Method for preparing a dc submarine cable factory joint and dc submarine cable

Abstract

The application relates to the technical field of direct-current submarine cables, and provides a preparation method of a direct-current submarine cable factory joint and a direct-current submarine cable. An elastic buffer layer is formed along the reaction force cone surface of a body insulating layer, a gradual change area is arranged at the end of the elastic buffer layer, the thickness of the gradual change area perpendicular to the reaction force cone surface gradually decreases from the center of the reaction force cone surface to both ends, an insulating recovery layer is finally formed on the surface of the elastic buffer layer and an inner semiconductive shielding recovery layer, the elastic buffer layer is clamped between the reaction force cone and the insulating recovery layer, the elastic buffer layer is arranged at the contact interface between the body insulating layer and the insulating recovery layer, the controllable deformation of the elastic buffer layer under the action of shearing stress can absorb the shearing stress of the contact interface, the stress is uniformly distributed, the electrical performance of the high-voltage direct-current submarine cable is improved, and the long-term interface stability of the high-voltage direct-current submarine cable during submarine laying and use is improved.

Description

Technical Field

[0001] This application relates to the field of DC submarine cable technology, and in particular to a method for preparing a DC submarine cable factory joint and a DC submarine cable. Background Technology

[0002] Factory-made joints (also known as flexible joints) for high-voltage direct current (HVDC) cross-linked polyethylene (XLPE) insulated submarine cables are key equipment for developing long-length cables and realizing long-distance, high-capacity flexible DC power transmission. During manufacturing, the conductors of two sections of the submarine cable are first welded together. After restoring the conductor shielding layer, newly molten XLPE is extruded and injected or wrapped around the tapered insulation of the submarine cable body. Vulcanization is then carried out under high temperature and high pressure conditions, causing the restored insulation to fuse with the original insulation. The electrical and mechanical properties of the flexible joint directly determine the reliability of the entire submarine cable.

[0003] In existing flexible joints, the restoring insulation and the bulk insulation are in direct contact on the inclined surface of the reactive force cone. Optimizing the smoothness of this inclined surface, as well as the vulcanization pressure and temperature, can reduce interfacial micropores and space charge accumulation, but the interface still faces significant mechanical stress problems. (1) Thermal cycling stress: During operation, submarine cables will undergo repeated heating-cooling cycles due to load changes and ambient temperature changes. XLPE material has a relatively high coefficient of thermal expansion, and the interfacial shear stress caused by thermal cycling can cause cumulative fatigue damage to the interface, which can easily lead to the initiation of microcracks.

[0004] (2) Axial laying tension: Submarine cables need to withstand large axial tensile forces during the laying process. As a weak link in the structure, the insulation interface of the flexible joint is prone to stress concentration under tension. Especially before the lead sheath and armor layer are restored, the insulation interface bears the maximum tensile load, which may lead to the propagation of micro-defects at the interface.

[0005] (3) Bending stress: There may be bending sections in the laying path of submarine cable. During the bending process, the interface is subjected to uneven shear and peeling stress. (4) Coupling effect of stress and electrical degradation: Micro-defects generated at the interface under shear stress will further become enrichment points of space charge and initiation points of electrical trees, accelerating insulation aging and eventually leading to breakdown.

[0006] Therefore, there is an urgent need for an improved solution that can actively absorb and disperse shear stress at the homogeneous interface while maintaining good electrical performance and long-term stability. Summary of the Invention

[0007] The purpose of this application is to provide a method for preparing a DC submarine cable factory joint and a DC submarine cable, so as to improve the technical problems mentioned in the background section.

[0008] For the purposes mentioned above, this application provides the following technical solution: The first aspect of this application provides a method for preparing a DC submarine cable factory joint, comprising: A reactive force cone and an inner semiconductive shielding recovery layer are formed at the flexible joint between the first and second submarine cables. An elastic buffer layer is formed along the surface of the reactive force cone, wherein a gradient region is formed at the end of the elastic buffer layer, and the thickness of the elastic buffer layer in the gradient region gradually decreases from the center of the reactive force cone surface toward both ends in a direction perpendicular to the surface of the reactive force cone. An insulating recovery layer is formed on the surfaces of the elastic buffer layer and the inner semiconductive shielding recovery layer, such that the elastic buffer layer is sandwiched between the reactive force cone and the insulating recovery layer; The elastic buffer layer and the insulating recovery layer are vulcanized and cooled to obtain a factory connector for connecting the first submarine cable and the second submarine cable.

