A power cable and a process for producing a power cable

Abstract

A power cable (1, 101) comprising: a conductor (2, 102) extending along a centre axis (C); an insulation system (5, 105) including at least a first semiconducting layer (3, 103) provided around the conductor (2, 102), and an insulation layer (4, 104) provided around the first semiconducting layer (3, 103); wherein the insulation layer (4, 104) is crosslinked or not crosslinked and comprises: ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol and LDPE or XLPE.

Description

TECHNICAL FIELD

[0001] The present disclosure generally relates to power cables.BACKGROUND

[0002] A typical power cable for medium and high voltage applications comprises a conductor surrounded by at least an inner semiconductive layer, an insulation layer and optionally an outer semiconductive layer, in that order. The power cables are typically produced by extruding the layers onto the conductor.

[0003] The polymer material in one or more of the layers of the power cable may be thermosetting, typically crosslinked, to improve e.g. heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance. However, crosslinking the polymer material is associated with several drawbacks, e.g. the need for degassing volatile compounds during the production of the power cable and difficulties in recycling the power cable.

[0004] Thermoplastic polymer material offers several advantages compared to a thermosetting polymer material, such as omitted degassing step and increased recyclability. However, the absence of a crosslinked material can lead to impaired material properties of the power cable. Thus, there is a need for power cables comprising polyolefin compositions which avoid, or at least reduces, some of the disadvantages associated with crosslinked material, but which also offer attractive material properties.

[0005] Thus, there is a need for power cables comprising polyolefin compositions which avoid the disadvantages associated with crosslinked material, but which also offer attractive material properties. There is also a need for power cables having crosslinked polymer material with improved electrical properties and / or reduced disadvantages associated with conventional crosslinked material.SUMMARY

[0006] A general object of the present disclosure is to provide a power cable that solves or at least mitigates the problems of the prior art.

[0007] According to a first aspect, there is hence provided a power cable comprising: a conductor extending along a centre axis; an insulation system including at least a first semiconducting layer provided around the conductor, and an insulation layer provided around the first semiconducting layer; wherein the insulation layer is not crosslinked and comprises: a) 50-84 wt% LDPE; b) 16-45 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, wherein the weight percentages are based on the insulation layer as a whole.

[0008] Hereby, an advantageous insulation layer is provided. Thus, an improved power cable is provided. Owing to the composition of the insulation layer, and in particular the polymer combination of LDPE and UHMWPE and its relative amount in the insulation layer, an insulation layer with improved material properties is provided. In particular, the insulation layer according to the second aspect may exhibit an advantageously high storage modulus at high temperatures (typically 120 °C). Without being bound by any theory, it is believed that the UHMWPE forms an additional physical network within the LDPE which contributes to the advantageously high storage modulus at high temperatures. Moreover, the insulation layer according to the first aspect may exhibit a combination of an advantageously low storage modules at low temperatures (typically at 30 °C) and an advantageously high storage modulus at high temperatures. Without being bound by any theory, it is believed that the relatively high amount of UHMWPE between 16 and 45 wt% contributes to the well-balanced combination of flexibility at lower temperatures and creep resistance at higher temperature of the insulation layer.

[0009] Storage modulus is a measure of the stored energy in a material during deformation, representing the elastic, or recoverable, portion of the material's response in dynamic mechanical analysis, DMA. It reflects the material's ability to store energy and return to its original shape after deformation. The storage modulus is a dynamic property specific to viscoelastic materials, e.g. the insulation layer of the present disclosure, and is measured under dynamic, oscillatory, conditions, wherein it is dependent on the frequency of the applied load. The storage modulus indicates how much energy is stored in the material and is recoverable during each deformation cycle.

[0010] It should be noted that the LDPE referred to herein is a low density polyethylene, typically a homopolymer of ethylene (LDPE homopolymer). The LDPE may e.g. have a density between 915 and 940 kg / m 3< , or between 917 and 930 kg / m 3< . Density is measured according to ISO 1183-1:2019.

[0011] According to one embodiment, the insulation layer comprises: a) 65-82 wt% LDPE, such as 70-82 wt% LDPE; b) 18-35 wt% UHMWPE, such as 18-30 wt% UHMWPE, wherein the weight percentages are based on the insulation layer as a whole. Hereby, the above mentioned improved material properties related to the storage modulus at low and / or high temperatures may be further improved. For example, the insulation layer comprises about 30 wt% of UHMWPE, or above 16 wt%, such as above 18 wt%, and below 45 wt% or below 35 wt%, such as below 30 wt% of UHMWPE.

[0012] According to one embodiment, the LDPE and UHMWPE constitutes at least 90 wt% of the insulation layer, such as at least 95 wt% of the insulation layer, such as at least 97 wt% of the insulation layer, based on the weight of the insulation layer as a whole.

[0013] According to one embodiment, the insulation layer has a storage modulus of between 300 and 500 MPa when measured at 30 °C, such as between 350 and 475 MPa, such as between 375 and 450 MPa, using the DMA Storage modulus method as defined herein. The DMA storage modulus method is described later. Thus, the insulation layer may exhibit an advantageously low storage modules ("flexibility") at low temperatures (at 30 °C). For example, the insulation layer may have a storage modulus of between 400 and 450 MPa when measured at 30 °C.

