HVDC cable accessory based on thermoplastic elastomer

Abstract

To provide a method for improving the breakdown strength of a high voltage direct current (HVDC) cable system that is assembled in field, and an HVDC cable accessory.SOLUTION: An HVDC cable accessory (an HVDC cable joint 1) comprising an electrical stress suppressing layer (3) configured to be connected to an HVDC cable (4) at a position is previously manufactured. The electrical stress suppressing layer (3) comprises a thermoplastic elastomer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to the field of high voltage direct current (HVDC) cable accessories, and more particularly to the interface between HVDC cable accessories and HVDC cables.

Background Art

[0002] High voltage direct current (HVDC) cables are advantageously used because they have the ability to transmit large amounts of power over long distances, and have less power loss and higher stability compared to alternating current power cables.

[0003] The installation of HVDC cables requires termination of the cables by cable terminations, and often also requires connection of multiple cables, for example via cable joints due to long distances.

[0004] When connecting an HVDC cable to a cable accessory such as a cable joint or a cable termination, the outer layer of the HVDC cable including the semiconductive screen is typically removed at the position of cable joining or termination (the interface between the cable and the accessory) to facilitate the connection. The semiconductive screen ensures that the electric field generated by the conductor is confined inside the conductor insulator of the HVDC cable. When the semiconductive screen is removed, a strong electric field is formed at the edge of the screen. If the electric field is not relaxed, damage may occur to the insulation layer of the HVDC cable, leading to dielectric breakdown. To prevent damage that can be caused by the electric field, cable joints and terminations are provided with a stress suppression layer, such as a rubber body. The stress suppression layer reduces the electric field stress at the interface between the cable and the accessory to an acceptable level, thus preventing partial discharge and insulation problems.

[0005] When the HVDC cable system is assembled on-site rather than in a factory, the stress relief layer is typically composed of an elastomer such as silicone rubber, EPDM, or EPR. Therefore, these HVDC cable systems rely only on achieving sufficient pressure to obtain a good dielectric interface between the HVDC cable and its accessories. Therefore, these interfaces are potential weak links in the HVDC cable system.

Summary of the Invention

[0006] Facing the above problems in existing HVDC cable systems, the inventors of the present invention recognized the need to improve the breakdown strength of HVDC cable systems assembled on-site rather than in a factory. Furthermore, the inventors of the present invention hypothesized that the breakdown strength of the system could be increased by improving the interface between the HVDC cable and its accessories.

[0007] Accordingly, in a first aspect, there is provided an HVDC cable accessory comprising an electrical stress relief layer configured to be connected to an HVDC cable at a location, the HVDC cable accessory being pre-manufactured, and the electrical stress relief layer comprising a thermoplastic elastomer.

[0008] The HVDC cable can comprise a conductor and an insulating layer, and the electrical stress relief layer is connected to the insulating layer of the HVDC cable.

[0009] By using TPE instead of elastomer in the electrical stress suppression layer of pre-manufactured HVDC cable accessories, it is possible to improve the dielectric interface between the HVDC cable and the HVDC cable accessories by heat-treating the interface in addition to using mechanical force. The heat treatment enables the TPE to melt together with the insulation of the HVDC cable. Therefore, the dielectric interface depends not only on the achievement of appropriate pressure but also on the entanglement between polymer chains within the TPE in the electrical stress suppression layer and within the insulation layer of the HVDC cable. The improved dielectric interface leads to an improvement in the breakdown strength of the HVDC cable system. The HVDC cable accessories are pre-manufactured. This enables the use of accessories when the HVDC cable system is installed on-site rather than in the factory.

[0010] The electrical stress suppression layer can comprise a stress cone.

[0011] Furthermore, the HVDC cable accessories can be HVDC cable terminations or HVDC cable joints.

[0012] The electrical stress suppression layer can contain at least 80% by weight of TPE based on the total dry weight of the electrical stress suppression layer. This enables a sufficient amount of the electrical stress suppression layer to melt together with the insulation layer of the HVDC cable, allowing for an extremely good interface.

