US3676578A - Electric conductor cables for use in overhead power transmissions - Google Patents

Abstract

Overhead conductor cable, comprising a steel core around which is wound an electrically conducting aluminum winding, in which the degree of sag of the cable during its working life is reduced by having its resistance to creep under tensile loading substantially increased.

Description

United States Patent Cahill 1 July 11,1972

[541 ELECTRIC CONDUCTOR CABLES FOR USE IN OVERHEAD POWER TRANSMISSIONS [72] Inventor: Terence Cahlll, Porthcawl, Wales [73] Assignee: G.K.N. Somerset Wire Limited, Cardiff,

Glamorgen County, Wales [22] Filed: 0ct.l4, 1970 [21] Appl.No.: 80,688

Related US. Application Data [63] Continuation-in-part of Ser. No. 800,223, Feb. 18,

1969, abandoned.

[52] US. Cl ..l74/128, 72/183 [51] Int. Cl. [58] Field of [56] References Cited UNITED STATES PATENTS 1,173,190 2/1916 l-loopes ..l74/128 3,068,353 12/1962 Hann 3,153,696 10/1964 Blanchard ..l74/l28 UX Primary Examiner-E. A. Goldberg Attorney-Kurt Kelman ABSTRACT Overhead conductor cable, comprising a steel core around which is wound an electrically conducting aluminum winding, in which the degree of sag of the cable during its working life is reduced by having its resistance to creep under tensile loading substantially increased.

12 Claims, 8 Drawing Figures PKTENTEDJULHIQR 3,676,578

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ELECTRIC CONDUCTOR CABLES FOR USE IN OVERHEAD POWER TRANSMISSIONS The present application is a continuation-in-part of my prior [15. Pat. application No. 800,223 filed Feb. 18, 1969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the invention This invention relates to electric conductor cables of the kind, herein referred to as the kind specified," which are used in the overhead transmission of electric power, and which are commonly referred to as overhead conductor wires, and which comprise a steel core around which is wound a plurality of electrically conducting wires or strands of wires, of commercially pure aluminum.

2. Description of the Prior Art Hitherto during the many years since electric conductor cables of the kind specified were first used in overhead power transmission, there has always been encountered the problem that after the cable is first put into service, as the aluminum wires which are wound around the core are necessarily under tension by reason of the tensile loading applied to the cable, the aluminum creeps, i.e. elongates permanently to an increasing extent throughout the working life of the cable, under the sustained tensile loading, while there is also a continued permanent elongation, i.e. creep, of the steel core for the same reason.

Consequently the initial sag invariably provided in the cable between successive pylons increases steadily during the cable life. By reason of this steady increase in the initial sag of the cable, the height of the pylons and the number required for a given route length must be made greater than would otherwise be necessary.

Also the elongation of the aluminum has hitherto been to a much greater extent than is the case with the steel core. The effect of this is to transfer to the steel core during the entire working life of the cable a steadily increasing proportion of the tensile loading initially taken by the aluminum wires.

For example, in a typical cable of the kind specified embodying a seven wire steel core around which were wound 54 commercially pure aluminum wires, a test showed that at the end of 1,000 hours the proportion of the total load taken by the steel core increased from 29percent to over 38percent. In the case of a further similar test on a cable of the kind specified in which the seven wire steel core was wound with twelve commercially pure aluminum wires the increase after 1,000 hours in the proportion of the total loading taken by the steel core was from 64.5percent to 73.6percent.

As the working life of an overhead cable of the kind specified is measured in years, as opposed to hours, the total extent of the load transferred from the aluminum to the steel core will be much greater in practice than is indicated by the above test figures.

The result of this progressive increase in the tensile loading of the core throughout the working life of the cable is to cause the core to elongate elastically to a steadily increasing extent so that for this reason alone the initial sag invariably provided in the cable between successive pylons increases steadily during the cable life with the consequent effect on pylon height and number per given route length as already mentioned.

It is further necessary to make the steel core of a larger cross section than would otherwise be necessary for a given tensile strength in order that the core shall still be capable of carrying the maximum increase in tensile loading obtaining at the end of the cable life which commonly is of the order of years.

SUMMARY OF THE INVENTION The present invention has for its primary object the provision of an improved form of overhead conductor cable of the kind specified, in which the extent to which the cable elongates during its working life is reduced to an extent such as to permit of economy being effected in the cross-section of the cables themselves and/or in their supporting pylons.

The present invention provides an electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than long tons per square inch, and a plurality of electrically conducting wires of commercially pure aluminum wound around the steel core, both the steel core and the aluminum wires being in such permanently plastically elongated condition that when the cable is subjected to a sustained tensile loading of substantially 40percent of the breaking load of the cable and for a period of 600 hours, the creep of the cable during at least the next successive 300 hours under said sustained tensile loading is negligable.

