NO140249B - STRONG CURRENT CABLE WHICH IS RESISTANT TO THE FORMATION OF WATER TREES IN THE INSULATION AND PROCEDURE FOR THE MANUFACTURE OF SUCH CABLE - Google Patents

Description

The present invention relates to high current cables which are

provided with extruded polymer insulation, as well as a method for producing such.

It is a well-known problem, particularly in connection with high-current cables for higher voltages, that so-called water trees or electrochemical trees formed in the insulation can weaken the electrical characteristics of the insulation. It is believed that contaminants and voids stimulate the growth of water trees which can cause penetration in the insulation. Many ways have previously been proposed to eliminate or reduce the formation of such water trees.

The problem of eliminating or reducing the formation of voids and water trees in polymer cable insulation has mostly been studied in connection with crosslinking processes. In such processes, the insulated conductor is passed through a zone of high temperature and high pressure and then through a cooled pressure zone. It has been shown that when

polymer insulation is exposed to a wet and a warm atmosphere such as

in a vapor cross-linking tube, a large number of voids will appear in the cable insulation.

While the problem of impurities in the insulation has mostly been attempted to be solved by improving the raw material, the problem of voids has

been processed during the cable manufacturing processes. Several methods have been proposed whereby the conventional cross-linking method is replaced by cross-linking under the influence of an inert gas, by cross-linking in long nozzles and by cross-linking in silicone oil. The heating methods have also been varied. Further methods attack the conventional water cooling systems to eliminate water and humidity at this stage as well. However, none of the known processes guarantee void-free insulation in the finished product. This is partly due to water molecules being present in the raw material, in part

of the fact that a small number of water molecules are usually formed during cross-linking

ing process by chemical reaction inside the polymer insulation, and partly because water molecules will very likely penetrate the insulation if the cable is not installed in a completely dry environment.

The problem of preventing or at least limiting the penetration of water into an installed cable has previously been attempted to be solved by introducing waterproof or impermeable metal sheaths on the outside of the insulated cable core.

In an article in Kunststoffe No. 5, 1977, pages 275 - 279 "Kabel-isolierungen aus Kunststoff" by Prof. Dr. G. Wanser and Dr. Ing. F. Wiznerowicz, this is mentioned as a possible solution to prevent the penetration of water into the cable. Such sheaths can either be made of extruded lead or aluminium, or bands or strips of steel or other metal can be used which are wound or folded around the cable core and welded longitudinally.

During and after installation of a metal-sheathed cable, there will always be a risk that the sheath may be mechanically damaged so that water or moisture can penetrate through the metal sheath. Many remedies have been proposed to prevent water from flowing long into the cable, so that the cable is not only exposed to water or moisture at the damaged location, but also for a not inconsiderable length on either side of the location. For this purpose, so-called water barriers have been placed at regular intervals along the cable, and it has also been proposed to completely fill all cavities in the cable with a water-repellent mass. To make such fillers as effective as possible, the metal casings can be corrugated or provided with an imprinted pattern. Remedies with these purposes in mind are e.g. disclosed in US Patent No. 3,943,271. Installation methods of the "pipe-type cable" type can also be mentioned.

From the above-mentioned article in Kunststoffe it appears that one first

in 1974, attention was drawn to the detrimental effects moisture could have on extruded insulation, and the term "water trees" first emerged at that time. However, another type of "wood formation" was discovered earlier, in connection with high-current cables with copper conductors being installed in humid environments rich in hydrobranch sulphide. This is discussed in BRD official document no. 2,049,105 from 1969. It was already known then that "trees" could form in a strong electric field, and the newly discovered trees were therefore called "sulphide trees" in contrast to the known electric "trees".

In this patent, it is described in particular that hydrogen sulphide (H2S) which is formed at the seabed penetrates through the polymeric cable insulation to the copper conductor where copper sulphides are formed. These sulphides in turn cause the formation of sulphide tree crystals in the insulation, whereby the insulation is destroyed.

While the formation of "sulphide trees" is dependent on the cable having a copper conductor and the surroundings being rich in sulphur, "water trees" are formed independently of the conductor material and the sulfur content of the surroundings. From the Kunststoffe article, however, it appears that the growth of "water trees" is stimulated by salt solution. Both types of tree formation (sulphide trees and water trees) can of course be remedied by using a waterproof metal jacket on the outside of the cable.

In the aforementioned BRD official document no. 2,049,105, it is proposed to remedy the problem of sulphide trees by placing layers containing preferably water-insoluble salts directly on the copper conductor or on the outside of the insulation. The purpose of these salt layers is to form a sulphide barrier, whereby any water-soluble sulphides from the surroundings connect with the salt layer and form a layer of water-insoluble sulphides. However, such a sulphide barrier will neither prevent nor reduce the ingress of water into the insulation so that "water trees" can nevertheless form.

