COMPOSITE TAPE FOR THE INSULATION OF ELECTRIC CABLES AND ELECTRIC CABLE USING SAID TAPE IN ITS INSULATION

Abstract: A tape for the insulation of electric cables comprising at least a layer (2, 4; 12; 22) of cellulose paper treated in such a way as to improve its electric characteristics, linked to a support of plastic material (3; 13; 23), the composite tape being wound up to form at least a part of the insulation (10) of a power cable (5) of the type in which the insulation is impregnated with fluid oil.

Description: The present invention concerns a composite tape for the insulation of electric cables, in particular of the oil-filled type, as well as an electric cable impregnated with fluid oil, in which at least a part of the insulation is obtained by windings of said tape. In the transmission of electric power at very high voltages, namely up to about 1000 kV, cables having an insulation of cellulose paper impregnated with fluid oil are normally used. At these voltage values, the dielectric losses in the insulation, and in particular in the paper, originate a heat development which requires the adoption of appropriate cooling systems, which are not quite practical and have a relevant cost or, alternatively, reduce considerably the power which can be transmitted by a given cable. The dielectric losses are substantially depending on the value of the so-called loss angle delta (δ ) (and more precisely the value of the tangent of said angle is considered) and, to a smaller extent, on the relative dielectric constant ε r of the cellulose paper. The delta tangent (tg δ ) value of the cellulose paper (and therefore of the corresponding dielectric losses) has been constantly reduced, in the last ten year periods, from 3% to 1.5% for the best papers, by virtue of improvements in the production process, as the use of deionized water for the mix preparation. However, these values are still too high when cellulose paper is used for very high voltages, so that the adoption of a forced cooling system is necessary to dissipate the heat due to the dielectric losses. To reduce said dielectric losses it was proposed to further reduce the delta tangent value of the insulation, providing a mixed insulation composed of tapes of paper and of tapes of plastic material, such as polypropylene, which has a very low delta tangent with respect to paper and also ε r values lower than those of cellulose paper. The insulation is effected by winding up alternatively the paper tapes and the plastic tapes or by making use of preformed laminated structures comprising one or two paper tapes coupled with a layer of plastic material obtained by extrusion, or by providing bonding agents between the layers. In these structures, it is convenient for the layer of plastic material to have a relevant thickness with respect to the total thickness of the tape, so that the resulting average value of delta tangent is reduced as much as possible, for instance to values of the order of 0.7 - 1%. A drawback of these layered structures is that the plastic material is not compatible with the impregnating oil, in contact with which it suffers a swelling. As the plastic material is present in high amounts, said swelling may involve dangerous stresses inside the insulating layer. In fact, owing to the swelling the layers of synthetic material, in particular when hot, exert radial pressures which may cause folds by collapse in the insulation, with the risk of a rupture in the cellulose layers. A further inconvenience is represented by the loss of flexibility of the cable. The layer of plastic material, extruded during the structure formation (for instance extruded between two paper layers in motion) has poor mechanical characteristics. The possible use of a bonding agent to couple plastic tapes to paper would prejudice the electric characteristics, which suffer a considerable deterioration for the presence of inadequate materials inside the insulation. The electric properties of cellulose paper, among which also the delta tangent value and the relative dielectric constant ε r value, can be appreciably improved by the addition of acids and/or mineral salts able to reduce the total electric valence of the material (namely the dipole number) by the formation of ionic compounds. These treated papers are obtained for instance by dipping the paper tapes into baths of appropriate substances, as boric acid and/or its salts, and then by drying them, and may show delta tangent values of the order of 0.4 - 0.5% and ε r values of the order of 1.8 - 2.0. However, these relevant improvements in the electric properties are obtained in consequence of a salt content which makes this paper unduly brittle (in dry condition) and therefore unsuitable to be used as insulation in power cables. In fact, even with not small thicknesses (for instance of 200 µm), there is a high risk of paper breaking already during the winding operations. Anyhow, it is above all in the finished cable that this paper, owing to its brittleness, cannot be used, for the possible risk of ruptures, wrinkles and other collapse phenomena, during the recovery or the paying-off of the cable itself. Therefore, in spite of a theoretical and often recommended possibility of use even in electric cables, these types of cellulose paper containing additives are really employed only for static systems or for systems, as transformers, condensers and the like, which are subjected to very reduced stresses. Consequently, the present invention aims at providing a tape-like