The present invention further provides an electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than 80 long tons per square inch, and a plurality of electrically conducting wires of commercially pure aluminum wound around the steel core, both the steel core and the aluminum wires being in such permanently plastically elongated condition that when the cable is subjected for a period of at least 500 hours to a sustained tensile loading of substantially 40percent of the breaking load of the cable, the resultant tensile strain in the cable is less than one half of that developed under the same tensile loading for the same time period in an otherwise identical cable of which neither the steel core nor the aluminum conducting wires have been permanently plastically elongated.

The aluminum wires forming the electrically conducting winding around the steel core are permanently elongated at elevated temperature so as to improve their creep resistance by effecting such elongation after the wires have been wound around the core, as it is not possible to wind the aluminum wires around the steel core after they have been permanently elongated to the required extent, without in the course of the winding operation risking impairment of the mechanical properties of the aluminum wires.

Also in effcting the permanent elongation of the aluminum wires the maximum temperature to which they are heated should not exceed 300 C and desirably is within the range of to 250 C. These temperatures are all below the optimum temperature range at which the medium to high carbon steel core is permanently elongated for which the minimum temperature to attain the desired results of this invention is about 300 C, unless the steel forming the core has already been subjected to a stress-relieving operation at a temperature not below 250 C, in which case the steel core may be effectively permanently elongated when at a temperature within the range of that useful in effecting permanent elongation of the aluminum wires, i.e. from 50 to 300 C.

If the steel forming the core is not subjected to a preliminary stress relieving operation at a temperature not below 250 C and desirably at a temperature higher than 250 C, the steel forming the core must be permanently elongated to the required extent, desirably within the range of 0.5 to 3percent increase on its original length, when at a temperature within the range of 300 to 500 C, desirably at about 400 C, so that in the absence of such prior stress-relieving step, two permanent elongation operations arerequired, first on the core before winding the aluminum wires therearound and afterwards on the fully wound cable. Such second elongation operation in no way impairs the creep resistance properties already imparted to the steel core by its first permanent elongatron.

From the foregoing it will be understood that the performance of the stress-relieving operation above mentioned is very advantageous in that it avoids the necessity of two successive permanent elongation operations on the core which if elongated after stress-relieving as above mentioned at the optimum temperature range for the aluminum wires develops creep resistance as good as that obtained at the higher temperature range of 300 to 500 C.

An especial economy in manufacture can be effected where the stress-relieving operation is effected in the course of providing the steel core as is desirable and customary with a corrosion resistance coating,provided that such coating is applied at a temperature not below 250 C, e.g. a vapordeposited coating of aluminum,or zinc by galvanizing the core at the customarily hot-galvanizing temperature of about 450 C.

The permanent elongation of the cable is preferably carried out at such an elevated temperature and tensile loading which are both so selected as to ensure that when the steel core cools and thus contracts lineally, it applies a compressive stress to the aluminum wires which compressive stress is, however, of a magnitude small enough to ensure that the aluminum wires are still in sufficiently close contact with the steel core as to assist in preserving the same against corrosion.

As a result, when the cable is put into service, under still air conditions, the tensile loading of the outer electrically conducting winding of aluminum will be quite small and its consequent tendency to creep will be very small, and the aluminum will only be subject to tensile loading substantial enough to produce significant creep over a long period of time, when the cable is swaying in a high wind or has a thick ice coating. Since these conditions obtain as a rule during a very minor proportion of the total working life of the cable, the proportion of the working life when the aluminum is liable to creep to any significant extent despite improvement in its creep resistance by this invention will be very small, thus further reducing the extent of transfer of the tensile loading to the steel cores.

Preferably, the amount of permanent elongation of the aluminum wire or strand is at least 0.75percent of the original length, and we have so far found that the best results are obtained. where the amount of permanent elongation of the aluminum is about lpercent of its original length.

With this invention, the extent of stress relaxation of the aluminum under typical tensile stress loadings to which the aluminum may be subjected in overhead power transmission, i.e. about 850 Kg. per square cm., is reduced from the present relaxation of 38percent to a value as low as percent, in the case where the aluminum is permanently elongated by an amount of between 0.5percent and l.0percent of its original length when subjected to such a tensile stress at a temperature of 100 C. as to effect stretching of the aluminum by this amount.

Thus the reduction in the amount of relaxation of the aluminum in a given period of time is nearly SOpercent, and experimental tests to date show that such reduction in the amount of relaxation is maintained over a period of at least l,000 hours.

This same proportion of reduction in relaxation, i.e. of nearly 50percent can accordingly be expected to be maintained during the overall useful life of the cable, i.e. customarily about years.

The effect of the foregoing is to reduce substantially the extent to which the initial tensile loading of the aluminum wires,when the cable is first in service, is transmitted to the steel core during the entire working life of the cable.

