Eskom places 66-kV and 132-kV overhead line circuits where acquiring rights-of-way is problematic.
The driving force behind Eskom's introduction and use of single-pole structures has been the need for structures with smaller footprints. Eskom experienced difficulty in obtaining rights-of-way (R/Ws) for new circuits and upgrading existing circuits in urban areas.
Until recently, the supporting towers used for 132-kV circuits in Eskom's western region were based on two designs: five-pole wood structures and steel lattice structures. Similarly, supporting towers used for 66-kV circuits were based on three designs: the wooden H-pole, five-pole structures and steel lattice towers. The area, or footprint, required to accommodate these structures is large compared with typical 11-kV and 22-kV single wood pole structures. As a result, Eskom developed, tested and used new 66-kV and 132-kV overhead line specifications that overcame R/W problems and offered technical and economic advantages.
Past Practice South Africa does not have indigenous trees that can be used as overhead power-line poles. The two locally grown alien species used for power-line structures are Pinus Radiata and Eucalyptus Saligna, and the biggest poles grown from these species are typically 55 ft (17 m) long with a top diameter of about 8 inches (20 cm). Generally, these poles are used for 22-kV circuits because, on average, they require a single 35-ft (11-m) pole with a top diameter of 7 inches (18 cm).
Until about five years ago, the use of steel poles was limited to streetlights and was primarily used by the then-new cellular telephone industry. The only major monopole transmission line erected in South Africa at that stage used imported monopole steel structures. At this time, Eskom used three alternative structure types for 66 kV (the wooden H-pole, five-pole structures and steel lattice towers) and two for 132 kV (the wooden five-pole and steel lattice). All these structures have large footprints, compared with typical 11-kV and 22-kV single wood pole structures. The multipole wood pole structure has a long transverse footprint, which measures 12 ft by 2 ft (4 m by 1 m) for 66 kV, and 21 ft by 2 ft (6 m by 1 m) for 132 kV. The lattice steel tower has a square footprint from 6 ft by 6 ft (2 m by 2 m) for 66 kV to 25 ft by 25 ft (8 m by 8 m) for 132 kV. To meet varying requirements in the different Eskom regions, the utility made several parallel developments that have produced new designs based on single supporting structures of both wood and steel.
Steel Monopoles: External Designs The first opportunity for steel monopole design was on the rebuild of a 66-kV line near Stellenbosch. Originally, this line had been built with single-circuit Truscon structures (originally designed by the Truscon Steel Co., Youngstown, Ohio, U.S., in the 1930s), as shown in Fig. 1. Reuse of the foundations retained the existing R/W, which was important because this 40-year-old circuit traveled through established vineyards, and renegotiation of the R/W was not a realistic option. Eskom specified the new structures and line conductors to be within the loading capability of the existing foundations. Horizontal-post silicone-rubber insulators made the design feasible. The manufacturer designed the steel poles, which had swagged joints, to meet the loading criteria, and introduced the keyhole plate. This plate enabled workers to attach the insulators to the pole via two keyholes, which were in a channel welded to the pole. Overall, the functional shapes of the intermediate and tension structures were similar, except that the latter were double-steel-pole stayed structures.
The second project, a new 66-kV line between an existing and new substation, was the first application to use bolted foundation self-supporting intermediates and tension towers. The monopole manufacturer produced this design, which was based on the monopole structures used for cell phone systems or high mast lighting. The design's octagonal foundations and tension structures with comparatively low taper earned them the nickname "water towers."
Steel Monopoles: In-House Designs Eskom, together with a structural engineer, developed new software for the design of the shaft. This design later expanded to include base-plate and holding-down bolt designs. The developers based the software design on the ASCE publication, Design of Steel Transmission Pole Structures, report No. 72. The first structure built using the new design software was tested to failure at Eskom's tower test station. The company used the data from this test to refine the software. Next, it redesigned and partially rebuilt the structure, which again tested to failure, meeting the design criteria.
