CA1270045A - Low loss alminum stranded wire with a steel core member - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A low loss aluminum stranded transmission line is composed of a steel core member formed by a plurality of steel wires for providing the tension strength of the line and an aluminum portion formed by a plurality of aluminum element wires twisted around the steel core member in at least one layer. The steel core member includes higher tension strength wires than the standard steel wires of a standard transmission line. Thus, the diameter of the steel core member can be reduced to be smaller than that of the standard steel core, without decreasing the break weight. In the aluminum portion, some, or all of the element wires have a fan-shaped cross section, so that the cross sectional space factor of the aluminum portion is actively increased to reduce the electric power loss during electrical transmission. Also, at least one projection of fin-shape form on at least one element wire of the aluminum portion is located in the most outer layer of the aluminum portion. This fin-shaped projection effectively prevents snow piling on the transmission line.

Description

~OW LOSS ALUMINUM STRANDED WIRE WITH A STEEL CORE MEMBER
BACKGROUND OF THE INVENTION
~ . _ The present invention relates to an improvement of helically stranded aluminum wire with a steel core member, widely used for overhead transmission lines. More particularly, the invention relates to an aluminum stranded transmission line in which the energy loss during electrical transmission can be greatly lowered and excessive accumulation of snow therearound can be effectively avoided.
Presently, in the construction of an electric power plant for the purpose of electrical transmission, serious problems regaLding the site selection may arise. General trends indicate that distances between power source point and demand point become greater with time. The long distance wires or transmission lines required for connection result in an unignorable energy loss during transmission. Many techniques such as the use of super high voltage for transmissions have been proposed, but further improvements are greatly needed.
Also, weather conditions should be considered.
Overhead transmission lines are used in many regions having highly varying climates which may include heavy snowfall. Snow frozen around the electric wires may grow gradually, and finally the electric wires may break. In the most serious case a steel transmission tower may be destroyed.

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The most common method for improving the afore-mentioned power loss in an electric transmission line is to lower the electrical resistance of the line. This method introduces firstly improving t~e conductivity and secondly enlarging the cross section of the wires.
The first conventional aluminum conductor steel reinforced cable (hereinafter defined "ACSR") comprises a steel core including steel wires which function mainly as a tension member and aluminum wires stranded around the steel core which function mainly as a conductor. This is very popular in the art and widely used in overhead transmission lines.
It is difficult to improve the conductivity of the ACSR presently employed. The aluminum wire currently used in ACSR is a hard-drawn aluminum conductor of the EC class and its conductivity is in the order of 62~ IACS. Even using high purity aluminum, for example 99.99% pure aluminum, which is obtainable from industrial refinement today, conductivity would hardly reach the order of 64%.
Even so, it is not advisable to adopt this type of aluminum for use in overhead transmission lines, since such high purity aluminum has low mechanical strength and extremely high price.

The second prior art teaching relates to enlarging the cross sectional area of the conductor. This necessitates the increase of the diameter and the weight ~270045 of the transmission line. These increases result in an increased wind load and a large sag results in the transmission line. Strong stretch tension and further reinforcement o~ the supporting steel towers would be re~uired to overcome this sag. Therefore, this system can become quite expensive to realize.
To avoid snow freezing and collecting on transmission lines, an apparatus was proposed whereby a heater wire or wires are twisted together into the element wires of the electric lines. Also, application of paints that repel snow and prevent freezing around the electric lines has been attempted. But, these methods are not sufficient since they can be difficult to apply to large transmission systems. Regardless of system size, in some instances the effect desired is much less pronounced than was expected from these methods.
The present invention endeavours to overcome these shortcomings.

In order to further discuss the prior art, the standard ACSR will now be discussed in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure l is a cross sectional view of a preferred embodiment of the transmission line of the present invention.

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Figure 2 is a cross sectional view of the prior art electric power transmission line.
Figure 3 is a diagram characterising the relation-ship between the diameter of the steel wire of ACSR, the tension strength of the steel wire of ACSR, and the break weight of the transmission line.
Figure 4 is a diagram illustrating the load of accumulating snow on the prior art transmission line and on the transmission line of the present invention.
Figure 5 illustrates a transmission line connection for observing the snow piling thereon.

ACSR STANDARDS
ACSRs are standardized and must adhere to specific values and dimensions. In compliance with the standard-ization of the ACSRs, steel towers for suspending the ACSRs, accessories therefor, and the like are also standardized. For this reason, any change or modification of ACSRs is limited with respect to the specified dimensions or values, in order to maintain the safety of the transmission line.
An exemplary cross section of the prior ACSR is illustrated in Figure 2. The wire consists of a steel core comprised of steel wires l' and two layers of

2~ stranded aluminum element wires 2 around the core. For a typical ACSR, the steel core consists of seven stranded . ~ .

i2700~5 steel wires 1' each having a diameter of 2.6 mm, their tension strength being 135 kg/mm . Thirty EC class aluminum element wires 2, having the diameter of 2.6 mm each, are wound around the steel core. The core has diameter d2 equal to 7.8 mm, with the outer diameter D2 of the entire ACSR being 18.2 mm. The tension weight of the ASCR is 6,980 kg, and the weight per unit length is 733 kg/km.

SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a low loss transmission line comprising a steel core comprisir.g steel wires each having a tension strength of more than 200 kg/mm , and aluminum wires disposed around the core to provide at least one layer of aluminum conductors, at least some of the aluminum conductors being fan shaped, the space-factor of the cross section of the aluminum wires being at least 85~.
The present invention also provides a transmission line which effectively avoids excessive snow piling thereon.
In the first aspect of the present invention, the steel wires used for the core are selected from materials having high tension strength so as to allow a reduction in the ratio of the cross sectional area of the steel core to the cross sectional area of the entire electric wire, or the steel occupation ratio. In addition, an increase in D
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the occupation ratio of the layer of the aluminum stranded wires, without enlarging the diameter of the entire electric wire is achieved.
In the second aspect of the invention, a portion, or preferably all of the aluminum element wires forming layers of aluminum stranded wire are made of a conductor having a fan-shaped cross section, so as to increase the space factor of the aluminum conductors. The space factor being the ratio of the cross sectional area of the aluminum wires to the space area of the aluminum layers, including dead space. That is, the space factor is the ratio of the aluminum portion of the cross section between the outer diameter of the line and the diameter of the steel core to the entire area between these two diameters, including any air space. This permits the increase of the effective cross sectional area of the aluminum portion as a conductive member, without increasing the outer diameter of the transmission line as a whole. Thus, the electric resistance of the wire is greatly decreased, and the electric power loss of the overhead transmission line is substantially reduced.
In the third aspect of the present invention, at least one of the element wires in the most outer layer of the aluminum stranded wire portion has a fin-shaped projection for substantially preventing the snow piling on the transmission line.

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Thus, it is an object of the present invention to provide a transmission line having a low loss aluminum stranded wire conductor and a steel core member capable of reducing the cross sectional area of the steel core member, without decreasing the standard break weight, by using a steel wire of higher tension strength than is standard for steel core wire.

DESCRIPTION OF THE PREFERRED EMBODIMENT
A preferred embodiment of the present invention consists of a transmission line shown in cross section in Figure 1. The cross sectional area of this line is chosen to be equivalent to the standard, that is 160 mm2. The outer diameter dl of the steel core portion may be selected as 6.3 mm. Thus, the cross sectional area of the steel core portion is 24.24 mm , instead of 37.16 mm2 as in the prior art steel core portion. Accordingly, the cross sectional area of the steel core is greatly reduced and the cross sectional area of the portion surrounding the core can be increased. This is desirable since the aluminum contributes greater conductivity to the transmission line than does the steel. Furthermore, the decrease of steel and the increase oE aluminum used does not lower the break weight of the wire, for which the steel portion is responsible. In fact the break weight can be maintained at the standard value.

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In order to accomplish this, steel wires, each having a tensile strength greater than that of the industry standard, are used.
From Figure 3 it can be seen that when a wire S strength of 200 kg/mm is selected for example, the diameter of the wire comprising the steel core may be in the order of 2.l mm. In the same manner when wire of 220 kg/mm strength is used, the diameter of the element wire comprising the steel core may be in the order of l.9 mm. Accordingly, it is understood that by adopting such large strength steel wire as the element material of the steel core, the outer diameter of the entire steel core can be reduced and the break weight of the electric wire can still be maintained at the standard value.
The inventors of the present invention have found, after a variety of experiments, that such steel wires of large strength and of high stretch characteristics can be obtained from well-known materials including above 0.8~ of C, 0.4-2.0% of Cr, and 0.8-1.5% of Si within a specific range of temperature and time.
Since the transmission line of this embodiment maintains the standard diameter, the wire does not require any modifications or special tools for connection, suspension and mounting. In addition, the wire does not promote an unordinary wind load or an increase of tension when suspended.

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Additionally, in accordance with the embodiment of Figure 1, the cross section of the aluminum wire 3 is formed in a fan-shape, instead of the circular cross section of the prior art wire 2 of Figure 2. Thus, the aluminum stranded wire of Figure 1 has a greater space factor than the wire in the prior art, and the conductors are more densely packed.
The cross sectional area of the aluminum portion, measured according to the embodiment of Figure 1, is of 207.8 mm whereas 159.3 mm is the cross sectional area of the aluminum portion of the standard ACSR.
As the transmission line of the present invention is composed to coincide in its outer diameter with the standard ACSR and to have a specific pattern of the aluminum stranded wire portion according to the present invention, many distinctive features, mentioned below, can be found.
The weight of the transmission line of this invention per unit length is of 766.3 kg/km, this being not substantiall~ different from the standard wire weight of 732.8 kg/km. This small difference does not affect the tension required for suspending the wire. The break weight of 7400 kgf according to this invention is larger than the standard wire break weight of 6980 kgf.
Further, the electric resistance, which is the most important feature of the present invention, is ~ L~1 . .
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1~'70045 0.140 ohm/km, instead of 0.182 ohm/km of the standard line. As a result, at a current of 454Ar the temperature of the wire comprising the present invention was found to be 81C, in contrast to the wire of the standard transmission line which was measured to have a temperature of 90 C. This excellent electrical characteristic permits a higher allowable current value of 518A at a wire temperature of 90C, rather than the 454A current of the standard line.
One aluminum wire of the most outer portion of the transmission line cross section of Figure 1 has a fin-shaped proiection 4. This projection 4 can be formed monolithically or separately. The purpose of the projection 4 is to prevent snow from freezing on the transmission line.

