EP0129755B1

EP0129755B1 - Electric power cable

Info

Publication number: EP0129755B1
Application number: EP19840106571
Authority: EP
Inventors: 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
Original assignee: Sumitomo Electric Industries Ltd; Toray Industries Inc
Priority date: 1983-06-09
Filing date: 1984-06-08
Publication date: 1988-03-09
Also published as: DE3469818D1; JPS59228313A; EP0129755A1; CA1220533A; JPH0239047B2

Description

The invention relates to an electric power cable as set forth in the preamble of claim 1. Such a cable is known from DE-A-2 412 222.
In this document electric power cables are disclosed 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.
From EP-A-1494 a dielectric insulating body made up of three parts is known, 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, however, have a disadvantage in that swelling with the insulating oil occurs to an exceptionally great extent. When the cable is used in any application involving oil immersion, it has entailed various restrictions. 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. As a measure of avoiding this trouble, 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.
When polypropylene film is impregnated with any of the alkylbenzene type oils which are preponderantly used as insulating oils in OF cables of the EHV class, the film swells as the temperature of the ambient air increases so that the film increases in thickness, possibly to the extent of notably increasing the interface pressure between the overlapping plies of film, causing the film to sustain rupture such as due to thermal expansion or contraction of the cable, and thus forcing upper plies of the film to fall into gaps formed between adjacent lower plies of the film and consequently causing damage. The result is generally degraded electrical characteristics.

Summary of the invention

Accordingly, 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/cm³, 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.
Other objects and characteristics of this invention will become apparent from the further disclosure of this invention to be made in the following detailed description of preferred embodiments, with reference to the accompanying drawings.

Brief description of the drawings

Figs. 1A and 1B are sectional views of typical insulation structures according to the present invention;
Fig. 2 shows a cross section of an oil-immersion insulated electric power cable;
Fig. 3 is a perspective view of an oil-immersion insulating layer in the oil-immersion insulated electric power cable of Fig. 2;
Fig. 4 is a cross section of a typical insulation structure contemplated by the invention, illustrating in an enlarged view a portion of the oil-immersion insulating layer involving a change of layer structure; and
Fig. 5 is a cross section of another embodiment of the invention.

Description of the preferred embodiments

The term "polypropylene (hereinafter referred to as "PP" for short) as used herein means 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. On the other hand, if the 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. In the PP species of the grade described above, 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. 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/cm³, preferably 0.907 to 0.912 g/cm³. 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. If the birefringence is less than the lower limit mentioned above, the swelling of the PP film with the insulating oil increases beyond the tolerable extent. If it exceeds the upper limit, the PP film is liable to sustain cracks which could cause dielectric breakdown. Such a PP film, therefore, does not meet the objects of this invention. 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. Conversely, if this ratio exceeds the upper limit, the differences of properties exhibited by the PP film in axes of varying directions increase beyond a tolerable extent, and consequently the workability of the PP film while being wound on the cable to form an insulator is notably degraded (for example, by stretching, wrinkling, or rupturing). The reasons for the film thickness being between 70 and 300 Ilm are given later.
Now, a typical method for producing the film to be used for the cable of this invention will be described by way of illustration. 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.
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. A typical method for limiting the ratio of thermal shrinkage in the liquid longitudinal direction residues in heating the PP film produced by the method described above to a temperature in a range of 80° to 140°C, preferably, 90° to 130°C, and retaining the film at this temperature for a period of 0.5 to 50 hours, preferably, 1 to 20 hours, with the film held in a tense state or allowed to slacken by 0.1 to 5% of the original size in the longitudinal direction. By this aging heat treatment, 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 _Ilm for the following reason. If the thickness is smaller than 70 µm, 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.
Generally, 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
cylindrical shape of the conductor and, in the produced OF cable, gives rise to abnormalities such as separation in the insulating layer and irregular distribution of gaps left between edges of adjacent turns of the tape, which tends to adversely affect the electrical properties of the cable as a whole.
Generally, for the formation of an insulating layer in a cable, 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. Thus, the thickness of oil layers formed in such gaps increases with the thickness of the tape. In an OF cable, 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.
In the light of the various conditions described above, 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.
The term "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.
Regarding the insulating oil for use in the cable of the invention, the inventors have found that alkylbenzenes containing an aromatic ring, particularly DDB (dodecyl benzene), which is commonly used in cables, best suits the purpose of the invention. Generally, the following conditions are adopted as criteria for selecting the insulating oil:

(A) The oil should be readily available at low cost.
(B) The oil should possess excellent and stable electrical properties.
(C) The oil should be highly compatible with the component materials of the insulating layer of the cable. Specifically, in the case of the invention, the oil should be amply compatible with the PP film.

