WO2017084709A1

WO2017084709A1 - Electric power cable and process for the production of electric power cable

Info

Publication number: WO2017084709A1
Application number: PCT/EP2015/077072
Authority: WO
Inventors: Hossein GHORBANI; Su Zhao; Anneli JEDENMALM; Chau-Hon HO; Markus Saltzer
Original assignee: Abb Hv Cables (Switzerland) Gmbh
Priority date: 2015-11-19
Filing date: 2015-11-19
Publication date: 2017-05-26

Abstract

The present invention relates to an electric power cable (1) comprising a metal conductor (2) and an electric insulation system (12) radially surrounding the conductor (2). The insulation system (12) comprises an inner semi-conducting layer (4) comprising a first base polymer material and a conductive filler, wherein the inner semi-conducting layer (4) surrounds the conductor (2). Further, the insulation system comprises an insulation layer (6) comprising a second base polymer material, wherein the insulation layer (6) is in contact with and surrounds the inner semi-conducting layer (4) radially outwards; and an outer semiconducting layer (8) comprising a third base polymer material and a conductive filler, wherein the outer semi-conducting layer is in contact with and surrounds the insulation layer (6) radially outwards. The first base polymer material comprises a non-cross-linked polypropylene base polymer and is different from the second base polymer material that comprises a cross- linked polyethylene base polymer, whereby a uniform electric field distribution in the insulation system (12) is obtained.

Description

ELECTRIC POWER CABLE AND PROCESS FOR THE PRODUCTION OF ELECTRIC POWER CABLE

TECHNICAL FIELD

The present invention relates to an electric power cable and to a process for the production of an electric power cable as defined in the appended claims.

BACKGROUND ART

Electric power cables are used to transmit electric power at a medium or high voltage. Electric power cables may be buried into the ground whereby they are called land cables. The electric power cables may also be buried into a sea bed or they may extend between two fixing points in sea water and cables of this type are called submarine, sea water or underwater power cables. Areas where energy is on the one hand needed and on the other hand produced may be located at a long distance from each other, which increases a need for safe power transfer.

In order to meet the demands for safe power transfer, the insulation systems in the cables need to be of high quality to ensure correct electrical and mechanical behaviour during the transmission of electric power. To electrically insulate the conductor, an insulation system is arranged to surround the conductor. Different insulation materials can be used in power cable applications, including paper and oil mass-impregnated cables and extruded polymeric materials.

Unless the power cables are appropriately insulated, significant leakage currents will flow in the radial direction of the cables, from the conductor to the surrounding grounded screen. Such leakage currents give rise to significant power losses, as well as to heating of the electrical insulation. The heating of the insulation can further increase the leakage current due to the reduction of the resistance with the increasing temperature. To avoid power losses and possible thermal runaway, the leakage current should therefore be kept as small and stable as possible.

Electric cables with extruded polymeric insulation systems normally comprise a conductor and a radially surrounding polymeric insulation system comprising an inner semi-conducting layer, an insulation layer and an outer semi-conducting layer. The insulation system is typically formed by extrusion directly on the conductor core. The insulation system normally has the same base polymer common to the semi-conducting and insulation layers in order to facilitate compatibility between the various layers of the insulation system. The properties of each layer can then be tuned using additives and fillers such as semi-conducting fillers, anti-scorch additives, antioxidants, etc., without modifying the base polymer per se.

A commonly used type of electric power cable is a cross-linked polyethylene insulated cable, which is usually called XLPE cable for short. This type of cable has an insulation system produced by triple extrusion of the inner semi-conducting, insulation and outer semiconducting layers. Each of these layers comprises a low density polyethylene (LDPE) base polymer comprising a cross-linking agent, most commonly an organic peroxide. Once extruded, the insulation system is then subjected to high temperature and pressure curing conditions in order to cross-link the LDPE, thus forming XLPE. After extrusion and curing, the cables are usually degassed by heat-treatment which helps to remove a portion of the cross- linking by-products from the cable insulation system. There have been many attempts to improve the electrical properties of insulation systems. For example, there have been attempts to reduce the conductivity of the insulation material, whilst ensuring the insulation material has good mechanical properties.

US 6,255,399 describes an electric cable comprising a conductor, an inner layer with semiconductive properties, an intermediate layer with insulating properties, an outer layer with semiconductive properties, a screen, and an outer sheath. At least the intermediate insulating layer comprises a polymer mixture comprising polypropylene as non-crosslinked base polymer material, mixed with a copolymer of ethylene with at least one alpha-olefin and optionally a diene, and in a preferred embodiment all of the insulating and semiconductive layers comprise such a polymer mixture. However, such a system suffers from higher dielectric losses compared to polyethylene-based systems.

US2012/0012362 describes a DC power cable comprising a conductor, an inner

semiconductive layer, an insulation and an outer semiconductive layer. At least one of the inner semiconductive layer or the outer semiconductive layer comprises a semiconductive composition comprising hybrid particles produced from carbon nano tubes and a spherical propylene base resin or a spherical low-density polyethylene base resin. The insulation comprises an insulation composition comprising inorganic nano particles and a polypropylene base resin or a low-density polyethylene base resin. The examples show the use of a low density polyethylene resin with a melt index of 1~2 as a base resin in the semiconductive composition and insulation composition. However, the system described requires the use of a number of expensive constituents that are complicated to manufacture and integrate into a cable manufacturing process, such as carbon nanotubes and surface-modified inorganic fillers.

WO2012150285 describes a crosslinked direct current (DC) power cable comprising a conductor surrounded by at least one layer, wherein the at least one layer comprises a polymer composition comprising a) a polyolefin which is other than low density polyethylene, b) a second polyolefin which is different from the polyolefin a), and c) an ion exchanger additive. However, also this system requires the use of inorganic fillers in the form of ion exchanger additives.