[0014] According to one embodiment, the insulation layer has a storage modulus of between 0.1 and 5 MPa when measured at 120 °C, such as between 0.2 and 5 MPa, such as between 0.3 and 2 MPa, using the DMA Storage modulus method as defined herein. Thus, the insulation layer may exhibit an advantageously high storage modulus at high temperatures (at 120 °C). For example, the insulation layer may have a storage modulus of between 0.75 and 2 MPa when measured at 120 °C. Typically, high storage modulus result in high creep-resistance.

[0015] Typically, the insulation layer has a storage modulus of between 300 and 500 MPa, such as between 350 and 475 MPa, such as between 375 and 450 MPa or of between 400 and 450 MPa when measured at 30 °C, and a storage modulus of between 0.1 and 5 MPa, such as between 0.2 and 5 MPa, such as between 0.3 and 2 MPa or of between 0.75 and 2 MPa when measured at 120 °C, using the DMA storage modulus method as defined herein. Thus, the insulation layer may exhibit a combination of an advantageously low storage modules at low temperatures (at 30 °C) and an advantageously high storage modulus at high temperatures (at 120 °C). Thus, a well-balanced combination of desirable storage modules at low and high temperatures is provided.

[0016] According to one embodiment, the insulation system is not crosslinked. Hereby, not only the insulation layer, but also the whole insulation layer is not crosslinked and may benefit from associated advantages (e.g. no degassing during the production of the insulation layer or insulation system, increased recyclability etcetera).

[0017] According to one embodiment, the insulation layer is thermoplastic, i.e. is formed of a thermoplastic composition. For example, the insulation layer does not comprise peroxide, or crosslinking residues. It should be noted that any layer of the power cable, such as e.g. the inner semiconductive layer, may additionally be thermoplastic. The first semiconducting layer typically comprises an electrically conductive compound, such as e.g. carbon black. According to one embodiment, the whole insulation system is thermoplastic. For example, the insulation system does not comprise peroxide, or crosslinking residues.

[0018] According to a second aspect of the present disclosure, there is provided a power cable comprising: a conductor extending along a centre axis; an insulation system including at least a first semiconducting layer provided around the conductor, and an insulation layer provided around the first semiconducting layer; wherein the insulation layer is crosslinked and comprises: a) 80-99.5 wt% XLPE; b) 1-20 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, wherein the weight percentages are based on the insulation layer as a whole.

[0019] Hereby, an advantageous insulation layer is provided. Thus, an improved power cable is provided. Owing to the composition of the insulation layer, and in particular the polymer combination of XLPE and UHMWPE and its relative amount in the insulation layer, an insulation layer with improved material properties is provided. In particular, the insulation layer according to the second aspect may exhibit an advantageously high storage modulus at high temperatures (typically 120 °C). Without being bound by any theory, it is believed that the UHMWPE forms an additional physical network within the XLPE which contributes to the advantageously high storage modulus at high temperatures. Moreover, by the addition of the UHMWPE to the XLPE, less crosslinking agent, such as peroxide, may be used for achieving corresponding material properties as compared to a corresponding insulation layer with no, or a different amount of, UHMWPE. It should be noted that also the UHMWPE may be crosslinked by the crosslinking agent.

[0020] It should be noted that the XLPE referred to herein is a low density polyethylene, typically a homopolymer of ethylene (LDPE homopolymer), which is crosslinked (i.e. has been crosslinked by using a crosslinking agent). In other words, the insulation layer has been crosslinked, preferably by using peroxide as crosslinking agent such that the LDPE is crosslinked into crosslinked polyethylene, XLPE. The LDPE used for the XLPE may e.g. have a density between 915 and 940 kg / m 3< , or between 917 and 930 kg / m 3< . Density is measured according to ISO 1183-1:2019. Thus, the XLPE is sometimes referred to as LDPE with crosslinking agent, e.g. LDPE and peroxide or LDPE and DCP. For example, the composition used for producing the XLPE may comprise 1 wt% peroxide (e.g. DCP).

[0021] According to one embodiment, the insulation layer comprises: a) 82-98 wt% XLPE, such as 85-94 wt% or 89-98 wt% XLPE; b) 3-18 wt% UHMWPE, such as 5-15 wt% or 5-10 wt% UHMWPE, wherein the weight percentages are based on the insulation layer as a whole. Hereby, the above mentioned improved material properties related to the storage modulus at high temperatures may be further improved. Moreover, by the relatively low amount of UHMWPE between 1 and 20 wt%, or even between 3 and 18 wt% or between 5 and 15 wt%, less crosslinking agent, such as peroxide, may be used for achieving corresponding material properties as compared to a corresponding insulation layer with no, or a different amount of, UHMWPE. For example, the insulation layer comprises about 10 wt% of UHMWPE, or above 1 wt%, such as above 5 wt%, and below 20 wt% or below 15 wt%, such as between 5 wt% and 10 wt% of UHMWPE.

[0022] According to one embodiment, the XLPE and UHMWPE constitutes at least 90 wt% of the insulation layer, such as at least 95 wt% of the insulation layer, such as at least 97 wt% of the insulation layer, based on the weight of the insulation layer as a whole.

[0023] According to one embodiment, the insulation layer has a storage modulus of between 250 and 350 MPa when measured at 30 °C, such as between 275 and 325 MPa, such as between 300 and 325 MPa, using the DMA Storage modulus method as defined herein. Thus, the insulation layer may exhibit an advantageously low storage modules ("flexibility") at low temperatures (at 30 °C).

