Water-tree-resistant cross-linked polyethylene insulating composition for high-voltage direct-current submarine cable as well as preparation method and application of water-tree-resistant cross-linked polyethylene insulating composition

Abstract

The invention belongs to the technical field of high-voltage cross-linked polyethylene compositions, and particularly relates to a water-tree-resistant cross-linked polyethylene insulating composition for a high-voltage direct-current submarine cable and a preparation method and application of the water-tree-resistant cross-linked polyethylene insulating composition. The insulating composition comprises the following components in parts by weight: 80-120 parts of low-density polyethylene resin with a melt index of 1.9-2.1 g / 10min; 0.2 to 0.3 part of a composite antioxidant; 1.25 to 1.85 parts of a composite cross-linking agent; 0.2 to 0.3 part of a scorch inhibitor; 8-12 parts of a water-inhibiting master batch; wherein the water-inhibiting master batch is prepared from 80 to 120 parts of composite polyolefin resin, 6 to 9 parts of composite water-inhibiting agent, 3.6 to 5.4 parts of maleic anhydride graft and 0.15 to 0.25 part of antioxidant; the composite water inhibitor comprises polyethylene glycol and oleamide in a mass ratio of (8-12): 1. The insulation composition can inhibit insulation scorching and resist insulation water tree growth, and is high in gel content, high in extrusion cabling speed and stable in production process.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-voltage cross-linked polyethylene compositions, and in particular relates to a water-tree-resistant cross-linked polyethylene insulation composition for high-voltage direct current submarine cables, and a preparation method and application thereof. Background Art

[0002] As the global economy continues to develop, electricity demand is rapidly increasing, and power security is crucial to national security. Cross-linked polyethylene (XLPE), due to its excellent mechanical, electrical, and heat-resistant properties, is currently widely used in the manufacture of HVDC cable insulation. In recent years, my country has continuously increased its offshore wind power infrastructure construction. HVDC cables insulated with XLPE, with their simple structure and high current carrying capacity, have been widely used for submarine-to-land DC transmission.

[0003] During the production and use of traditional high-voltage chemically cross-linked polyethylene insulation, it was discovered that due to the structural limitations of low-density polyethylene resins, the cross-linker content needed to be increased to achieve better mechanical and heat resistance. However, upon decomposition during vulcanization and cross-linking, the cross-linker produces byproducts such as water and micropores containing low-molecular-weight gases. Under the combined effects of factors such as high-voltage electric fields, operating time, and high seabed humidity, water byproducts in the cable insulation, or moisture intruding from outside, can migrate freely, forming dendritic channels that continuously grow in the form of dendrites. This gradually degrades the electrical performance of the cable insulation. Once the water dendrites grow and reach a certain size, they transform into electrical dendrites, ultimately leading to cable insulation breakdown.

[0004] At the same time, submarine cables for large-span power transmission require a long continuous extrusion cycle for cable insulation, and a high cross-linking agent content will also increase the risk of pre-cross-linking during the cable insulation extrusion process, reducing the cable production cycle and efficiency; more importantly, these pre-cross-linking defects and the pores produced by cross-linking by-products will be superimposed, which will further aggravate the formation of insulation water trees, adversely affecting the overall performance of the cable.

[0005] The information disclosed in this background technology section is only intended to deepen the understanding of the overall background technology of the present invention and should not be regarded as an admission or any form of suggestion that the information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0006] In order to solve the above technical problems, the first object of the present invention is to provide a high-voltage insulation composition that can inhibit insulation scorch, resist insulation water tree growth, has a high gel content, a fast extrusion cabling speed, and a stable production process.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable comprises the following components in parts by weight:

[0009] 80-120 parts of polyethylene resin;

[0010] 0.2-0.3 parts of compound antioxidant;

[0011] 1.25-1.85 parts of composite cross-linking agent;

[0012] 0.2-0.3 parts of scorch inhibitor;

[0013] 8.0-12 parts of jellyfish-inhibiting particles;

[0014] The polyethylene resin is a low-density polyethylene resin having a melt index of 1.9-2.1 g / 10 min; for example, the melt index of the low-density polyethylene resin is 1.9 g / 10 min, 2.0 g / 10 min, or 2.1 g / 10 min.

[0015] More preferably, the low-density polyethylene resin used is BASF-Yangtze 2220HSC or Shanghai Petrochemical J182C.

[0016] In the present invention, the melt index and melt index range of the low-density polyethylene resin ensure the electrical performance, environmental stress cracking resistance, and extrusion processability of the HVDC water-tree-resistant cross-linked polyethylene insulation composition. The use of a single, stable low-density polyethylene resin ensures stable insulation thickness for high-voltage cables, especially submarine cables, during long extrusion processes. This overcomes the problem of using composite resins, which can lead to large insulation thickness fluctuations due to uneven mixing or melt index deviations, and can result in substandard insulation at the thinnest point.

[0017] Preferably, the composite cross-linking agent includes dicumyl peroxide and triallyl isocyanurate, and the mass ratio of dicumyl peroxide to triallyl isocyanurate is 1-1.5:0.25-0.35; the scorch inhibitor is 2,4-diphenyl-4-methyl-1-pentene, and the mass ratio of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide and triallyl isocyanurate is 1:4.2-5.8:1.05-1.3.

[0018] Although 2,4-diphenyl-4-methyl-1-pentene can improve the rigidity of the polyethylene molecular chain and slow down the movement rate of the molecular chain through the benzene ring group when used alone, due to the lack of cross-linking agent to initiate cross-linking, a sufficient cross-linking network cannot be formed in the subsequent cross-linking stage of the insulation layer, resulting in serious deficiencies in the thermal extension and mechanical properties of the insulation material. Moreover, the scorch inhibitor itself cannot inhibit the local aggregation and overheating caused by the random thermal motion of the molecular chain, and long-term extrusion may still indirectly cause scorch tendencies.