Despite attempts in the prior art to reduce conductivity of the insulation material, there is still the need for improvement of the polymeric insulation systems. SUMMARY OF THE INVENTION

It is an object of the present invention to improve upon the prior art electrical cables. For example it is desirable to obtain as uniform electric field in the cable as possible. It is also desirable to obtain a cable with better electric field distribution with improved mechanical properties and thus to provide a cable with a more robust insulation system, especially at permanent or transient higher operating temperatures. Moreover, it is desirable to obtain a cable that is economical to produce and does not require the use of expensive and

complicated constituent fillers and additives.

Therefore, it is an object of the present invention to provide a power cable with as uniform electric field distribution as possible whilst maintaining or even improving the mechanical properties of the cable. Especially, it is an object to provide a cable with more uniform electric field distribution both at high temperatures and also during thermal dynamics, i.e. high-low temperature cycles. Also, by providing a more uniform electric field distribution, it will be possible to increase the cable voltage level and average electric field, as well as operate at higher conductor temperatures. Furthermore, it is an object to provide a cable with a more robust insulation system.

It is also an object of the present invention to provide a transmission power cable with an electrical insulation system having improved insulation properties and which is suitable for high voltage or medium voltage power cable applications. It is a further object of the invention to provide an improved and modified insulation system which is simple and cost-efficient. More specifically, it is an object of the invention to provide an insulation system that does not require the use of additives or fillers, such as nanotubes, nanoparticles, surface-modified inorganic particles, that are expensive and complicated to manufacture and handle in or integrate into the cable manufacturing process. A still further object of the present invention is to provide reliable transfer of electrical power.

It is also an object of the present invention to provide a power cable, and especially an HVDC or MVDC cable having improved electrical properties.

The inventors of the present invention have realized that the by-products from the cross- linking agents used in modern XLPE insulation systems have a negative effect on the conductivity of the insulation system. The inventors have discovered that by-products formed from the bis(i-butylperoxyisopropyl)benzene (Vul-Cup^®) cross-linking agent, typically used in the inner and outer semi-conducting layers, has an especially dominating negative effect on the conductivity. Moreover, the inventors have realized that during heat-treatment and operation at elevated temperatures, the cross-linking agent by-products can diffuse or migrate between layers. Therefore, the by-products from the cross-linking agent in the semiconducting layers can migrate into the insulation layer and in this manner affect the electrical properties of the entire insulation system. The inventors have discovered that degassing is insufficient to remove these detrimental by-products from the portions of the insulation system in closest proximity to conductor. Furthermore, the inventors have discovered that the presence of these Vul-Cup by-products in the insulation causes very high dynamics in the conductivity upon cooling, which leads to a lower robustness for the insulation system.

According to the present invention defined in the appended claims, it has been surprisingly found that by using a combination of a non-cross-linked polypropylene as the base polymer in the inner semi-conducting layer, together with a cross-linked polyethylene base polymer in the insulation layer, the formation of the cross-linking by-products that are most detrimental to the insulation properties is avoided, whilst the excellent mechanical properties of the cable insulation system are maintained or even improved.

Thus, the above-mentioned objects are achieved by an electric power cable comprising a metal conductor and an electric insulation system radially surrounding the conductor, which insulation system comprises:

An inner semi-conducting layer comprising a first base polymer material and a conductive filler, wherein the inner semi-conducting layer surrounds the conductor;

- an insulation layer comprising a second base polymer material, wherein the insulation layer is in contact with and surrounds the inner semi-conducting layer radially outwards; and

an outer semi-conducting layer comprising a third base polymer material and a conductive filler, wherein the outer semi-conducting layer is in contact with and surrounds the insulation layer radially outwards.

The first base polymer material and the second base polymer material are different from each other. The first base polymer material comprises a non-cross-linked polypropylene-based base polymer, and the second base polymer material comprises a cross-linked polyethylene-based base polymer.

By combining the use of a non-cross-linked polypropylene base polymer in the inner semiconducting layer together with a cross-linked polyethylene (XLPE) base polymer in the insulation layer, a number of advantages are obtained. By propylene-based base polymer is meant in this connection that the repeating units in the polymer are mainly constituted of propylene monomers. No cross-linking by-products detrimental to the insulation electrical properties are formed in the insulation system in proximity to the cable core. Thus, excellent electrical properties are achieved, including a more uniform electrical field distribution in the cable and a greater tolerance of thermal dynamics. This leads to a cable that is more robust and can operate at higher conductor temperatures and therefore higher voltages. At the same time, the combined use of non-cross-linked polypropylene-based base polymer in the inner semi-conducting layer and cross-linked polyethylene-based base polymer in the insulation layer ensures that the cable insulation system has excellent mechanical properties. By polyethylene-based base polymer is meant in this connection that the repeating units in the polymer are mainly constituted of ethylene monomers. The polypropylene base polymer of the inner semi-conducting layer has good mechanical properties per se, and since the adjacent base polymer of the insulation layer is cross-linked, the mechanical properties are improved. Further, the cross-linking agent can diffuse to some extent over the layer interface, whereby excellent layer bonding and compatibility are ensured. The excellent mechanical and electrical properties of the insulation system are achieved without resorting to the use of expensive and complicated fillers such as nano-tubes or surface modified particles, thus ensuring ease of manufacture and reducing costs. Because the insulation layer is XLPE based, greater compatibility with existing XLPE cable systems is achieved.

According to another feature of the present invention, the non-cross-linked polypropylene- based base polymer of the first base polymer material comprises, or consists of, an isotactic polypropylene, a polypropylene copolymer, a polypropylene terpolymer, or mixtures thereof, preferably a polypropylene copolymer. This ensures that the inner semi-conducting layer has the required electrical and mechanical properties, as well as sufficient compatibility with the insulation layer. According to yet another feature of the present invention the polyethylene-based base polymer of the second base polymer material comprises, or consists of, low density polyethylene, ultra-low density polyethylene, linear low density polyethylene, high density polyethylene, ultra-high density polyethylene, or a mixture thereof, preferably low density polyethylene. This ensures that the cable insulation layer, which is often the thickest layer of the insulation system, has the required electrical and mechanical properties, as well as sufficient compatibility with the inner and outer semi-conducting layers.