BRD official document no. 2,537,283 deals with a solution to the problem of water trees in the cable insulation. In this paper, one believes to have found that the reason for the formation of water trees is the following: In the plastic insulation there are small cavities that absorb water from the surroundings, and when the water's chemical potential is reduced by an electric field, the small water-filled cavities will increase so that water trees are formed. With the aim of reducing the electric field and thereby reducing wood formation, an electrolyte is mixed into the insulation material. Great emphasis is placed on the electrolyte being distributed as evenly as possible throughout the insulation, and a mixing ratio of 10 - 1% by weight of the insulation material is specified. Such a solution must be considered to be very risky, as it must be assumed that any addition of electrolytic material to the insulation will be able to stimulate the formation of water trees.

As previously mentioned, several alternative cross-linking methods have been proposed to reduce the formation of voids in the insulation material. As, however, conventional steam cross-linking followed by water cooling represents an economical and simple process, mention should also be made of an article in Fujikura Technical Review 1974, pages 40 - 57,

"The new cross-linking method of crosslinked polyethylene cables with ultrasonic wave", where it is stated that the cross-linking process can take place faster, in order to reduce the time that the insulation material is exposed to moisture, by using ultrasound during the process itself, and that the insulation material during cross-linking and the cooling process is provided with an undefined moisture-absorbing layer.

In Fujikura's BRD official publication no. 2,519,574, it is claimed that the moisture-absorbing substance calcium oxide (CaO) has a very good effect.

The patent mentions a number of moisture-absorbing substances, of which they have concentrated on CaO (quick lime), MgO (magnesia), CaS04 (-2H20) (gypsum) and Si02 gel (silica gel). All these substances have very low solubility in water, and the relative humidity above a saturated solution is close to 100%. This means that when a layer containing CaO or one of these substances becomes saturated, the layer and its surroundings will have a relative humidity of 100%. The main purpose of the invention described there is to prevent the penetration of water vapor into the insulation during the vapor cross-linking process, so that small cavities (micro-voids) are not formed in the insulation. The invention according to Fujikura is based on the theory that if you avoid the formation of cavities in the insulation during the cross-linking process, water trees will not be able to form in the finished cable even if the cable is installed in a humid environment. The invention therefore assumes that the CaO layer is placed before the steam cross-linking process begins and the layer has played its role when this process is finished, because the active substance will then be saturated with water vapor and will no longer have any moisture-absorbing effects. According to the writing, the thickness of the moisture-absorbing layer must be adapted to the time the cable is in the cross-linking stage, i.e. the longer the cable insulation is in the vapor cross-linking tube, the thicker the layer should be. This is to ensure that the layer's effect is effective as long as the vapor cross-linking process is ongoing. However, experiments with a cable sample provided with a combined semiconductor/CaO layer on the outside of the cable insulation (as well as between conductor and cable insulation) are mentioned, where the insulation showed no signs of water wood formation after the 30 days the cable sample was immersed in water. However, the trial period must be considered far too short to convince that the method will provide any effective long-term protection of the cable if it is installed in a humid environment. In addition, CaO must be considered unsuitable for providing long-term protection due to mentioned high relative humidity. It is even pointed out in the aforementioned patent that the overlying moisture-absorbing layer can be removed after the cross-linking of the insulation, and that if the layer is retained, it can (should) be covered by a cover, i.e. a moisture-proof cover. In contrast to what is proposed in the said patent, the present invention aims to provide a cable which does not depend on moisture-proof sheaths, but which nevertheless protects the insulation against the formation of water trees, during the entire expected life of the cable, usually 30 years.

This printed document gives no indication that compounds other than CaO have the intended effect, since most of the moisture-absorbing substances mentioned are rejected in the first selection, and the compounds MgO, CaSO4 («2^0) and SiO2 gel are rejected in the second selection .

The main purpose of the invention is to provide a high-current cable which is resistant to the formation of water trees in the insulation, and which does not have the same disadvantages as previously known cables.

The invention appears from the claims set out below, and preferred embodiments are specified in subordinate claims.

Experiments have shown that if the relative humidity in the insulation is lower than a certain limit, water trees will not start to grow even if the insulation contains voids and contaminants. This limit further appears to be approximately 70%, which corresponds to the humidity in air over water that is saturated with NaCl.

