FI64866C - FRAME RELEASE CABLE OVER FREED CABLE - Google Patents

Description

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y (51) K ».ik.3 / int.a.3 H 01 B 7/28 ENGLISH I - FI N LAN D (21) Pwenttlhtkumu * - PK« ntaiw6knlns 781186 (22) Haktmlspilvi - AraBknlnfadag l8.04. J8 (^ 0 (23) Alkupihri —GUtlfhetsdag l8.04.78 (41) Become public - Blivit offentllg 29-10.78

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Patent · and registration register Amekan utlafd och uti.skriften publtcsnd J ^ J

(32) (33) (31) privilege — Btfird priorltet 28.04.77

Norway-Norway (NO) 771479 (71) Forskningsinstitutt Elektrisitetsforsyningens, 7034 Trondheim,

Norway-Norway (NO) (72) Jarle Sletbak, Trondheim, Arild Botne, Trondheim, Norway-Norway (NO) (74) Oy Kolster Ah (54) Vesicle-resistant power cable and its manufacturing method -Vattenbläsbeständig starkströmskabel och förfarande för dess fram The present invention relates to power cables and in particular to power cables with extruded polymeric insulation. In the case of high-voltage power cables in particular, the problem is well known that the so-called water blisters or electrochemical blisters formed in the insulation can impair the electrical properties of the insulation. It is believed that contaminants and voids contribute to the growth of blisters, which can lead to insulation breakage. Many remedies have been proposed in the prior art to eliminate or reduce the formation of such vesicles.

The problem of eliminating or reducing the formation of pores and blisters in polymeric cable insulation has mostly been considered in the context of the crosslinking process. Such processes include passing the insulated conductor through a high temperature, high pressure zone and then through a cooling zone, which may also be pressurized. It has been shown that when subjected to polymeric insulation in a humid and hot environment, such as in a steam-operated vulcanization tube, a large number of pores are formed in the insulation of the cable.

When the problem of insulation impurities has most often been solved by improving the raw material, the problem of pores hits the cable manufacturing process. Several methods have been proposed in which conventional steam crosslinking is replaced by neutral gas crosslinking, a long compression vulcanizer, and even crosslinking with silicone oil. Heating methods vary. Additional methods eliminate conventional water cooling systems to eliminate water and moisture at this stage as well. However, none of the known processes guarantee the porosity of the final product insulation. This is partly due to the fact that the water molecules are in the raw material, and partly due to the fact that small amounts of water molecules are formed in the vulcanization process due to the chemical reaction in the polymeric insulation, and partly due to the fact that water molecules are more likely to penetrate the insulation.

An attempt has been made to solve the problem of preventing or at least limiting the ingress of water into the installed cable according to the prior art by placing watertight or water-impermeable metal socks around the insulated cable core. In his article "Kabelisolierungen aus Kunststoffe" of the magazine kunststoffe no. 5, 1977, pages 275-279, Prof. Dr. G. Wanser and Ph.D. Such socks may be in the form of extruded lead or aluminum, or may be made of tapes or strips of steel or other metals bent and welded around the core of the cable.

During and after the installation of metal sock cables, there is always a risk that the sock will be mechanically damaged so that water or moisture will penetrate the sock. A few aids are recommended to be used to prevent the longitudinal flow of water inside the cable, as the core of the cable is not exposed to water or moisture in its damaged location but also at a considerable distance from this location in both directions. To this end, 3 64866 water traps have been placed at regular intervals in the cable and attempts have also been made to fill all the spaces of the cable with a waterproof mass. In order to make such filling compounds as efficient as possible, metal socks can be corrugated or provided with certain types of teeth. Such means are described, for example, in U.S. Patent No. 3,943,271. Finally, pipe-type cable insulation methods must be mentioned.

It is apparent from the aforementioned Kunststoff article that it was not until 1974 that the detrimental effect of moisture on extruded insulation was invented, and the concept of "vesicles" was coined. However, a different "blistering" was previously observed when installing copper conductor power cables in an environment rich in hydrogen sulfide (I ^ S). This is described in patent application DT-OS No. 2,049,105 of 1969. At that time it was already known that "blisters" could form in a strong electric field and newly detected blisters were called "sulphite blisters" as opposed to well known electric blisters.

