US 3080446 A
Description (OCR text may contain errors)
March 5, 1963 V. F. VOLK HIGH VOLTAGE CABLE Filed Jan. 16, 1961 INVENTOR. #4 f. M
United States Patent 3,tl8il,446 HIGH VOLTAGE CABLE Victor F. Volk, Hastings on Hudson, N.Y., assignor to Anaconda Wire and Cable Company Filed Jan. 16, 1961, Ser. No. 82,819 Claims. (Cl. 17425) My invention relates to pipe-type cable and particularly to pipe-type cable having the insulation thereof protected by a dielectric skid wire.
In the manufacture of high voltage cables of the type where electric conductors are wrapped with a plurality of layers of insulating material such as, for example, a plurality of paper tapes, and subsequetnly pulled into a steel pipe which is then filled with an insulating fluid, such as hydrocarbon oil or pressurized nitrogen it is known to protect the cable cores during the pulling operation with spiral windings of bronze skid wires. These known skid wires are most conveniently formed in halfround sections with the flat side laid against the insulation and the rounded surface making low friction contact with the inner walls of the pipe.
Cables of the type described in my application are generally known as pipe-type cables and will be so styled in this application although it will be understood that my invention is not limited by the nature of the enclosure comprising the containing element of my cable.
Most commonly, said containing element is an eight-inch steel pipe but it may be comprised of a nonmetallic ma.- terial such as cement or plastic and may have various degrees of flexibility.
The known construction of pipe-type cables having bronze skid wires has proven to be satisfactory in almost all respects but has the one serious shortcoming that the currents induced in the skid wire may constitute a serious power loss accounting, in some instances, to as much as 2% of the power losses in the cable.
It is an object of my invention to provide a pipe-type :cable wherein the skid wire is free from induced current losses.
It is another object of my invention to provide a pipetype cable in which the sldd wire will remain wrapped around the insulated conductor in the event of an ac cidental severing of the skid wire during installation.
I have invented a high-voltage electric cable comprising a conductor, electrical insulation surrounding said conductor, and a dielectric skid wire applied spirally over the insulation. The outer surface of my skid wire is preferably in compression so that the skid wire grips the insulation and will not unwind from the insulation even if it should be broken at some point. The insulated conductor and the skid wire of my cable are surrounded by a tubular container, overall. In a preferred embodiment of my invention the skid wire is comprised of linear polyethylene and in another preferred embodiment it is comprised of nylon. My cable has an electric shield over the insulation and in a preferred embodiment a plurality of conducting means are spaced at intervals alongthe skid wire making electrical contact between the shield and the tubular container which in this embodiment is metallic. The conducting means will preferably extend for a complete turn of the skid wire so as to be certain to contact the container regardless of the position of rotation of the insulated conductor.
I have also invented a process for manufacturing a pipe-type cable comprising the steps of insulating an electrical conductor, extruding a length of dielectric skid wire, forming the skid wire into a helix with an inside diameter not substantially larger than the diameter over the insulated conductor and wrapping the skid wire around the insulated wire while compressing the outer of which linear polyethylene is a preferred example. purpose of the skid wires l7, 18 is to protect the shield 3,089,446 Patented Mar. 5, 1353 "ice surface of the skid wire. Advantageously, the skid wire is annealed and cooled While in a helical form. When the skid wire is comprised of linear polyethylene, I prefer to anneal it at a temperature of 220-230 F.
A more thorough understanding of my invention will be obtained from a study of the appended drawing.
In the drawing:
The FIGURE is a lengthwise cut-away view of a cable made in accordance with my invention.