[0009] Furthermore, the elastic buffer layer has a thickness of 1.2 to 3 mm along the center perpendicular to the surface of the reactive force cone, the bevel angle of the gradient region is 15° to 20°, and the length of the gradient region along the axial direction of the first submarine cable and the second submarine cable is 20 to 30 mm.

[0010] Furthermore, the elastic buffer layer is formed of a low-modulus elastomer, with a Young's modulus of 2-5 MPa, a Shore A hardness of 35-50, and an elastic recovery rate of not less than 90%.

[0011] Furthermore, the elastic buffer layer comprises a base polymer, reinforcing filler, crosslinking agent, heat stabilizer, space charge inhibitor, and processing aid, specifically including, by weight: The mixture contains 100 parts of base polymer, 25-40 parts of reinforcing filler, 0.5-1.5 parts of crosslinking agent, 1-3 parts of heat stabilizer, 5-12 parts of space charge inhibitor, and 0.5-1.0 parts of processing aid.

[0012] Further, the base polymer, reinforcing filler, crosslinking agent, heat stabilizer, space charge inhibitor and processing aid are compounded at a predetermined temperature to obtain a compound rubber. The compounded rubber is dissolved in an organic solvent to obtain an elastic buffer layer slurry with a solid content of 20-40 wt%. The elastic buffer layer slurry is then applied to the surface of the reactive force cone to form the elastic buffer layer.

[0013] Further, the step of applying the obtained elastic buffer layer slurry to the surface of the reactive force cone to form the elastic buffer layer also includes: The surface of the reactive force cone is cleaned and surface activated to achieve a surface roughness of 5-15. ; A silane coupling agent is coated on the surface of the reactive force cone to form a coupling agent base coating. After the coupling agent base coating dries, the elastic buffer layer slurry is applied to the surface of the reactive force cone to form the elastic buffer layer.

[0014] Furthermore, the vulcanization process includes a first-stage vulcanization and a second-stage vulcanization. The first-stage vulcanization is used to enable the elastic buffer layer to complete cross-linking and curing, and the second-stage vulcanization is used to enable the insulation recovery layer to complete cross-linking and curing.

[0015] Furthermore, in the first stage of vulcanization, the prepared elastic buffer layer and the insulating recovery layer are heated to a first predetermined temperature range and maintained at a first predetermined nitrogen pressure to complete the crosslinking and curing of the elastic buffer layer; In the second stage of vulcanization, the elastic buffer layer and the insulating recovery layer are heated to a second predetermined temperature range while maintaining the first predetermined nitrogen pressure to complete the crosslinking of the insulating recovery layer; wherein the second predetermined temperature range is greater than the first predetermined temperature range.

[0016] Further, the vulcanization and cooling treatment of the elastic buffer layer and the insulating recovery layer includes: The elastic buffer layer and the insulating recovery layer after vulcanization are subjected to gradient cooling, and the elastic buffer layer and the insulating recovery layer are cooled from the vulcanization temperature to room temperature at a first rate, a second rate and a third rate decreasing sequentially, respectively.

[0017] The second aspect of this application provides a DC submarine cable, including a DC submarine cable factory connector prepared by the method for preparing a DC submarine cable factory connector as described in the first aspect of this application.

[0018] The method for preparing a DC submarine cable factory joint described above, as provided in this application, can achieve at least the following technical effects: This application provides an elastic buffer layer between the main insulation layer and the insulation recovery layer of the submarine cable. The elastic buffer layer is laid on the surface of the reactive force cone cut from the main insulation layer, and the thickness of the elastic buffer layer decreases in the gradient area at both ends. This allows the elastic buffer layer to undergo controllable deformation under shear stress, absorbing the interfacial shear stress generated by the submarine cable under thermal cycling, bending and laying tension, so as to achieve stress uniformity while maintaining good electrical performance and long-term interfacial stability. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 A flowchart illustrating a method for preparing a DC submarine cable factory joint is provided in the embodiments of this application. Figure 2 This is a partial structural cross-sectional view of a DC submarine cable factory connector provided in the embodiments of this application; Reference numerals: 100, DC submarine cable factory joint; A, first submarine cable; B, second submarine cable; 10, first conductor; 11, second conductor; 12, inner semiconducting shielding layer; 121, inner semiconducting shielding recovery layer; 13, main body insulation layer; 131, reactive force cone; 14, insulating shielding layer; 15, outer sheath; 16, elastic buffer layer; 17, insulating recovery layer. Detailed Implementation

