US3211821A - Electric cable - Google Patents

Description

Oct 12, 1965 F. w. WAKEFIELD 3,211,821

ELECTRIC CABLE Filed June 18, 1962 Lead Standard Standard Lead Sheath Conductor Conductor Semconduct- 6 ing Fahr/'c Tape 0/'/ Saturated 8 Semiconduct/'ng 517550 ,.0 n Semca /)ducting 01'/ Saturated Barr/'er 50/9" PGM-f /nsu/at/'ah Lead Sheath Bind/'ng Tape Crushed Saturated Paper Fi//er FREDERICK W WAKEF/ELD Attorney United States Patent O United States Steel Corporation, a corporation of New Jersey Filed June 18, 1962, Ser. No. 203,168 13 Claims. (Cl. 174-26) This application which is a continuation-impart of my copending application Serial No. 64,852 filed October 25, 1960 now abandoned, which in turn is a continuation-inpart of my abandoned application Serial No. 803,940 filed April 3, 1959, relates to an electric cable and more particularly to an oil impregnated solid type metal covered power cable. Such cables have been extensively used for a long time and have the advantages of having low losses and being relatively cheap to manufacture. However, they have certain disadvantages which reduce their service life. The saturating oil completely permeates the paper insulation and stranded copper conductor with a chemical reaction taking place between the saturating oil and the copper conductor which adversely affects the electrical properties of the saturating oil. When the cable transmits power the copper conductor becomes hot, thus accelerating the chemical reaction and causing the deteri orated -oil to migrate into the paper insulation. This condition -is especially bad because the paper insulation nearestthe conductor is most affected, this being the position where the voltage stress across the insulation is maximum.

When a solid type paper cable is installed on an incline the saturating oil migrates, by gravity, from the high points to the low points. Most of this migration takes place through the spaces in the stranded conductor, which is the hottest part of the cable during load cycles and where the resistance to axial flow is less. The dry paper insulating at the cable high spots result in ionization and hot spots which eventually lead to a voltage failure caused by thermal instability.

When a solid type paper cable is heated, the increase in volume due to the expansion of the lead covering is much less than the volume increase of the saturating oil, with the result that high pressure builds up inside the cable. This internal pressure is sufficient to cause permanent expansion of the lead covering or sheath. During periods of light load the cable cools off and the saturating oil contracts, thus causing very low pressure gas voids inside the cable. Severe ionization takes place in these low pressure voids and usually starts a tracking type failure near the conductor where the volts per mil stress is the highest. Once the first few tapes near the conductor have been punctured it is easier for the carbon track to continue through the insulation and cause a voltage failure.

In some instances a layer or layers of various types of .material have been used between the conductors and the paper insulation, but these materials have not been satisfactory for various reasons. The material may not be impervious to the oil so that the oil will migrate into the conductor, the material may not be oil resistant so that -it will contaminate the oil and raise its power factor, and/or the material may not be sufciently resilient to prevent the formation of low pressure voids in the paper insulation.

In order to overcome `some of these difliculties oil filled cables have been provided, but these require oil reservoirs at frequent 4intervals and require gas supply apparatus which must be maintained and supervised. Also pressure is usually applied to the outside of the cable insulation. Even after draining the cable core at an elevated temperature there is usually enough saturant remaining in the insulation to gather in the pressure tubes Patented Oct. 12, 1965 ICC and follow to the lowest point of the installed cable. Once the pressure tubes are filled for a considerable distance with the Viscous saturant it is difficult for pressure compensation to take place, especially during light cable loads when it is most required to prevent tracking. Manufacturing cost is high and in case of gas pressure failure damage may occur due to tracking before the tube can be corrected.

It is therefore an object of my invention to provide an improved oil impregnated insulated cable which eliminates or greatly reduces the aforementioned disadvantages. g

Another object is to provide such a cable having builtin pressure compensation.

Still another object is to provide such a cable which has a much longer life than the solid type cable previously used.