The polyethylene-based base polymer of the second base polymer material can be cross- linked by an organic peroxide cross-linking agent, preferably selected from dicumyl peroxide, bis(i-butylperoxyisopropyl)benzene, i-butyl cumyl peroxide, 2,5-di(t-butylperoxy)-2,5- dimethylhexane, n-butyl-4,4'-di(i-butylperoxy)valerate, l,l'-di(t-butylperoxy)-3,3,5- trimethylcyclohexane, or mixtures thereof, most preferably dicumyl peroxide. This ensures that the insulation layer is well cross-linked at the selected curing temperature and has excellent mechanical properties. It also assists in bonding the insulation layer to the inner and outer semi-conducting layers. According to an embodiment of the present invention, the third base polymer material is the same as the first base polymer material. Thus, the third base polymer material may comprise a non-cross-linked polypropylene-based base polymer. The non-cross-linked polypropylene- based base polymer may comprise, or consists of, an isotactic polypropylene, a polypropylene copolymer, a polypropylene terpolymer, or mixtures thereof, preferably a polypropylene copolymer. By using same materials as the first and third base polymer materials the manufacturing process can be simplified, since only a single semi-conducting polymer material is required for the manufacture of the cable.

According to another embodiment of the present invention, the third base polymer material is the same as the second base polymer material. Thus, the third base polymer material can comprise, or consist of, a cross-linked polyethylene-based base polymer. Therefore, it is possible to use polyethylene-based base polymers in both the insulation layer and the outer semi-conducting layer. Since the outer semi-conducting layer can be efficiently degassed, the use of a cross-linked polyethylene-based base polymer in this layer is possible and provides excellent mechanical properties whilst minimally impacting the electrical properties of the cable. The polyethylene-based base polymer of the third base polymer material may comprise, or consist of, low density polyethylene, ultra-low density polyethylene, linear low density polyethylene, high density polyethylene, ultra-high density polyethylene, or a mixture thereof, preferably low density polyethylene. According to a further feature of this embodiment, the polyethylene-based base polymer is cross-linked by an organic peroxide cross-linking agent, suitably selected from bis(i-butylperoxyisopropyl)benzene, i-butyl cumyl peroxide, 2,5-di(t- butylperoxy)-2,5-dimethylhexane, n-butyl-4,4'-di(i-butylperoxy)valerate, l,l'-di(i- butylperoxy)-3,3,5-trimethylcyclohexane, or mixtures thereof, most preferably bis(i- butylperoxyisopropyl)benzene. Since the outer semi-conducting layer can be efficiently degassed, the use of a cross-linked polyethylene in this layer provides excellent mechanical properties whilst minimally impacting the electrical properties of the cable system. The first and the second semi-conducting layers may comprise conductive filler in an amount of from 10 to 40 % by weight, based on the total weight of the first and/or the second semiconducting layer, respectively. This ensures that these layers possess the requisite electrical properties.

According to a further feature of the present invention, the inner semi-conducting layer and/or outer semi-conducting layer comprises carbon black as the conductive filler and at most 0.2 % by weight of carbon nanotubes. Nanoparticles such as carbon nanotubes are difficult to handle in a manufacturing process. By using carbon black which is essentially free of carbon nanotubes, or has a maximum content of 0.2% by weight it can be ensured that the semi-conducting layers possess adequate conductive properties without resorting to the use of expensive or complicated additives or fillers such as carbon nanotubes.

According to still another feature of the present invention, the insulation layer does not comprise inorganic filler. This means that the insulation layer possesses the requisite electrical and mechanical properties without resorting to the use of expensive and complicated additives or filler materials such as inorganic nano-particles or surface-modified particles.

According to one more feature of the present invention, the electric power cable is a high voltage direct current (HVDC) cable having a rated voltage of 50kV or higher. The excellent electrical and mechanical properties of the cable of the present invention are preferred for transferring such high voltages.

The objects above are also attained by a process for the production of an electric power cable comprising the steps of:

i) providing a conductor;

ii) extruding an inner semi-conducting layer comprising a first base polymer

material comprising a non-cross-linked polypropylene-based base polymer to surround the conductor radially outwards;

iii) extruding an insulation layer comprising a second base polymer material

comprising cross-linked polyethylene-based base polymer to be in contact with the first semi-conducting layer and to surround the first semi-conducting layer radially outwards;

IV extruding an outer semi-conducting layer comprising a third base polymer material to be in contact with the insulation layer and to surround the insulation layer radially outwards;

v) subjecting the cable to a curing procedure.

The extrusion of the inner semi-conducting layer, insulation layer and outer semi-conducting layer can be performed as a triple extrusion. This ensures a high-quality insulation system with good bonding and compatibility between layers, and minimal defects.

According to another feature, the process comprises a further step of:

vi) heat treating and degassing the cable to remove cross-linking by-products. This removes detrimental cross-linking by-products from the cable insulation system. The invention will now be further described with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples according to the present invention will be described below with reference to the accompanied drawings in which:

Fig. 1 is a side view of a single phase electric power cable according to the present invention; Fig. 2 is a cross-section of a single phase electric power cable according to the present invention;

Fig. 3 is a flow chart showing the steps of a process for the production of the electric power cable according to the present invention.

Fig. 4 shows results from a test showing accumulation of cross-linking by-products with respect to the radial position.

Fig. 5 shows a temperature curve used during measurements shown in Example 2 and Fig. 6.

Fig. 6 shows conductivity of samples obtained during a temperature-treatment shown in Fig. 5. Further features and advantages will be described in the following detailed description. DETAILED DESCRIPTION

Electric power cables, also called electric transmission power cables, are aimed for

transmitting electric power. The demands with regard to electric and mechanical properties are increasing due to increased demand for electricity and long distances electricity needs to be transmitted.

