Modular insulation recovery method for dc submarine cable and dc submarine cable

Abstract

The application relates to the technical field of direct-current submarine cables, and provides a modular insulation recovery method of a direct-current submarine cable and the direct-current submarine cable. The method is used for preparing a cross-linked polyethylene (XLPE) composition according to a preset group, mixing and granulating XLPE to obtain cross-linked polyethylene particles, and preparing a tubular blank. The tubular blank is pre-cross-linked by adopting a segmented heating vulcanization process to obtain an insulation recovery module in a pre-cross-linked state. A conical butt joint surface matched with a reaction force conical surface of a body insulation of a submarine cable joint part is formed. A positioning structure and a microstructure of an exhaust structure are formed on the conical butt joint surface. The insulation recovery module is sleeved to the submarine cable joint part, the conical butt joint surface is tightly attached to the reaction force conical surface, the insulation recovery module is fused and re-cross-linked, and a factory joint after insulation recovery is formed in the submarine cable joint part. The application prolongs the service life, electrical reliability, insulation recovery efficiency and quality of the factory joint, and simplifies the installation process.

Description

Technical Field

[0001] This application relates to the field of DC submarine cable technology, and in particular to a modular insulation restoration method for DC submarine cables and a DC submarine cable. Background Technology

[0002] In high voltage direct current (HVDC) submarine power transmission projects, in order to achieve long-distance (e.g., more than 200 km) continuous laying of submarine cables, the factory joints (flexible joints) of the submarine cables must be treated to restore their insulation performance and mechanical strength.

[0003] Existing on-site molding processes require extruding and high-temperature vulcanizing XLPE material in the reactive force cone region. Since the insulation thickness of 500kV joints exceeds 30mm, the enormous heat capacity leads to significant temperature differences between the inner and outer layers during vulcanization, easily causing uneven cross-linking. Furthermore, the secondary melting history of the restored insulation differs from the bulk insulation in terms of crystallinity and thermal elongation, easily generating shear stress under thermal cycling conditions, leading to microcracks and insulation aging. In addition, on-site molding processes have high requirements for environmental cleanliness, temperature control accuracy, and large-scale tooling. The highly variable offshore construction environment makes it difficult to guarantee the quality of multi-joint projects. Existing precast joints are heterogeneous interfaces, relying on the radial pressure of the elastomer to maintain contact, lacking chemical cross-linking. Significant pressure changes under thermal cycling result in micro-air gaps, easily leading to a decline in electrical performance, making them unsuitable for factory joints in high-voltage DC submarine cables. Therefore, a modular insulation restoration method for DC submarine cables and a DC submarine cable solution are urgently needed to address these technical problems. Summary of the Invention The purpose of this application is to provide a modular insulation restoration method for DC submarine cables and a DC submarine cable, so as to improve the technical problems mentioned in the background section.

[0004] For the purposes mentioned above, this application provides the following technical solution: This application involves manufacturing a pre-crosslinked insulation restoration module in a factory. This module is then processed to form a conical mating surface that mates with the reactive force cone of the cable joint's body insulation, creating a microstructure with positioning and venting structures. The insulation restoration module is then fitted tightly to the cable joint and fused and re-crosslinked to form a factory-jointed connector with restored insulation. This process eliminates shear stress caused by thermal expansion and contraction mismatch, improves the factory joint's thermal cycling resistance and service life, effectively reduces the residual concentration of byproducts and eliminates air gaps while maintaining electrical performance. This improves electrical reliability, insulation restoration efficiency and quality, simplifies the installation process, and reduces installation difficulty.

[0005] The first aspect of this application provides a modular insulation restoration method for DC submarine cables, comprising: A cross-linked polyethylene (XLPE) composition is prepared according to a preset formula, the cross-linked polyethylene composition is mixed and granulated to obtain cross-linked polyethylene particles, and the cross-linked polyethylene particles are prepared into tubular blanks. The tubular blank is pre-crosslinked using a segmented heating vulcanization process to obtain an insulation recovery module in a pre-crosslinked state, which is between an uncrosslinked state and a fully crosslinked state. The insulation restoration module is processed to form a conical mating surface that matches the reactive force cone surface of the body insulation of the submarine cable joint. Microstructures of positioning structure and venting structure are formed on the conical mating surface. The insulation restoration module is fitted onto the submarine cable joint, so that the conical mating surface fits tightly against the reactive force conical surface. The insulation restoration module is then fused and re-crosslinked to form a factory joint with restored insulation at the submarine cable joint.

[0006] Preferably, the preset components include a matrix resin, a crosslinking agent, and a heat stabilizer; The matrix resin is configured as 100 parts by weight of ultra-clean grade low-density polyethylene (LDPE), the crosslinking agent is configured as 1.6 to 2.0 parts by weight of dicumyl peroxide (DCP), and the heat stabilizer is configured as 0.3 to 0.6 parts by weight of antioxidant.

[0007] Preferably, the preset component may further include a space charge inhibitor, which is configured as less than or equal to 3.0 parts by weight of nano-SiO2.

[0008] Preferably, the cross-linked polyethylene composition is mixed and granulated to obtain cross-linked polyethylene particles, and the cross-linked polyethylene particles are then prepared into tubular preforms, comprising: The cross-linked polyethylene composition is mixed and granulated in an internal mixer at 120~130℃ to obtain cross-linked polyethylene particles, and the cross-linked polyethylene particles are extruded into tubular preforms using a tubular extruder. The tubular blank has a hollow structure, with an inner diameter 0.8~1.5mm larger than the inner diameter of the body insulation of the submarine cable joint to be connected, and an outer diameter 0.5~1.0mm larger than the outer diameter of the body insulation of the submarine cable joint to be connected.