Referring now to the figure:
A pipe-type cable designated generally at 10 has a conductor 11 Wrapped with strand shielding 12 of semiconducting carbon black paper and insulated with a heavy wall 13 built up of a plurality of layers of insulating paper. The wall of insulation 13 is covered with a conducting shield 14 of copper tapes. The shield 14 is shown applieddirectly over the insulation 13 but there may be intermediate layers of carbon black tape or of impervious tapes applied for the purpose of preventing moisture from entering the insulation 13 during the processes of storing, shipping, and installing the cable. The conductor 11, strand shielding 12, insulation 13, shield 14, and any intermediate layers between the insulation 13 and the shield 14 constitute a unit 16 that I shall hereinafter cell an insulated conductor. Over the insulated conductor 16 I have applied two skid wires l7, 1%, apart. Although I have chosen to apply two skid wires my invention is not restricted to this number. For example a single skid wire might be applied at /2 the pitch that I have chosen for my two wires or a larger number such as three or four might be applied having a longer pitch.
The skid wires 17, 18 have each a substantially flat surface 19 having the insulated conductor 16 and a rounded outer surface 21. A steel pipe 22; constitutes a tubular container for the insulated conductor 16 covered by skid wires'll, 13. Although for the sake of simplicity I have shown only one insulated conductor 16 in the pipe 22, three such insulated conductors are generally inclosed in the pipe, each of the conductors being identical to the insulated conductor 16 and having skid wires applied identically to the skid wires 1'7, 18. The pipe 22 is filled with an insulating fluid such as an insulating oil which can penetrate the insulation 13 or a gas such as nitrogen maintained under super-atmospheric pressure.
it is a feature of my invention that the skid wires 17, 18 are comprised of a dielectric, c-r insulating material The 14 and insulation 13 from the effects of abrasion against the pipe 22 during installation. The method of installation requires that the insulated conductor 16 be unwound frorn reels and dragged into the pipe 22 by means of a cable fastened to the conductor 11. Commonly three insulated conductors such as the one designated 16 are pulled into the pipe 22 simultaneously from three different reels.
Prior to my invention bronze armor wires similar to my skid wires l7, 15 were wrapped around the core 16 to protect the insulated conductor during installation. These known armor wires had the disadvantages that they readily picked up induced electric current from the conductor 11, that they added considerably to the weight of the insulated conductor 16, and that they had a relatively high coefficient of friction against the walls of the pipe 22 compared to the skid wires 17, 18 of my invention. My skid wires 17, 18 are formed of a tough, resinous material which will not transmit electric current, has high resistance to abrasion, and has a. very low coefficient of friction. Polyethylene may be used for my skid wires 17, 18 or the vinyl resin known as rigid polyvinyl chloride. However, I prefer to form my skid wires from 3 7 linear -polyethylene or from. nylon. By linear polyethylene I refer to polyethylene resin'with a density of about 0.941 or higher. This polyethylene is made by a low pressure process such as the Ziegler process and is extremely tough and abrasion resistant.
Nylon is a generic term for a family of polyamide resins characterized by high tensile strength, toughness and abrasion resistance and by a low co eflicient of friction. Both the linear polyethylene and nylon which I prefer-for the skid wires of my invention are well known materials of commerce for which I make no claims of invention as such.
Where the pipe 22 is a metallic pipe and it is con sidered desirable to ground the shield 14 periodically along the length of the cable, one or both of the skid wires 17, 18 are wrapped at spaced intervals with metal bands 23, 24. The bands 23, 24 are conveniently applied to the skid wires 17, 18 at spaced intervals prior to the application of theskid wires to the insulated conductor 16. Although I prefer to use bands 23, 24 made from bronze or stainless steel it will be understood that helical windings 'ofmetal wire may be substituted for the bands or the surface of the skid wire may be coated with conducting paint. One of the principal advantages of my invention resides in the high electrical resistance of the dielectric skid wires 17, 18 as co'mpared'with the bronze armor wires known to prior art. This high electrical resistance prevents induced electrical currents in the skid wires. It is important, therefore, that the "conducting bands 23, 24 should be applied only at spaced intervals on the skid wires 17, 18 and not form a con tinuous conducting path along the cable. By spacing my conducting means '23, 24 I effectively prevent any appreciable power losses from induced currents. The pipetype cable constitutes relatively expensive engineering installation requiring considerable advance planning and preparation for a successful installation. If, in the course of pulling the insulated conductor 16. into the pipe 22, any accident were to befall the conductor 16 requiring the Withdrawal of the conductor from'the pipe and its return to the factory the financial losses would be great.