The projection 4 has a cross section having dimensions 1.5 mm x 1.5 mm and is twisted in helical form according to the pitch of the strand of the outer aluminum element wire on which it is formed.
The effectiveness of projection 4 is affirmed by an experiment illustrated in Figure 5. Three steel towers A, B, and C are prepared for suspending the line 10 of the present invention and the line 10a of the standard, respectively. These lines 10 and 10a are connected and suspended in parallel to each other between the steel towers A and B and between the steel towers B and C, the .

~70~3a.5 lines being suspended by insulators 11 at towers A and C.
The lines at the intermediate tower B are suspended by a loadcell 12 which can detect the weight of piling snow.
The experiment was performed in an area of significant snow fall. Piled snow was detected by means of the loadcell 12 and was automatically recorded by a suitable device. The weight of the snow piled onto line 10 of the present invention and line 10a of the standard ACSR were measured respectively and recorded every hour in chart form.
Figure 4 shows a chart measuring snow piling over the course of almost 20 hours. In the first one hour, it appears that the snow piling of the prior line is greater but this occurs because the piling of the two lines was different at the outset of the experiment.
In the above measured result, one can clearly see that the snow piling on the line of the present invention increases to about 0.3 kg/m, but at that value, snow falls off and piling does not increase above 0.3 kg/m. Thus, projection 4, formed around the line of the present invention effectively limits the piling of snow.
Snow piles to a much higher degree on the standard line, however, since when the snow piles up to a certain amount it rotates downward by its own weight but does not - 25 fall off immediately because of the surface tension between the wire and the snow. New snow falls on the wire and ., ~
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rotates downward to unite with the old rotating snow to form a cylinder. This occurs successively to gradually develop a concentric cylinder of snow around the wire with increasing rotational movement. The constant gradient of the curve relating to the prior wire in Figure 4 typically shows such a growth of snow as time elapses.
The transmission line of the present invention, and more specifically its fin-shaped projection, rejects this rotation of snow and therefore the growth of the snow around the wire. The rejection of the rotation of snow results in uneven snow weight around the transmission line and the snow will fall by its own weight much more easily.
In the experiment described above, the critical weight of surrounded snow to prevent its rotation around the wire and to fall from the line by its own weight is about 0.3 kg/m. The curve relating to the present invention in Pigure 4 shows that the snow growing over the critical weight point falls from the line.

Features of the low loss transmission line with a spiral fin of the present invention as compared with the standard aluminum stranded wire can be more clearly understood from the tables I to VI appended below.
It will be apparent to those skilled in the art that various changes and modifications may be made within the spirit of the above teachings. For example, in the above embodiment all of the aluminum element wires have ., , , . ~.. . .
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~27(~1~45 fan-shaped cross sections, but this is not necessarily a requirement. Note that to obtain sufficient results of the present invention the space factor of the aluminum portion is preferably over 85~.
The number of fin-shaped projections is not limited to one as is disclosed in the above embodiment. Indeed, the effect can still be expected by incorporating a plurality of such projections. Further, the fin-shaped projection also has the side benefit of reducing the turbulance of the air around the wire, so that wind noise and wire vibration are diminished.
The aluminum element wire used is not limited to the above EC class type. Various heat proof aluminum alloys, high strength system aluminum alloys, and so on can also be used for purposes of the present invention.
Any decrease in conductivity, resulting from the use of different materials, can be compensated for by further increase of the space factor of the conductors in the manner discussed above.
Accordingly, the transmission line of the present invention can greatly reduce the electric power loss of overhead transmission lines and can effectively prevent snow piling on the line.
In view of the above, the present invention may be embodied in other specific formswithout departing from the : ., .. ,::, , -:
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Claims (6)

CLAIM

1. A low loss stranded transmission line comprising:
a steel core comprising steel wires each having a tensile strength of more than 200 kg/mm2; and aluminum wires disposed around said core to provide at least one layer of aluminum conductors, at least some of said aluminum conductors being fan shaped.

2. The line according to claim 1, whereby the space factor of the cross section of said aluminum wires is at least 85%.

3. A line according to claim 1 wherein said tensile strength of the steel wires is higher than that of standard steel wires to decrease the cross sectional area occupied by said steel wires as compared to a standard transmission line, and the outer diameter of the line is no greater than that of a standard transmission line.

4. A line according to claim 1, wherein an outer one of said aluminum conductors is provided with an outward projection.

5. A line according to claim 4, with the aluminum conductor having said projection extending helically along the line.

6. A line according to claim 1 or 2, wherein said steel core member includes materials having more than 0.8% of C, more than 0.4-2.0% of Cr, and more than 0.8-1.5% of Si.