CA2423215A1 - Carbon-core transmission cable - Google Patents

Abstract

A high-voltage transmission cable having a carbon fiber core. The outer conductor is aluminum. The carbon core is enshrouded in a sheath to prevent the formation of a galvanic cell at the aluminum-core interface. The carbon core transmission cable has an invariant sag, is operable at greater ampacity and greater temperature than an ACSR cable of comparable size, and is a cost-effective replacement for conventional ACSR cables, as a means of increasing the power that can be distributed over the existing power transmission grid.

Description

CARBON-CORE TRANSMISSION CABLE
BACKGROUND INFORMATION
FIELD OF THE INVENTION
[0001] The field of the invention relates to electrical overhead transmission cable.
More particularly, the invention relates to very high-voltage transmission cable.
DESCRIPTION OF THE PRIOR ART

[0002] The conventional overhead transmission line conductor or cable currently in use in 95°~ of the transmission lines used in the United States and Europe is an Aluminum Conductor Steel Reinforced (ACSR) cable. With most ACSR cable, the aluminum outer conducting layer and the steel inner core share the structural load, with the load-bearing ratio of aluminum to steel varying nominally between 25175°~ and 50150°r6, depending on the cable configuration. There are numerous cable configurations which have been designed to offer a wide range of structural and electrical capabilities. Each configuration has a steady-state thermal rating, which is the maximum allowable temperature, and an ampacity rating that represents the maximum allowable continuous current canying capacity of the cable for that steady-state thermal rating. Typically, most ACSR cable is rated for operation at a maximum steady-state temperature of 75 C. For example, the Drake, a commonly used ACSR transmission cable, has an ampacity rating of 907 Amps for a steady-state thermal rating of 75°C. At times of peak demand, the utilities are allowed to operate their transmission cables at emergency temperatures above 75 C for only short periods. Careful consideration is given to not allow a transmission cable to remain at elevated temperatures for extended duration. Not only will the cable experience additional line sag, which may present a danger of arcing to ground, but the structural properties of the aluminum andlor steel may also degrade. Table 1 below shows the ampacity ratings for several conventional transmission cables used in the field.
Type Size Strand Diameter Weight StrengthAmpacity kcmil* (AIlSteel)in Ibl10001t Ib amps ACSR/Drake 795 26/7 1.108 1093 31,500 907 (75C) ACSR/Bluebird2156 84/19 1.762 2508 60,300 1623 (75C) AAC/Lilac 795 61/- 1.028 746 14,300 879 (75C) AAC/Sagbrush2250 91/- 1.729 2128 37,500 1612 (75C) Table 1 ACSR - Aluminum Conductor Steel Reinforced AAC - All Aluminum Conductor * - Standard unit of aluminum cross section [0003] These overhead transmission cables are strung on towers and stretch along transmission corridors that crisscross the countryside and form the power distribution grid. Today, utility providers face the dilemma of an ever increasing demand for power along with fierce opposition to any plan to expand the existing transmission grid by adding new corridors. One solution would be to increase the loads that are pushed along the transmission cables as a way of providing greater electrical power using the existing transmission grid. The problem with that solution is that, as the current flow increases along a conductor, so do the resistive losses, with the result that the conductor heats up to higher temperatures. As indicated above, the allowable operating temperature of a particular size of cable may not exceed the steady-state thermal rating, except for brief periods. With both steel and the aluminum, the sag on the transmission line resulting from greater temperatures may be great enough to present the danger of arcing to the ground. By way of example, based on an experimental value of the coefficient of thermal expansion of the steel in the ACSR Drake cable at 85°r6 of the theoretical strength of the steel, the drake cable has a line sag of 21.3 ft/1000 ft at 23 °C, 29.5 ft at 200 °C, and 48.5 ft at 262 °C. Thus, the amount of power that can safely flow across a transmission conductor at any given voltage is limited.

(0004] Another solution to providing greater amounts of electric power over the existing transmission grid is to replace the existing ACRS cable with cable that is operable at higher temperatures and with invariant line sag. The new cable would, however, also have to be cost-effective, that is, not be more costly than the cost of using the conventional cable in an expanded transmission grid.

