TECHNICAL FIELD

The present invention relates to a composite insulation material for an electric power cable, to a use of the composite insulation material for an insulation system in an electric power cable, to a process for the production of an electric power cable comprising a conductor and an insulation system comprising the composite insulation material surrounding the conductor, to an electric power cable comprising the composite insulation material, and to an electric power cable produced according to the process, as defined in the appended claims. BACKGROUND ART

High voltage electric power cables, herein referred to as electric power cables, are used to transmit electric power of medium or high voltage. Such electric power cables may be buried into the ground and are called land cables, or the electric power cables may be buried into a sea bed or they may be arranged to extend between two fixing points in sea water. Cables used in sea applications are called submarine, sea water or underwater power cables.

Electric power cables generally comprise a conductor covered by an insulation system and a protective jacket. The insulation system usually comprises at least one semi-conductive layer and an insulation layer. In addition, a protection system is applied to protect the insulation system against e.g. moisture penetration, i.e. to block water from penetrating to the insulation and further to provide protection against mechanical wear or forces during for example production and installation.

The insulation system may comprise or consist of multiple layers of paper-based material, which form a semi-conductive layer and an insulation layer of the insulation system. A semi- conductive layer in that case typically comprises paper comprising semi-conducting filler particles, such as carbon black, that renders the paper material semi-conductive. The paper may also be metallized and can comprise a layer of conductive material that renders the paper material semi-conductive. The layers of semi-conductive paper and layers of insulation layer are lapped around a conductor to form the insulation system. After the paper-based layers have been applied onto the cable, impregnation of cables is performed by using an impregnation liquid which can be a dielectric fluid. The dielectric fluid, such as high viscosity oil, is used to protect the insulation system against moisture pick-up and to fill up all pores and voids or other interstices in the insulation system. After impregnation, the insulation system is usually directly provided with a moisture barrier to keep the oil inside the insulation system and to protect the insulation system from moisture and air from the outside environment. Usually, the moisture barrier is provided in the form of an extruded lead sheath. The extrusion is performed onto the cable directly after it is lifted from the

impregnation oil. The process for the production of a Electric power cable with a paper-based insulation system is further described for example in "Submarine Power Cables: Design, Installation, Repair, Environmental Aspects", Thomas Worzyk, Springer-Verlag Berlin

Heidelberg, 2009, ISBN 978-3-642-01269-3, pp 126-130.

Impregnation liquids having high viscosity limit the leaking risk and possible axial flow of the liquid along the cable during the operation of the cable. However, impregnation with high viscosity impregnation liquid requires a long impregnation time and there may be a risk that not all voids in the insulation material are completely filled. For this reason impregnation is often performed at elevated temperatures which results in decreased viscosity of the impregnation liquid and thus accelerated impregnation process. However, rising of the temperature entails a long cooling process which is needed to decrease a risk for creation of voids. Long impregnation and cooling times complicate the manufacturing process and increases the costs. Also, since high current causes intensive cable heating and elevated temperature, the liquid viscosity is decreased and undesired internal liquid circulation is enhanced. Further, when using high viscosity impregnation liquids there is a risk for internal voids due to thermal shrinkage of the cable when the power transition is switched off and the cable is rapidly cooled down. Therefore, the working temperature of a cable with such insulation system is somewhat limited which also limits the maximum power level that can be transmitted.

Thus, during the manufacture it would be desirable that the impregnation liquid had a low viscosity to enable quicker impregnation. Also, low viscosity is desirable when the

impregnation is performed since low impregnation temperatures can be used. However, when using low viscosity impregnation liquids there is a risk for drainage of the impregnation liquid. Further, the maximum power level that can be transmitted is limited since the liquid viscosity is further decreased when the temperature in the cable rises.

To increase the maximum power level that can be transmitted, so called high temperature impregnated cables having a maximum working temperature of up to about 80°C are available on the market. In these cables a paper insulation tape is replaced with a polypropylene laminated paper (PPLP) insulation tape. However, the PPLP-tape still requires impregnation and the problems relating to viscosity of the impregnation are the same as mentioned above.

Thus, properties of the impregnation liquid or fluid and in particular the viscosity of the impregnation liquid are of high importance in manufacturing of high-voltage cables having a multilayer insulation system comprising or consisting of layers of paper-based material.