[0024] According to one embodiment, the insulation layer has a storage modulus of between 0.1 and 1 MPa when measured at 120 °C, such as between 0.2 and 0.8 MPa, such as between 0.3 and 0.7 MPa, using the DMA Storage modulus method as defined herein. Thus, the insulation layer may exhibit an advantageously high storage modulus at high temperatures (at 120 °C).

[0025] Typically, the insulation layer has a storage modulus of between 250 and 350 MPa when measured at 30 °C, such as between 275 and 325 MPa, such as between 300 and 325 MPa when measured at 30 °C, and a storage modulus of between 0.1 and 1 MPa when measured at 120 °C, such as between 0.2 and 0.8 MPa, such as between 0.3 and 0.7 MPa when measured at 120 °C, using the DMA storage modulus method as defined herein. Thus, the insulation layer may exhibit a combination of an advantageously low storage modules at low temperatures (at 30 °C) and an advantageously high storage modulus ("creep resistance") at high temperatures (at 120 °C). Thus, a well-balanced combination of desirable storage modules at low and high temperatures is provided.

[0026] According to one embodiment, the insulation layer has been crosslinked by using peroxide as crosslinking agent. For example, the peroxide is dicumyl peroxide or DCP. As mentioned, earlier, by the addition of the UHMWPE to the XLPE, less crosslinking agent, such as peroxide or DCP, may be used for achieving corresponding material properties as compared to a corresponding insulation layer with no, or a different amount of, UHMWPE. Also, the addition UHMWPE contributes in requiring less crosslinking agent (e.g. DCP) in XLPE based insulation layer as compared to an insulation layer with no UHMWPE.

[0027] According to one embodiment, applicable to both the first and second aspects of the disclosure, the UHMWPE has a Mw of 2 000 000 - 8 000 000 g / mol. However, according to another example, the UHMWPE has a Mw of 1 500 000 - 8 000 000 g / mol, such as between 3 000 000 g / mol and 6 000 000 g / mol.

[0028] According to one embodiment, applicable to both the first and second aspects of the disclosure, the insulation layer is an extruded insulation layer, e.g. a co-extruded insulation layer. For example, the insulation layer may have been extruded onto the first semiconducting layer. Alternatively, the insulation layer may have been co-extruded together with the first semiconducting layer.

[0029] According to one embodiment, applicable to both the first and second aspects of the disclosure, the extruded insulation layer has been produced by compounding and subsequent extrusion. Thus, the insulation layer may have been obtained from a composition comprising a) 50-84 wt% LDPE; b) 16-45 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, or from a composition comprising a) 80-99.5 wt% XLPE; b) 1-20 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, the weight percentages being based on the composition as a whole.

[0030] According to one embodiment, applicable to both the first and second aspects of the disclosure, the insulation layer, or the composition used for obtaining the insulation layer, comprises at most 3 wt% of one or more additives, such as antioxidant(s), stabilizer(s), processing aid(s), inorganic filler(s), the weight percentage being based on the insulation as a whole, or based on the composition as a whole.

[0031] According to one embodiment, applicable to both the first and second aspects of the disclosure, the insulation layer, or the composition used for obtaining the insulation layer, consists of LDPE, UHMWPE and / or one or more additives, or consists of XLPE, UHMWPE and / or one or more additives, typically in amounts specified above.

[0032] According to one embodiment, applicable to both the first and second aspects of the disclosure, the power cable is a medium voltage or high voltage power cable, preferably a medium voltage or high voltage power cable.

[0033] According to one embodiment, applicable to both the first and second aspects of the disclosure, the power cable further comprises a second semiconducting layer. For example, the first semiconducting layer may be an inner semiconducting layer arranged in direct contact with, and radially inside of, the insulation layer, and the second semiconducting layer may be an outer semiconducting layer arranged radially outside of the insulation layer.

[0034] According to one embodiment, applicable to both the first and second aspects of the disclosure, the second semiconducting layer is comprised in the insulation system. The second semiconducting layer may be arranged in direct contact with, and radially outside of, the insulation layer. The second semiconducting layer may form an insulation screen. The first semiconducting layer may form a semiconducting conductor shield. The first semiconducting layer may be arranged closest to the conductor (i.e. closest with regards to the insulation system).

[0035] According to a third aspect of the present disclosure, there is provided a process for producing a power cable comprising the steps of: providing a conductor; applying an insulation system surrounding the conductor, wherein the insulation system includes at least a first semiconducting layer and an insulation layer surrounding the first semiconducting layer; wherein the insulation layer is not crosslinked and comprises: a) 50-84 wt% LDPE; b) 16-45 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, wherein the weight percentages are based on the insulation layer as a whole, or wherein the insulation layer is crosslinked and comprises a) 80-99.5 wt% XLPE; b) 20 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, wherein the weight percentages are based on the insulation layer as a whole.

[0036] According to one embodiment, the insulation system is applied by extrusion, such as co-extrusion. For example, the insulation layer may be extruded onto the first semiconducting layer. Alternatively, the insulation layer may be co-extruded together with the first semiconducting layer.

