Insulating material for ultrahigh-voltage direct-current cable based on gradient interface structure nano-composite and preparation method of insulating material

Abstract

The invention discloses an insulating material for an ultrahigh-voltage direct-current cable based on a gradient interface structure nano-composite and a preparation method of the insulating material. The insulating material is prepared from the following components in parts by weight: 80 to 100 parts of high-density polyethylene, 0 to 20 parts of low-density polyethylene, 1 to 3 parts of functionalized nano filler, 1.5 to 2.5 parts of dicumyl peroxide, 0.1 to 0.3 part of antioxidant and 0.1 to 0.3 part of lubricant. The core innovation lies in that the functionalized nano filler is nano magnesium oxide with a gradient interface structure constructed on the surface, the structure forms a poly (glycidyl methacrylate)-poly (1-octylene) block polymer chain through two-step graft polymerization, and continuous transition of modulus and polarity from the filler to a polyethylene matrix is realized. According to the insulating material, the maximum space charge density can be reduced to 1.2 * 10 < 3 > C / m < 3 >, the direct-current breakdown strength is larger than or equal to 550 kV / mm, the direct-current / alternating-current breakdown strength ratio reaches 0.74, the strength retention rate after 1000-hour aging is larger than or equal to 94%, and the insulating material has excellent mechanical performance and processing adaptability, is suitable for an ultrahigh-voltage direct-current cable with the voltage grade larger than or equal to 320 kV, can meet the requirement for long-distance high-capacity power transmission, and has a wide application prospect. And long-term safe and stable operation of the cable is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cable materials, in particular to an insulation material for ultra-high voltage direct current (UHVDC) cables based on a gradient interface structure nanocomposite and a preparation method thereof. BACKGROUND

[0002] With the increasing demand for global energy structure adjustment and cross-regional energy allocation, the ultra-high voltage direct current (UHVDC) cable technology has become one of the core technologies for solving long-distance power transmission due to its advantages of large transmission capacity, low loss and flexible control. As the "core component" of the UHVDC cable, the performance of the insulation material directly determines the voltage level, transmission capacity and service life of the cable, and is the key bottleneck restricting the development of UHVDC power transmission technology.

[0003] Currently, the XLPE insulation materials for UHVDC cables in the commercialization and research and development stages mainly face the following three technical problems:

[0004] 1) Space charge accumulation problem: Under the synergistic effect of direct current high voltage electric field and temperature gradient field, space charge injection and accumulation easily occur in the insulation medium and the filler-matrix interface, leading to serious distortion of the local electric field, greatly accelerating the aging process of the insulation material, and even causing insulation breakdown, which is the primary factor limiting the voltage level improvement of the cable.

[0005] 2) Mismatch between direct current and alternating current electrical strength: The direct current breakdown strength of existing XLPE materials is generally lower than the alternating current breakdown strength, resulting in the actual operating voltage of the direct current cable being much lower than the alternating current resistance limit of the material, causing waste of material performance and limiting the transmission capacity and operating safety of the direct current cable.

[0006] 3) Nanofiller dispersion and interface combination defects: In order to suppress space charge, the industry widely studies the modification of XLPE using nanofillers (such as MgO, SiO2, etc.), but the high specific surface area of nanofillers makes them prone to agglomeration, and the large polarity difference between the fillers and the polyethylene matrix leads to weak interface combination. Agglomerated fillers or defective interfaces will form new charge traps and breakdown initiation points, which will worsen the electrical properties of the material and fail to achieve the modification purpose.

[0007] In the prior art, the improvement ideas for the above problems mainly focus on two directions: one is to pursue the "ideal uniform dispersion" of nanofillers in the matrix, and to reduce agglomeration by optimizing the dispersion process or adding a dispersing agent; the other is to strengthen the "strong interface combination" of the filler and the matrix, and to improve the interface compatibility by coupling agent treatment. However, it has been proved in practice that the "ideal dispersion" alone cannot solve the problem of charge injection caused by the sudden change of interface polarity, and the "strong interface combination" will form a single deep charge trap, which is easy to cause irreversible accumulation of charges, and neither of them can fundamentally solve the core technical problems of super-high voltage DC insulation material. Therefore, it has become a technical problem to be solved in the field to develop an insulation material that can break through the existing limitations from the aspects of charge transport mechanism and interface structure design, and has the ability to suppress space charge, high DC electrical strength and long-term operation stability. SUMMARY

[0008] The purpose of the present application is to overcome the defects of the existing XLPE insulation material for super-high voltage DC cables, such as serious accumulation of space charge, insufficient DC electrical strength, poor interface stability and low long-term operation reliability, and to provide an insulation material for super-high voltage DC cables based on gradient interface structure nanocomposite and a preparation method thereof. The insulation material is designed by innovative gradient interface structure, which fundamentally optimizes the charge transport behavior, realizes efficient suppression of space charge, improves the DC breakdown strength, improves the matching of DC and AC electrical strength, and has excellent thermal mechanical properties and long-term operation stability, meeting the use requirements of ≥320kV super-high voltage DC cables.

