The cross rope suspension (CRS) concept whereby conductor phases are suspended from transverse spanning wire rope(s) instead of from rigid crossarms is finding new applications. The cross rope concept began about 40 years ago as a solution to avalanche problems in a mountainous area in British Columbia, Canada. Recently, the concept was successfully applied by Eskom, Johannesburg, South Africa, to a 400-kV line. Eskom has modified the concept of suspending conductors without crossarms and sometimes without towers. These variations include an emergency CRS tower and a delta-configuration with lower electrical and magnetic fields. Additional work is under way on refining maintenance techniques for CRS lines.
The first major application of a cross rope was in 1955 after a massive avalanche wiped out five towers of the two circuits of the 315-kV lines of Alcan that delivered power from the Alcan generating station at Kemano, British Columbia, to a smelter about 31 km (50 miles) away at Kitimat, British Columbia. Safe tower locations were difficult to find because the area was rugged and mountainous with the potential for extreme winds and ice loads that call for an ice specific of 60 kg per m (40 lb per ft).
Relocating towers to "really safe" locations (the stated mandate for the repairs) was not possible because of the magnitude of the avalanche threat. A casual remark that the problem might be resolved with a "sky hook" led to the ideal solution. Two 77 mm (3.03 inch) steel strandings spanning 1200 m (3937 ft) across the valley suspended the more than 2 km (1.2 miles) of six phases of 58-mm (2.3-inch) conductors well above the avalanche threats that passed below (Fig. 1).
With the exception of a low-voltage CR system built in Hawaii, the CRS concept remained dormant for almost 20 years, although the structural efficiency of wires and lattice masts found application in the guyed V tower. The guyed V tower was developed in the late 1950s as a means of converting the guyed portal structure used in the relatively flat terrain of northern Europe and the more difficult terrain in Canada and the northeastern United States.
Guyed V towers were often used for EHV lines, but their limitations became apparent when used at the 750-kV level. As voltages rise, towers become top heavy because the crossarm width increases almost directly with voltage while height increases more slowly. The weight of 750-kV class guyed towers exceeds the lifting capacity of almost all helicopters now and most certainly did in the early 1970s when the 735-kV James Bay project of Hydro-Quebec was examined.
Thousands of kilometers of guyed V lines at voltages from 230- to 765-kV were already in place where tower erection at the higher voltages had almost always been by mobile crane working from temporary access roads. However, a tower failure from whatever cause could become a major problem for repair if the sometimes long access roads were not immediately available. Long bush roads are difficult to maintain throughout the year where some of these projects had been and were soon to be built.
Another tower type or construction method was desired for the James Bay project. A concept derived from the original CRS in the mountains evolved into the transmission tower system now known as the CRS and called the Chainette (little necklace) in Quebec where the first major installations were on three of Hydro Quebec's James Bay lines, totaling 2000 km (1243 miles) in length.
The CRS of Hydro Quebec uses a six-part cross rope system, a complex of wires made necessary by the threat of galloping in icing areas. The triangulated suspension system prevents oscillatory forces at one support point from being transmitted to other phase support points.
South African Cross-Rope Tower Experiences After Bonneville Power Administration constructed a 60 km (37.3 mile) stretch of 500-kV CRS in Oregon in 1982, the next significant CRS activity occurred in South Africa. Dr. Roberto Behncke developed the adaptation of the CRS technology for the South African conditions, initially at a voltage of 400 kV, from and with the assistance of White, who proposed the single rope solution and the pre-cut and fitted guys discussed below. The 400-kV Camden-Duvha line, with a length of 112 km (69.6 miles), was the first Eskom line constructed using CRS towers. The CRS was used for all tangent and small angle suspension positions of the 112 km (69.6 mile) line.
The standard Eskom CRS tower is a simple construction consisting of two masts of various heights connected by a spacer cable and a single cross-rope. This design was practical since icing and galloping are not problems in South Africa.
The insulator strings and conductors are suspended from the cross-rope by inverted suspension clamps. Mast lengths are between 29 m (95 ft) and 38 m (125 ft) in 1.5 m (4.9 ft) increments, using common end tapered pieces, with various combinations of 6, 7.5 and 9 m (19.6, 24.6 and 29.5 ft) extensions. This allows the height of the tower to be adjusted, as well as allowing sitting on steep side slopes by using unequal mast lengths. The tops of the masts are 29 m (95 ft) apart, and the tallest tower occupies an area 70 m by 40 m (230 ft by 131 ft). The masts have a common slope of 1.10 for all tower heights. Because the mast bases are about 21 m (69 ft) apart, farming and other activities are still possible under and through the tower. The right-of-way width between towers is much reduced because of the reduced phase spacing permitted by the CRS. The guy wires and cross-ropes are 24 mm (0.94 inches) in diameter, while the spacer cable is 16 mm (0.63 inches) in diameter.