(0005] What is needed therefore, is a high-voltage transmission cable that is operable at higher temperatures and yet has an invariant line sag. What is further needed, is such cable that provides increased ampacity. What is yet further needed, is such cable that provides a significantly greater strength-to-weight ratio.
Finally, what is needed is such cable that is eoonomicai to use as a replacement for the conventional high-voltage transmission cable.
BRIEF SUMMARY OF THE INVENTION

[0006] For reasons stated above, it is an object of the present invention to provide a high voltage transmission cable that has an invariant line sag. It is a further object to provide such a cable that provides increased ampacity. It is a yet further object to provide such a cable that is operable at higher temperatures. it is a still yet further object to provide such a cable that has a better strength-to-weight ratio and that is a cost-effective replacement for the conventional ACSR cable.
(O~TJ The objects are achieved by providing a carbon-core (C-C) transmission cable according to the invention comprising a carbon core and an aluminum conductor.
The aluminum conductor is similar in structure and material to the aluminum conductor of a conventional high-voltage transmission cable. The carbon core may be a braided rope, a core of unidirectionally aligned fibers, or a core of carbon composite rods.
Either a fiber coating or a core sheath, such as KEVLAR ~, is recommended to prevent the formation of a galvanic cell at the carbon-aluminum intertace.
[0008] Carbon has an extremely small coefficient of thermal expansion. The use of a carbon core in a transmission cable enables steady-state operation at temperatures far above currently allowable temperatures. The principal temperature-limiting consideration in a transmission grid structure using C-C cable is the effect on line hardware of operating at elevated temperature. Generally, the temperature limit for existing line hardware components is considered to be 220°C. The "Drake" cable, one of the most prevalent conductors in the field today, was chosen for use as a reference baseline cable during the development and assessment of the C-C cable according to the invention. The benefits of increasing the ampacity of the transmission cable are shown in the graph shown in FIG. 9, using the Drake cable as an example. By increasing the operating temperature from 75°C to 200°C , the rated ampacity of the cable increases by a factor of 1.8, and by increasing it to 300°C, the ampacity increases by a factor of 2.2. As stated previously, the steel core cable (Drake) cannot be operated safely and/or effectively at these elevated temperatures.
[0009] The carbon core in the C-C cable according to the invention includes various architectural configurations of carbon fiber. The core may be made of a braided rope of carbon filament, a rope of longitudinally-aligned carbon fibers, or carbon composite rods comprising carbon fibers fixed in a high temperature matrix. A high-temperature, high-performance polymer, such as PEEKT"", is a suitable matrix material. Carbon filament has a very high tensile strength, much greater than that of steel, but is relatively weak against diametric shear. Thus, it is important in the construction of the carbon core, in order to obtain the highest possible strength characteristics of the carbon core, that the carbon filaments be twisted as little as possible during the processing to make the core and in operation. Some twist is necessary, as the cable must have some ability to flex so that it can be wound on a spool.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 illustrates a first embodiment of the C-C cable according to the invention.
(0011] FIG. 2 illustrates a second embodiment of the C-C cable according to the invention.
(0012] FIG. 3 illustrates a third embodiment of the C-C cable according to the invention.
[0013] FIG. 4 is an illustration of the carbon core braid.
[0014] FIG. 5 is an illustration of the carbon core rope of FIG. 2, covered with the sheath.
i0015j FIG. 6 illustrates the CTE test setup.
(0016] FIG. 7is a graph of the results of the CTE test on carbon core braid and carbon core rope.
[0017] FIG. 8 is a graph of the results of the RBS test on the carbon core braid and the carbon core rope.
.(0018] FIG. 9 is a graph illustrating the increased ampacity of a conductor operating at higher temperatures.

