GB2037060A

GB2037060A - Electric power cables

Info

Publication number: GB2037060A
Application number: GB7934194A
Authority: GB
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1978-10-02
Filing date: 1979-10-02
Publication date: 1980-07-02
Also published as: DE2939971A1; FR2438324B1; GB2037060B; FR2438324A1

Abstract

An electric power cable for burial in direct contact with soil and gravel has a central power conductor (12) surrounded by a tubular sleeve of eg. cross linked polyethylene (16) and a plurality of neutral conductor wires (18) helically wound around the insulating sleeve concentrically therewith. The neutral conductor wires are composed of a good electrically conductive material such as copper or aluminium (22) and are provided with corrosion protection means in the form of a sheath (20, Fig. 2 not shown) of ferrous material such as a low carbon steel, which is solid phase bonded to the copper core. The cross sectional area of the copper sheath relative to the total cross sectional area of neutral conductor wire is at least 10% to provide long term corrosion protection and can be as high as 40% with an optimum range between 18% and 25%. An optional additional protective layer of a galvanically more active material may be provided, such as zinc, magnesium, aluminium or alloys thereof. <IMAGE>

Description

SPECIFICATION Electric power cables This invention relates generally to electrically conductive cables or wires and more particularly to electric power cables adapted to be buried in the ground and to the neutral conductors associated with such power cables.

It is common practice, at least in U.S.A., to place such cables in the ground in direct contact with soil and gravel. So locating the cables offers many advantages such as obviating weather problems, generally providing long life expectancy, avoiding the cluttering of air space to mention a few; however, this practice also results in a serious limitation.

That is, it has been found that the cables, and in particular the neutral conductors, have a marked proclivity for uneven and catastrophic corrosion. Copper, and sometimes aluminium, are generally used for the neutral conductors due to the electrical conductivity requirements of the neutral conductors and these materials react with many of the constituents of the soil so that such conductors, when unprotected, can corrode completely through in one or more places along the length of the wire in a very sort period of time, in a matter of months in some cases, thereby interrupting the electrical continuity of the conductor and impairing or destroying its effectiveness. This problem became generally recognized in the 1 960's when various power cables were dug up and the corrosion of the neutral conductors was noted.Attempts have been made to obviate or at least mitigate this corrosion problem by providing a protective coating or layer on the neutral conductor. For example, one of the most commonly employed protective measures taken with copper conductors has been to tin the copper. While tin provides a certain degree of protection, if the tin coating is not continuous corrosion can quickly proceed at the point of discontinuity. Discontinuities, even if they are not originally present when the cable is laid, can be caused after installation, for instance by rodents, such as moles or the like, biting the conductor, or by mechanical damage during installation.

Another protective measure which has been employed is to dispose a carbon filled cross linked polyethylene sleeve about the conductor; however, this significantly increases the cost of the product as well as rendering it more difficult to handle, particuarly in pulling the cable through ducts and the like. A further disadvantage of using this material is the fact that its availability and cost has been effected by world politics relative to the marketing and delivery of oil which is used to make the polyethylene. In addition, increased corrosion damage to surrounding less noble metals will occur due to galvanic effects.

Still another protective measure which has been used is the employment of cathodic protectionv which can be very effective but suffers from certain inherent limitations including the fact that it is relatively expensive.

That is, anodes must be spaced along the length of the neutral resistivity conductor. The specific spacing is based on the materials chosen, the resistivity of the wire and the particular soil conditions at a given location but, for example, may be in the order of every ten to twenty feet. Another factor which adds to the expense is due to the low polarizability of copper. That is, a fairly significant amount of current is required to provide effective protection.

According to a first aspect of the invention, there is provided: An electric power cable comprising a central conductor and tubular insulator means for said central conductor, an axially extending neutral conductor being supported by said insulator means, said neutral conductor composed of a core of a first material having good electrically conductive characteristics and having a ferrous sheath thereabout.

According to a second aspect of the invention, there is provided: An electric power cable comprising a centrally disposed conductor, a tubular sleeve of electrically insulative material disposed about the conductor and neutral conductor means supported on the tubular sleeve, the neutral conductor means including a core of a first material having good electrically conductive characteristics and having a sheath of a second dissimilar ferrous material thereabout and solid phase bonded thereto.

According to a third aspect, this invention provides an improved electric cable for the underground distribution of electric power by providing a cable having a centrally disposed electrical power conductor composed of suitable electrically conductive material such as copper surrounded by a tubular insulating sleeve of cross linked polyethylene or other suitable electrically insulative material supporting a plurality of neutral conductor wires helically wound around the sleeve concentrically therewith. The neutral conductor wires are preferably composed of a core of copper, although aluminium may be used, encased by a ferrous jacket. The cross sectional area of the ferrous material compared to the cross sectional area of both the core and jacket range from 10% to 40% with a preferred range of 18% to 25%.The wire can additionally be provided with a protective layer of a galvanically more active material such as zinc, magnesium, aluminium or alloys thereof.