A 66-kV line near the Cape Town Airport was the first double-circuit line (ash conductors) designed with the revised software package. Its design used lattice angle-iron crossarms. The software calculated the cost for each structure, using the total weight and the unit steel cost. It found the total structure cost to be directly proportional to the wall thickness of the structure. In order to optimize wall thickness to give the required structure strength at the lowest cost, Eskom used a wider base for the tapered towers, particularly for large angle positions. To correct a defect, the contractor took down all 11 of the intermediate structures and reinstalled them within two days, demonstrating the ease of installation the monopole structures provide.
Further development of the design and its use increased the number of different circuit applications, as illustrated by the following projects:
* Crews built a 66-kV double-circuit Upas line specifically for a steel rolling mill. They built these double-circuit structures with tubular tapered crossarms and an access ladder. The structures for this power line had an optical fiber ground wire (OPGW). The structures' overall heights ranged from 60 ft to 98 ft (18.2 m to 29.8 m).
* A project replaced a section of 132-kV underground cable feeding the docks at Cape Town with an overhead 132-kV line, introducing another application of steel monopoles. The line was routed across national roads, suburban roads, two railway lines and a bridge ramp. All the foundations were piled, the largest foundation being 6 ft by 6 ft (2 m by 2 m), which in one or two tower positions was the only space available. Manufacturers increased the thickness of the steel for some of the structures to make them as slim as possible. Crews then used lattice structures at the start of the line to blend with existing installations in the area.
* Workers constructed a 4-mile (7-km) 66-kV line crossing the hills to the east of Stellenbosch on steel monopoles (bolted foundations for strains and planted foundations for the intermediates) from the substation to the tee-off position for a future line. A single wood pole 66-kV line continued the circuit over the mountains to the Franschhoek Valley for another 10 miles (16 km).
The principal differences and advantages that steel monopole structures offer, compared with conventional steel lattice structures (Fig. 3) include:
* A more compact size, horizontally and vertically
* A smaller footprint or area
* A lower installation cost
* More options for strain structures (bolted base or planted base and stays)
* More options for intermediate structures (bolted base or planted base)
* A flexible design - the design methodology enables structures to be optimized for a particular line or application, for example, height, size, load capacity and number of circuits
* More versatility and easier installation, from the foundations to the complete assembly.
Wooden Single Poles The Eskom standard wood pole supports for 66-kV lines, prior to 1995, were confined to H-pole and five-pole wood structures. The first 66-kV line Eskom built with single wood poles crossed the wheat lands north of Cape Town to supply a new substation. Difficulty in obtaining a new R/W for the standard structures prompted the design change. Eskom used a derivation of the Cape A-frame for the 66-kV line (Fig. 4). However, this method quickly fell into disrepute because of its perceived threat to birds, which required further redevelopment. Pole twisting was also a problem, hence, the company did not consider using single wood pole structures with horizontal post insulators (as used on the steel monopole lines) a viable proposition.
Examples of recently commissioned wood pole circuits include:
* A T-frame design used on four lines in rural areas in the western cape used suspension insulators for the outer phases to overcome pole twisting. However, to provide the necessary strength on the strain structures, the stays (usually two inline in each direction) had to support the top section of the pole above the crossarm. Unfortunately, this method caused problems with stay-to-phase clearances.
A wood pole 66-kV line design in the Oudtshoorn area, which included Fox conductors and an overhead shield wire, employed a new innovation to obviate most of the effects of pole twisting - a strap inserted in the conductor suspension hardware. The lengthened suspension hardware allows some 15 degrees rotational freedom in both directions before the insulator is strained. Additionally, Eskom used twin wood poles for the first time for the angle strain structures, mainly because of the load imposed by the overhead shield wire. This method kept the stay attachment points below the crossarm level (Fig. 2).