[0009] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0010] An insulation material for super-high voltage DC cables based on gradient interface structure nanocomposite is composed of the following components by weight:

[0011]

[0012]

[0013] The functionalized nanofiller is a nanometer magnesium oxide with a gradient interface structure on the surface, the gradient interface structure is a transition layer with continuous change of modulus and polarity from the surface of the filler to the polyethylene matrix, which is formed by two-step grafting polymerization of polyglycidyl methacrylate-poly-1-octene block polymer chains on the surface of the filler, the polyglycidyl methacrylate is a high-polarity short-chain segment, and the poly-1-octene is a weak-polarity long-chain segment.

[0014] The core innovation of the functionalized nano-filler lies in its gradient interface structure on the surface: the structure is formed by grafting two polymer chains with different polarity and molecular chain length (poly (glycidyl methacrylate) (PGMA) and poly (1-octene) (POctene)) on the surface of nano-magnesium oxide, which builds a transition layer with continuous change of modulus and polarity from the filler surface to the polyethylene matrix, and realizes precise regulation of charge transport behavior.

[0015] Preferably, the high-density polyethylene has a melt index of 0.5-2.0 g / 10 min under the test conditions of 190°C and 2.16 kg.

[0016] Preferably, the low-density polyethylene has a melt index of 1-3 g / 10 min under the test conditions of 190°C and 2.16 kg, and a melting point of 105-110°C.

[0017] Preferably, the average particle size of the nano-magnesium oxide is 20-50 nm.

[0018] Preferably, the antioxidant is one or a combination of both of Irganox 1010 and Irgafos 168.

[0019] Preferably, the lubricant is one of calcium stearate, ethylene bis-stearamide, and zinc stearate.

[0020] Further, the construction of the gradient interface structure includes the following steps:

[0021] P1, silanization treatment: disperse the nano-magnesium oxide in anhydrous ethanol, after ultrasonic dispersion for 30 minutes, add a silane coupling agent containing amino groups (γ-aminopropyl triethoxysilane), reflux and stir at 75-85°C for 5-7 hours to amino-functionalize the surface of the nano-magnesium oxide; after centrifugal washing, vacuum dry to obtain the amino-functionalized filler; after the reaction, wash with ethanol for 3 times by centrifugation, and vacuum dry at 80°C for 12 hours to obtain the amino-functionalized nano-magnesium oxide;

[0022] P2, disperse the amino-functionalized nano-magnesium oxide in xylene under nitrogen protection, first heat to 80-85°C, add glycidyl methacrylate (GMA) monomer and azobisisobutyronitrile (AIBN) initiator, react for 4-6 hours to graft "hard segment" PGMA (high polarity, strong interaction with the filler) on the surface of the filler; then increase the temperature to 85-90°C, add 1-octene monomer, continue to react for 6-8 hours to graft "soft segment" POctene (non-polar long chain, highly compatible with the polyethylene matrix); after the reaction is completed, pour the product into a large amount of ethanol for precipitation, separate by high-speed centrifugation, then use a Soxhlet extractor to continuously extract with acetone for 40-48 hours (to remove homopolymer impurities), and finally vacuum dry at 60°C to constant weight to obtain the functionalized nano-filler.

[0023] A preparation method of an insulation material for an ultra-high voltage direct current cable based on a gradient interface structure nanocomposite, comprising the following steps:

[0024] S1, preparing a functionalized nanofiller: constructing a gradient interface structure on the surface of nanometer magnesium oxide according to the gradient interface structure construction step, to obtain a functionalized nanofiller;

[0025] S2, preparing a master batch: mixing the functionalized nanofiller, 10-30 parts by weight of high-density polyethylene, and an antioxidant, and then granulating by melt blending to obtain a master batch;

[0026] S3, final mixing: mixing the master batch with the remaining high-density polyethylene, low-density polyethylene, dicumyl peroxide, and a lubricant to obtain a mixture;

[0027] S4, hot pressing and crosslinking: hot pressing the mixture into a sheet, and then performing crosslinking treatment at high temperature to obtain the required insulation material for an ultra-high voltage direct current cable.