[0009] Preferably, the tubular blank is pre-crosslinked using a segmented heating vulcanization process to obtain an insulation recovery module in a pre-crosslinked state. The pre-crosslinked state is between an uncrosslinked state and a fully crosslinked state, including: The segmented heating vulcanization process includes a preheating section, a low-temperature cross-linking section, a medium-temperature cross-linking section, and a cooling section; The target gel content in the pre-crosslinked state is between the gel content in the uncrosslinked state and the fully crosslinked state, and is configured to be 45%~55%; When the torque of the tubular preform reaches a preset torque threshold corresponding to the target gel content, it enters the cooling section.

[0010] Preferably, the insulation restoration module is processed to form a conical mating surface that mates with the reactive force cone surface of the submarine cable joint portion's body insulation, including: The insulation restoration module is cut at both ends by a CNC lathe to form a conical mating surface that matches the reactive force cone surface of the submarine cable joint. The surface roughness of the conical mating surface is configured to be less than or equal to 3.2 μm.

[0011] Preferably, the microstructure forming the positioning structure and the venting structure on the conical mating surface includes: The positioning structure is an annular groove, which is evenly distributed radially along the conical mating surface. The depth of the groove is configured to be less than or equal to 0.5 mm, the width of the groove is configured to be less than or equal to 0.8 mm, and the radial spacing of the groove is configured to be less than or equal to 6 mm. The exhaust structure is intersecting and connected with the positioning structure. The exhaust structure is a groove structure, and the depth of the groove structure is configured to be less than or equal to 0.3 mm.

[0012] Preferably, the insulation restoration module is fitted onto the submarine cable joint portion, such that the tapered mating surface is tightly fitted to the reactive force tapered surface, including: An annular protrusion is formed on the reactive force cone surface to cooperate with the positioning structure, so that the positioning structure of the cone-shaped mating surface is tightly fitted to the annular protrusion on the reactive force cone surface.

[0013] Preferably, the insulation restoration module is fused and re-crosslinked to form a factory joint with restored insulation at the submarine cable joint portion, including: The insulation restoration module is preheated and diffused fused using a segmented heating mold; The fused module is subjected to high-pressure vulcanization. One or more parameters, including time, temperature, and gel content, are monitored during the vulcanization process and compared with the calculation results in the preset simulation model to obtain the optimal parameter values ​​for re-crosslinking. Based on the optimal parameter values, a factory joint with restored insulation is formed at the submarine cable joint.

[0014] A second aspect of this application provides a DC submarine cable, comprising: a factory joint formed by the modular insulation restoration method for DC submarine cables according to any of the foregoing embodiments.

[0015] This application, through the modular insulation restoration method for DC submarine cables and the DC submarine cable described above, can achieve at least the following technical effects: The modular insulation restoration method and DC submarine cable provided in this application involve manufacturing pre-crosslinked insulation restoration modules in a factory. These modules are then processed to form a conical mating surface that mates with the reactive force cone surface of the cable joint's body insulation, creating a microstructure with positioning and venting structures. The insulation restoration modules are then fitted tightly to the cable joint, and fused and re-crosslinked to form a factory-jointed cable with restored insulation. This method eliminates shear stress caused by thermal expansion and contraction mismatch, improves the factory joint's thermal cycling resistance and service life, effectively reduces the residual concentration of byproducts and eliminates air gaps while ensuring electrical performance. This improves electrical reliability, insulation restoration efficiency and quality, simplifies the installation process, and reduces installation difficulty. Attached Figure Description

[0016] 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.

[0017] Figure 1 A flowchart illustrating the modular insulation restoration method for DC submarine cables provided in this application embodiment; Figure 2 This is a structural diagram of a DC submarine cable provided in an embodiment of this application; Figure 3 This is a structural diagram of the insulation restoration module provided in the embodiments of this application; Figure 4 This is a cross-sectional view of the insulation restoration module provided in an embodiment of this application; Figure 5 Another flowchart of the modular insulation restoration method for DC submarine cables provided in the embodiments of this application; Reference numerals: 10-DC submarine cable; 11-conductor; 12-inner semiconducting shielding layer; 121-inner semiconducting shielding recovery layer; 13-body insulation layer; 131-reactive force cone surface; 132-annular protrusion structure; 14-insulating shielding layer; 15-outer sheath; 16-insulation recovery module; 161-positioning structure; 162-venting structure. Detailed Implementation

[0018] 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.

[0019] All terms used in this application (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0020] For example, the terms "first" and "second" used in this application are only used to distinguish similar objects and differentiate the first object from another object, rather than to describe a specific order or sequence, nor should they be interpreted as indicating or implying relative importance.

[0021] This application proposes a modular insulation restoration method for DC submarine cable 10, combined with Figure 1 As shown, the method includes: Step S101: Prepare a cross-linked polyethylene (XLPE) composition according to a preset formula, mix and granulate the cross-linked polyethylene composition to obtain cross-linked polyethylene particles, and prepare the cross-linked polyethylene particles into tubular blanks.