The electric power cable is preferably of a type single phase electric power cable. For example, the cable may be high voltage direct current (HVDC) cable, extra high voltage cable (EHV), medium-voltage cable or low-voltage cable. The electric transmission power cables comprise a conductor, which is usually mainly constituted by a metal such as copper or aluminum. The conductor is surrounded by an electric insulation system which comprises an inner semi-conducting layer, an insulation layer and an outer semi-conducting layer. An insulation layer is located between the semiconducting layers. Single phase cables comprise one conductor. Normally, the conductor has a generally circular cross section, even though alternative shapes might be conceived. The radially surrounding electric insulation system with insulation and semi-conducting layers usually has a cross-section with an outer peripheral shape

corresponding to the outer peripheral shape of the conductor, normally a generally circular outer periphery, and the insulation system surrounds the conductor radially and

concentrically. In this way uniform insulation in the cable can be obtained and electrical properties of the cable can be improved.

The cables may be underwater power cables or the cables may be land cables. The cable is preferably a power transmission cable having a rated voltage of 50 kV or higher, and is thus suitable for use as a high voltage transmission power cable. Preferably, the cable is a high voltage direct current (HVDC) cable.

According to the present disclosure, the conductor is surrounded by an electric insulation system which comprises an inner semi-conducting layer, an insulation layer and an outer semiconducting layer. In the insulation system, the insulation layer or layers should have insulation properties and essentially no conductivity or very low conductivity. The semi-conducting layers can be rendered semi-conducting by using for example fillers having conducting properties.

By insulation layer is meant a layer of a material that 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 layer is meant a layer of 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 semiconducting material may be typically of larger than 10^~5 S/m at 20 °C, such as up to about 10 or 10² S/m. Typically, the conductivity is less than 10³ S/m at 20 °C.

By conductivity is meant the property of transmitting electricity. The conductivity of a conducting material is more than about 10³ 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 10⁸ S/m at 20 °C. Fig. 1 is a partially cut side view of an electric cable 1 according to the present invention, and Fig. 2 shows a radial cross section thereof. The cable 1 comprises a conductor 2, an inner semiconducting layer 4 radially innermost and closest to the conductor 2, an insulation layer 6 radially surrounding and in contact with the inner semi-conducting layer 4 and an outer semiconducting layer 8 radially outermost from the conductor and in contact with the insulation layer. The inner semi-conducting layer 4, the insulation layer 6 and the outer semi-conducting layer 8 together form an insulation system 12 (shown only in Fig. 1) for the transmission power cable 1.

The inner semi-conducting layer comprises a first base polymer material, the insulation layer comprises a second base polymer material and the outer insulation layer comprises a third base polymer material. By base polymer material it is meant the polymeric matrix that comprises the bulk of each insulation system layer. Additives such as cross-linking agents and conductive filters can be added to the base polymer material to provide the material used for each layer of the insulation system. The material used for each layer of the insulation system typically comprises at least 60 wt% of a base polymer. There may be more than one insulation layer and there may be more than two semiconducting layers in the insulation system, such as 1-4 insulation layers and 1-4 semiconducting layers. The transmission power cable 1 in Fig. 1 and 2 is surrounded by an outer sheath 10. According to one embodiment, the insulation system can be directly attached to and arranged to be in contact with the conductor. In this way effective insulation can be provided. The conductor may be also indirectly surrounded by the polymeric insulation system, i.e. the electric power cable may comprise at least one material layer between the conductor and the insulation system. In this way it is possible to e.g. customize cables. The conductor and the insulation system can be surrounded by further material or layers of material. Further materials and layers may have different tasks such as that of holding the different cable parts together, giving the 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, or sheath-like barriers to provide a water barrier for the cables.

In the insulation system, the first base polymer material, that is to say the base polymer material of the inner semi-conducting layer, comprises or consists of a non-cross-linked polypropylene-based base polymer and can be selected from polypropylene homopolymers such as isotactic polypropylene, polypropylene copolymers, polypropylene terpolymers, or mixtures thereof. If copolymers or terpolymers are used, propylene is the predominant monomeric constituent of the polymer. That is to say that the molar proportion of propylene monomer in the polymer is in excess of the molar proportion of any other single monomer in the polymer. The polypropylene-based polymer may be mixed or blended with further polymers in order to provide a base polymer material with the desired chemical, mechanical and electric properties. For instance, the polypropylene-based polymer may be blended with an elastomeric polymer, such as a propylene-based elastomer, in order to obtain improved mechanical properties. The first base polymer material comprises greater than 50 wt% of polypropylene-based polymer, preferably at least 60 wt% polypropylene, such as 70 wt% or 80 wt% polypropylene- based polymer.

The second base polymer material, that is to say the base polymer material of the insulation layer, comprises or consists of polyethylene-based base polymer and can be selected from low density polyethylene, ultra-low density polyethylene, linear low density polyethylene, high density polyethylene and ultra-high density polyethylene or mixtures thereof. The

polyethylene can comprise homopolymers, copolymers or terpolymers of ethylene, as long as ethylene is the predominant monomeric constituent of the polymer. That is to say that the molar proportion of ethylene monomer in the polymer is in excess of the molar proportion of any other single monomer in the polymer. Preferably, the polyethylene polymer is low density polyethylene. The polymeric material renders the insulation system relatively thermally stable while an effective insulation property is obtained.

The second base polymer material comprises greater than 50 wt% polyethylene, preferably at least 80 wt% polyethylene, even more preferably at least 95 wt% polyethylene. Most preferably, the second base polymer consists of polyethylene.

The third base polymer material, that is to say the base polymer material of the outer semiconducting layer, comprises or consists of a polypropylene-based base polymer or

polyethylene- based base polymer as defined above. The third base polymer may be the same as either the first base polymer or the second base polymer. However, if the third base polymer is the same as the second base polymer, the third base polymer in any case may contain no added cross-linking agent, or may differ from the second base polymer in the cross- linking agent used. Since it is possible to remove essentially all the cross-linking by-products from the outer semi-conducting layer by degassing, the outer semi-conducting layer may most preferably comprise polyethylene as the insulation layer does, but with a different cross- linking agent as compared to the insulation layer.

