Although aluminum conductor was used for overhead transmission as early as 1898, its widespread use did not occur A until the 1940s, when copper was designated as a vital war material and was no longer available for use by electric utilities. To obtain the desired strength required for transmission lines, the lightweight aluminum was combined with the high tensile strength of steel in the development of aluminum conductor steel reinforced (ACSR). Today, most overhead transmission lines use this conductor construction, which consists of stranded hard-drawn aluminum wires over a stranded core of high-strength steel.
On a continuous basis, ACSR usually can be operated at temperatures up to 100°C (212°F) and, for limited-time emergencies, at temperatures as high as 125°C (257°F) without any significant change in the conductor's physical properties related to a reduction in tensile strength because of annealing of aluminum. These temperature limits constrain the thermal rating of a typical 230-kV line, strung with 795 kcmil ACSR “Drake” conductor, to about 400 MVA, corresponding to a current of 1000 A.
Increasing the Thermal Rating
Because it is increasingly difficult to obtain rights-of-way (R/Ws) for new transmission lines, and the financial motivation to do so may be questionable, a great deal of interest surrounds the concept of increasing the thermal rating of existing lines. Among the ways to achieve this increase are to:
Increase the maximum allowable operating temperature to 100°C. For example, if the line is limited to a modest temperature of 50°C to 75°C (122°F to 167°F), and the electrical clearance is sufficient to allow an increase in sag for operation at a higher temperature, then the thermal rating of the line can be increased. If sufficient clearance does not exist in all spans, then conductor attachment points may be raised, conductor tension increased or other mechanical methods applied to obtain the necessary clearance at the higher temperature.
Use dynamic ratings or less-conservative weather conditions relating to wind speed and ambient temperatures. For example, if the existing line is already rated at a temperature near 100°C, and a modest increase of 5% to 15% is desired, then monitors can be installed and the higher ratings used when wind speed is higher than the standard 2 ft/sec and the ambient temperature is lower than 40°C (104°F).
Replace with a larger conductor or with a conductor capable of continuous operation above 100°C. These solutions would be ideal if the line was already limited to 100°C, and the thermal rating increased by more than 25%.
Given the low cost, high conductivity and low density of aluminum, no other high-conductivity material is presently used. Therefore, replacement with a larger conductor will result in an increased load on existing structures because of an increase of wind/ice and tension.
Figure 1 shows how the thermal rating of an existing line can be increased about 50% by using a replacement conductor that has twice the aluminum area of the original conductor. It is important to note that the larger conductor doubles the original strain structure tension loads and increases transverse wind/ice conductor loads on suspension structures by about 40%. Such large load increases typically would require structure reinforcement or replacement. This drawback to the use of a larger conductor may be avoided by using the high-temperature, low-sag (HTLS) conductor, which can be operated at temperatures above 100°C while exhibiting stable tensile strength and creep elongation properties. Practical temperature limits of up to 200°C (392°F) have been specified for some conductors. Using the HTLS conductor — which has the same diameter as the original — at 180°C (356°F) also increases the line rating by 50% but without any significant change in structure loads (Fig. 1). If the replacement conductor has a lower thermal elongation rate than the original, then the structures will not have to be raised.
Although the use of a larger conductor provides a reduction in losses over the life of the line while operating temperatures remain at a modest level, the use of the HTLS conductor reduces capital investment by avoiding structure modifications. In either case, replacing the existing conductors should improve the reliability of the line because the conductor, connectors and hardware will all be new.
Figure 2 shows that the original conductor's “initial installed sag” increases to a final “everyday” sag, typically at 60°F (16°C) with no ice or wind, as a result of both occasional wind/ice loading and the normal aluminum strand creep elongation that is a result of tension over time. This final sag may increase occasionally because of ice/wind loading or high electrical loads, but these effects are reversible.
For most transmission lines, maximum final sag is the result of electrical rather than mechanical loads. It is important that any replacement conductor is installed so its final sag under maximum electrical or mechanical load does not exceed the original conductor's final sag and the existing structures need not be raised or new structures added. Under these circumstances, where structure reinforcement or replacement is to be avoided, HTLS conductors are used to advantage.
Attractive Material Properties
As previously noted, transmission conductors are constructed from helically stranded combinations of individual wires where galvanized steel wires are used for mechanical reinforcement, aluminum wires for the conduction of electricity, and hard-drawn aluminum for both mechanical and electrical purposes.
In the ACSR, both the steel and the aluminum strands provide mechanical strength as demonstrated by the popular 26/7 construction where 43% of the conductor strength is provided by the hard-drawn aluminum strands. Desirable properties for reinforcing core-wire material include a high elastic modulus, a high ratio of tensile strength to weight, the retention of tensile strength at high temperatures, a low plastic and thermal elongation, a low corrosion rate in the presence of aluminum and a relatively high electrical conductivity. The material must be easy to fabricate into wire for stranding.
Among the choices available for HTLS conductors are:
Aluminum conductor steel supported (ACSS) and a variation of this construction where the aluminum strands are formed to produce a trapezoidal shape (ACSS/TW).
Special zirconium high-temperature aluminum alloy conductor Invar steel reinforced (ZTACIR).
