Despite the fact the concrete pole is more expensive than the wood pole, the direct economic advantages outweigh the higher pole cost.
The long-term advantages, such as the reduction of maintenance costs and outage reduction, will provide additional long-term savings. For example, all intermediate poles were single wood structures, and yet a capital cost savings of 14% is achieved with the spun concrete poles. In many areas, intermediate poles comprise double wood pole structures, so replacing them with concrete poles offers a 30% savings. Further cost savings can be achieved with designs that eliminate crossarms. In the latter case, K-Line silicon rubber insulators are mounted directly off the pole and fitted with a full-range 6-mm to 34-mm (0.2-inch to 1.3-inch) diameter conductor clamp. Although this insulator design is more expensive, it is more than compensated by the elimination of crossarms altogether. In addition to being even cheaper, this design also avoids crossarm corrosion, which is of particular concern in the Sûr area of Oman.
The introduction of spun concrete poles has improved the reliability of overhead networks in Oman. The use of concrete poles has eliminated outages caused by pole fires, and concrete poles require no maintenance, have improved specification insulation and employ other corrosion-resistant materials. In addition, concrete poles are used to cross wadi riverbeds, which, in the past, have washed away wood pole structures following heavy rains. In general, PDO and the distribution utilities are reducing the line construction costs, have less maintenance and have a much stronger, more reliable network by using the locally produced concrete poles.
This combination of appropriate insulators, spun concrete poles and conductor choice has created solutions to provide a stable and reliable power supply in the desert areas and cities of Oman in environmental conditions that are amongst the most severe in the world.
Brian Wareing ([email protected]) holds a bachelor's degree and a Ph.D. degree in physics. He worked for 35 years with EA Technology, UK before establishing Brian Wareing Tech Ltd., an overhead line and lightning protection consultancy. Wareing currently delivers master's degree course modules in power engineering at Newcastle University, in addition to courses in overhead line design, wood pole overhead lines and lightning protection worldwide. He is currently the convenor of CIGRÉ Study Committee B2-AG-06, WG48 on the performance of new conductor types. Wareing is the author of the Institution of Engineering and Technology book Wood Pole Overhead Lines.
Assumptions for a Cost Price Comparison of Wood and Concrete Pole Lines
|Wood pole line||Europoles concrete pole line with armless construction|
|Two section poles||Two section poles|
|70-m span length (Panther conductor)||100-m span length (Panther conductor)|
|= 13 intermediate structures + 2 section structures||= 8 intermediate structures + 2 section poles|
|Section poles: double structures||Section poles: single structure|
|Section pole with four stays||Section poles without stays|
Cost Comparison of 33-kV Overhead Line Construction in Wood and Concrete
|Item||Wood pole line||Concrete pole line|
|Quantity||Unit price||Total price||Quantity||Unit price||Total price|
Petroleum Development Oman Co. www.pdo.co.om
RAY International www.rayoman.com