[0019] The combination of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide, and triallyl isocyanurate maximizes the anti-scorch effect. The benzene ring groups in the 2,4-diphenyl-4-methyl-1-pentene molecule anchor some molecular chains during thermal absorption and the initial stages of insulation extrusion, reducing their thermal activity and delaying localized overheating and premature crosslinking caused by random collisions of molecular chains. Furthermore, the benzene ring structure, when incorporated into the molecular chain, enhances rigidity and heat resistance. It is only during the subsequent crosslinking stage of cable insulation preparation that dicumyl peroxide gradually decomposes to produce a large number of free radicals. At this point, the pre-inhibition effect of the scorch inhibitor is complete, ensuring that free radicals only initiate effective crosslinking after the insulation layer is formed, reducing the risk of scorch. Triallyl isocyanurate disperses free radical active sites through polyallyl groups, avoiding micro-scorching caused by localized aggregation of free radicals. At the same time, it promotes the formation of a uniform and dense cross-linked network. Combined with the enhanced molecular chain rigidity of 2,4-diphenyl-4-methyl-1-pentene, it further enhances the heat resistance and deformation resistance of the insulating material, fundamentally reducing the risk of water treeing / electrical treeing during long-term operation.

[0020] Furthermore, the mass ratio of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide, and triallyl isocyanurate is preferably 1:5:1.15-1.25. This ratio prevents the scorch inhibitor from failing due to an excess of dicumyl peroxide, nor does it lead to insufficient crosslinking and low gel content due to insufficient dicumyl peroxide. This ensures both long-cycle extrusion and complete crosslinking. Furthermore, the number of allyl groups in triallyl isocyanurate is just enough to capture and transfer the majority of the free radicals from dicumyl peroxide decomposition, reducing ineffective free radical loss and achieving a more uniform distribution of crosslinking points, significantly improving the consistency of material properties.

[0021] The scorch inhibitor has a 2,4-diphenyl-4-methyl-1-pentene content of ≥99%, an impurity particle size of ≤70 μm, and a number of impurities per kilogram of ≤5. For example, the scorch inhibitor may be AMSD-MBL from Huazhongrong.

[0022] Dicumyl peroxide is used as a crosslinking agent; the crosslinking agent has a dicumyl peroxide content ≥ 99%, a total active oxygen content ≤ 5.92%, an impurity particle size ≤ 70 μm, and a number of impurities per kilogram ≤ 5. For example, the crosslinking agent can be dicumyl peroxide from Arkema.

[0023] Triallyl isocyanurate is used as a crosslinking agent. Specifically, the crosslinking agent has a triallyl isocyanurate content of ≥99%, an impurity particle size of ≤70 μm, and a number of impurities per kilogram of ≤5. For example, the crosslinking agent can be TAIC from Fangruida.

[0024] In the present invention, because long-span submarine cables for power transmission require long continuous extrusion cycles for cable insulation, a specific amount of composite crosslinking agent cannot effectively balance the thermal extension, mechanical, and scorch resistance of the cable insulation. By blending a clean scorch inhibitor with the composite crosslinking agent, the benzene ring groups in the scorch inhibitor are incorporated into the polyethylene molecular chains during the cable insulation crosslinking process, increasing the rigidity of the polyethylene molecular chains and effectively ensuring the heat resistance and mechanical properties of the insulation composition at a specific composite crosslinking agent dosage. This also reduces the occurrence of scorch on the polyethylene molecular chains during cable insulation extrusion, improving the cable extrusion production cycle and efficiency.

[0025] The specific addition amounts of dicumyl peroxide and triallyl isocyanurate in the present invention help reduce the total crosslinking agent content in the high-voltage direct current water-tree resistant cross-linked polyethylene insulation composition, reduce pre-crosslinking or old glue generated during the extrusion process of the insulation material, and reduce pores and other by-products generated during the cross-linking reaction. This prevents the formation of dendritic and continuously growing branch structure channels in the cable insulation due to pores or other by-products under the combined effects of multiple factors such as high-voltage electric fields and operating time, and is beneficial to improving the electrical performance of the cable.

[0026] The water-inhibiting masterbatch is obtained by mixing and granulating composite polyolefin resin, composite water-inhibiting agent, maleic anhydride graft and antioxidant in a mass ratio of 80-120:6.0-9.0:3.6-5.4:0.15-0.25; the composite water-inhibiting agent includes polyethylene glycol and oleamide in a mass ratio of 8-12:1.

[0027] Preferably, the total oleamide content in the water-inhibiting masterbatch is ≥98.5%, the impurity particle size is ≤150 μm, and the number of impurities per kilogram is ≤10. For example, the oleamide can be oleamide from Zhilian New Materials. The average molecular weight of the polyethylene glycol is 20,000, for example, PEG-20000 from Beko Chemical. The weight proportion of polyethylene glycol in the water-inhibiting masterbatch is 5-7%.

[0028] If the polyethylene glycol dosage is too high, it is easy to precipitate from the matrix during processing or long-term use, forming surface frost or internal aggregation. The precipitated polyethylene glycol may undergo abnormal reactions during high-temperature extrusion, or the local concentration may be too high, resulting in uneven melting state, increasing the probability of scorching, generating old glue points or hard particles, and destroying the uniformity of the insulating layer. The anti-water tree effect of polyethylene glycol depends on the large number of hydroxyl groups (-OH) in its molecular chain to form hydrogen bonds with water molecules, thereby fixing the moisture that invades the insulating layer and blocking the "water source" for the growth of water trees. When the dosage is too low, the number of hydroxyl groups is insufficient and it is impossible to effectively capture moisture. Under the action of the electric field, moisture will migrate and aggregate along the resin defects, accelerating the formation and expansion of water tree branches, resulting in a significant decrease in anti-water tree performance. The present invention controls the proportion of polyethylene glycol to 5-7%. The number of hydroxyl groups within this range is sufficient to effectively fix moisture, and combined with the dispersing effect of oleic acid amide, it ensures that polyethylene glycol is evenly distributed in the resin matrix and blocks the water tree growth path.

[0029] Preferably, the composite polyolefin resin in the water-suppressing masterbatch comprises a low-density polyethylene resin with a melt index of 1.9-2.1 g / 10 min and a polyolefin elastomer resin with a melt index of 4.5-5.5 g / 10 min, with the mass ratio of the two being 1-1.5:1. Specifically, the low-density polyethylene resin employed has a dielectric loss tangent of ≤0.0003. For example, the low-density polyethylene resin may be BASF-Yangzi 2220HSC. Specifically, the polyolefin elastomer resin employed is an ethylene-octene copolymer, for example, the polyolefin elastomer may be Dow 8200.

[0030] Preferably, the melt index of the maleic anhydride grafted material in the water-inhibiting masterbatch is 1.0-3.0 g / 10 min. Specifically, the maleic anhydride grafted material is maleic anhydride grafted polyethylene, and the maleic anhydride MAH group content is ≥0.5%. For example, the maleic anhydride grafted polyethylene can be Nengzhiguang MC226.

