EP0129755B1 - Elektrisches Starkstromkabel - Google Patents

Elektrisches Starkstromkabel Download PDF

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Publication number
EP0129755B1
EP0129755B1 EP19840106571 EP84106571A EP0129755B1 EP 0129755 B1 EP0129755 B1 EP 0129755B1 EP 19840106571 EP19840106571 EP 19840106571 EP 84106571 A EP84106571 A EP 84106571A EP 0129755 B1 EP0129755 B1 EP 0129755B1
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EP
European Patent Office
Prior art keywords
film
kraft paper
electric power
power cable
cable according
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
EP19840106571
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English (en)
French (fr)
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EP0129755A1 (de
Inventor
Toshiya C/O Toray Industries Inc. Yoshii
Kenji C/O Toray Industries Inc. Tsunashima
Satoshi C/O Toray Industries Inc. Horiuchi
Ryosuke Sumitomo Electric Ind. Ltd. Hata
Shosuke Sumitomo Electric Ind. Ltd. Yamanouchi
Masayuki Sumitomo Electric Ind. Ltd. Hirose
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
OFFERTA DI LICENZA AL PUBBLICO;PUBBLICO
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Sumitomo Electric Industries Ltd
Toray Industries Inc
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Publication of EP0129755A1 publication Critical patent/EP0129755A1/de
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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01BCABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B9/00Power cables
    • H01B9/06Gas-pressure cables; Oil-pressure cables; Cables for use in conduits under fluid pressure
    • H01B9/0611Oil-pressure cables
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01BCABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B7/00Insulated conductors or cables characterised by their form
    • H01B7/02Disposition of insulation
    • H01B7/0208Cables with several layers of insulating material

Definitions

  • the invention relates to an electric power cable as set forth in the preamble of claim 1.
  • a cable is known from DE-A-2 412 222.
  • electric power cables comprising a conductor, an insulating layer formed on said conductor comprising various combinations of layers of paper and polypropylene films. Further, the insulating layers are impregnated with an insulating oil.
  • dielectric insulating body made up of three parts, each of which comprises laminate tapes of sandwiched polypropylene/paper.
  • the insulating layer formed thereby is impregnated by an insulating oil.
  • Polypropylene film or alkene polymer film as used in the known insulating layers provides a considerable increase of dielectric breakdown voltage with respect to insulating paper and has further advantages such as a low dielectric loss factor and a dielectric constant approximating the dielectric constant of the insulating oil.
  • the conventional polypropylene insulated cables have a disadvantage in that swelling with the insulating oil occurs to an exceptionally great extent.
  • the cable When the cable is used in any application involving oil immersion, it has entailed various restrictions.
  • the film When a cable is wrapped in polypropylene film and the resultant polypropylene insulated cable is immersed in insulating oil, for example, the film swells in order to tighten its pressure on the cable and consequently deprive the cable of its flexibility and impair the fluidity of the insulating oil between the insulating film turns.
  • the film may be more loosely wound around the conductor. When the film is loosely wound, however, there is a possibility of the film slipping out of place or being wrinkled.
  • Another disadvantage suffered by the conventional polypropylene insulated cable is that, where edges of the insulating film overlap, the fluidity of the insulating oil between the adjacent turns is liable to be low, possibly to the extent of inducing dielectric breakdown.
  • a primary object of the invention is the provision of an oil-immersion electrically insulated cable suffering only nominal loss and enjoying excellent insulation by having an insulation layer formed on a conductor by winding at least a sheet of improved polypropylene film on the conductor, thereby eliminating the above-noted defects, including excessive swelling.
  • Another object of the invention is to provide an oil-immersion electrically insulated cable of the type above, which has an insulation layer formed on a conductor by alternately winding at least a sheet of polypropylene film and at least a sheet of kraft paper, thereby, in addition to the elimination of swelling problem, eliminating insufficient fluidity of the insulating oil in the insulating layer.
  • This invention accomplishes the object noted above by providing a cable which is characterized by having an insulator formed of polypropylene film of an oil-immersion electric insulation grade having a density in the range of 0.905 to 0.915 g/cm 3 , birefringence in the range of 0.020 to 0.035, a ratio of strengths in two axial directions (tensile strength in longitudinal direction/tensile strength in lateral direction) in the range of 5 to 15, and a thickness in a range of 70 to 300 Ilm.
  • polypropylene hereinafter referred to as "PP" for short
  • PP polypropylene of a grade having an isotacticity of at least 90%, preferably at least 95%, and more preferably at least 97%, and a melt index in the range of 0.5 to 40 g/10 minutes, preferably 1 to 20 g/10 minutes.
