Power cable with improved filling compound
BACKGROUND AND SUMMARY OF THE INVENTION 
     High-voltage power transmission cables have a center conductor surrounded by a semi-conducting shield between the conductor and the insulation of the cable. A semi-conducting shield surrounds the insulation, and there is an outer shield, usually a corrugated metal shield, around the semi-conducting shield of the insulation. 
     Such transmission lines are subjected to operating conditions not met with in other types of cables. It is important to prevent water or water vapor from gaining access to the insulation. Polyethylene insulation, which is the insulation most commonly used on such cables, is damaged by the formation of electro-chemical trees when moisture is present at regions of high electrical stress in the insulation. The voltages used for high-voltage power transmission lines are such as to cause regions of high electrical stress in the insulation. 
     Power transmission lines carry much heavier loads when demand is high; and the loads are greatly reduced at other times. This variation in the amount of current flowing through the conductors causes substantial variations in temperature at different times. 
     This &#34;load cycling&#34; results in expansions and contractions of the cable. When successive layers of the cable construction do not maintain their adhesion to one another, the expansion and contraction of the cable causes cracking at the interfaces of the layers and results in voids in which moisture can accumulate, and travel lengthwise of the cable where the voids intersect one another and provide longitudinal clearance between layers of the cable structure. 
     It has been the practice to use filler compounds within high voltage power transmission cables to prevent formation of voids and migration of moisture lengthwise of the cable between successive cable layers. 
     Such fillers or flooding compounds may be satisfactory on new cables; but as the cable ages, the compound may lose its adhesive qualities or may harden and fracture and portions of it may migrate into other layers of the cable, particularly the insulation layer. Such migration may facilitate the formation of electrochemical trees and greatly shorten the useful life of the cable. 
     There are many fillers and flooding compounds available commercially. I have tested the most promising of these compounds in extensive cable research tests and could not find any compound that had and retained all of the desired characteristics for a high-voltage transmission cable over long periods of time when subject to load cycling; high temperatures encountered under overload conditions; exposure to moisture, particularly at elevated temperatures and aging that tends to reduce adhesiveness and flexibility. 
     This invention is based on the discovery that certain compounds of isobutylene rubber do provide all of the desirable filler characteristics for a high-voltage power transmission cable. Specific compounds and their correlation in the cable construction will be explained more fully as the description proceeds. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views: 
     FIG. 1 is a diagrammatic view showing the way in which the compound of this invention is preferably introduced into a stranded conductor of a cable in accordance with this invention; 
     FIG. 2 is a cross-section of a high-voltage power transmission cable using the filler material of this invention; and 
     FIG. 3 is an enlarged sectional view taken on the line 3--3 of FIG. 2. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     High voltage power transmission cables are generally made with stranded conductors for greater flexibility. Individual strands designated in FIG. 1 by the reference character 10 are passed through a guide plate 12 to a forming die 14 which brings the strands together. Beyond the forming die 14 the stranded conductor passes through an extruder 16 which is used to apply a semi-conducting plastic shield over the stranded conductor. Beyond the extruder 16, there is other apparatus indicated generally by the reference character 18 and which may be conventional cable-making equipment for applying insulation, an insulation shield, filler or flooding compound and a metallic shield or other outer coating for the cable. This structure is illustrated diagrammatically in FIG. 1, and it will be understood that the stranding machine is ordinarily equipped to give the strands a helical lay, such apparatus being well known and no more detailed description of it being necessary for purposes of this application. 
     FIG. 1 also shows a container 20 from which the filler of this invention is supplied to the cable. A heater 22 raises the temperature of the compound in the container 20 to a temperature which permits pumping of the filler compound through tubing to a compound applicator 24. 
     A pump 26 driven by a motor 28 withdraws compound from the container 20 and pumps it through tubing 30 to the applicator 24 through which the strands 10 pass on their way to the forming die 14. Check valves 32 and 34 are preferably provided in the tubing 30. The compound coats the strands 10 in the compound applicator 24, and when the strands are brought together in the forming die 14, the compound fills all spaces between the strands and excess filler material drains from the applicator to a sump 36 from which it is recovered and reused. The finished cable is designated in FIG. 1 by the reference character 40, and FIG. 2 shows a greatly enlarged sectional view of the cable. 