[0013] The electrical stress suppression layer may further contain polymers other than TPE, such as polyolefins and polycarbonates, and additives such as nitrides, metal oxides, crosslinking agents, and / or carbon-based additives.

[0014] The TPE can have a peak melting point within the range of 100 to 180 °C determined in accordance with ISO 11357-1:2023, and / or a melt index within the range of 0.05 to 30 g / 10 min (at 190 °C with 5 kg) determined in accordance with ASTM D1238-23a. This helps in forming a more homogeneous melt with the insulation layer of the HVDC cable and improves the dielectric interface.

[0015] The TPE can be polyolefin-based. This enables better compatibility with the insulation layer of HVDC cables, typically including LDPE such as XLPE.

[0016] The polyolefin-based TPE can be an ethylene copolymer such as an ethylene block copolymer. Preferably, the ethylene copolymer is an ethylene / 1-olefin copolymer such as an ethylene / 1-olefin block copolymer. More preferably, the ethylene / 1-olefin copolymer is an ethylene / 1-octene copolymer such as an ethylene / 1-octene block copolymer. These TPEs have high compatibility with the insulation layer of HVDC cables and have characteristics particularly suitable for use in the electric stress suppression layer.

[0017] The HVDC cable accessory can further include an insulation layer disposed around the electric stress suppression layer. Further, the HVDC cable accessory can include a semiconductive layer disposed around the electric stress suppression layer and, if present, around the insulation layer of the HVDC cable accessory.

[0018] The electric stress suppression layer can further include at least one field grading material (FGM) layer. The FGM layer is configured to be connected to the HVDC cable. This enables better electric field grading ability of the HVDC cable accessory and reduction of electric field stress can be obtained. The at least one FGM layer includes TPE and preferably includes a filler.

[0019] According to a second aspect, an HVDC cable system is provided. The HVDC system comprises at least one HVDC cable comprising a conductor and an insulating layer disposed around the conductor, HVDC cable accessories according to the first aspect and the electrical stress suppression layer is in direct contact with the insulating layer of the HVDC cable at at least one location.

[0020] The system according to the second aspect has a higher and more stable breakdown strength.

[0021] According to a third aspect, a method for installing an HVDC cable system is provided. The method comprises a) providing at least one HVDC cable comprising a conductor and an insulating layer disposed around the conductor; b) providing HVDC cable accessories according to the first aspect; c) joining the HVDC cable and the HVDC cable accessories such that the electrical stress suppression layer is in direct contact with the insulating layer of the HVDC cable at at least one location, thereby obtaining an interface between the insulating layer of the HVDC cable and the electrical stress suppression layer; d) heating the interface between the insulating layer of the HVDC cable and the electrical stress suppression layer to obtain a molten interface and

[0022] By applying the method according to the third aspect, an HVDC cable system having a higher and more stable dielectric breakdown strength can be obtained on-site.

[0023] The HVDC cable may further comprise a semiconductive screen removed from the HVDC cable prior to step c) at least at the location where the electrical stress suppression layer is in direct contact with the insulating layer of the HVDC cable.

[0024] The joining in step c) is preferably carried out by pushing the HVDC cable into the HVDC accessory. Thus, a mechanical force is applied, and the applied pressure can result in an improved interface.

[0025] The HVDC accessory may be a cable joint. Preferably, at least two HVDC cables are provided in step a), and a connected HVDC cable system is obtained in step d).

[0026] The heat in step d) may be provided as induction heating and / or a heat map.

[0027] Next, some aspects and embodiments will be described by way of example with reference to the accompanying drawings.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0029] Here, aspects of the present disclosure will be further described in more detail below with reference to the accompanying drawings showing specific embodiments of the invention.

[0030] However, these aspects can be embodied in many different forms and should not be construed as limiting. 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 all aspects of this disclosure to those skilled in the art. Throughout the specification, like numbers refer to like elements.

