Gener is supplying power to northern Chile via a new transmission line from Salta in Argentina. The design parameters for the line, which is carrying power generated in a new combined cycle plant, were primarily determined by the need to cross the Andean mountain range at an altitude of up to 4800m above sea level. G. Gheorghita, W Roemer, G-H Dian and S Grunwald, Fichtner, Stuttgart, Germany
The Chilean electricity company (Gener) has extended its high voltage transmission system in the northern Atacama region of the country to provide power to develop the regional mining industry. Studies of alternative sources of power resulted in construction of a 345 kV single circuit overhead transmission line between Gener’s Salta combined cycle power plant (Guâmes) in Argentina (see MPS, August 1998) and an existing substation in Atacama.
The line length of the trans-Andean scheme is approximately 408 km and the transmission system is routed roughly parallel to the Tropic of Capricorn. To improve the stability of power transmission on this line, a 490 MVAr series compensator has been installed at the Atacama substation compensating 70 per cent of the line reactance. A transverse profile of the route of the line is shown in Figure 1.
The mechanical and electrical parameters of the project had to be optimised to marry high reliability with low cost. This involved using standard components where possible. The inhospitable terrain made it important to keep operation and maintenance requirements to a minimum and the project has to achieve the minimum environmental impact.
Selection of the design parameters had to take account of various geographical and geophysical conditions prevailing along the line route. This route, traversing regions with altitudes of between 900 m and 4800 m above sea level, comprises a variety of terrains. These included: flat and hilly areas of forest and agricultural land with easy access; rough and mountainous areas with extremely difficult access; rivers and valleys where crossings had to allow for spans of 900 m and above; and salt lakes and deserts.
Crossing such diverse geophysical regions, the route is subject to significant variations in a variety of physical parameters, all of which had to be taken into account when selecting the electrical and mechanical design parameters. These parameters included:
isoceraunic levels (ie incidence of thunder);
In order to meet the levels of reliability and availability necessary for the project, the physical conditions required selection of some unusual design parameters. These parameters, determined by the geological and geophysical conditions, included:
increased leakage distance at insulators to avoid need to wash insulators in arid areas;
minimum clearance of 3 m horizontally and 9 m vertically between conductors and earthwires in order to reduce line outages resulting from conductor galloping, asymmetrical ice loads of conductors or conductor jumping;
strengthening of the line elements to take account of predicted wind and ice loads;
reduced average spans at high altitudes, to limit the effects of conductor galloping;
increase in exceptional loadings margins for suspension towers to prevent cascading;
enhanced exceptional loading margins for tension towers to prevent a possible difference in conductor tensions; and design features to accommodate hot line maintenance.
In fact, as a consequence of the high altitude and corresponding low air density, the dimensions of the 345 kV line corresponded to those of a 500 kV line.
Transmission line design
The line route was divided into two zones for design purposes. The characteristics of the two zones are primarily determined by their climatic conditions. These two zones, which required both different mechanical and different electrical design considerations for the transmission line elements, were defined as follows:
Zone I. The lower Salta area with altitudes ranging from approximately 900 to 1500 m above sea level, and encompassing roughly 40km of the line.
Zone II. From Salta to the Atacama, where altitude ranges from approximately 1500 m to 4800 m above sea level covering the remaining 370 km of the line.
The phase conductors of the transmission line had to be capable of transmitting a maximum of 700 MVA at 345 kV nominal voltage and 4000 m average altitude and to provide satisfactory radio interference (RI), audible noise (AN) and corona loss performance. To meet the necessary mechanical performance, a steel-reinforced aluminium conductor (SRAC) with a greased core was selected.
Variants with 2 to 4 SRAC subconductors and with aluminium/steel ratios of 4.3, 6 and 7.7 were analysed. The variant with 2 SRAC “Curlew” per phase was selected. This configuration allowed transmission of 700 MVA under all climatic conditions while limiting the corridor with disturbed radio reception to 65 m in lower altitude regions and 350 m in higher altitudes. The corridor with audible noise pressure levels above 55-60 dBA was restricted to 65 m at lower altitudes and 300 m in higher altitudes, while the corona loss performance was considered satisfactory.
For the earthwires, an aluminium clad steel (ACS) conductor with a cross section of 95 mm2 was selected for both mechanical and electrical reasons. Two such earthwires, installed on top of the towers, provide shielding angles of 20°.
The differential sagging between phase conductors and earthwires ensures lower shielding angles at mid-span with mid span clearances of 10 m. Such a configuration eliminates mid-span flashovers.
The general principles of insulation design for the project were based on American standards. The evaluation of the dielectric strength of the external insulation was derived from the American experience outlined in EPRI’s 1982 reference (Transmission line reference book, 345 kV and above, second edition, revised EPRI/1982) and CIGRE recommendations (Guidelines for the evaluation of the dielectric strength of external insulation, CIGRE, WG 33.07/1991).
Taking into consideration areas with high pollution and altitude correction factors, the leakage distance based on maximum phase-to-earth voltage (rms) was established as 3.5 cm/kV for the lightly polluted Zone 1 near Salta, Argentina, covering approximately 40 km of the line length and 5.0 cm/kV to take account of the heavy pollution affecting the rest of the line.