Gapped high-temperature aluminum alloy conductor extra high-strength steel reinforced (GTACSR).
Aluminum conductor composite reinforced (ACCR).
Aluminum stranded conductors reinforced with fiberglass and graphite composites also are being studied.
The ACSS and ACSS/TW conductors are made of annealed aluminum, which increases its electrical conductivity slightly but whose physical properties are unaffected by temperatures in excess of 200°C. For these conductors, the mechanical load is carried almost entirely by the steel core. For the ACSS/TW construction, the aluminum strands are formed to produce the required trapezoidal shape that increases the aluminum cross section without increasing the overall diameter.
Many electric utilities have used these two constructions, which have an extensive history of good experience. The main limitation on these constructions is a relatively low strength and modulus that limit use in regions where heavy ice loads occur. This limitation may be overcome to some extent by using a larger steel core in the ACSS/TW.
The ZTACIR is commercially available from Japan where it has been widely used. There has been little field experience in the United States, but extensive laboratory test data are available on both the Invar steel core and the zirconium aluminum alloy wire material. There are no special problems with installation and termination of the conductor, but Invar steel is somewhat weaker than conventional steel core wire, which limits its use in high ice and wind load areas. Operating temperatures are in the range of 150°C to 200°C (302°F to 392°F).
The GTACSR construction, designed for operation at 150°C, uses trapezoidal-shaped high-temperature aluminum alloy strands with a grease-filled gap over the steel core. Commercially available from Japan, there has been limited field experience at National Grid installations in England, Korea and Japan. Extensive laboratory data and detailed installation instructions are available. The installation of this conductor is more complex and labor intensive than ACSR, requiring special semi-strain-type suspension fittings for long lines.
The ACCR, for operation at 200°C, employs high-temperature zirconium alloy strands over an alumina fiber composite, high conductivity, low thermal expansion core. It is available in limited quantities from the 3M Company where extensive tests have been performed on several sizes in the laboratory. There has been limited field testing in the United States where installation has required careful handling and the use of special large blocks.
The selection process for HTLS replacement conductor is unique to each line uprating, but the most important issue usually involves sag as a function of temperature. For example, considering an existing line with the 795 kcmil ACSR “Drake” installed in a 1000 ft (305 m) ruling span to an initial unloaded tension equal to 20% of its rated breaking strength at 60°F, the initial tension is 15,300 lbs (6940 kg) under maximum ice/wind loading. The everyday initial ruling span sag of 21.8 ft (6.6 m) will increase to 25.7 ft (7.8 m) over the life of the line as a result of normal creep elongation.
At the original maximum conductor temperature of 100°C, the ruling span sag of Drake ACSR is 31.7 ft (9.7 m). To avoid having to raise the existing structures, the sag of any HTLS replacement conductor also will be limited to 31.7-ft sag at its maximum temperature. Using the standard ambient conditions of a wind speed of 2 ft/sec, an air temperature of 40°C and solar heating for summer at noon, the “Drake” conductor has a thermal rating of 990A for a conductor temperature of 100°C. The sag behavior of HTLS replacement conductors are compared in Fig. 3, where each of the HTLS alternatives has the same diameter as the “Drake” conductor and the same unloaded sag at 60°F. As the figure shows, the ACCR conductor, with its very low thermal elongation, attains the highest operating temperature of 190°C (370°F), corresponding to a rating of 1550A. ACSS/TW, with its greater aluminum cross sectional area, reaches the maximum sag at 120°C (248°F), corresponding to a rating of 1270A. TACIR reaches maximum sag at 132°C (270°F), which corresponds to a rating of 1220A and the GTACSR reaches the sag limit at 124°C (255°F) for a thermal rating of 1170A.
For all of these replacement conductors, the conductor temperature at the maximum sag of 31.6 ft is well below the continuous operating temperature limit. The thermal-rating comparison could be different if the line was not clearance limited or limited to a higher sag. Similarly, the results could be different if the final everyday sags of the HTLS conductors were different because of differences in vibration damping or because of structure tension load limits. In any uprating application, however, the use of HTLS conductor offers the possibility of a large increase in thermal capacity for minimum capital investment.
An IEEE Panel Session on “Applications and Economics of New Conductor Types” was held in Vancouver, British Columbia, in July 2001. After the panel session, EPRI funded a brief study of high-temperature, low-sag conductor alternatives. The data in this article summarize the presentation by Douglass and the EPRI project report managed by Edris.
Dale Douglass received the PhD degree in 1967 from Carnegie Mellon University. He has held positions at Boeing, Bell Labs and Kaiser Aluminum and Chemical Corp. At Kaiser, he was responsible for the development and engineering application of bare and insulated power conductors, continuing this work at Power Technologies Inc. until 1999 when he joined Power Delivery Consultants Inc.
Abdeal-Aty Edris received the BS degree from Cairo University, the MS degree from Ain-Shams University in Egypt and the PhD degree from Chalmers University of Technology in Sweden. He has worked for ABB in Sweden and the United States in the development and application of reactive-power compensators and high-voltage dc transmission systems. He joined EPRI in 1992 as manager of FACTS Technologies and is presently technical leader of Transmission and Substation Asset Utilization Target of EPRI's Science and Technology Development.