[0037] According to one embodiment, the extruded insulation layer has been produced by compounding and subsequent extrusion. Thus, the insulation layer may have been obtained from a composition comprising: a) 50-84 wt% LDPE; and b) 16-45 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol and not using any crosslinking agent (for providing a non-crosslinked insulation layer), or from a composition comprising a) 80-99.5 wt% XLPE; and b) 20 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol and using a crosslinking agent (for providing a crosslinked insulation layer), wherein the weight percentages are based on the insulation layer as a whole.

[0038] According to one embodiment, the LDPE and UHMWPE have been premixed prior to the compounding without a crosslinking agent. It is advantageous to premix the LDPE and UHMWPE to obtain a good mixture. In case of crosslinking, the crosslinking agent, such as peroxide (e.g. DCP) is added subsequently. The addition of crosslinking agent is preferably achieved by a soaking method.

[0039] The examples and embodiments discussed above in connection to the first and second aspects applies mutatis mutandis to the third aspect.

[0040] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise.BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 schematically shows a radial cross section of a power cable of an example embodiment; Fig. 2 schematically shows a side view of at least a part of the power cable of Fig. 1; and Fig. 3 schematically shows a radial cross section of a power cable of an example embodiment; Fig. 4 schematically shows a side view of at least a part of the power cable of Fig. 3; and Fig. 5 is a flow-chart showing the steps of a process for producing a power cable according to at least some example embodiments; Fig. 6 is a graph showing the Dynamic Mechanical Analysis (DMA) storage modulus (E') for various comparative and inventive example samples; Fig. 7 is a graph showing the DMA storage modulus for various comparative and inventive example samples; Fig. 8 is a graph showing the DMA storage modulus for various comparative and inventive example samples; and Fig. 9 is a graph showing the DMA storage modulus for various comparative and inventive example samples. DETAILED DESCRIPTION

[0042] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description.

[0043] Figs. 1 and 2 show an example of a power cable 1. The power cable 1 is in this example a single power core power cable. However, it should be mentioned that the power cable could comprise several identical power cores to form a multi-core power cable, such as three-phase power cable. The power cable 1 may be a HVDC power cable or a HVAC power cable, for example for voltages higher than 35 kV, or higher than 110 kV, or higher than 450 kV, or higher than 550 kV, or higher than 800 kV. The power cable 1 may alternative be a medium power cable (DC or AC), for example for voltages between 5 kV and 35 kV. However, according to one alternative embodiment, the power cable 1 is a medium power cable (or high voltage cable in the medium voltage span) for voltages between 1 kV and 35 kV.

[0044] In Fig. 1, a radial cross section of an embodiment of a power cable 1 is shown. The radial cross section is a cross section in the radial direction r of the power cable 1, i.e. a plane perpendicular to a centre axis of the power cable 1 (extending into the paper). The power cable 1 may e.g. be defined by cylindrical coordinates (by a radial distance r, azimuth φ which is the angle along the circumferential direction, and an axial coordinated along the longitudinal axis).

[0045] In Fig. 2, a side view of the power cable 1 of Fig. 1, in which the power cable 1 extends along the centre axis C (or centre longitudinal axis).

[0046] In the following, the power cable 1 will be described with reference to both Figs. 1 and 2. The power cable 1 comprises a conductor 2 extending along the centre axis C. The conductor 2 may for example be stranded, segmental of Milliken type, solid, or a profile wire conductor. The conductor 2 may for example comprise copper or aluminum.

[0047] The power cable 1 further comprises an insulation system 5 provided around, and covering, the conductor 2.

[0048] The insulation system 5 comprises a first semiconducting layer 3 provided around the conductor 2, an insulation layer 4 provided around the first semiconducting layer 3, and optionally, a second semiconducting layer 9 provided around the insulation layer 4. Thus, the first semiconducting layer 3 surrounds the conductor 2, the insulation layer 4 surrounds the first semiconducting layer 3, and the optional second semiconducting layer 9 surrounds the insulation layer 4.

[0049] In the embodiment of Figs. 1 and 2, the insulation layer 4 is arranged to be in direct contact with, and radially outside of the first semiconducting layer 3. Moreover, the optional second semiconducting layer 9 is arranged to be in direct contact with, and radially outside of, the insulation layer 4. Thus, the first semiconducting layer 3 may form a semiconducting conductor shield and be referred to as an inner semiconducting layer. The optional second semiconducting layer 9 may form an insulation screen and be referred to as an outer semiconducting layer.

[0050] In the embodiment of Figs. 1 and 2, the power cable 1 further comprises a water barrier structure 17 surrounding the insulation system 5. Thus, in the embodiment of Fig. 1, the water barrier structure 17 is in direct contact with the insulation system 5. In the example, the water barrier structure 17 is arranged in direct contact with, and radially outside of, the optional second semiconducting layer 9. The water barrier structure 17 is typically arranged to prevent, or at least greatly reduce, water penetrating into the insulation system 5 from the outside. The water barrier structure may e.g. exhibit a water permeability of below 0.1 g / (m2*24 h) or below 0.05 g / (m2*24 h). The water barrier structure may e.g. be a tape wrapped around the insulation system 5, and / or may comprise a metallic foil.