  • An isotacticity below the lower limit mentioned above is undesirable because such increases the degree of swelling with the insulating oil. If the melt index is below the lower limit mentioned above, the amount of swelling with the insulating oil also increases.
  • melt index is above the upper limit mentioned above, the amount of the polymer dissolved in the insulating oil is increased, and consequently the viscosity of the insulating oil rises.
  • those materials which prove suitable for the manufacture of the cable of this invention fulfill the requirement that the temperature of melt crystallization (T me) be in the range of 105° to 120°C, preferably 108° to 118°C.
  • T me temperature of melt crystallization
  • a PP species having a T below the lower limit mentioned above suffers from a great increase of the degree of swelling with the insulating oil.
  • a PP species having a T me exceeding the upper limit mentioned above exhibits an inferior film-forming property and produces a homogeneous film with difficulty and, consequently, aggravates dielectric faults.
  • the PP film to be used for the cable of this invention is required to have a density in the range of 0.905 to 0.915 g/cm 3 , preferably 0.907 to 0.912 g/cm 3 . If the density is below the lower limit mentioned above, the degree of swelling with the insulating oil is increased. Conversely, if the density is above the upper limit mentioned above, the PP film becomes brittle and the mechanical strength of the insulating layer of the cable is insufficient.
  • the birefringence of the PP film to be used for the cable of this invention is required to fall in the range of 0.020 to 0.035, preferably 0.025 to 0.032.
  • the ratio of strengths in the two axial directions of the PP film to be used for the cable of this invention namely, the quotient of the tensile strength of the film in the longitudinal direction divided by the tensile strength thereof in the lateral direction, is required to be in the range of 5 to 15, preferably 7 to 12. If this ratio is less than the lower limit mentioned above, the swelling of the PP film with the insulating oil increases beyond a tolerable extent.
  • the PP resin is melted, extruded in the form of a sheet through an extrusion die, wound on a cooling drum, and left to cool and solidify.
  • the PP sheet thus obtained is passed between a set of reducing rolls and rolled with a rolling ratio (the quotient of the thickness of sheet after rolling divided by the thickness of sheet before rolling) in a range of 5 to 12, preferably 7 to 10.
  • the pressure of rolling is desirably in a range of 10 to 3000 kg/cm, preferably 100 to 1000 kg/cm, and the temperature of the reducing rolls is desirably in a range of 60° to 160°C.
  • the PP sheet can be easily rolled uniformly at a high rolling ratio by wetting the surface of the PP sheet with a suitable liquid (such as water, an aqueous solution of surface active agent, alkylene glycol, polyalkylene glycol, glycerin, or an electrically insulating oil) while the PP sheet is entering the reducing rolls.
  • a suitable liquid such as water, an aqueous solution of surface active agent, alkylene glycol, polyalkylene glycol, glycerin, or an electrically insulating oil
  • the PP film obtained by the foregoing rolling treatment (generally in a thickness in a range of 10 to 300 microns) is again heated to 100° to 150°C and subjected to heat treatment at this temperature for a period of 1 to 20 seconds until it slackens by 0.5 to 10% of the original size in the longitudinal direction.
  • the present invention is characterized by possessing the features described above.
  • the oil-immersion electrically insulated cable of the present invention can be obtained in a more desirable form by limiting the ratio of thermal shrinkage of the PP film in the longitudinal direction to the range of 0.1 to 5%, preferably 0.5 to 3%. if the ratio of thermal shrinkage exceeds the upper limit mentioned above, a disadvantage results in that the insulating layer is liable to tighten and consequently wrinkle. If this ratio is less than the lower limit, the film in the insulating oil is liable to stretch in the longitudinal direction and the insulating layer wound on the cable slacken.
  • the ratio of thermal shrinkage in the longitudinal direction can be confined within the range of 0.1 to 5%, preferably, 0.5 to 3%.
  • the thickness of the PP film sheet is limited to the range of 70 to 300 Il m for the following reason. If the thickness is smaller than 70 ⁇ m, there is a fair possibility that the PP sheet will sustain fracture and consequently fail to provide a mechanical strength required for permitting the wrapping the PP tape around a conductor to produce an insulating layer and, in the finished OF cable, fail to retain a strength necessary for enabling the cable to resist flexing, and cause the resultant insulating layer to sustain abnormalities such as wrinkles, dents, and collapses which adversely affect the electrical properties of the cable.