[0031] In certain cases, an HVDC cable system that includes cables, joints, and / or terminations requires on-site assembly rather than factory assembly. The assembly of HVDC cable accessories to HVDC cables on-site is typically performed using mechanical force. These systems rely only on achieving sufficient pressure to obtain a good dielectric interface between the HVDC cable and the HVDC cable accessories. The present disclosure provides HVDC cable accessories that enable heat treatment of surfaces that are pre-manufactured to form a dielectric interface between the HVDC cable and the HVDC cable accessories. The heat treatment enables a better dielectric interface in addition to mechanical force and thus improves the breakdown strength of the HVDC cable system.

[0032] A first aspect of the present disclosure provides an HVDC cable accessory comprising an electrical stress suppression layer configured to contact an HVDC cable at at least one location, the HVDC cable accessory being pre-manufactured and the electrical stress suppression layer comprising TPE. Thus, the electrical stress suppression layer is configured to be disposed around the HVDC cable and to contact the HVDC cable directly, preferably contacting the insulation layer of the HVDC cable directly. The direct contact results in a dielectric interface between the electrical stress suppression layer and the HVDC cable.

[0033] The HVDC cable accessory is pre-manufactured, i.e., the entire HVDC cable accessory is manufactured separately from the HVDC cable, and the HVDC cable accessory and the HVDC cable are joined on-site.

[0034] By using TPE in the electrical stress suppression layer, it becomes possible to heat-treat the dielectric interface formed between the insulating layer of the cable and the electrical stress suppression layer. The heat treatment melts the thermoplastic portion of the TPE and the insulating layer of the HVDC cable together to form a uniform interface. This improves the dielectric breakdown strength of the dielectric interface and thus also improves the dielectric breakdown strength of the entire HVDC cable system.

[0035] Preferably, the electrical stress suppression layer contains at least 80% by weight of TPE, more preferably at least 90% by weight of TPE, based on the total dry weight of the electrical stress suppression layer. This amount of TPE in the electrical stress suppression layer enables a sufficient amount of the electrical stress suppression layer to melt together with the insulating layer of the HVDC cable, thus enabling an extremely good interface. The electrical stress suppression layer may contain 100% by weight of TPE based on the total dry weight of the electrical stress suppression layer.

[0036] The electrical stress suppression layer may further contain polymers other than TPE, such as polyolefins and polycarbonates, and additives such as nitrides, metal oxides, crosslinking agents, and / or carbon-based additives. Specific examples of the additives may be boron nitride, aluminum nitride, ZnO, MgO, Al2O3, graphene, graphene oxide, and carbon black.

[0037] TPE is a polymer having both elastic and thermoplastic properties. These materials combine the properties of rubber with the ability to be processed like plastic. Therefore, by using TPE instead of an elastomer in the electrical stress suppression layer, it is possible to have rubber-like properties while enabling processing such as heat treatment.

[0038] The heat treatment of the dielectric interface can be improved by selecting a TPE having a peak melting point and / or melt index similar to that of the insulating layer of the HVDC cable. Similar melting points and / or melt indices enable the formation of a more uniform melt between the electrical stress suppression layer and the insulating layer of the HVDC cable. Therefore, a peak melting point in the range of 100 - 150 °C determined according to ISO11357-1:2023, and / or a melt index in the range of 0.05 - 30 g / 10 min (at 190 °C with 5 kg) determined according to ASTM D1238-23a are preferred.

[0039] The TPE preferably has a low crystallinity. The crystallinity is preferably in the range of 5 - 30%, more preferably in the range of 10 - 20% as determined by ISO11357-1:2023. Low crystallinity results in a TPE having very good rubber elasticity.

[0040] The insulating layer of the HVDC cable typically includes LDPE such as XLPE. Therefore, preferably, the TPE in the electrical stress suppression layer is a polyolefin-based polymer, more preferably a polyolefin-based block copolymer. A polyolefin-based block copolymer is a polyolefin having alternating blocks of raised crystalline "hard" segments and elastomeric "soft" segments. The alternating block structure gives the polyolefin block copolymer good flexibility compared to random polyolefin copolymers.

