U.S. patent number 3,666,879 [Application Number 05/082,248] was granted by the patent office on 1972-05-30 for power cable.
This patent grant is currently assigned to International Standard Electric Corporation. Invention is credited to Christian Worm Hirsch, John Normann Johnsen.
United States Patent |
3,666,879 |
Hirsch , et al. |
May 30, 1972 |
POWER CABLE
Abstract
The geometric dimensions and power losses of a power cable are
maintained constant along its length, notwithstanding changing
ambient thermal conditions. This is made possible by changing the
conductor material at different locations, for example, copper to
aluminum, having different thermal resistivity. The sections of
different conductor material are joined by conventional techniques
such as welding, soldering, etc. Copper is used at the ends to
simplify splicing.
Inventors: |
Hirsch; Christian Worm
(Lysakfr, NO), Johnsen; John Normann (Oslo,
NO) |
Assignee: |
International Standard Electric
Corporation (New York, NY)
|
Family
ID: |
26647376 |
Appl.
No.: |
05/082,248 |
Filed: |
October 20, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Feb 6, 1970 [NO] |
|
|
417/70 |
Nov 8, 1969 [NO] |
|
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4437/69 |
|
Current U.S.
Class: |
174/128.1;
174/126.1; 174/126.2; 174/130 |
Current CPC
Class: |
H01B
7/14 (20130101); H01B 9/006 (20130101) |
Current International
Class: |
H01B
9/00 (20060101); H01b 005/08 () |
Field of
Search: |
;174/126R,126CP,128,130,11R,25R,94 ;338/330 ;333/99S |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Dummer et al., Wires & R.F. Cables, Pitman, 1968, p.
129.
|
Primary Examiner: Goldberg; E. A.
Claims
What is claimed is:
1. A power cable subjected to varying thermal characteristics along
a route comprising a plurality of adjoining parallel longitudinal
electrical conductors each having longitudinal sections of aluminum
and copper jointed in series, said sections each having a
predetermined length and disposition selectively matching the
changes in the thermal characteristics along said route to maintain
said conductors below a predetermined maximum temperature and to
minimize power loss therealong, and said conductors having constant
cross-sectional dimensions along their length.
2. A power cable according to claim 1, wherein a substantial
portion of a length of each said conductor is made of aluminum and
the end portions thereof are made of copper.
3. A power cable according to claim 2, wherein said conductors are
multistrand conductors arranged in layers.
4. A power cable according to claim 3, wherein said plurality of
conductors include the same number of wires throughout the cable
length.
5. A power cable according to claim 4, wherein the geometrical
cross-section and shape of each of the wires are substantially
uniform throughout the cable length.
6. A power cable according to claim 5, wherein the cross-section of
the cable comprises aluminum wires and copper wires selectively
arranged in inner and outer layers along different longitudinal
sections to provide gradual changes of material along the length of
cable.
7. A power cable according to claim 6, wherein said aluminum wires
are in inner layers and said copper wires in outer layers.
8. A power cable according to claim 6, wherein the joints of
adjacent wires in a multistrand layered conductor are spaced apart
within a predetermined length of the cable.
9. A power cable according to claim 8, wherein said joints of
adjacent wires around the circumference of said cable are
stagger-ed to provide an interleaved pattern along the length of
the layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power cables which are
prefabricated to be installed along a certain cable route and
particularly to the maintenance of relatively constant properties
therealong.
2. Description of the Prior Art
The current carrying capacity of power cable is determined by
factors such as the properties of the conductor, of the insulation
and of the cable surroundings. The most critical of these factors
are the properties of the insulation, in particular, its abilities
to withstand heat. The cable insulation will in most cases lose its
original insulating properties if it is subjected to excessive heat
for any period of time. The heat affecting the insulation is
determined by the conductor resistance and the heat dissipation
properties of the cable insulation and those of the surroundings. A
particular cable installed at a particular place must therefore not
be loaded to such an extent as to cause deterioration of the
insulation. In order to be on the safe side, overdimensioning of
cable conductors are quite common.
Power cables are usually designed so that the power losses -- i.e.,
losses in the conductor, dielectric losses in the insulation,
sheath losses etc. are kept constant along the cable at constant
current and constant voltage. However, there are two factors which
may cause higher temperature than desirable if measures are not
taken for compensation of such losses. First, the heat dissipation
properties of the surroundings may vary considerably along a cable
route, causing increased cable temperature at places where the heat
dissipation properties are poor. Second, the conductor temperature
may increase at places such as joints where two cable lengths are
joined together. Problems caused by these two factors shall be
considered in detail below.