This initial loading of the aluminum wires is quite often as high as about 70percent to 35percent of the total loading (according to the design of the cable). Having regard to the increased resistance to creep of the steel core, the practical effect of the foregoing is for a given total cross section of aluminum wire corresponding to the requisite electrical loading of the cable, to reduce significantly the cross section of the steel core, thereby effecting a significant reduction in both the weight and cost of the cable per unit length. Such reduction in the weight of the steel core per unit length has the effect of reducing the total tensile loading on a cable of given length and cross section of the aluminum wires thereby still further reducing the tendency thereof to creep.

Alternatively, without reducing the cross sectional area of the steel core, it is possible without increasing the overall tensile loading of the cable safely to reduce the height of the pylons and/or to increase the distance between adjacent pylons with consequent substantial saving in the cost of a given length of transmission line.

It is further believed that by reducing the initial sag in the cable between pylons of height and spacing as hitherto used, larger currents than hitherto can be transmitted, producing temperatures in the cable high enough to result by thermal expansion of the cable during operation in the same total sag as has hitherto been customary. Thus the cable can be employed more efi'tciently than heretofore.

Still a further very important advantage of this invention is that by reason of the significant decrease in the creep of the cable during its working life, the invention permits of a further pair of cables embodying the invention to be strung beneath an already existing overhead cable of the hitherto conventional form without risk of the increase in sag of the further pair of cables during their working life being great enough to result in less than the minimum safe ground clearance beneath such further cables during their working life.

The present invention is based on my discovery that when a length of a medium or high carbon steel or of a commercially pure as opposed to a pure aluminum is permanently and thus plastically elongated under a tensile loading sufficiently high as to effect such elongation, such permanent plastic elongation occurs as a result of relative slip in parts of the metal along slip planes therebetween, but that in the case of a medium to high carbon steel or commercially pure aluminum such slip after it commences is resisted, so as to preclude further elongation under the applied tensile loading, by the interlocking along the slip planes and along opposite faces of such planes of carbon atoms as well as of certain other non-ferrous atoms where present in typical medium to high carbon steels, e.g. nitrogen atoms, so as to preclude further slip, and thus further permanent elongation under the applied tensile loadmg.

Similar action occurs in respect of at least some of the atoms of the impurity elements invariably present in very small amounts in commercially pure as opposed to pure aluminum so as in like manner to medium or high carbon steel to preclude further slip and thusfurther elongation under the applied tensile loading.

ln consequence, with a cable in which both the medium to high carbon steel core as well as the commercially pure aluminum wire windings have been permanently elongated at the required elevated temperature in accordance with this invention, the resistance of the so formed cable to creep under a sustained tensile loading of a substantial value, e.g. 7,250 lbs for a peripheral cable diameter of about three-fourths inch, embodying seven steel wires in the core each of diameter 0.110 inches and eight aluminum conducting wires each of diameter 0.1 10 inches, is greatly improved as compared with an otherwise identical cable in which neither the steel core or the aluminum windings have been permanently elongated in accordance with this invention.

Thus after a period as low as even 400 hours, during which the cable in accordance with this invention is subjected to a sustained tensile loading which is substantially 40percent, e. g. 4lpercent of the breaking load, the resultant tensile strain in the wound cable is not more than substantially one half of that developed under the same tensile loading for the same time period in an otherwise identical cable of which neither the steel core nor the aluminum conducting wires have been permanently plastically elongated. Over longer periods of sustained tensile loading, e.g. 700 hours, the foregoing comparative improvement in the creep resistant properties of the cable according to this invention is more marked and it is envisaged that this comparative improvement will be still greater over the customary useful life of the cable, i.e. 25 years above mentioned.

Also when the cable of which both the medium to high steel core and the commercially pure aluminum windings have been permanently plastically elongated by this invention is subjected to a sustained tensile loading of substantially 40percent, e. g. 4lpercent, of the breaking load of the cable and for a period of 600 hours, the creep of the cable during at least the next successive 300 hours under this same sustained tensile loading is negligible. By negligible" is meant that the creep during the next successive 300 hours is so small as to be undetectable within the limits of the experimental error of extensometers of the types commonly employed for measuring the increase in length on a gauge length of approximately 18 inch. In contrast, experimental evidence indicates that in the case of an otherwise identical cable under the same sustained loading and of which neither the core nor the winding have been permanently plastically elongated by this invention, not only is the creep strain more than twice that obtained with the invented cable at the expiration of the first 600 hours, but during the next successive 300 hours the cable is continuing to creep to a measurable, i.e. more than negligible extent.

Over the entire useful life of 25 years following the first 600 hours above specified, doubtless the creep of the invented cable will be measurable i.e. more than negligible, but for the reasons above indicated the rate of creep can be expected to compare very favorably with the comparative cable not embodying this invention as above referred to.

BRIEF DESCRIPTION OF DRAWINGS FIGS. 1, 2, 3 and 4 show graphs illustrating test results;

FIG. 5 is a plan view, partly cut away, of a cable according to one embodiment of the invention;

FIG. 6 is a cross-sectional view of the cable of FIG. 5;

FIGS. 7 and 8 are views corresponding to FIGS. 5 and 6 respectively of a cable according to a further embodiment of this invention.