[0041] The polyolefin copolymer may be an ethylene copolymer, preferably a block copolymer. Ethylene can be copolymerized with a 1-olefin such as 1-octene to form an ethylene / 1-olefin such as an ethylene / 1-octene copolymer. The copolymer is preferably a block copolymer. This enables a TPE that has properties particularly suitable for use in the electrical stress suppression layer while being compatible with the insulating layer of the HVDC cable.

[0042] In the case of a preferred ethylene / 1-octene block copolymer, the hard segments typically contain a low molar concentration of 1-octene, while the soft segments contain a high molar concentration of 1-octene.

[0043] In addition to the electrical stress suppression layer, the HVDC cable accessory can further include an insulating layer disposed around the electrical stress suppression layer. Further, the HVDC cable accessory can include a semiconductive layer disposed around the electrical stress suppression layer and, if present, around the insulating layer of the HVDC cable accessory. The semiconductive layer and / or the insulating layer of the HVDC cable accessory can include a TPE, preferably the same TPE as the TPE of the electrical stress suppression component. The semiconductive layer preferably further includes a filler.

[0044] The electrical stress suppression layer can further include at least one FGM layer. The FGM layer is configured to be connected to the HVDC cable. Preferably, the FGM layer is disposed around the HVDC cable, more preferably around the insulating layer of the HVDC cable. This enables better electric field gradient ability and reduction of electric field stress of the HVDC cable accessory. The FGM layer includes a TPE. In addition to the TPE, the FGM layer may further include a filler such as SiC, ZnO, and / or graphene oxide.

[0045] The HVDC cable accessory can be an HVDC cable termination or an HVDC cable joint.

[0046] According to a second aspect, an HVDC cable system is provided. The HVDC system includes at least one HVDC cable including a conductor and an insulating layer disposed around the conductor, and an HVDC cable accessory according to the first aspect and the electrical stress suppression layer is in direct contact with the insulating layer of the HVDC cable at one position.

[0047] The HVDC cable system is assembled on site using pre-manufactured HVDC cable accessories.

[0048] The system according to the second aspect has a higher and more stable breaking strength.

[0049] The HVDC cable typically further comprises a semiconductive screen disposed around the insulating layer of the HVDC cable. This semiconductive screen is removed from the HVDC cable at least at a position where the electrical stress suppression layer is in direct contact with the insulating layer of the HVDC cable prior to step c). The semiconductive screen is typically removed from most of the HVDC cable within the HVDC cable accessory to facilitate connection between the HVDC cable and the HVDC cable accessory during on-site installation.

[0050] The HVDC cable may further comprise other layers such as, but not limited to, additional semiconductive layers, water stop tapes, and outer layers.

[0051] According to a third aspect, a method for installing an HVDC cable system is provided. The method comprises a) providing at least one HVDC cable comprising a conductor and an insulating layer disposed around the conductor; b) providing an HVDC cable accessory according to the first aspect; c) joining the HVDC cable and the HVDC cable accessory such that the electrical stress suppression layer is in direct contact with the insulating layer of the HVDC cable at at least one position and an interface between the insulating layer of the HVDC cable and the electrical stress suppression layer is obtained; d) heating the interface between the insulating layer of the HVDC cable and the electrical stress suppression layer to obtain a molten interface and includes.

[0052] The installation of the HVDC system is carried out on site.

[0053] In step c), the HVDC cable and the HVDC cable accessories are joined such that when assembled, the electrical stress suppression layer is disposed around the insulation layer of the cable. This is preferably obtained by pushing the HVDC cable into the HVDC cable accessories. The pushing applies pressure to the system and creates a mechanical force that forms an interface between the HVDC cable and the electrical stress suppression layer.

[0054] By applying the method according to the third aspect, an HVDC cable system having a higher and more stable dielectric breakdown strength is obtained.

