WO2015197686A1

WO2015197686A1 - An insulation system for hv cable joint, a method for forming a joint and a cable joint

Info

Publication number: WO2015197686A1
Application number: PCT/EP2015/064245
Authority: WO
Inventors: Hossein GHORBANI; Tobias QUIST; Eric REBILLARD
Original assignee: ABB Technology AG
Current assignee: ABB Technology AG
Priority date: 2014-06-25
Filing date: 2015-06-24
Publication date: 2015-12-30
Anticipated expiration: 2016-12-25

Abstract

An insulation system in a high voltage cable joint comprising a first cable end (12) of a first high voltage electric cable (10), and a second cable end (22) of a second high voltage electric cable (20). The first cable end comprises a first cable insulation layer (13) surrounding a first electric conductor (14) provided with a conductor screen (16), and the second cable end comprises a second cable insulation layer (23) surrounding a second electric conductor (24) provided with a conductor screen (26). The first and second electric conductors are electrically and mechanically connected to each other thereby forming a conductor joint (30).The first and second cable insulation layers comprise a polymer, and the insulation system further comprises a joint insulation layer (50) arranged to cover the conductor joint (30) and the cable insulation layer (13; 23) of the respective cable ends (12; 22). The joint insulation layer comprises a non-crosslinked polymer material, and the insulation system further comprises an insulation screen (60) layer provided over the joint insulation layer. Further, an intermediate layer (40) is arranged between at least apart of the respective cable insulation layers (12; 23) and the joint insulation layer (50), which intermediate layer comprises a crosslinking agent of a content that is higher than any content of crosslinking agent in the polymer material in the joint insulation layer.

Description

AN INSULATION SYSTEM FOR HV CABLE JOINT, A METHOD FOR FORMING A JOINT AND A CABLE JOINT

Technical field of the invention

The present invention relates to an insulation system in a high voltage cable joint and a method for forming a joint between two high voltage electric cables. The present invention also relates to a high voltage cable joint.

Background

High voltage, HV, cables are used for power distribution on land and in the sea.

Such cables are usually extruded and comprise an electric conductor that is surrounded by a number of layers of different materials having different purposes and uses, e.g. as many as eight to nine layers. The innermost layers usually comprise a conductor screen closest to the conductor and an insulation layer externally of the conductor screen.

When a cable has to be spliced with another cable, all of the cable layers of one cable must be spliced, jointed or connected in some way to the corresponding layers of the other cable. The present invention concerns in particular issues related to the cable insulation in a cable joint.

A common type of HV cable is the crosslinked polyethylene cable, which is usually called XLPE cable for short. This type of cable has an insulation layer of a polymer that is a cross linked polyethylene. This polymer in the cable insulation is a polymer that has been vulcanized or crosslinked, and which originally contained e.g. peroxide in order to obtain crosslinking of the polymer in a crosslinking process. When two cables are spliced, first of all the conductor ends are denuded of all external layers. Then the conductors are electrically and mechanically connected to each other, e.g. by welding, and then the conductor screens are restored. In the next step, the cable insulation should be restored. Usually, the cable ends would have been prepared such that the cable insulation would have been tapered down and have a conical shape with the conductor emerging from the top of the cone. In the prior art, a joint between the insulation materials in the two cables is obtained by applying a non-crosslinked joint insulation material of polymer over the ends of the insulation materials in the cables to be joined and also over the conductor splice located there between. The joint insulation material usually contains a peroxide as a crosslinking agent. After this, the joint would be vulcanized, and the joint insulation material should then be able to crosslink with the already vulcanized material in the cable insulation. Then the insulation screen and all other layers are restored. The idea is to recreate the cable in the joint by building it from inside out and restore all layers of the cable in the joint. This type of joint is called a flexible vulcanized joint, FVJ. It is also known as the factory joint.

Today there is a development towards higher and higher voltages in the power industry. A reduction of the content of crosslinking agent, e.g. peroxide, in the cable insulation contributes to improved electric properties that make it possible to increase the voltage in the cable.

However, if there is only very little peroxide left in the cable insulation material, the crosslinking level across the interface between the joint insulation and the cable insulation will be inadequate and the adherence between the insulation materials in the two layers may not be as robust as needed. The result may be failure, e.g. when subjecting the joint to high voltage and temperature variations. In US 4084307 is described a method of joining two cables having an insulation of crosslinked polymer. The conductors are joined and the joint is then wrapped with a tape of the same polymer containing a crosslinking agent. The joint is then enclosed in one or two tubes and then enclosed in a casing and subjected to heating and all-sided pressure exerted by a fluid. In GB 1204989 is disclosed a cable joint wherein an intermediate layer of rubber is provided between the cable insulation and a cast resin body, which cast resin body encloses the two spliced cable ends. The rubber contains a crosslinking or vulcanizing agent, which is peroxide.

A similar problem occurs when jointing cables of a type having an insulation material in which there is no crosslinking agent present at all. Such cables are cables with an insulation layer of e.g. polyethylene (PE) or polypropylene (PP). Summary of the invention

An object of the present disclosure is to provide a joint insulation system in a high voltage cable joint that displays an improved adhesion between the insulation layers in the cable joint.