At least the inner semi-conducting layer, which is located closest to the conductor and surrounds the conductor, is non-cross-linked. However, during the manufacture of the cable, it is possible that minor amount of a cross-linking agent will diffuse from the surrounding insulation layer to the inner semi-conducting layer. The cross-linking agent may mainly diffuse to the surface of the semi-conducting layer. Thus, when the cable is cured the diffused cross- linking agent can react and some cross-linking may occur at the interface between the surface of the inner semi-conducting layer and the insulation layer. This may lead to bonding between the inner semi-conducting layer and the insulation layer which further strengthens the mechanical properties of the cable. The diffusion of the cross-linking agent from the insulation layer occurs naturally. By non-cross-linked is thus meant in this application that no cross- linking agent is actively added to the base polymer of the inner semi-conducting layer.

The inner semi-conducting layer is thus essentially free of a cross-linking agent. This means that it contains less than 0.2 wt. % of a cross-linking agent. As explained above, small amounts of a cross-linking agent may naturally diffuse from the neighboring insulation layer which comprises an added cross-linking agent to the inner insulation layer. In addition, if any of the polymers used in the base polymer material is synthesised by radical polymerization, residual traces of peroxide polymerization initiator may be present. The inner semi-conducting layer has thermoplastic properties and contains no cross-linking by-products that diffuse into the neighboring insulation layer. Cross-linking by-products are a group of chemicals which are formed during cross-linking of polymers, for example polyethylene or polypropylene, due to chemical reactions of radicals forming from cross-linking agents, such as peroxide, and the polymer.

By assuring that at least the inner semi-conducting layer is non-cross-linked the insulation system will contain a reduced amount of cross-linking by-products that can negatively affect the electrical properties of the insulation system, especially compared to insulation systems in which all layers are cross-linked. Also, by using a polypropylene base polymer in the inner semi-conducting layer, the mechanical properties of the cables are affected minimally. By cross-linking the base polymer in the insulation layer it is rendered more resistant against softening and loss of shape at higher temperatures, such as above 70°C, especially in case the polymer matrix is a low density polyethylene (LDPE).

The insulation layer and optionally the outer semi-conducting layers are cross-linked.

According to one embodiment the outer semi-conducting layer is non-cross-linked and is thus essentially free of a cross-linking agent. According to an alternative embodiment, the outer semi-conducting layer is cross-linked. Cross-linking ensures that sufficient mechanical strength for the cable is provided.

The cross-linking agent for the base polymer in the insulation layer and optionally in the second semi-conducting layer may be any cross-linking agent suitable for use in connection with a polyethylene polymer or copolymer thereof, such as a peroxide-based, silane-based cross-linking agent or azo-compounds. The cross-linking may also be performed by radiation. The cross-linking agent in the insulation layer and in the second semi-conducting layer may be different from each other. In this way the cross-linking agent can be adjusted to the specific needs of the respective materials in the layers. Preferably, the cross-linking agent is peroxide- based. For example, the insulation layer may be cross-linked with dicumyl peroxide (Di-Cup^®) and the semi-conducting layer with another type of peroxide-based compound, such as a highly active bisperoxide, bis(tert-butyldioxyisopropyl)-benzene (Vul-Cup^®).

The amount of the cross-linking agent can be from 0.3-2.0 % by weight, based on the weight of the base polymer, to ensure sufficient cross-linking. Different additives and fillers can be added to the base polymers to render the polymeric material with desired properties. Additives may be for example stabilizers such as

antioxidants, nucleating agents, cross-linkers, cross-linking boosters such as 2,4,6-triallyl cyanurate, scorch retard agents and flame retardants. Stabilizers, particularly antioxidants prevent negative effects of oxidation. Preferably, the insulation layer is free from inorganic fillers, especially surface-modified particles or nanoparticles that are troublesome and expensive to manufacture and integrate into the cable manufacturing process.

The semi-conducting layers may comprise conductive fillers, such as conductive particles that render the semi-conducting layer the desired conductivity. The conductive particles may be of any kind, such as metallic conductive filler particles or carbon black. The content of the particles may vary e.g. between 10 to 40 % by weight, based on the total weight of the semiconducting layer. Carbon black is often used due to its stability also at high temperatures. The conductive filler preferably does not comprise added carbon nanotubes, as these are expensive and troublesome to manufacture and handle during the power cable manufacturing process. However, small quantities of carbon nanotubes may be present as impurities in the carbon black filler, in amounts up to 0.2 weight %. The process for the production of the present electric power cable is schematically illustrated in the flow chart of Fig. 3. The process comprises the following steps: i) providing a conductor 2;

ii) extruding an inner semi-conducting layer 4 comprising a first base polymer material comprising a non-cross-linked polypropylene-based base polymer to surround the conductor 2 radially outwards. No cross-linking agent is added to the base polymer in the first semi-conducting layer 4;

iii) extruding an insulation layer 6 comprising a second base polymer material comprising a cross-linked polyethylene-based base polymer and a cross-linking agent to be in contact with the inner semi-conducting layer 4 and to surround the inner semi-conducting layer 4 radially outwards;

iv) extruding an outer semi-conducting layer 8 comprising a third base polymer material and optionally a cross-linking agent to be in contact with the insulation layer 6 and to surround the insulation layer 8 radially outwards; v) subjecting the cable 1 to a curing procedure.

The conductor may be of the kind described above. The extrusion may be performed by using any of the available common extrusion technologies, which are well known for the skilled person and not described in detail herein. The extrusion steps ii) to iv) may be performed simultaneously or in sequence. To facilitate the control of the process, the extrusion steps are preferably performed simultaneously, by triple-extrusion.

In the steps iii) and v) the base polymer used in the insulation layer 6 can be cross-linked by using dicumyl peroxide as a cross-linking agent. The dicumyl peroxide cross-linking agent provides improved mechanical and thermal properties for the insulation layer.