     FIG. 2 shows a stranded conductor 40 to which a semi-conducting conductor shield 42 is applied. A layer of insulation 44, preferably polyethylene, surrounds the conductor shield 42; and there is a semi-conducting insulation shield 46 around the outside of the insulation 44. A corrugated metal shield 48 extends around the other structure of the cable, and there is a layer of filler compound 50 for sealing the space between the insulation shield 46 and the corrugated conductor 48, as will be more fully explained in connection with FIG. 3. 
     The filler material, that fills all spaces of the stranded conductor 40, is designated in FIG. 2 by the reference character 52. This material is a compound of low molecular weight polyisobutylene rubber or a low molecular weight copolymer of isobutylene-isoprene rubber. To either of these isobutylene rubber materials, 40 to 150 parts by weight of electrical conductive carbon black or graphite material is added for each 100 parts of the isobutylene rubber material. 
     The addition of this carbon makes the filler material semi-conductive, and it also serves an important function, in that it prevents flow of the isobutylene rubber material at temperatures up to 200° C. Thus the filler material can withstand temperatures encountered during heavy loads on the power transmission lines without softening and having its viscosity become so low that it will flow out of the cable at cable ends or flow lengthwise of the corrugated metal shield at locations where the cable is on a substantial slope. 
     Some additional material can be added, if necessary, as a processing aid; for example, a hydrocarbon oil, such as used in rubber compounding, or a chlorinated paraffin or isobutylene liquid plasticizer can be used to bring the isobutylene rubber compound to a pumping consistency without utilizing excessive heat. It is preferable, however, to use as little processing air as possible or none at all when it is not necessary for obtaining a pumping consistency. 
     The disadvantages of the processing aids are that they may migrate into the insulation shield and cause swelling and a consequent reduction in the conductivity of the shield. 
     The amount of electrical conductive carbon black or graphite material which is mixed with the isobutylene rubber material is from 40 to 150 parts by weight of the carbon to 100 parts of the isobutylene rubber compound; and the preferred range is from 40 to 50 parts of the carbon in order to obtain the desired conductivity. 
     The filler compound of this invention is of a puttylike consistency at room temperatures, and it has good adhesiveness. One of the outstanding advantages of the filler of this invention is its retention of adhesiveness in spite of repeated heating and cooling resulting from load cycling of high voltage power transmission cables. It is essential for a filler compound to retain both its flexibility and its adhesiveness in order to be satisfactory. If the compound becomes brittle, it will separate easily from an adjacent face of the material or fracture as the result of expansion and contraction of the cable. 
     The filler compound of this invention retains its flexibility and adhesiveness so that it prevents the cable, with which it contacts, during qualification testing. 
     For use between the outer interstices of the stranded conductor and the conductor shield or insulation and between the metallic shield and the cable core, the filler material of this invention also retains its resistivity. The volume resistivity of the filling compound applied, as above described, preferably does not exceed 50,000 ohm-centimeters, a value established by the cable industry for semi-conductive material used in cables. 
     The filler material of this invention is not intended as a replacement for the conductor shield 42 or the insulation shield 46. Its primary purpose is to provide an adhesive filler that sticks to the conductor and to the inside surface of the conductor shield 42. If the conductor is stranded, as is usually the case, then the filler of this invention eliminates voids in spaces between the strands of the conductor. This filler material is also used to provide an adhesive layer between the insulation shield 46 and the metal shield 48. By having these adhesive layers, any migration of water lengthwise of the cable is prevented. 
     Referring to FIG. 3, the filler compound 50 is shown adhered to the insulation shield 46 and also adhered to the inside surface of the metallic shield 48. The corrugations of the shield 48 are preferably circumferential. It is intended that the filler compound fill the entire space between the metallic shield 48 and the cable core. 
     Since the filler compound of this invention is semiconducting, it could be used to replace the conductor shield and/or the insulation shield; though not primarily intended for such use. If it is to be used to replace the conductor shield or insulation shield, then the filler material must be applied in such a way that an excess of the material surrounds either the conductor or insulation for a radial distance on all sides equal to the thickness of a semi-conducting shield that it replaces. 
     The preferred embodiment of this invention has been illustrated and described, but changes and modifications can be made and some features can be used in different combinations without departing from the invention as defined in the claims.