In particular, this patent describes how hydrogen sulfide (HjS) generated in the sea penetrates through the insulation of a polymeric cable into a copper conductor where copper sulfites are formed. These sulfites, in the form of sulfite vesicles, create vaults in the insulation, whereby the insulation deteriorates.

When the formation of "sulfite blisters" is dependent on the copper-conducting cable and the presence of a lot of sulfur in the environment, the formation of "water blisters" is independent of the conductor material and the sulfur content of the environment. From an article by Kunststoff

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however, it appears that saline conditions promote the growth of vesicles. The formation of both types of blisters (sulfite blisters and vesicles) is resisted by using a waterproof metal sock.

Said patent DT-OS No. 2,049,105 proposes to resist sulfite blisters by applying a layer of water-insoluble salts directly to the copper conductor or outside the insulator. The purpose of these salt layers is to provide a sulfite layer such that any water-soluble sulfites in the environment react with the salt layer to form a layer of water-insoluble sulfides. However, such a salt layer does not prevent or reduce the passage of water into the insulation, thereby further the formation of vesicles.

4,64866

Patent application DT-OS No. 2,537,283 describes one solution to the vesicle problem. This patent states that the cause of the formation of vesicles is as follows: an insulator made of plastic has small bubbles that absorb water from the environment, and when an electric field reduces the chemical potential of water, small water-filled pores grow and form vesicles. To reduce the electric field and Selma to reduce the blisters, electrolyte is mixed into the insulating material. It is important that the electrolyte is distributed as evenly as possible. The ratio of mixing to the insulating material 7a is mentioned as 10 to 1% by weight of the insulating material. This solution must have a rather high risk because the addition of electrolytic material to the insulation is most likely to increase the growth of vesicles.

As previously mentioned, several different crosslinking methods are proposed to reduce the pore formation of the precursor material. However, conventional steam crosslinking followed by water cooling represents an economical and simple process, and this is the subject of the Fujikura Technical Review 1974 on pages 40-57: "The new cross-linking method of crosslinked polyethylene cables with Ultrasonic wave" . This article states that it is possible to accelerate the crosslinking process in order to reduce the exposure time of the insulating material to water by using ultrasonic waves during the process and protecting the insulating material with an undefined moisture absorbing layer during the crosslinking and cooling process.

Fujikura's patent application DT-OS 2,519,574 states that calcium oxide (CaO) is very effective as a moisture absorbing agent. This patent mentions a number of moisture absorbing agents focused on CaO, MgO, CaSO 4 (-21 2 O) and SiO 2 gel. The water solubility of all these substances is low and the relative humidity of the saturated solution is close to 100%. Thus, when the layer containing CaO or some other such substance is saturated, the relative humidity of the layer and its surroundings is 100%. The main object of the invention described here is to prevent the propagation of steam into the insulation during the steam crosslinking process so that no micropores are formed in the insulation. According to Fujikura, the invention is based on the theory that if the formation of micropores in the insulation is prevented during crosslinking, no blisters will form in the finished game, even if placed in a humid or wet environment. The invention therefore requires that the CaO layer be applied before steam crosslinking and that the layer has no effect when said process is completed, because the active substance is saturated with steam and is no longer able to absorb moisture. According to said patent, the thickness of the moisture-absorbing layer should be adjustable according to the time the cable is in the crosslinking phase, i.e. the longer the cable insulation is in the steam crosslinking pipe, the thicker the layer must be. This ensures that the layer is effective as long as the steam crosslinking process lasts. However, experiments in which the cable is provided with a combined semiconductor / CaO layer outside the cable insulation (and between the conductor and the cable insulation) in which no water blisters were visible in the insulation after the cable samples were immersed in water for 30 days have also been reported. However, the test period must be considered far too short to show that the method would provide effective long-term cable protection if the cable is installed in humid conditions. In addition, CaO must be considered unsuitable for providing long-term protection due to said high relative humidity. It is further mentioned in said patent application that the outer moisture absorbing layer may be removable after vulcanization of the insulation and that if the layer is left in place it should be covered with a sock, presumably moisture impermeable. Contrary to what is suggested in said patent application, the present invention seeks to provide a cable which does not require moisture-impermeable socks, but which still protects the insulation from the formation of vesicles throughout the life of the cable, i.e. at least 30 years.

The main object of the present invention is to provide power cables which are not susceptible to the growth of water vesicles in the insulation and which do not have the disadvantages of previously known cables.