[0028] Preferably, the melt blending granulation in step S2 uses a co-rotating twin-screw extruder, and the processing temperature is 150-180℃, and the screw rotation speed is 200-300rpm.

[0029] Preferably, the materials in step S3 are mixed in a 70-80℃ internal mixer for 10-20 minutes;

[0030] Preferably, the hot pressing in step S4 is performed at 120-150℃ and 10MPa, the mixture is placed on a flat curing press, preheated for 5-10 minutes, degassed 5-8 times (to remove bubble impurities), and then hot pressed for 5-10 minutes to form a sheet with the required thickness; the crosslinking treatment is performed at 180-200℃ for 15-20 minutes to simulate the actual crosslinking process of the cable insulation layer, to obtain the final insulation material.

[0031] The application of an insulation material for an ultra-high voltage direct current cable in an ultra-high voltage direct current cable, wherein the voltage grade of the ultra-high voltage direct current cable is ≥320kV.

[0032] Compared with the prior art, the technical scheme of the present application has the following advantages:

[0033] A, extremely inhibiting space charge accumulation, greatly improving insulation reliability: the "buffer zone" and "deep-shallow trap complex" constructed by the gradient interface structure make the maximum space charge density of the material as low as 1.2×10 3 C / m 3Compared with traditional MgO / XLPE composite insulation material, the charge accumulation phenomenon is reduced by one order of magnitude, and there is almost no charge accumulation phenomenon; the severe injection and accumulation of charges in the medium and interface under direct current high voltage are effectively avoided, the risk of local electric field distortion is eliminated, the insulation aging is delayed from the root, and the breakdown accident rate is significantly reduced, thereby providing a core guarantee for the long-term safe operation of the ultra-high voltage direct current cable.

[0034] B. Optimize the matching degree of direct current / alternating current electrical performance, and break through the bottleneck of voltage level: the direct current breakdown strength of the material reaches 550 kV / mm or above, which is much higher than that of pure XLPE (421 kV / mm) and traditional composite insulation material (485 kV / mm), and the direct current / alternating current breakdown strength ratio is increased to 0.74, which is significantly close to 1 compared with the prior art, thereby solving the key pain point that the direct current electrical strength and alternating current electrical strength of the traditional insulation material are not matched; the material can support a ≥320 kV ultra-high voltage direct current power transmission system, and can even be adapted to the demand of a ±400 kV or even ±800 kV higher voltage level in the future, thereby effectively improving the cable transmission capacity and power transmission efficiency.

[0035] C. Excellent long-term operation stability, and prolong the service life of the cable: after 1000 hours of partial discharge aging test, the direct current breakdown strength retention rate is as high as 94%, which is much higher than 82-85% of the traditional composite insulation material; the gradient interface structure can effectively prevent the migration and agglomeration of the nano filler under long-term electric and thermal stress, and always maintain the uniform dispersion of the filler and the interface integrity, so that the space charge suppression performance decays slightly, thereby ensuring that the electrical performance and mechanical performance of the material are stable in the complex service environment, greatly prolonging the service life of the ultra-high voltage direct current cable, and reducing the operation and maintenance cost.

[0036] D. Consider the mechanical enhancement and processing adaptability, and have high industrial transformation value: the gradient interface structure realizes efficient stress transmission of the nano filler and the polyethylene matrix, the tensile strength of the material is increased to 25.8 MPa or above, which is significantly enhanced compared with that of pure XLPE (21.5 MPa) and traditional composite insulation material (23.1 MPa), and the elongation at break is maintained in an acceptable range of 510-530%, which takes into account the strength and toughness; the raw materials and processes such as silanization treatment, solution grafting, double screw extrusion and hot press crosslinking are all mature industrial technologies, without the need to add special equipment, and are easy to mass produce, and the technical pain points of the traditional nano filler, such as difficult dispersion and weak interface combination, are solved, so that the material has wide industrial application prospects. DETAILED DESCRIPTION

[0037] The present application can be implemented in many different forms, and should not be understood as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the inventive concept to those skilled in the art. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the present application.

