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.

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.

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.