[0055] When the HVDC cable and the HVDC cable accessories are assembled on-site and an interface is formed between the insulation layer of the HVDC cable and the electrical stress suppression layer, heat is applied to the interface. The heating can be brought about, for example, by induction heating, and further / or by external means such as a heat map (for example, using a mat with heating elements). The heating is applied until a melt is formed between the electrical stress suppression layer and the insulation layer of the HVDC cable and the two layers at least partially fuse with each other.

[0056] The HVDC cable accessories may be cable terminations, and thus, in step d), a terminated cable can be obtained.

[0057] The HVDC accessories may also be cable joints. Thus, preferably, at least two HVDC cables are provided in step a) and a connected HVDC cable system is obtained in step d).

[0058] FIG. 1 is a schematic longitudinal sectional view of an installed HVDC cable joint 1 provided with an electrical stress suppression layer 3 (i.e., an HVDC cable joint that is pre-manufactured and installed on an HVDC cable on-site). The installed HVDC cable joint 1 extends along the longitudinal symmetry axis 20.

[0059] The installed HVDC cable joint 1 comprises an HVDC cable 4 having a conductor 5, a cable insulation layer 7 (previously referred to in this specification as the insulation layer of the HVDC cable) disposed around the conductor 5, and a semiconductive screen 9 disposed around the cable insulation layer 7. The HVDC cable 4 extends along the longitudinal symmetry axis 20.

[0060] Typically, the semiconductive screen 9 is removed from most of the cable 4 within the cable joint 1 during installation to facilitate the connection between the HVDC cable 4 and the HVDC cable joint 1.

[0061] The installed HVDC cable joint 1 may further comprise a connector 11 provided as a current connection device and an inner deflector 13. The connector 11 is configured to receive a connector 5 at a first end and to receive another connector connected to the HVDC cable 4 at a second end. Thus, two HVDC cables to be joined are electrically connected via the connector 11. The inner deflector 13 may be electrically connected to the conductor 5 via the connector 11. The connector 11 may be conductive, while the inner deflector 13 may be semiconductive. The inner deflector 13 may comprise a TPE and, preferably, a filler, and more preferably, the same TPE as the electrical stress suppression layer 3.

[0062] When the HVDC cable joint 1 is installed, the electrical stress suppression layer 3 is provided around the cable insulation layer 7 and thus directly contacts the cable insulation layer 7 during installation. The electrical stress suppression layer 3 comprises a TPE.

[0063] According to FIG. 1, the electrical stress suppression layer 3 is arranged along the cable insulation layer 7 and is electrically connected to the semiconductive screen 9 of the HVDC cable 4 at the first axial end. The semiconductive screen 9 ends immediately after contact with the electrical stress suppression layer 3. At the second axial end, the electrical stress suppression layer 3 is electrically connected to the conductor 5 of the HVDC cable by being electrically connected to the inner deflector 13.

[0064] In the direction from the first axial end to the second axial end, the electrical stress suppression layer 3 has a first portion and second and third portions following the first portion. The first portion has a linear shape with a constant radial thickness, the second portion has a non-linear shape in which the outer radius of the electrical stress suppression layer 3 increases non-linearly to reach a peak and then decreases non-linearly at the peak, and the third portion has a linear shape with a constant radial thickness. The first portion extends from the semiconductive screen 9 and transitions to the second portion. The second portion transitions to the third portion that extends to the inner deflector 13. The non-linear shape may be the shape of a stress cone. The geometric shape of the electrical stress suppression layer 3 is further helpful for suppressing the electric field.

[0065] The HVDC cable joint 1 can include an insulating layer 15 that may be arranged around the electrical stress suppression layer 3. The insulating layer 15 preferably starts from the peak of the second portion of the electrical stress suppression layer 3 and extends along the longitudinal axis 20. The insulating layer 15 may have a tapered shape in the axial direction from the third portion of the electrical stress suppression layer 3 towards the second and first portions. The insulating layer 15 can include TPE and preferably can include the same TPE as the electrical stress suppression layer 3.