[0038] Example 1

[0039] 1. Raw materials (by weight)

[0040] High-density polyethylene (HDPE): 100 parts, grade HDPE M6210, melt index 1.0 g / 10 min (190°C, 2.16 kg), ash content < 50 ppm;

[0041] Low-density polyethylene (LDPE): 0 parts;

[0042] Nano-magnesium oxide: 1 part (as functionalized nano-filler), average particle size 20 nm;

[0043] Dicumyl peroxide: 1.5 parts, purity ≥ 99%;

[0044] Antioxidant: 0.1 part, Irganox 1010;

[0045] Lubricant: 0.1 part, calcium stearate;

[0046] Auxiliary reagents: silane coupling agent KH-550, AIBN, PGMA monomer, 1-octene, anhydrous ethanol, dimethylbenzene (all industrial grade).

[0047] 2. Preparation steps

[0048] S1. Preparation of functionalized nano-filler:

[0049] P1: Disperse 10 g of nano-magnesium oxide in 200 mL of anhydrous ethanol, ultrasonic for 30 minutes, add 1.2 g of KH-550, reflux and stir at 75°C for 7 hours, centrifugal wash for 3 times, vacuum drying at 80°C for 12 hours, obtain aminated MgO;

[0050] P2: Under nitrogen protection, disperse 5 g of aminated MgO in 150 mL of dimethylbenzene, add 0.04 g of AIBN, graft PGMA at 80°C for 6 hours, then increase the temperature to 85°C and graft 1-octene for 8 hours; the product is precipitated with ethanol, centrifuged, then Soxhlet extracted with acetone for 48 hours, vacuum dried at 60°C to constant weight, obtain functionalized MgO@PGMA-b-PO;

[0051] S2. Preparation of master batch: mix 1 part of functionalized MgO@PGMA-b-PO, 10 parts of HDPE, and 0.1 part of Irganox 1010, melt and granulate through a double screw extruder, processing temperature 150°C, screw rotation speed 200 rpm;

[0052] S3, final mixing: add the masterbatch and the remaining 90 parts of HDPE, 1.5 parts of DCP, and 0.1 parts of calcium stearate into a 70°C internal mixer, and mix for 20 minutes;

[0053] S4, hot pressing and crosslinking: preheat the mixed material at 120°C and 10 MPa for 10 minutes, exhaust 5 times, and hot press for 10 minutes to form a 100 μm sheet; crosslink in an oven at 180°C for 20 minutes to obtain the finished insulation material.

[0054] Example 2

[0055] 1, raw materials (by weight)

[0056] High-density polyethylene (HDPE): 90 parts, brand ExxonMobil HDPE HTA-001, melt index 1.5 g / 10 min (190°C, 2.16 kg);

[0057] Low-density polyethylene (LDPE): 10 parts, melt index 2.0 g / 10 min (190°C, 2.16 kg), melting point 108°C;

[0058] Nano-magnesium oxide: 2 parts (as functionalized nano-filler), average particle size 30 nm;

[0059] Dicumyl peroxide: 2.0 parts;

[0060] Antioxidant: 0.2 parts, Irganox 1010 and Irgafos 168 in a mass ratio of 1:1;

[0061] Lubricant: 0.2 parts, ethylene bis-stearamide (EBS);

[0062] Auxiliary reagents are the same as in Example 1.

[0063] 2, preparation steps

[0064] S1, preparation of functionalized nano-filler:

[0065] P1: disperse 10 g of nano-magnesium oxide in 200 mL of anhydrous ethanol, ultrasonic for 30 minutes, add 1.5 g of KH-550, reflux and stir at 80°C for 6 hours, centrifuge and wash, and then vacuum dry at 80°C for 12 hours;

[0066] P2: under nitrogen protection, disperse 5 g of the aminated filler in 150 mL of xylene, add 0.05 g of AIBN, graft PGMA at 82°C for 5 hours, and then raise the temperature to 88°C to graft 1-octene for 7 hours; the subsequent precipitation, extraction, and drying steps are the same as in Example 1.

[0067] S2, Masterbatch preparation: 2 parts of functionalized filler, 20 parts of HDPE, 0.2 parts of composite antioxidant were mixed and granulated by twin-screw extruder, processing temperature 165°C, screw rotation speed 250 rpm;

[0068] S3, Final mixing: the masterbatch was added into a 75°C internal mixer with the remaining 70 parts of HDPE, 10 parts of LDPE, 2.0 parts of DCP and 0.2 parts of EBS, and mixed for 15 minutes;

[0069] S4, Hot pressing and crosslinking: preheating at 135°C for 8 minutes, 10 MPa, exhaust 6 times, hot pressing for 8 minutes to form a 150 μm sheet; crosslinking at 190°C for 18 minutes to obtain the finished insulation material.