The double-circuit 66-kV line from Knysna to Plettenberg Bay proved to be the most difficult line in terms of terrain, design and construction. The route passes through 16 miles (26 km) of hilly, forested terrain from Knysna, crossing the Garden Route national road before passing through bush and forest areas to Plettenberg Bay. Originally, sections of the route were inaccessible to the construction teams, and estimates for lattice or steel structures were rejected because they were too expensive. The design Eskom developed drew from the utility's experience with twin poles and clearance problems on strain structures. The company selected an oak conductor and a twin wood pole (two poles bolted together) for the intermediate structures, since the locally grown timber does not have sufficient strength to use as a single pole. Crews used insulator mounting brackets instead of bolting the insulators directly to the pole and installed extension arms to enable the use of span lengths up to 1670 ft (509 m), as shown in Fig. 5.
The strain structures also were a new development in steel poles. They were designed to be fully stayed, light (about 2 tons for a complete structure) and easy to erect, and their foundations are minimal. The pole bottom ends have universal joints, so there is no resulting moment on the foundations, either during erection or in the case of conductor failure. The strain structures also were designed to have electrical spans of up to 1670 ft (509 m), see Fig. 6. The line is possibly the only double-circuit wood pole line ever built in Africa.
Eskom built a single-circuit 7-mile (11-km) long line, using Chickadee conductor, in the mountains above Ceres. This area often becomes covered in the snow in winter. The loose anti-twist conductor supports should prevent snow buildup on the conductors (Fig. 7).
The main differences between the conventional structure - for example, either the multipole wood pole or steel structures, lattice or monopole - and the single wood pole structure are:
* A considerably lower cost
* A smaller footprint
* Strain structures that include a wide variety - from the older designs to the new options
* Options for intermediate structures (single or twin wood pole)
* Easier installation than older types of structures.
Standard Range of Overhead Line Conductors. Until recently, Eskom used conductors based on the British standards for ACSR and AAAC conductors. Now, the range has been extended to include Canadian standard conductors. Table 1 shows the conductors used for the new range of 66-kV or 132-kV overhead lines.
Financial Benefits Cost reduction is one of the key factors for promoting the use of single-pole structures. However, monopole steel lines are not always the most cost-effective option; sometimes they are the only option.
Over the last two years, Eskom has instituted cost-related performance indicators for the installed costs of 66-kV and 132-kV overhead lines and assessed regional performance against these indicators. The extent of cost savings achieved with the use of single-pole structures is illustrated in the following examples:
* Using the single-pole design on the 66-kV circuit through the vineyards near Stellenbosch effectively reduced R/W costs by R600,000 (US$87,000). (Eskom pays servitude for land traversed by the line on circuits above 33 kV, the usual cost being one-third the value of land. Eskom obtains wayleaves for all overhead line routes for circuits up to and including 33 kV.)
* A double-circuit 66-kV line, 2.5 miles (4 km) long, with Kingbird conductors. Line supports include 11 angle strains and 10 intermediate poles. The cost target is R875,000 per km (US$203,000 per mile), with an estimated cost for line with steel lattice structures of R944,500 per km (US$220,000 per mile). The constructed cost for line with steel monopole structures is R741,000 per km (US$172,000 per mile).
* The Knysna-Plettenberg Bay double-circuit 66-kV line is 16 miles (26 km) long with oak conductors. Its wood pole supports include 35 angle strains and 89 intermediates. The cost target is R232,000 per km (US$54,000 per mile). The estimate for steel lattice supports is R561,000 per km (US$130,000 per mile). Estimate for wood pole supports is R429,000 per km (US$100,000 per mile), and constructed cost for twin wood-pole plus steel stayed strains is R223,000 per km (US$52,000 per mile).
Community and Environmental Benefits The small size of the steel monopoles enables energy companies to use them inside substations (for example, as line terminal poles). Pressure from farmers, particularly when it became known that single-pole structures were available, also forced consideration of monopole supports that offer more options for the selection of a line route.
Eskom has successfully developed an acceptable environmental solution to ease the problem of upgrading and extending its existing 66-kV and 132-kV overhead line system. By combining innovation with technology, the company has achieved its objective cost effectively, satisfying the business objective set by all utilities to control the customer's cost per kilowatt-hour.