[0031] Preferably, the antioxidant in the water-inhibiting mother particles is a thiophenol antioxidant. Thiophenol antioxidants, such as Antioxidant 300 and Antioxidant 1035, contain both phenolic hydroxyl groups and sulfur atoms (-S-) in their molecules. They combine the free radical-scavenging ability of hindered phenols with the hydroperoxide-decomposing function of sulfur. These composite antioxidants are particularly effective in high-temperature or harsh environments.

[0032] For long-cycle extruded submarine DC cables, traditional chemically cross-linked insulation materials, scorch-resistant chemically cross-linked insulation materials, or nano-modified DC chemically cross-linked insulation materials cannot effectively inhibit the formation and growth of water trees in cable insulation, which will inevitably affect the service life of high-voltage submarine cables.

[0033] In the present invention, the polyolefin elastomer in the water-inhibiting masterbatch exhibits excellent compatibility with low-density polyethylene (LDPE) and can be effectively and evenly dispersed in the non-crystalline regions of the LDPE, promoting compatibility between the water-inhibiting masterbatch matrix and the composite water-inhibiting agent. The maleic anhydride groups of the maleic anhydride-grafted polyethylene undergo an esterification reaction with the hydroxyl groups of the polyethylene glycol, and the polyethylene segments exhibit excellent compatibility with the polyethylene matrix. Under the action of the maleic anhydride-grafted polyethylene, the high-molecular-weight polyethylene glycol in the composite water-inhibiting agent effectively links with the LDPE resin, making it less likely to precipitate in the cable insulation. The hydroxyl groups in the polyethylene glycol exhibit excellent hydrophilicity, immobilizing water molecules in the cable insulation and significantly reducing the formation and growth of water dendrites in the insulation. Furthermore, oleamide reduces internal friction during the preparation of the water-inhibiting masterbatch, improving the fluidity of the composite polyolefin resin in its molten state, and thereby reducing uneven mixing of the polyethylene glycol with the composite polyolefin resin after premature melting due to its lower melting point.

[0034] As a preferred method, the preparation method of the water-inhibiting mother particle comprises:

[0035] S1: mixing a low-density polyethylene resin and a polyolefin elastomer resin to prepare a composite polyolefin resin, and mixing polyethylene glycol and oleamide to prepare a composite water inhibitor;

[0036] Specifically, the melt index of low-density polyethylene resin and polyolefin elastomer resin is quite different. Pre-mixing can ensure that they are more evenly dispersed in the subsequent preliminary mixing in the high-speed mixer and twin-screw melt extrusion, avoiding stratification or local performance unevenness caused by differences in density and fluidity; the ratio of polyethylene glycol to oleamide is 8-12:1. Oleamide can be first crushed into fine powder, then mixed with polyethylene glycol particles in proportion to form a composite water inhibitor, and then put into the high-speed mixer to make oleamide evenly dispersed in the polyethylene glycol, avoiding product performance fluctuations caused by local aggregation of additives in subsequent processes.

[0037] S2: putting the composite polyolefin resin, composite water inhibitor, maleic anhydride grafted product and antioxidant into a high-speed mixer, using a low-speed mixing mode, and mixing at a speed of 30±2rpm for 40-60s;

[0038] Specifically, the low speed is used to achieve a preliminary uniform mixing of the raw materials, avoiding excessive friction, heating, or stratification of the raw materials caused by high-speed stirring. The 40-60s mixing time ensures that the four raw materials are initially dispersed evenly in a short period of time, preparing for subsequent melt extrusion.

[0039] In step S3, the mixed material after low-speed mixing in step S2 is fed into a twin-screw extruder through a feeding device. At a shear speed of 25-35 rpm, the screw temperature is controlled to gradually increase from the feed section to the die head, and the temperature of the final section is 155-165°C, and the material temperature is 160-180°C. After filtering through a multi-layer filter installed on the die head, the mixed material is granulated and dried by water drawing to obtain clean water-inhibiting mother particles. The filter screen includes at least one layer of 300-mesh high-precision filter screen.

[0040] Specifically, three layers of filter screens are installed, of which the high-precision filter screen has a mesh size of 300 and can filter out impurities larger than 50 μm. Through water-stretching granulation, a production capacity of 200-300 kg / h is achieved.

[0041] Preferably, the composite antioxidant includes at least two of hindered phenol antioxidants, thioester antioxidants, and thiophenol antioxidants.

[0042] Specifically, hindered phenol antioxidants, such as Antioxidant 1010, contain a sterically hindered phenolic hydroxyl group (-OH) in their molecular structure, which interrupts the oxidation chain reaction by capturing free radicals. Thioester antioxidants, such as DLTP (dilauryl thiodipropionate), decompose hydroperoxides (ROOH) to form stable products, which synergize with hindered phenol antioxidants to enhance antioxidant efficiency. Thiophenol antioxidants include Antioxidant 300 and Antioxidant 1035.

[0043] Specifically, the antioxidants used are antioxidant 300 and antioxidant 1010 in a mass ratio of 1-2:1; or the antioxidants used are antioxidant 300, antioxidant 1010, and antioxidant DLTP in a mass ratio of 1-1.5:1-1.5:1; or the antioxidants used are antioxidant 1035, antioxidant 1010, and antioxidant DLTP in a mass ratio of 0.8-1.2:1-1.5:1.

[0044] In the present invention, a specific antioxidant can effectively improve the heat resistance and anti-scorch properties of the HVDC water-tree-resistant cross-linked polyethylene insulation composition. For the long-term extrusion processing of high-voltage submarine cable insulation, even an excessive amount of a single antioxidant is insufficient to protect the material's heat aging resistance and anti-scorch properties during production, cable insulation extrusion, and use. Instead, excessive antioxidant migration and precipitation during long-term operation can affect the cable's performance, heat resistance, and electrical properties.

[0045] Another object of the present invention is to provide a method for preparing the above-mentioned water-tree-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables, wherein polyethylene resin, composite antioxidant, and water-inhibiting mother particles are fed into a BUSS high-speed shearing machine, and the composition is plasticized, finely filtered, granulated underwater, and blended with the composite cross-linking agent and the scorch inhibitor for thermal insulation absorption to obtain the water-tree-resistant cross-linked polyethylene insulation composition for high-voltage DC submarine cables.