[0022] In this embodiment, in a factory environment with a cleanliness level of Class 10000 or higher, a cross-linked polyethylene (XLPE) composition is first prepared according to a preset composition. The preset components include a matrix resin, a cross-linking agent, and a heat stabilizer. Specifically, the matrix resin refers to the base polyethylene resin in the XLPE composition, which is the continuous phase and main insulating component of the insulating material. It can be low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), or a blend of both. Preferably, 100 parts by weight (a unit representing the relative mass ratio between components) of ultra-clean grade low-density polyethylene (LDPE) is used to prepare the matrix resin. The cross-linking agent is an additive that forms a three-dimensional cross-linked network between polyethylene molecular chains. Commonly, it is an organic peroxide that decomposes upon heating to generate free radicals, which abstract hydrogen atoms from the PE molecular chains, connecting different molecular chains by C–C bonds to form a cross-linked network. Preferably, 1.6 to 2.0 parts by weight of dicumyl peroxide (DCP) is used to prepare the cross-linking agent. Heat stabilizers refer to additives that inhibit the thermal oxidative degradation of polyethylene during high-temperature processing and cross-linking, preventing yellowing, scorching, and performance degradation. In XLPE insulation formulations, they typically refer to antioxidants, such as pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (antioxidant 1010) and tris(2,4-di-tert-butylphenyl) phosphite (antioxidant 168). Preferably, 0.3 to 0.6 parts by weight of antioxidants are used to form the heat stabilizer, wherein the antioxidant is a compound of antioxidant 1010 and antioxidant DSTDP in a mass ratio of 1:1.

[0023] In addition, the preset components may also include space charge inhibitors, which are used to suppress the accumulation of space charge in the insulation layer of high-voltage DC cables, improve the electric field distribution, and reduce the risk of electrical treeing. In the XLPE insulation system, commonly used inorganic nanoparticles include nano-silica (nano-SiO2), nano-alumina (nano-Al2O3), nano-montmorillonite (nano-MMT), and nano-barium titanate (nano-BaTiO3). Preferably, nano-SiO2 is used in a formulation of less than or equal to 3.0 parts by weight to form the space charge inhibitor.

[0024] Next, the cross-linked polyethylene composition formed above is mixed and granulated. Mixing and granulation is the process of uniformly dispersing powdered / liquid additives into the PE matrix. Specifically, various raw materials are physically mixed in a mixer or high-speed mixer, then melted, mixed, and extruded into strips using an extruder, and then cut into uniform granules. Preferably, the cross-linked polyethylene composition is mixed and granulated in a mixer at 120~130℃ to obtain cross-linked polyethylene granules, which are then extruded into tubular preforms using a tubular extruder. The tubular preform has a hollow cylindrical structure. To achieve an interference fit and meet the interfacial clamping force after fusion shrinkage, preferably, the inner diameter of the tubular preform is designed to be 0.8~1.5mm larger than the inner diameter of the main body insulation of the submarine cable joint to be connected, and the outer diameter is designed to be 0.5~1.0mm larger than the outer diameter of the main body insulation of the submarine cable joint to be connected. This reduces the installation difficulty and ensures that the main body insulation of the submarine cable joint can be smoothly fitted in, avoiding damage to the interface by forced installation. The extruded billet is immediately placed in a cooling water bath to set and cool to room temperature, providing a basis for subsequent processes.

[0025] Step S102: The tubular blank is pre-crosslinked using a segmented heating vulcanization process to obtain an insulation recovery module 16 in a pre-crosslinked state, wherein the pre-crosslinked state is between an uncrosslinked state and a fully crosslinked state.

[0026] In this embodiment, the segmented heating vulcanization process includes a preheating section, a low-temperature crosslinking section, a medium-temperature crosslinking section, and a cooling section. The preheating section operates at 120-130°C for 3-5 minutes under normal pressure to ensure uniform temperature inside and outside the tube blank, eliminate internal stress, and soften the material through heat conduction, providing thermodynamic conditions for the crosslinking reaction. At this stage, the DCP crosslinking agent has not yet decomposed significantly. The low-temperature crosslinking section operates at 140-148°C for 5-8 minutes under a pressure of 0.1-0.2 MPa. This temperature is close to the critical decomposition temperature of DCP. Under a slightly positive pressure environment, oxygen is isolated, and DCP begins to slowly decompose, generating free radicals to initiate the crosslinking reaction. This stage forms a sparse initial crosslinking network, preventing the reaction from becoming too violent and difficult to control due to excessively high temperatures. The medium-temperature crosslinking section operates at 52-158°C for 3-5 minutes under a pressure of 0.1-0.2 MPa. This section appropriately increases the temperature to accelerate the free radical reaction, achieving the target degree of crosslinking. Maintaining a slightly positive pressure prevents the tube blank from bulging and deforming due to the vaporization of internal volatiles. The cooling section rapidly cools the material to less than or equal to 80°C for 5 to 10 minutes to quickly remove heat, freeze the DCP decomposition rate at the current level, completely terminate the crosslinking reaction, suppress grain coarsening, and preserve the material's toughness.