According to the invention is defined an insulation system in a high voltage cable joint comprising a first cable insulation layer of a first cable end of a first high voltage electric cable, and a second cable insulation layer of a second cable end of a second high voltage electric cable, the first cable insulation layer surrounding a first electric conductor provided with a conductor screen of the first electric cable, the second cable insulation layer surrounding a second electric conductor provided with a conductor screen of the second electric cable, said first and second cable insulation layers comprising a polymer material, the insulation system further comprising a joint insulation layer arranged to cover the cable insulation layer of the respective cable ends and a conductor joint in which said electric conductors and the respective conductor screens are connected, and the joint insulation layer comprising a non-crosslinked polymer material, and the insulation system further comprising an insulation screen layer provided over the joint insulation layer, characterized in that an intermediate layer is arranged between at least a part of the respective cable insulation layers and the joint insulation layer, that the intermediate layer comprises a crosslinking agent of a content that is higher than any content of a crosslinking agent in the polymer material in the joint insulation layer. The first and second electric conductors are being electrically and mechanically connected to each other by respective insulation-free ends and the respective conductor screens are being connected and thereby forming the conductor joint.

Through this is achieved that some of the crosslinking agent in the intermediate layer will migrate into the cable insulation layer and provide good crosslinking between these two layers, while there will still be crosslinking agent left in the intermediate layer that can react with the joint insulation layer and also provide adequate crosslinking with the joint insulation layer. The crosslinking between the layers will ensure good

adherence between the layers in the insulation system. This can be achieved

irrespective of if the insulation layer material in the respective cable insulation or the joint insulation contains a crosslinking agent or does not contain a crosslinking agent, i.e. the content of crosslinking agent is zero. With regard to the joint insulation layer, this is reflected in the claim by the wording "any content of crosslinking agent in the polymer material in the joint insulation layer", thus meaning that the joint insulation may contain a crosslinking agent or may not contain a crosslinking agent, i.e. the content of

crosslinking agent may be zero. For example, a cable having an insulation layer made of a thermoplastic material, i.e. a non-crosslinked polymer material containing no crosslinking agent, can be made to adhere to a joint insulation layer, of a thermoplastic polymer or a crosslinked polymer, since the crosslinking agent in the intermediate layer will migrate into the thermoplastic insulation layer of the cable and also into the joint insulation layer and provide good crosslinking between the layers, at least in the areas of the respective insulation layers that are in contact with the intermediate layer.

Thus it will be possible to have cables with an insulation having very low content of crosslinking agent, and which cables still have a good adherence between the insulation layers in the cable joints. Such cables are in demand for use in high voltage installations, since the low content of crosslinking agent will provide improved electric properties that makes it possible to increase the voltage in the cable.

A further advantage is that different types of cables having different insulation materials can be jointed successfully, and also the insulation material in the joint insulation can differ from one or both cable insulation materials and still good adherence can be obtained between the insulation layers. For example, older types of cables can be jointed to newer types of cables. Examples of cables are XLPE-cables (PEX cables), and cables with other crosslinked insulation such as EPR (ethylene propylene rubber) cables and EPDM (ethylene propylene diene rubber) cables. Other examples of cables are thermoplastic cables with non-crosslinked insulation, e.g. PE (polyethylene) cables such as LDPE (low density polyethylene) cables, LLDPE (linear low density

polyethylene) cables, VLDPE (very low density polyethylene) cables, HDPE (high density polyethylene) cables, HMWPE (high molecular weight polyethylene) cables and also polypropylene cables. It may be a DC cable or an AC cable. Further examples are cables having insulation of polyethylene crosslinked with silane (Si-XLPE). With regard to cables having an insulation layer of crosslinked polymer, the intermediate layer containing a crosslinking agent will also provide the advantage that any initial content of crosslinking agent in the cable insulation layer and the joint insulation layer can be reduced, or in some instances eliminated entirely, and as a result the amounts of residual products from the crosslinking process will be reduced. As an example, the joint insulation layer may contain 0,55% dicumyl peroxide, e.g. Di-Cup®, which can be compared to the traditional levels of 1 -2% peroxide. Further, as an example, the content of crosslinking agent in the intermediate layer may be at least 20% higher than the content of crosslinking agent in the joint insulation layer, before any crosslinking process, and preferably at least 35% higher. By content of crosslinking agent is meant the concentration of crosslinking agent as a weight percentage of the entire material in the respective layer. Generally, for a joint insulation polymer material having a content of crosslinking agent of between approximately 0% and 1 ,5%, the intermediate layer should preferably contain a content of crosslinking agent of 1 -3%, before performing the crosslinking process. The intermediate layer that is arranged between at least a part of the respective cable insulation layers and the joint insulation layer can be arranged to cover the entire cable insulation of the two cables or cover only a part thereof, though preferably it should cover a major part, e.g. at least in the region of 80%.

According to one feature, in analogy with the above, the joint insulation layer may comprise a non-crosslinked polymer material comprising a crosslinking agent.