[0046] A method for preparing a water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable comprises:

[0047] A1 feeds polyethylene resin, compound antioxidant, and water-inhibiting masterbatch into a BUSS high-speed shearing machine through a vector weighing feeding system. After plasticization and impurities filtration, the material is formed through an underwater granulation process. After granulation, the particles are dehydrated and heated to dry.

[0048] A2 adds a composite cross-linking agent and a scorch inhibitor at 65°C-75°C to a mixing system with a rotation speed of 2-6 rpm, and the particles, composite cross-linking agent and scorch inhibitor are blended, dried, and then heat-insulated and absorbed to obtain a high-voltage DC water-tree-resistant cross-linked polyethylene insulation composition.

[0049] Preferably, in step A1, the speed of the BUSS high-speed shearing machine is 270-350 rpm, the screw temperature is 80-120°C, the barrel temperature is 100-140°C, the material temperature is 185-205°C, and the material is filtered through a multi-layer filter installed on the head, and then obtained by underwater granulation and drying. The filter includes at least one layer of 500-mesh high-precision filter.

[0050] Specifically, four layers of filters can be installed, of which the high-precision filter has a mesh size of 500 and can filter impurities larger than 25 μm, and the production capacity of 2.5 T / h-3.5 T / h can be achieved through underwater granulation.

[0051] Preferably, in step A2, the insulation absorption time is 10-14 hours, and the insulation temperature is 70-80°C.

[0052] The present invention implements high-precision filtration designs for the preparation stages of water-inhibiting masterbatch and insulating composition, respectively, to ensure the insulation performance of high-voltage DC submarine cables. Specifically, three layers of filter screens are set up during the preparation of the water-inhibiting masterbatch to filter impurities larger than 50 μm, ensuring the cleanliness of the water-inhibiting masterbatch itself and ensuring the uniform dispersion of polyethylene glycol, laying the foundation for the water-tree resistance of the subsequent insulating layer. During the preparation of the insulating composition, four layers of filter screens are set up to filter impurities larger than 25 μm. The resulting insulating composition is directly used to form the submarine cable insulation layer, avoiding local discharge and electric field distortion caused by the long-term operation of the high-voltage DC submarine cable insulation layer in high electric field and humid environments. The insulating material's microscopic uniformity is improved to ensure uniform electric field distribution and reduce the risk of local breakdown. Combined with a low content of composite cross-linking agent, the design further reduces pores and byproducts, synergistically improving the electrical performance of the insulating layer.

[0053] Another object of the present invention is to provide a submarine DC power cable capable of achieving a long-term operating temperature of 90°C. This cable is prepared from the aforementioned high-voltage DC water-tree-resistant cross-linked polyethylene insulation composition. It should be noted that the cable includes, but is not limited to, a high-voltage DC submarine cable.

[0054] The cable formed by the water-tree resistant cross-linked polyethylene insulation composition for high-voltage direct current submarine cable of the present invention can achieve a long-term operating temperature of 90°C.

[0055] The beneficial effects of the present invention are:

[0056] (1) The present invention adopts a single low-density polyethylene resin matrix with a stable melt index, which can keep the insulation thickness of the submarine cable stable during the long-term extrusion process, and overcome the problem that the insulation thickness fluctuates greatly due to uneven mixing or melt index deviation when using composite resins, thereby making the thinnest point of the high-voltage cable insulation thickness unqualified; the introduction of composite antioxidants can delay the thermal oxidation aging of the material, avoid performance degradation during processing and long-term operation, and improve the production qualification rate and product reliability. At the same time, by limiting the coordinated use of composite cross-linking agents and scorch inhibitors, the pre-cross-linking or old glue generated during the extrusion of the insulation material and the pores and other by-products generated during the cross-linking reaction are reduced, and the generation of dendritic and continuously growing dendritic structure channels is avoided, which is beneficial to the improvement of the electrical performance of the cable, and can also reduce the generation of polyethylene molecular chain scorch during the cable insulation extrusion process, thereby improving the cable extrusion production cycle and efficiency.

[0057] (2) The polyolefin elastomer in the water-inhibiting masterbatch has good compatibility with low-density polyethylene and can be effectively and evenly dispersed in the non-crystalline region of low-density polyethylene, promoting the compatibility between the water-inhibiting masterbatch matrix and the composite water-inhibiting agent; under the action of maleic anhydride grafted polyethylene, the high molecular weight polyethylene glycol in the composite water-inhibiting agent is effectively linked to the low-density polyethylene resin, making it difficult to precipitate in the cable insulation; through the hydroxyl groups with excellent hydrophilicity in the polyethylene glycol, the water molecules in the cable insulation are fixed, which will greatly reduce the formation and growth of insulating water dendrites; at the same time, oleic acid amide can reduce the internal friction of the water-inhibiting masterbatch during the preparation process, improve the fluidity of the composite polyolefin resin in the molten state, and thus reduce the mixing unevenness of the polyethylene glycol with the composite polyolefin resin after it melts in advance due to its low melting point. This scheme ensures that the water-inhibiting component is evenly dispersed in the resin matrix by introducing pre-prepared water-inhibiting masterbatch, so that the insulating material can still maintain stable insulation performance in a long-term humid electric field environment, thereby extending the service life of the cable.

[0058] (3) The anti-water-tree type insulation composition for high-voltage DC submarine cables of the present invention has a high and evenly distributed gel content, ensuring that the cross-linked network of the insulation layer is dense and stable, providing solid support for electrical performance. At the same time, the material has excellent adaptability, can be quickly extruded into cables, and the insulation thickness fluctuation during the production process is small, the process stability is strong, and the long-term operation reliability of the high-voltage submarine cable is effectively guaranteed. The corresponding preparation method is mature and stable. Through the optimized pre-mixing, melt plasticization and thermal insulation absorption process, large-scale mass production is achieved, and the output can reach 2.5T / h-3.5T / h. The parameters of each link are highly controllable, which greatly reduces the risk of batch fluctuations, takes into account both production efficiency and product quality consistency, and is suitable for the industrial production needs of large-span submarine cables. DETAILED DESCRIPTION

[0059] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0060] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used in this specification are for the purpose of describing specific embodiments only and are not intended to limit the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0061] Example 1

[0062] This embodiment provides a water-tree-resistant cross-linked polyethylene insulation material for a high-voltage direct current submarine cable, which includes the following components in parts by weight:

[0063] 100 parts of low-density polyethylene resin; 0.25 parts of composite antioxidant; 1.55 parts of composite cross-linking agent; 0.25 parts of 2,4-diphenyl-4-methyl-1-pentene; 10 parts of water-inhibiting mother particles;

[0064] The composite antioxidant includes 0.15 parts of antioxidant 300 and 0.1 parts of antioxidant 1010;

[0065] The composite crosslinking agent includes 1.25 parts of dicumyl peroxide and 0.3 parts of triallyl isocyanurate;

[0066] The water-inhibiting masterbatch is made by mixing and granulating a composite polyolefin resin, a composite water inhibitor, maleic anhydride-grafted polyethylene, and antioxidant 300 in a mass ratio of 100:7.5:4.5:0.25. The composite water inhibitor comprises polyethylene glycol and oleamide in a mass ratio of 10:1. The composite polyolefin resin contains low-density polyethylene resin and ethylene-octene copolymer in a mass ratio of 1.2:1.