[0066] The HVDC cable joint 1 can further include a semiconductive layer 17 extending along the longitudinal axis, and the semiconductive layer 17 is disposed around the joint insulation layer 15 and around the electrical stress control layer 3. Thus, the semiconductive layer 17 covers both the electrical stress control layer 3 and the insulation layer 15. Thereby, an outer triple point 19 can be defined at the intersection of the electrical stress control layer 3, the joint insulation layer 15, and the semiconductive layer 17. At one end, the semiconductive layer 17 contacts the semiconductive screen 9 of the HVDC cable 4. The semiconductive layer 17 can include TPE, and preferably can include the same TPE as the electrical stress control layer 3.

[0067] The HVDC cable 4 and the HVDC cable joint 1 can be a cylindrically symmetric device having a rotational symmetry axis 20. Each of the components of the HVDC cable 4 and the cable joint 1 is rotationally symmetric about the longitudinal symmetry axis 20.

[0068] Furthermore, there exists a mirror symmetry plane 10 in the radial direction that mirrors the components of the HVDC cable 4 and the HVDC cable joint 1 to obtain a second HVDC cable connected to the HVDC cable 4 and a second portion of the HVDC cable joint.

[0069] FIG. 2 is a schematic longitudinal cross-sectional view of the installed HVDC cable termination 2 (i.e., the HVDC cable termination pre-manufactured and installed on the HVDC cable at the site) provided with the electrical stress control layer 3.

[0070] The HVDC cable termination 2 includes an outer housing 23 having a tubular outer shell with an inner circumference, and the outer shell has a longitudinal axis 20.

[0071] The HVDC cable termination 2 includes the HVDC cable 4 having a conductor 5, a cable insulation layer 7 disposed around the conductor, and a semiconductive screen 9 disposed around the cable insulation layer 7. The HVDC cable 4 extends along the longitudinal axis 20 within the outer housing 23 of the HVDC cable termination 2.

[0072] The semiconductive screen 9 is typically removed from most of the cable extending into the HVDC cable end fitting 2 to facilitate the connection between the HVDC cable 4 and the HVDC cable end fitting 2 during installation.

[0073] The electrical stress relief layer 3 comprises a stress cone 3' and an FGM layer 3". The FGM layer 3" is disposed around the cable insulation layer 7 when the HVDC cable end fitting 2 is installed. The FGM layer 3" is electrically connected to the semiconductive screen 9 at one end and extends along the longitudinal axis 20. Preferably, the semiconductive screen 9 terminates just where the FGM layer 3" begins. The FGM layer 3" contains TPE.

[0074] When the HVDC cable end fitting 2 is installed, the stress cone 3' is electrically connected to the semiconductive screen 9 at one end and then disposed around the FGM layer 3". Thereby, the stress cone 3' contacts the cable insulation layer 7 through the FGM layer 3" during installation, and an interface between the electrical stress relief layer 3 and the cable insulation layer 7 is formed through the FGM layer 3".

[0075] The stress cone 3' contains the same TPE as the FGM layer 3". The stress cone 3' and the FGM layer 3" may contain TPE and a filler, that is, the stress cone 3' and the FGM layer 3" may have the same composition. The stress cone 3' extends along the longitudinal axis 20 to cover a portion of the HVDC cable 4 within the outer housing 23.

[0076] The HVDC cable end fitting 2 can further comprise a semiconductive layer 17 disposed partially around the stress cone 3'. The semiconductive layer 17 is connected to the semiconductive screen 9 at one end. The semiconductive layer 17 can contain TPE and preferably a filler.

[0077] The space 25 may be provided along the longitudinal axis 20 between the inner circumference of the outer housing 23 and the electrical stress relief layer 3, the semiconductive screen 9, or the semiconductive layer 17. The space 25 is preferably filled with an electrically insulating fluid.

[0078] The outer housing 23 further includes a first flange 27, preferably made of metal, located at the first end 31 of the outer housing 23, and a second flange 29, preferably made of metal, located at the second end 33 of the outer housing 23. The HVDC cable 4 enters the outer housing through the first opening 35 of the first flange 27 and exits the outer housing 23 through a second opening 37 that may be provided in the second flange 29.