A switching impulse phase-to-earth withstand voltage of at least 850 kV was selected. This best matches the other insulation levels of the transmission system. The levels were adopted for the two zones are shown in Table 1. These voltages result in minimum clearances between live and grounded parts of 4.2 m at 4800 m altitude.
A lightning impulse phase-to-earth withstand voltage of 1175 kV was selected. This results in a minimum clearance between live and grounded parts of 4.2 m at 4800 m altitude.
The lightning performance depends largely on the earthing resistance of the towers. The tripout rate with assumed earthing resistances of 15 V and 25 V, was calculated in the preliminary design to be 0.66 tripouts/100 km year. Soil resistivity measurements during the line survey revealed that earthing resistances of 25 V were not practically achievable. New calculations with resistances above 25 V resulted in a predicted failure rate of 1.42 tripouts/100 km year.
The insulation leakage distance and the switching impulse insulation level govern the design of the insulation. A short duration power frequency withstand voltage of 1.4 x maximum permissible operating voltage required a minimum clearance between live and grounded parts of 1.50 m at 4800 m altitude.
The basic mechanical design data for the 345kV transmission line is shown in Table 2.
Phase conductors and earthwires
The clamp component of the conductor and earthwire tensile stress should not exceed the permitted maximum tensile stress, calculated for a safety factor of 2. The limiting conditions considered were:
–5°C with 550 N/m2 wind pressure and 2 cm ice with a density of 0.9 kg/dm3;
–30°C without additional loads;
+0°C and 1500 N/m2 wind pressure;
Taking into consideration the use of armour rods, spacer dampers and vibration dampers, the every-day stress was selected as 20 per cent of the nominal breaking load of the conductors at +15°C without additional loads. The dimensions of the vibration dampers and spacer dampers were selected to assure a 100 year life expectancy of conductors and earthwires.
While normal SRAC cable with a greased galvanised steel core was selected for the largest portion of the line, alumoweld core wires of the same size were used in the salt polluted area near the Salar del Rincon – a distance of approximately 40 km.
Cap and pin (disc) type toughened glass insulators were used for the project. The suspension towers are equipped with V-strings on the middle phase and I-strings on the outer phases. For increased safety, double strings were used at various crossings. The tension towers were generally equipped with double tension strings utilising two different connection points.
A safety factor of 2.5 was selected for the mechanical stress calculation of insulator strings in order to achieve a high reliability of the line during operation. The electric design required 26 insulator units with 170 mm spacing. Tension insulator strings were equipped with 2 additional insulators in order to provide sufficient electric safety margin in the case of damage to individual discs.
All insulator strings were equipped with life-end guard rings. Earth-end grading rings were mounted onto all insulator strings within a distance of 1.5 km from substations.
The tower geometry selection was based on the following criteria:
cost of the materials needed for the structures (and weight of structures per component);
foundation types and costs;
ease of transport of structural elements;
reliability under severe meteorological conditions;
ease of maintenance and replacement of components;
performance under cascade failures;
sensitivity to vandalism;
performance when subject to lateral rain or snow conditions;
resilience to stone or snow avalanches;
ease and stability of erection on steeply sloping ground;
performance when subject to differential earth movement;
electrical parameters, joule and corona losses
area of tower base line corridor;
lightning performance; and environmental impact.
Taking all these into account, either self-supporting or guyed towers were chosen, with a horizontal conductor arrangement and two earthwires at the top of the structures. Self-supporting structures were used in mountainous regions and at locations with high mechanical loading. Guyed towers were used in flat or hilly regions with easy access. The tower configurations are shown in Figure 2.
Tower types were selected on the basis of known line angles. With a 2° line deviation angle, self-supporting and guyed Ve suspension towers were used. Where the deviation angle was 8°, self-supporting towers were selected. Where there was a 30° deviation angle, a 60° line deviation angle and a dead end, tension towers were chosen. Transposition towers (2 complete transpositions), consisted of a combination of special tension towers with special transposition insulator string arrangements.
Safety factors for tower design were 1.8 for normal loading cases and 1.4 for exceptional loading cases. IEC recommendations (IEC 826, Loading and strength on overhead transmission lines, 1991-04) were considered when selecting these factors.
Foundation design was determined following a geological survey. Various foundation types were used depending on the conditions encountered. The safety factors adopted were 2.0 for normal loading and 1.5 for exceptional loading.
Loading parameters and safety factors were selected in such a way as to achieve the following failure sequence of the transmission line elements:
1. Suspension insulator sets.
2. Line conductors.
3. Tension insulator sets.
4. Suspension towers.
5. Tension towers.
Figure 3 shows a series of load limit curves for the various line elements plotted on a graph of wind velocity vs ice thickness for a suspension tower.
Following the completion of the design phase, construction contracts for the new line were awarded in April 1997, and the line entered operation two years later. The main contractors for the project were Teyma Abengoa (Argentina), Abengoa (Chile), and Siemens AG (Germany). Since entering operation, the line has behaved fully in accordance with the design expectations.
TablesInsulation levels selected for the two zones of the project Mechanical design data