By way of example only, certain illustrative embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a fragmentary view of a cable embodying this invention with certain components thereof broken away at various locations ta highlight the details thereof; Figure 2 is a cross sectional view of a neutral conductor helically wound about the cable of Fig. 1; and Figure 3 is a cross-sectional view similar to Fig. 2 of a modified neutral conductor.

Reference is now made to Fig. 1 of the drawings in which is illustrated an exemplary embodiment of the invention for the underground distribution of AC electrical power in which numeral 10 is used to designate a cable used for such purposes. Cable 10 comprises a central core 1 2 of suitable electrically conductive material such as copper or aluminium which serves as the power conductor and, as is common in the industry, is preferably surrounded by a relatively thin shield 14 of cabon loaded cross-linked polyethylene.Tubular member 1 6 of electrically insulative material such as cross-linked polyethylene or other suitable material encases sleeve 1 4 and core 1 2 and supports a plurality of neutral conductive wires 18 which are helically wound concentrically thereon and which extend substantially along the entire length of the cable for ultimate connection with suitable ground rods or the like. Another thin shield of carbon loaded cross-linked polyethylene (not shown) can be provided intermediate member 16 and wires 18, if desired.

In normal use the ends of cable 10 are connecteld to various electrical equipment such as transformers while essentially the remaining portion may be placed underground in direct contact with the soil or subsoil. The ends of the neutral conductors are also connected to the electrical equipment as well as to the above mentioned ground rods. Although the opposite ends of the neutral conductors are normally connected to ground rods, it is also desired that current be permitted to flow directly to ground from the neutral conductors throughout their length therefore the neutral conductors are placed in direct contact with the soil and consequently are subjected to varying corrosive conditions depending on the particular soil conditions in which they are placed.In order to prevent localized and catastrophic corrosion of neutral wire 1 8 it is provided with a sheath 20 of ferrous material disposed about its core 22.

Use of sheath 20 results in several advantages including a predictable and uniform corrosion behavior. That is, the ferrous material tends to corrode at a uniform rate along its buried length. Another significant advantage is that ferrous material such as steel is generally anodic relative to the copper core and thus provides galvanic protection of copper core 22 so that even if a break occurs in the steel sheath somewhere along its length the copper will not be subjected to corrosion until the steel sheath has corroded away completely around the periphery of conductor 1 8.

Yet another advantage that the ferrous sheath provides is that the amount of noble metal in the underground plant is reduced thereby reducing undesirable galvanic effects. Should it be desirable to prevent even the uniform corrosion of the ferrous sheath as by way of cathodic protection it will be noted that the current requirements for effective protection are significantly less, by a factor of up to 10, than would be required for unsheathed copper. In some instances the extra strength resulting from the use of the ferrous sheath will also be advantageous. Essentially any ferrous material can be used in carrying out the invention and therefore the choice frequently will be based on economic considerations. Typical materials useful in carrying out the invention include low carbon steels, copper containing steel alloys, chromium and nickel containing alloys and the like.It will be appreciated that the amount of galvanic protection provided by the ferrous sheath will depend not only on the material used for core 22 but also the particular alloy used for sheath 20. In some cases high nickel and chromium containing alloys such as stainless steel will not provide effective galvanic protection and thus should not be chosen. That is in sulphur containing soils stainless steel can become cathodic relative to copper so that in such cases it would not protect the copper core from corrosion once a break in the sheath occurred. In the event that aluminium is employed for core 22 the material chosen for sheath 20 should be anodic relative to core 22 for optimum protection. For instance, there are aluminium alloys which could be selected for use as a sheath with an aluminium core which would provide desired galvanic protection.

Corrosion resistance can be further enhanced by providing an additional layer 24 of various thicknesses of protective material as seen in Fig. 4 by coating the sheath wire with an active metal such as zinc, aluminium, magnesium or alloys thereof. Zinc and magnesium can be applied by conventional methods such as electroplating or hot dip galvanizing while aluminium can be conventionally coated by spraying or hot dipping.

The neutral conductor 1 8 is preferably made by bonding a pair of ferrous strips around the copper or aluminium core by solid phase bonding techniques known in the art.

For example the clad conductor can be formed in a continuous operation by bending the strips to form half cylinders and bonding the strips to each other and to the core in the solid phase by rolling as set forth in U.S.

Patent No. 3,355,795 which issued December 5, 1 967 to the present applicants.

During the bonding process, edge fins are formed which project radially from the sheathed core composite. Squeeze rolls of the type disclosed in the patent by applying sufficient pressure which is generally more than enough to effect bonding can be used to sever these edge fins. However, it has been found that when used with ferrous sheath material it is preferred to apply only enough pressure to produce a satisfactory bond and leave the protruding fins for removal by separate severing means downstream of the squeezing rolls once the composite has cooled somewhat. This modification results in very significant improvement in expected roll life.