material, suitable to form the insulation of electric cables for very high voltages, which shows reduced dielectric losses. Further, the invention aims at providing said material with the maximum possible paper content, so that a cable insulated with said material is flexible, easy to be impregnated and substantially devoid of any stress due to the swelling of the synthetic materials. In other words, the plastic material is mainly employed as a mechanical support for the cellulose material. Also, the invention aims at providing a material of the above indicated type in which the hot bonding between the plastic material and the cellulose material does not cause any degradation of the mechanical characteristics both of the plastic material and of the paper. Finally, a further aim of the invention is that of providing an electric cable whose insulation is at least partially built up by windings of the above indicated tape-like material. Accordingly, the subject matter of the invention is a composite tape for the insulation of high voltage electric cables, comprising a paper layer bonded to at least one surface of a film of polymeric plastic material, characterized in that said paper has a content of mineral, salts ranging between 0.1 and 3% on the paper weight, and that the thickness of the plastic material ranges between 10 and 30% of the tape total thickness. In a preferred embodiment of the invention, the film of plastic material is made of bi-oriented polypropylene and is covered by a thin layer of propylene-ethylene copolymer. The invention provides moreover an electric power cable at least partially insulated with composite tapes of the above described type. The invention will now be described with reference to some embodiments illustrated in the attached drawings, in which: - figure 1 is a partial cross section of a composite tape according to the invention, - figure 2 is a partial cross section of an alternative embodiment of a composite tape according to the invention, - figure 3 is an enlarged partial section of a further alternative embodiment of the composite tape according to the invention, and - figure 4 represents an oil-filled cable, whose insulation is obtained by the tape according to the invention. Figure 1 shows in cross section the structure of a composite tape according to the invention; the tape is indicated in its whole with the reference numeral 1 and is formed of two paper layers 2, 4 between which is sandwiched and bonded a tape or film 3 of polymeric plastic material having good general electric characteristics. The paper layers 2 and 4 have preferably the same thickness and are made of an additive-treated paper, obtained for instance by dipping into a solution of boric acid or magnesium borates or other salts, and by a subsequent drying. The content by weight of the additive so introduced in the paper ranges between 0.1 and 3%, and preferably between 0.5 and 1% of the paper weight. The thickness of the plastic film, with respect to the total tape thickness, ranges between 10 and 30%, and preferably between 15 and 25%. Said tape total thickness may vary according to the requirements; however, it ranges preferably between 50 and 200 µm, which values are corresponding to those normally in use. For instance, in a tape obtained according to the structure represented in figure 1 and having a total thickness of 110 µm, the film of plastic material may have a thickness of 20 µm and each of the two paper layers a thickness of about 45 micron, which corresponds to a 18% ratio of plastic material to the total tape thickness. In the embodiment represented in figure 2, the composite tape 11 is instead formed of only one layer of additive-treated paper 12, bonded to a film of plastic material 13. By building up a tape having a thickness of 100 µm, the thickness of the plastic material remains of 20 µm, whilst the thickness of the paper layer becomes of 90 µm. Polyolefines are the polymeric plastic materials more suitable for the plastic layer, by virtue of their good electric characteristics (they are not polar and have a good dielectric strength), thermal properties (sufficiently high softening temperature) and mechanical features. A particularly appropriate plastic material proved to be polypropylene. Another material giving good results is polymethyl-pentene, which melts at a temperature higher than that of polypropylene and offers therefore a further advantage during the cable sheathing with aluminium. Figure 3 represents in cross section the structure of a further embodiment of the composite tape according to the invention. It comprises a layer of additive-treated paper 22, bonded to a tape or film 23 of bi-oriented polypropylene, the molecules of which have been oriented by stretching along two orthogonal directions. The paper can be treated with additive as in the preceding examples, or with other salts as for instance silicates, whose content is anyhow contained in the above indicated ranges. The coupling of the two layers is obtained by means of a thin layer or coat 25 of propylene-ethylene copolymer which is applied for instance by extrusion on the tape of bi-oriented polypropylene. This surface microlayer of polypropylene copolymer has a thickness of the order of a few micron, for instance 2 µm, and is particularly suitable to achieve the thermal bonding between polypropylene and paper. Preferably, its propylene content is higher than 80% by weight. In fact, the material constituting the microlayer adheres by affinity to polypropylene