[0079] The HVDC cable 4 and the HVDC cable termination 2 may be a cylindrically symmetric device having a rotational symmetry axis 20. Each of the components of the HVDC cable 4 and the HVDC cable termination 2 is rotationally symmetric about the longitudinal symmetry axis 20.

[0080] Although various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and not of limitation, and the scope is defined by the following claims.

Claims

1. An accessory (1, 2) for an HVDC cable, comprising an electrical stress suppression layer (3) configured to be connected to an HVDC cable (4) at a position, The accessory (1, 2) for an HVDC cable is pre-manufactured, and the electrical stress suppression layer (3) contains a thermoplastic elastomer.

2. The accessory (1, 2) for an HVDC cable according to claim 1, wherein the electrical stress suppression layer (3) comprises a stress cone.

3. The accessory (1, 2) for an HVDC cable according to claim 1 or 2, which is a cable joint (1) or a cable termination (2).

4. The accessory (1, 2) for an HVDC cable according to any one of claims 1 to 3, wherein the electrical stress suppression layer (3) contains at least 80% by weight of a thermoplastic elastomer based on the total dry weight of the electrical stress suppression layer (3).

5. The accessory (1, 2) for an HVDC cable according to any one of claims 1 to 4, wherein the thermoplastic elastomer has a peak melting point in the range of 100 to 180 °C determined according to ISO 11357-1:2023, and / or a melt index in the range of 0.05 to 30 g / 10 min when measured at 190 °C with 5 kg and determined according to ASTM D1238-23a.

6. The accessory (1, 2) for an HVDC cable according to any one of claims 1 to 5, wherein the thermoplastic elastomer is an olefin copolymer, preferably an olefin block copolymer.

7. The accessory (1, 2) for an HVDC cable according to claim 6, wherein the olefin copolymer is an ethylene copolymer, preferably an ethylene block copolymer.

8. The accessory (1, 2) for an HVDC cable according to claim 7, wherein the ethylene copolymer is an ethylene / 1-olefin copolymer, preferably an ethylene / 1-olefin block copolymer.

9. The accessory (1, 2) for an HVDC cable according to claim 8, wherein the ethylene / 1-olefin copolymer is an ethylene / 1-octene copolymer, preferably an ethylene / 1-octene block copolymer.

10. The electric stress suppression layer (3) comprises at least one electric field gradient material layer (3") containing the thermoplastic elastomer, and the electric field gradient material layer is configured to be connected to the HVDC cable (4) at one position. The HVDC cable accessory (1, 2) according to any one of claims 1 to 9.

11. At least one HVDC cable (4) comprising a conductor (5) and an insulating layer (7) disposed around the conductor (5), The HVDC cable accessory (1, 2) according to any one of claims 1 to 10 and comprising The electric stress suppression layer (3) is in direct contact with the insulating layer (7) of the HVDC cable (4) at one position. An HVDC cable system.

12. A method for installing an HVDC cable system, comprising: a) preparing at least one HVDC cable (4) comprising a conductor (5) and an insulating layer (7) disposed around the conductor (5); b) preparing the HVDC cable accessory (1, 2) according to any one of claims 1 to 10; c) joining the HVDC cable (4) and the HVDC cable accessory (1, 2) such that the electric stress suppression layer (3) is in direct contact with the insulating layer (7) of the HVDC cable (4) at at least one position, and an interface between the insulating layer (7) of the HVDC cable (4) and the electric stress suppression layer (3) is obtained; d) heating the interface between the insulating layer (7) of the HVDC cable (4) and the electric stress suppression layer (3) to obtain a molten interface. A method including

13. The method according to claim 12, wherein the HVDC cable (4) further comprises a semiconductive screen (9) removed from the HVDC cable (4) prior to step c) at least at a position where the electric stress suppression layer (3) is in direct contact with the insulating layer (7) of the HVDC cable (4).

14. The method according to claim 12 or 13, wherein the heating is provided by induction heating and / or a heat map.

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