According to one feature, the crosslinking agent in the polymer material of the joint insulation layer may be the same crosslinking agent as in the intermediate layer. This will also contribute to a homogenous insulation system. An example of a suitable crosslinking agent is dicumyl peroxide, e.g. Di-Cup® as previously mentioned. Another example of crosslinking agent is Vul-Cup®.

According to one embodiment, the intermediate layer may comprise a non- crosslinked polymer material comprising a crosslinking agent. By providing a polymer as a carrier for the crosslinking agent, the handling is facilitated and this is advantageous from an environmental point of view. E.g. the intermediate layer can be provided in the form of a polymer tape that is wound around the cable insulation ends. The intermediate layer does not have to cover the cable insulation ends completely, and there may exist smaller gaps between the wound turns.

The intermediate layer may comprise the same non-crosslinked polymer material as the joint insulation layer, but with a higher content of crosslinking agent. It is advantageous to have as homogenous an insulation system as possible and this will contribute towards that.

According to an alternative embodiment, the intermediate layer may comprise a coating of pure crosslinking agent. This may be applied by brush application of liquid crosslinking agent.

According to another alternative embodiment, the intermediate layer may comprise a crosslinking agent dissolved in a solvent, e.g. a volatile solvent. This will facilitate obtaining an even layer of crosslinking agent and will further enhance the diffusion or migration of crosslinking agent into the cable insulation layers.

According to one feature, the polymer material in the joint insulation layer may be the same polymer material as in at least one of the cable insulation layers. Generally, it is advantageous for the insulation system to be as homogenous as possible in order not to negatively affect the electrical field. To have the same polymer material in the joint insulation and in at least one of the cable insulations will contribute to a more

homogenous insulation system. In addition, it would be advantageous to have the same polymer composition in the joint insulation layer and the cable insulation layer to obtain an even more homogenous insulation system.

According to one feature, the polymer material in the joint insulation layer may be a different polymer material as to at least one of the cable insulation layers. For example, the non-crosslinked polymer material in the joint insulation layer may be a non- crosslinked variant of the polymer material of at least one of the cable insulation layers. Examples of such polymer materials for the insulation layer are crosslinkable polyolefins such as XLPE, and crosslinkable polypropylene grades such as ethylene propylene rubber (EPR) or ethylene propylene diene rubber (EPDM). This would result in the advantage of having the same polymer material in the joint insulation and at least one of the cable insulations after the crosslinking process, thus resulting in a more

homogenous joint. According to one feature, the polymer material of the insulation layer of the first cable and the polymer material of the insulation layer of the second cable may be the same polymer materials, or alternatively different polymer materials. By enabling the use of a variety of polymer materials, modern cables can be joined with legacy cables, or cables from different manufacturers can be joined.

According to one feature, the polymer material of the respective insulation layer of the first cable and the second cable is a crosslinked polymer material or a non- crosslinked polymer material, with or without a crosslinking agent. For example, the cable insulation may be formed from non-crosslinked polyolefins. Such non-crosslinked polyolefins may be thermoplastic polyolefins completely lacking a crosslinking agent, such as high molecular weight polyethylene (HMWPE) and other examples already mentioned above. The non-crosslinked polyolefins may also be polyolefins comprising a crosslinking agent that have yet to be subjected to crosslink-forming conditions. This may, for example, be an XLPE or EPDM precursor that has not yet been subjected to curing.

According to yet another feature, the non-crosslinked polymer material comprises peroxide as a crosslinking agent.

The above defined insulation system is defined as such prior to the undertaking of any crosslinking process such as vulcanization.

According to another aspect of the inventions is defined a method for forming a joint between two high voltage electric cables, each cable comprising an electric conductor provided with a conductor screen and surrounded by a cable insulation layer comprising a polymer material, and an insulation screen layer, the method comprising:

- preparing cable ends of the two electric cables to be joined by removing the cable insulation layer from the conductors adjacent the ends thereof to be joined so as to obtain insulation-free ends of the conductors,

- joining the insulation-free ends of said conductors and connecting the respective conductor screens thereby obtaining a conductor joint,

- applying an intermediate layer to cover at least a part of the cable insulation layer adjacent the conductor joint, said intermediate layer comprising a crosslinking agent, - applying a joint insulation layer over the intermediate layer and the conductor joint, which joint insulation layer comprises a non-crosslinked polymer material,

- applying an insulation screen layer over the cable insulation layers and the joint insulation layer of the finished joint,

wherein the content of crosslinking agent in the intermediate layer is higher than any content of crosslinking agent in the polymer material in the joint insulation layer.

This method has advantages corresponding to the advantages previously described in connection with the insulation system.

According to one feature, the method may comprise applying a joint insulation layer comprising a non-crosslinked polymer material comprising a crosslinking agent.

According to another feature, the method may comprise applying a joint insulation layer comprising a polymer material comprising a crosslinking agent that is the same crosslinking agent as in the intermediate layer.

According to yet another feature, the method may comprise applying an intermediate layer comprising a non-crosslinked polymer material comprising a cross- linking agent.

Additionally, the method may comprise applying an intermediate layer comprising the same non-crosslinked polymer material as the joint insulation layer, but with a higher content of crosslinking agent.