  • the required thickness of the insulating layer is obtained by adjusting the number of plies of film tape wrapped on the conductor. If the thickness of the tape is small, the number of plies of the tape is proportionally increased, with the result that the size of the equipment needed increases and the amount of work involved in mounting, replacing, and splicing film tapes increases. If the thickness of the PP film exceeds 300 pm, the PPtape has an excessively high stiffness such that the tape, when being wound on the conductor to produce an insulating layer thereon, offers resistance in conforming to the contour of the
  • the film tape is wound on the conductor in such a manner as to permit the occurrence of gaps between edges of adjacent turns of film tape.
  • the thickness of oil layers formed in such gaps increases with the thickness of the tape.
  • the electrical strength of the oil layer is lower than the insulating strength of the tape portion. The fact that the oil layers notably increase in size, therefore, does not prove very favorable.
  • OF cable is produced by preparing PP films of varying sheet thicknesses in the range of 70 to 300 um, cutting these PP films into PP tapes of a suitable width, winding on the inner side of the insulating layer (the side bordering on the conductor and, therefore, experiencing severe electrical stress) PP tapes of smaller thickness, which are rather inferior mechanically but quite superior electrically, and, on the outer side of the insulating layer (the side where the electrical stress is less toward the outside and the effects of flexing exerted thereon increase toward the outside), PP tapes of greater thickness, which are rather inferior electrically but quite superior mechanically.
  • kraft paper as used herein means ordinary insulating paper which has been conventionally used in OF cables of the EHV class.
  • the thickness of the kraft paper to be used in the cable of this invention is limited to the range of 7.0 to 300 Ilm for the same reasons given above with respect to the thickness of the PP film.
  • alkylbenzenes containing an aromatic ring particularly DDB (dodecyl benzene), which is commonly used in cables, best suits the purpose of the invention.
  • DDB dodecyl benzene
  • DDB proves to be an ideal insulating oil. With respect to condition (C), however, DDB is not ideal in that it causes swelling of the PP film.
  • the degree of compatibility between a film and insulating oil is determined by their respective SP values (index of solubility); the similarity between the film and the insulating oil in a particular combination increases and the ability of the insulating oil to swell the film also increases as the SP values of the film and the insulating oil approach each other.
  • SP values index of solubility
  • Both. PP and DDB have SP values approximating 8.
  • their combination has been heretofore held to be less desirable than the other combinations, such as PP and polybutene oil or PP and silicone oil, because it has high mutual compatibility and entails a high degree of swelling.
  • the inventors have found that, since the swelling of the PP film by the insulating oil is caused by this oil penetrating the amorphous phase of the film, the fortification of the amorphous phase, which constitutes one electrically weak point of the PP film, renders the lubricating oil in the combination susceptible to heavy swelling of the PP film more desirable from the electrical point of view.
  • DDB proves all the more desirable electrically in the sense that it possesses a benzene ring which causes it to excel in gas absorbing properties and resistance to corona discharge.
  • the impulse breakdown value of one PP film layer is about 0.8 in the combination with polybutene, and 0.6 to 0.7 in the combination with silicone oil. This trend applies to the AC breakdown strength of the PP film.
  • the cable is left standing at the maximum expected actual working temperature (generally in a range of 85° to 95°C) for 24 to 48 hours to allow the PP film to swell to saturation prior to the shipment of the cable. This conditioning is effective to ensure the cable possesses good electrical properties from the outset of its service.
  • the amount of swelling of the PP film with DDB can be restrained by optimizing the film's density, birefrigence, and ratio of strength in two axial directions within the ranges mentioned above.
  • actual measurements indicate that the ratio of increase in the thickness of the film by swelling in the present combination of PP film and DDB is one-half that in the combination of homo-casting PP film and DDB.
  • the surfaces of either or both the kraft paper and PP film are embossed to produce bosses of a size sufficient for absorbing an increase of the thickness of the PP film due to swelling, preventing the pressure within the layer of the insulating tape from abnormally rising, and maintaining the fluidity of the lubricating oil.
  • the surface-roughness R max thus produced is required to be in a range of 1 to 50 pm, preferably 2 to 40 pm. If the surface roughness is less than the lower limit mentioned above, the amount of swelling of the PP film to be absorbed by the bosses is insufficient and the fluidity of the insulating oil in the layer is deficient, thereby inducing dielectric breakdown.
  • the PP film may be impaired by the embossing treatment and the bosses formed occupy too much volume and therefore continue their existence even after the-swelling of the film, with a possible result that oil passages may occur between overlapping film tapes, which effect degrades the electrical strength of the layer.
  • the coarsening of the surface of the PP film may be effected by an embossing treatment, for example.