[0027] The tubular billet was pre-crosslinked using the segmented heating vulcanization process described above, resulting in an insulation recovery module 16 in a pre-crosslinked state. The pre-crosslinked state lies between the uncrosslinked and fully crosslinked states. The uncrosslinked state refers to a state where the crosslinking agent in the crosslinked polyethylene material has not fully decomposed, the polyethylene molecular chains are basically linear or slightly branched, and a continuous three-dimensional crosslinked network has not yet been formed. Its characteristics include: the polyethylene molecular chains mainly rely on van der Waals forces and molecular chain entanglement to maintain their shape; there are no large numbers of CC crosslinking bonds; it is easy to soften and flow when heated; its shape retention ability is poor; crosslinking agents such as DCP have not fully decomposed, or the free radicals generated by decomposition have not triggered sufficient crosslinking reactions; the gel content is very low; and the thermal elongation is very high, significantly higher than that of the fully crosslinked state.

[0028] Gel content refers to the percentage of the three-dimensional cross-linked network portion of cross-linked polyethylene that is insoluble in solvents such as xylene, which constitutes the sample mass and is an indicator of the degree of cross-linking in XLPE. Specifically, the gel content is determined using a solvent extraction method (thermal extraction method). Thin slices or fragments are cut from the pre-cross-linked tubular preform and weighed. Pack the sample into a mesh bag or filter paper bag of known mass, weigh the total weight to calculate the net mass of the sample, and place it in a Soxhlet extractor or reflux apparatus. Extract with xylene under boiling reflux for 24 hours. Remove the mesh bag / filter paper bag and dry it in a vacuum oven at 120-140℃ to remove residual xylene. After cooling to room temperature, weigh the sample. gel content Where w1 is the mass of the insoluble residue after extraction and drying. The sample dry mass before extraction is indicated by the gel content. A higher gel content indicates a higher proportion of three-dimensional cross-linked networks and a higher degree of cross-linking in the material.

[0029] Thermal elongation refers to the percentage of elongation of a specimen within a specified time under a specified high temperature (usually 200℃) and a specified tensile load (usually 0.2MPa), relative to the original gauge length. It reflects the resistance of the cross-linked network to high-temperature creep; the more developed the cross-linked network, the more difficult it is for the molecular chains to slip relative to each other. Specifically, thin sheets or scraps are cut from pre-cross-linked tubular blanks and punched into dumbbell-shaped specimens (original gauge length). Typically 20 mm), or a tubular specimen can be used directly. Measure the width and thickness of the gauge section, calculate the cross-sectional area, clamp the specimen into the upper and lower clamps of the thermal extension apparatus, and hang a weight at the lower end to subject the specimen to a tensile stress of 0.2 N / mm² (i.e., 20 N / cm²). Place the entire apparatus in an oven preheated to 200℃±3℃ and maintain for 15 minutes (starting from when the temperature is reached). Read the distance between the gauges through the observation window of the oven. Remove the load and leave the sample in the oven for 5 minutes without load. Then remove it and allow it to cool naturally to room temperature. Calculate the thermal elongation. .in This is the original gauge length (usually 20 mm). The gauge length is the result of applying a load of 0.2 N / mm² at 200℃ for 15 minutes. The more complete the cross-linking, the lower the thermal elongation; conversely, insufficient cross-linking makes the material more prone to elongation under high temperature and external force.

[0030] Fully cross-linked polyethylene (CXL) refers to a state where the cross-linking agent in the CXL material has essentially decomposed, and the polyethylene molecular chains form a continuous three-dimensional network through C-C cross-linking bonds. This results in a material exhibiting high cross-linking density and good heat deformation resistance. Its characteristics include: a large number of polyethylene molecular chains connected by cross-linking bonds to form a three-dimensional network structure; resistance to viscous flow during heating; strong dimensional stability and shape retention; near-complete depletion or complete reaction of the cross-linking agent; a high gel content, typically reaching 60%–80%; and low thermal elongation, usually controlled below 80%.

[0031] The pre-crosslinked state is between the uncrosslinked state and the fully crosslinked state. Preferably, the corresponding gel content is in the range of 45% to 55%, and the thermal elongation (at 200℃ / 0.2MPa load) is about 120% to 140%.

[0032] During the pre-crosslinking process, to ensure the tubular preform reaches a semi-crosslinked state, a torque rheometer is used to test the crosslinked polyethylene composition with the same formulation before process implementation. A curve showing the change in torque value as a function of the crosslinking reaction is plotted, and the gel content corresponding to different torque points is measured, thus establishing a torque-gel content mapping relationship. Torque changes are monitored in real time using an online torque monitoring system. When the torque of the tubular preform reaches a preset torque threshold, the gel content reaches 48%. With a target gel content of 3%, the product immediately enters the cooling section, with the cooling rate controlled at 15~25℃ / min. Rapid cooling freezes the decomposition reaction of the DCP crosslinking agent instantly, preventing the degree of crosslinking from exceeding the standard due to thermal inertia. It locks the product in a semi-crosslinked state to inhibit the further generation of crosslinking byproducts and grain coarsening.

[0033] This embodiment avoids insufficient or excessive crosslinking, keeping the gel content and thermal elongation within the target range, improving the material's microstructure and processing performance, and enhancing the bonding quality and joint reliability.

[0034] Step S103: The insulation restoration module 16 is processed to form a conical mating surface that matches the reactive force cone surface of the body insulation of the submarine cable joint. Microstructures of positioning structure 161 and venting structure 162 are formed on the conical mating surface.