DETAILED DESCRIPTION OF THE INVENTION
[0019] FIGS. 1 to 3 illustrate various embodiments of the present invention.
FIG. 1 illustrates a C-C cable 10 comprising an outer conductor 16, and a braided carbon core 12. FIG. 2 illustrates a C-C cable 20 comprising the outer conductor layer 16, a sheath 14, and a carbon core rope 22 that comprises minimally twisted strands of carbon filament rope, wherein the carbon filaments are substantially longitudinally aligned. FIG.
3 illustrates a C-C cable 30 comprising the outer conductor layer 16 and a carbon composite core 32 comprising rods formed of carbon filaments embedded in a matrix.
The rods are structurally aligned and have been rigidized by the matrix. The matrix used in this embodiment is the polymer PEEK"', although it is within the scope of the invention to use other suitable substances as matrix material. It is noted that, for test purposes, the C-C cable 10 and C-C cable 30 were assembled without including the sheath 14, as shown in FIGS. 1 and 3, but that, when the respective cable is manufactured for actual use in the field, the core is enshrouded in the sheath 14. As a general note, reference designations remain the same for identical components used in the various embodiments.
[0020] The outer conductor 16 in the embodiments shown is typically an aluminum conductor of the type used for high-voltage transmission lines. The sheath 14 is shown as a woven or wrapped sheath, the purpose of which is to prevent the formation of a galvanic cell at the area of contact between the carbon and aluminum. A
suitable material for the sheath is KEVLAR~. It is understood that other suitable material may be used for the sheath, instead of the KEVLAR~.
[0021] The carbon core braid 12 shown in FIG. 4 was fabricated from a high modulus, commercial grade PAN (polyacrylonitride) based carbon fiber from Zoltek, Panex 33~, with a 48K-tow filament. A "tov~'° is an assemblage of filaments, often referred to as a °yarn." This fiber was used to produce a 12-strand, single braid rope.