Such an accident would be the severing of one or both of the skid wires 17, 18 due to cutting edges on the inside of the pipe 22 or to foreign objects in the pipe. If the skid wires 17, 18 were merely wrapped around the insulated conductor 16 the outside surface 21 of the skid wires 17, 18 would be under tension and the inside surface 19 would be under compression in accordance with well known bending formulas and the skid wires 17, 18 would spring away from the insulated conductor 16 at. any point where a break should occur and would, indeed, become loose for the full length of the cable. It would be impossible to make repairs in thefield where the skid wires 17, 18 had become loosened over .any long length of the cable. It-should be further noted that the skid wires 17, 18 protect the cable .not only during the process of pulling the insulated conductor '16 into the pipe 22 but also after the pipe has been filled with insulating fluid and the .cable energized the skid wires 17, 18 are relied upon to protect the shield 14 and insulation 13 during the writhings that take place because of If the skid wires 17, 18 were applied so as to produce tension in the outside surface 21 not only would the skid wires tend to spring open if they were broken but any small cuts or abrasions of the surface would become focal points for tearing under the influence of the tension or stretch in the surface and such tearing might eventually extend through the entire thickness of the skid wire-and result in abreak. I have overcome the above mentioned obstacles by so applying the skid 'wires 17, '18 that the outer surfaces are in compression rather than tension. This has the twofold advantage of making the skid wires more resistant to abrasive action and of guaranteeing that they will remainsnugly boundto the insulated conductor 16 even though a break may occur at some point. If a break should occur'in a skid wire in the field it is a relatively simple matter to repair it so long as the skid wire does not spring open and the delay and expense of "returning the cable to the factory is avoided.
In order to produce compression rather than tension in the outer surface of my skid wires 17, 18 I first form them .into' a helix and anneal them in this form.
I clairn'i 1'. A pipe-type electric cable comprising a'conductor, electrical insulationsurrounding said conductor, a dielectric skid Wire applied helicall'y over said insulation the outer surface of said skid wire. being in compression, said skid wire gripping said insulation, and a tubular container surrounding said insulation and said skid wire.
2. Thecable of claim 1 wherein said skid Wire is comprised of linear polyethylene.
3. The cable-of-claim 1 wherein said skid wir eis comprised of nylon.
4. A pipe-type electric cable comprising a conductor, electrical insulation-surrounding said conductor, an electrical shield surrounding said insulation, a dielectric skid wire applied helically over "said shield, a plurality of conducting means wrapped around said skid wire at spaced intervals along said skid wire, and a metallic tubular container surrounding said shield and said skid wire, said "conducting means making electrical contact between said shield and said container. I
5. A pipe-type'electric cable comprising a conductor, electrical insulation surrounding'said conductor, an electrical shield surrounding said insulation, a dielectric skid wire applied helically in a plurality of turns over said shield, a plurality of conducting means wrapped around said skid wire 'at spaced intervals along said skid wire and extendingfor at least'one of said turns, and a metallic tubular container surrounding said shield and said skid wire, said conducting means making electrical contact between said shield'and said container.
References Cited in' the file of this patent UNITED STATES PATENTS .-2,'236,-286 .Dunsheath Mar. 25, 1941 2,516,747 Bennett July '25, 1950 2,665,328 Atkinson et al. Jan. 5, 1954 2,823,152 Mildner etal. e Feb. 11, 1958 FOREIGN PATENTS 67,330- Sweden June 18, 1927 Archer Feb. 11, 1958 I