[0051] The power cable 1 may furthermore comprise a polymeric jacket 11 provided around the insulation system 5. In the embodiment of Figs. 1 and 2, the polymeric jacket is arranged in direct contact with, and radially outside of, the water barrier structure 17. Thus, the water barrier structure 17 is arranged in between the insulation system 5 and the polymeric jacket 11. However it should be noted that the polymeric jacket 11 may be arranged in direct contact with, and radially outside of, the insulation system 5, in case the water barrier structure 17 is omitted.

[0052] The polymeric jacket 11 may e.g. comprise a polymeric material that is extruded around the water barrier structure 17 and / or the insulation system 5. The polymeric jacket 11 of the embodiment of Figs. 1 and may be wholly or partly water impermeable.

[0053] The insulation system 5 may be extruded and comprise thermosetting and / or thermoplastic polymer material. For example, the layers 3, 4, 9 of the insulation system 5 may be different in the aspect of being made of thermosetting and thermoplastic polymer material. However, at least the insulation layer 4 is made of thermoplastic polymer material and is hence not crosslinked. Thus, the insulation layer 4 typically does not comprise peroxide or other crosslinking agents or crosslinking residues. Typically, the insulations system 5 is not crosslinked. The first semiconducting layer 3, and the optional second semiconducting layer 9, typically comprises an electrically conductive compound, e.g. carbon black.

[0054] The insulation layer 4 comprises a) LDPE and b) ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol. The insulation layer may further comprise c) one or more additives. The one or more additives may include antioxidant(s), stabilizer(s), processing aid(s) and inorganic filler(s). According to one embodiment, the insulation layer 4 consists of a) LDPE and b) UHMWPE and optionally c) one or more additives.

[0055] For example, the insulation layer 4 comprises a) 50-84 wt% LDPE, b) 16-45 wt% UHMWPE, optionally together with 0 to 3 wt% one or more additives. The weight percentages are based on the insulation layer 4 as a whole. For example, the insulation layer 4 comprises a) 65-82 wt% LDPE, such as 70-82 wt% LDPE, b) 18-35 wt% UHMWPE, such as 18-30 wt% UHMWPE, and optionally together with 0 to 3 wt% of one or more additives. For example, the LDPE and UHMWPE constitutes at least 90 wt% of the insulation layer 4, such as at least 95 wt% of the insulation layer 4, such as at least 97 wt% of the insulation layer 4, based on the weight of the insulation layer as a whole.

[0056] The LDPE referred to herein is a low density polyethylene, typically a homopolymer of ethylene (LDPE homopolymer). The LDPE may e.g. have a density between 915 and 940 kg / m 3< , or between 917 and 930 kg / m 3< . Density is measured according to ISO 1183-1:2019. The UHMWPE may have a Mw of between 1 500 000, or 2 000 000 and 8 000 000 g / mol.

[0057] As will be shown in the examples later, but which is briefly mentioned here, the insulation layer 4 has advantageous material properties, such as an advantageously high storage modulus at high temperatures (typically 120 °C). It is believed that the UHMWPE forms an additional physical network within the LDPE which contributes to the advantageously high storage modulus at high temperatures. Moreover, the insulation layer 4 may exhibit a combination of an advantageously low storage modules at low temperatures (typically at 30 °C) and an advantageously high storage modulus at high temperatures, i.e. a well-balanced combination of flexibility at lower temperatures and creep resistance at higher temperatures. For example, the insulation layer exhibit a storage modulus of between 0.1 and 5 MPa when measured at 120 °C, such as between 0.2 and 5 MPa, such as between 0.3 and 2 MPa of between 0.75 and 2 MPa, using the DMA Storage modulus method as defined herein. Additionally or alternatively, the insulation layer exhibit a storage modulus of between 300 and 500 MPa when measured at 30 °C, such as between 350 and 475 MPa, such as between 375 and 450 MPa or of between 400 and 450 MPa, using the DMA Storage modulus method as defined herein.

[0058] Turning to Figs. 3 and 4 showing another example of a power cable 101. The power cable 101 is in this example a single power core power cable as that described with reference to Figs. 1 and 2. However, it should be mentioned that the power cable could comprise several identical power cores to form a multi-core power cable, such as three-phase power cable. The power cable 101 in Figs. 3 and 4 are in large corresponding to the power cable 1 of Figs. 1 and 4, why mainly the differences between the power cables 1, 101 are described in the following. Like number are used for like references, but with an additional value of "100" for each feature, i.e. "power cable 1" of Figs. 1 and 2 relates to "power cable 101" of Figs 3 and 4.

[0059] In Fig. 3, a radial cross section of an embodiment of a power cable 101 is shown, and in Fig. 4, a side view of the power cable 101 of Fig. 3, in which the power cable 101 extends along the centre axis C (or centre longitudinal axis) is shown. In the following, the power cable 101 will be described with reference to both Figs. 3 and 4. The power cable 101 comprises a conductor 102 extending along the centre axis C, an insulation system 105 provided around, and covering, the conductor 102, a water barrier structure 117 surrounding the insulation system 105 and a polymeric jacket 11 provided around the insulation system 105 (which in the examples of Figs. 3 and 4 also includes being provided around the water barrier structure 17). All layers, except for the insulation system 105 may be corresponding, or be the same, as the corresponding layers of the insulation system 5 of Figs. 1-2.

[0060] The insulation system 105 comprises a first semiconducting layer 103 provided around the conductor 102, an insulation layer 104 provided around the first semiconducting layer 103, and optionally, a second semiconducting layer 109 provided around the insulation layer 104, in a corresponding structure as for the power cable 1 of Figs. 1 and 2.