The thermal resistivity of the surroundings depends on whether a
cable is laid in sand, soil, clay, water, ducts etc. Such varying
conditions are taken into account when designing a cable
installation, allowing heavier loading of a cable having good heat
dissipation possibilities than of one having poor heat dissipation.
However, often the thermal resistivity of the surroundings will
vary along the cable route. When in such cases the same type of
cable with the same dimensions, especially, the same conductor
cross-section and material, is used along the whole cable route,
the conductor temperature will vary along the cable. Since the
maximum conductor temperature must not exceed a certain limit
depending on the type of the cable, the operating voltage etc.,
these conditions must be taken into account during the dimensioning
of the cable.
The part of the route having the poorest heat dissipation
properties will therefore be the critical factor when determining
the cable dimensions. As a result of this, the part of the cable
passing through areas with better heat dissipation properties will
have a lower conductor temperature than permissible, because these
parts are not loaded to their full current carrying capacity. Such
overdimensioning is usually very expensive. This is especially the
case for long submarine cables, where most of the cable is exposed
to very favorable thermal conditions -- namely the part of the
cable which is lying in the water -- while only a small part at
each end of the cable is in the ground under relatively poor
thermal conditions and therefore being the critical factor for
dimensioning the whole cable.
In order to avoid poor utilization due to variation of the heat
dissipation properties along the cable route, it has been suggested
to alter the conductor cross-section by joining together cable
lengths with different cross-sections by means of special cable
joints. This procedure is normally elaborate and costly,
particularly in the case of an oil-filled cable. Further such
special joints are normally undesirable for technical and
electrical reasons especially in connection with submarine cable.
Thus, however carefully a joint is designed and executed, the
dielectric strength of the cable at or near the joint will in many
cases be somewhat lower than that of the insulation in the rest of
the cable. Furthermore, due to the added wall of insulation
required at a joint, the conductor may often run hotter in the
middle of a joint than in the rest of the cable.
In order to obtain the best possible cable installation, a cable
should be designed such that the operating temperature on the cable
insulation at all places is just below the maximum allowable
temperature. It might, however, be desirable, at joints, to specify
a somewhat lower temperature.
Conductors for insulated cables in the past normally have been made
with copper because of its high conductivity of electric current
and low power loss capability. However, in more recent years, its
high cost has prompted a change to less expensive aluminum in spite
of the fact that its conductivity is only about 60 percent of that
of copper. But the change from copper to aluminum for certain types
of applications, such as cables for high voltages has been slow.
This has been due to certain disadvantages of aluminum as compared
to copper, such as its higher thermal coefficient of expansion and
more complicated procedure for the joining of conductors.
For cables which are to be directly buried, the above disadvantages
are not of significance, since a buried cable is confined so that
the heating of the cable during loading cannot result in a
longitudinal or transverse movement of the cable.
Also in the case of submarine cables the problem is minor, since
the portion of the cable which is submerged will normally heat up
very little due to the efficient cooling provided by the water,
while the shore ends normally are directly buried.
For cables installed in ducts the condition are quite different in
that the cable is allowed to move longitudinally in the ducts
during heating and cooling. To take up this expansion it is
normally necessary to provide underground chambers or manholes at
each splicing point in order to allow the cable to be bent in the
shape of a "U" on each side of the splice. This practice has been
used for many years in the case of cables with copper conductors
and methods have been found to join the two copper conductors in a
manner which, from experience, has proven to make the conductor
stand this repeated bending and tensioning. The method of joining
the two copper conductors normally consists of placing a copper
sleeve over the conductors and compressing this sleeve by means of
a hydraulic press, the sleeve thus providing the necessary support
to help the conductors stand the repeated bending.