The main advantages of the present invention will become apparent from the following claims.

Experiments have shown that when the relative humidity of the insulation is below a certain limit, the vesicles do not start to grow, even if there are pores and contaminants in the insulation. This limit also appears to be about 70%, which corresponds to the humidity of the air above the water saturated with NaCl.

6 4 8 6 6 One of the advantages of kale with a layer having the properties of the present invention is that the formation of water vesicles in the insulation is greatly reduced even if there are pores and impurities in the insulation. The present invention does not make the improvements used to reduce pores and contaminants unnecessary but substantially increases the means of producing better and safer cables.

Utilizing this invention further, it is possible to provide cables with expensive waterproof metal sock11a.

To achieve the desired effect, the moisture-reducing material must be evenly distributed and cover the entire surface of the cable.

Several materials are known which are capable of reducing relative humidity. The following should be mentioned: 1. Non-volatile substances which, when dissolved in water, reduce the vapor pressure above the solution in relation to that which would be above pure water of the same temperature. Water-soluble salts are an example of suitable substances.

2. Stable hydrates are formed by salts such as CaCl 2 + 2H 2 O = CaCl 2 * 2H 2 O, MgCl 2 and Mg (ClO 4) 2.

3. Acid and base anhydrides, such as, for example, P2O5 + 3H2O = 2H3PO4, H2SO27 (= H2SO4 + SO3) + H2O = 2H2SO4.

4. Substances capable of physical adsorption, such as silica gels and so-called "molecular filters", which adsorb water to the surface. The latter group of substances, namely silica gel and "molecular sieves" having a very large surface area or porosity, have the disadvantage that, once the surface is saturated with water, they no longer have a moisture-reducing effect. These substances are therefore not useful when a long-term effect is desired.

Focusing on the long-term effect, experiments have shown that the first two groups of substances are the most useful. Experiments have also shown that these substances are very resistant to leaching.

Examples of salts having the desired effect are mentioned in R.G. Wylie, "The properties of water-salt systems in relation to humidity," published in 1965 in A. Wexler's book "Humidity et Moisture," Volume III, Chapman and Hall. Of these salts, only those with a relative humidity of less than 70% are useful, but the list of such water-soluble salts cannot be considered exhaustive. The water-soluble materials useful in the present invention are placed, as mentioned, outside and away from the outer semiconductor layer of the cable and do not act as a tight water barrier. The purpose of these materials is to reduce the relative humidity of the insulation to a certain percentage. The relative humidity of the insulator near water decreases to the value characteristic of the substance used, i.e. the relative humidity becomes the same as in the air above the aqueous solution saturated with the substance used.

The above and other objects and features of the present invention will become more apparent from the following detailed description with reference to the drawings, in which Figure 1 shows a cross-section of a cable according to the invention, and Figure 2 schematically shows an apparatus for manufacturing a cable according to Figure 1.

The core 1 of the electric power cable to which the present invention can be applied can be any conventional core, i.e. any shape of conductor and any material, and the core is covered with a semiconductor material 2 followed by one or more extruded polymeric insulating layers 3 of the desired thickness and outer layer of semiconductor material 4.

Layer 5 contains a material which is known to be able to reduce and stabilize the relative humidity of the environment to a certain value determined by the material, and this layer is hereinafter referred to as an active layer. The active layer 5 can be extruded as a fourth polymer layer. Furthermore, one or more active layers can be extruded or placed on top of each other. Alternatively, the active layer may be made into a strip which is wound or folded around the core of the cable. In addition, two or more strip layers or coils or combinations of extruded layers and twisted layers may be present. A cover layer 6, which is preferably a polymeric material, is arranged outside the active layer.

The ratio of the thickness of the active layer to the diameter of the cable may be equal to the thickness of the semiconductor layers. Significant to the duration of the moisture stabilization layer is that the 8 64866 cable should have a sufficient amount of active agent per unit area. The amount of active substance must therefore be dimensioned according to a certain duration under certain conditions (ambient humidity and temperature, water permeability to the coating layer 2 substance). For example, an amount of 0.01 to 0.1 g / cm is considered to give good cable life results. It should now be mentioned that although the active material is saturated under moisture, the moisture stabilization and reduction action remains as long as the active substance remains. The relative humidity of the insulator only gradually increases the growth values of the vesicles as the active substance gradually dilutes.

