Note: Descriptions are shown in the official language in which they were submitted.
~3~3~
This invention relates to composite conductors of the
type used in power transmission lines and is primarily concerned
with ACSR (Aluminum Co~ductor Steel Reinorced) conductors. The
invention is, however, also applicable to all-aluminum stranded
conductors, which hit~erto have not been satis~actory in long
spans owing to creep problems inherent in their design.
The current design of ACSR conductors is such as to pro-
duce excessive sags at higher operating temperatures. Also these
conductors are sùbject to additional sag due to long term creep.
From a study of these problems it has been found by applicants
that the effects of temperature and creep can be greatly reduced
by certain changes to the conductor structure.
When an ACSR conductor is strung in a span o-E a transmission
line, the aluminum and steel strands share the resultant mechani-
cal tension in a manner determined by the aluminum to s-teel
ratio, temperature and creep. When heated by the line current,
the conductor is elongated as the aluminum and steel expand at
their respective rates, and this results in an increase in sag
and a reduction in total conductor tension. However, tests
on conventional ACSR conductors at higher operating temperatures
have shown that the measured sags are considerably greater than
one would expect from calculations by accepted methods. In the
accepted methods of sag calculation it is assumed that as the
conductor temperature increases the aluminum becomes completely
unloaded at a certain temperature, due to its having a higher
coefficient of thermal expansion than the steel, and plays no
further role byond this temperature except for its effect on the
total conductor mass. Accordingly, it is assumed that at elevate~
temperatures the conductor sag is determined entirely by the
thermal e~pansion characteristics of the steel. EIowever, tests
have shown that the stress in the aluminum does not remain zero
above the critical temperature but reverses, that is to say,
the aluminum goes into compression after the zero load condition
~ 1 --
has been reache~. The compressive load in the aluminum results
in increasec1 tensile stress in the s-teel core and so produces
additional elongation and sag. At some higher temperature the
aluminum strands ~inally de~orm by birdcaging and the internal
stresses are relieved, the compressive stress in the aluminum at
which this occurs depending mainly on the conductor structure.
In some conductors a compressive stress in the aluminum as high
as 2500 psi has been observed.
Creep in conventional ACSR conductors is a term used to
describe the long term elongation of a conductor under tension.
Naturally, any elongation will result in additional sags which
must be considered in the line design. Creep is normally
attributed to elongation of the aluminum under tension, resulting
in transfer of load to the steel which elongates elastically under
increased loads. Studies of the creep phenomena have shown that
at normal operating tensions and temperatures the tensile creep
in the aluminum is very small and does not account for the ob-
served creep in composite ACSR conductors. Applicants' ex-
periments and analysis show that the creep of ACSR conductors is
actually due to radial deformation of aluminum strands, which
deformation results in a transfer of load from the aluminum to
the steel. This deformation is the result of axial tension being
transformed into radial compressive forces between helically
wound strands and it takes place at the strand crossings at which
the load bearing areas of contact between the strands are
insufficient.
The present invention relates to composite conductor
structures based on the foregoing considerations, the structures
being such as to minimize the effects of temperature and creep
on conductor sag.
According to one aspect of the invention, in an ACSR con-
ductor having a steel core with one or more layers of helically
wound aluminum strands thereon, the aluminum strands are spaced
-- 2 --
L3~L
circumferentially from one ano-ther, thereby providing
clearances -to accommodate thermal expansion of the strands and
so prevent additional tensile loading of -the steel core and
thus reduce sags at high temperatures.
The aluminum strands may be of round cross-section, the
circumferential clearances therebetween being at least 0.005 inch,
and the steel strands may be of round or ~runcated segmental
cross-section. Alternatively, the aluminum strands may be of
truncated segmental cross-section, the circumferential clearances
threbetween being at least 0.003 inch. To minimize creep due
to radial compressive forces between the conductor strands a-t
the strand crossing points the strands are preferably of trun-
cated segmental cross-section to provide large areas of contact
at the crossing points. To set the temperature at which the
aluminum becomes unstressed a controlled amount of slack can be
provided in the aluminum strands during manufacture, and accord-
ing to the present invention this is accomplished by interposing
a deformable spacing means between the steel core and the
aluminum strands. The result is that at low temperatures when
the tension is high the aluminum bears some of the load but at
higher temperatures the aluminum birdcages and the sag is re-
duced. Creep is also reduced because the slack lowers the
aluminum strass.
According to another aspect of the invention, as applied to
an all-aluminum stranded conductor, the strands are arranged as
a plurality of concentric layers and are of truncated segmental
cross-section to provide large areas of contact at the crossing
points. In this way the radial compressive forces whiah give
rise to creep, and which have made all-aluminum conductors un-
satisfactory in the past, can be greatly reduced.
Several embodiments of the invention will now be described,by way of example, with reference to the accompanying drawings
in which:
53~
Figure 1 illustrates a portion of a first ACSR conductor
in accordance with the invention;
Figure 2 illus-trates a portion of a second ACSR conductor
in accordance with the invention;
Figure 3 illustrates a portion of a third ACSR conductor
in accordance with the invention;
Figure 4 illustrates a detail of the structure shown in
Figure 3;
` Figure 5 illustrates a portion of a fourth ACSR conductor
in accordance with the invention;
Figure 6 illustrates a detail of the structure shown in
Figure 5;
Figure 7 illustrates a portion of a fifth ACSR conductor
in accordance with the invention;
Figure 8 illustrates a portion of an ACSR conductor
similar to that of Figure 5 but having only one layer of
aluminum strands; and
Figure 9 illustrates a portion of an all-aluminum stranded
conductor in accordance with the invention.