  • the PP film of the invention is passed between embossing rolls held at 90° to 140°C to coarsen either or both of the opposite surfaces of the PP film, with the surface roughness R max falling in the range of 1 to 50 ⁇ m, preferably 2 to 40 ⁇ m.
  • the combination of rolling and embossing treatments proves to be most desirable.
  • other methods may be used.
  • the rolling treatment of the aforementioned combination may be replaced by a combination of rolling and stretching treatments or by a stretching treatment using closely spaced rolls.
  • the embossing treatment of the aforementioned combination may be replaced by a sand blasting process or etching process to effect the desired surface coarsening.
  • an embossing treatment with embossing rolls is most desirable. Otherwise, a process of spraying water drops may be utilized.
  • the cable which is obtained is suitable for use as EHV through UHV classes of 275 to 1000 KV, particularly the UHV class.
  • the alternate winding of kraft paper and PP film is essential for the manufacture of the cable of this invention for the following two reasons: First, this winding provides the produced cable with improved mechanical strength which a cable insulated exclusively with PP film does not easily attain. Secondly, the interposition of kraft paper containing a polar group between the opposed surfaces of PP film and the distribution thereof throughout the entire insulating layer serves to improve the electrical strengths, particularly impulse strength, especially, positive impulse strength.
  • the thermal expansion coefficient of kraft paper is extremely small, in fact, about two orders of magnitude lower than the thermal expansion coefficient of PP film.
  • the Young's modulus of Kraft paper is small compared with that of PP film.
  • a sheet of kraft paper When a sheet of kraft paper is combined with a sheet of PP film in such a manner that it has at least one surface thereof bordering on the sheet of PP film, there are derived numerous advantages, including the fact that the cushioning effect of kraft paper greatly facilitates the control of the inner pressure between adjacent tapes, permits the conditions of cable production to be selected in very wide ranges, and renders the production easy such that the adjacent tapes in the produced cable are allowed to slide smoothly over each other and do not suffer mutual displacement because the kraft paper absorbs flexion exerted thereon during handling prior to actual installation of the cable. Further, the kraft paper very smoothly absorbs any increase of the thickness of the PP tape due to swelling and thermal expansion and permits the inner pressure between the adjacent tapes to be easily maintained at the optimum level and discourages formation of gaps between the adjacent tapes.
  • plastic film which lacks a polar group and has carbon and hydrogen atoms arranged very neatly and orderly, is slightly inferior in resistance to corona discharge to kraft paper, which contains a polar group and has carbon and hydrogen atoms distributed randomly.
  • the reason for this difference remains yet to be clarified.
  • This trend is conspicuous particularly with respect to the impulse strength, especially, the positive impulse strength.
  • the inventors have found that the combination of kraft paper and the PP film of this invention manifests outstanding properties because the kraft paper gives rise to uniformly distributed barrier interfaces in the insulating layer. This discovery has led to the provision of a cable of excellent electrical strength.
  • Fig. 2 is a cross section of an oil-immersion electrically insulated cable.
  • reference numeral 1 denotes a path for oil
  • 2 a conductor
  • 3 an oil-immersion insulating layer wound on the conductor
  • 4 a metallic sheath of aluminum or lead enclosing the oil-immersion insulating layer
  • 5 a corrosion-proofing layer superposed on the sheath 4.
  • Fig. 1A is a cross section illustrating a typical insulation structure according to the present invention. It depicts the portion Z of the cross section of Fig. 2 in the form of an enlarged model.
  • Fig. 1A represents one version of the insulation structure wherein sets each composed of one sheet of PP film 3a and one sheet of kraft paper 3b are repeated throughout the entire insulating layer. Since the ratio of PP film and kraft paper in this structure is roughly 1:1, the value of ⁇ ⁇ tan 6 of the completed cable is intermediate the respective values of ⁇ ⁇ tan 6 of the two materials.
  • this structure Since this structure has sheets of kraft paper providing an excellent cushioning effect, each interposed between adjacent sheets of PP film, it can cope easily with enlargement of the PP film due to swelling. This structure is produced most easily because it offers ample allowance for surface coarsening of the kraft paper and provides good control of the winding tension of the tapes. Even for the sake of flexibility and other mechanical properties of the completed cable, the fact that the kraft paper manifests an excellent cushioning effect is a highly desirable merit. Further, since sheets of kraft paper are interposed between adjacent sheets of PP film and thus are distributed throughout the entire thickness of the insulating layer, providing the effect of a barrier, virtually no loss occurs in the electric breakdown strengths, specifically, the positive impulse breakdown strength measured with the conductor side with the higher stress being the positive pole. The thickness effect of the plastic film, i.e., the loss of breakdown strength which occurs when a layer formed exclusively of sheets of PP film is given an increased thickness is eliminated. Thus, the cable using this insulation structure also excels electrically.