Figure 2 schematically shows a preferred method of manufacturing a power cable according to the present invention. The core of the cable is obtained from the coil 10 in one or more nozzles 11, 12, 13, in which the inner semiconductor 2, the insulator 3 and the outer semiconductor 4 are extruded, while the vulcanization layer or layers are joined and cooled at the vulcanization site 14. After obtaining the moisture stabilizing layer 5, a surface layer is added. 15, after which the cable is wound on a coil 16. If steam is used in the crosslinking, the moisture stabilizing layer 5 is placed e.g. in the nozzle 17 just after the cable with insulation 3 and semiconductor layers 2 and 4 has ignored the vulcanization site. Thus, it is manufactured that the layer is fully effective as soon as the cable is completed. If steam and possibly also water for cooling are not used in the crosslinking process, it may be possible to connect the moisture stabilizing layer 5 sequentially in connection with the outer semiconductor before the vulcanization process, as shown by the nozzle 18.

The active ingredient may be added to the die in the form of a fine powder or granule. Such an active powder layer can also be made by passing the cable through a box or housing containing such a powder before extruding the outer shell or attaching the cover tape. Alternatively, the active agent may be ingested from a solution through which the die granules pass prior to extrusion. Such an active solution can also be adsorbed onto the outer semiconductor layer extruded into the insulating layer by passing the cable through such a solution. However, the active layer can also be made into a semiconductor. An active rotatable tape can also be made in this way. The material can also be sintered to a turntable or extrudable layer to prevent rapid rinsing. Experiments have shown that the leaching time of the active material under such conditions is substantially longer than the expected life of the cable.

Example Long-term testing of cables has been carried out as a practical test of the principle of the present invention, in which water-soluble substances are used as a moisture stabilizing layer. The cables to be tested are rated for a nominal voltage of 12 kV and have a 35 mm aluminum conductor, an extruded inner semiconductor, a 3.4 mm polyethylene insulation cross-linked with steam and an extruded outer semiconductor (triple extruded). A layer of creped carbon paper is wound on top of the outer semiconductor with a coverage of 75%. The active layer of cable No. 1 is obtained by passing a pre-twisted cable through a vessel with an aqueous solution saturated with CaCl 2, after which the cable is dried, a 0.5 mm thick shrink sleeve is pulled over the cable and shrunk to provide a test cable. The space between the dry carbon paper web layer of cable No. 2 and the shrink stocking was filled with a saturated aqueous solution of CaCl 2 before shrinking the stocking. Cable No. 3 is a reference cable similar to Cable No. 2 and has been prepared accordingly except that pure water has been used instead of CaCl 2 solution. All cables are placed in water and tested at room temperature. At intervals according to Table 1, 5 mm long samples have been taken from the insulation and examined under a microscope to count the vesicles and measure their lengths. The mean size x is expressed in micrometers and s is the scatter.

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Claims (10)

An electric strong current cable that is resistant to forming "water trees" in the insulation, comprising a conductor provided with one or more layers of extruded polymeric insulating material, comprising an inner and an outer semiconducting layer and an outer protective layer of polymeric material, characterized in that and separated from the insulation there is provided a coherent layer containing water-soluble salts which remain water-soluble during the life of the cable and which in a known manner limit and stabilize the relative humidity of the conductor insulation to a value not exceeding 70%. 2. A high current cable according to claim 1, characterized in that the salts are selected from the group of salts which form stable hydrates. 3. A high current cable according to claim 2, characterized in that the water-soluble salt is Ca 4. A high current cable according to claim 2, characterized in that the water-soluble salt is MgC 5. A high current cable according to any of the preceding claims, characterized in that the water-soluble salts are contained in the semiconducting layer. 6. A high current cable according to any of the preceding claims, characterized in that the water-soluble salts are contained in a helically wound band. 7. A high current cable according to any of the preceding claims, characterized in that the amount of water-soluble 2 salts is at least 0.01 g / cm of the surface of the layer. The method of producing a strong electric current cable according to claim 1, wherein an electrical conductor is passed through one or more extruder nozzles for applying an inner semiconducting layer, one or more insulating layers and an outer semiconducting layer, at least one of which consists of cross-linked polymeric material. , whereupon the cable undergoes a crosslinking process such that one or more of said layers are crosslinked, and an outer protective layer of polymeric material is applied], characterized in that the cable is off-welded and separated from the insulation by winding,