Regarding conditions (A) and (B), 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.
Generally, 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. Both. PP and DDB have SP values approximating 8. Thus, 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.
After much study in this respect, 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. Moreover, 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. According to the inventors' studies, the impulse breakdown value of one PP film layer, with the value in the combination with DDB taken as unity (1), 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. For the outstanding electrical properties of DDB in combination with the PP film to be retained intact without any sacrifice of the compatibility of the DDB with the PP film, the following solution has been devised:
Since, as described above, thorough impregnation of the PP film with DDB is advantageous from the standpoint of electrical properties, 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.
Through a study of the PP film, it has been ascertained that 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. To be specific, 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.
To remedy the insufficiency, 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.
When one or both surfaces of the PP film and the kraft paper to be used in the cable of the invention are coarsened by embossing, 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. Conversely, if the surface roughness exceeds the upper limit mentioned above, 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. To be specific, 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 µm, preferably 2 to 40 µm.
For the production of the film for use in the cable of the present invention, the combination of rolling and embossing treatments proves to be most desirable. Optionally, other methods may be used. For example, 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. Also, the embossing treatment of the aforementioned combination may be replaced by a sand blasting process or etching process to effect the desired surface coarsening.
For the purpose of coarsening the surface of the kraft paper, an embossing treatment with embossing rolls is most desirable. Otherwise, a process of spraying water drops may be utilized.
As the number of sheets of PP film superposed on the insulating layer is increased, there are obtained improvements in the dielectric loss tangent (tan 6) and dielectric constant (s), in addition to such advantages as lowered swelling of the film with the insulating oil, improved fluidity of the insulating oil within the insulating layer, improved mechanical properties of the insulating layer and enhanced workability of the PP film when the film is wound on the conductor, and fewer occurrences of excessive tightening or loosening of the winding of the layer. Consequently, 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. When a plurality of sheets of kraft paper are superposed and exposed to changes of temperature due to load variation, the kraft paper exhibits an extremely high flexibility. When the kraft paper is cut into tapes, the edge faces of the tapes are very smooth and do not form rigid cutting edges as observed in cut edge faces of PP film. 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.
Where two cable ends are joined, it is usual that the winding of the insulation layer and the subsequent winding of tapes across the joint of the two cable ends are both carried out manually. In this case, if the entire insulating layer is formed exclusively of PP film tapes, it is rather difficult for the inner layer of PP tapes to be tightly wound manually. When PP film and kraft paper are alternately wound as contemplated by this invention, the inner layer can be very easily tightened with the kraft paper and the tightly wound condition of the inner layer can be easily retained intact. Thus, the alternate winding facilitates the work of cable production and stabilizes and enhances the quality of the produced cable. All these factors enhance the mechanical and electric properties of the insulating layer.
Purely from the electrical point of view, 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. As a result of much study, 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.
From the standpoint of material costs, even a PP film, one of rather inexpensive plastic materials, is still more than twice as expensive as kraft paper. Thus, the economy of the cable improves as the proportion of kraft paper in the combination of kraft paper and PP film is increased. Also for thorough coordination between the performance (ε. tan 6) and the economy of the cable, the cable of this invention, having alternate windings of kraft paper and PP film, manifests its outstanding effects to the fullest extent.