In order to produce a rope of representative size duplicating the Drake baseline reference conductor, it was necessary to evaluate the cross sectional area with multiple tows. Small sample lengths were hand twisted to various levels of twists per inch (tpi).
[0022] First, single ends, then multiple ends were combined, twisted, and measured.
In producing rope and cordage with high modulus synthetic fibers a certain amount of twist was necessary to balance the strand length and align the tows to maximize the tenacity of the fiber. There is loss in strength when converting fiber filaments into strands and then finally into a rope due to handling the fiber during processing. The amount and direction of twist will generally minimize the loss caused by handling the fiber. It was determined by these tests that a small tpi yielded the best results.
(0023) Once the twist level in combination with the resulting tow size was selected, fibers were set-up on a small flier arm twister. In order to minimize the fiber fly and surface abrasion, as well as adding moisture resistance, the fibers were pulled through a coating bath containing a water-based polymer. Soft silicone dies were used to regulate the amount of coating retained. In order to balance a braided structure, half of the individual tows, which will make up the braid, are twisted in each direction, clockwise and counter clockwise. During the twisting process a single long length bobbin in each direction was produced. The length of the fiber on each twister bobbin was such that it can be rewound onto individual braider bobbins of each direction. Each twister bobbin was then brought to a winding station for transfer onto the braider bobbins. The braider used has a carrier pattern similar to a maypole dance configuration. The carriers hold the individual bobbins of twisted and wound yarn, and are designed to allow the payoff of the yarn under tension by having the yarn traverse over a series of spring loaded sheaves.
(0024) The speed of the carriers and the speed of the capstan that pulls out the rope are regulated by the combination of gears selected and used in the braider.
The resulting braid angle of the carbon fiber rope, i.e., the angle between the fibers as they pass back and forth, was designed to allow the individual tows within the bundle to align parallel with the centerline of the rope. The fiber alignment was critical in producing the highest possible conversion efficiency for the filaments. Figure 4 shows the final product of the carbon core braid.
[0025] The resulting carbon core braid had a total of 2 towlstrand (45,700 fibersltow) x 12 strands for a total of 1,096,800 carbon fibers. Approximately 150 ft of the carbon core braid was fabricated for subsequent test and evaluation.
[0026 The carbon core rope 22 was fabricated from a high modulus (HM) commercial grade of Amooo T300 grade 12K tow carbon fiber. The design concept of the carbon core rope 22 employed a unidirectional fiber reinforcement architecture.
Having previously evaluated a larger range of fiber twists, a .25 tpi was selected to increase the translation efficiencies. The fibers were run through the same coating bath as mentioned above with the carbon core braid 12 and twisted on a flier arm twister.
The bobbins were positioned on a ladder creel unit with individual tensioning, for constant back drag. The ends were pulled together through a central round die and fed up into the center of a 24-carrier braider. The fiber bundles were aligned uni-axially to duplicate the cross sectional area of the Drake steel core. To facilitate the compaction of the unidirectional carbon fiber core, a layer of Kevlar was braided over the core to compress the fibers to a nominal 609~b fiber volume. KEVLAR ~ 29, approximately 7500 denier per end, was utilized. Finally, the carbon core rope 22 was pulled up into the braid by the Kevlar to produce a double braid with a parallel core of HM
carbon fiber.
The parallel core was proposed as a method of further increasing the strength of the rope by not passing the ends over and under one another, which increases the shear and subsequently reduces the axial tensile load bearing capability. Figure 5 shows the final product of the uni-directional carbon core with a KEVLAR~ sheath. The resulting unidirectional carbon core rope 22 had a total of 7 towlstrand (12,000 fibersltow) x 10 s strands for a total of 840,000 carbon fibers. Approximately 150 ft of the unidirectional carbon core rope 22 was fabricated for subsequent test and evaluation.
(0027 In order to document the strength, weight and temperature characteristics of the carbon fiber core, the inventor conducted a series of strength and thermal expansion tests of trial samples of the carbon core braid 12 and the carbon core rope 22. The inventor selected two commercially available brands of carbon filament for use in the cores. Table 2 below shows the material properties for the various types of cable core materials, including steel, aluminum, and two brands of carbon filament.
As can be seen in Table 2, both types of carbon fiber material have a significantly higher ultimate strength than that of the steel or aluminum. Also shown is the coefficient of thermal activity for the steel, the aluminum, and the two carbon materials. The PANEX
33~ is manufactured by Zoltec and the T-300 by Amoco.
Density Modulus Ultimate StrengthCTE Resistivity Material Ib~n Mpsi ksi 1~c mmlmmlqC ~c .fin Aluminum 0.098 10.0 13.9 25.0 7.1 Steel 0.282 29.0 254.5 12.9 210.5 PANEX 33~ 0.064 33.5 529.4 -0.6 4572.0 T-300 0.065 33.1 551.1 -0.6 3937.0 Table 2 [0028] Tests to determine the coefficient of thermal exansion (CTE test) of several configurations of the carbon core material were conducted. FIG. 6 illustrates the CTE
test setup for the carbon core braid 12. The tests incorporated a number of different preload and applied tension values. These values were atl determined as a percentage of the rated breaking strength (RBS) of the carbon core. The heat load was applied to the carbon core braid 12 by heating the aluminum pipe with a DC current. A
typical test cycle is summarized as follows:

~ Preload applied to specimen (0°~, 30°~, 50%) ~ Applied load applied to specimen (15°r6, 20°r6, 25°~) ~ Temperature is raised to 200°C while applied load is maintained ~ At 200°C temperature is maintained for 15 minutes ~ Specimen is allowed to cool to 70°C
~ At 70°C temperature is raised to 200°C
~ At 200°C temperature is maintained for 15 minutes ~ Specimen is allowed to cool to ambient [0029] The data for the carbon core braid 12 was reduced by first averaging the two end thermocouples and the center thermocouple on the rope. The strain was calculated using the laser transducer data and dividing by the length of carbon core braid 12 in the aluminum-heating pipe. Then, the strain versus temperature was plotted for each heating and cooling cycle and a trend line was developed for each cycle. A
typical strain-temperature plot is shown in Figure 7. The complete CTE results are listed in Table 3. The average CTE for braided rope was -1.345 mm/mml°C.
Test # Cool Cycle Heat Cycle Coof Cycle Average #1 #2 #2 1~ mm/mm/iC 1~u mmlmml~ i,u mmlmml9G 1,u mmlmmlqC

1 -1.5 -1.9 -1.0 -1.48 2 -1.3 -1.4 -1.4 -1.37 3 -1.3 -0.3 -0.9 -0.86 4 -2.2 -0.9 -1.8 -1.67 ~ No Data ~ No Data I - - No Data No Data I