[0035] In this embodiment, firstly, the insulation restoration module 16 in a semi-crosslinked state is machined at both ends using a CNC lathe. During the machining process, the inner hole and one end face of the insulation restoration module 16 are used as a unified reference for clamping and positioning. Typically, a double-center mating or a dedicated expansion mandrel is employed to ensure high coaxiality of the workpiece during cutting (error controlled within 0.02mm). Based on the preset electric field distribution requirements, the corresponding taper algorithm and axial length parameters (typically 120mm~200mm) are input into the CNC system, driving the diamond-coated forming tool to perform precise linkage cutting along the axial and radial directions to form a tapered mating surface that mates with the reactive force tapered surface of the submarine cable joint. By optimizing the cutting process parameters, such as setting the spindle speed to 300~500 r / min and controlling the feed rate to 0.05~0.1 mm / r, the surface roughness (Ra) of the tapered mating surface is controlled to be less than or equal to 3.2μm. Surface roughness (Ra) refers to the arithmetic mean of the absolute values ​​of the surface profile offset within a certain sampling length. It is used to characterize the microscopic unevenness of the tapered mating surface of the insulation restoration module 16. The smaller the value, the smoother the surface; the larger the value, the rougher the surface.

[0036] Then, microstructures of positioning structure 161 and venting structure 162 are formed on the conical mating surface. The microstructures are millimeter-scale structures with dimensions much smaller than the total insulation thickness. The design aims to retain the positioning and venting functions without affecting the electrical performance. Compared to the overall wall thickness of the insulation recovery module 16 and the relatively small size of the microstructures with insulation distance, the impact on the macroscopic electric field distribution is negligible.

[0037] The positioning structure 161 is an annular groove. The groove shape is rectangular or trapezoidal, and it is evenly distributed radially along the conical mating surface. The depth of the groove is configured to be less than or equal to 0.5 mm, the width of the groove is configured to be less than or equal to 0.8 mm, the radial spacing of the groove is configured to be less than or equal to 6 mm, and the number of grooves is 6 to 8. The positioning structure 161 is used to achieve mechanical positioning and prevent axial movement during the assembly stage.

[0038] The exhaust structure 162 is intersecting and connected with the positioning structure 161 to form a gas escape channel. The exhaust structure 162 is a groove structure, which can be a spiral groove or a herringbone groove. The depth of the groove structure is configured to be less than or equal to 0.3 mm, preferably 0.2~0.3 mm. It is used to achieve the discharge of three types of volatiles during the on-site docking stage: interfacial air, adsorbed water vapor, and cross-linking byproducts.

[0039] Next, after machining, the conical surface is finished using water jet or abrasive flow equipment. Although the preceding CNC cutting process has achieved a surface quality of Ra≤3.2 μm, the cutting process leaves fine parallel tool marks, micro-chipping, and a weak boundary layer on the conical surface (slight oxidation of the surface polyethylene caused by cutting heat and enrichment of cross-linking byproducts). These problems lead to two negative effects: first, the peaks of the micro-tool marks hinder the direct contact of XLPE molecular chains, reducing the effective bonding area at the interface; second, the weak boundary layer becomes the starting point for interfacial delamination, which can easily lead to space charge accumulation during long-term operation. To remove the above defects, the conical surface is finished. Specifically, low-pressure water jet finishing can be used at a pressure of 20MPa with 80-mesh alumina abrasive slurry, tangentially sprayed and scanned twice, followed by rinsing and drying. This results in a surface finish Ra of 1.6μm~3.2μm on the finished conical mating surface, improving the fusion wettability with the cable reactive force cone.

[0040] This embodiment improves the matching accuracy between the reactive force cone surface and the cable body insulation, optimizes the interface bonding quality and mechanical strength, and improves the reliability of submarine cable operation.

[0041] Step S104: The insulation restoration module 16 is fitted onto the submarine cable joint portion, so that the conical mating surface is tightly fitted to the reactive force conical surface, and the insulation restoration module 16 is fused and re-crosslinked to form a factory joint with insulation restoration at the submarine cable joint portion.

[0042] In this embodiment, under a clean construction environment, the first step is to process the submarine cable joint to match the insulation restoration module 16. First, according to the design drawings, the outer sheath 15 (which may include multiple layers including semi-conductive resistive water tape and a metal sheath) and the insulation shielding layer 14 of the cable end are stripped sequentially to the specified length. A special tool is used during the stripping process to ensure a smooth cut and avoid excessive damage to the underlying material. Then, a CNC or high-precision manual chain cutting device is used to perform tapering on the XLPE body insulation layer 13, controlling the cone angle between 25° and 30° to ensure a smooth transition of the electric field along the cone surface. The length of the cone surface is determined according to the voltage level. After the cutting is completed, a special polishing tool and anhydrous ethanol are used to finely polish the cone surface, so that the surface roughness Ra of the reactive cone surface 131 reaches 1.6~3.2μm. Next, at the large end of the reactive cone surface 131 (near the cut-off point of the insulating shielding layer 14), one or more annular protrusions 132 are machined using precision turning or a special forming tool. The axial position, width, and height of these protrusions are precisely configured to form an interference fit with the positioning structure 161 on the insulation restoration module 16. The conductors 11 at both ends are reliably welded using argon arc welding or exothermic welding. The welding process is carried out under inert gas protection to prevent oxidation. After welding, the weld points are finely polished using a special grinding tool to ensure a smooth, burr-free, and uneven surface, thus eliminating the potential for electric field concentration. Finally, a semi-conductive shielding material of the same material as the cable body is extruded under high pressure using a special mold, or semi-overlapped and wrapped using high-quality semi-conductive tape in a clean environment. After wrapping or extrusion, a special tool is used to perform surface vulcanization treatment or hot air shaping to form a continuous, smooth, and non-peelable inner semi-conductive shielding restoration layer 121. The surface of this layer needs to be cleaned and polished to ensure its smoothness.