Alternatively, the method may comprise applying the intermediate layer in the form of a coating of pure crosslinking agent.

According to another alternative, the method may comprise applying an intermediate layer comprising a crosslinking agent dissolved in a solvent.

In addition, the method may comprise having a content of crosslinking agent in the intermediate layer that is at least 20% higher than the content of crosslinking agent in the polymer material in the joint insulation layer.

According to one feature the method may comprise applying a joint insulation layer comprising the same polymer material as the polymer material in at least one of the cable insulation layers.

The non-crosslinked polymer material may comprise peroxide as a crosslinking agent. The method above concerns the forming of a cable joint prior to the undertaking of any crosslinking process such as vulcanization. The method may also comprise submitting the cable joint to a crosslinking process in one or more steps and thereby obtaining crosslinking between the intermediate layer and the respective cable insulation layer, and crosslinking between the intermediate layer and the joint insulation layer.

If necessary, any surplus material is naturally removed and the surfaces are evened out before applying the next layer.

According to yet another aspect of the present invention is defined a high voltage cable joint obtained in accordance with the method defined in the method claim defining submitting the cable joint to a crosslinking process in one or more steps.

The above method claims have advantages corresponding to already described advantages in connection with the claims directed to an insulation system.

According to yet another aspect of the present invention is defined a high voltage cable joint comprising a first cable end of a first high voltage electric cable, a second cable end of a second high voltage electric cable, the first cable end comprising a first cable insulation layer surrounding a first electric conductor provided with a conductor screen, the second cable end comprising a second cable insulation layer surrounding a second electric conductor provided with a conductor screen, said first and second electric conductors being electrically and mechanically connected to each other by respective insulation-free ends and the respective conductor screens being connected and thereby forming a conductor joint, said first and second cable insulation layers comprising a polymer, the cable joint further comprising a joint insulation layer arranged to cover the conductor joint and the cable insulation layer of the respective cable ends, and the joint insulation layer comprising a polymer material, and the cable joint further comprising an insulation screen layer provided over the joint insulation layer, characterized in that an intermediate layer is arranged between at least a part of the respective cable insulation layers and the joint insulation layer, that the intermediate layer and the respective cable insulation layer is connected by means of crosslinking and that the intermediate layer and the joint insulation layer is connected by means of crosslinking. This cable joint is consequently a cable joint in which the previously defined insulation system is used and after performing a crosslinking process such as vulcanization.

According to one feature, the intermediate layer may comprise a crosslinked polymer material. According to one feature, the joint insulation layer may comprise the same crosslinked polymer material as the intermediate layer.

According to one feature, the polymer material in the joint insulation layer may be the same polymer material as in at least one of the cable insulation layers.

According to yet another feature, the polymer material in the joint insulation and/or the respective cable insulation layer is a non-crosslinked polymer material or a crosslinked polymer material.

According to a further feature, a portion of the cable insulation layer of the respective cable, which portion faces the intermediate layer, may display a higher degree of crosslinking than the rest of the cable insulation layer.

The above cable joint claims have advantages corresponding to already described advantages in connection with the claims directed to an insulation system, and in particular the cable joint displays good adherence between the cable insulation and the joint insulation.

To summarize, the disclosed cable joint insulation system can be used for many different types of cables. The cables for which the disclosed cable joint insulation system can be used are cables with similar or different insulation systems of crosslinked or non- crosslinked polymer insulation materials, with or without a crosslinking agent. The cables can for example be high voltage land cables or sea cables. Examples of cables with crosslinked insulation are XLPE-cables (PEX cables) and EPDM (ethylene propylene diene rubber) cables. Other examples of cables with non-crosslinked insulation are PE (polyethylene) cables such as LDPE (low density polyethylene) cables, HMWPE (high molecular weight polyethylene) cables, LLDPE (linear low density polyethylene) cables, VLDPE (very low density polyethylene) cables, HDPE (high density polyethylene) cables, HMWPE (high molecular weight polyethylene) cables, and also polypropylene cables. It may be a DC cable or an AC cable. By high voltage is meant voltages from 36 kV and higher. Further examples are cables for medium voltages with insulation system comprising polyethylene crosslinked with silane (Si-XLPE). The joint insulation in the insulation system, i.e. prior to any crosslinking process, can be similar or different from the cable insulation and can comprise the above mentioned types of insulation comprising a non-crosslinked polymer material, with or without a crosslinking agent.

The disclosed insulation system can also be used when the insulation layer of the first cable and the insulation layer of the second cable are of different thicknesses, and therefore of different diameters. This may be the case, for example, where the cable insulation layers comprise different polymer materials and therefore must have different insulation thickness to obtain the same cable rating. Again, this means that cables from different eras or manufacturers can be joined.

Further features and advantages of the invention will also become apparent from the following detailed description of embodiments.