[0021] The technical solutions of this application will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] like Figure 1 The diagram shown is a schematic flowchart of a method for manufacturing a DC submarine cable connector 100. Figure 2 The diagram shows a partial cross-sectional view of a DC submarine cable factory connector 100. The method for manufacturing the DC submarine cable factory connector 100 provided in this embodiment involves setting an elastic buffer layer 16 between the main insulation layer 13 and the insulation recovery layer 17 of the submarine cable, and setting the thickness of the elastic buffer layer 16 to decrease in the gradient region at both ends. This allows the elastic buffer layer 16 to undergo controllable deformation under shear stress, absorbing the interfacial shear stress generated by the submarine cable under conditions such as thermal cycling, bending, and laying tension, achieving stress homogenization while maintaining good electrical performance and long-term interfacial stability. A specific method for manufacturing a DC submarine cable factory connector 100 includes the following steps: Step S100: A reactive force cone 131 and an inner semiconductive shielding recovery layer 121 are formed at the flexible joint between the first submarine cable A and the second submarine cable B.

[0023] Specifically, submarine cable expansion joints (factory joints) are key components in submarine cable systems used to connect two cable sections or repair damaged sections. Their core function is to enable continuous production over long distances, ensuring the continuity and reliability of the cable system. For example... Figure 2 As shown, the first submarine cable A and the second submarine cable B are two insulated submarine cables to be connected. In preparing the DC submarine cable factory connector 100, the outer covering structure of the conductors in the predetermined areas at the ends of the first submarine cable A and the second submarine cable B is first stripped. This includes sequentially stripping the outer sheath 15 (including multiple protective layers including a metal sheath), the insulating shielding layer 14, part of the body insulation layer 13, and the inner semiconductive shielding layer 12, exposing the first conductor 10 inside the first submarine cable A and the second conductor 11 inside the second submarine cable B. The body insulation layer 13 is cut to obtain a reactive force cone 131 with a continuous slope. Then, the exposed first conductor 10 and second conductor 11 are fused together, and an inner semiconductive shielding recovery layer 121 is formed on the outside of the first conductor 10 and the second conductor 11 to restore the inner semiconductive shielding layer 12. The structure prepared above is then subjected to preliminary vulcanization treatment, completing the pretreatment for preparing the DC submarine cable factory connector 100.

[0024] Preferably, the reaction force cone 131 forms a continuous slope along the axial direction of the first submarine cable A and the second submarine cable B, respectively. The angle between the continuous slope and the axial direction of the first submarine cable A and the second submarine cable B is 20° to 25°. The length of the continuous slope along the axial direction of the first submarine cable A and the second submarine cable B is 3 to 5 times the thickness of the body insulation layer 13.

[0025] Preferably, the first submarine cable A and the second submarine cable B are 500kV DC XLPE (polyethylene) insulated submarine cables. In this embodiment, exothermic welding or argon arc welding is used to fuse the first conductor 10 and the second conductor 11, and the connection is smooth and burr-free by grinding the welding point.

[0026] Optionally, a prefabricated semi-conductive half-sleeve or a wound semi-conductive strip is used to form the inner semi-conductive shielding restoration layer 121. The prefabricated semi-conductive half-sleeve is a factory-prefabricated semi-conductive rubber or plastic tube of a specific shape, and the inner semi-conductive shielding restoration layer 121 formed by it has stable shape and electrical properties; the wound semi-conductive strip is a self-adhesive thin strip with semi-conductive properties used by construction workers to wrap around the part that needs to be restored with a certain tension and overlap rate. This method can adapt to surfaces with various complex shapes.

[0027] Step S200: An elastic buffer layer 16 is formed along the surface of the reactive force cone 131. A gradient region is formed at the end of the elastic buffer layer 16. The thickness of the gradient region perpendicular to the surface of the reactive force cone 131 gradually decreases from the center of the surface of the reactive force cone 131 towards both ends.