  • the cable of the insulation structure of Fig. 1A is stable and excellent both mechanically and electrically.
  • it is suitable for use in EHV to UHV classes of 275 to 1000 kV.
  • the cable is required to possess a still lower value of ⁇ ⁇ tan 6.
  • the inventors have consequently developed the insulation structure illustrated in Fig. 1B. More specifically, in this structure, sets, each consisting of two sheets of PP film 3a and one sheet of kraft paper 3b interposed therebetween, are repeated throughout the entire insulating layer. In this insulation structure, the cushioning effect of the kraft paper and the resistance offered to corona discharge by the barriers of kraft paper are excellent.
  • the cable attains the desired reduction of dielectric loss tangent at substantially no sacrifice of other mechanical and electrical properties.
  • the cable suffers a slight loss in its positive impulse property, it retains the barrier effect of the kraft paper and the resistance to corona discharge as expected.
  • the cushioning effect of the kraft paper is derived more safely and desirably by using raw kraft paper containing water in a ratio of 3 to 6% above the level of the moisture in the air or moisture:adjusted kraft paper having its thickness increased in advance by the addition of water rather than dry kraft paper having its moisture content lowered to 1 % or lower in advance, as has been done for insulation layers made solely of kraft paper.
  • the winding contemplated by this invention is easy to perform and quite inexpensive.
  • the invention has further improved the breakdown property of the cable as follows. Specifically, they have found it highly desirable to use sheets of kraft paper having a high dielectric constant and high resistance to corona discharge in several, for instance, three to ten, lowermost plies closest to the conductor and consequently subjected the most electric stress. With this technique, particularly the positive impulse strength of the cable can be improved without suffering any discernible rise of the value of ⁇ ⁇ tan 6 of the cable as a whole.
  • Fig. 3 is a perspective view of the oil-immersion insulating layer in the oil-immersion electrically insulated cable.
  • 6 denotes a lower (left-hand) oil-immersion insulating layer
  • 7 an upper (right-hand) oil-immersion insulating layer
  • 8 a portion where a change of layers (change of taping head) occurs in the gap winding oil-immersion insulating layer and where the depth of the oil gap equals the thickness of two plies of tape. This particular portion constitutes another weak point of the cable.
  • the present invention uses tapes of kraft paper, as shown in Fig. 4, in the plies destined to be exposed to the portions 8a of the layer change in the oil-immersion insulating layer (the portions indicated by the arrow in Fig. 4) where the depth of the oil gap equals the thickness of two plies of tape, so that any local breakdown of the oil gap 8a will be prevented from readily developing into total breakdown of the cable by the barrier effect of the kraft paper.
  • plies of PP film are used at the vicinity of layer changes.
  • these two plies may be formed of kraft paper so that a total of four plies of kraft paper are present, two above and two below each point of change of layer. This structure has been demonstrated to be quite effective.
  • Arranging plies of kraft paper at areas of layer change is more effective closer to the conductor side where the electric stress is prominent.
  • the mixing ratio of kraft paper is desired to be lowered to reduce the value of s ⁇ tan 6, it is advantageous to adopt this approach at the areas of layer change in only the lowermost five or so layers from the boundary of the conductor.
  • the amount of surface coarsening of the PP film and kraft paper contemplated by this invention varies widely with the class of voltage, the size of the conductor, the kind of the cable, and the insulating oil to be used. It is particularly affected by the combination of the specific combination of PP film and insulating oil.
  • bosses of a size of 6 to 20 ⁇ m on all sheets of the PP film it suffices to form bosses of a size of 6 to 20 ⁇ m on all sheets of the PP film, to form bosses of a size of 20 to 40 pm on every other sheet of PP film, or to form bosses of a size of 5 to 10 Ilm on all the sheets of PP film and bosses of a size of 1 to 5 pm on the sheets of kraft paper.
  • bosses of this size it is possible to optimize the inner pressure between the adjacent tapes throughout the entire insulating layer by adjusting the tape width and. controlling the tape winding tension.
  • this inner pressure was determined by holding a given cable at the highest working temperature (85° to 95°C, for example) for 24 hours, thereby to amply swell the layer of PP film, then bending the cable twice alternately in opposite directions into a loop of a diameter about 20 times the outermost diameter of the insulating layer, and disassembling the cable and visually examining the insulating tapes in the insulating layer for possible sign of irregularities.