[0061] The insulation system 105 may be extruded and comprise thermosetting and / or thermoplastic polymer material. For example, the layers 103, 104, 109 of the insulation system 105 may be different in the aspect of being made of thermosetting and thermoplastic polymer material. However, at least the insulation layer 104 is made of thermosetting polymer material and is hence crosslinked. Thus, the insulation layer 104 typically comprises peroxide or other crosslinking agents or crosslinking residues. Typically, the insulation system 105 is crosslinked. The first semiconducting layer 103, and the optional second semiconducting layer 109, typically comprises an electrically conductive compound, e.g. carbon black.

[0062] The insulation layer 104 comprises a) XLPE and b) ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol. The insulation layer may further comprise c) one or more additives. The one or more additives may include antioxidant(s), stabilizer(s), processing aid(s) and inorganic filler(s). According to one embodiment, the insulation layer 104 consists of a) XLPE and b) UHMWPE and optionally c) one or more additives.

[0063] For example, the insulation layer 104 comprises a) 80-99.5 wt% XLPE, b) 1-20 wt% UHMWPE, optionally together with 0 to 3 wt% one or more additives. The weight percentages are based on the insulation layer 104 as a whole. For example, the insulation layer 4 comprises a) 82-98 wt% XLPE, such as 85-94 wt% or 89-98 wt% XLPE, b) 3-18 wt% UHMWPE, such as 5-15 wt% or 5-10 wt% UHMWPE, and optionally together with 0 to 3 wt% of one or more additives. For example, the XLPE and UHMWPE constitutes at least 90 wt% of the insulation layer 104, such as at least 95 wt% of the insulation layer 104, such as at least 97 wt% of the insulation layer 104, based on the weight of the insulation layer as a whole.

[0064] The XLPE is a crosslinked low density polyethylene, typically a homopolymer of ethylene (LDPE homopolymer) which is crosslinked (i.e. has been crosslinked by using a crosslinking agent such as peroxide). The LDPE used for the XLPE may e.g. have a density between 915 and 940 kg / m 3< , or between 917 and 930 kg / m 3< . Density is measured according to ISO 1183-1:2019. The UHMWPE may have a Mw of between 1 500 000, or 2 000 000 and 8 000 000 g / mol.

[0065] As will be shown in the examples later, but which is briefly mentioned here, the insulation layer 104 has advantageous material properties, such as an advantageously high storage modulus at high temperatures (typically 120 °C). It is believed that the UHMWPE forms an additional physical network within the XLPE which contributes to the advantageously high storage modulus at high temperatures. Moreover, by the addition of the UHMWPE to the XLPE, less crosslinking agent, such as peroxide, may be used for achieving corresponding material properties as compared to a corresponding insulation layer with no, or a different amount of, UHMWPE. It should be noted that also the UHMWPE may be crosslinked by the crosslinking agent. For example, the insulation layer exhibit a storage modulus of between 0.1 and 5 MPa when measured at 120 °C, such as between 0.1 and 1 MPa when measured at 120 °C, such as between 0.2 and 0.8 MPa, such as between 0.3 and 0.7 MPa, using the DMA Storage modulus method as defined herein. Additionally or alternatively, the insulation layer exhibit a storage modulus of between 250 and 350 MPa when measured at 30 °C, such as between 275 and 325 MPa, such as between 300 and 325 MPa, using the DMA Storage modulus method as defined herein.

[0066] Each one of the insulation layers 4, 104 is, as previously mentioned, preferably an extruded insulation layer, i.e. has been produced by extrusion, such as co-extrusion. Preferably, the extruded insulation layer has been produced by compounding, preferably by premixing the LDPE and UHMWPE, or XLPE and UHMWPE, prior to the compounding.

[0067] Fig. 5 is a flow-chart describing a process for producing a power cable according to at least some examples. The power cable is for example that of Figs. 1 and 2, or that of Figs. 3 and 4. Thus, in the following, reference is made to both power cable 1 and power cable 101.

[0068] In a first step S10, a conductor 2, 102 is provided. The conductor 2, 102 may for example be stranded, segmental of Milliken type, solid, or a profile wire conductor, and / or comprise copper or aluminum.

[0069] In a second step S20, being subsequent to the first step S10, an insulation system 5, 105 is applied to surround the conductor 2, 102. The insulation system includes at least a first semiconducting layer 3, 103 and an insulation layer 4, 104 surrounding the first semiconducting layer 3, 103. The insulation system 5, 105 may e.g. be applied to surround the conductor 2, 102 by extrusion, or co-extrusion. For example, at least two of the layers of the insulation system 5, 105 is co-extruded onto the conductor 2, 120.

[0070] As previously mentioned, the insulation layer may either be a non-crosslinked layer comprising 50-84 wt% LDPE and 16-45 wt% UHMWPE, or the insulation layer may be a crosslinked layer comprising 80-99.5 wt% XLPE and 1-20 wt% UHMWPE.Examples

[0071] In the following, various samples were prepared and analyzed by specific material parameters. Each sample correspond to a previously described insulation layer.Example 1

[0072] LDPE (LD 101BA from ExxonMobil) was mixed with UHMWPE (429015 from Sigma-Aldrich) with an average Mw of 3 000 000-6 000 000 g / mol. A sample containing only UHMWPE and a sample containing only LDPE were also prepared. The samples were produced by melt compounding at 180 °C, 50 rpm for 6 minutes using a microcompounder (HAAKE ™< MiniCTW Micro-Conical Twin Screw Compounder). No crosslinking agent was added.

