CH635700A5 - Heavy-current cable, and a method for its production - Google Patents

Description

The present invention relates to a power cable according to the preamble of the first claim.

It is a well-known problem that so-called water trees or electrochemical trees develop, especially with high-voltage cables for higher voltages in the insulation65

hen that affect the electrical properties of the insulation. It is believed that contaminants and cavities stimulate the growth of the saplings, which can lead to breakdown of the insulation. Various countermeasures have been proposed in the past to eliminate or at least reduce the formation of such water trees.

The problem of eliminating or reducing the formation of cavities and saplings in polymer cable insulation was mostly considered in connection with crosslinking processes. Such processes consist of the insulated conductor being passed through a zone of high temperature and pressure and then through a cooling zone, the latter of which can also be under pressure. It has been shown that when a polymer insulation is exposed to a humid and hot atmosphere, such as occurs in a steam cross-linking cylinder, a large number of voids occur in the cable insulation.

While efforts to solve the contaminant problem mostly go to improving the raw material, the blow hole problem is addressed during cable manufacturing. Various methods have been proposed in which the usual method of heating with steam is replaced by heating with inert gas, by using a long nozzle and utilizing the resulting frictional heat, or by heating with silicone oil. The methods of heating have been changed several times. Other processes deal with the usual water cooling systems in order to switch on water and moisture also in this stage. Nevertheless, none of the known processes guarantee void-free insulation in the end product.

This is partly due to the fact that water molecules are present in the raw material, but also partly due to the fact that a small number of water molecules usually arise during the crosslinking process through chemical reaction within the polymer insulation, and partly due to the The fact that water molecules will most likely enter the insulation if the cable is not installed in a completely dry environment.

The solution to the problem of preventing or at least reducing the entry of water into an installed cable is described in detail in the “prior art” by applying waterproof metal shields around the cable core. In an article «Plastic cable insulation» by Prof. Dr. G. Wanser and Dr. F. Wiznero-wics in “Kunststoffe 67” (1977) 5, pages 275-279, this is mentioned as a possible solution for preventing water from entering the cable. Such shields can be made of extruded lead or aluminum, but can also be steel or other metal strips, which are wound around the cable core and welded.

During and after commissioning a cable shielded with metal, there is always a risk that the shield will be mechanically damaged so that water or moisture can penetrate through the shield. Various countermeasures have been proposed to prevent longitudinal water flow in the cable, i.e. to prevent the condition in which the cable core is exposed not only to water or moisture at the point of damage, but also over a considerable length on both sides of the damage. For this purpose, so-called water stops were arranged along the cable at regular intervals, and there were also proposals which provided for all the cavities in the cable to be completely filled with a water-repellent compound. In order to make such compound fillings as effective as possible,

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the metal shields are corrugated or have other suitable shapes. Such countermeasures have been e.g. in U.S. Patent No. 3,943,271. Finally, the cable installation in tight pipes can be mentioned.

From the above-mentioned article in "Kunststoffe 67" it can be seen that the destructive effect of moisture in extruded insulation was only discovered in 1974 and that the term "water tree" was created during this time. Another type of sapling has previously been identified in connection with the installation of power cables with copper conductors in environments with high hydrogen sulfide content. This was described in DT-AS No. 2,049,105 in 1969. It was already described at that time that a strong electric field, called a sapling, could arise, and these saplings were called sulfide saplings to distinguish them from the known electrical saplings.

The last-mentioned patent particularly described how hydrogen sulfide, which is present on the sea floor, penetrates into the polymer cable insulation to the copper conductor, where copper sulfides are then produced. These sulfides cause sulfide tree crystals to form in the isolation, thereby destroying the isolation.

While the formation of sulfide trees depends on the presence of a copper conductor and the environment being enriched with hydrogen sulfide, water trees are generated regardless of the conductor material and the hydrogen sulfide content of the environment. However, it is known from the aforementioned article in “Kunststoffe 67” that the growth of the water trees is stimulated by salt solutions. Both types of tree formation (sulfide and water trees) can of course be overcome if a waterproof metal screen is used.

In the above-mentioned DT-OS 2 049 105 it was proposed to overcome the problem of the sulfide trees by arranging a layer of water-insoluble salts directly on the copper conductor or outside the insulation. The purpose of these salt layers is to create a sulfide barrier so that possible water-soluble sulfides in the environment interact with the salt layer and form a layer of water-insoluble sulfides. However, such a sulfide barrier neither prevents nor reduces the penetration of water into the insulation, so that water trees can continue to arise.