Therefore, there is a need for improvements in manufacturing processes of paper-based cable insulation systems. There is also a need to improve electrical properties of power cables comprising a paper-based insulation system.

SUMMARY OF THE INVENTION In view of the problems stated above, there is thus a need to improve insulation materials and manufacturing processes used in connection with the insulation systems and in the manufacture of electric power cables. There is thus also a need to improve the manufacturing process of electric power cables. Further, there is a need to improve the electrical and mechanical properties of electric power cables having an insulation system comprising multiple layers of insulation material based on impregnable and porous insulation material. The porous insulation material could be a paper-based insulation material.

The object of the present invention is thus to provide an insulation material for an insulation system of an electric power cable which is easy and quick to impregnate and which forms an electrically and mechanically stable insulation system for an electric power cable. It is a further object of the present invention to provide a process for the manufacture of a high voltage cable which provides for an easier and quicker impregnation of a dielectric fluid into the pores of the insulation material. It is a further object to provide a process which provides an electric power cable with an insulation system which tolerates higher temperatures during use, and thus increases the maximum power level that can be transmitted with the electric power cable.

It is still another object of the present invention to provide a process for the manufacture of an electric power cable in which use can be made of a low viscosity impregnation liquid. Also, it is an object to minimize the problems caused by the low viscosity impregnation liquids during operation. Further, it is an object to enable better control of the impregnation process during the manufacture of an electric power cable.

According to the present invention, the objects above are attained by a composite insulation material for an electric power cable comprising a porous base material and a curing agent for a cable insulation impregnation liquid, which is curable or cross-linkable. The curing agent is embedded in a carrier material and forms a curing agent capsule with the carrier material. The curing agent is arranged to be released from the carrier material. Since the curing agent is embedded in the carrier material, it will not come into an immediate contact with an impregnation liquid during the manufacturing process of an electric power cable. The curing agent is released from the capsule by means of a pre-determined action.

Since the insulation material comprises a curing agent, the impregnation liquid can be made to cure and thus it is possible to provide a solid insulation system for electric power cables having an insulation system based on a porous, e.g. a paper-based insulation material. This leads to many advantages compared to the prior art solutions. For example, the cable will gain a high breakdown strength, which is provided by the multi-layered and solid insulation structure since each layer can act as a barrier for an arc during the breakdown. It will be also possible to increase the working temperature of the electric power cable, which leads to an ability to transmit higher current rates. Also, the problems relating viscosity-temperature dependency of the impregnation liquid will be considerably decreased, or completely eliminated. The composite insulation material allows for the use of an impregnation liquid with low viscosity so that the impregnation process can be quicker while the problems associated with low viscosity in the working conditions, such as undesired internal liquid circulation, can be avoided.

The carrier material may comprise wax, such as a paraffin wax, which is optionally chemically modified. The carrier material may also comprise a polymer-based thermoplastic material. Further carrier material may comprise an inorganic material, such as silica-based material, or a polymer-based thermosetting material. The carrier material may also consist of any one of the above-mentioned materials or combinations thereof. The materials are chosen such that they do not negatively affect the properties of curable impregnation liquids in electric power cable insulation systems and are thus suitable for use in cable applications.

The curing agent may be arranged to be released from the carrier material by means of application of increased temperature or heating, radiation or mechanical action, such as increased pressure. Thus, it is possible to trigger curing of the cable by means of a certain predetermined action, whereby the good controllability of a manufacturing process can be provided. Since the curing agent is releasable from the curing agent capsule by means of action, e.g. heating, the curing process of the impregnation liquid can be better controlled during the manufacturing process.

Waxes and thermoplastic materials may have relatively low melting temperatures. Therefore, the capsules can be temperature-activated, i.e. the curing agent may be released by means of increased temperature, while the carrier material, e.g. paper is not deteriorated. Thus, a reliable and effective control of the impregnation process can be provided. Therefore, according to an embodiment, the curing agent is released by means of application of increased temperature, whereby the carrier material melts. The carrier material suitably comprises or consists of a wax or thermoplastic material. The carrier material, such as the wax or thermoplastic material, can have a melting temperature of from 40-200°C. By using a temperature activated carrier material, i.e. by arranging the curing agent to be released from the carrier material by means of increased temperature, a simple and effective control of the conditions leading to release of the curing agent can be provided. In this way it can be assured that the curing agent is inactive until a certain temperature is reached. According to another embodiment, the curing agent is released by means of radiation or mechanical action, such as increased pressure, whereby the carrier material decomposes and the curing agent released. The carrier material may comprise or consist of an inorganic material, such as silica-based material, or a polymer-based thermosetting material. By using a material which can be activated by radiation or mechanical action, the chemical properties of the curable impregnation liquid are minimally affected. The porous base material may be fiber-based. Fibers enable formation of a porous net structure. The fibres may consist of or comprise cellulosic fibres, polymeric fibres, silica-based fibres or any combination thereof. Other types of inorganic fibres than silica-based fibres could be also used. Such fibres have good insulation properties and therefore are suitable for use in electric power cable insulation systems.