Brief description of the drawings

A detailed description of the present invention and embodiments thereof, given as examples only, will now be made with reference to the schematic accompanying drawings, in which:

Fig. 1 illustrates schematically parts of a high voltage cable joint;

Fig. 2 illustrates schematically parts of a high voltage cable joint;

Fig. 3 illustrates schematically an embodiment of an insulation system in a high voltage cable joint according to the invention;

Fig. 4 illustrates schematically the insulation system of Fig. 3 with an insulation screen layer, and

Fig. 5 illustrates schematically steps of a method according to the invention.

Detailed description

A typical extruded high voltage cable for the sea includes many different layers, such as, from inside and out, a conductor e.g. of copper or aluminium, a conductor screen for example of a semiconducting polymer, an insulation of for example a dry cured polymer, an insulation screen of for example a semi-conducting polymer, a bedding, a lead sheath, an inner sheath, a tensile armour layer and an outer sheath. A land cable would have similar layers, but without the armour layer.

As already described above, typical materials used for cable insulation are polymer materials comprising polyolefin materials such as differing grades of

polyethylene, e.g. XLPE, LDPE (LLDPE, VLDPE) and HMWPE, differing grades of polypropylene, polyethylene copolymers, polypropylene copolymers, and copolymers of ethylene and propylene, e.g. EPR and EPDM, and polyethylene crosslinked with silane. The cable insulation layer is commonly a polymer material that has been crosslinked. In that case, the polymer originally contained a crosslinking agent, e.g. a peroxide, and after crosslinking there may still be a small amount left of the crosslinking agent in the material.

In some cases, the cable insulation layer may contain no crosslinking agent. For example, the cable insulation may be formed from non-crosslinked polymer materials comprising non-crosslinked polyolefins. Such non-crosslinked polyolefins may be thermoplastic polyolefins completely lacking a crosslinking agent, such as high molecular weight polyethylene (HMWPE). The non-crosslinked polyolefins may also be polyolefins comprising a crosslinking agent that have yet to be subjected to crosslink- forming conditions. This may, for example, be an XLPE precursor that has not yet been subjected to curing.

The material comprising the cable insulation layer may also contain further components such as fillers, pigments, stabilizers, scorch retardants, antioxidants, UV- absorbers, anti-statics, lubricant and silanes.

In the following description of embodiments of the insulation system of the invention, it should be understood that the mentioned first cable and second cable may have any type of insulation as described above, and the type of insulation may very well be different in the two cables. Further, the joint insulation may also be of any one of the above described types comprising a non-crosslinked polymer material, with or without a crosslinking agent, and it may or may not be of the same type as one or both of the cable insulations. The embodiments below of the insulation system are described prior to the undertaking of any crosslinking process such as vulcanization. In Fig. 1 is schematically illustrated a first high voltage cable 10 and a second high voltage cable 20. The first cable end 12, of the first high voltage cable 10, and the second cable end 22, of the second high voltage cable 20, have both been prepared for jointing. All of the outer layers have been removed close to the cable end, down to the respective cable insulation screen layers 1 1 , 21 , which are of a semi-conductive material. Internally of the respective insulation screen layer is the cable insulation layer 13, 23 that surrounds the respective electric conductor 14, 24, provided with a conductor screen 16, 26. In the following, the insulation screen layer, the cable insulation layer, the conductor screen, and the conductor of the first cable 10 will be referred to as the first insulation screen layer 1 1 , the first cable insulation layer 13, the first conductor screen 16, the first conductor 14, etc. and the insulation screen layer, the cable insulation layer, the conductor screen, and the conductor of the second cable 20 will be referred to as the second insulation screen layer 21 , the second cable insulation layer 23, the second conductor screen 26, the second conductor 24, etc.

The end part 17 of the first cable insulation layer 13, i.e. the cable insulation layer of the first cable 10 that surrounds its electric conductor named the first electric conductor 14, which end part 17 is adjacent the first cable end 12, has been gradually removed in order to expose the first conductor 14 and obtain a first conductor end 15 that is free of insulation. Usually the end part 17 of the cable insulation layer is then given a tapered or conical form, with the end 15 of the conductor projecting from the top of the cone. The second cable end 22 of the second electric cable 20 has been prepared in a corresponding way, and its cable insulation layer, named the second cable insulation layer 23, has been gradually removed at an end part 27 to expose its conductor, named the second conductor 24, and the second conductor end 25. The respective conductor screen 16, 26 surrounding the respective conductor has also been removed in order to obtain the denuded conductor ends 15, 25.

In Fig. 2 is illustrated the same cables 10, 20 as in Fig. 1 where the ends 15, 25 of the respective conductors 14, 24 have been mechanically and electrically joined, e.g. by welding, and after that the conductor screen 16, 26 has been restored and a conductor joint 30 is formed. The next measure is to also restore the rest of the insulation system of the two cables 10, 20. The insulation system according to the invention is illustrated in Figs. 3 and 4. An intermediate layer 40 comprising a crosslinking agent is provided between the respective cable insulation layers 13, 23 and a joint insulation layer 50 that is applied over the conductor joint 30 and the cable insulation layers 13, 23 of the cable insulation end parts 17, 27 of the two cables 10, 20. The joint insulation layer 50 comprises a non- crosslinked polymer material. The joint insulation material may comprise a crosslinking agent or it may not comprise a crosslinking agent, i.e. the content of crosslinking agent may be zero. In, both cases, the content of crosslinking agent in the intermediate layer 40 is higher than any content of crosslinking agent in the joint insulation layer 50. This has the result that, when the cable joint is submitted to a crosslinking process, an adequate crosslinking can be obtained between the intermediate layer 40 and the joint insulation layer 50, even though some of the crosslinking agent in the intermediate layer will react with the cable insulation layers 13, 23. All in all, good crosslinking will be achieved between all three layers in the insulation system, namely between the cable insulation layers 13, 23 and the intermediate layer 40, as well as between the

intermediate layer 40 and the joint insulation layer 50.