One of the advantages of high current cables provided with a layer according to the invention is that the formation of water trees in the insulation is greatly reduced, even if the insulation contains voids and contaminants. The invention does not make remedies that are carried out to reduce voids and contamination unnecessary, but significantly contributes to the purpose of producing better and safer high-current cables.

By using the present invention, however, it will not be necessary to equip the cables with expensive waterproof metal sheaths.

To achieve the desired effect, the moisture-reducing material should be distributed evenly over the entire surface of the cable.

Salts soluble in water are used as materials that have the property of reducing relative humidity: 1. They are non-volatile compounds which, when dissolved in water, reduce the vapor pressure above the solution compared to what it would be for pure water at the same temperature. 2. Salts that form stable hydrates, such as e.g. CaCl2 + 2H2O = CaCl2 • 2H2O; MgCl2 + 6H2O = MgCl2 • 6H2O.

When a long-term effect is desired, experiments have shown that such compounds are suitable. Experiments have also shown that these compounds exhibit a high resistance to being washed out.

Examples of salts that will have the desired effect are mentioned in an article by R.G. Wylie "The properties of water-salt systems in relation to humidity", published in a book by A. Wexler: "Humidity & Moisture", Volume III, Chapman and Hall, 1965.

Of the salts mentioned, only those which remain water soluble for the life of the cable and which have a relative humidity lower than 70% will be

.suitable. However, the list of water-soluble salts cannot be considered

to be complete.

When choosing the active connection, it must be taken into account that the connection must remain water-soluble, as it must not react to become water-insoluble with substances that are either found in the cable construction itself, or that one must (can) expect to need in from the surroundings during operation.

Of the substances found in the cable itself, this primarily applies to the carrier material itself (polymer, possibly with filler) and additives to this or to the other construction materials in the form of anti-oxidants, plasticizers, fire-retardant additives or the like.

Substances that can penetrate from the cable surroundings will primarily be low-molecular gases such as atmospheric gases (oxygen), ozone from the discharge, hydrogen sulphide and ammonia from organic decay, but mineral oils will also be able to diffuse in through the sheath and bind to the active layer.

The water-soluble compounds which are applicable according to the present invention are, as mentioned, placed in a continuous layer on the outside and separated from the cable's insulation, but do not form a waterproof barrier. The purpose of this layer is to reduce the relative humidity in the insulation to a predetermined percentage. In the presence of water, the relative humidity in the insulation will be reduced to a value that is characteristic of the compound used in the layer, i.e. the relative humidity will be the same as in an atmosphere above a saturated solution in water of the compound used .

The above-mentioned purpose and special features of the invention will become clearer from the detailed description below with reference to the drawings, where Fig. 1 shows a cross-section of a cable according to the invention, and Fig. 2 shows a sketch of a plant for producing a cable according to Fig. 1.

The core 1 in high current electric cables for which the present invention can be used can be of any conventional type, such as e.g. a metallic conductor of any shape and material surrounded by a layer of semi-conducting material 2, then surrounded by one or more layers of extruded polymer insulation 3 of the desired thickness and an outer layer of semi-conducting material 4.

The layer 5, which contains compounds which in a known manner are capable of reducing and stabilizing the relative humidity of the insulation to a value determined by the material itself, will hereinafter be called the active layer. The active layer 5 can be extruded as a fourth layer of polymeric material. Furthermore, several active layers can be provided which are extruded or placed on top of each other. Alternatively, the active layer can be formed in the form of a band that is wound or folded around the cable core. Furthermore, two or more wrapped or folded bands can be used, or a combination of extruded layers and wrapped (folded) layers. A covering layer 6 of polymeric material is placed on the outside of the active layer.

The thickness of the active layer in relation to the cable diameter can be of the order of magnitude the same thickness as the semi-conducting layers. The decisive factor for the lifetime of the moisture stabilizing layer is that the cable contains a sufficient amount of active material per unit surface area. The quantity of the active material must therefore be dimensioned based on the desired lifetime under specified conditions (temperature, humidity in the surroundings, water permeability of the casing material). For example, a content of 0.01 - 0.1 g/cm is assumed to give good results throughout the cable's lifetime. It should be noted here that even if the active material is saturated by being exposed to moisture, the moisture stabilizing and reducing effect will be present as long as active material is present. The relative humidity in the inner insulation material will only gradually increase to values where water trees form, as the active material is diluted.