Referring firstly to the graph of FIG. 1, a test was performed to establish the improvement in the resistance to creep of commercially pure aluminum wire as used in overhead conductor cables, when such wire is permanently elongated by stretching it at an elevated temperature, by taking a length of the wire of initial diameter 3.25 mms. and straightening it by drawing it down slightly to a diameter of 3.l8 mms. Some of these straightened wires were then permanently elongated by stretching at a temperature of 200 C. using the method of U.S. Pat. No. 3,068,353, and a series of comparative measurements was carried out between the so permanently elongated wires and the wires which had been merely drawn down without permanent elongation, by loading all of the wires with a tensile loading of 34 Kgs. and measuring the strain over varying periods of time up to 500 hours with the wire at temperatures of C. and 50 C.

The results of this test are set out on the accompanying graph marked FIG. 1, in which the permanently elongated wire and the as drawn wire are respectively referred to by the terms elongated and as drawn, and the results were plotted on the accompanying graph.

From this graph it will be seen that at any given time interval the strain indicating the extent of creep for the as drawn specimen was more than twice the strain of the permanently elongated specimen maintained at the same temperature during the test.

Thus at the end of 500 hours the as drawn specimen maintained at a temperature of 50 C. has crept i.e. increased in length by 0.004 cm. per cm. of specimen length, while in the case of the elongated specimen maintained at a temperature of 20 C. during the test, the increase in length during the same time interval was one-tenth of the foregoing, i.e. 0.0004 cm. per cm. of specimen length, equal to a percentage extension of 0.04 percent.

One less preferred form of the method of this invention which involves the use of the method of the foregoing prior U.S. Pat. No. 3,068,353 will now be described.

The employment of the method referred to in the preceding paragraph possesses the advantage that ,as will be apparent from an examination of the specification of the Pat. No.

.' 3,068,353, the strain and thus the resultant permanent elongation applied to the cable can be most carefully controlled.

Referring to FIGS. 5 and 6 of the drawings, in one specific application of such last mentioned method, a length of electric conductor cable 10 of the kind specified was formed, such cable comprising a steel core 11 formed as a length of steel strand of outside diameter 9.53 mms. embodying a central steel wire 12, and six further steel wires 13 wound helically therearound as a single layer, each individual wire 12, 13 having a diameter of 3.18 mms. with the pitch of each complete helix being 17.5 cms.

The steel from which the core forming wires were made was a plain, i.e. non-alloy medium to high carbon steel having a specific composition within the following composition limits:

Carbon 0.4 to 0.85 wt. Manganese 0.5 to 0.9 wt. Silicon 0.1 to 0.35 wt.

Sulphur and Phosphorus in insignificant amounts.

Balance Iron, together with other elements including Nitrogen customarily present in plain i.e. non-alloy medium to high carbon and in amounts customarily present in such steel.

This steel core was subjected to a preliminary stress-relieving operation involving the heating of the core or the wires forming the same to a temperature of at least 250 C, for example by hot galvanizing the steel wires before forming the core,at a temperature of about 450 C.

Around this central steel core 11 were wound helically three concentric layers 14, 15, 16, of commercially pure aluminum wire 17, each aluminum wire 17 having a diameter of 3.18 mms., with the inner layer 14 being wound in a direction opposite to the direction of winding of the wires 13 forming the steel core, with the middle layer 15 of the aluminum wire helically wound in a direction opposite to the inner layer 14 and with the outer layer 16 wound in a direction the same as the inner layer 14 i.e. opposite to the intermediate layer 15. The inner, middle and outer layer of helical aluminum wires contained l2, l8 and 24 wires respectively, with the pitch of the helices of the inner, middle and outer layers being respectively 19 cm., 26 cm. and 32 cm.

The nature of the helical winding of the three layers of aluminum wire was such that as in the customary commercial manufacture of conductor cables of the kind specified, the inner layer 14 of aluminum wires was wound into tight contact with the exterior of the steel core 11, with the middle and outer layers 15, 16 of aluminum wires wound respectively in tight contact with the inner and middle layers 14, 15 respectively of the aluminum wires.

Such cable was then advanced through apparatus as described in the foregoing prior U.S. Pat. No. 3,068,353, with the diameters of the various V-section pulley grooves so chosen as at the maximum temperature of heating, namely 200 C. to produce a maximum elongation in the cable during its passage through the apparatus of lpercent of its original length.

At the above temperature of heating, the strain of lpercent produced in the above described apparatus resulted in a tensile loading of the aluminum wires great enough to produce a permanent elongation thereof of about lpercent which for the above elongation temperature range of 50 to 300 C will be somewhat greater than that obtaining for the steel core because the steel has an appreciably higher elastic limit at such temperature range than the aluminum. In other words during the permanent stretching of the wound cable, the recoverable elastic stretching of the steel will be appreciably greater than that of the aluminum.