[0043] The second step involves fitting the insulation restoration module 16 onto the submarine cable connector, ensuring that the annular protrusion 132 on the reactive force cone surface 131 of the main insulation layer 13 tightly engages with the positioning structure 161 on the insulation restoration module 16, thereby achieving a tight fit between the conical mating surface and the reactive force cone surface. Furthermore, the insulation restoration module 16 is designed with an open structure, allowing it to be pried open and directly fitted into the submarine cable connector where the conductor 11 connection and inner semiconductive shielding have been restored. This avoids external diameter obstacles on the cable threading path at the end, simplifying the assembly process and reducing installation difficulty.

[0044] The third step involves fusing and re-crosslinking the insulation restoration module 16 to form a factory joint with restored insulation at the submarine cable joint.

[0045] In this embodiment, the insulation restoration module 16 is fused and re-crosslinked to form a continuous structure between the restoration insulation and the main body insulation at the molecular level, and finally a factory joint with the same material as the main body insulation and no interface traces is formed at the joint.

[0046] This application involves manufacturing a pre-crosslinked insulation restoration module 16 in a factory. This module is then processed to form a conical mating surface that mates with the reactive force cone surface of the submarine cable joint's body insulation. Microstructures, including a positioning structure 161 and an venting structure 162, are formed. The insulation restoration module 16 is then fitted tightly to the submarine cable joint and fused and re-crosslinked to form a factory-jointed joint with restored insulation. This eliminates shear stress caused by thermal expansion and contraction mismatch, improves the factory joint's thermal cycling resistance and service life, effectively reduces the residual concentration of byproducts and eliminates air gaps while ensuring electrical performance. This improves electrical reliability, insulation restoration efficiency and quality, simplifies the installation process, and reduces installation difficulty.

[0047] like Figure 2 The diagram shows the structural composition of a DC submarine cable 10. The DC submarine cable 10 includes a conductor 11, an inner semiconductive shielding layer 12, a main body insulation layer 13, an insulating shielding layer 14, and an outer sheath 15. The conductor 11 is located at the innermost layer of the cable. The inner semiconductive shielding layer 12 covers the conductor 11, the main body insulation layer 13 covers the inner semiconductive shielding layer 12, the insulating shielding layer 14 covers the inner main body insulation layer 13, and the outer sheath 15 covers the insulating shielding layer 14. The outer sheath 15 can be multiple layers, including a semiconductive resistive water tape and a metal sheath. An insulation restoration module 16 is located at the joint to restore insulation continuity and electrical performance at the joint. The inner semiconductive shielding layer 12 includes an inner semiconductive shielding restoration layer 121, which is the restoration structure of the inner semiconductive shielding layer 12 at the joint. It is located outside the conductor 11 connection structure and inside the main body insulation layer 13, and is used to restore the inner shielding continuity at the joint. The main body insulation layer 13 includes a reactive force cone surface 131 and an annular protrusion structure 132. The reactive force cone surface 131 is a conical transition structure formed by machining the end of the main body insulation layer 13. It is located between the main body insulation layer 13 and the inner semiconducting shielding recovery layer 121, so that the thickness of the insulation layer gradually decreases along the axial direction, allowing the electric field to transition smoothly along the insulation surface and avoiding sudden concentration of the electric field at the insulation break. The annular protrusion structure 132 is located on the reactive force cone surface 131 and is used to cooperate with the positioning structure 161 on the insulation recovery module 16 to achieve axial limiting, anti-movement and assembly position calibration.

[0048] like Figure 3 The image shows the insulation restoration module 16 after cutting, including a positioning structure 161 and an venting structure 162. The insulation restoration module 16 is installed at the submarine cable joint to restore the insulation structure interrupted during joint construction, and to reconstruct the insulation thickness and electric field distribution at the joint. The positioning structure 161 is disposed on the tapered mating surface of the insulation restoration module 16, and is used to cooperate with the annular protrusion structure 132 to achieve axial limiting, anti-movement, and assembly position calibration.

[0049] like Figure 4 The diagram shows a cross-sectional view of the insulation restoration module 16. The venting structure 162 is also located on the conical mating surface of the insulation restoration module 16 and communicates with the positioning structure 161 to form a gas escape channel. This channel is used to expel interfacial air, moisture, volatiles, and cross-linking byproducts during on-site docking, vacuum treatment, and high-pressure vulcanization, thereby reducing the interfacial air gap. Furthermore, the insulation restoration module 16 has an open structure, facilitating docking with the submarine cable after the conductor 11 connection and internal semiconducting shield restoration have been completed.

[0050] Optionally, such as Figure 5 As shown, in step S104, the insulation restoration module 16 is fused and re-crosslinked to form a factory joint with restored insulation at the submarine cable joint portion, including: Step S501: The insulation restoration module 16 is preheated and diffused through a segmented heating mold.