Preferably, the composite insulation material is provided in the form of a tape. Other types to provide the material could be possible, such as for example sheets of material, webs of material, bands or threads. However, tapes are commonly used in the prior art manufacturing apparatus, whereby the use of existing manufacturing equipment can be utilized. The curing agent capsules are preferably microcapsules or nanocapsules having an average largest dimension in a two-dimensional plane visible in a TEM (Transmission Electron

Microscopy) image of less than ΙΟΟμιη. When the size, i.e. the largest dimension of the capsules, is limited to less than ΙΟΟμιη, it will be possible to embed or place the capsules in pores of the base material. The curing agent capsules can be applied on the surface of the porous base material. The application may be performed by using any coating technology, e.g. by spray coating, or for example by printing. Application to the surface of the base material is relatively simple and the use of any kind of base material is enabled. Alternatively or additionally, the curing agent capsules may be located in pores of the porous base material. This can be accomplished e.g. by adding the curing agent capsules to a suspension used to manufacture the base material, i.e. to a cellulosic suspension used to manufacture a paper-based insulation material. The capsules that are located in the pores of the base material will provide for solidification of the impregnation liquid throughout the whole three-dimensional extension of the base material. Thus, an insulation material with improved electrical and mechanical properties can be provided.

The curing agent may be a catalytic substance, i.e. a catalyst. The curing agent can be a platinum catalyst, such as platinum divinyltetramethyldisiloxane complex or platinum cyclovinylmethylsiloxane complex. In this way the curing procedure will be fast. Alternatively or additionally the curing agent may be any other suitable reaction initiator, e.g. a peroxide, as long as the aimed curing procedure can be obtained. The curing agent is adapted to initiate curing reaction for the curable impregnation liquid that is used in the manufacturing process. According to one variant the curing agent is adapted to initiate a curing reaction for a silicone resin-based impregnation liquid. Silicone resin-based impregnation liquids are preferable in electronical applications since they are inert in electrical applications, provide effective insulation and do not negatively affect the electrical properties of power cables. Also, a quick impregnation of the composite insulation material can be provided.

The composite insulation material can be electrically insulating. In this way it can be used to form a multiple insulation layer structure. Alternatively, the composite insulation material can be electrically semiconducting. In this way it can be used to form a multiple semiconducting layer structure.

The present invention also relates to a use of the composite insulation material as defined above for an insulation system in an electric power cable.

The present invention further relates to a process for the production of an electric power cable comprising a conductor and an insulation system surrounding the conductor, wherein the process comprises the steps of: i) providing a conductor for the electric power cable;

ii) applying the composite insulation material of the present invention as

defined above to surround the conductor and thus provide an insulation system for the electric power cable;

iii) impregnating the insulation system surrounding the conductor with a

curable or cross-linkable impregnation liquid which is a dielectric fluid and thus provide an impregnated cable;

iv) subjecting the impregnated cable from the step iii) to an increased temperature, radiation, or increased pressure step to release the curing agent from the curing agent capsule, whereby curing of the impregnation liquid is initiated when the impregnation liquid comes into contact with the curing agent;

v) curing the impregnated cable from the step iv); vi) terminating the curing and providing an electric power cable with a solid insulation system;

vii) optionally providing protective materials to cover the insulation system, such as armouring and/or protective sheaths. According to the process, use can be made of the composite insulation material defined above and a solid, multilayered insulation system can be provided. This is a huge advantage, since in this way the problems with viscosity-temperature dependency can be considerably decreased or eliminated. The use of a low viscosity impregnation liquid is now possible, and thus the impregnation process and cooling down of the cable after impregnation is significantly shorter than in the prior art processes. Since the impregnation liquid is cured, high working temperatures of the cable will not affect the viscosity of the impregnation liquid and lead to problems for example with internal liquid distribution caused by the decreased viscosity of the impregnation liquid at high temperatures. Also, the manufacturing process and especially the initiation of the curing of the impregnation liquid in contact with the composite insulation material can be easily controlled.