In the step iv) a cross-linking agent is preferably added, in order to cross-link the base polymer in the outer semi-conducting layer 8 during step v). The cross-linking agent is preferably different from the cross-linking agent used to cross-link the insulation layer 6. Preferably, the base polymer in the outer semi-conducting layer 8 is cross-linked by using a highly active bisperoxide-compound, bis(tert-butyldioxyisopropyl)benzene. Thus, different properties to the insulation layer and the outer semi-conducting layer can be rendered by the use of different cross-linking agents. Preferably, the process comprises a further step of: vi) heat treating and degassing the cable to remove cross-linking by-products. Optionally, the cable can also be subjected to a second heat-treatment step after applying a cable sheath.

Heat treating and degassing may be performed during the production process when deemed necessary. The heat-treating may be performed in an oven or by using any other technology known in the art and apparent to the skilled person. In this way, the amount of by-products can be decreased. It is known that e.g. polar chemicals, such as water and cross-linking byproducts, affect the conductivity in insulation polymeric materials. Thus, there is a desire to limit the amount of such chemicals in the insulation system of the power cables. Also, any methane formed as a by-product during the cross-linking procedure must be removed in order to avoid a fire hazard. The cable may also be subjected to a second heat treatment step after applying a sheath to the cable. This second heat treatment step is intended to redistribute the diffusive chemicals in the insulation system in order to obtain a more even distribution.

The inventors of the present invention have noted that since the chemicals can leave the insulation system only through the outer semi-conducting layer of the cable, it will lead to a non-uniform distribution of by-products in the insulation system so that the radially inner parts of the insulation system in the cable, i.e. layers in close proximity to the conductor will contain a higher amount of by-products than the radially outer parts of the cable, i.e. for example the outermost semi-conducting layer of the power cable. The by-products may redistribute by time and heat, either purposefully or through use, and the distribution may become more uniform, but problems with non-uniform distribution of the by-products lead to e.g. problems with locally high electric field before the by-products are redistributed uniformly.

For example, referring to prior art cables comprising a cross-linked XLPE inner semi- conducting, insulation and outer semi-conducting layers, the different peroxides used for cross-linking produce different set of by-products which may have different effects on conductivity. For example, prior art insulation systems commonly comprise an insulation layer comprising a cross-linked polyethylene polymer (XLPE) cross-linked with dicumyl peroxide DCP, known as Di-Cup^®, CAS number 80-43-3. The polyethylene base polymer is low density polyethylene, LDPE. The XLPE which is commonly used as the semi-conducting material layers in the cable contains another type of peroxide, for example a bisperoxide, bis(tert- butyldioxyisopropyl)benzene, which is highly active and on the market known as Vul-Cup^® peroxide, CAS number 25155-25-3. Since there are two different peroxides, the cross-linking by-products produced in the insulation layer and semi-conducting layers during the cable production are different. After extrusion and cross-linking that occurs during curing, the cables are usually heat-treated which helps to remove a portion of the cross-linking by-products from the cable insulation system. However, not all of the cross-linking by-products can be removed, especially from the inner parts of the cable, whereby the electrical properties of the cable are negatively affected. Still referring to prior-art cable insulation systems, most of the by-products in the outer semiconducting layer can be degassed during the heat treatment. But the by-products in the inner semi-conducting layer will diffuse into the innermost parts of the insulation layer adjacent to the inner semi-conducting layer. The by-products originating from the inner semi-conducting layer in the cable insulation can be measured. This will be shown below in Example 1. Example 1

In this example, investigating the properties of prior art insulation systems, the inner and the outer semi-conducting layers of the insulation system were cross-linked with a bisperoxide bis(tert-butyldioxyisopropyl)benzene known with a trade name Vul-Cup^® peroxide, CAS 25155-25-3. Different Vul-Cup by-products after cross-linking were identified, namely:

1.3- diacetyl benzene

1.4- diacetyl benzene 1,3-dihydroxy isopropyl benzene 1,3-acetylhydroxy isopropyl benzene 1,4-dihydroxy isopropyl benzene

As it is shown in Fig. 4, the concentration of the by-products from semi-conducting layers is highest in the inner parts and negligible in the outer parts. The graph shows the radial concentration distribution of the above-mentioned by-products from Vul-Cup^®, as measured in a cable using GC-FID (gas chromatography with flame ionization detector). The measurement point at the smallest radius is in the inner semiconducting layer and the measurement point at the largest radius is in the outer semiconducting layer. The

intermediate points are within the insulation layer. The measured by-products have obviously diffused from the inner semiconducting layer into the insulation layer. In the outer

semiconducting layer and in the insulation layer nearby, the by-products could not be detected, so the by-products that were formed during cross-linking of the outer

semiconducting layer have been removed during the manufacturing process, such as during degassing.

Example 2 A series of experiments was carried out in order to investigate the effect of different inner semi-conducting layers on the electrical properties of the insulation system The apparent conductivity as function of time was measured using the method described in: H. Ghorbani, C. 0. Olsson, J. Andersson, V. Englund, "Robust characterization of the DC-conductivity of HVDC insulation materials at high electric fields", 2015, JiCable conference 15, Versailles, France. A commercially available XLPE polymer, Borlink LS4258DCE from Borealis, comprising an LDPE base polymer material and dicumyl peroxide (DiCup) as the cross-linking agent, was used as the model insulation layer in all experiments. In order to investigate the effect of Vul-Cup in the inner semi-conducting layer, a commercially available XLPE semi-conducting polymer mixture, Borealis Borlink LE0550-06, comprising an LDPE base polymer, carbon black and Vul- Cup cross-linking agent was used.