[0051] In this embodiment, the insulation restoration module 16 is preheated and diffused through a segmented heating mold. During the preheating stage, the temperature is slowly increased to 125-140°C, which falls within the high-elasticity range of the semi-crosslinked XLPE and is far below the rapid decomposition temperature of the DCP crosslinking agent. At this point, the material only softens and rebounds without undergoing a violent crosslinking reaction. This ensures that the conical mating surface fully fills the micro-gap while preventing excessive material flow from collapsing the exhaust channels. Simultaneously, the vacuum system is activated and maintained for 18-25 minutes to allow the conical mating surface to fully fuse and degas, achieving module softening, conical surface fusion, and vacuum extraction of interfacial air and some crosslinking byproducts, preparing for the next stage of deep fusion. During the diffusion stage, the temperature rises to 150-158°C, which falls within the rapid decomposition range of the DCP crosslinking agent. At this point, the modulus of the semi-crosslinked XLPE further decreases, and the molecular chain segment mobility is significantly enhanced. The XLPE molecular chains on both sides of the interface can interpenetrate and entangle. Simultaneously, the residual DCP decomposes to generate new free radicals, forming new CC crosslinking bonds at the interface, thus insulatingly bonding the module to the body. Continue vacuum evacuation for 8-12 minutes to promote molecular chain mutual solubility, deep interfacial fusion, and further degassing.

[0052] Step S502: High-pressure vulcanization is performed on the fused module. One or more parameters, including time, temperature, and gel content, are monitored during the vulcanization process and compared with the calculation results in the preset simulation model to obtain the optimal parameter values ​​for re-crosslinking. Based on the optimal parameter values, a factory joint with restored insulation is formed at the submarine cable joint.

[0053] In this embodiment, after the diffusion fusion stage ends and deep degassing is completed, the vacuum system is shut down and evacuation is stopped. High-purity nitrogen (purity ≥99.999%) or saturated steam is introduced into the heating jacket and vulcanization mold to remove residual air and prevent thermal oxidation degradation of the insulation at high temperatures. The high-pressure vulcanization process is then initiated, gradually increasing the system pressure to 1.6~1.8 MPa. This pressure range is sufficient to suppress the foaming tendency of XLPE at high temperatures and force the molten material to fully fill the microstructure voids. Simultaneously, the temperature is further increased from 150~158℃ in the diffusion stage to 168~175℃, entering the temperature range for complete decomposition and rapid crosslinking of the DCP crosslinking agent. During the vulcanization process, one or more parameters, including time, temperature, and gel content, are monitored in real time and compared with the calculation results in the preset simulation model.

[0054] The preset simulation model can be established based on the existing COMSOL Multiphysics finite element software to create a multiphysics coupling model. The model input parameters may include: cable specifications (conductor cross-section, insulation thickness), target gel content of insulation recovery module 16 (48%±3%), ambient temperature, heating jacket power distribution, etc. Those skilled in the art should understand that the specific implementation of the preset simulation model does not affect the specific implementation of the embodiments of this application, and this application does not limit the specific implementation method of the preset simulation model.

[0055] For example, during the vulcanization process, 12-16 thermocouples arranged along the mold axis monitor the temperature of each section in real time, comparing the measured temperature field with the ideal temperature field in the preset simulation model. If the temperature in the interface area is detected to be too low or the heating is delayed (which may lead to insufficient crosslinking and high thermal elongation), the system automatically increases the power of the heating jacket in that area or appropriately extends the holding time. If the local temperature is detected to be too high (which may lead to excessive crosslinking, low thermal elongation, or even scorching), the system automatically reduces the heating power. The holding time is not a fixed value, but is dynamically calculated by the simulation model based on real-time temperature data; a typical holding time is 12-20 minutes. The model predicts the time required to reach the gel content and thermal elongation in the pre-crosslinked state under the current process conditions, obtaining the optimal parameter values ​​for re-crosslinking, which serve as the basis for terminating vulcanization. Based on the optimal parameter values, a factory joint with restored insulation is formed at the submarine cable joint section.

[0056] After the factory joint with restored insulation is formed, the heating system is shut off, and the temperature is lowered to ≤80℃ (at which point XLPE is basically set and below the residual decomposition temperature of DCP). Then, the pressure relief valve is slowly opened to gradually release the internal high-purity nitrogen or vapor pressure to atmospheric pressure, avoiding internal bubbles or microcracks caused by sudden pressure drops. After depressurization, the heating jacket and vulcanization mold are removed, exposing the joint body with restored insulation. Internal defects are screened without damaging the joint. Two complementary non-destructive testing techniques, X-ray digital imaging (DR) and ultrasonic phased array (PAUT), are used to inspect the fusion quality. Specifically, X-ray digital imaging is used to check whether there are residual air gaps, impurities, micropores, and metallic foreign objects (such as cutting debris accidentally left during construction) in the interface area between the insulation restoration module 16 and the main body insulation layer 13. The judgment criteria are uniform image background, no continuous or discontinuous black lines (air gap characteristics) in the interface transition area, and no abnormal spots. Ultrasonic phased array can distinguish between fully fused, partially fused, and unfused regions, and can quantitatively assess the filling status of microstructures. The judgment criteria are that the interface echo signal is continuous and uniform, without abnormal missing or enhanced signals, indicating that the interface is not delaminated and the microstructure is densely filled.

[0057] After all the above quality tests are passed, the insulation shielding layer 14 and the outer sheath 15 (which may include multiple layers such as semi-conductive resistive water tape, metal sheath, and various external protective layers) are restored sequentially according to the conventional factory joint manufacturing process, thus completing the preparation of the entire factory joint. This improves the electrical reliability, insulation restoration efficiency, and quality of the DC submarine cable factory joint.