[0073] The various samples of example 1 are summarized in Table 1. Table 1. Compositions of the various samplesCE1CE2CE3CE4CE5IE1UHMW PELDPEUHMWP E:LDPE (1:99)UHMWP E:LDPE (5:95)UHMWP E:LDPE (10:90)UHMWP E:LDPE (30:70)UHMW PE wt%100151030LDPE wt%10099959070CE: Comparative Example; IE: Inventive Example

[0074] With reference to Fig. 6, the DMA storage modulus is shown on a logarithmic scale (Log storage modulus vs sample temperature) for the samples of Table 1. As is shown in Fig. 6, IE1 exhibits improved storage modulus at high temperatures (about 120 °C) compared with CE2, CE3, CE4 and CE5. Moreover, as compared to CE1, IE1 exhibits improved storage modulus at low temperatures (about 30 °C), almost in parity with CE2, CE3, CE4 and CE5. Thus, IE1 exhibit a well-balanced combination of flexibility at lower temperatures and creep resistance at higher temperature. Notably, the storage modulus of IE1 at about 120 °C is of several orders of magnitude higher than the storage modulus of CE2, CE3, CE4 and CE5. At an even higher temperature (about 145 °C), the storage modulus of IE1 is higher than CE2, CE3, CE4 and CE5. That is, it has been surprisingly realized that mixing of UHMWPE with LDPE in a ratio of the inventive example provides a synergistic composition having an even higher storage modulus than LDPE, or LDPE mixed with UHMWPE, but with a lower content of UHMWPE. For CE5, i.e. at a UHMWPE content of about 10 wt%, there is no significant difference to CE3 or CE4. However, for IE1, i.e. at a UHMWPE content of about 30 wt%, there is a significant improvement. Thus, it is reasonable to assume that for a UHMWPE content of above 10 wt%, such as e.g. at, or above 16 wt%, the improvement with regards to the storage modulus at high temperatures is present.Example 2

[0075] LDPE (LD 101BA from ExxonMobil) was compounded with UHMWPE (429015 from Sigma-Aldrich) with an average Mw of 3 000 000-6 000 000 g / mol by melt compounding at 180 °C, 50 rpm for 6 minutes using a microcompounder (HAAKE ™< MiniCTW Micro-Conical Twin Screw Compounder). The peroxide (DCP) was firstly dissolved in methanol, and the compounded LDPE and UHMWPE were soaked with the solution with different concentration of DCP. The soaked samples were dried at 70 °C in an oven overnight and then hot-pressed at 180 °C and 200 bar forming crosslinked samples . A sample containing only XLPE being crosslinked LDPE (LD 101BA from ExxonMobil) with 1 wt% DCP and a sample containing only XLPE being crosslinked LDPE (LD 101BA from ExxonMobil) with 2 wt% DCP were also prepared by the above-mentioned soaking and hot-press processes.

[0076] The various samples of example 2 are summarized in Table 2. Table 2. Compositions of the various samplesCE6CE7IE2IE3IE4XLPE 1 (LDPE 1% DCP)XLPE 2 (LDPE 2% DCP)UHMWP E:LDPE: DCP (1:98:1)UHMWP E:LDPE: DCP(5:94 1)UHMWP E:LDPE: DCP (10:89:1)UHMWP E wt %1510LDPE wt%9998989489DCP %12111CE: Comparative Example; IE: Inventive Example

[0077] With reference to Figs. 7, 8 and 9, the DMA storage modulus is shown on a logarithmic scale (Log storage modulus vs sample temperature) for the samples of Table 2. As is shown when comparing the graphs of Figs. 6, 7 and 9, IE2, IE3 and IE4 exhibits improved storage modulus at high temperatures (about 120 °C) with increased amount of UHMWPE. For example, for IE4 shown in Fig. 9, the storage modulus is greatly improved compared with CE6, i.e. for the same amount of DCP (1 wt%) and even better than CE7 being crosslinked with a higher amount of DCP (2 wt%). Thus, by the addition of the UHMWPE to the XLPE (LDPE and DCP), less crosslinking agent, such as peroxide or DCP, may be used for achieving corresponding material properties, here related to the storage modulus at high temperatures, as compared to CE6 and CE7. It is believed that the UHMWPE forms an additional physical network within the XLPE which contributes to the advantageously high storage modulus at high temperatures, and the lower amount of DCP needed.Methodology

[0078] The above DMA storage modulus were performed according to the method as described in the following.

[0079] Storage modulus was measured using Dynamic Mechanical Analysis (DMA). The samples were hot-pressed (180 °C, 200 bar) to a thickness of 1 mm and the samples were then cut to pieces with length and width dimensions of 10×8 mm. DMA was carried out using a DMA1 from METTLER TOLEDO in tensile mode . A temperature sweep was conducted using a frequency of 1 Hz and oscillation displacement of 10 µm with a temperature ramp of 30 °C to 180 °C (1 °C / min).