[0028] Specifically, based on step S100, a reactive force cone 131 structure is obtained, and the surface of the reactive force cone 131 is cleaned and surface activated to achieve a surface roughness of 5~15μm, which is used to achieve a firm adhesion of the elastic buffer layer 16. Then, a silane coupling agent is coated on the surface of the reactive force cone 131 to form a coupling agent undercoat, which is used to improve the interfacial adhesion strength between the reactive force cone 131 and the elastic buffer layer 16. After the coupling agent undercoat dries, the elastic buffer layer 16 is formed on the surface of the reactive force cone 131. The elastic buffer layer 16 continuously and completely covers the surface of the reactive force cone 131, with gradient regions at both ends. The thickness of the elastic buffer layer 16 in the gradient regions decreases from the center (of the reactive force cone 131) to both ends. The thickness of the elastic buffer layer 16 is the height perpendicular to the surface of the reactive force cone 131. In this embodiment, an elastic buffer layer 16 is provided, with a thickness gradient region at both ends, and its stiffness also transitions smoothly. When shear stress is transmitted, the elastic buffer layer 16 undergoes shear deformation. Since there is no abrupt change in stiffness, the deformation is continuous and smooth, thereby achieving a more uniform distribution of stress throughout the gradient region, thus avoiding electric field / stress concentration and improving long-term stability.

[0029] Preferably, the surface of the reactive force cone 131 is cleaned using isopropanol or a special cleaning agent; the surface of the reactive force cone 131 is then activated by light mechanical polishing or plasma activation treatment. Optionally, γ-aminopropyltriethoxysilane or a special XLPE-silicone rubber adhesive primer is selected as the silane coupling agent.

[0030] Preferably, the center thickness of the elastic buffer layer 16 is perpendicular to the surface of the reactive force cone 131. Figure 2 In section d), the preferred thickness is 1.2~3mm. The slope of the elastic buffer layer 16 within the gradient region ranges from 15° to 20°, and the length of the elastic buffer layer 16 along the axial direction of the first submarine cable A and the second submarine cable B within the gradient region ranges from 20 to 30mm. By setting an elastic buffer layer 16 with a gradient bevel that gradually decreases in thickness at the center and ends, the buffer layer can undergo controllable elastic shear deformation when the interface is subjected to shear stress, and the stress peak is dispersed from the abrupt change point to the entire gradient region, maximizing the uniform distribution of stress and significantly reducing the risk of interface failure.

[0031] In this embodiment, the center thickness of the elastic buffer layer 16 is respectively set as ( Figure 2Four groups of samples (d) with thicknesses of 1.0 mm, 1.5 mm, 2.0 mm, and 3.0 mm were used, with other parameters remaining constant. Finite element simulation and experimental testing revealed a near-positive correlation between the center thickness d of the elastic buffer layer 16 and the stress dispersion effect: when the thickness increased from 1.0 mm to 2.0 mm, the peak interfacial shear stress decreased significantly from 2.4 MPa to 1.3 MPa; when the center thickness d of the elastic buffer layer 16 increased from 2.0 mm to 3.0 mm, the peak shear stress decreased to 1.1 MPa, with the rate of decrease slowing down. Therefore, considering that an excessively thick elastic buffer layer 16 would increase the axial and radial dimensions of the joint and potentially negatively impact the electric field distribution, the preferred center thickness d of the elastic buffer layer 16 in this embodiment is 1.5~2.0 mm.

[0032] Furthermore, the elastic buffer layer 16 is formed using a low-modulus elastomer (optionally addition-cured liquid silicone rubber (LSR) or high-consistency silicone rubber (HCR)). The resulting elastic buffer layer 16 has a Young's modulus of 2-5 MPa, a Shore A hardness of 35-50, and an elastic recovery rate of not less than 90%. The elastic buffer layer 16 formed in this embodiment has a much lower Young's modulus than the 100-400 MPa of the XLPE body insulation layer 13 in the prior art.

[0033] Preferably, the elastic buffer layer 16 comprises a base polymer, reinforcing filler, crosslinking agent, heat stabilizer, space charge inhibitor, and processing aid, specifically comprising, by weight: 100 parts of base polymer, 25-40 parts of reinforcing filler, 0.5-1.5 parts of crosslinking agent, 1-3 parts of heat stabilizer, 5-12 parts of space charge inhibitor, and 0.5-1.0 parts of processing aid.