The above joining procedures has proven to be less reliable in the
case of aluminum conductors because of the lower tensile strength
of an aluminum sleeve together with the before mentioned greater
coefficient of thermal expansion of aluminum. Joints in aluminum
conductors for high tension cables are therefore normally made by
welding or soldering. While this procedure has proven entirely
satisfactory for cables directly buried for the reasons given
above, the softening of the aluminum metal at the weld may make
this procedure less suitable in the case of a cable installed in
ducts. With the conductor stranded from half-hard or three quarters
hard aluminum wires, the soft area at the joint will constitute a
weak spot when the cable is subjected to the bending cycles
described above.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a more efficient
cable by preventing the overdimensioning which follows from using
conventional dimensioning methods and to avoid undesirable extra
joints. The overdimensioning problem may be taken care of by
changing the electrical resistance of the cable conductor in
accordance with the thermal resistivity along the cable route. The
thermal resistivity or heat dissipation properties should be
measured along the cable route, or estimated, at the early planning
stage in order to decide which conductor resistance value should be
used on the various parts of the route in order to ensure optimum
utilization of the current carrying capacity on all parts of the
cable.
Another object of the present invention is to provide a power cable
comprising a conductor made wholly or partly of aluminum, designing
the cable lengths such that the temperature at the joints between
succeeding lengths is kept as low as possible. This is obtained
according to the present invention by gradually changing the
aluminum portion of the conductor cross-section to copper at the
ends of each length.
According to the present invention, the cable conductor is designed
such that the electrical resistance per unit length of the
conductor, at any point along the cable route, is chosen in
accordance with the desired conductor temperature at that
particular point. The required change of the electrical resistance
of the conductor is obtained by changing partly or wholly from one
conductor material to another and vice versa in the longitudinal
direction of the conductor. The conductor materials are preferably
aluminum and copper and the change of the electrical conductor
resistance is achieved while maintaining the geometrical dimensions
of the conductor.
By designing a power cable in accordance with the present
invention, the conductor temperature within the joint will be much
lower than it would otherwise be. This again means that the
insulation in and near the joint will operate at a lower
temperature than the insulation in the remaining part of the cable.
In most cases this will result in lower dielectric losses. An
additional advantage is that the problem of joining two lengths of
cable together is greatly reduced, since it is easier to join two
copper conductors than it is to join two made of aluminum.
It is a feature of the invention that the conductor resistance
along a cable route where the thermal resistivity of the cable
surroundings vary is changed in accordance with the change of
thermal resistivity of the cable surroundings by changing the
conductor material such that the geometrical dimensions of the
cable is kept constant throughout the cable route.
By designing a power cable according to the present invention one
obtains optimum utilization of the current carrying capacity on all
parts of the cable without using cable sections of different
dimensions and special joints between such sections.
The present invention avoids the mentioned disadvantages of using
aluminum as conductor material, while it maintains the savings
which are possible by using this conductor material instead of
copper. In this case of duct installed cables only a few per cent
of the cable length need to have copper as conductor material.
The above mentioned and other features and objects of the present
invention will clearly appear from the following description of
embodiments of the invention taken in conjunction with the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a, 1b and 1c show cross-sections of a conductor where the
conductor materials in the various layers are changed gradually
from aluminum to copper,
FIG. 2 is a longitudinal section through a conductor along the
conductor axis showing the different cross-sections as illustrated
in FIGS. 1a, 1b and 1c, and
FIG. 3 shows six conductor wires of one layer where the conductor
material of the wires is changed over a certain length.
DETAILED DESCRIPTION OF THE INVENTION
In FIGS. 1a, 1b and 1c are shown a power cable conductor consisting
of a hollow tubular core 1 and two layers 2 and 3 of a plurality of
strands of profiled wires. In FIG. 1a, the tubular core 1 and the
inner profiled layer 2 are made of aluminum, while the outermost
layer 3 is made of copper. In FIG. 1b the conductor material of the
inner layer 2 has been changed to copper, while in FIG. 1c the
tubular core 1 is also made of copper. The cross-sections shown in
these three figures represent power cable conductors, the conductor
material of which is chosen in accordance with a desired
temperature on the conductor.
The change in conductor material is preferably obtained by
designing conductors which in the longitudinal direction is
constituted by wires of at least two different conductor
materials.
For large cables the conductor is usually designed as a multistrand
conductor, the number of wires constituting the conductor and the
geometrical cross-section and shape of these wires are maintained
throughout the cable route.
When the conductor is a multistrand conductor the desired changes
of the conductor resistance is obtained by changing the conductor
material in at least one of the wires. The desired change from one
conductor material to another is obtained by splicing wires of
different conductor materials together by welding, soldering,
compression joining or similar processes prior to working of the
wire or conductor to its final cross-section and shape. An
advantage of this is that the joint is automatically tested during
the final working of the cable, for instance, during the drawing of
the cable.