[0070] Example 3

[0071] 1, Raw materials (by weight)

[0072] High-density polyethylene (HDPE): 80 parts, grade Basell HDPE M300054, melt index 2.0 g / 10 min (190°C, 2.16 kg);

[0073] Low-density polyethylene (LDPE): 20 parts, melt index 3.0 g / 10 min (190°C, 2.16 kg), melting point 110°C;

[0074] Nano-magnesium oxide: 3 parts, average particle size 50 nm;

[0075] Dicumyl peroxide: 2.5 parts;

[0076] Antioxidant: 0.3 parts, Irgafos 168;

[0077] Lubricant: 0.3 parts, zinc stearate;

[0078] Auxiliary reagents are the same as in Example 1.

[0079] 2, Preparation steps

[0080] S1, Preparation of functionalized nano-filler:

[0081] P1: 10 g of nano-magnesium oxide was dispersed in 200 mL of anhydrous ethanol, ultrasonic for 30 minutes, 1.8 g of KH-550 was added, refluxed at 85°C for 5 hours, centrifuged and washed, and then vacuum dried at 80°C for 12 hours;

[0082] P2: 5 g of amino-functionalized filler was dispersed in 150 mL of xylene under nitrogen protection, 0.06 g of AIBN was added, PGMA was grafted at 85°C for 4 hours, and 1-octene was grafted at 90°C for 6 hours; the subsequent precipitation, extraction and drying steps were the same as in Example 1;

[0083] S2, masterbatch preparation: 3 parts of functionalized filler, 30 parts of HDPE, 0.3 parts of Irgafos 168 are mixed, and a twin-screw extruder is used for granulation, the processing temperature is 180℃, and the screw rotation speed is 300rpm;

[0084] S3, final mixing: the masterbatch is added into an 80℃ internal mixer together with the remaining 50 parts of HDPE, 20 parts of LDPE, 2.5 parts of DCP, and 0.3 parts of zinc stearate, and mixed for 10 minutes;

[0085] S4, hot pressing and crosslinking: preheating at 150℃ for 5 minutes, 10MPa, 8 times of exhaust, hot pressing for 5 minutes to form a 1mm sheet, and crosslinking at 200℃ for 15 minutes.

[0086] Comparative Example

[0087] In order to verify the creativity of the application, the following three groups of comparative examples are set up:

[0088] Comparative Example 1 (pure XLPE): no functionalized nanofiller, the remaining components and preparation process are the same as Example 2;

[0089] Comparative Example 2 (traditional composite material): replace the functionalized filler with ungrafted silanized MgO, and the rest is the same as Example 2;

[0090] Comparative Example 3 (physical blending material): untreated MgO is physically mixed with PGMA and POctene homopolymer and then added to the matrix, and the rest is the same as Example 2.

[0091] Performance Test

[0092] The insulating materials of Examples 1-3 and Comparative Examples 1-3 are tested for performance, and the test results are shown in Table 1 below:

[0093] Table 1 Performance Test Results

[0094]

[0095]

[0096] Result Analysis

[0097] The comparison data show that:

[0098] 1) The application realizes efficient suppression of space charge by constructing a gradient interface structure, the charge density is reduced by 96.6% compared with pure XLPE, and by 85.9% compared with traditional composite material;

[0099] 2) The DC breakdown strength is increased by 34.2% compared with pure XLPE, and by 16.5% compared with traditional composite material, and the AC / DC strength ratio reaches 0.74, which is significantly better than the prior art;

[0100] 3) The performance retention rate after long-term aging is as high as 94%-95%, which is much higher than that of the comparative examples, proving that the gradient interface structure can effectively avoid filler aggregation and guarantee the interface stability.

[0101] The above examples fully prove that the technical scheme of the present application solves the core technical bottleneck of the existing super-high voltage DC cable insulating material from the root by innovative gradient interface structure design, and is mature in process, easy to industrialize, and has extremely high industrial application value.

[0102] Industrial application

[0103] The super-high voltage DC cable insulating material of the present application is suitable for super-high voltage DC power transmission systems with voltage grade ≥320kV, and can be widely applied to the manufacture of DC cables with voltage grade of ±400kV, ±660kV, ±800kV and higher, especially to long-distance, large-capacity cross-regional power transmission projects, and can significantly improve the power transmission efficiency, operation safety and service life of the cable, and has a broad market prospect.