Accordingly after the cable had-been cooled and de-tensioned by its passage around the pulley groove of progressively decreasing diameter as described in the last mentioned U.S. Pat. No. 3,068,353, the aluminum wires were now found to be very slightly in compression, but still closely, although no longer so tightly, wound around the steel core, thus producing a cable having the various advantages earlier mentioned, and with the steel having an ultimate tensile strength of not less than 80 long tons per square inch and a ductility expressed as an elongation of at least 4percent before fracture as measured over a specimen length of 10 inches.

In a test on a length of cable wound to the particular configuration above specified in reference to FIGS. 5 and 6, this was permanently elongated not by the method of the above US. Pat. No. No. 3,068,353 but by stretching it in a form of tensile testing machine while heated uniformly to a temperature of 200 C., so as to be stretched to an extent such as to produce a permanent elongation of lpercent of the original length, and thus permanently to elongate the aluminum wires by substantially lpercent of their original length. Comparison tests were then carried out to measure the creep resistance under prolonged tensile loadings of this particular cable as compared with an otherwise identical cable which, including the component wires thereof, had not been subjected to any stretching to produce permanent elongation.

A test length in each case 1.52 meters long, of these two cables was then subjected while at room temperature, to a sustained tensile loading by stretching the test length by an amount specified in each case as a percentage of its original length,by subjecting the length of cable to a sustained tensile stress, the value of which at the start of each test was 2,835 Kgs. 1

Measurements of the increase in length i.e. the creep strain were taken at the end of lOO hours and also at the end of 200 hours, the results being set out in Table I as follows:

TABLEI Nature of Test piece.

Cable of known type.

Cable embodying invention. 0.0080% 0.00857:

From the results set out in Table I the improvement in creep resistance properties of a cable incorporating the present invention as compared with an otherwise similar cable not incorporating the present invention will be apparent.

In a further series of tests of a cable in accordance with this invention, this was of the configuration depicted in FIGS. 7 and 8 comprising a core 11 embodying a central steel wire 12 and six further steel wires 13 wound helically therearound, each of these seven steel core wires being of diameter 0.1 inches and being formed of a plain non-alloy medium carbon steel within the medium to high carbon analysis range above specified so as to have the following specific analysis:

Carbon wt. .567! Manganese wt. .6l% Silicon wt. .2%

Sulphur and Phosphorous in insignificant amounts.

Balance Iron together with other elements including Nitrogen customarily present in plain, i.e. non-alloy medium carbon steel and in amounts customarily present in such steel.

The steel wires forming the core were first stress-relieved by hot dipping in a galvanizing bath with the molten zinc at a temperature of 450 C, after which the so galvanized steel wires were wound together to form the core 11 of the configuration above described and shown in FIGS. 7 and 8.

Around the so formed core were wound helically two concentric layers 14, 16 of commercially pure aluminum wires 17 as customarily used in overhead conductor cables, each of these aluminum wires having a diameter of .110 inches with the inner layer 14 being wound in a direction opposite to the direction of winding of the wires 13 forming the steel core, with the outer layer 16 wound in a direction opposite to the inner layer l4.

The inner and outer layers of the helically wound aluminum wires contained 12 and 18 wires respectively.

The nature of the helical winding of the two layers of aluminum wire was such that as in the customary commercial manufacture of conductor cables of the kind specified, the inner layer 14 of aluminum wires was wound into tight contact with the exterior of the steel core 11, with the outer layer 16 wound in tight contact with the inner layer 14.

Selected lengths of the so wound cable were then permanently elongated in some cases by the use of the method and apparatus described in the specification of the foregoing US. Pat. No. 3,068,353 and in other cases with the entire length to be tested in straight configuration and clamped at opposite ends between two sets of clamping jaws of which one set was movable under external pressure means away from the other set in a direction aligned with the straight length of cable between the two sets of jaws. Provision was made for passing an electric heating current along the length of the cable between the two sets of jaws.

In each of these further series of tests, with either method of permanently elongating the wound cable including the steel core and the aluminum winding, the cable was heated to maximum temperature of 200 C and the apparatus according to the above US. Pat. No. 3,068,353 and also of the form last described was so arranged and operated as in each case to produce a permanent elongation of the cable amounting to an increase in length of lpercent of the original length when heated to the above temperature.

Two samples produced by each of the above two methods of permanent elongation, i.e. four samples in all and each 5 feet in length were then subjected to a constant force creep strain test by applying to each sample a sustained constant tensile loading of 7,250 lbs. which was 4lpercent of the breaking load of the cable i.e. 17,700 lbs. This creep strain test was performed with the specimen length of cable maintained at a temperature of 20 C corresponding to a typical service temperature of the cable. The creep strain was measured by an extensometer connected to two positions spaced apart from one another along the length of each specimen by a distance of just over l8 inches so as to measure the increase in length of such 1 8 inch length at different time intervals.