  • the cable be manufactured by setting the amount of surface coarsening in the range of 1 to 50 pm, depending on the application of the cable and the type of insulating oil, and that the taping conditions be coordinated with the selected amount of surface coarsening. Once the cable is produced, it is then desirable and necessary to have the PP film swell at the highest working temperature of the cable.
  • the cable of the present invention provides the following outstanding features by producing a cable for which the values of density, birefringence, ratio of strengths in two axial directions, and surface roughness of the PP film used in the cable are within their respective ranges herein defined, superposing sheets of the PP film and sheets of kraft paper in the described manner, and properly coarsening either or both of the opposite surfaces of the tapes:
  • the film is required to be rated as Rank A.
  • a film rated as Rank B may be acceptable.
  • a film rated as Rank C should be rejected as an oil-immersion insulating material.
  • a volume of PP resin pellets having an isotatic structure content of 97.6%, melt index of 6 g/10 minute, and a T mc of 110.5°C were supplied to an extruder and melt extruded through a T-shaped die at 260°C in the form of a sheet.
  • the molten sheet was wound on a cooling drum at 30°C and allowed to cool and solidify to produce a sheet about 1000 pm in thickness.
  • This sheet was passed between a set of reducing rolls (roll diameter 250 mm) and rolled to about 9 times the original length. The rolling was carried out with a rolling pressure of 500 kg/cm and roll temperature of 140°C.
  • the sheet surface was wetted with polyethylene glycol.
  • the rolled film about 90 ⁇ m in thickness was introduced into an atmosphere at 130°C and subjected to a 10-second heat treatment, causing it to slacken by 1 % in the longitudinal direction. Then, the film was passed through embossing rolls held at 130°C to transfer print a sand blast pattern about 100 mesh in surface roughness on both surfaces of the film. The film, in a tense state, was held for 10 hours in an atmosphere at 120°C to carry out an aging heat treatment and thereafter left to cool gradually to room temperature. This film was cut into tapes 22 mm in width.
  • the properties exhibited by the PP film were as follows:
  • the properties of the kraft paper used in combination of the PP film were as follows:
  • the insulating layer of this invention swells only slightly with the insulating oil; shows good fluidity of the insulating oil and good bending properties, possesses a high impulse strength, experiences only nominal loss of positive impulse strength, permits attainment of the desired value of e . tan 6 and, therefore, proves highly advantageous for use in the production of an oil-immersion electrically insulated cable.
  • the inventors have succeeded in realizing an oil-impregnated insulating power cable which is of high quality and high practicability by using a specific combination of PP film and kraft paper.
  • the loss ( ⁇ ⁇ tan 6) is limited by the thickness limitation of the kraft paper as mentioned previously. In order to reduce the loss, it is necessary to reduce the thickness of the paper as a whole below this lower limit value of about 70 pm, which is impossible for reasons mentioned before.
  • a thinner kraft paper can be used together with PP film.
  • the PP film to be used together with the thinner kraft paper should be a low swelling PP film having roughened surfaces.
  • PP laminated paper sandwich the low swelling PP film to form a multiple (in this case three) layer laminated structure, referred to as PP laminated paper, to be used as a substitute for the kraft paper.
  • Table 2 shows examples of specifications of the PP laminated paper.
  • ⁇ ⁇ tan 6 of a cable having an insulating layer prepared by winding a combination tape of one PP laminated paper and two PP films is:
  • the use of the PP laminated paper according to the present invention results in a remarkably reduced loss in the cable.
  • Table 3 shows comparative data of cables having insulating layer composed of PP films and the PP laminated papers.
  • sample No. 1 is the same as sample No. 1 in Table 1.
  • test results for samples Nos. 10 and 11 show that these are usable in practice.
  • the high stress produced around the conductor 10 of an AC cable can be minimized to further improve the dielectric breakdown strength of the insulating layer thereof by employment of a so-called "s-graded insulating layer" in which a portion of the insulating layer 11 adjacent the conductor 10 has a value of s which is reduced with the distance from the conductor.
  • the thickness of the insulating layer can be reduced correspondingly, and thus the size of such a cable can be reduced. It is as important as the reduction of ⁇ ⁇ tan 5 to reduce the size of the cable by reducing the thickness of the insulating layer. This is particularly true for cable used in the EHV to UHV ranges.