The composite insulation material is suitably in the form of a tape that is wound around the conductor. The tape can entrap the curing agent capsules such that the curing agent capsules are not freely distributed to the complete volume of the impregnation liquid surrounding the cable. The insulation system comprises inner semi-conducting layers surrounding the conductor, insulation layers surrounding the inner semi-conducting layers and outer semi-conducting layers surrounding the insulation layers. The composite insulation material is electrically insulating in the insulation layers and electrically semi-conductive in the semi-conducting layers. The composite insulation material can thus be made both insulating and

semiconducting, for example by adding filler material that renders the material semiconducting. Therefore, a process that is easy to control and in which existing equipment can be used, is provided.

The curing agent capsules can be applied on the surface of the porous base material or embedded in pores of the porous base material. Thereby the curing of the impregnation liquid occurs in a volume of the impregnation liquid in contact with the released curing agent. Thus, since the curing material is located on the surface of the base material or in the pores of the base material of the insulation material, the curing will only occur where the impregnation liquid is in contact with the insulation material. Thus, the remaining impregnation liquid will not cure and can be used for several impregnations. Thereby the process is also economical. According to one feature, the process comprises in the step ii) applying an outermost layer of insulation material which is free of curing agent capsules. In this way the entrapment of the curing agent capsules can be further improved.

According to one embodiment, the carrier material in the curing agent capsules is

temperature activated and melts when the temperature is elevated to equal or higher temperature than the melting temperature of the carrier material. In this way a simple process control by alternating the process temperature can be obtained.

The present invention also relates to an electric power cable comprising a conductor and an insulation system comprising the composite insulation material as defined above. Also, the present invention relates to an electric power cable comprising a conductor and a solid insulation system obtained by the process as defined above.

The electric power cable can be a high voltage direct current (DC) cable.

The electric power cable can alternatively be a high voltage alternating current (AC) cable.

Further features and advantages will be described with reference to the appended detailed description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. la is a partially cut side view of the layers of a cable comprising a paper-based insulation system for use with the present invention;

Fig. lb is a radial cross section of the layers of a cable comprising a paper-based insulation system for use with the present invention;

Fig. 2 is a radial cross section view of a HVAC cable comprising cables with an insulation system according to the present invention; Fig. 3 is a flow chart illustrating the steps of the process for the production of the cable according to the present invention.

DETAILED DESCRIPTION

The electric power cables of the present invention generally comprise a metal conductor, an insulation system and optionally a protection system arranged to protect the insulation system and the cable against mechanical forces and/or moisture.

The conductor is usually mainly constituted by a metal such as copper or aluminium. The conductor may be solid or stranded. Normally, the conductor has a generally circular cross section, even though alternative shapes might be conceived. The conductor is surrounded by an insulation system. The insulation system may have a cross- section with an outer peripheral shape corresponding to the outer peripheral shape of the conductor, normally a generally circular outer periphery. The conductor may be directly or indirectly surrounded by the insulation system, i.e. the electric power cable may comprise at least one material layer between the conductor and the insulation layer, e.g. a conductive tape.

The electric insulation system comprises a first, inner semi-conducting layer or layers. The semiconducting layers are surrounded by an insulation layer or layers. The insulation layers are normally surrounded by a second, outer semi-conducting layer or layers. The insulation system according to the present invention is based on a porous impregnable insulation material, which can be for example paper. Such insulation systems usually comprise multiple layers of semiconducting or insulating material. The insulation material has insulating properties, i.e. no conductivity or very low conductivity in the insulating layers and semiconducting properties in the semiconducting layers. The semi-conducting properties can be obtained by the use of conductive filler particles. The filler may be for instance carbon black. By insulating properties of a material is meant that the material resists electricity. The conductivity of the insulation material may be for example of from about 1*10 ^~8 to about 1*10 ²⁰ S/m at 20 °C, typically from 1*10 ⁹ to 1*10 ¹⁶ , depending of the magnitude of the electric field. By semi-conducting properties of a material is meant a material that has an electrical conductivity that is lower than that of a conductor but that is not an insulator. The

conductivity of the semi-conducting material may be typically of larger than 10 ^~5 S/m at 20 °C, such as up to about 10 or 10 ² S/m. Typically, the conductivity is less than 10 ³ S/m at 20 °C. By conductivity is meant the property of transmitting electricity. The conductivity of a conducting material is more than about 10 ³ S/m at 20 °C. For example, carbon black has a conductivity of about 1000 S/m. Basically there is no upper limit, but in practical solutions the upper limit is about 10 ⁸ S/m at 20 °C.