These and other objects will be more apparent after referring to the following specification and attached drawings, in which:

FIGURE l is a cross section of a preferred embodiment of the cable of my invention;

FIGURE 2 is a view, similar to FIGURE 1, showing a second embodiment of my invention; and

FIGURE 3 isa cross sectional view of a multi conductor cable utilizing my invention.

Referring more particularly to FIGURE 1 of the drawings, reference numeral 1 indicates a conductor which is preferably stranded copper wire. According to my invention I provide a resilient oil impervious and'oil resistant semi-conducting barrier around said conductor. As shown in FIGURE 1 this barrier includes a semi-conducting fabric tape 2 adjacent the conductor 1- and an insulating layer 3 .around the fabric tape 2'. The` insulator 3 is preferably irradiated polyethylene, but may be cross linked polyethylene. Oil impregnated paper insulation 4 surrounds the layer 3 and a lead sheath 5 surrounds the insulation 4. The materials of layer 3 are oil resistant, relatively impermeable to air and oil, and able to withstand temperatures of at least approximately 115 C. without substantial deterioration. Other physical properties are as follows:

Properties: irradiated polyethylene Percent power factor-60 cycles 0.05%. S.-I. C 60 cycles 2.3. Electric strength l800 volts per mil; Volume resistivity 1015 ohm-cm. Moisture resistance Good.

The oil is that commonly used in electric cables. One particular .type often used is Sun Oils XX Heavy Cable Impregnating Oil which is a mineral oil having the following properties:

Of the above properties the viscosity and the initial percent power factor are the most significant. To prevent undue migration of the oil the viscosity should not be below and for manufacturing reasons the viscosityshould not exceed 1000. This is true of all impregnating oils used in solid type impregnated paper cable.

.occur during heat cycles.

In making the cable it is preferred that the layers 2 and 3 be applied in tape form. The layer 3 may al-so be applied to the conductor 1 by extrusion. The paper insulation 4 is thenapplied in the usual manner and the assembly is placed in an oil saturating tank where heat and vacuum is applied to remove moisture from the assembly. During the dryingprocess ysuflicient heat is supplied to bond the semi-conducting tapes into a semi-conducting layer. The ends of the dry assembly are positioned in the tank above the saturating oil level, the ends of the copper strands Isealed vWith solder and the entire cross-section of the ends sealed with oil resistant tape, thus preventing the oil from entering the stranded conductor 1. After cornpleting the oil wsaturation process the assembly is allowed to cool and the lead sheath 5 immediately applied to complete the cable.

Since the saturating oil does not contact the conductor 1 there can be no chemical .action between the saturating oil and the copper. Since the oil in the conductor is eliminated, less than normal expansion of the lead sheath will When the cable cools olf the air entrapped in the conductor 1 will expand and prevent any very 'low pressure rvoids'fro'm forming in the paper insulation 4, especially near the conductor 1. When the that the vnormal rate of oxidation of the oil is not increased at elevated temperatures and hence there is little or no effect on the power factor. The life Iof such cable is approximately '2l/2 times the life of ordinary solid type cable.

In the embodiment of my invention shown in FIGURE 2 a stranded conductor 6 is surrounded by a resilient oil impervious and oil resistant semi-conducting barrier 7 which is preferably irradiated polyethylene made semiconducting by adding carbon particles thereto. Oil impregnated paper insulation 8 surrounds the barrier 7 and a lead vsheath 9 surrounds the insulation 8. 'Ilhis cable is made in essentially the same manner and will function in essentiallythe same manner as the cable of FIGURE l.

In the embodiment of my invention shown in FIGURE 3 three spaced apart electrical conductors 11 are provided. A resilient oil impervious and oil resistant semi-conducting barrier 12 surrounds each conductor and Ioil impregnated `paper insulation 13 surrounds the'barrier 12. The memlmetallicground Wires, but lsare'preferably non-metallic material ysuchas manila rope which can Vbe deformed under pressure. lA lbarrier 16 of the same type as barrier 12 is arranged `aroundea'ch stranded 'member 15. Crushed oil :saturated paper ller Y17 `fills vthe voids 'between shielding 1 4 and barriers 16. VBinding tape 18 is arranged over the "ent-ire assembly beneath la'lead sheath 19.