  • the insulating layer is composed of five layers 11 to 15, in which the layer 11 is formed of kraft paper, a layer 12 of an alternating combination of a sheet of kraft paper and a sheet of the PP film, a layer, 3 of an alternating combination of a sheet of kraft paper and two sheets of the PP film, a layer of an alternating combination of a sheet of the PP laminated paper and a sheet of the PP film and a layer of an alternating combination of a sheet of the PP laminated paper and two sheets of the PP films.
  • Table 4 shows data of a typical example of the e-graded cable shown in Fig. 5.
  • the dielectric loss (e . tan 6) of the kraft paper, the PP laminated paper and the low swelling PP film are about 3.4x0.2 (%), 2.8x0.1 (%) and 2.2x0.02 (%), respectively.
  • One or more layers among the layers 1 to 5 in Table 4 may be omitted, if necessary, according to the class of the cable.
  • the cable having the e-graded insulating layer exhibits an improvement of the dielectric breakdown voltage by 3 to 10% relative to a cable having no e-graded layer.
  • the insulating oil impregnated power cables according to the present invention which utilize as at least portions of its insulating layer a PP film having a low swelling and good mechanical properties and a kraft paper of a natural polar material or a PP laminated paper, with at least portions thereof being roughened, exhibits remarkable improvements in dielectric loss, dielectric breakdown voltage, and reliability.

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Claims (20)

1. Elektrisches Starkstromkabel, enthaltend:
einen Leiter (2) und eine den Leiter (2) umgebende Isolierung (3), die wenigstens teilweise durch Umwicklung des Leiters (2) mit Isolierpapier (36) und Polypropylenfilm (3a) gebildet ist, wobei die Isolation (3) mit Isolieröl imprägniert ist, dadurch gekennzeichnet, daß der Polypropylenfilm (3a) eine Dichte im Bereich von 0,905 bis 0.915 g/cm3, eine Doppelbrechung im Bereich von 0,020 bis 0,035, ein Verhältnis von Längsdehnungsfestigkeit zu Querdehnungsfestigkeit im Bereich von 5 bis 15 und eine Dicke im Bereich von 70 bis 300 pm hat.
2. Elektrisches Starkstromkabel nach Anspruch 1, dadurch gekennzeichnet, daß das Isolieröl Dodecylbenzen ist.
3. Elektrisches Starkstromkabel nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß das Isolierpapier Kraftpapier (36) ist, das um den Leiter alternierend mit dem Polypropylenfilm (3a) gewickelt ist.
4. Elektrisches Starkstromkabel nach Anspruch 3, dadurch gekennzeichnet, daß ein Film (3a) und Kraftpapier (3b) Kombination aus einer Lage Polypropylenfilm (3a) und einer Lage Kraftpapier (3b) ist.
5. Elektrisches Starkstromkabel nach Anspruch 3, dadurch gekennzeichnet, daß eine kleinste Einheit einer abwechselnden Wicklung aus Polypropylenfilm (3a) und Kraftpapier (3b) eine Kombination aus zwei Lagen Polypropylenfilm (3a) und einer Lage Kraftpapier (3b) ist.
6. Elektrisches Starkstromkabel nach Anspruch 2, dadurch gekennzeichnet, daß ein Teil von einer oder beiden Flächen von wenigstens einem Teil des Polypropylenfilms (3a) und des Kraftpapiers (3b) eine Rauhigkeit im Bereich von 1 bis 50 um aufweist.
7. Elektrisches Starkstromkabel nach Anspruch 1, dadurch gekennzeichnet, daß die Isolierung aus mehreren Lagen (11 bis 15) unterschiedlicher Arten besteht, wobei die Reihenfolge dieser Lagen (11 bis 15) vom Leiter (10) ausgehend vorbestimmt ist.
8. Elektrisches Starkstromkabel nach Anspruch 7, dadurch gekennzeichnet, daß die Isolierlagen unterschiedliche Dielektrizitätskonstanten haben, wobei ihre Reihenfolge vom Leiter (10) ausgehend derart ist, daß die Dielektrizitätskonstante der Isolierung abgestuft ist, mit dem größten Wert an der am meisten innenliegenden Isolierlage und fortlaufend abnehmend mit dem Abstand von dem Leiter (10).