In order to reduce registration problems, i.e., problems in maintaining correct alignment of the strips relative to one another and to the core, the ferrous strips can be preformed so that they are trough shaped with a groove dimensional to snugly fit around the conductor core. After appropriate preparation, e.g., cleaning, the grooved strips which can be guided more easily and accurately, and the core are then fed between squeeze rolls to effect reduction in the outside radii of the strips and core with concomitant solid phase bonding between the strips and between the core and the strips as set forth in U.s. Patent No. 3,320,666 which issued May 23, 1 967 to the present applicants.

A typical neutral conductor wire was formed using two strips of 1006 low carbon steel approximately 0.4 inch wide by 0.010 inch thick and a copper core having a diameter of 0.0225 inches for a resulting composite in which the cross sectional area of the sheath was approximately 18% of the cross-sectional area of the composite (sheath plus core). The strips and core were thermally cleaned by resistance heating with the temperature of the steel strips being raised to a point within the range of 1400 -1600'F at the time they entered the nip of the squeeze rolls and the copper to a somewhat lower temperature. The temperature of the composite leaving the nip of the squeezing rolls was approximately 1400'F. The reduction in cross sectional area to effect the desired solid phase bond was approximately 10%.The resulting composite has excellent electrical conductivity both axially, along its length, as well as radially through the sheath.

It will be realized that various size material can be employed in making the composite.

For example for an efficient production module the squeezing rolls can be made to accomodate a core having a diameter of 0.298 inches and ferrous strips having a thickness of 0.016 inch.

For effective corrosion protection it is preferred to provide a cross sectional sheath area of at least approximately 10% of the total cross sectional area of the composite. More protection can be afforded by increasing this percentage, however, the electrical conductivity of the composite is concomitantly decreased as the ratio of the area of the ferrous sheath is increased so it is preferred that the cross sectional area of the sheath not exceed approximately 40% of the total cross sectional area of the composite with a preferred range being approximately 18-25%.

Various changes can be made in the above products and methods without departing from the scope of the invention as defined by the appended claims.

The described cables have the following advantages: the neutral conductors are not prone to localized or catastrophic corrosion when buried under the surface of the ground, the cable has a significantly improved life expectancy over conventional prior art cables yet is economical to produce and easy to handle and install, the concentric neutral is easily polarized and thus readily protected by cathodic protection, and the concentric neutral allows fewer wires to be used and thus reduces the cable cost.

Claims

1. An electric power cable comprising a central conductor and tubular insulator means for said central conductor, an axially extending neutral conductor being supported by said insulator means, said neutral conductor composed of a core of a first material having good electrically conductive characteristics and having a ferrous sheath thereabout.

2. An electric power cable according to claim 1 in which the neutral conductor is provided with an additional layer of protective material surrounding the ferrous sheath.

3. An electric power cable according to claim 2 in which the additional layer is composed of a zinc or zinc alloy.

4. An electric power cable according to claim 2 in which the additional layer is composed of an aluminium or aluminium alloy.

5. An electric power cable according to claim 2 in which the additional layer is composed of magnesium or magnesium alloy.

6. An electric power cable according to any preceding claim in which the first material is copper.

7. An electric power cable according to any of claims 1 to 5 in which the first material is aluminium.

8. An electric power cable according to any preceding claim in which the cross sectional area of the sheath of the neutral conductor relative to the total cross sectional area of the core and sheath of the neutral conductor is between approximately 10% and 40%.

9. An electric power cable according to claim 8 in which the cross sectional area of the sheath of the neutral conductor relative to the total cross sectional area of the core and sheath of the neutral conductor is between approximately 18% and 25%.

1 0. An electric power cable comprising a centrally disposed conductor, a tubular sleeve of electrically insulative material disposed about the conductor and neutral conductor means supported on the tubular sleeve, the neutral conductor means including a core of a first material having good electrically conductive characteristics and having a sheath of a second dissimilar ferrous material thereabout and solid phase bonded thereto.

11. An electric power cable according to any preceding claim in which the neutral conductor means comprises a plurality of elongated wires helically wrapped around the tubular sleeve, each wire having a core and a ferrous sheath, the ferrous sheath having a cross sectional area relative to the cross sectional area of the core and sheath of at least approximately 10%.

1 2. An electric power cable substantially as herein described with reference to and as illustrated by the accompanying drawings.

1 3. An electric power cable as claimed in claim 1 2 substantially as herein described with reference to and as illustrated by Figs. 1 and 2 of the accompanying drawings.

14. An electric power cable as claimed in claim 1 3 but modified substantially as herein described with reference to and as illustrated by Fig. 3 of the accompanying drawings.