In making the cable of vFIGURE 3the barrier 12 is applied in Athe manner set 'forth above'and will bond to k'the paper 'insulation 13,"but'n-otto the conductor 11. The

paper 'insulation 13`is applied 'in the usual manner and lthe shielding 14, which is preferably copper tape, is ap plied atthe same time as the paper. The insulated con- Aductors'are then stranded together and the covered members positioned between the insulated conductors 11 as shown. The crushed paper fillers are then applied acording to standard practice and the metallic binder tape 18 applied overall. The cable is then positioned in the saturating tank with the cable ends sealed with a material, such as an epoxy resin, and positioned above the oil level in the saturating tank. The cable is vacuum dried at a temperature between and 120 C. which is suiiicient to heat seal the polyethylene around the stranded conductors 11 and around the stranded members 15. The hot saturating oil is excluded from the conductors 11 and members 15 by means of the oil barriers 12 and 16. The entrapped air in the conductor strands compensates for pressure changes on the inside area of the cable insulation and the entrapped air in the stranded members 16 compensates for pressure changes occuring on the outside area of the cable insulation. The members 15 should be as large as possible. If the members 15 are manila rope the cable pressure will be reduced by the rope assuming an elliptical shape so as to reduce its cross sectional area. Thus, the barriers 12 and 16 are forced into the outside surface depressions of members 11 and 15 due to 'increasing internal cable pressure caused by the saturating oil, especially when it is heated. When the saturating oil cools and contracts, the entrapped air in members 11 and 15 will expand barriers 12 and 16, respectively, and thus tend to adjust the oil-paper volume within the sheath 19. No excessive strain will be exerted on the lead sheath 19 because the barriers 12 and 16 will change their volumes at much lower pressures than the lead sheath 19. The cable pressure and vacuum peaks are also at a minimum because the oil is excluded from the members 11 and-15 so that there is less saturant per unit length in the cable than that normally present.

Ordinarily most saturant migration due to gravity takes place inside the copper'strand land since no oil is present in the copper strands saturant migration is greatly reduced. Also, there is not contact of the oil with the copper which acts as a catalyst to aid deterioration of the oil.

While several embodiments of my invention have been shown and described, it will be apparent that other adaptations and modifications may be made Without departing from the scope of the following claims.

I claim:

1. An electric cable comprising -a plurality of spaced apart stranded electrical conductors, a resilient oil impervious and oil resistant semi-conducting barrier surrounding each conductor, said barrier being made of a material able to withstand temperatures of at least approximately C. without substantial deterioration,

a solid insulating material impregnated with a mineral oil surrounding each barrier, shielding surrounding said insulating material, a plurality of 'stranded members arranged between said conductors 'on the outside of said insulating material, a resilient oil impervious and oil Iesistant semi-conducting barrier surrounding each of said stranded members, a metal sheath surrounding said last named barriers and shielding, and an oil impregnated iiller between said metal sheath and said last named barriers and shielding, said barriers confining s aid oil tothe space outside of said conductors.

2. An electric cable according to claim 1 in which said lirst named semi-conducting barriers include a semi-con` ducting layer adjacent the conductor surrounded by `a; layer of insulating irradiated polyethylene.

3. An electric cable according to claim 1 in which said semi-conducting barriers include irradiated polyethylene impregnated with conducting material.

4. An electric cable'according to claim 1 in which said stranded members are made of compressible material.

5. An electric cable according to claim 4 in which said first named semi-conducting barriers include a semi-conducting layer adjacent the conductor surrounded by a layer of insulating irradiated polyethylene.

6. An electric cable according to claim 4 in Which said semi-conducting barriers include irradiated polyethylene` impregnated with conducting material.