As an example, in the case when the joint insulation layer comprises a polymer material comprising a crosslinking agent, the content of crosslinking agent in the intermediate layer is at least 20% higher, or at least 40%, or at least 60% or even at least 80% higher, than the content of crosslinking agent in the joint insulation layer, before any crosslinking process. By content of crosslinking agent is meant the concentration of crosslinking agent as a weight percentage of the entire material in the respective layer.

As an example the joint insulation can contain 0,55% of crosslinking agent, e.g. a peroxide such as dicumyl peroxide such as Di-Cup®.

In the case of a cable insulation or joint insulation of no or very little content of crosslinking agent, the amount of crosslinking agent in the intermediate layer should be in the order of at least 0,3% by weight. For example between 0,3% and 2% by weight.

Generally, for a joint insulation polymer material having a content of crosslinking agent of between approximately 0% and 1 ,5%, the intermediate layer should preferably contain a content of crosslinking agent of 1 -3%, before performing the crosslinking process.

The intermediate layer 40 can be provided in the form of a polymer tape that is wound around the cable insulation ends 17, 27. The intermediate layer 40 does not have to cover the cable insulation ends completely, and there may exist smaller gaps between the wound turns.

In an advantageous embodiment, the polymer in the joint insulation layer is the same polymer as the polymer in at least one of the cable insulation layers, or at least a similar type of polymer.

According to one embodiment, the intermediate layer 40 is of a polymer material. This polymer material is advantageously the same non-crosslinked polymer or at least of a similar polymer material as the material in the joint insulation layer, but with a higher content of crosslinking agent.

In an alternative embodiment, the intermediate layer 40 comprises pure crosslinking agent that for example may be applied as a coating on the surface of the cable insulation layer. As another alternative embodiment, the intermediate layer 40 may comprise a crosslinking agent dissolved in a solvent that is applied to the surface of the cable insulation layer.

Preferably the crosslinking agent in the polymer material of the joint insulation layer is the same or at least a similar type of crosslinking agent as the crosslinking agent of the intermediate layer.

An example of a crosslinking agent is peroxide. A suitable crosslinking agent containing peroxide has been found to be dicumyl peroxide, for example the one sold under the trademark Di-Cup®. However, also other crosslinking agents are possible.

As shown in Fig. 4, the external surface of the joint insulation layer 50 should be level with the external surface of the respective cable insulation screen layers 1 1 , 21. If there is surplus material of the joint insulation layer, as shown in fig. 3, this should be removed and an even surface should be formed. If the two cables are of differing diameters then a smooth taper from the larger diameter to the smaller diameter should be formed. Finally, an insulation screen layer 60 is provided over the joint insulation layer. The method, according to the present invention, for forming a joint between two high voltage electric cables 10, 20, each cable comprising an electric conductor 14, 24 provided with a conductor screen 16, 26 and surrounded by a cable insulation layer 13, 23 comprising a polymer material, and an insulation screen layer 1 1 , 21 , comprises the following steps:

preparing cable ends 12, 22 of the two electric cables to be joined by removing the cable insulation layer 13, 23 from the conductors adjacent the ends thereof to be joined so as to obtain insulation-free ends 15, 25 of the conductors (A),

joining the insulation-free ends 15, 25 of said conductors and connecting the respective conductor screens 16, 26 thereby obtaining a conductor joint 30 (B),

applying an intermediate layer 40 to cover at least a part of the cable insulation layer 13, 23 adjacent the conductor joint 30, said intermediate layer 40 comprising a crosslinking agent (C), which intermediate layer and the polymer material of a joint insulation layer 50 have been chosen such that the intermediate layer has a content of crosslinking agent that is higher than any content of crosslinking agent in the polymer material in the joint insulation layer,

applying the joint insulation layer 50 over the intermediate layer 40, the conductor joint 30 and the cable insulation layer 13, 23 adjacent the conductor joint (D), applying an insulation screen layer 60 over the joint insulation layer 50 in order to make the insulation system complete (E).

The method may comprise more detailed steps comprising applying an intermediate layer and a joint insulation layer in analogy with the different variants of the described insulation system.