Table 3 [0030] Rated Breaking Strength (RBS) of the carbon core: Tow and strand tests were performed to determine fundamental strength characteristics of the carbon fiber.
The tests were performed both dry (without a matrix) and with a matrix (epoxy) to determine the effect of a matrix material on shear load transfer between fibers. Both the carbon core braid 12 and the unidirectional carbon core rope 22 were tested to determine their respective RBS. Samples were potted in open wire rope spelter sockets using West System Resin and Epoxy 105/205, two-part system. The open spelter socket was then pinned in a 1 in thick steel plate. A typical core test consisted of preloading the sample to 100 Ibf and then cycling the samples to 10% of RBS, three (3) times. The core was then tested to failure under displacement control with a load rate of 0.1 in/min for the braided rope and 0.05 in/min for the unidirectional core.
[0031] The results of the tow test determined an average dry strength of 133 Ib and epoxied strength of 324 Ib. The results for a dry seven (7) tow strand was 934 Ib. The complete results are shown in Table 4. Plots of the load-deflection curves for the RBS
tests of the carbon fiber core are shown in Figure 8. The results of these tests show the carbon core braid 12 was about' the stiffness of the unidirectional carbon core rope 22. This difference in stiffness is due to the braid architecture. The average RBS of the carbon core braid 12 was 7,450 Ibf and the unidirectional carbon core rope 22 was 7,440 Ibf. The results of the test are shown below in Table 4.
Test Material RBS RBSITheory RBS

ibf ratio Braided Rope #1 PANEX~9 33 7,400 0.194 Braided Rope #2 PANF,C~ 33 7,510 0.197 Unidirectional RopeThomel~ T-300 7,110 0.268 #1 Unidirectional RopeThomel~ T-300 7,840 0.296 #2 Unidirectional RopeThomel~ T-300 7,360 0.278 #3 Average Tow (Dry) Thomel~ T-300 106 0.329 Average Tow (Epoxy)Thomel~ T-300 324 0.856 Average Strand (Dry)Thomel~ T-300 934 0.353 Table 4 (0032] The theoretical RBS values are derived from the manufacturer's published ultimate strength value and normalized for the number of carbon fibers present in each core. The results of the RBS tests show a reduced strength without the use of a matrix material in the tow tests. Tow tests with the use of a matrix material tested to 85°~ of the theoretical fiber strength, whereas the dry tow and strands tested to 33%
and 35%
of the theoretical fiber strength, respectively. Results of the full core tests show the unidirectional dry carbon core rope 22 failed at 28°r6 of the theoretical fiber strength, and the dry carbon core braid 12 failed at 196 of the theoretical fiber strength.
The additional decrease in strength, as compared to dry tow data is likely attributable to fiber damage during the additional processing of the carbon core braid 12 during braiding. It should be noted that for the two trial samples listed above, the actual volume fraction of carbon fiber was 51.5°~ for the carbon core braid 12 based on a core diameter of 0.4135 in and 69.3°r6 for the unidirectional carbon core rope 22 based on a diameter of 0.3035 in. The actual Drake ACSR cable has a steel core diameter of 0.408 in and a steel volume fraction of 24.3°~b. In order to make direct comparisons in the following sections, the carbon core's diameter and volume fraction are assumed to be equal with that of the steel core of the Drake.
(0033] Using the experimental value of CTE, predicted sag values were determined.
Two comparisons were performed, one at the upper limit (85~b) and the lower limit (35°r6) of the theoretical core strength. In accordance with standard practices, the sag calculations assume 2096 of the RBS for an initial tension and fixed end supporks. At the upper limit in Table 5, the carbon core conductor at 75°C shows approximately 2I3 less sag than the ACSR. At 200°C the sag is approximately 6.6 times less. Over 262°C
at the upper limit of the core, the carbon core rope would break. This is due to the high initial load applied at 20°r6 of 45,100 Ib or 9020 Ib. These results show that the traditional ACSR conductor sag increases at elevated temperatures, whereas the carbon core conductor sag actually decreases (invariant sag).
Conductor Weight RBS Sag ~ 23C Sag ~ 75C Sag ~ 200C Sag ~ 262C