Now, working examples of the invention will be described with reference to the accompanying drawings. Fig. 2 is a cross section of an oil-immersion electrically insulated cable. In this diagram, 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, and 5 a corrosion-proofing layer superposed on the sheath 4.
One version of the alternate winding is illustrated in Fig. 1A. 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. Specifically, 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 ε · tan 6 of the completed cable is intermediate the respective values of ε · tan 6 of the two materials. Since this value for kraft paper is 3.4xO.2% and that of PP film is about 2.2×0.02%, the overall value of the complete cable is equivalent to 2.8×0.1%. With this structure, the dielectric loss tangent is reduced (to the order of (2.8x0.1 %)/(3.4x0.2%)=0.₄1.) compared with that of the conventional cable using kraft paper exclusively in the insulating layer. Thus, the cable of the present invention proves highly useful for the EHV class of 275 to 500 kV.
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.
As described above, the cable of the insulation structure of Fig. 1A is stable and excellent both mechanically and electrically. Thus, it is suitable for use in EHV to UHV classes of 275 to 1000 kV.
For a further reduction in the dielectric loss tangent, which is proportional to the square of the transmission voltage and e . tan 5, the cable is required to possess a still lower value of ε · tan 6. To meet this demand, 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. In the cable using this insulation structure, the dielectric constant (e) is (2x2.2+3.4)/3=2.₆ and the dielectric loss tangent (tan 5) is (2x0.02 %+0.2 %)/3=0.087 %. Thus, the value of e . tan 6 of this cable is (2.6x0.087)/(3.4x0.2)=0.33 as compared with the cable using the insulating layer made exclusively of kraft paper. Thus, the cable attains the desired reduction of dielectric loss tangent at substantially no sacrifice of other mechanical and electrical properties.
In this insulation structure, since the mixing ratio of kraft paper as a cushioning component is decreased both locally and overall to two-thirds that in the structure of Fig. 1A, it becomes necessary to slightly increase the aforementioned amount of surface coarsening of the PP film and kraft paper and to increase slightly the tape winding tension. Nevertheless, since all the sheets of the PP film border on kraft paper and thus make the most of the cushioning effect of the kraft paper, the produced cable proves amply practicable from the standpoint of manufacture and flexibility.
From the electrical point of view, although 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. Unlike the winding of dry kraft paper which entails an extra process for drying, special storage designed to keep the paper dry, and a taping machine specially designed to permit the paper to be wound in a dry state, 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 ε · 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. In the diagram, 6 denotes a lower (left-hand) oil-immersion insulating layer, 7 an upper (right-hand) oil-immersion insulating layer, and 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.
To overcome this weak point, 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. In Fig. 4, plies of PP film are used at the vicinity of layer changes. Optionally, 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. Where 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.
Concerning particularly the reduction of the positive impulse property, which demands due attention in the application of the PP film to the cable, improvements are attained notably by the arrangement of the kraft paper. These improvements add all the more to the effectiveness of the cable of the present invention.
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.
In the cable of the present invention, the combination of PP film with DDB has been demonstrated to enable the cable to attain excellent electrical properties, although other insulating oils can also provide excellent results. For a POF cable, for example, polybutene-type insulating oils of high viscosity are most often used. When such an insulating oil is used, since the amount of swelling of the PP film is small, it suffices to form bosses of a size of 2 to 10 pm, for example, only on the sheets of kraft paper used in the insulation structure of Fig. 1A. Of course, it otherwise suffices to form bosses of a size of about 5 µm only on the sheets of PP film. In the case of the insulation structure of Fig. 1B involving the combination of PP film with DDB, it suffices to form bosses of a size of 6 to 20 µm 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. With 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.
The appropriateness of 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.
In any event, it is essential that 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:

(1) The swelling of the cable with the insulating oil is minimal.
(2) The fluidity of the insulating oil between the overlapping plies of the film is satisfactory.
(3) The film exhibits outstanding mechanical properties for an insulating layer and enjoys good workability while being wound on the conductor.
(4) The insulating layer wound on the conductor neither tightens nor slackens easily.
(5)The formation of an insulating layer across a joint of two cable ends is easy and the insulating layer so formed is reliable.
(6) The film excels in both dielectric constant and dielectric loss tangent so that required properties are provided at no sacrifice of economy.
(7) The cable excels in breakdown properties. It particularly is excellent in the positive property.

The terms and methods of measurement as used in the present invention will now be described below.

(1) Isotacticity: A given PP sample is extracted from boiling N-heptane. The weight of the extracted residue is divided by the original weight of the sample. The quotient is multiplied by 100. The product, expressed as a percent, is used to represent the isotacticity.
(2) Melt index: This physical property is measured under the conditions L of ASTM D-1238-73.
(3) Temperature of melt crystallization (T_mc): A sample 5 mg in weight is placed in a tester, for instance, Model DSC-II, made by Perkin Elmer Corp., with the atmosphere inside the test displaced with nitrogen. Then, the sample is heated to raise its temperature at a rate of 20°C/minute to 200°C and then held at this level of 200°C for five minutes. Then, the hot sample is cooled at a rate of 20°C/minute, causing in the meantime the tester to describe a peak of the heat generated as a consequence of the crystallization of the molten sample. The temperature at the apex of the curve so described is reported as T_mc.
(4) Density: This property is measured in accordance with ASTM D-1505.
(5) Birefringence: By the use of an Abbe refractometer, the refractive index in the longitudinal direction (Ny) and that in the lateral direction (N_x) of a given sample of film are measured. The difference obtained by subtracting N_x from Ny is the birefringence. In this measurement, a sodium D ray is used as the light source and methyl salicylate as the mounting medium.
(6) Ratio of strengths in two axial directions: The tensile strength in the longitudinal direction, ay (kg/mm²), and the tensile strength in the lateral direction, σ_x (kg/mm²) of a sample of film are measured by the method of ASTM D-882-67). The quotient of ay divided by _6x represents the ratio of strengths.
(7) Surface roughness (R_max): The roughness, Rmax, of a sample of film is measured by the method described in JIS B-0601-1976. The cutoff value is fixed at 0.5 mm.
(8) Ratio of thermal shrinkage: A specimen 200 mm in length and 10 mm in width is cut from a sample of film, with the longitudinal direction of the specimen taken as the direction of measurement of this ratio. The specimen is held for 15 minutes in an oven in which hot air at 120°C is circulated. After this, the specimen is removed from the oven and measured for length at room temperature. With L representing the length (in mm) found by the measurement, the ratio of thermal shrinkage is defined by the following equation:
(9) Degree of swelling of cable insulating layer with insulating oil: A lamination of a desired number of specimens each 30 mmx30 mm is subjected to a pressure of about 1 kg/cm² by a spring. The thickness of the lamination in the wound state is noted as t₁. In this state, the lamination is dried to a desired extent and then immersed in insulating oil. It is heated to a temperature to be tested, for example, 85 to 95°C, and kept as it is for 4 to 24 hours to swell the PP film completely. The thickness of the lamination swelled completely is noted as t₂. Then, the degree of swelling (%) is found in accordance with the following equation:
(10) Fluidity of insulating oil: A sample of film is wrapped around a conductor to form a cable. This cable is immersed in the insulating oil and then impregnated with the oil under a vacuum. Thereafter, the cable is disassembled and visually examined to determine whether or not the insulating oil has been dispersed in all gaps in all the plies of the film. A rating is made on a three-point scale defined as follows.
- Rank A Thorough uniform dispersion of the oil throughout the gaps.
- Rank B: There are points at which slight insufficiency of oil dispersion occurs.
- Rank C: There are planes in which total absence of oil dispersion occurs.

For use as an oil-immersion insulating material, the film is required to be rated as Rank A. In applications to low voltage cables, a film rated as Rank B may be acceptable. A film rated as Rank C should be rejected as an oil-immersion insulating material.

(11) Electrically insulating oil: This is a generic term applicable to all known electrically insulating oils such as mineral oil, castor oil, cottonseed oil, alkylbenzene, diallyl alkanes, polybutene oil, and silicone oil.

The present invention will now be described more specifically below the reference to actual examples and comparative examples.
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 µm 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:
For comparison, a commercially available nonstretched PP film and a biaxially oriented PP film were tested for the same properties. The results were as follows:
The properties of the kraft paper used in combination of the PP film were as follows:
Tapes 22 mm in width of the kraft paper were wound with one-third overlap on a stranded conductor 200 mm² in cross section in a varying structure as indicated in the Table below. The cables consequently obtained were dried then impregnated with DDB at room temperature and left to stand at 100°C for 48 hours for thorough swelling of the PP film. The cables were allowed to cool to normal room temperature, bent twice alternately in opposite directions into a loop of a diameter about 20 times the outside diameter of the insulating layer, and disassembled for visual inspection of the condition of the insulating layers.
From these results, it is noted that 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.
As mentioned hereinbefore, 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 (ε · 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.
According to the present invention, 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.
Two sheets of kraft 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.
Assuming a value of ε · tan δ of the PP laminated paper of 2.8×0.1 %, the value of ε · tan δ of the cable having an insulating layer prepared by alternately winding the PP film and the PP laminated paper is:
The value of ε · 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 ratios of these values to those of a cable having an insulating layer composed of only the kraft paper are, respectively, as follows:
and
As is clear from these ratios, 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. In Table 3, sample No. 1 is the same as sample No. 1 in Table 1.
As is clear from Table 3, the test results for samples Nos. 10 and 11 show that these are usable in practice. In producing a cable on the basis of either sample No. 10 or No. 11, it may be necessary to roughen the surfaces of the PP film to the extent of 20 to 40 µm or to roughen the surfaces of the PP film to the extent of 5 to 10 µm and those of the PP laminated paper to the extent of 3 to 10 µm, while the tension of the tape when wound is controlled to be sufficiently small.
According to another embodiment of the present invention, which is shown in Fig. 5, 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. According to this embodiment, 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 ε · 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.
. In Fig. 5, 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.
In Table 4, 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.
As described hereinbefore, 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.