After these steps an intermediate cable joint product is obtained which can then be submitted to a crosslinking process (F) in order to finalize the insulation system of the cable joint. The non-crosslinked polymer material of the intermediate layer and the joint insulation layer will then become connected by crosslinking. If the polymer material of the joint insulation contains a crosslinking agent, then the joint insulation will also be crosslinked. Concerning the cable insulation, connection to the intermediate layer by crosslinking will in particular occur in the interface portions of the cable insulation, i.e. in the portion of the cable insulation that is in contact with the intermediate layer. This would be the case irrespective of if the respective cable insulation material contains any small amounts of crosslinking agent or if it is a thermoplastic. The cable insulation will therefore display a higher degree of crosslinking in the portion of the insulation that is in contact with the intermediate layer than in the rest of the insulation, if the rest of the insulation is even crosslinked at all. In a similar way, if the joint insulation material contains no crosslinking agent, then there will only be crosslinking in the interface portion of the joint insulation that is in contact with the intermediate layer. Ideally, the degree of crosslinking in the joint insulation layer, the intermediate layer and the interface portion of the cable insulation layers will be essentially the same, resulting in a homogenous insulation system, although this may not always be possible, such as when the original cables are of non-crosslinked HMWPE. Also ideally, the crosslinking agent in the polymer of the intermediate layer and the joint insulation will be used up by the crosslinking process, but it is foreseen that there may still be some smaller amounts of residual crosslinking agent left.

The remaining layers of the cable can then be restored and the completed cable joint with all its layers is obtained.

The crosslinking process may be performed in one or more steps. For example, after the conductors have been jointed, the conductor screen will be restored and a first crosslinking is then performed after the conductor screen has been restored, in order to obtain a smooth surface of the conductor screen. A second crosslinking step may be performed after the joint insulation material has been applied, in order to obtain a smooth surface on the cable insulation and the joint insulation, without any pronounced transition therebetween. A third crosslinking step can be performed after the insulation screen has been applied. One purpose of the crosslinking process is of course to achieve the crosslinking between the different layers but it also has the positive effect of producing even surfaces, which is also very important in order for the different layers of a cable to contact each other without any gaps.

The invention shall not be considered limited to the illustrated embodiments, but can be modified and altered in many ways, as realised by a person skilled in the art, without departing from the scope defined in the appended claims.

Claims

Patent Claims

1. An insulation system in a high voltage cable joint comprising a first cable insulation layer(13) of a first cable end (12) of a first high voltage electric cable (10), and a second cable insulation layer (23) of a second cable end (22) of a second high voltage electric cable (20), the first cable insulation layer (13) surrounding a first electric conductor (14) provided with a conductor screen (16) of the first electric cable, the second cable insulation layer (23) surrounding a second electric conductor (24) provided with a conductor screen (26) of the second electric cable, said first and second cable insulation layers comprising a polymer material, the insulation system further comprising a joint insulation layer (50) arranged to cover the cable insulation layer (13; 23) of the respective cable ends (12; 22), and a conductor joint (30), in which said electric conductors and the respective conductor screens are connected, and the joint insulation layer comprising a non-crosslinked polymer material, and the insulation system further comprising an insulation screen (60) layer provided over the joint insulation layer, characterized in that an intermediate layer (40) is arranged between at least a part of the respective cable insulation layers (12; 23) and the joint insulation layer (50), and that the intermediate layer comprises a crosslinking agent of a content that is higher than any content of crosslinking agent in the polymer material in the joint insulation layer.

2. The insulation system according to claim 1 , wherein the joint insulation layer (50) comprises a non-crosslinked polymer material comprising a crosslinking agent.

3. The insulation system according to claim 2, wherein the crosslinking agent in the polymer material of the joint insulation layer (50) is the same crosslinking agent as in the intermediate layer (40).

4. The insulation system according to any one of the preceding claims, wherein the intermediate layer (40) comprises a non-crosslinked polymer material comprising a cross-linking agent.

5. The insulation system according to claim 4, wherein the intermediate layer (40) comprises the same non-crosslinked polymer material as the joint insulation layer (50), but with a higher content of crosslinking agent.

6. The insulation system according to any one of claims 1 -3, wherein the intermediate layer (40) comprises a coating of pure crosslinking agent.

7. The insulation system according to any one of claims 1 -3, wherein the intermediate layer (40) comprises a crosslinking agent dissolved in a solvent.

8. The insulation system according to any one of the preceding claims, wherein the content of crosslinking agent in the intermediate layer (40) is at least 20% higher than the content of crosslinking agent in the polymer material in the joint insulation layer (50).

9. The insulation system according to any one of the preceding claims, wherein the polymer material in the joint insulation layer (50) is the same polymer material as in at least one of the cable insulation layers (13; 23).

10. The insulation system according to any one of claims 1 -8, wherein the polymer material in the joint insulation layer (50) is a different polymer material as to at least one of the cable insulation layers (13; 23).

1 1 . The insulation system according to any one of claims 1 -10, wherein the polymer material of the insulation layer (13) of the first cable (10) and the polymer material of the insulation layer (23) of the second cable (20) are the same polymer material.

12. The insulation system according to any one of claims 1 -10, wherein the polymer material of the insulation layer (13) of the first cable (10) and the polymer material of the insulation layer (23) of the second cable (20) are different polymer materials.

13. The insulation system according to any one of claims 1 -12, wherein the polymer material of the respective insulation layer of the first cable and the second cable is a crosslinked polymer material or a non-crosslinked polymer material, with or without a crosslinking agent.