To investigate the effect of a non-cross-linked polypropylene base polymer in the inner semiconducting layer, a polypropylene-based semi-conducting polymer mixture was prepared. The mixture comprised 75 wt% base polymer material and 25 wt% carbon black as the conductive filler. The base polymer material comprised a 70/30 blend by weight of two commercially available propylene polymers, Bormed SC820CF and Tafmer PN3650. Bormed SC820CF is a soft random heterophasic propylene copolymer. Tafmer PN3650 is a propylene-based, propylene- compatible elastomer. The semi-conducting polymer mixture was prepared by compounding the ingredients in a BUSS compounder at 180-200 °C for 5 minutes. This mixture was then pelletized and stored for later use. A series of tests were performed with three different samples. All samples used a 1mm thick plate of LS4258DCE as the model insulation material, pressed by melting at 130 °C for 6 minutes and vulcanized at 180 °C for 12 minutes under 300 bar pressure, thus ensuring a cross-linked material. - The reference sample used no semi-conducting layer. A high voltage electrode was affixed directly to one side of the insulation plate and measurement and guard electrodes affixed to the other side.

- The comparative sample used as a semi-conducting layer a plate formed from Borlink LE0550-06 by the same press program as for the insulation layer described above. A single circular disc of this XLPE semi-conducting plate was cut and placed in contact with the insulation plate. The high-voltage electrode was affixed to the semiconducting layer. The measurement and guard electrodes were affixed to the insulation layer on the side opposite to the semi-conducting layer.

- The inventive sample used as a semi-conducting layer the polypropylene-based semi- conducting polymer mixture described above. This was pressed by melting at 170 °C for 6 minutes and further pressed at 190 °C for 11 minutes under 200 bar pressure. A single circular disc of this polypropylene semi-conducting plate was cut and placed in contact with the insulation plate. The high-voltage electrode was affixed to the semiconducting layer. The measurement and guard electrodes were affixed to the insulation layer on the opposite side to the semi-conducting layer.

In the experiment behind Fig. 5 and Fig. 6, the applied voltage across each sample was 30 kV, and the temperature was varied according to the curve shown in Fig. 5. This means that the temperatures illustrated as a function of time in Fig. 5 were used in the conductivity measurements as shown in Fig. 6. The test program starts at room temperature, then the oven temperature is increased to 70 °C for 24 hours followed by cooling to room temperature and heating back to 70 °C and consequently 90 °C; finally the system is cooled down to room temperature. During the whole thermal cycling program a constant voltage of 30 kV is applied to the samples and the leakage current is measured. Using the sample thickness,

measurement electrode area, voltage and leakage current the DC conductivity of the samples was calculated. The results for the three different samples are shown in Fig. 6. As described above, for the comparative sample, the semiconducting plate was made of a XLPE containing Vul-Cup^®. For the inventive sample, the semiconducting plate was made of a polypropylene-based base polymer material without added cross-linking agent. As seen in Figure 6, the measured conductivity of the comparative sample is highest due to the substances originating from the inner semi-conducting material with Vul-Cup^®. Note that the conductivity scale in Fig. 6 is logarithmic.

The leakage current was measured for each sample and apparent conductivity was calculated for each sample as illustrated in Fig. 6. An arrow 61 points at a line which shows the reference sample of insulation layer only. An arrow 65 points at a line which shows the inventive sample of a non-cross-linked polypropylene semi-conducting layer together with an XLPE insulation layer, and this corresponds to the cable comprising an insulation system according to the present invention. An arrow 63 points at a line which shows the comparative sample of a Vul- Cup cross-linked XLPE semi-conducting layer together with an XLPE insulation layer, and this corresponds to prior art XLPE insulation systems. Comparing the apparent conductivity of the samples the following observations were made:

At 70 °C the conductivity of both the inventive and reference samples is clearly higher than for the reference sample, but in the case of the comparative sample the conductivity is considerably higher. The 24 h value of the reference sample is around 3 fS/m, for the inventive sample it is 26.5 fS/m and for the comparative sample it is as high as 175.5 fS/m which is around 60 fold higher than the reference sample and around 7 fold higher than the inventive sample.

- The comparative sample demonstrates peaks in the apparent conductivity after cooling from 70 °C and even more so from 90 °C onwards. This effect is so strong that, after having been at 90 °C, even after cooling to room temperature the conductivity is higher than the 70 °C value measured after 24 h. This effect is expected to happen in prior art XLPE HVDC cables close to the inner semi-conducting layer and can have major consequences on the electric field distribution in the cable during cooling after being at 90 °C. This is a major hinder to such prior art cables achieving 90 °C conductor temperature. In the case of the inventive sample this conductivity peak effect is not present to the same extent as for the comparative sample. Therefore, this indicates that the non- cross-linked polypropylene semi-conducting material has a smaller effect on the insulation system conductivity as compared to XLPE. It is notable that even for the Polypropylene sample, a peak in the conductivity is observed after 90 °C, but this peak has a different nature and decays much faster. The polypropylene-based base polymer material used in the sample also meets the required mechanical requirements for a semiconducting layer in a cable insulation system.

Considering the facts and problems the present inventors have realized and the measurement results shown above, the effect of using different inner semi-conducting materials can be evaluated and conclusions can be made.

Since the by-products concentration originating from the inner semi-conducting layer is normally higher in the inner parts of a cable insulation system, this can cause a local increase of conductivity in the inner parts of the cable insulation system. As shown in the results, chemicals from inner semi-conducting layers cross-linked with Vul-Cup^® affect the apparent conductivity of XLPE considerably. This means that in the prior art cable with an inner semiconducting layer containing Vul-Cup^®, the conductivity of the inner parts of the insulation system will be much higher than the conductivity of the outer parts. This will lead to a less uniform electric field in which the field at the inner parts is reduced and instead the field at the outer parts will increase. This will increase the local field at the outer parts of the insulation system, specifically in the interface to the outer semi-conducting layer. Hence the risk of failure is increased.

Besides, considering the abnormal increase of apparent conductivity during cooling in the comparative sample comprising XLPE in contact with Vul-Cup^® cross-linked semi-conducting material, such an insulation system will lead to dynamics, i.e. variations, in the electric field and to increased risk of instability in the insulation system.