[0067] The preparation method of water-inhibiting jellyfish comprises:

[0068] S1: mixing a low-density polyethylene resin and a polyolefin elastomer resin to prepare a composite polyolefin resin, and mixing polyethylene glycol and oleamide to prepare a composite water inhibitor;

[0069] S2: putting the composite polyolefin resin, composite water inhibitor, maleic anhydride grafted product and antioxidant into a mixer and mixing at a speed of 30 rpm for 50 seconds;

[0070] S3 feeds the mixed material into a twin-screw extruder at a shear speed of 30 rpm. The screw temperature is gradually increased from the feed section to the die head, with the final section temperature at 160°C and the material temperature at 160-180°C. After being filtered through three layers of filter screens installed in the die head, the mixed material is pelletized by water stretching. The filter screens include at least one layer of 300-mesh high-precision filter screen. The high-precision filter screen can filter impurities larger than 50 μm.

[0071] A method for preparing a water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable comprises:

[0072] A1 is produced using the "four-screw, four-pin" BUSS production line imported from Switzerland. Polyethylene resin, compound antioxidant, and water-inhibiting masterbatch are fed into a BUSS high-speed shear through a vector weighing feeding system. The BUSS high-speed shear speed is 310rpm, the screw temperature is 80-120℃, the barrel temperature is 100-140℃, and the material temperature is 185-205℃. After filtering through four layers of filter screens installed on the die head, the product is granulated and dried underwater. The filter screens include at least one layer of 500-mesh high-precision filter screen. After granulation, the particles are dehydrated and heat-dried. The 500-mesh high-precision filter screen removes impurities larger than 25μm.

[0073] A2 added a composite cross-linking agent and a scorch inhibitor at 70°C into a mixing system with a rotation speed of 4 rpm. The particles, composite cross-linking agent and scorch inhibitor were blended and dried, and then thermally absorbed. The thermal absorption time was 12 hours, and the thermal insulation temperature was 75°C to obtain a high-voltage DC water-tree resistant cross-linked polyethylene insulation composition.

[0074] Example 2

[0075] This embodiment provides a water-tree-resistant cross-linked polyethylene insulation material for a high-voltage direct current submarine cable. The preparation method is identical to that of Example 1, except that the raw material components are different. The material comprises the following components in parts by weight:

[0076] 80 parts of low-density polyethylene resin; 0.2 parts of composite antioxidant; 1.25 parts of composite cross-linking agent; 0.2 parts of 2,4-diphenyl-4-methyl-1-pentene; 8 parts of water-inhibiting mother particles;

[0077] The composite antioxidant includes 0.075 parts of antioxidant 300, 0.075 parts of antioxidant 1010, and 0.05 parts of antioxidant DLTP;

[0078] The composite crosslinking agent includes 1.0 part of dicumyl peroxide and 0.25 part of triallyl isocyanurate;

[0079] The water-inhibiting masterbatch is made by mixing and granulating a composite polyolefin resin, a composite water inhibitor, maleic anhydride-grafted polyethylene, and antioxidant 1035 in a mass ratio of 90:6.5:3.8:0.2. The composite water inhibitor comprises polyethylene glycol and oleamide in a mass ratio of 9:1. The composite polyolefin resin contains low-density polyethylene resin and ethylene-octene copolymer in a mass ratio of 1.1:1.

[0080] Example 3

[0081] This embodiment provides a water-tree-resistant cross-linked polyethylene insulation material for a high-voltage direct current submarine cable. The preparation method is identical to that of Example 1, except that the raw material components are different. The material comprises the following components in parts by weight:

[0082] 120 parts of low-density polyethylene resin; 0.3 parts of composite antioxidant; 1.85 parts of composite cross-linking agent; 0.3 parts of 2,4-diphenyl-4-methyl-1-pentene; 12 parts of water-inhibiting mother particles;

[0083] The composite antioxidant includes 0.09 parts of antioxidant 1035, 0.12 parts of antioxidant 1010, and 0.09 parts of antioxidant DLTP;

[0084] The composite crosslinking agent includes 1.5 parts of dicumyl peroxide and 0.35 parts of triallyl isocyanurate;

[0085] The water-inhibiting masterbatch is made by mixing and granulating a composite polyolefin resin, a composite water inhibitor, maleic anhydride-grafted polyethylene, and antioxidant 300 in a mass ratio of 110:8.5:5.2:0.28. The composite water inhibitor comprises polyethylene glycol and oleamide in a mass ratio of 11:1. The composite polyolefin resin contains low-density polyethylene resin and ethylene-octene copolymer in a mass ratio of 1.4:1.

[0086] Table 1 Physical property test results of Examples 1-3 (refer to JB / T 10437)

[0087]

[0088]

[0089] Comparative Example 1

[0090] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formula components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that: instead of using a composite antioxidant, 0.3 parts of antioxidant 1010 are added.

[0091] Table 2 Physical property test results of comparative example 1 (reference JB / T 10437)

[0092]

[0093] Comparative Example 2

[0094] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for a high-voltage DC submarine cable. The formula components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that 0.6 parts of a composite cross-linking agent are added, and the weight ratio of the two is the same as in Example 1. At this time, the mass ratio of 2,4-diphenyl-4-methyl-1-pentene, diisopropyl peroxide, and triallyl isocyanurate is 1:1.936:0.464.