DT-OS No. 2 537 283 describes a solution to the water tree problem. This patent states that the reason for the emergence of water trees is as follows: Small cavities are present in the plastic insulation, which absorb water from the environment, and when the chemical potential of the water is reduced by an electric field, the small ones become water-filled blowholes larger and form water trees. In order to reduce the electric field and thereby the tree formation, an electrolyte is mixed into the insulating material. It is important that the electrolyte is distributed as evenly as possible in the insulating material, and a mixing ratio of 10 ~ 7: 1% by weight of the insulating material is mentioned. This solution must be seen as rather risky, since the addition of electrolytic material for insulation most likely stimulates the growth of water trees.

As already mentioned, various types of cross-linking were specified in order to reduce the formation of voids in the insulating material. The usual steam networking followed by water cooling is a cheap and simple process and is the subject of an article "The new crosslinking method of crosslinked Polyethylene cables with ultrasonic wave" in "Fujikura Technical Review"

(1974), pages 40-57. This article states that the crosslinking process should be accelerated to reduce the time that the insulating material is exposed to moisture by using ultrasound energy during the process, and that the insulating material during the crosslinking and cooling process by moisture-absorbing layer is protected.

DT-OS No. 2,519,574 states that the moisture absorbing agent calcium oxide (CaO) is very effective. Although various moisture absorbing agents are mentioned in this patent, it is mainly concentrated on CaO, MgO, CaS04 (.2H20) and Si02 gel. All of these agents have very low solubility in water and the relative humidity of a saturated solution is close to 100%. This means that when a CaO or a layer containing these agents is saturated, the layer and its surroundings have a relative humidity of 100%. The main purpose of the invention described in this document is to prevent steam from entering the insulation during the steam crosslinking process so that no micro voids are formed in the insulation. The invention is based on the theory that if the formation of micro-voids in the insulation is avoided during the crosslinking process, no water trees develop in the finished cable, even if this is installed in a damp or wet environment. The invention therefore prescribes that the CaO layer is applied before the steam crosslinking and that the layer has no effect when the crosslinking process is complete because the active agent is saturated with steam and no longer has a moisture-absorbing ability. According to this patent application, the thickness of the moisture-absorbing layer should therefore be matched to the time during which the cable is in the crosslinking stage, i.e. The longer the cable insulation remains in the steam cross-linking cylinder, the thicker the layer has to be. This is to ensure that the layer is effective as long as the steam crosslinking process lasts. However, tests are also mentioned with a cable which is provided outside the cable insulation (and between the conductor and the cable insulation) with a combined semiconductor / CaO layer, in which the insulation showed no water trees after the cable had been immersed in water for 30 days was. However, this test time must be considered far too short to prove that the method provides effective long-term protection for the cable when installed in a humid environment. CaO must also be considered unsuitable for long-term protection because of the high relative humidity already mentioned. It is even said that the outer moisture-absorbing layer can be removed after the insulation has been crosslinked, and that if the layer remains, it should be covered, probably a waterproof cover.

Contrary to what was proposed in the mentioned DT-OS No. 2,519,574, the present invention aims to produce a cable which is independent of moisture-proof screens, but which protects the cable against the formation of water trees during the entire life of the Cable, ie protects for at least 30 years.

The main purpose of the present invention is to obtain a power cable which is not susceptible to the formation of water trees in the insulation and which does not have the aforementioned disadvantages of known cables.

This object is achieved by the features mentioned in the characterizing part of claims 1 and 11. In dependent claims, advantageous developments of the cable and the method for producing this cable are specified.

Experiments have shown that when the relative humidity5

10th

15

20th

25th

30th

35

40

45

50

55

60

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insulation is below a certain limit,

no water trees start to grow, even if there are voids and dirt in the insulation.

This limit appears to be around 70% relative humidity, which corresponds to the humidity in air above water which is saturated with NaCl.

One of the advantages of power cables with a layer in accordance with the features of the present invention is that the formation of water trees in the insulation is greatly reduced, even if the insulation contains voids and contaminants. The invention dispenses with unnecessary countermeasures to reduce blowholes and contamination, but indicates significant advances in the production of better and safer power cables.

Furthermore, when using the present invention 15, it is not necessary to provide the cables with expensive waterproof metal shields.

In order to achieve the desired effect, the moisture reducing material should have an even distribution, i.e. the entire surface of the cable should be covered with 20.

The water-soluble materials are applied to an outer layer separated from the outer semiconducting layer of the cable and do not form a watertight barrier. The purpose of these materials is to limit the relative humidity of the insulation to a certain percentage. The relative humidity of the insulation is reduced in the presence of water to a value which is typical of the material used, i.e. the relative humidity is the same as in an atmosphere above a saturated solution of the material used in water.