and, taking advantage of the fact that the softening temperature of the copolymer is lower than that of polypropylene, a stable coupling can be obtained between the paper 22 and the tape 23 without the melting of the latter. In other words, the laminated structure is effected at a temperature lower than the melting temperature of polypropylene, which therefore maintains all of its characteristics of mechanical resistance. In particular, the bi-oriented structure of polypropylene is not altered so that it has a greater resistance to swelling in contact with the impregnating agent and imparts a mechanical resistance to the composite tape. Obviously, still according to the invention, the composite tape could be built up with bi-oriented polypropylene covered on both faces by a polypropylene copolymer so as to obtain a composite tape having paper layers on both faces, analogously to the embodiment shown in figure 1. Of course, it is possible to use other bi-oriented plastic materials and/or other copolymers to obtain the thermal bonding. Figure 4 illustrates a cable 5 conveying energy at about 700 kV, which comprises a conductor 6 formed by a plurality of keystone-shaped wires or straps 7 which define an inner duct 8 for the oil, and by an insulation 10 wound up around the conductor. Between the conductor 6 and the insulation 10, there is a semi-conductive screen 9, and an external semi-conductive screen 11 is provided between the insulation 10 and the protective metal sheath 15 of the cable. According to the invention, at least a part of the insulation 10 of the illustrated oil-filled cable is obtained by winding composite tapes of the above indicated type. If only a part of the insulation 10 is built up by using the illustrated composite tape, said part is preferably the innermost one, namely the one nearest the conductor. The following table shows a comparison between the mechanical characteristic of a laminated structure according to the invention, formed of a layer of 20 micron of polypropylene, covered only on one face with paper containing borates, and those of a conventional laminate, having a thickness of polypropylene of 52 micron sandwiched between two paper layers. In both laminates the total thickness is 125 µm. Tensile strength (N/mm²) Ultimate elongation % Longit. Transvers. Longit. Transvers. Paper-polypropylene-paper laminate 48 24 2 5 Tape according to the invention 105 50 4 12 The invention achieves the proposed goals. In fact, the composite tape according to it has low tangent and ε r values which, in particular, are lower than those of the conventional paper-polypropylene-paper structures. Further, the reduced thickness of the plastic material and its bi-orientation minimize the swelling caused by the impregnating oil when the composite tape is used to insulate a cable, which cable is further provided with the required flexibility, since the plastic material is present in a reduced amount. Moreover, the tape has very good characteristics of mechanical resistance which prove advantageous both during the conductor winding and in the subsequent operations to which the cable is submitted. In particular, for example, the composite tape has very good characteristics of tearing resistance, due to the film of bi-oriented polypropylene which has not suffered alterations during the coupling, since the softening has concerned only the microlayer of copolymer. In a composite tape according to the invention there are not additional constraints to the paper density, which can have values near the unity, whilst in the paper-polypropylene-paper laminates the paper is preferably of the low density type.

Claims: A composite tape for the insulation of high voltage electric cables, comprising a paper layer bonded to at least one surface of a film of polymeric plastic material, characterized in that said paper (2, 4; 12; 22) has a content of mineral salts ranging between 0.1 and 3% on the paper weight, and that the thickness of plastic (3; 13; 23) material ranges between 10 and 30% of the tape total thickness. A tape as in claim 1, characterized in that the content of mineral salts ranges between 0.5 and 1% by weight of the paper and that the thickness of the plastic material ranges between 15 and 25% of the tape total thickness. A tape as in claims 1 or 2, characterized in that said mineral salts are borates. A tape as in claims 1 or 2, characterized in that said mineral salts are silicates. A tape in any of claims 1 to 4, characterized in that said plastic material (3; 13; 23) is bi-oriented. A tape as in any of claims 1 to 5, characterized in that a thin layer (25) of copolymer having a softening temperature lower than that of the polymer (23) is provided between the paper (22) and the polymeric material (23). A tape as in claim 6, characterized in that the plastic material is a polyolefine. A tape as in claim 7, characterized in that the plastic material is polypropylene. A tape as in claim 7, characterized in that the plastic material is polymethyl-pentene. A tape as in claim 8, characterized in that said copolymer is a propylene-ethylene copolymer and that the thin layer has a thickness of the order of 2 µm. An electric cable for conveying energy at high voltage, comprising at least one conductor (6) insulated at least partially with windings (10) of a composite tape according to any of the preceding claims.

Inventors: BOSISIO, CLAUDIO
Applicants: PIRELLI CAVI S.P.A.
Application year: 1986
Main classification: H01B3
IPC codes: H01B7, H01B9, H01B17, H01B3