[0095] Table 3 Physical property test results of comparative example 2 (refer to JB / T 10437)

[0096]

[0097]

[0098] Comparative Example 3

[0099] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage direct current submarine cables. The composition and preparation method are essentially the same as those in Example 1, with the only difference being the addition of 2.0 parts of a composite cross-linking agent. The weight ratio of the composite cross-linking agent and the composite cross-linking agent is the same as in Example 1. The mass ratio of 2,4-diphenyl-4-methyl-1-pentene, dicumyl peroxide, and triallyl isocyanurate is 1:6.452:1.548.

[0100] Table 4 Physical property test results of comparative example 3 (refer to JB / T 10437)

[0101]

[0102] Comparative Example 4

[0103] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. The formulation and components thereof are substantially the same as those in Example 1, and the preparation method is exactly the same. The only difference is that the composite cross-linking agent is a single auxiliary cross-linking agent, and 1.55 parts of triallyl isocyanurate are added.

[0104] Table 5 Physical property test results of comparative example 4 (refer to JB / T 10437)

[0105]

[0106]

[0107] Comparative Example 5

[0108] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. The formulation and components thereof are substantially the same as those in Example 1, and the preparation method is exactly the same, with the only difference being that no scorch inhibitor is added.

[0109] Table 6 Physical property test results of comparative example 5 (reference JB / T 10437)

[0110]

[0111] Comparative Example 6

[0112] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. The formulation and components thereof are substantially the same as those in Example 1, and the preparation method is exactly the same, with the only difference being that no water-inhibiting mother particles are added.

[0113] Table 7 Physical property test results of Comparative Example 6 (refer to JB / T 10437)

[0114]

[0115]

[0116] Comparative Example 7

[0117] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formula components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that the composite polyolefin resin in the water-inhibiting mother particles is a single low-density polyethylene.

[0118] Table 8 Physical property test results of Comparative Example 7 (reference JB / T 10437)

[0119]

[0120] Comparative Example 8

[0121] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formula components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that the composite water inhibitor in the water-inhibiting masterbatch is a single polyethylene glycol.

[0122] Table 9 Physical property test results of Comparative Example 8 (reference JB / T 10437)

[0123]

[0124]

[0125] Comparative Example 9

[0126] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formula components are basically the same as those in Example 1, and the preparation method is exactly the same. The only difference is that no maleic anhydride graft is added to the water-inhibiting motherbatch.

[0127] Table 10 Physical property test results of comparative example 9 (reference JB / T 10437)

[0128]

[0129] Comparative Example 10

[0130] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. Its formula components are exactly the same as those in Example 1, except that: no water-inhibiting masterbatch is prepared, and its formula components are manually weighed and put into a high-speed mixer for mixing. After mixing for a certain period of time, the material is fed into a twin-screw extruder for melt mixing and granulation.

[0131] Table 11 Physical property test results of Comparative Example 10 (reference JB / T 10437)

[0132]

[0133]

[0134] Comparative Example 11

[0135] This comparative example provides a water-tree-resistant cross-linked polyethylene insulation material for high-voltage DC submarine cables. The formulation components are exactly the same as those in Example 1, and the preparation method adopts a conventional process: the formulation components are manually weighed and put into a high-speed mixer for mixing. After mixing for a certain period of time, the material is fed into a twin-screw extruder for melt mixing and granulation.

[0136] Table 12 Physical property test results of Comparative Example 11 (reference JB / T 10437)

[0137]

[0138] As shown in Example 1 and Comparative Example 1, a single antioxidant 1010 cannot ensure that the insulation material meets the thermal aging requirements and cannot meet the insulation material requirements for HVDC submarine cables. This is primarily because the hydroxyl hydrogen atoms on the benzene ring of the hindered phenol antioxidant 1010 are easily combined with peroxyl radicals (ROO·), alkoxy radicals (RO·), and hydroxyl radicals (HO·), terminating the polyethylene molecular chain oxidation process and developing antioxidant capacity. However, antioxidant 1010 has a low melting point and is rapidly consumed during high-temperature extrusion processing and long-term thermal aging, failing to provide long-term protection for the insulation material's performance. Curing tests show that the insulation material has a short curing time and low scorch resistance, making it unsuitable for long-term extrusion production of HVDC submarine cables. Excessive addition of antioxidant 1010 can cause the antioxidant to precipitate "blooming" from the insulation material or migrate to the insulation material's surface, reducing the insulation material's electrical performance and the cable's service life.

[0139] It can be seen from Example 1 and Comparative Examples 2 and 3 that the amount of composite cross-linking agent used is too low, resulting in insufficient cross-linking of the material, low gel content, decreased mechanical properties of the material, and excessive thermal elongation after cross-linking. Although the vulcanization time in the vulcanization test is relatively long, this is mainly because the amount of composite cross-linking agent is too little, the content of free radicals generated by the decomposition of peroxide is relatively low, resulting in a low cross-linking density of the material, the material cannot be fully cross-linked, the creep of the polyethylene molecular chain in the non-cross-linked area increases, and water trees are easily formed and become longer. If the amount of composite cross-linking agent is excessively increased, the content of free radicals generated by the decomposition of peroxide will be too high. On the one hand, it will consume the antioxidants in the material and reduce the thermal aging performance of the material; on the other hand, it will increase the cross-linking density of the material, the thermal elongation of the insulation material will be low, and during the vulcanization process, the polyethylene molecular chains will cross-link rapidly, and the anti-scorch performance will be reduced; at the same time, after the excessive decomposition of the composite cross-linking agent, the by-products such as water and low molecular weight gas micropores in the insulation material will increase, the probability of water tree formation will increase and the length of the water tree will become longer, which will greatly affect the electrical properties of the material and the service life of the cable; in addition, the composite cross-linking agent that is not fully decomposed and has polarity can be converted into impurities in the material, which will also lead to a decrease in the dielectric properties and volume resistivity of the material.

[0140] As shown in Example 1 and Comparative Example 4, triallyl isocyanurate relies on free radicals generated by the decomposition of dicumyl peroxide to initiate the crosslinking reaction. Triallyl isocyanurate alone cannot generate free radicals independently, thus failing to effectively crosslink the polyethylene molecular chains. Consequently, the material cannot undergo vulcanization crosslinking, and its mechanical properties, thermal aging, and thermal elongation properties fail to meet the insulation requirements for HVDC submarine cables. Furthermore, the uncrosslinked polyethylene molecular chains creep under heat, resulting in a long water tree.