Considering the results mentioned above, and noting the conductivity of XLPE in contact with a non-cross-linked polypropylene semi-conducting material, it is clear that using a non-cross- linked polypropylene material as the inner semi-conducting layer will help to make the electric field in the cable more uniform. This will lead to a cable with better field distribution and therefore a more robust insulation system.

Since the outer semi-conducting layer is effectively degassed during the cable heat-treatment, the negative influence of the Vul-Cup by-products from the outer semi-conducting layer is less than compared to the inner semi-conducting layer. Therefore it is possible to replace only the inner semi-conducting layer with a polypropylene alternative, and use the prior-art XLPE semiconducting material in the outer semi-conducting layer.

It should be understood that the above description of preferred embodiments has been made in order to exemplify the invention, and that alternative solutions will be obvious for a person skilled in the art, however without departing from the scope of the invention as defined in the appended claims supported by the description and the drawings.

Claims

1. Electric power cable (1) comprising a metal conductor (2) and an electric insulation system (12) radially surrounding the conductor (2), which insulation system (12) comprises: - an inner semi-conducting layer (4) comprising a first base polymer material and a conductive filler, wherein the inner semi-conducting layer (4) surrounds the conductor (2);

an insulation layer (6) comprising a second base polymer material, wherein the insulation layer (6) is in contact with and surrounds the inner semi-conducting layer (4) radially outwards; and

an outer semi-conducting layer (8) comprising a third base polymer material and a conductive filler, wherein the outer semi-conducting layer is in contact with and surrounds the insulation layer (6) radially outwards,

characterized in that the first base polymer material and the second base polymer material are different from each other, and the first base polymer material comprises a non-cross-linked polypropylene-based base polymer, and the second base polymer material comprises a cross-linked polyethylene-based base polymer.

2. Electric power cable according to claim 1, wherein the non-cross-linked polypropylene- based base polymer of the first base polymer material comprises an isotactic polypropylene, a polypropylene copolymer, a polypropylene terpolymer, or mixtures thereof, preferably a polypropylene copolymer.

3. Electric power cable according to claim 1 or 2, wherein the polyethylene-based base polymer of the second base polymer material comprises low density polyethylene, ultra-low density polyethylene, linear low density polyethylene, high density polyethylene, ultra-high density polyethylene, or a mixture thereof, preferably low density polyethylene.

4. Electric power cable according to any one of the preceding claims, wherein the

polyethylene-based base polymer of the second base polymer material is cross-linked by an organic peroxide cross-linking agent, preferably selected from dicumyl peroxide, bis(t-butylperoxyisopropyl)benzene, ί-butyl cumyl peroxide, 2,5-di(t-butylperoxy)-2,5- dimethylhexane, n-butyl-4,4'-di(i-butylperoxy)valerate, l,l'-di(t-butylperoxy)-3,3,5- trimethylcyclohexane, or mixtures thereof, most preferably dicumyl peroxide.

5. Electric power cable according to any one of the preceding claims, wherein the third base polymer material comprises a non-cross-linked polypropylene-based base polymer.

6. Electric power cable according to claim 5, wherein the non-cross-linked polypropylene- based base polymer of the third base polymer comprises an isotactic polypropylene, a polypropylene copolymer, a polypropylene terpolymer, or mixtures thereof, preferably a polypropylene copolymer.

7. Electric power cable according to any one of claims 1-4, wherein the third base

polymer material comprises a cross-linked polyethylene-based base polymer.

8. Electric power cable according to claim 7, wherein the polyethylene-based base

polymer of the third base polymer material comprises a low density polyethylene, ultra-low density polyethylene, linear low density polyethylene, high density polyethylene, ultra-high density polyethylene, or a mixture thereof, preferably low density polyethylene.

9. Electric power cable according to claim 8, wherein the polyethylene-based base

polymer of the third base polymer is cross-linked by an organic peroxide cross-linking agent, preferably selected from bis(i-butylperoxyisopropyl)benzene, i-butyl cumyl peroxide, 2,5-di(t-butylperoxy)-2,5-dimethylhexane, n-butyl-4,4'-di(i- butylperoxy)valerate, l,l'-di(i-butylperoxy)-3,3,5-trimethylcyclohexane, or mixtures thereof, most preferably bis(i-butylperoxyisopropyl)benzene.

10. Electric power cable according to any one of the preceding claims, wherein the first and the second semi-conducting layers comprise conductive filler in an amount of from

10 to 40 % by weight, based on the total weight of the first and/or the second semiconducting layer, respectively.

11. Electric power cable according to any one of the previous claims, wherein the inner semi-conducting layer and/or outer semi-conducting layer comprises carbon black as the conductive filler and at most 0.2 % by weight of carbon nanotubes.

12. Electric power cable according to any one of the previous claims, wherein the

insulation layer does not comprise inorganic filler.

13. Electric power cable according to any one of the preceding claims, wherein the cable (1) is a high voltage direct current (HVDC) cable having a rated voltage of 50kV or higher.

14. Process for the production of an electric power cable (1) according to any one of claims 1-13 comprising the steps of: i) providing a conductor (2);

ii) extruding an inner semi-conducting layer (4) comprising a first base polymer material comprising a non-cross-linked polypropylene-based base polymer to surround the conductor (2) radially outwards;

iii) extruding an insulation layer (6) comprising a second base polymer material comprising a cross-linked polyethylene-based base polymer, to be in contact with the first semi-conducting layer (4) and to surround the first semiconducting layer (4) radially outwards;

iv) extruding an outer semi-conducting layer (8) comprising a third base polymer material to be in contact with the insulation layer (6) and to surround the insulation layer (8) radially outwards;

v) subjecting the cable (1) to a curing procedure.

15. Process according to claim 14, wherein steps ii), iii) and iv) of extruding the inner semiconducting layer, insulation layer and outer semi-conducting layer are performed as a triple extrusion.

16. Process according to any one of claims 14-15 comprising a further step of:

vi) heat treating and degassing the cable to remove cross-linking by-products.