9. Elektrisches Starkstromkabel nach Anspruch 8, dadurch gekennzeichnet, daß die Isolation wenigstens zwei unterschiedliche Lagen aus den Lagengruppen aufweist, die in der Reihenfolge vom Leiter (10) ausgehend umfaßt:
a) eine Lage (11) aus Kraftpapier (3b),
b) eine Lage 12 aus wenigstens einer Kombinationseinheit aus einer Lage Kraftpapier (3b) und einer Lage Polypropylenfilm (3a),
c) eine Lage (13) aus wenigstens einer Kombinationseinheit aus einer Lage Kraftpapier (3b) und zwei Lagen Polypropylenfilm (3a),
d) eine Lage (14) aus einer abwechselnden Kombination aus einer Lage Polypropylenfilm und einer Lage eines elektrisch isolierenden Papierlaminats aus zwei Lagen elektrisch isolierenden Papiers und einer Polypropylenlage zwischen den Papierlagen,
e) eine Lage (15) aus einer abwechselnden Kombination aus zwei Lagen Polypropylenfilm und einem Laminat der vorgenannten Art.
10. Elektrisches Starkstromkabel nach Anspruch 9, dadurch gekennzeichnet, daß in dem Laminat die Lagen aus elektrisch isolierendem Papier klebend mit der Polypropylenlage verbunden sind, die zwischen die genannten Lagen schmelzextrudiert ist.
11. Elektrisches Starkstromkabel nach Anspruch 9, dadurch gekennzeichnet, daß in dem Laminat die Polypropylenlage ein Polypropylenfilm ist, der zwischen den Lagen aus elektrisch isolierendem Papier liegt.
12. Elektrisches Starkstromkabel nach einem der Ansprüche 2 bis 11, dadurch gekennzeichnet, daß das Kraftpapier (3b) ein rauhes Kraftpapier ist.
13. Elektrisches Starkstromkabel nach einem der Ansprüche 2 bis 11, dadurch gekennzeichnet, daß das Kraftpapier (3b) ein feuchtigkeits-konditioniertes rauhes Kraftpapier ist.
14. Elektrisches Starkstromkabel nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß das Kabel durch Erwärmen für 24 bis 48 Stunden bei einer höchsterwarteten Betriebstemperatur vor der Auslieferung konditioniert ist.
15. Elektrisches Starkstromkabel nach Anspruch 9, dadurch gekennzeichnet, daß eine innerste (11) der genannten Lagen 3 bis 10 Einzellagen aus Kraftpapier enthält.
16. Elektrisches Starkstromkabel nach Anspruch 9, dadurch gekennzeichnet, daß in innersten fünf Lagen abwechselnde Lagen in unterschiedlichen Richtungen gewunden sind und daß an Spalten, die an Überschneidungen zwischen benachbarten der Lagen ausgebildet sind, Querlagen benachbart zu dem Spalt aus Kraftpapier gebildet sind.
17. Elektrisches Starkstromkabel nach einem der Ansprüche 9 bis 16, dadurch gekennzeichnet, daß ein Teil einer oder beider Flächen von wenigstens einem Teil des Polypropylens und des Laminats eine Rauhigkeit im Bereich von 1 bis 50 um hat.
18. Elektrisches Starkstromkabel nach einem der Ansprüche 9 bis 17, dadurch gekennzeichnet, daß das Laminat ein rauhes Laminat ist.
19. Elektrisches Starkstromkabel nach einem der Ansprüche 9 bis 17, dadurch gekennzeichnet, daß das Laminat ein feuchtigkeits-konditioniertes rauhes Laminat ist.
20. Elektrisches Starkstromkabel nach einem der Ansprüche 9 bis 19, dadurch gekennzeichnet, daß jede der Lagen aus einer Mehrzahl von Einzellagen aus Streifen besteht.
EP19840106571 1983-06-09 1984-06-08 Elektrisches Starkstromkabel Expired EP0129755B1 (de)

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JP10393783A JPH0239047B2 (ja) 1983-06-09 1983-06-09 Yushindenkizetsuenkeeburu
JP103937/83 1983-06-09

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EP0129755A1 EP0129755A1 (de) 1985-01-02
EP0129755B1 true EP0129755B1 (de) 1988-03-09

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IT1173045B (it) * 1984-01-17 1987-06-18 Pirelli Cavi Spa Cavo elettrico ad olio fluido perfezionato
KR101867168B1 (ko) 2016-08-18 2018-06-12 엘에스전선 주식회사 전력 케이블

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US3378419A (en) * 1964-06-22 1968-04-16 Anaconda Wire & Cable Co Method of making synthetic-film insulated high-voltage cable
GB1442634A (en) * 1973-03-16 1976-07-14 Bicc Ltd Electric cables
IT1105990B (it) * 1977-09-29 1985-11-11 Bicc Ltd Cavi elettrici per alte tensioni

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CA1220533A (en) 1987-04-14
EP0129755A1 (de) 1985-01-02

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