The composite insulation material of the present invention is based on a porous material. After impregnation the pores of the base material are filled. In the final insulation system, the impregnation fluid is cured and thus a solid insulation system is obtained. By porous is meant a material that contains pores such that the medium is permeable to fluids. In this way the porous base material can be impregnated with an impregnation liquid. The porous base material can be fiber-based or other type of porous material, e.g. the porous base material may be polymeric tape, e.g. a polypropylene-based tape. The fibre-based base material can comprise cellulosic fibres, polymeric fibres, silica-based fibres, other inorganic fibres or any combination thereof. Silica-based fibres tolerate many chemicals and high temperatures. Polymeric fibres are commonly used in different nonwoven materials that are suitable as base materials in the present insulation material. Commonly used porous insulation base materials are paper-based and are thus based on cellulosic fibres. Cellulose has very good electrical properties and is a good insulator. Paper usually has a grammage of from about 60-190 g/m ² , but is not limited to this specific range. The insulation paper may be manufactured for example from unbleached kraft pulp. The insulation paper, which is normally provided in the form of paper tapes, is applied to the cable by means of paper lapping equipment comprising a row of lapping heads. The insulation paper is then wrapped around the cable while the tension of the paper is continuously controlled. The semi-conducting paper comprises filler particles of for example carbon black and is applied to the cable in a similar manner. Paper absorbs humidity from the surrounding air and if the water content of the paper during the manufacture is high, e.g. between 6-12%, the cable needs to be dried before impregnation. A cable with an insulation system based on a porous insulation base material is impregnated with a curable or cross-linkable impregnation liquid that is a dielectric fluid, such as a silicone- based impregnation liquid, to protect the insulation against moisture pick-up and to fill up all pores and voids or other interstices in the insulation system. The impregnation liquid according to the invention may be a liquid silicon mixture, polyurethanes, epoxies or other blends.

It is important that there is substantially no air or water present in the insulation system. Impregnation is performed by feeding the cable including the conductor and the surrounding paper-based insulation system into a vessel containing the dielectric fluid. The curing agent is adapted to initiate a curing reaction for the dielectric fluid, which is preferably a silicone resin- based impregnation liquid. The impregnation liquid may be for example a mixture of methyl, vinyl and hydroxyl terminated polydimethylosiloxanes having a low viscosity, preferably in the range of 50 -2000 cSt (at room temperature, measured by means of a standard rheometer). Liquid silicon mixtures or polyurethanes are also an option for the curable impregnation liquid. The curing agent can be for example a catalytic substance, such as a platinum catalyst, e.g. a platinum divinyltetramethyldisiloxane complex or platinum cyclovinylmethylsiloxane complexes that are especially suitable for use with silicone resin-based impregnation liquid. Also peroxides could be used as the curing agent or catalyst.

The curing agent is embedded in a carrier material. The curing agent and the carrier material are manufactured so as to form a curing agent capsule. The curing agent is releasably arranged in the carrier material. The curing agent is arranged to be released from the carrier material by means of a pre-determined action, e.g. by means of applying increased

temperature, i.e. heating, radiation or mechanical action, such as by means of increased pressure. Preferably, the curing agent is released by means of increased temperature, whereby the carrier material melts. The carrier material suitably comprises or consists of a wax or thermoplastic material that has a melting temperature of from 40-200°C, and thus, when the cable after impregnation is heated to the melting temperature, the carrier material melts and the curing agent is released. The curing agent may also be arranged to be released by means of radiation or mechanical action, such as increased pressure, whereby the carrier material decomposes or ruptures and the curing agent is released. The carrier material may comprise or consist of an inorganic material, such as silica-based material, or a polymer-based thermosetting material. By using a material which can be activated by radiation or mechanical action, the chemical properties of the curable impregnation liquid are minimally affected. The carrier material may be any material that does not deteriorate the properties of the impregnation liquid or the insulation properties of the insulation material. The wax may be for example a paraffin wax or polyethylene wax, which are optionally chemically modified. The a polymer-based material can be a thermoplastic material, which becomes soft when heated and re-harden on cooling, or a thermosetting material, which are materials that cure irreversibly. Any type of thermosetting or thermoplastic material could be used as a carrier material as long as the material can be activated so as to release the curing agent. The carrier material may also comprise emulsifiers and other additives to enhance particle formation. Further the carrier material may be an inorganic material, such as a silica-based material. Any known technology to obtain particles may be used, such as by means of phase separation, spray drying, vibration nozzle or interfacial polycondensation. The capsule can be of a core- shell or multicore or matrix type.