[0058] In one embodiment, this application also provides a DC submarine cable 10, including a factory joint formed by the modular insulation restoration method of the DC submarine cable 10 described in any of the above embodiments.

[0059] This application achieves at least the following technical effects through the above embodiments: 1. This application eliminates shear stress caused by thermal expansion and contraction mismatch by manufacturing pre-crosslinked insulation restoration modules in the factory, thereby improving the thermal cycling resistance and service life of the factory joints.

[0060] 2. The present application processes a microstructure that forms a conical mating surface and a positioning and venting structure. This effectively reduces the residual concentration of by-products and eliminates air gaps while ensuring electrical performance. It also improves electrical reliability, insulation recovery efficiency and quality, simplifies the installation process, and reduces installation difficulty.

[0061] 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 modular insulation restoration method for DC submarine cables, characterized in that, include: A cross-linked polyethylene (XLPE) composition is prepared according to a preset formula. The XLPE composition is mixed and granulated in an internal mixer at 120-130°C to obtain XLPE granules. The XLPE granules are then extruded into tubular preforms using a tubular extruder. The tubular preforms have a hollow structure, with an inner diameter 0.8-1.5 mm larger than the inner diameter of the main body insulation of the submarine cable joint to be connected, and an outer diameter 0.5-1.0 mm larger than the outer diameter of the main body insulation of the submarine cable joint to be connected. The preset formula includes a matrix resin, a cross-linking agent, a heat stabilizer, and a space charge inhibitor. The matrix resin is configured as 100 parts by weight of ultra-clean low-density polyethylene (LDPE), the cross-linking agent is configured as 1.6-2.0 parts by weight of dicumyl peroxide (DCP), the heat stabilizer is configured as 0.3-0.6 parts by weight of an antioxidant, and the space charge inhibitor is configured as less than or equal to 3.0 parts by weight of nano-SiO2. The tubular blank is pre-crosslinked using a segmented heating vulcanization process to obtain an insulation recovery module in a pre-crosslinked state, which is between an uncrosslinked state and a fully crosslinked state. The insulation restoration module is processed to form a conical mating surface that matches the reactive force cone surface of the body insulation of the submarine cable joint. Microstructures of positioning structure and venting structure are formed on the conical mating surface. The insulation restoration module is fitted onto the submarine cable joint, so that the conical mating surface fits tightly against the reactive force conical surface. The insulation restoration module is then fused and re-crosslinked to form a factory joint with restored insulation at the submarine cable joint.

2. The modular insulation restoration method for DC submarine cables according to claim 1, characterized in that, The tubular blank is pre-crosslinked using a segmented heating vulcanization process to obtain an insulation recovery module in a pre-crosslinked state. This pre-crosslinked state is between an uncrosslinked state and a fully crosslinked state, and includes: The segmented heating vulcanization process includes a preheating section, a low-temperature cross-linking section, a medium-temperature cross-linking section, and a cooling section; The target gel content in the pre-crosslinked state is between the gel content in the uncrosslinked state and the fully crosslinked state, and is configured to be 45%~55%; When the torque of the tubular preform reaches a preset torque threshold corresponding to the target gel content, it enters the cooling section.

3. The modular insulation restoration method for DC submarine cables according to claim 1, characterized in that, The insulation restoration module is processed to form a conical mating surface that mates with the reactive force cone surface of the submarine cable joint portion's body insulation, including: The insulation restoration module is cut at both ends by a CNC lathe to form a conical mating surface that matches the reactive force cone surface of the submarine cable joint. The surface roughness of the conical mating surface is configured to be less than or equal to 3.2 μm.

4. The modular insulation restoration method for DC submarine cables according to claim 3, characterized in that, The microstructures forming positioning and venting structures on the conical mating surface include: The positioning structure is an annular groove, which is evenly distributed radially along the conical mating surface. The depth of the groove is configured to be less than or equal to 0.5 mm, the width of the groove is configured to be less than or equal to 0.8 mm, and the radial spacing of the groove is configured to be less than or equal to 6 mm. The exhaust structure is intersecting and connected with the positioning structure. The exhaust structure is a groove structure, and the depth of the groove structure is configured to be less than or equal to 0.3 mm.

5. The modular insulation restoration method for DC submarine cables according to claim 4, characterized in that, The insulation restoration module is fitted onto the submarine cable joint, ensuring that the tapered mating surface fits tightly against the reactive force tapered surface, including: An annular protrusion is formed on the reactive force cone surface to cooperate with the positioning structure, so that the positioning structure of the cone-shaped mating surface is tightly fitted to the annular protrusion on the reactive force cone surface.

6. The modular insulation restoration method for DC submarine cables according to claim 5, characterized in that, The insulation restoration module is fused and re-crosslinked to form a factory joint with restored insulation at the submarine cable joint, including: The insulation restoration module is preheated and diffused fused using a segmented heating mold; The fused module is subjected to high-pressure vulcanization. One or more parameters, including time, temperature, and gel content, are monitored during the vulcanization process and compared with the calculation results in the preset simulation model to obtain the optimal parameter values ​​for re-crosslinking. Based on the optimal parameter values, a factory joint with restored insulation is formed at the submarine cable joint.

7. A DC submarine cable, characterized in that, include: Factory joints formed by the modular insulation restoration method for DC submarine cables according to any one of claims 1-6.

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