[0080] The storage modulus at 30 and 120 °C were used as stiffness indicators. A lower storage modulus, i.e. stiffness, at low temperature (about 30 °C) is an indicator of higher flexibility. A higher storage modulus, i.e. stiffness, at high temperature (about 120 °C), is an indicator of a higher creep-resistant behaviour and / or high temperature integrity.

[0081] The DMA storage modulus may alternative be performed by ISO 6721-1 or ASTM D4065,

[0082] Any standard or qualifications mentioned in the present application are to be based on instructions valid on the date of priority of the present application.

[0083] The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims. For example, in case the power cable comprises several identical power cores to form a multi-core power cable, such as three-phase power cable, each power core may correspond to that described with reference to the power cable described in Figs. 1 and 2. Thus, each power core may at least comprise a conductor 3, an insulation system 5 as previously described. The power cores may be stranded and surrounded by a polymeric jacket.

Claims

1. A power cable (1) comprising: - a conductor (2) extending along a centre axis (C); - an insulation system (5) including at least a first semiconducting layer (3) provided around the conductor (2), and an insulation layer (4) provided around the first semiconducting layer (3); wherein the insulation layer (4) is not crosslinked and comprises: a) 50-84 wt% LDPE; b) 16-45 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, wherein the weight percentages are based on the insulation layer (4) as a whole.

2. The power cable (1) of claim 1, wherein the insulation layer (4) comprises: a) 65-82 wt% LDPE, such as 70-82 wt% LDPE; b) 18-35 wt% UHMWPE, such as 18-30 wt% UHMWPE, wherein the weight percentages are based on the insulation layer (4) as a whole.

3. The power cable (1) of any of claims 1-2, wherein the LDPE and UHMWPE constitutes at least 90 wt% of the insulation layer (4), such as at least 95 wt% of the insulation layer (4), such as at least 97 wt% of the insulation layer (4), based on the weight of the insulation layer as a whole.

4. The power cable (1) of any of claims 1-3, wherein the insulation layer (4) has a storage modulus of between 300 and 500 MPa when measured at 30 °C, such as between 350 and 475 MPa, such as between 375 and 450 MPa, using the DMA Storage modulus method as defined herein.

5. The power cable (1) of any of claims 1-4, wherein the insulation layer (4) has a storage modulus of between 0.1 and 5 MPa when measured at 120 °C, such as between 0.2 and 5 MPa, such as between 0.3 and 2 MPa, using the DMA Storage modulus method as defined herein.

6. The power cable (1) according to any of claims 1-5, wherein the insulations system (5) is not crosslinked.

7. A power cable (101) comprising: - a conductor (102) extending along a centre axis (C); - an insulation system (105) including at least a first semiconducting layer (103) provided around the conductor (102), and an insulation layer (104) provided around the first semiconducting layer (103); wherein the insulation layer (104) is crosslinked and comprises: a) 80-99.5 wt% XLPE; b) 1-20 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, wherein the weight percentages are based on the insulation layer (104) as a whole.

8. The power cable (101) of claim 7, wherein the insulation layer (104) comprises: a) 82-98 wt% XLPE, such as 85-94 wt% or 89-98 wt% XLPE; b) 3-18 wt% UHMWPE, such as 5-15 wt% or 5-10 wt% UHMWPE, wherein the weight percentages are based on the insulation layer (104) as a whole.

9. The power cable (101) of any of claims 7-8, wherein the XLPE and UHMWPE constitutes at least 90 wt% of the insulation layer (104), such as at least 95 wt% of the insulation layer (104), such as at least 97 wt% of the insulation layer (104), based on the weight of the insulation layer as a whole.

10. The power cable (101) of any of claims 7-9, wherein the insulation layer (104) has a storage modulus of between 250 and 350 MPa when measured at 30 °C, such as between 275 and 325 MPa, such as between 300 and 325 MPa, using the DMA Storage modulus method as defined herein.

11. The power cable (101) of any of claims 7-10, wherein the insulation layer (4) has a storage modulus of between 0.1 and 1 MPa when measured at 120 °C, such as between 0.2 and 0.8 MPa, such as between 0.3 and 0.7 MPa, using the DMA Storage modulus method as defined herein.

12. The power cable (101) of any of claims 7-11, wherein the insulation layer (4) has been crosslinked by using peroxide as crosslinking agent.

13. The power cable (1, 101) of any of claims 1-12, wherein the UHMWPE has a Mw of 2 000 000 - 8 000 000 g / mol.

14. The power cable (1, 101) of any of claims 1-13, being a medium voltage or high voltage power cable.

15. A process for producing a power cable (1, 101) comprising the steps of: - providing a conductor (2, 102); - applying an insulation system (5, 105) surrounding the conductor (2, 102), wherein the insulation system includes at least a first semiconducting layer (3, 103) and an insulation layer (4, 104) surrounding the first semiconducting layer (3, 103); wherein the insulation layer (4) is not crosslinked and comprises: a) 50-84 wt% LDPE; b) 16-45 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, wherein the weight percentages are based on the insulation layer (4) as a whole, or wherein the insulation layer (104) is crosslinked and comprises: a) 80-99.5 wt% XLPE; b) 1-20 wt% ultra-high molecular weight polyethylene, UHMWPE, having a Mw of at least 1 000 000 g / mol, wherein the weight percentages are based on the insulation layer (104) as a whole.

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