The curing agent capsules can be applied on the surface of the porous base material.

Alternatively or additionally, the curing agent capsules can be added so that they will be located in pores of the porous base material. For example, the capsules can be applied either to the surface of an insulation paper available on the market or they can be added to the paper pulp during paper manufacturing process. If the capsules are added during the papermaking process, the capsules will be embedded in the whole paper volume. Since the curing agent is incorporated in the base material, the curing agent will be entrapped when the composite insulation material is wound around a conductor in a cable manufacturing process. Therefore, the curing agent does not disintegrate to the surrounding impregnation liquid, and curing occurs only locally in proximity of the curing agent. The capsules are suitably microcapsules or nanocapsules having an average largest dimension in a two-dimensional plane visible in a TEM image of less than ΙΟΟμιη.

The cable is after impregnation usually provided with a moisture barrier to keep the dielectric fluid inside the insulation system, and to protect the insulation system from water. The moisture barrier is water resistant but is also preferably oil-resistant. By "resistant" is meant that the material provides a barrier against e.g. water, oil and/or air, but which is not necessarily completely impermeable to e.g. water, oil and/or air. Moisture barriers are needed since the inherent properties of the electric insulation are such that the insulation may deteriorate and loose its insulation effect if the insulation is subjected to water for a long period. The conductor, the insulation layer and the moisture barrier arrangement can be surrounded by further material or layers of material that can be included in the protection system of the cable. Further materials and layers may have different tasks such as that of holding the different cable parts together, giving the electric power cable mechanical strength and protecting the cable against physical as well as chemical attacks, e.g. corrosion. Such materials and layers are commonly known to the person skilled in the art. For example, such further materials may include armouring, for example steel wires, and outer protective sheaths.

The electric power cables according to the present invention can be single phase or three-phase electric power cables. Single phase cables comprise one conductor surrounded by an insulation system. Three-phase cables comprise three conductors, each of which is surrounded by a separate electric insulation system. The three phase electric power cable may also comprise further material and layers arranged around and enclosing the rest of the cable as described above. Such further material and layers may have different tasks such as that of holding the different cable parts, as described above, together, and giving the cable mechanical strength and protection, against physical as well as chemical attack, e.g. corrosion, and are commonly known to the person skilled in the art.

The electric power cables according to the invention may be underwater power cables or the cables may be land cables. The cable is preferably an electric power cable having a rated voltage of 50 kV or higher, and is thus suitable for use as a high voltage transmission power cable. For example, the cables may be high voltage direct current (HVDC) cables, high voltage alternating current (HVAC) cables, extra high voltage cables (EHV), medium-voltage cables and low-voltage cables.

Fig. la and Fig. lb show an example of an electric power cable comprising an insulation system. In Fig. la, an electric power cable 10 is shown in a partially cut side view and in Fig. lb in a radial cross-section view and reference is made equally to both figures. The electric power cable comprises a metal conductor 1, which may be a solid or stranded metal conductor of conductive metal, such as aluminium or copper. The cable 10 further comprises an insulation system 2 (indicated in Fig. la) comprising an inner semi-conducting layer 4 comprising multiple layers of the composite insulation material of the present invention, which surrounds the conductor 1 coaxially and radially outwards of the conductor 1. The inner semi-conducting layer 4 may be in direct contact with the conductor 1, or a layer of for example conductive tape (not shown) may be arranged in between the conductor 1 and the inner semi-conducting layer 4. The inner semi-conducting layer 4 can be paper-based and comprises carbon black as filler to render the paper-based material semi-conducting. The inner semi-conducting layer 4 is surrounded coaxially and radially outwards by an insulating layer 5 which comprises multiple layers of the composite insulation material of the present invention having insulating properties. The insulating layer 5 can also be paper-based. Both the inner semi-conducting layer 4 and the insulating layer 5, when paper-based, are suitably provided in the form of a paper tape and are helically wrapped around the conductor 1 by means of rows of lapping heads (not shown). The insulating layer 5 is surrounded coaxially and radially outwards by a second, outer semi-conducting layer 7 which comprises multiple layers of the composite insulation material of the present invention having semi-conducting properties. The cable 10 further comprises a protection system comprising a moisture barrier 8, which can be a welded metal layer and which can be corrugated or smooth.