14. The insulation system according to any one of the preceding claims, wherein the non-crosslinked polymer material comprises peroxide as a crosslinking agent.

15. A method for forming a joint between two high voltage electric cables (10, 20), each cable comprising an electric conductor (14; 24) provided with a conductor screen (16;

26) and surrounded by a cable insulation layer (13; 23) comprising a polymer material, and an insulation screen layer (1 1 ; 21 ), the method comprising:

- preparing cable ends (12; 22) of the two electric cables to be joined by removing the cable insulation layer from the conductors adjacent the ends thereof to be joined so as to obtain insulation-free ends (15; 25) of the conductors,

- joining the insulation-free ends (15; 25) of said conductors and connecting the respective conductor screens (16; 26) thereby obtaining a conductor joint (30),

- applying an intermediate layer (40) to cover at least a part of the cable insulation layer (13; 23) adjacent the conductor joint, said intermediate layer (40) comprising a crosslinking agent,

- applying a joint insulation layer (50) over the intermediate layer (40) and the

conductor joint (30), which joint insulation layer comprises a non-crosslinked polymer material,

- applying an insulation screen layer (60) over the joint insulation layer (50), wherein the content of crosslinking agent in the intermediate layer is higher than any content of crosslinking agent in the polymer material in the joint insulation layer.

16. The method according to claim 15, comprising applying a joint insulation layer (50) comprising a non-crosslinked polymer material comprising a crosslinking agent.

17. The method according to claim 16, comprising applying a joint insulation layer (50) comprising a polymer material comprising a crosslinking agent that is the same crosslinking agent as in the intermediate layer (40).

18. The method according to any one of claims 15-17, comprising applying an intermediate layer (40) comprising a non-crosslinked polymer material comprising a cross-linking agent.

19. The method according to claim 18, comprising applying an intermediate layer (40) comprising the same non-crosslinked polymer material as the joint insulation layer (50), but with a higher content of crosslinking agent.

20. The method according to any one of claims 15-17, comprising applying the intermediate layer (40) in the form of a coating of pure crosslinking agent.

21 . The method according to any one of claims 15-17, comprising applying an intermediate layer (40) comprising a crosslinking agent dissolved in a solvent.

22. The method according to any one of claims 15-21 , comprising having a content of crosslinking agent in the intermediate layer (40) that is at least 20% higher than any content of crosslinking agent in the polymer material in the joint insulation layer (50).

23. The method according to any one of claims 15-22, comprising applying a joint insulation layer (50) comprising the same polymer material as the polymer material in at least one of the cable insulation layers (13; 23).

24. The method according to any one of claims 15-23, in which the non-crosslinked polymer material comprises peroxide as a crosslinking agent.

25. The method according to any one of claims 15-24, comprising submitting the cable joint to a crosslinking process in one or more steps and thereby obtaining crosslinking between the intermediate layer and the respective cable insulation layer, and

crosslinking between the intermediate layer and the joint insulation layer.

26. A high voltage cable joint obtained in accordance with the method defined in claim 25.

27. A high voltage cable joint comprising a first cable end (12) of a first high voltage electric cable (10), a second cable end (22) of a second high voltage electric cable (20), the first cable end comprising a first cable insulation layer (13) surrounding a first electric conductor (14) provided with a conductor screen (16), the second cable end comprising a second cable insulation layer (23) surrounding a second electric conductor (24) provided with a conductor screen (26), said first and second electric conductors being electrically and mechanically connected to each other by respective insulation-free ends (15; 25) and the respective conductor screens being connected and thereby forming a conductor joint (30), said first and second cable insulation layers comprising a polymer material, the cable joint further comprising a joint insulation layer (50) arranged to cover the conductor joint (30) and the cable insulation layer (13; 23) of the respective cable ends (12; 22), and the joint insulation layer comprising a polymer material, and the cable joint further comprising an insulation screen (60) layer provided over the joint insulation layer, characterized in that an intermediate layer (40) is arranged between at least a part of the respective cable insulation layers (12; 23) and the joint insulation layer (50), that the intermediate layer and the respective cable insulation layer is connected by means of crosslinking and that the intermediate layer and the joint insulation layer is connected by means of crosslinking.

28. The high voltage cable joint according to claim 27, wherein the intermediate layer (40) comprises a crosslinked polymer material.

29. The high voltage cable joint according to claim 28, wherein the joint insulation layer (50) comprises the same crosslinked polymer material as the intermediate layer (40).

30. The high voltage cable joint according to any one of claims 27-28, wherein the polymer material in the joint insulation layer (50) is the same polymer material as in at least one of the cable insulation layers (13; 23).

31 . The high voltage cable joint according to claim 27-28, wherein the polymer material in the joint insulation and/or the respective cable insulation layer is a non-crosslinked polymer material or a crosslinked polymer material.

32. The high voltage cable joint according to any one of claims 27-30, wherein a portion of the cable insulation layer of the respective cable, which portion faces the intermediate layer, displays a higher degree of crosslinking than the rest of the cable insulation layer.