The composite conductor shown in Figure 1 comprises a
steel core 10 of round cross-section, the core being compose~ o~
helically wound steel strands 11 which are individually of round
cross-section. Aluminum strands 12, which are of conventional
round cross-section, are helically wound in a single layer on
the steel core, the aluminum strands crossing the steel strands
at crossing points 13 which provide relatively small elliptical
areas of contact. The invention is characterized by the act
that the strands 12 are circumferentially spaced from one another
to provide clearances 14. With this structure, at low tempera-
tures the total tensile stress of the conductor is shared
between the aluminum strands and the steel strands, the tensile
stress inthe aluminum strands being reduced as the strands
elongate with increasing temperature. However, up to a certain
-- 4
~ .53~3~
temperature for which -the conductor is ~esigned the clearances
14 can accommodate elongations o~ the aluminum strands so that
they are not placed in compression as in the case of a convention-
al ACSR conductor and so do not transfer an additional tensile
load to the steel core. It has been calculated that, ~or a con-
ductor of the construction illustrated in Figure 1, the clearances
between the aluminum strands should be at least 0.005 inch to
accommodate thermal expansions up to a maximum temperature of
200C.
It will be appreciated that the structure shown in Flgure
1 may be modified by adding one or more further layers of helical-
ly wound aluminum strands, the strands of each layer being spaced
circumferentially from one another as in the innermost layer,
the layers of strands being helically wound in opposite direc-
tions alternately.
The struc~ure illustrated in Figure 2 is basically similar
to that of Figure 1, having a stranded steel core 10 with indi-
vidual strands 11 of round cross-section. The aluminum strands
15, however, are of truncated segmental cross-section, that is
to say, the cross-section is defined by a short length of an
annulus. With this construction the strand crossing points 16
provide larger load bearing areas than do the crossing points
13 of the èarlier construction, so that the radial compressive
stresses are greatly reduced and the creep prohlem is reduced.
In this case, the strands 15 are circumferentially spaced from
one another to provide clearances 14, and to accommodate dif-
ferential elongation of the aluminum strands up to a temperature
of 200C these clearances should be at least 0.003 inch.
As in the construction earlier described, this structure
may be modified by the addition of one or more layers o~ alumin~m
strands, the layers of strands being helically wound alternately
in opposite directions, and the strands of each layer being
circumferentially spaced from one another.
.
~S3434
Referriny now ~o Figures 3 and 4, -the third ACSR conductor
has a stranded steel core of round cross-section composed of
steel strands 17 which are oE truncated segmental cross-section.
The aluminum strands 15 are also oE truncated segmental cross-
section, and are helically wound in two concentric layers 18, 19,
the strands of each layer being circumferentially spaced to
provide clearances 14. It will be appreciated that the structure
may be modified by the addition of further concentric layers of
aluminum strands, or by the provision of just a single layer 18.
This embodiment of the invention is characterized by the fact
that the strands 15 of the innermost layer 18 are formed with
int:egral deformable pointed projections 20 e~tending radially
inwardly from their inner surfaces and bearing on the outer
strands 17 of the steel core. These projections 20 act as de-
formable spacing means interposed be~ween-the steel core and
the aluminum strands, to provide a certain amount of spacing
prior to installation of the conduc~or, the projections yielding
under compressive forces when the conductor is suspPnded and so
furnishing a controlled amount of slack in the aluminum strands.
The reason for providing the deformable spacing means is to con-
trol the initial slack which affects the load distribution of the
conductor. With a controlled amount of slack the aluminum will
bear some of the load at low temperatures but will birdcage at
higher temperatures. The thermal expansion after birdcaging
temperature occurs only at the rate of the steel and so sags are
reduced. Reduced creep is an added benefit of the lower aluminum
tension.
Figure 5 illustra ~s a modification of this structure, in
which the steel core is composed of strands 11 of round cross-
section, as in the earlier embodiments, and in which the helicallywound aluminum strands 15 are of truncated segmental cross-
section, the strands of each layer being circumferentially spaced
from one another to provide the clearances 14. In this
-- 6 --
31 ~53~
cons-truction, the s-trands of -the innermost layer 18 are formed
on their inner faces wi-th integral longitudinally extending
ridges 21, which act as deformable spacing means exactly as the
projections 20 (Fig. 4) of the previous embodiment. Figure 6
shows a detail o~ this cons-truction, in which a strand 15 of
the innermost layer 18 is formed with the deformable ridges 21
which bear against the outer strands 11 of the steel core at
crossing points 22.
Figure 7 illustrates yet another construction which is very
similar to that of Figure 5, having a steel core of round cross-
section with strands 11, and at least one layer of helically
wound aluminum strands 15 which are spaced circumferentially
from one another to provide the clearances 14. In this con-
struction, however, instead of projections or ridges being formed
on the inner faces of the aluminum strands, a layer of plastical-
ly deformable material 22 is interposed between the steel core
11 and the aluminum conductor strands 15. The material 22 can
be practically any material which will serve as a spacer initial-
ly and yield to radial loads so as to provide the required
amount of slack in the aluminum strands, polyvinyl chloride
being a suitable material which is readily available.
The structure shown in Figure 8 is essentially the same as
that of Figure 5, the only difference being that the aluminum
strands are arranged in a single layer 18.
Figure 9 shows an all-aluminum conductor of round cross-
section consisting of a plurality of concentric layers of
aluminum strands 23. The aluminum strands 23 are of truncated
segmental cross-section. The central strand 24 is of circular
cross-section. It will be seen that the strands of each layer
engage the strands of the adjacent layers over large areas of
contact, thereby minimizing the radial compressive loads and so
minimizing creep problems. The strands are, of course, helically
wound the helices of adjacent layers being of opposite hand.
7 --
