Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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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.
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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.
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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
CA 02423215 2003-03-20
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
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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:
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~ 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
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[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
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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
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CA 02423215 2003-03-20
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
23C
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.
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(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.
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