[0034] Preferably, the base polymer is vinyl polydimethylsiloxane; the reinforcing filler is fumed silica with a BET specific surface area of ​​200-300 m² / g; the crosslinking agent is a hydrogen-containing silicone oil + platinum catalyst system; the heat stabilizer is cerium oxide or iron oxide; the space charge inhibitor is surface-modified Mg(OH)₂ nanoparticles with a particle size of 50-200 nm; and the processing aid is zinc stearate.

[0035] Further, the base polymer, reinforcing filler, crosslinking agent, heat stabilizer, space charge inhibitor, and processing aid in the above-mentioned weight ratio are added to a mixer and mixed for 15-30 minutes at a predetermined temperature (preferably 80-120°C) to obtain a uniform compound. The obtained compound is then dissolved in an organic solvent (optionally toluene or xylene) to obtain an elastic buffer layer 16 slurry with a solid content of 20-40 wt%. The obtained elastic buffer layer 16 slurry is then applied to the rotating first submarine cable A end (or second submarine cable B end) using a micro single-screw extruder or a dedicated coating die. The coating has a center thickness of 1.2-3 mm (preferably 1.5 mm) and a length of 20-30 mm along the axial direction of the submarine cable in the gradient region at both ends. The extrusion temperature is controlled at 80-100°C to prevent premature crosslinking of the elastic buffer layer 16. Afterward, the coated elastic buffer layer 16 is preliminarily surface-leveled to ensure it is free of bubbles and impurities.

[0036] Step S300: An insulating recovery layer 17 is formed on the surfaces of the elastic buffer layer 16 and the inner semiconductive shielding recovery layer 121, such that the elastic buffer layer 16 is sandwiched between the reactive force cone 131 and the insulating recovery layer 17.

[0037] Specifically, an insulating recovery layer 17 is formed on the surfaces of the elastic buffer layer 16 and the inner semiconductive shielding recovery layer 121, such that the elastic buffer layer 16 forms a homogeneous interface between the main insulation layer 13 and the insulating recovery layer 17 of the first submarine cable A and the second submarine cable B. The submarine cable end coated with the elastic buffer layer 16 as described in step S200 is placed in a dedicated insulating extrusion die (the die is preheated to 85~95°C). An XLPE insulating material (containing a peroxide crosslinking agent, such as dicumyl peroxide) compatible with the main insulation layer 13 is extruded into the dedicated insulating extrusion die through an extruder, covering the elastic buffer layer 16 and the inner semiconductive shielding recovery layer 121 respectively, forming the insulating recovery layer 17. Through the aforementioned dedicated insulating extrusion die, a good coating is ensured between the elastic buffer layer 16 and the main insulation layer 13 and the insulating recovery layer 17.

[0038] Step S400: The elastic buffer layer 16 and the insulation recovery layer 17 are vulcanized and cooled to obtain a DC submarine cable factory connector 100 for connecting the first submarine cable A and the second submarine cable B.

[0039] Further, in step S400, the formed elastic buffer layer 16 and insulating recovery layer 17 undergo a two-stage co-curing treatment, including a first-stage curing and a second-stage curing. The first-stage curing is used to ensure the elastic buffer layer 16 completes cross-linking and curing, and the second-stage curing is used to ensure the insulating recovery layer 17 completes cross-linking and curing.

[0040] Specifically, a two-stage vulcanization process is performed by placing a dedicated insulating extrusion die into a vulcanization apparatus and pressurizing it with nitrogen to 1.0~1.5 MPa. In the first stage of vulcanization, the formed elastic buffer layer 16 and insulating recovery layer 17 are heated to a first predetermined temperature range (preferably 140~160°C and held for 30~60 min), while maintaining a first predetermined nitrogen pressure (preferably 1.2 MPa), allowing the elastic buffer layer 16 to fully crosslink and form a strong interfacial bond with the surface of the main insulating layer 13, thus completing the crosslinking and curing of the first stage elastic buffer layer 16. In the second stage of vulcanization, the formed elastic buffer layer 16 and insulating recovery layer 17 are heated to a second predetermined temperature range (preferably 170~180°C, or 235~245°C for conventional XLPE processes; held for 60~120 min), while maintaining the aforementioned first predetermined nitrogen pressure (preferably 1.2 MPa), to complete the crosslinking of the insulating recovery layer 17.

