SIGNIFICANT ADVANCES have been made in the optimization of suspension structures used to support extra-high-voltage (EHV) transmission lines that employ technology developed by French and Canadian engineers. During the last two decades, transmission engineers in South Africa have also advanced the design of suspension towers with the introduction of the guyed-vee structure in 1985, which resulted in a 35% cost savings compared with conventional suspension structures. This was followed by the adoption of the cross-rope suspension structure, which produced a 50% effective savings over self-supporting towers.

In 1998, a compact chainette using a delta configuration was introduced for longer links. Although not cheaper on a per-structure basis, the delta-phase configuration does offer significant operational cost savings for lines in excess of 200 km (125 miles) due to the favorable capacitance and inductance characteristics.

All of these developments reduced costs considerably for suspension structures and circuits where all line bends are supported by self-supporting structures. However, in 2000, Eskom received research funding to investigate the potential benefits in guyed-strain or angle structures.

STRAIN STRUCTURE INVESTIGATION

Literature research and international consultation revealed there were no known utilities using a guyed-strain structure on a single pylon on EHV transmission lines; the closest instances of this were triple-mast 400-kV structures being developed in Brazil and Canada. This finding seems surprising because there are numerous guyed-strain structures being used to support lower-voltage overhead lines.

A second important research finding was that the potential savings were significantly higher than first assumed due to the high relative weight of strain structures. For example, the average 400-kV chainette suspension tower weighs 4.5 tons; however, 400-kV strain structures can weigh anywhere from 12 tons to 70 tons. Hence, considerable scope exists for reducing project costs by optimizing strain structures, which can typically make up 40% to 50% of the structure weight of a 400-kV transmission line.

The design philosophy adopted for structural optimization takes into consideration the multitude of mechanical, structural and electrical engineering variables that are interdependent and have an impact on the cost of the pylon. For example, installing guys with a higher pretension can reduce stresses in the superstructure by limiting deflection, but imposing higher loadings on foundations. Hence, the optimal value of this variable can only be determined if the cost impacts of all civil- and mechanical-related components are evaluated. Therefore, the design process was undertaken using an integrative approach rather than the conventional sequential design process.

The tower evaluation study revealed the following two winning designs:

  1. For light deviation angles, a double-masted chainette system.

  2. For large deviation angles, a single-masted system.

DESIGN ACHIEVEMENTS FOR INSULATED CHAINETTE STRUCTURE

The main design achievements can be summarized as follows:

Material savings

Eliminating jumpers decreases material requirements and construction time. For example, 72 steel and aluminum joints on strain assemblies are eliminated in the new design when used on a single-circuit, quad-bundle conductor configuration.

Environmental benefits

On conventional towers, bird guards are required to prevent large birds from perching on the area above live conductors. No such protection is required on the new tower design. The visual impact of the towers is considerably reduced, and the new towers reduce steel usage by 50%.

Structural efficiency

The erected steel of the double-mast structure is 42% of a conventional supporting steel, even though the new tower is designed to accept higher loadings (e.g. tornado loads of 250 km/h [155 mph] on the tower and 188 km/h [116 mph] on the conductors). The selection of high-strength 350-W (350-Mpa yield stress) steel on the main legs results in smaller sections and lower-mast wind loads.

Productivity improvements

The reduced tower weight and elimination of jumpers saves time in construction. Contractors using the new tower designs reported that structure erection and stringing was four times the rate of that achieved with conventional structures. Jumper elimination proved to be a major design challenge, and attempts to eliminate the natural tendency of the suspended-conductor bundle to tilt proved impractical. The design that was eventually developed allows the conductor bundle to tilt freely. Productivity enhancements in the design of the mast peaks replacing the use of welding with bolted components removed the need for dye penetration and X-ray testing of the welds.

Live-line maintenance

The end users of the new structures specified that insulators should be replaced using live-line procedures. Due to the relative importance of the insulation as a structural component of the tower, double redundancy built into the assembly allows complete failure of any single insulator. Live-line insulator replacement is achievable using a compression lever system.

SINGLE-MAST DESIGN ACHIEVEMENTS

The design achievements of this structure were similar to those for the double-masted structure. The live-line maintenance constraint prevented the use of the insulated chainette for the larger deviation design angles. Hence, for larger angles, a vertical mast configuration with separate assemblies supporting each phase was incorporated in the new design.

The single-mast structure for medium angles — 15 degrees to 30 degrees — utilizes a flying strain assembly with short crossarms that maintain electrical clearance to the structure during reverse high-wind conditions. Hardware for this structure is considerably simplified by the use of flying strain assemblies that require half the insulation and less than 50% of the hardware required for conventional strain assemblies, thus satisfying the foremost priority of cost optimization. Double redundancy has been built into the insulation configuration, allowing for complete failure of one insulator.

FOUNDATION DESIGN

The foundation design received particular attention so that the interrelationships between mechanical and civil designs were accurately modeled and cost estimated. Developed internally, the software program automates the foundation design process, optimizes the geometry of the footing and automates final drafting, including a scaled 3-D presentation of the most cost-effective foundation.