A comparative creep strain test was carried out with a length of the same cable as that just described i.e. with the wires forming the steel core galvanized and wound with conducting aluminum wires as above described with reference to FIGS. 7 and 8 but with the cable not permanently elongated.

The results of the foregoing tests are set out in Table No. II below, both for the non-permanently elongated material, for the same material permanently elongated when in straight configuration as above described and also when permanently elongated by the method and apparatus of the US. Patent No. 3,068,35 3 already mentioned.

TABLE II Creep strain l0'-" inches per inch of specimen length after Test N0.Method of permanent elongation 10 hr. 25 hr. hr. hr. 200 hr.

3722 Materialnotper- 7.3 l5 18 27 30 32.5

manently elongated 3756 Material per- 4.5 8.5 l0.5 15.5

manently elongated in straight form 3762 The results of these creep strain tests are plotted on the graph forming FIG. 2 on which is indicated the numbers of the tests to which the various creep strain tests refer.

From FIG. 2 and the related table it will be seen that after a time interval as low as 200 hours, the creep strain of the permanently elongated material is, except for test No. 3762, already less than half that obtaining with the non-elongated material denoted by test No. 3722 and the rate of increase in the strain of the non-elongated material after 200 hours up to at least 260 hours was so much greater than that of each of the permanently elongated specimens that well before the expiration of the first 500 hours of sustained tensile loading even with test No. 3762 the creep strain would be not more than substantially one half of and indeed less than half that of the non-permanently elongated material.

Thus well before the expiration of the first 500 hours of sustained tensile loading at about 40percent of the breaking load the creep resistance of each of the permanently elongated specimens was more than twice that of the otherwise identical material which had not been permanently elongated.

Indeed this condition is shown to be satisfied for test nos. 3756, 3764 and 3768 after the expiration of only 200 hours.

In respect of test No. 3722, non-permanently elongated material and test No. 3768 of permanently elongated material, the creep strain tests just described were continued for a period of time substantially longer than that shown in FIG. 2 and the related table. The results of this are depicted in the graph of FIG. 3, from which it will be seen that under the sustained tensile loading of 4lpercent, i.e. about 40percent of the breaking load, at the expiration of 600 hours from the commencement of the test the creep strain of the non-elongated material was considerably more than twice that of the material in the permanently elongated condition, i.e. the resistance of the latter to creep was then more than twice that of the otherwise identical non-elongated material.

Also after the expiration of some 500 to 600 hours the nonelongated material was continuing measurably to stretch i.e. to creep under the foregoing sustained tensile loading, whereas after about 550 hours and certainly after 600 hours, for the next successive 300 hours, there was no measur'able increase in the strain of the permanently elongated material, i.e. the creep of such material during this next 300 hour period was negligible. (see FIG. 3).

The improvement in creep resisting properties of a conductor cable in accordance with this last form of the invention in which the steel core, as well as the aluminum electrically conducting windings, are permanently elongated as above described, can on tests carried out be defined as a reduction of about 50percent in the creep strain as compared with an otherwise identical conductor which had not been permanently elongated as above described. It is considered that if such permanently elongated conductor cable were in service, over a period of 25 years, the common working life of an overhead conductor cable, the increase in sag would correspond to that obtained by thermal expansion by a temperature increase of less than C. The increase in sag over the same period of time with an identical conducting cable which had not been permanently elongated but was installed under similar conditions, would correspond to that obtained by a temperature increase of 30 C. From the foregoing the improved creep resistance of a conductor cable of the preferred form permanently elongated in accordance with this invention will readily be understood.

FIG. 4 shows a graph on which is plotted for a number of cables containing in their cross section different relative proportions of steel and commercially pure aluminum, the percentage creep, i.e. increase in length per unit length after the cable has been subjected over a period of 30 years to a sustained tensile loading of percent of the ultimate tensile strength (1. .T.S.) of the cable. Such sustained tensile loading is in practice regarded as the safe working load of existing cables under normal atmospheric conditions which are considered to obtain during the major part of the entire life of the cable, i.e. absence of high winds and freedom from accumulations ofice or snow. The period of thirty years is considered to be the maximum useful life of an overhead conductor cable. The foregoing data is plotted in FIG. 4 for the two types of cables, namely:

i Both the steel core and the outer winding of aluminum are conventional, i.e. neither have their creep resistance increased in accordance with this invention. This data which is represented by the continuous line in FIG. 4 is taken from already known experimental results.

ii The outer winding of commercially pure aluminum but not the steel has its creep resistance increased by permanently elongating the aluminum wires. This data is represented by the dashed line in FIG. 4. Necessarily. for the period of thirty years indicated, such data is estimated, the basis for the estimation being that tests so far carried out show that over extended periods of time with a sustained tensile loading as high as 50percent of the ultimate tensile strength, the percentage creep of commercially pure aluminum wire which has been permanently elongated is approximately one half of that of otherwise identical aluminum wire which has not been permanently elongated.