[0041] Furthermore, the elastic buffer layer 16 and the insulating recovery layer 17 after the two-stage vulcanization treatment are subjected to gradient cooling, and are cooled from the vulcanization temperature to room temperature at successively decreasing first, second and third rates, respectively.

[0042] Preferably, the first cooling rate is 10°C / min, used to cool the vulcanized elastic buffer layer 16 and insulation recovery layer 17 from the vulcanization temperature to 150°C and hold for 5 minutes. The second cooling rate is 8°C / min, used to cool from 150°C to 100°C and hold for 5 minutes. The third cooling rate is 3°C / min, used to cool from 100°C to room temperature. Through the above gradient cooling process, thermal stress is gradually released during the preparation of the factory joint. After the temperature drops below 100°C and is sufficiently cooled, pressure is released and the mold is demolded to release the pressure in the special insulation extrusion mold. Then the mold is opened and the factory joint is removed. The temperature gradient between the ends of the first submarine cable A and the second submarine cable B is maintained by a cooling water system to prevent thermal stress damage caused by end effects.

[0043] Furthermore, after completing the preparation of the elastic buffer layer 16 and the insulation recovery layer 17, the factory joint is cleaned, coated with semi-conductive paint, and wrapped with semi-conductive self-adhesive tape to prepare the insulation shielding layer 14. Then, multiple outer sheaths 15, including a metal sheath, are prepared by wrapping with semi-conductive self-adhesive tape, thus completing the preparation of the DC submarine cable factory joint 100.

[0044] In this embodiment, when the center thickness d of the elastic buffer layer 16 is preferably 1.5~2.0mm, finite element analysis and testing show that the DC breakdown field strength and partial discharge initiation voltage meet the electrical performance requirements of the 500kV DC submarine cable factory joint 100. Finite element analysis shows that under typical operating conditions (conductor temperature 90℃, ambient temperature variation ±30℃, minimum bending radius laying, axial tensile load), after adding the elastic buffer layer 16 provided in this embodiment, the maximum shear stress at the contact interface between the main insulation layer 13 and the elastic buffer layer 16 is reduced by 35%~50%, the von Mises stress distribution is more uniform, and the peak region of the shear stress shifts to the interior of the elastic buffer layer 16.

[0045] This embodiment also provides a DC submarine cable, such as Figure 2 As shown, the assembly includes a first submarine cable A, a second submarine cable B, and a DC submarine cable factory connector 100 to be connected. The DC submarine cable factory connector 100 is manufactured using a DC submarine cable factory connector manufacturing method described above in this embodiment.

[0046] This application achieves the following technical effects through the above embodiments: 1. In traditional flexible joints, XLPE material exhibits a high elastic modulus (approximately 100~400MPa) after vulcanization and cross-linking. Both sides of the interface between the main insulation layer and the insulation recovery layer are rigid materials. When subjected to shear stress, the stress cannot be effectively dissipated through material deformation, but instead concentrates in the weakest area of ​​the interface, resulting in a high stress concentration factor. This application introduces a low-modulus, high-elasticity elastic buffer layer (Young's modulus 2~5MPa) at the interface between the main insulation layer and the insulation recovery layer. Therefore, when the flexible joint is subjected to shear stress generated by axial tension, bending, or thermal cycling, the elastic buffer layer first undergoes controllable shear deformation, dispersing the peak stress at the stress concentration point throughout the buffer layer area, achieving a uniform stress distribution. Simultaneously, the high elastic recovery rate (≥90%) of the buffer layer ensures that it can recover its original shape after stress unloading, avoiding interface delamination. This improves the electrical performance and long-term interface stability of high-voltage DC submarine cables when laid on the submarine.