Type Ibl1000r1"Ib R ft ft f~

ACSR/Drake1093 31,50021.3 29.5 43.1 48.5 ACCF 811 45,10011.2 10.0 6.1 2.3 Table 5: Upper Limit [0034] At the lower limit the carbon core conductor at 75°C has approximately 10%
less sag than the ACSR. At 200°C and 300°C, the carbon core has approximately 36°~
and 50°r6 less sag, respectively. See Table 6. The lower limit construction of carbon conductor at 300°C is still within allowable strength limits. This is due to the low load applied at 20°~ of 18,600 Ib or 3720 Ib. This also shows that, as the sag on the traditional cable increases, the sag on the carbon core cable decreases.
Conductor Weight RBS Sag (~ Sag ~ 75C Sag (~ 200CSag ~ 300C

Type Ita11000ftIb ft f~ t~ ft ACSRIDrake1093 31,50021.3 29.5 43.1 51.5 ACCF ~ 811 ~ 18,60027.3 ~ 26.8 ~ 25.6 ~ 24.6 ~

Table 6: Lower Limit (0035] In addition to the thermal behavior, the carbon core also exhibits a lower overall conductor weight per unit length. This is because the carbon core is 4.4 times lighter that the steel core. This translates to a 26°~6 weight savings in the Drake transmission cable and a strength-to-weight ratio that is potentially 2 times greater than that of steel.

(0036] It is understood that the embodiments described herein are merely illustrative of the present invention. Variations in the construction of the C-C cable may be contemplated by one skilled in the art without limiting the intended scope of the invention herein disclosed and as defined by the following claims.

Claims (19)

1. A high-voltage transmission cable comprising:
an aluminum conductor, an electrically insulative sheath, and a carbon core, wherein said aluminum conductor surrounds said sheath and said sheath surrounds said carbon core.

2. The transmission cable of claim 1, wherein said sheath is made of a material capable of withstanding an operating temperature greater than 150 degrees C.

3. The transmission cable of claim 2, wherein said sheath is made of PTFE.

4. The transmission cable of claim 2, wherein said sheath is made of a material from the group consisting of poly-paraphenylene terepththalmide, poly p-phenylene, aramid fiber, and combinations thereof.

5. The transmission cable of claim 2, wherein said sheath has a low coefficient of friction and provides a slip plane to reduce wear between said aluminum conductor and said carbon core.

6. The transmission cable of claim 1, wherein said carbon core comprises a carbon-fiber reinforced composite rod.

7. The transmission cable of claim 6, wherein said carbon-fiber reinforced composite rod comprises carbon fiber pultruded in a high-temperature polymeric material.

8. The transmission cable of claim 6, wherein said high-temperature polymeric material includes materials from the group consisting of thermoset polymers, thermoplastic polymers, and combinations thereof.

9. The transmission cable of claim 6, wherein said carbon core includes a plurality of said carbon-fiber reinforced composite rods.

10. The transmission cable of claim 9, wherein one or more of said rods are substantially trapezoidal in shape.

11. The transmission cable of claim 6, wherein said carbon core is a bundle of said plurality of said carbon-fiber reinforced composite rods, and wherein said rods are twisted slightly axially.

12. The transmission cable of claim 6, wherein said plurality of said carbon core is a bundle of said plurality of carbon-fiber reinforced composite rods, and wherein said rods are axially aligned.

13. The transmission cable of claim 1, wherein said carbon core comprises a braid of dry carbon fibers.

14. The transmission cable of claim 1, wherein said carbon core comprises a rope of unidirectionally aligned dry carbon fibers.

15. The transmission cable of claim 1, wherein said aluminum conductor includes a plurality of aluminum rods.

16. The transmission cable of claim 15, wherein said plurality of aluminum rods are twisted slightly relative to an axial direction of said cable.

17. The transmission cable of claim 15, wherein said plurality of aluminum rods are wrapped axially about said core and said sheath.

18. The transmission cable of claim 1, wherein said aluminum conductor is a sectioned aluminum coating over said sheath and said carbon core.

19. The transmission cable of claim 18, wherein said sectioned aluminum coating is applied over said sheath and said carbon core.