[0141] As can be seen from Example 1 and Comparative Example 5, due to the long continuous extrusion cycle required for cable insulation in submarine cables for long-span power transmission, the composite cross-linking agent cannot effectively balance the thermal extension performance, mechanical properties, and scorch resistance of cable insulation. The scorch inhibitor inserts the benzene ring group into the polyethylene molecular chain during the cable insulation cross-linking process, improving the rigidity of the polyethylene molecular chain, effectively ensuring the heat resistance and mechanical properties of the insulating composition under a specific composite cross-linking agent addition amount, reducing the generation of scorch of the polyethylene molecular chain during the cable insulation extrusion process, and improving the cable extrusion production cycle and efficiency. When the scorch inhibitor is added to the insulation material, during the process of cross-linking the polyethylene molecular chain by decomposing the cross-linking agent to generate free radicals, some free radicals will insert the scorch inhibitor molecular chain into the polyethylene molecular chain, reducing the excessive cross-linking of the polyethylene molecular chain, inhibiting the generation of macromolecular polyethylene gel, and improving the scorch resistance of the material. It can be seen from the vulcanization test that when there is no scorch inhibitor, the scorch resistance of the material is poor and cannot be used for the long-cycle extrusion production of high-voltage direct current submarine cables.

[0142] As shown in Example 1 and Comparative Example 6, byproducts of crosslinker decomposition, such as micropores of water and low-molecular-weight gases, can lead to the formation of dendritic channels in the cable insulation under the combined effects of high-voltage electric fields, operating time, and other factors. This can gradually degrade the electrical performance of the cable insulation. When water tree dendrites reach a certain size, they transform into electrical dendrites, ultimately leading to cable insulation breakdown. Without the addition of water tree inhibitors, water tree growth is significant, seriously impacting the service life of HVDC submarine cables.

[0143] As can be seen from Examples 1 to 3 and Comparative Example 7, the polyolefin elastomer in the water-inhibiting masterbatch composite polyolefin resin has a molecular structure in which the octene chain is longer than the ethylene chain. This octene chain acts as a linker and buffer in the insulating material composition, increasing the impact resistance and elongation at break of the insulating material. Furthermore, the polyolefin elastomer has a narrow molecular weight distribution and good fluidity, which can improve the fluidity and melt viscosity of the insulating material blend system, enhance the compatibility of additives such as the composite water inhibitor with the insulating material resin matrix, and improve the resistance of the insulating material and cable insulation to water tree growth.

[0144] It can be seen from Examples 1 to 3 and Comparative Example 8 that, because the oleic acid amide in the water-inhibiting motherbatch has a certain polarity, during the extrusion processing of the composite polyolefin resin and the insulating material, the friction in the melt can be reduced, and the fluidity of the composite polyolefin resin and the insulating material in the molten state can be improved, thereby reducing the uneven mixing of the polyethylene glycol with the composite polyolefin resin after the polyethylene glycol is melted in advance due to its low melting point; at the same time, a trace amount of oleic acid amide migrates to the surface of the insulating material to form a film-like structure, which hinders the migration of the polyethylene glycol with strong polarity to the surface of the insulating material, thereby affecting the electrical properties of the insulating material and the growth of water trees in the cable insulation, resulting in a shortened service life of the cable.

[0145] As shown in Examples 1-3 and Comparative Example 9, due to the poor compatibility of the polar composite water inhibitor with the low-density polyethylene resin (LDPE) used as the insulation material, the polyethylene glycol in the composite water inhibitor migrates to the surface during the extrusion process of the insulation material or cable insulation, affecting the electrical properties of the insulation material and cable insulation and also reducing the cable insulation's resistance to water tree growth. The polar groups (maleic anhydride groups) of the maleic anhydride-grafted polyethylene in the water inhibitor masterbatch undergo an esterification reaction with the hydroxyl groups of the polyethylene glycol, forming a chemical bond. This effectively bonds the non-polar polyethylene segments to the LDPE resin used as the insulation material, thus preventing the composite water inhibitor from migrating from the insulation material.

[0146] As can be seen from Examples 1-3 and Comparative Example 10, without preparing water-inhibiting masterbatch, all raw materials were weighed and mixed directly in a high-speed mixer, followed by melt-mixing and granulation in a twin-screw extruder. Due to the low melting point of the composite water-inhibiting agent, some of the composite water-inhibiting agent adhered to the inner wall or agglomerated in the high-speed mixer. Furthermore, during the initial stages of twin-screw extrusion, the partially melted composite water-inhibiting agent could not be effectively mixed with the low-density polyethylene resin. Furthermore, the grafting reaction of the maleic anhydride-grafted polyethylene with the composite water-inhibiting agent requires a certain temperature and time. Consequently, the composite water-inhibiting agent was unevenly dispersed in the insulation material, resulting in a reduced composite water-inhibiting agent content. Furthermore, ineffectively grafted composite water-inhibiting agent migrated to the insulation material surface, affecting the insulation material's electrical properties and resistance to water treeing, and reducing production efficiency.

[0147] It can be seen from Examples 1 to 3 and Comparative Example 11 that the production capacity is greatly reduced by using the traditional twin-screw melt-mixing granulation method. Since the composite cross-linking agent and the scorch inhibitor are injected into the mixed melt in liquid form, the melt temperature is relatively low. The pressure bearing capacity and precision of the traditional twin-screw equipment filtration system are insufficient, and it cannot adapt to the fine filtration requirements of the 500-mesh high-precision filter screen, resulting in the material being unable to meet the impurity requirements of the high-voltage submarine cable; at the same time, the use of manual operation reduces product stability and consistency, and reduces production efficiency.

[0148] In summary, it can be seen from the embodiments and comparative examples that specific antioxidants can effectively improve the heat resistance and scorch resistance of the insulating material, and the specific addition amount of the composite cross-linking agent helps to reduce the cross-linking agent content in the high-voltage DC anti-water tree cross-linked polyethylene insulation composition, reduce the pre-cross-linking or old glue generated during the extrusion process of the insulating material and the pores and other by-products generated during the cross-linking reaction, and avoid the combined effect of multiple factors such as high-voltage electric field and operating time. The cable insulation is prevented from producing dendritic and continuously growing dendritic structure channels due to pores or other by-products, and is beneficial to the improvement of the electrical performance of the cable; at the same time, because the submarine cable for large-span power transmission requires a long continuous extrusion cycle of cable insulation, the composite cross-linking agent cannot effectively take into account the thermal extension performance, mechanical properties and scorch resistance of the cable insulation. The clean scorch inhibitor is blended with the composite cross-linking agent, and the benzene ring group in the scorch inhibitor is inserted into the polyethylene molecular chain during the cross-linking process of the cable insulation, thereby improving the rigidity of the polyethylene molecular chain, effectively ensuring the heat resistance and mechanical properties of the insulating composition under the specific addition amount of the composite cross-linking agent, reducing the generation of scorch of the polyethylene molecular chain during the extrusion of the cable insulation, and improving the cable extrusion production cycle and efficiency. Water-inhibiting motherbatch can immobilize water molecules in cable insulation, significantly reducing the formation and growth of water dendrites in the insulation and extending the service life of high-voltage cables. The use of a Swiss-imported "four-screw, four-pin" BUSS production line not only significantly improves material production efficiency and stability, but also ensures the stability of continuous extrusion of high-voltage cable insulation.