Fig. 2 shows an example of an alternating current (AC) electric power cable 20 comprising three conductors 21. All conductors are identical, but for clarity reasons, only one conductor has been provided with reference signs. Each conductor 21 is surrounded coaxially and radially by an insulation system 22 comprising multiple layers of the insulation material. Similarly, only one insulation system 22 is provided with reference signs, but all insulation systems are identical. Each insulation system 22 comprises an inner semi-conducting layer 24, an insulation layer 25 and an outer semi-conducting layer 27, whereby each of the layers comprise multiple layers of the insulation material of the present invention. The insulation system 22 is in turn surrounded coaxially and radially outwards by a protection system comprising a moisture barrier 28 (only one reference sign in figures, but all moisture barriers are identical), which can be a metal layer that can be corrugated or smooth. The three conductors 21 are surrounded by an outer shield 29 that keeps the three conductors 21 together within the AC cable 20. In Fig. 3, the process according to the present invention is illustrated in a flow chart. In the first step i) of the process a conductor for the electric power cable is provided. The conductor can be any of the kind described above.

In the second step ii) of the process a composite insulation material as defined above is applied to surround the conductor. Thus an insulation system comprising insulating and semiconducting layers for the electric power cable can be provided. The insulation system comprises multiple layers of the composite insulation material of the present invention. The insulation material may have a base material of for example paper or a woven or nonwoven fabric-type or foil-type material, which can comprise polymeric materials and/or glass fibres. The insulation material is preferably applied by wrapping or lapping the material around the conductor by means of a wrapping equipment. Such equipment can be of any type known in the art.

The insulation material contains microcapsules comprising a curing agent which is suitably a catalyst. The microcapsules comprise a carrier material which has a certain pre-determined melting temperature. For example, the carrier material can have a melting temperature of 130°C, whereby it is able to release the catalyst at elevated temperature of 130 °C. Thus, the crosslinking of the polymer is not possible at lower temperatures. Preferably, the last layer of the insulation system is wrapped or lapped by using a prior art insulation material not containing curing agent capsules. This further prevents solidification of the rest of the liquid in the tank, since the capsules are effectively entrapped inside the insulation system. If the base material in the composite insulation material is based on paper, there may be a need to dry the insulation system before impregnation. This can be done for example by means of vacuum drying in a vacuum drying apparatus known in the art.

In the next step iii) the insulation system surrounding the conductor is impregnated with a curable impregnation liquid which is a dielectric fluid. The dielectric fluid is curable or cross- linkable. The impregnation liquid according to the invention may be a liquid silicon mixture, polyurethanes, epoxies or other blends. Other curable polymeric dielectric fluids could be used. Preferably, the dielectric liquid is a silicone resin- based impregnation liquid. The impregnation liquid may be for example a mixture of methyl, vinyl and hydroxyl terminated polydimethylosiloxanes having a low viscosity, preferably in the range of 50 -2000 cSt measured with at room temperature, i.e. 20°C at atmospheric pressure. The viscosity may be measured by using a standard rheometer, e.g. by means of an apparatus having a trade name Kinexus from a company Malvern. Liquid silicon mixtures or polyurethanes could be also used. The impregnation may be performed for example by means of vacuum impregnation, which includes immersion of the wrapped conductor in an impregnation liquid until the

impregnation liquid penetrates substantially all the pores of the wrapped insulation material and all voids are filled with the liquid.