[0104] The unmentioned part of the present application is applicable to the prior art.

[0105] Obviously, the above examples are only examples for clearly illustrating, but not limit the embodiments. For those skilled in the art, other different forms of changes or variations can be made on the basis of the above description. Here, it is not necessary and impossible to enumerate all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. An insulating material for ultra-high voltage DC cables based on a gradient interface structure nanocomposite, characterized in that, Composed of the following components in parts by weight: 80-100 parts of high-density polyethylene; 0-20 parts of low-density polyethylene; 1-3 parts of functionalized nanofiller; 1.5-2.5 parts of dicumyl peroxide; Antioxidant 0.1-0.3 parts; Lubricant 0.1-0.3 parts; The functionalized nanofiller is magnesium oxide nanoparticles with a gradient interface structure on their surface. The gradient interface structure is a transition layer from the filler surface to the polyethylene matrix with continuously changing modulus and polarity. It is constructed by two-step graft polymerization on the filler surface to form poly(glycidyl methacrylate)-poly(1-octene) block polymer chains. The poly(glycidyl methacrylate) is a high-polarity short-chain segment, and the poly(1-octene) is a weakly polar long-chain segment.

2. The insulating material for ultra-high voltage DC cables according to claim 1, characterized in that, The high-density polyethylene has a melt index of 0.5-2.0 g / 10 min, and the test conditions are 190℃ and 2.16 kg. The low-density polyethylene has a melt index of 1-3 g / 10 min, and the test conditions are 190℃, 2.16 kg, and a melting point of 105-110℃.

3. The insulating material for ultra-high voltage DC cables according to claim 1, characterized in that, The average particle size of the nano-magnesium oxide is 20-50 nm.

4. The insulating material for ultra-high voltage DC cables according to claim 1, characterized in that, The construction of the gradient interface structure includes the following steps: P1. Silanization treatment: Disperse nano-magnesium oxide in anhydrous ethanol, add an amino-containing silane coupling agent, and reflux and stir at 75-85℃ for 5-7 hours to aminate its surface; after the reaction is completed, centrifuge and wash 3-4 times with anhydrous ethanol, and vacuum dry at 70-80℃ for 10-12 hours to obtain the aminated filler. P2. Two-step graft polymerization: Under nitrogen protection, aminated nano-magnesium oxide is dispersed in xylene. First, the temperature is raised to 80-85℃, and glycidyl methacrylate monomer and azobisisobutyronitrile initiator are added. The reaction is carried out for 4-6 hours. Then, the temperature is raised to 85-90℃, 1-octene monomer is added, and the reaction is continued for 6-8 hours. After the reaction is completed, the product is poured into a large amount of ethanol for precipitation and separation. It is then continuously extracted with acetone for 40-48 hours. Finally, it is vacuum dried at 60℃ to constant weight to obtain functionalized nanofiller.

5. A method for preparing the insulating material for ultra-high voltage DC cables according to any one of claims 1-4, characterized in that, Includes the following steps: S1. Preparation of functionalized nanofillers: According to the method described in claim 4, a gradient interface structure is constructed on the surface of nano-magnesium oxide to obtain functionalized nanofillers; S2. Preparation of masterbatch: After mixing functionalized nanofillers, 10-30 parts by weight of high-density polyethylene and antioxidant, the masterbatch is obtained by melt blending and granulation. S3. Final mixing: The masterbatch is mixed with the remaining high-density polyethylene, low-density polyethylene, dicumyl peroxide, and lubricant to obtain a mixture. S4. Hot pressing and cross-linking: The mixture is hot-pressed into sheets, and then cross-linked at high temperature to obtain the required insulation material for ultra-high voltage DC cables.

6. The method according to claim 5, characterized in that, The melt blending granulation described in step S2 uses a co-rotating twin-screw extruder with a processing temperature of 150-180℃ and a screw speed of 200-300 rpm.

7. The method according to claim 6, characterized in that, In step S3, the materials are mixed in a 70-80℃ internal mixer for 10-20 minutes.

8. The method according to claim 6, characterized in that, The hot pressing in step S4 is carried out at 120-150℃ and 10MPa, with preheating for 5-10 minutes, venting 5-8 times, and hot pressing for 5-10 minutes; the crosslinking treatment is carried out at 180-200℃ for 15-20 minutes.

9. The application of the insulating material for ultra-high voltage DC cables according to any one of claims 1-5 in ultra-high voltage DC cables, characterized in that, The voltage level of the ultra-high voltage DC cable is ≥320kV.

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