Design The first CRS towers were designed to support a triple Bersfort conductor (686 mm2, 1354 mcm) but have since been used for twin Bersfort and triple Tern conductor (403 mm2, 795 mcm) by adjusting the maximum wind and weight spans.
Because of the pin-ended conditions of the masts, they need only resist mainly axial compression loads and some bending due to the wind forces. This simplifies the design considerably and allows for the use of common member sizes for all the body extensions. Due to the wind effect on the masts, the taller towers have reduced permissible wind spans, compared to the shorter towers.
The CRS towers have also been checked for failure-containment conditions to ensure against a cascade of towers in the event of a failure of one tower. Longitudinal loads for this case are reduced due to the swing of the cross-rope, which then acts as an additional length of insulator string.
"Tornado loads" consisting of high wind forces on the tower were also applied to allow for the effect of high-intensity winds associated with South Africa's prevalent thunderstorms and the main cause of line failures.
The various components of the tower are designed to different strength factors to ensure that in case of an overload, the masts fail first, followed by the guys, foundations and lastly the conductor attachments. This means that a tower that fails because of overloading will still have the guys, cross-rope and foundations intact, so only new masts are needed.
After all the components were designed and detailed in-house, the tower was tested at the Tower Test Station in Rosherville, Johannesburg. The final test was a destruction test; failure occurred at 110% of the design transverse load. Behncke left Eskom in 1993 to join the National Grid in England and further development has been done by Ritky.
Construction of CRS Lines Initially, erection of the towers was slow, but as the contractor's personnel gained experience, the number of towers erected in a day increased to an average of eight.
Three of the guys are non-adjustable with the fourth used to take up any slack. Accurate survey and good communication between the site and the factory or field shop are required to ensure that the guys are the correct length. (Note: Precutting saves not only the cost of three adjusting devices, but also eliminates the need for temporary attachments and take-ups as well as the entire operation of plumbing and straightening the erected tower, an operation that can engage a crew of five or six.)
All guys and the cross ropes are tested to 83% of UTS (ultimate tensile strength) before sending to site to check the compression fittings and strands. This causes some permanent elongation, which must be accommodated in the rope length calculations.
The towers are erected in the following steps: - Both masts are assembled next to the fittings
- The mast with the two fixed guys is erected and held with a third temporary guy to the opposite footing
- The second mast is erected, and the spacer cable connected
- The temporary guy is removed and the slack taken up in the adjustable guy
- Later, the cross-rope, complete with insulators, fittings and the pilot cables in the pulleys, is pulled up and connected to the tops of the masts
- Stringing, regulating and clamping in as for any other line.
Maintenance of CRS Lines Most lines are maintained under live-line conditions using under-slung techniques. Some special tools had to be designed to fit onto the cross-rope for insulator replacement. Techniques were perfected on each type of cross-rope before energizing, and the live-line teams foresee no problems in maintenance.
Development in South Africa A number of recent projects encompass the CRS principle. An emergency, two-mast CRS tower has been designed to replace damaged towers, suspension and strain, which can be erected within 18 hours, including stringing. A number of these towers are stored around the country, together with precast anchor blocks for use in emergencies.
The insulated cross-rope type allows deviations of up to 60 deg for light conductors, decreasing to 20 deg for the heavier types (Fig 2).
A delta-configuration has been designed which requires narrower servitude than the standard CRS, and has lower electrical losses and magnetic fields. This is used on a 240-km, 400-kV line under construction. The delta configuration allows the line to be designed without capacitors. However, increased insulation cost means that this tower is cost effective only for long lines or where line compaction is necessary (Fig. 3). All the towers described above have the same masts, which simplifies ordering spares and allows for speedy repairs of failures.
A small-scale version of the first cross-rope system is planned between two small hills on the skyline of a mountain range near Cape Town. Due to high visual impact in a scenic area, it will not be possible to position a standard tower on the skyline. The cross-rope will carry two circuits of a 400-kV line over the pass.
H. Brian White a consulting engineer living in Hudson, Quebec, Canada has 46 years of transmission line experience obtained from work in more than 16 countries.
Franz Ritky is chief engineer, transmission line technology, Eskom.