7. An electric cable comprising three spaced apart electrical conductors, a resilient oil impervious and oil resistant semi-conducting barrier surrounding each conductor, said barrier being made of a material able to withstand temperatures of at least approximately 115 C. without substantial deterioration, a solid insulating material impregnated with a mineral oil surrounding said barrier, a shielding layer surrounding said insulating material, three stranded members arranged between said conductors on the outside of said insulating material, a resilient oil impervious and oil resistant semi-conducting barrier surrounding each of said stranded members, a metal sheath surrounding said last named barriers and shielding, a binding tape on the inside of Said metal sheath, and an oil impregnated filler between said binding tape and said last named barriers and shielding, said barriers conning said oil to the space outside of said conductors.

8. An electric cable according to claim 7 in which said first named semi-conducting barriers include a semiconducting layer adjacent the conductor surrounded by a layer of insulating irradiated polyethylene.

9. An electric cable according to claim 7 in which Said semi-conducting barriers include irradiated polyethylene impregnated with conducting material.

10. An electric cable according to claim 7 in which said stranded members are made of compressible material.

11. An electric cable according to claim 10 in which said rst named semi-conducting barriers include a semiconducting layer adjacent the conductor surrounded by a layer of insulating irradiated polyethylene.

12. An electric cable comprising a plurality of spaced apart stranded electrical conductors, a resilient oil impervious and oil resistant semi-conducting barrier surlrounding each conductor, a solid insulating material impregnated with a mineral oil surrounding each barrier, shielding tape surrounding said insulating material,

a plurality of stranded members arranged between said conductors on the outside -of said shielding tape, a resilient oil impervious and oil resistant semi-conducting barrier surrounding each of said stranded members, a metal sheath surrounding said last named barriers and shielding, an an oil impregnated filler between said metal sheath and said last named barriers and shielding, each of said rst named barriers including a semi-conducting fabric tape and a tape of a compound of the class consisting 5 lof irradiated polyethylene and cross linked polyethylene surrounding said semi-conducting tape, said barriers coniining said oil to the space outside of said conductors.

13. An electric cable comprising three spaced apart stranded electrical conductors, a resilient oil impervious and oil resistant semi-conducting barrier surrounding each conductor, a solid insulating material impregnated with a mineral oil surrounding said barrier, a shielding tape surrounding said insulating material, three stranded members arranged between said conductors on the outside of said insulating material, a resilient oil impervious and oil resistant semi-conducting barrier surrounding each of said stranded members, a metal sheath surrounding said last named barriers and said tape, a binding tape on the inside of said metal sheath, and an oil impregnated ller between said binding tape and said last named barriers and said shielding tape, each of said barriers including a semi-conducting fabric tape and a tape of a compound of the class consisting of irradiated polyethylene and cross linked polyethylene surrounding said semiconducting tape, said barriers confining said oil to the space outside of said conductors.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 7/36 Great Britain. 7/ 45 Great Britain,

LARAMIE E. ASKIN, Primary Examiner.

JOHN P. WILDMAN, JOHN F. BURNS, Examiners.

Claims (1)

1. AN ELECTRIC CABLE COMPRISING A PLURALITY OF SPACED APART STRANDED ELECTRICAL CONDUCTORS, A RESILIENT OIL IMPERVIOUS AND OIL RESISTANT SEMI-CONDUCTING BARRIER SURROUNDING EACH CONDUCTOR, SAID BARRIER BEING MADE OF A MATERIAL ABLE TO WITHSTAND TGEMPERATURES, OF AT LEAST APPROXIMATELY 115*C. WITHOUT SUBSTANTIAL DETERIOURATION, A SOLID INSULATING MATERIAL IMPREGNATED WITH A MINERAL OIL SURROUNDING EACH BARRIER, SHIELDING SURROUNDING SAID INSULATING MATERIAL, A PLURALITY OF STRANDED MEMBERS ARRANGED BETWEEN SAID CONDUCTORS ON THE OUTSIDE OF SAID INSULATING MATERIAL, A RESILIENT OIL IMPERVIOUS AND OIL RE-

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