The invention will now be explained in more detail on the basis of exemplary embodiments. The drawing shows:

1 shows a cross section of a cable according to the invention; and 35

2 schematically shows a system for producing the cable according to FIG. 1.

There are various known materials that can be used to illustrate this invention and that have the ability to reduce relative humidity 40. The following can be mentioned:

1. Non-volatile materials which, when dissolved in water, reduce the vapor pressure above the solution relative to that for pure water at the same

Temperature would be present. Water-soluble salts are an example of suitable materials.

2. Salt-forming stable hydrates, e.g. CaCh + 2H2O = CaCb • 2H2O MgCh and Mg (C104> 2.

3. Acidic and basic anhydrides, e.g. P2O5 + 3H2O = 2H3PO4 H2S2O7 (= H2S04 + SO3) + H2O - 2H2SO4. 50

4. Materials suitable for physical adsorption, such as Silica gel and so-called molecular sieves, which adsorb water on the surface.

The latter group of materials, namely silica gel and molecular sieves, which have a very large surface 55 or porosity, have the disadvantage that once the surface is saturated with water, they no longer have a moisture-reducing effect. These materials are therefore unsuitable if long-term effectiveness is desired. 60

In terms of long-term effectiveness, tests have shown that the first two groups of materials are the most suitable. Tests have also shown that these materials have a high resistance to washing out. 65

Examples of salts that have the desired effect are published in an article "The properties of water-salt systems in relation to humidity" by R. G. Wylie

1965 in a book by A. Wexler: "Humidity and Moisture", Volume III published by Chapman and Hall. Of the salts mentioned, only those with a relative humidity below 70% are suitable, but the list of such water-soluble salts cannot be regarded as complete.

The core 1 of an electric power cable to which the present invention can be applied may be of conventional construction, i.e. a metallic conductor of any configuration and material, which is coated with a layer of semiconducting material 2, followed by one or more layers of extruded polymeric insulation 3 of the desired thickness and an outer layer of semiconducting material 4. A layer 5 contains a material which has a known ability to reduce and stabilize the relative humidity of its surroundings to a value determined by the material, which layer is hereinafter referred to as the active layer. The active layer 5 can be extruded as a fourth layer of polymeric material. Furthermore, more than one active layer can be provided, which are extruded or arranged on one another. As a variant, the active layer can be provided in the form of a tape, which is wound or folded around the cable core. Again, it is possible to wrap or fold two or more tapes, or a combination of extruded layers and wound or folded layers can also be provided. Outside the active layer, a cover layer 6, preferably made of polymeric material, is provided.

The thickness of the active layer with respect to the cable diameter can be of the same order of magnitude as that of the semiconducting layers. It is important for the life of the active layer that the cable should have a sufficient amount of active material per surface unit. The amount of active material must therefore be designed in such a way that a desired service life is obtained under certain conditions (temperature, ambient humidity, water permeability of the cover layer material). A content of 0.01-0.1 g / cm2 is considered as an example, which gives good results for the life of the cable. It should now be noted that even if the active material is saturated by exposure to moisture, the moisture stabilizing and reducing effect is retained as long as the active material remains. The relative humidity of the insulating material will only gradually increase to values at which water trees grow when the active material is gradually used up.

2 schematically shows an advantageous method for producing a power cable according to the invention. The cable core 1 is led from a decoiler 10 to one of several extruders 11, 12, 13 for applying the inner semiconductor layer 2, the insulation 3 and the outer semiconductor layer 4, whereupon the crosslinkable layer (s) are crosslinked in a crosslinking device 14 ( will).

If the cable is provided with the active layer 5, it is provided with an outer layer 6 in an extruder 15, whereupon the cable is wound up on a take-up reel 16. If steam is used during the crosslinking process, the active layer 5 will e.g. applied in an extruder 17, precisely after the cable with the insulation 3 and the semiconducting layers 2 and 4 has passed the crosslinking device. This ensures that the active layer is fully effective once the cable is finished. If no steam is used in the crosslinking process and possibly also no water cooling, it is possible to apply the active layer 5 directly after the application of the outer semiconductor layer and before the crosslinking process, as is indicated by the extruder 18.