In the next step iv) the impregnated cable from the step iii) is subjected to activation by means of increased temperature, radiation, or increased pressure step to release the curing agent from the curing agent capsule. Therefore, curing of the impregnation liquid is initiated since the impregnation liquid can come into contact with the curing agent. For example, after the impregnation, the cable can be heated for a short period of time above a pre-determined temperature allowing for release of the curing agent or catalyst from the curing agent capsules embedded in the composite insulation material. That initiates the cross-linking reaction of the impregnation liquid and finally a fully solid insulation system is formed. After activation, in the next step v) of the process curing the impregnated cable is performed. The curing or solidification of the liquid can be performed inside an impregnation tank because the curing agent is released only locally inside the lapped insulation system. Thus, the liquid surrounding the insulation system does not cure. In another embodiment the curing or crosslinking of the cable can be made outside of the tank. For instance, it may be performed by passing the cable through a special oven which provides for the elevated temperature necessary to release the catalyst from the composite insulation material. According to one embodiment, the impregnation liquid is silicone resin-base and the curing agent is a Pt- catalyst. When curing occurs, silicone rubber is formed, created in the reaction of addition between vinyl terminated polydimethylsiloxane and hydride functional polymer (e.g.

methylhydrosiloxane - dimethylsiloxane copolymer trimethylsiloxy terminated.) The reaction is initiated by Pt based catalyst a shown below:

— o -O O- 1 I

— 0 -Si -CH ₃ + H ₂ C =CH -Si -CH2CH2— Si -

I 1 I

CH3 Π3 uii After curing, a step vi) of the curing is terminated. Termination can be done for example after a pre-determined time period for example by decreasing the temperature of the cable. Thus, an electric power cable with a solid insulation system is provided. In the nest step vii) protective materials to cover the insulation system are optionally provided, such as armouring and/or protective sheaths.

The process according to the present invention provides a well-controlled solidification process for the insulation system in sense of time of its initiation and of the space in which the curing or crosslinking takes place. This allows for solidification of the impregnation liquid only at certain areas. According to the present invention the area in which solidification takes place is the space in between lapped layers of the insulation material and the space inside the pores of the material used for lapping.

Example 1

Use was made of curing agent capsules comprising a thermoplastic carrier material and a platinum catalyst. The capsules had an average largest dimension in a two-dimensional plane visible in a TEM image of less than ΙΟΟμιη. The capsules were dispersed in water at concentration of O.lg capsules/4g water and a paper-based insulation material was wetted thoroughly in the water-capsule dispersion. 24 flat multilayer samples were formed and the samples were dried by means of vacuum drying in 80°C, i.e. below the melting point of the thermoplastic carrier material. The samples were then impregnated under vacuum with a curable silicone resin-based impregnation liquid. No cross-linking or curing and thus solidification of the impregnation liquid occurred initially. The samples were then heated to a temperature of 120°C in the impregnation bath, which is above the melting temperature of the thermoplastic carrier material. Thus, the carrier material melts and the curing agent is released. Solidification of the silicone resin-based impregnation liquid started between the paper layers. No solidification outside of the samples was noticed.

Electrical tests according to standard IEC 60243 of the samples were performed. The results are shown in Tables 1 and 2 below. Sample U [kV] Mean thickness Estimated Probe

of the samples, Withstand configuration d [mm] [kV/mm] [mm/mm]

1 46 0.443 103.9 25/75

2 50 0.396 126.3

3 58 0.408 142.1

4 56 0.433 129.3

5 50 0.440 113.7

6 58 0.441 131.7

7 50 0.494 101.3

8 52 0.438 118.8

9 54 0.534 101.1

10 40 0.457 87.5

11 50 0.430 116.2

12 62 0.425 146.0

Table. 1. The average estimated dielectric withstand ¾ 118.2 kV/mm.

Sample U [kV] Mean Thickness, Estimated Probe

d [mm] Withstand configuration

[kV/mm] [mm/mm]

13 62 0.455 136.4 25/25

14 64 0.551 116.3

15 78 0.589 132.5

16 80 0.455 175.8

17 66 0.431 153.1

18 58 0.370 156.8

19 74 0.414 178.7

20 72 0.581 123.9

21 74 0.625 118.4

22 80 0.430 186.2

23 68 0.411 165.3

24 71 0.430 165.2 Table 2. The average estimated dielectric withstand ¾ 150.7 kV/mm.

From the tests it can be seen that paper can be used as a porous base material and the carrier material may be a thermoplastic material that melts when the temperature is increased and thereby releases the curing agent. Especially, it could be noted that curing appeared only in between paper layers and not in the rest of the impregnation liquid which proves that only localised curing process was obtained. Thereby an easily controllable impregnation process can be obtained by using the composite insulation material according to the present invention.