Claims

1. An electric power cable comprising:

A conductor (2) and

insulation (3) surrounding said conductor (2), said insulation (3), having at least portions thereof formed by winding on said conductor (2), insulating paper (36) and polypropylene film (3a) and insulating oil with which said insulation (3) is impregnated, characterised in that

said polypropylene film (3a) has a density in a range of 0.905 to 0.915 g/cm³, a birefringence in a range of 0.020 to 0.035, a ratio of a lengthwise tensile strength to a widthwise tensile strength in a range of 5 to 15, and a thickness in a range of 70 to 300 pm.

2. The electric power cable according to claim 1, characterised in that said insulating oil comprises dodecyl benzene.

3. The electric power cable according to claim 1 or 2, characterised in that said insulating paper comprises kraft paper (36), said kraft paper (36) being wound around said conductor alternately with said polypropylene film (3a).

4. The electric power cable according to claim 3, characterised in that a film (3a) and kraft paper (3b) is a combination of one sheet of polypropylene film (3a) and one sheet of kraft paper (3b).

5. The electric power cable according to claim 3, characterised in that a minimum unit of alternate winding of polypropylene film (3a) and kraft paper (3b) is a combination of two sheets of polypropylene film (3a) and one sheet of kraft paper (3b).

6. The electric power cable according to claim 2, characterised in that a portion of either or both surfaces of at least a portion of said polypropylene film (3a) and kraft paper (3b) have a roughness in a range of 1 to 50 µm.

7. The electric power cable according to claim 1, characterised in that

said insulation comprises a plurality of layers (11 to 15) of different kinds, the order of said layers (11 to 15) from said conductor (10) being predetermined.

8. The electric power cable according to claim 7, characterised in that said insulating layers have different dielectric constants, their order from said conductor (10) being such that the dielectric constant of said insulation is graded with the largest value at the innermost insulating layer and decreasing successively with distance from said conductor (10).

9. The electric power cable according to claim 8, characterised in that the insulating comprises at least two different layers of the group of layers listed in the order from said conductor (10) comprising:

a) a layer (11) comprising kraft paper (3b),

b) a layer (12) comprising at least one combination unit consisting of one sheet of kraft paper (3b) and one sheet of said polypropylene film (3a),

c) a layer (13) comprising at least one combination unit consisting of one sheet of kraft paper (3b) and two sheets of said polypropylene film (3a),

d) a layer (14) formed of an alternating combination of one sheet of said polypropylene film and of one sheet of an electrically insulating paper laminate consisting of two sheets of electrically insulating paper and a polypropylene layer between said paper sheets,

e) a layer (15) formed of an alternating combination of two sheets of polypropylene film and of a laminate as set forth before.

10. The electric power cable according to claim 9, characterised in that in said laminate said sheets of electrically insulating paper are adhesively bonded together with said polypropylene layer melt extruded between said sheets.

11. The electric power cable according to claim 9, characterised in that in said laminate said polypropylene layer is a polypropylene film sandwiched by said sheets of electrically insulating paper.

12. The electric power cable according to any one of claims 2 to 11, characterised in that said kraft paper (3b) comprises raw kraft paper.

13. The electric power cable according to any one of claims 2 to 11, characterised in that said kraft paper (3b) comprises moisture-conditioned raw kraft paper.

14. The electric power cable according to any one of the preceding claims characterised in that said cable is conditioned by heating for 24 to 48 hours at a highest expected operating temperature prior to shipment.

15. The electric power cable according to claim 9, characterised in that an innermost one (11) of said layers comprises 3 to 10 plies of said kraft paper.

16. The electric power cable according to claim 9, characterised in that, in an innermost five layers, alternate layers are wound in opposite directions, and wherein, at gaps formed at intersections between adjacent ones of said layers, plies laterally adjacent to said gap are formed of said kraft paper.

17. The electric power cable according to any one of claims 9 to 16, characterised in that a portion of either or both surfaces of at least a portion of said polypropylene and said laminate have a roughness in a range of 1 to 50 pm.

18. The electric power cable according to any one of claims 9 to 17, characterised in that said laminate comprises raw laminate.

19. The electric power cable according to any one of claims 9 to 17, characterised in that said laminate comprises moisture-conditioned raw laminate.

20. The electric power cable according to any one of claims 9 to 19, characterised in that each of said layers comprises a plurality of plies of tapes.