[0149] The results in Tables 1 to 12 indicate that the water-tree-resistant cross-linked polyethylene insulation material for HVDC submarine cables, produced using a "four-spiral, four-pin" bussing line imported from Switzerland, inhibits insulation scorching, resists water tree growth, exhibits a high gel content, achieves rapid extrusion, and produces a stable production process, making it suitable for HVDC submarine cable production. This method for preparing water-tree-resistant cross-linked polyethylene insulation material for HVDC submarine cables offers high yield and a stable process, with a production capacity of up to 3.5 tons per hour. Currently, the company is the only manufacturer to use this imported "four-spiral, four-pin" bussing line to produce HVDC submarine cable insulation material. This breakthrough surpasses conventional twin-screw production processes, significantly improving production efficiency and making it a promising material for future HVDC submarine cables.

[0150] Those skilled in the art should understand that the above embodiments are merely illustrative of the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent variations or modifications made in accordance with the spirit and substance of the present invention are intended to be encompassed within the scope of protection of the present invention.

Claims

1. A water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable, characterized in that: The composition comprises the following components in parts by weight: 80-120 parts of polyethylene resin; 0.2-0.3 parts of compound antioxidant; 1.25-1.85 parts of composite cross-linking agent; 0.2-0.3 parts of scorch inhibitor; 8.0-12 parts of jellyfish-inhibiting particles; Wherein, the polyethylene resin is a low-density polyethylene resin having a melt index of 1.9-2.1 g / 10min; The water-inhibiting masterbatch is obtained by mixing and granulating a composite polyolefin resin, a composite water-inhibiting agent, a maleic anhydride graft and an antioxidant in a mass ratio of 80-120:6.0-9.0:3.6-5.4:0.15-0.25; the composite water-inhibiting agent comprises polyethylene glycol and oleamide in a mass ratio of 8-12:

1.

2. The water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable according to claim 1, characterized in that: The scorch inhibitor is 2,4-diphenyl-4-methyl-1-pentene, the composite crosslinking agent includes dicumyl peroxide and triallyl isocyanurate, and the mass ratio of the 2,4-diphenyl-4-methyl-1-pentene, the dicumyl peroxide and the triallyl isocyanurate is 1:4.2-5.8:1.05-1.

3.

3. The water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable according to claim 1, characterized in that: In the water-inhibiting masterbatch, the total amide content of the oleic acid amide is ≥98.5%, the average molecular weight of the polyethylene glycol is 20,000, and the weight proportion of the polyethylene glycol in the water-inhibiting masterbatch is 5-7%.

4. The water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable according to claim 3, characterized in that: In the water-inhibiting masterbatch, the composite polyolefin resin comprises a low-density polyethylene resin with a melt index of 1.9-2.1 g / 10 min and a polyolefin elastomer resin with a melt index of 4.5-5.5 g / 10 min, and the mass ratio of the two is 1-1.5:1; the melt index of the maleic anhydride graft is 1.0-3.0 g / 10 min; and the antioxidant is a thiophenol antioxidant.

5. The water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable according to claim 4, characterized in that: The preparation method of the water-inhibiting mother particle comprises: S1: mixing a low-density polyethylene resin and a polyolefin elastomer resin to prepare the composite polyolefin resin, and mixing polyethylene glycol and oleamide to prepare the composite water inhibitor; S2: putting the composite polyolefin resin, the composite water inhibitor, the maleic anhydride grafted product and the antioxidant into a mixer and mixing them at a speed of 30±2 rpm for 40-60 seconds; S3 feeds the mixed material into a twin-screw extruder. At a shear speed of 25-35 rpm, the screw temperature is controlled to gradually increase from the feed section to the die head, and the final section temperature is 155-165°C, and the material temperature is 160-180°C. After filtering through a multi-layer filter installed on the die head, the mixed material is granulated and dried through water drawing to obtain clean water-inhibiting mother particles. The filter includes at least one layer of 300-mesh high-precision filter.

6. The water-tree-resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable according to claim 1, characterized in that: The composite antioxidant comprises at least two of hindered phenol antioxidants, thioester antioxidants and thiophenol antioxidants.

7. The method for preparing a water-tree resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable according to any one of claims 1 to 6, characterized in that: include: A1 feeds the polyethylene resin, the composite antioxidant, and the water-inhibiting masterbatch into a BUSS high-speed shearing machine, plasticizes and filters out impurities, and forms the particles through an underwater granulation process. After granulation, the particles are dehydrated and heated to dry; A2 adds a composite cross-linking agent and a scorch inhibitor at 65°C-75°C into a mixing system with a rotation speed of 2-6 rpm, and the particles, the composite cross-linking agent and the scorch inhibitor are blended, dried, and then heat-insulated and absorbed to obtain the high-voltage DC water-tree-resistant cross-linked polyethylene insulation composition.

8. The method for preparing a water-tree resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable according to claim 7, characterized in that: In step A1, the speed of the BUSS high-speed shearing machine is 270-350 rpm, the screw temperature is 80-120°C, the barrel temperature is 100-140°C, the material temperature is 185-205°C, and the material is filtered through a multi-layer filter installed on the head, and then obtained by underwater granulation and drying. The filter includes at least one layer of 500 mesh high-precision filter.

9. The method for preparing a water-tree resistant cross-linked polyethylene insulation composition for a high-voltage direct current submarine cable according to claim 7, characterized in that: In step A2, the insulation absorption time is 10-14 hours, and the insulation temperature is 70-80°C.

10. A cable, prepared from the high-voltage direct current water-tree resistant cross-linked polyethylene insulation composition according to any one of claims 1 to 6.

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