Alternatively, a short piece of wire made of one of the two metals
may be spliced with a short piece made of the other metal in
advance, if necessary under laboratory conditions to produce good
quality joints, so that only joints between wires of similar metals
have to be made during the stranding operation.
Another variation of the invention is that in any cross-section of
the cable comprising aluminum wires as well as copper wires, the
aluminum wires are arranged in the core and in the inner layers
while the copper wires are arranged in the other layers.
In FIG. 2 is shown a cut through a conductor similar to that shown
in FIGS. 1a-1c, with sections A--A, B--B and C--C corresponding to
those of FIGS. 1a, 1b and 1c respectively. At the left the complete
conductor is made of aluminum, while at the right the whole
cross-section is changed to copper. In this figure it is shown that
the conductor material is not changed abruptly in the whole
conductor cross-section. Layers 4, 5 and 7 indicate the aluminum
portions of the outermost layer, the inner profiled layer and the
tubular core respectively, while 6, 8 and 9 indicate the copper
portions.
When the conductor consists of two or more wires each with a joint
of two different metals it is preferable to space the joints of the
two metals evenly over a certain length of the conductor in order
to avoid abrupt changes of the mechanical and electrical properties
of the said conductor. Preferably the distance between each joint
should be at least 10 cm.
In FIG. 3 is shown six of the plurality of profiled wires
representing six of the 18 wires on the outer multistranded
profiled wire layer of the conductor similar to that shown in FIGS.
1a, 1b, 1c and FIG. 2. It is considered advantageous to undertake
the change from one conductor material to another over a certain
length, and in FIG. 3 is illustrated how this may be effected in
individual strands. At the extreme left all wires are of aluminum,
while at the extreme right all wires are of copper. As will be seen
from the drawing each wire joint is placed at a certain distance
from the neighboring joints, thereby obtaining an interleaved
pattern. The joint of wire 13 is placed between unjointed portions
of the neighbor wires 12 and 14.
Another factor which will help in spreading the wire joints over a
certain length is the cable stranding effect. If all joints were
aligned on the wire pay-off equipment the joints will appear
staggered where the wires are stranded on to the hollow core or
onto a lower layer.
In the case of long oil-filled submarine cables which are
manufactured in discrete lengths and jointed together in the
factory, it will also be advantageous to change to copper just
before a joint. An advantage is that a mechanically stronger joint
is obtained. This is of particular importance in connection with
pipe-type cable and duct cable.
In the case of pipe-type cables, which normally consist of several
lengths joined together, it is very important that the tensile
strength of the joints is not lower than that of the cable. The
mechanical strain on the pipe-type cable is considerable when the
cable expands and shrinks within the confinement of the pipe due to
temperature variations.
Regrading duct cables, lengths of which are joined together in
special manholes, the change to copper should preferably be
undertaken at points where the cable is still within the duct.
Thereby the whole portion of the cable which is subjected to the
most severe bending stresses, e.g., the portion located within the
manhole, has copper as a conductor material and the many years of
experience gained with duct cables with copper conductors may be
applied also to an aluminum/copper conductor cable. The change to
copper should however, be made close to the entrance of the
manhole, for example, one meter within the duct, in order to use
the least possible amount of copper.
It is also of advantage to change to copper near a joint even when
the cable is not installed in a duct or pipe and subjected to the
added mechanical strain associated to these types of installation.
This is particularly true in the case of submarine cable since
these joints may have to be made under less than ideal conditions,
such as on board a ship, and the quality of the joint therefore may
not be the very best. For submarine cables which are subjected to
unusual strain, either during laying (deep water) or later, the
added strength of the copper joint, may also warrant the adoption
of this conductor construction.
By keeping the geometrical dimensions of the cable constant
throughout the cable route, the manufacturing process is
facilitated. Once the conductor has been made, it may be passed
through the further manufacturing steps, such as insulating,
sheathing and armouring processes, in one pass.
It should be noted that the patterns shown in the drawings are
merely for illustrations of the appearance of the conductor. Many
other changes may be made in the design and configuration of the
cable without departing from the spirit and scope of the present
invention.
* * * * *