It is therefore considered a very reasonable estimation that at a lower sustained tensile loading of 20percent of the ultimate tensile strength over a period of thirty years, the percentage creep of the permanently elongated commercially pure aluminum wire will be not more than one half of that of the otherwise similar aluminum wire which has not been permanently elongated and that the dashed line curve is properly indicative of what will happen in practice for the sustained loading and period of time indicated in FIG. 4.

Although one satisfactory way of permanently elongating the wound cable is by the method and apparatus of the foregoing US. Pat. No. 3,068,353, another method which it is considered is to be preferred in that it does not involve bending the aluminum conducting wires during their permanent elongation at elevated temperatures is to stretch long lengths of the wound cable in straight configurations. This would preferably be done with opposite ends of such straight lengths clamped between clamping jaws displaceable away fromone another by hydraulic pressure operated rams or like means by a distance sufficient to effect the required percentage permanent elongation of the wound cable, the clamping jaws at opposite ends of each such cable length being connected to a heating current supply circuit so as to effect electric resistance heating to the desired temperature e.g. 200 C of each length of cable which is being stretched and thus permanently and plastically elongated by the relative apart movement of the said clamping jaws.

I claim:

I. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than long tons per square inch, and a plurality of electrically conducting wires of commercially pure aluminum wound around the steel core, both the steel core and the aluminum wires being in such permanently plastically elongated condition that when the cable is subjected to a sustained tensile loading of substantially 40 percent of the breaking load of the cable and for a period of 600 hours, the creep of the cable during at least the next successive 300 hours under said sustained tensile loading is negligible.

2. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than 80 long tons per square inch, and a plurality of electrically conducting wires of commercially pure aluminum wound around the steel core, both the steel core and the aluminum wires being in such permanently plastically elongated condition that when the cable is subjected for a period of at least 500 hours to a sustained tensile loading of substantially 40 percent of thebreaking load of the cable, the resultant tensile strain in the cable is less than one half of that developed under the same tensile loading for the same time period in an otherwise identical cable of which neither the steel core nor the aluminum conducting wires have been permanently plastically elongated.

3. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than 80 long tons per square inch, and a plurality of electrically conducting wires of commercially pure aluminum wound around the steel core, both the steel core and the aluminum wires being in such permanently plastically elongated condition that the wound cable is permanently stretched by an amount of at least 0.5 percent as compared with an otherwise identical cable of which both the steel and the aluminum are in the as drawn condition.

4. A cable according to claim 3, wherein the permanently stretched steel core has a ductility expressed as an elongation of at least 4 percent before fracture as measured over a specimen length of inches.

5. A cable according to claim 3 wherein said wound cable is in permanently stretched condition by an amount of between 0.5 percent and 3 percent as compared with an otherwise identical cable of which both the steel and the aluminum are in the as drawn condition.

6. According to claim 3, wherein the wound cable is permanently stretched by up to 3 percent as compared with an otherwise identical cable of which both the steel and the aluminum are in the as drawn condition and, when the cable is subjected for a period of at least 500 hours to a sustained tensile loading of substantially 40 percent of the breaking load of the cable, the resultant tensile strain in the cable is less than one half of that developed under the same tensile loading for the same time period in an otherwise identical cable of which both the steel core and the aluminum conducting wires are in the as drawn condition.

7. A cable according to claim 1 wherein said aluminum wires are compressively stressed to a degree small enough to maintain contact between the steel core and the surrounding envelope of aluminum wires, the degree of compression being such that the aluminum wires are tensioned merely by the weight of the cable when the same is first put into service.

8. A cable according to claim 2 wherein said aluminum wires are compressively stressed to a degree small enough to maintain contact between the steel core and the surrounding envelope of aluminum wires, the degree of compression being such that the aluminum wires are tensioned merely by the weight of the cable when the same is first put into service.

9. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than long tons per square inch and a plurality of electrically conducting wires wound around the steel core and formed of commercially pure aluminum so as to contain traces of impurity elements, with some of the carbon atoms in the steel, and some of the atoms of the impurity elements in the aluminum at positions along the slip planes of the steel and aluminum respectively being so displaced as a result of permanent plastic stretching of both the steel and the aluminum as to so resist further slip along said slip planes such that when the cable is subjected to a sustained tensile loading of substantially 40 percent of the breaking load of the cable and for a period of 600 hours, the creep of the cable during at least the next successive 300 hours under said sustained tensile loading is negligable.

10. A cable according to claim 9 wherein the steel forming the core is characterized by:

a carbon content of 0.4 to 0.85 percent a manganese content of 0.5 to 0.9 percent a silicon content of 0.1 to 0.35 percent said percentages being by weight.

11. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than 80 long tons per square inch and a plurality of electrically conducting wires wound around the steel core and formed of commercially pure aluminum so as to contain traces of impurity elements, with some of the carbon atoms in the steel, and some of the atoms of the impurity elements in the aluminum at positions along the slip planes of the steel and aluminum respectively being so displaced as a result of permanent plastic stretching of both the steel and the aluminum as to so resist further slip along said slip planes such that when the cable is subjected for a period of at least 500 hours to a sustained tensile loading of substantially 40 percent of the breaking load of the cable, the resultant tensile strain in the cable is not more than substantially one half of that developed under the same tensile loading for the same time period in an otherwise identical cable of which neither the steel core nor the aluminum conducting wires have been permanently elonated. g 12. A cable according to claim 11 wherein the steel forming the core is characterized by:

a carbon content of 0.4 to 0.85 percent a manganese content of 0.5 to 0.9 percent a silicon content of0.l to 0.35 percent said percentages being by weight.

Claims (11)

1. 2. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than 80 long tons per square inch, and a plurality of electrically conducting wires of commercially pure aluminum wound around the steel core, both the steel core and the aluminum wires being in such permanently plastically elongated condition that when the cable is subjected for a period of at least 500 hours to a sustained tensile loading of substantially 40 percent of the breaking load of the cable, the resultant tensile strain in the cable is less than one half of that developed under the same tensile loading for the same time period in an otherwise identical cable of which neither the steel core nor the aluminum conducting wires have been permanently plastically elongated.
2. 3. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than 80 long tons per square inch, and a plurality of electrically conducting wires of commercially pure aluminum wound around the steel core, both the steel core and the aluminum wires being in such permanently plastically elongated condition that the wound cable is permanently stretched by an amount of at least 0.5 percent as compared with an otherwise identical cable of which both the steel and the aluminum are in the as drawn condition.
3. 4. A cable according to claim 3, wherein the permanently stretched steel core has a ductility expressed as an elongation of at least 4 percent before fracture as measured over a specimen length of 10 inches.
4. 5. A cable according to claim 3 wherein said wound cable is in permanently stretched condition by an amount of between 0.5 percent and 3 percent as compared with an otherwise identical cable of which both the steel and the aluminum are in the as drawn condition.
5. 6. According to claim 3, wherein the wound cable is permanently stretched by up to 3 percent as compared with an otherwise identical cable of which both the steel and the aluminum are in the as drawn condition and, when the cable is subjected for a period of at least 500 hours to a sustained tensile loading of substantially 40 percent of the breaking load of the cable, the resultant tensile strain in the cable is less than one half of that developed under the same tensile loading for the same time period in an otherwise identical cable of which both the steel core and the aluminum conducting wires are in the as drawn condition.
6. 7. A cable according to claim 1 wherein said aluminum wires are compressively stressed to a degree small enough to maintain contact between the steel core and the surrounding enveloPe of aluminum wires, the degree of compression being such that the aluminum wires are tensioned merely by the weight of the cable when the same is first put into service.
7. 8. A cable according to claim 2 wherein said aluminum wires are compressively stressed to a degree small enough to maintain contact between the steel core and the surrounding envelope of aluminum wires, the degree of compression being such that the aluminum wires are tensioned merely by the weight of the cable when the same is first put into service.
8. 9. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than 80 long tons per square inch and a plurality of electrically conducting wires wound around the steel core and formed of commercially pure aluminum so as to contain traces of impurity elements, with some of the carbon atoms in the steel, and some of the atoms of the impurity elements in the aluminum at positions along the slip planes of the steel and aluminum respectively being so displaced as a result of permanent plastic stretching of both the steel and the aluminum as to so resist further slip along said slip planes such that when the cable is subjected to a sustained tensile loading of substantially 40 percent of the breaking load of the cable and for a period of 600 hours, the creep of the cable during at least the next successive 300 hours under said sustained tensile loading is negligible.
9. 10. A cable according to claim 9 wherein the steel forming the core is characterized by: a carbon content of 0.4 to 0.85 percent a manganese content of 0.5 to 0.9 percent a silicon content of 0.1 to 0.35 percent said percentages being by weight.
10. 11. An electric conductor cable for use in overhead electric power transmission comprising a central core formed of medium to high carbon steel having an ultimate tensile strength of not less than 80 long tons per square inch and a plurality of electrically conducting wires wound around the steel core and formed of commercially pure aluminum so as to contain traces of impurity elements, with some of the carbon atoms in the steel, and some of the atoms of the impurity elements in the aluminum at positions along the slip planes of the steel and aluminum respectively being so displaced as a result of permanent plastic stretching of both the steel and the aluminum as to so resist further slip along said slip planes such that when the cable is subjected for a period of at least 500 hours to a sustained tensile loading of substantially 40 percent of the breaking load of the cable, the resultant tensile strain in the cable is not more than substantially one half of that developed under the same tensile loading for the same time period in an otherwise identical cable of which neither the steel core nor the aluminum conducting wires have been permanently elongated.
11. 12. A cable according to claim 11 wherein the steel forming the core is characterized by: a carbon content of 0.4 to 0.85 percent a manganese content of 0.5 to 0.9 percent a silicon content of 0.1 to 0.35 percent said percentages being by weight.

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