[0047] 2. In the DC submarine cable factory joint preparation provided in this application, the above technical effects can be achieved simply by adding a buffer layer coating / extrusion step to the existing factory joint manufacturing process and optimizing the vulcanization cooling regime. No major modifications to the production line are required, which has the advantages of simple implementation and low modification cost for industrialization.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for preparing a DC submarine cable factory joint, characterized in that, include: A reactive force cone and an inner semiconductive shielding recovery layer are formed at the joint of the first and second submarine cables; An elastic buffer layer is formed along the surface of the reactive force cone, wherein a gradient region is formed at the end of the elastic buffer layer, and the thickness of the gradient region perpendicular to the surface of the reactive force cone gradually decreases from the center of the surface of the reactive force cone towards both ends. An insulating recovery layer is formed on the surfaces of the elastic buffer layer and the inner semiconductive shielding recovery layer, such that the elastic buffer layer is sandwiched between the reactive force cone and the insulating recovery layer; The elastic buffer layer and the insulating recovery layer are vulcanized and cooled to obtain a factory connector for connecting the first submarine cable and the second submarine cable.

2. The method for preparing a DC submarine cable factory joint according to claim 1, characterized in that, The elastic buffer layer has a center thickness of 1.2 to 3 mm perpendicular to the surface of the reactive force cone, the bevel angle of the gradient region is 15° to 20°, and the length of the gradient region along the axial direction of the first submarine cable and the second submarine cable is 20 to 30 mm.

3. The method for preparing a DC submarine cable factory joint according to claim 1, characterized in that, The elastic buffer layer is formed of a low-modulus elastomer. The Young's modulus of the elastic buffer layer is 2~5 MPa, the Shore A hardness is 35~50, and the elastic recovery rate is not less than 90%.

4. The method for preparing a DC submarine cable factory joint according to claim 3, characterized in that, The elastic buffer layer comprises a base polymer, reinforcing filler, crosslinking agent, heat stabilizer, space charge inhibitor, and processing aid, specifically including, by weight: The mixture contains 100 parts of base polymer, 25-40 parts of reinforcing filler, 0.5-1.5 parts of crosslinking agent, 1-3 parts of heat stabilizer, 5-12 parts of space charge inhibitor, and 0.5-1.0 parts of processing aid.

5. The method for preparing a DC submarine cable factory joint according to claim 4, characterized in that, The base polymer, reinforcing filler, crosslinking agent, heat stabilizer, space charge inhibitor, and processing aid are compounded at a predetermined temperature to obtain a compound rubber. The compounded rubber is dissolved in an organic solvent to obtain an elastic buffer layer slurry with a solid content of 20-40 wt%. The elastic buffer layer slurry is then applied to the surface of the reactive force cone to form the elastic buffer layer.

6. The method for preparing a DC submarine cable factory joint according to claim 5, characterized in that, The step of applying the elastic buffer layer slurry to the surface of the reactive force cone to form the elastic buffer layer includes: The surface of the reactive force cone is cleaned and surface activated to achieve a surface roughness of 5-15. ; A silane coupling agent is coated on the surface of the reactive force cone to form a coupling agent base coating. After the coupling agent base coating dries, the elastic buffer layer slurry is applied to the surface of the reactive force cone to form the elastic buffer layer.

7. The method for preparing a DC submarine cable factory joint according to claim 1, characterized in that, The vulcanization process includes a first-stage vulcanization and a second-stage vulcanization. The first-stage vulcanization is used to cross-link and cure the elastic buffer layer, and the second-stage vulcanization is used to cross-link and cure the insulating recovery layer.

8. The method for preparing a DC submarine cable factory joint according to claim 7, characterized in that, In the first stage of vulcanization, the prepared elastic buffer layer and the insulating recovery layer are heated to a first predetermined temperature range and maintained at a first predetermined nitrogen pressure to complete the crosslinking and curing of the elastic buffer layer; In the second stage of vulcanization, the elastic buffer layer and the insulating recovery layer are heated to a second predetermined temperature range while maintaining the first predetermined nitrogen pressure to complete the crosslinking of the insulating recovery layer; wherein the second predetermined temperature range is greater than the first predetermined temperature range.

9. The method for preparing a DC submarine cable factory joint according to claim 1, characterized in that, The step of vulcanizing and cooling the elastic buffer layer and the insulating recovery layer includes: performing gradient cooling on the vulcanized elastic buffer layer and the insulating recovery layer, cooling the elastic buffer layer and the insulating recovery layer from the vulcanization temperature to room temperature at successively decreasing first, second and third rates respectively.

10. A DC submarine cable, characterized in that, Includes a DC submarine cable factory joint prepared by the preparation method of a DC submarine cable factory joint as described in any one of claims 1 to 9.

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