Fig. 2 shows an advantageous method for producing high current cable according to the present invention. The cable core 1 is fed from an unwinding drum 10 to one or more extruders 11, 12, 13 for the application of inner semiconductor 2, insulation 3 and outer semiconductor 4, after which the crosslinkable layers are crosslinked and cooled in a crosslinking plant 14. When the cable is provided with a moisture stabilizer layer 5 is applied to a cover layer 6 in an extruder 15, after which the cable is wound up on a drum 16. When water vapor is used during the crosslinking process, the moisture stabilizing layer 5 must be applied, e.g. in an extruder 17 after the cable's insulation 3 and semiconducting layer 2, 4 have passed the crosslinking facility. This ensures that the layer has full effect from the time when the cable has been manufactured. If a cross-linking process is used without water vapor and possibly also without water cooling, it will be possible to apply the moisture-stabilizing layer 5 in tandem with the application of the outer semiconductor already before the cross-linking process, as indicated by extruder 18.

The active material can be added to an extruder in the form of a finely divided powder or granules. A layer of such active powder can also be produced by passing the cable through a container or box containing such powder before extruding an outer covering layer or applying a covering tape. Alternatively, the active material can be dissolved in a solution through which the extruder granulate passes before extrusion. Such an active solution can also be adsorbed in an insulating layer which is extruded outside the outer semi-conductive layer by passing the cable through such a solution. Incidentally, the active layer can also be made semi-conductive. An active wrapable band can also be made in this way. The material can also be sintered on with a wrapped band or an extruded jacket on the outside to prevent rapid leaching. Experiments have shown that the leaching time for the material under such conditions will be significantly longer than the expected lifetime of the cable.

" Example

As a practical test of the principle of using water-soluble compounds according to the invention as a moisture stabilizing layer, long-term tests have been carried out on cables. The cables are for 12kV nominal voltage with 35 mm 2 aluminum conductor, extruded inner semiconductor,

3.4 mm insulation of (vapor) cross-linked polyethylene and extruded outer semiconductor (triple extruded). A layer of carbon paper (creped) is wrapped around the outer semiconductor with approx. 75% overlap. The active layer on cable no. 1 is applied by passing the fully wound cable through a container with a saturated water solution of CaC^ which is then dried before pulling over a 0.5 mm thick shrink tube and crimping it firmly. For cable no. 2, the space between the dry band ring and the shrink tube was filled with a saturated water solution of CaC^ before the tube is crimped. Cable no. 3 is a reference cable with the same design and method as cable no. 2, except that pure water has been used instead of CaC^ solution. All the cables are put to the test immersed in water at room temperature. At the indicated test times, 5 mm long samples of the insulation have been taken, which have been microscoped, in order to count trees and measure their length. x denotes the mean value in yum and s the spread. The results of the surveys are given in table 1."

Claims (10)

1. Electric high-current cable that is resistant to the formation of water trees in the insulation, comprising a conductor provided with one or more layers of extruded polymeric insulating material as well as an inner and an outer semi-conductive layer, and comprising an outer protective layer of polymeric material, characterized in that outside and separated from the insulation is placed a continuous layer containing water-soluble salts which remain water-soluble during the life of the cable, and which in a manner known per se limits and stabilizes the relative humidity in the conductor insulation to a value that does not exceed 70%. 2. High current cable according to claim 1, characterized in that the salts are selected from the group of salts which form stable hydrates. 3. High current cable according to claim 2, characterized in that the water-soluble salt is CaC^. 4. High current cable according to claim 2, characterized in that the water-soluble salt is MgC^. 5. High current cable according to any one of the preceding claims, characterized in that the water-soluble salts are contained in a semi-conducting layer. 6. High current cable according to any one of the preceding claims, characterized in that the water-soluble salts are contained in a helically wound ribbon. 7. High current cable according to any one of the preceding claims, characterized in that the amount of water-soluble salts amounts to at least 0.01 g/cm 2 of the layer's surface. 8. Method for producing high current electric cable according to claim 1, whereby an electric conductor is passed through one or more extruders for applying an inner semi-conductive layer, one or more insulating layers and an outer semi-conductive layer, wherein at least one of the layers consists of cross-linkable polymeric material, after which the cable undergoes a cross-linking process so that one or more of the aforementioned layers are cross-linked, and an outer protective layer of polymeric material is applied, characterized in that the cable, outside and separated from the insulation, by winding, extrusion, or on another way, a continuous layer is applied which contains water-soluble salts which remain water-soluble during the life of the cable, and which, in a manner known per se, limits and stabilizes the relative humidity in the conductor insulation to a value that does not exceed 70%. 9. Method according to claim 8, characterized in that the moisture stabilizing layer is applied after cross-linking of the said layer(s), possibly before the cross-linking process, if this does not include treatment with steam. 10. Method according to claim 8, characterized in that the water-soluble salts in mixture with granulated polymeric material are applied by extrusion.