The objective of the foundation design software is to minimize the installed cost of the footing in terms of the most efficient shape to transmit forces to the soil safely and the optimal tradeoff, for example, between the quantities of concrete and steel reinforcing used in the foundation. Typically, a foundation design is optimized by considering the permutations of footing geometry by varying up to five dimension variables, which in turn influence the cost of excavation, surface preparation, reinforcing, shuttering, concrete and backfill. These cost variables stay within the practical limitations of construction equipment and crews.

The software achieves optimization by using an iterative procedure and linear programming, a method that can reduce the cost of foundation material by 15% to 40%.

TOWER TESTING

Full-scale prototype towers have been subject to a rigorous test program to verify the electrical and mechanical design specification as follows:

Electrical tests

These are performed at the NETFA HV Laboratory in Johannesburg, South Africa, where the structures exhibited relatively high electrical strength due to the phase-to-phase design clearances.

Mechanical tests

Full-scale load testing was undertaken on erected structures at Eskom's Tower Test Station facility in Johannesburg. Two structures were tested to withstand the loads imposed on the structures for quad Tern conductors (27-mm [1.06-inch] diameter).

LIMITATIONS

Although the new tower designs are considerably cheaper, they cannot be used to replace self-supporting structures, as the large footprint taken up by the new towers makes it impractical for positioning in excessively sloping or mountainous terrain. Also, the towers cannot be used in situations where conductors will experience uplift.

As a result of the large footprint taken up by cross-rope towers (and to a lesser extent other guyed structures), these towers are not suitable for urban, built-up areas, or areas where land is expensive. Eskom's cross-rope towers can occupy a 75-m × 45-m (245-ft × 145-ft) footprint compared to the 9-m × 9-m (30-ft × 30-ft) footprint for an equivalent self-supporting tower, but the net area affected that limits land use varies with the land usage. When used for grazing, there is no difference in the area affected. But, where land is cultivated, some farmers use the area inside the footprint, and in these situations, the guy positions are highlighted with yellow plastic sleeves. However, most farmers prefer self-supporting towers even though the size of the foundations for guyed towers is 50% smaller.

It is normal practice for Eskom to negotiate a 55-m (180-ft) right-of-way, compensating the landowner for 100% of the market value for the strip, but the owner is advised that the use of cross-rope towers could result in guys being positioned some 20 m (65 ft) beyond the boundary of the right-of-way.

Eskom experienced a number of minor teething problems with both towers when constructed for use, but the most significant is the need for additional care when changing from a vertical- to a horizontal-phase configuration.

SUMMARY

The impact of the new tower designs is best illustrated by a comparison of the costs for the conventional and new towers based on a recent successful tender that included the component costs for both new and conventional designs. Fig. 5 shows that cost savings for the new structures compared with conventional towers is between 46% to 52%. When related to the number of new transmission lines planned by Eskom for construction in the next five years, the total cost savings attributable to the use of the new structures is in excess of US$20 million. The total research and development cost for the two towers was $700,000; hence, the return on this investment was considerable.

Eskom first erected the new towers in Natal, South Africa, on the Athlene — Pegasus project that was completed in December 2003, where cost savings of $2.53 million were realized. Currently, the structures are being utilized on three separate 400-kV projects under construction.

The design of the final structure in the series, the 528E tower (for deviation angles between 30 degrees to 60 degrees), has been completed and is awaiting final approval for fabrication. Cost savings associated with this structure are projected to be the most promising in the series.

ACKNOWLEDGEMENT

The authors wish to acknowledge the help and support received from all the suppliers, contractors, engineers and specialists who contributed to this project.

Pierre Marais is the lead structural engineer in Trans Africa Projects and leader of the Guyed Strain Tower Project. Marais' experience ranges from project management to structural/geotechnical design and templating. He is a registered professional engineer with 14 years experience in overhead power line design and project management. He holds BSCE and MBA degrees, and a national diploma in project management and an MBA. pierrem@taprojects.co.za

D. Muftic is a corporate consultant with 35 years experience in overhead power line development. He obtained a BSEE degree and a Ph.D. in engineering in 1989. He is a member of CIGRE's Study Committee B2 (Overhead Lines), and CIGRE's Working Groups 06 (Principles of OHL Design) and 12 (Electrical Aspects of OHL). Muftic is actively involved in several Eskom research projects and training in overhead power line design.

Jose Diez Serrano is head of the structural section in Trans Africa Projects. Serrano has a national diploma in mechanical engineering and a bachelor's degree in business economics. He has 21 years experience in overhead line design, 10 of which he served as tower test station manager, and he is a former member of CIGRE's Study Group B2 and Working Group 08.

Riaz Vajeth is business manager at Trans Africa Projects, where he is responsible for a large team of technical experts involved with planning and design work for Eskom and a number of international customers. He has 14 years experience in power system planning and related fields, having worked in the system analysis, network operations and the distribution and transmission planning departments of Eskom. He was awarded the BSEE degree by the University of Natal in South Africa in 1990 and is a registered professional engineer in South Africa. riazv@taprojects.co.za