The active material can be an extruder in the form of fine

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distributed powder or granules. An extruded semiconductor layer, 3.4 mm insulation of layer with such active powder can also be achieved by steam cross-linked polyethylene and an extruded outer by extruding the cable from the outer semiconductor layer (triple extruded). A layer of creped soot paper was passed through a container containing such powder over the outer half-lead cover or the application of a cover tape through a layer. 5 wrapped with 75% overlap. The active layer of test-As a variant, the active material distributed in a solution, cable no. 1 was obtained by being the fully wound cable through which the extruder granulate passes through a container with a saturated aqueous solution prior to extrusion. Such an active solution can also be drawn in an Isola from CaCh, which layer is then adsorbed drying layer, which has been halved over the outer, after which a 0.5 mm thick shrink tube is extruded over the conductive layer by the Cable was pulled through the cable and shrunk onto it. While running the solution. The active layer can also be made semi-test cable No. 2, the space between the dry conductive. An active roll-up tape can also be produced in this way by wrapping with soot paper and the shrink tube. The active material is filled with a saturated aqueous solution of CaCh before it can also be shrunk into a coiled tape or into an extruded tube. Test cable no. 3 was a layer sintered to quickly wash out reference cables similar to cable no. 2 and manufactured in the same way. Experiments have shown that the wash-out time for the, with the exception that, instead of the CaCh solution, pure water-active material was used under such conditions, much higher. All test cables were in water as the expected life of the cable. As practical dipped and tested at room temperature. In the test of the principle of using water-soluble intervals shown in Table I, 5 mm long patterns of the insulation materials according to the present invention were removed as active layer 20 and long-term experiments with cables were carried out under the microscope to count the water. The test trees and for measuring their length. The middle cables were designed for a nominal voltage of 12 kV with size «x» in (in, while «s» indicates the distribution of an aluminum conductor with a cross-section of 35 mm2, one inside the water tree.

Cable tests in water at room temperature (20 ° C) voltage 20 kV frequency: 50 Hz

Cable active test sapling in isolation sapling from semiconductor layers

(Loop-shaped) inner layer outer layer

No. Shift interval number length vert. Number Length Vert. Number Length Vert.

(h) per mm x (| im) s per mm x (| im) s per mm x (| im) s

1

Dry CaCh

5312

0

0

0

2nd

Saturated CaCh

8602

0

0

0

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no

1921

21.6

20th

13

0.8

50

14

0

9495

15

50

0.5

160

0.5

260

The results of Table I show that the active layer of the aqueous solution is present, a considerable amount is fully effective against the formation of water trees. 40 influence, it can be seen that even such a simple one-If one also takes into consideration that with cable pattern no. 2 this exemplary embodiment has a considerable lifespan because of the fact that CaCh only has a saturated form.

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1 sheet of drawings

Claims (14)

635 700 2nd PATENT CLAIMS 1. Water-resistant electrical power cable with an inner conductor (1), which is provided with one or more layers (3) of polymeric insulating material as well as with an inner (2) and an outer (4) semiconducting 5 layer, characterized in that an active layer (5) is present outside the outer semiconductor layer (4) and separated therefrom, which contains water-soluble material which limits and stabilizes the relative humidity of the cable insulation to a value not exceeding 70%. 2. Cable according to claim 1, characterized in that the water-soluble material are water-soluble salts. 3. Cable according to claim 1, characterized in that the water-soluble material is a salt from the group of Sai is zen, which form stable hydrates. 4. Cable according to claim 3, characterized in that the material is CaCh. 5. Cable according to claim 3, characterized in that the material is MgCk. 20th 6. Cable according to one of claims 1 to 5, characterized in that the active layer (5) is semiconducting. 7. Cable according to one of claims 1 to 6, characterized in that an insulating layer (6) made of polymeric material is applied over the active layer. 25th 8. Cable according to one of claims 1 to 7, characterized in that the active layer (5) is applied in the form of a helical-spiral-shaped band. 9. Cable according to one of claims 1 to 8, characterized in that the active layer contains at least 30 g of active material per cm2 of its surface. 10. Cable according to claim 1, characterized in that the active layer (5) contains water-soluble material which limits and stabilizes the relative humidity of the cable insulation to a value not exceeding 50%. 35 11. The method for producing an electrical power cable according to claim 1, characterized by the steps: passage of the electrical conductor (1) through one or more extruders (11-13) for applying the inner semiconducting layer (2), one or more insulation 40th layers (3) and the outer semiconducting layer (4), at least one of said layers (2-4) consisting of crosslinkable polymeric material; either crosslinking the crosslinkable layers (2-4) and subsequent application of the active layer (5), or application of the active layer (5) 45 and subsequent crosslinking of the crosslinkable layers (2-4). 12. The method according to claim 11, characterized in that the active layer (5) is applied before crosslinking, and that no steam is used in the crosslinking. 50 13. The method according to claim 11, characterized in that for the application of the active layer (5) by extrusion the granulated polymeric material to be extruded, the active material is added as finely divided crystals. 14. The method according to claim 11, characterized in that the active material is used to produce a saturated solution which is adsorbed by the granulated polymeric material before the extrusion of the active layer (5).