A high voltage direct current submarine cable insulation material and a method of making the same

Abstract

The application belongs to the technical field of polymer composites, and particularly relates to a high-voltage direct-current submarine cable insulation material and a preparation method thereof. The high-voltage direct-current submarine cable insulation material is prepared from raw materials including low-density polyethylene, fillers and functional additives. The fillers include micron fillers, submicron fillers and nanometer fillers. By selecting micron, submicron and nanometer fillers with three average particle sizes, the three fillers with different sizes construct wide-distribution trap energy levels in the matrix, effectively inhibit the injection and accumulation of space charges under a direct-current electric field, and the targeted surface modification of the three fillers can significantly improve the dispersibility and interface compatibility of the fillers. The prepared insulation material has significantly reduced space charge accumulation and controllable electric field distortion rate in a wide temperature range, and is suitable for high-voltage direct-current submarine cables.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite materials technology, specifically relating to an insulation material for high-voltage direct current submarine cables and its preparation method. Background Technology

[0002] With the continuous growth in demand for large-scale offshore wind power grid connection and long-distance transoceanic power transmission, the use of ±320kV~±525kV flexible direct current (HVDC) submarine cables has increased dramatically. Cross-linked polyethylene (XLPE) has become the mainstream insulation material for high-voltage cables due to its excellent electrical strength and mechanical properties. However, under the long-term action of a DC electric field, the non-polar matrix of polyethylene lacks effective charge traps, and space charges are easily injected from the conductor shielding layer and accumulate in the insulation layer, leading to severe distortion of the local electric field, accelerating insulation aging, and even causing breakdown. In addition, the DC volume resistivity of existing XLPE insulation materials is highly sensitive to temperature. Under wide temperature range operating conditions from low seawater temperature (about -2℃) to full-load high temperature (about +70℃), the conductivity increases exponentially with increasing temperature, the leakage current increases, and it is easy to form thermo-electric positive feedback runaway, which significantly reduces the long-term operational reliability of the insulation.

[0003] To suppress space charge accumulation in DC cables, current research attempts to introduce inorganic nanofillers (such as nano-Al2O3) into the polyethylene matrix to construct charge traps. However, single-scale or single-type fillers often only provide a narrow distribution of trap energy levels, which is insufficient for capturing carriers of different energies and rates over a wide temperature range, easily leading to charge capture-release imbalance. At the same time, the poor interfacial compatibility between inorganic fillers and organic matrices makes them prone to aggregation, which in turn introduces new interfacial defects and local conductive channels into the matrix, degrading insulation performance.

[0004] Therefore, there is an urgent need to develop a high-voltage DC cable insulation material that has a wide distribution of trap energy levels, excellent interfacial compatibility, and long-term operational stability over a wide temperature range. Summary of the Invention

[0005] The purpose of this invention is to provide an insulation material for high-voltage DC submarine cables that can effectively suppress space charge accumulation and electric field distortion within a wide temperature range of −2℃ (seawater environment) to +70℃ (full load condition), thereby making the electric field more stable.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of the present invention provides an insulation material for high-voltage direct current submarine cables, the raw materials for which are prepared include low-density polyethylene, fillers, and functional additives.

[0007] Preferably, the low-density polyethylene has a melt mass flow rate of 1.8-2.2 g / 10 min at 190°C and a DC volume resistivity of >10 PΩcm at 23°C.

[0008] In some alternatives, the low-density polyethylene is sourced from Borealis, Borlink™ LS4258DCS.

[0009] Preferably, the amount of filler added is 1.6%-2.7% of the mass of low-density polyethylene.

[0010] Preferably, the filler includes micron-sized fillers, submicron-sized fillers, and nano-sized fillers.

[0011] Preferably, the mass ratio of the micron-sized filler, the submicron-sized filler, and the nano-sized filler is (8-12):(5-9):(3-6).

[0012] Preferably, the micron-sized filler includes one or two of modified magnesium oxide and modified aluminum oxide.

[0013] The method for preparing the modified magnesium oxide includes the following steps: micron-sized magnesium oxide is added to an aqueous ethanol solution, ultrasonically dispersed, a silane coupling agent is added, the mixture is refluxed and stirred to react, separated, washed 2-3 times with anhydrous ethanol, dried and pulverized to obtain the final product.

[0014] Preferably, the average particle size of the micron-sized magnesium oxide is 3-8 μm.

[0015] In some alternative solutions, the micron-sized magnesium oxide is sourced from Bohuas Nanotechnology (Ningbo) Co., Ltd.

[0016] Preferably, in the ethanol-water solution, the volume ratio of ethanol to deionized water is 9:1, and the pH value is adjusted to 4.5-5.0 with acetic acid.

[0017] Preferably, the solid-liquid ratio of the micron-sized magnesium oxide and the ethanol aqueous solution is 1 g: (5-10) mL.

[0018] Preferably, the specific parameters for ultrasonic dispersion are: ultrasonic power of 150-250W, ultrasonic frequency of 40kHz, and time of 30-40 min.

[0019] In some alternative embodiments, the parameters for ultrasonic dispersion described in this invention are all consistent.

[0020] Preferably, the silane coupling agent includes hexadecyltrimethoxysilane, γ-methacryloxypropyltrimethoxysilane, or vinyltrimethoxysilane.

[0021] Preferably, the amount of the silane coupling agent added is 1.0% to 1.5% of the mass of micron-sized magnesium oxide.

[0022] Preferably, the specific parameters of the reflux stirring reaction are: temperature of 70~75℃, stirring speed of 300~400r / min, and reaction time of 2~3 h.

[0023] Preferably, the specific parameters for the drying and pulverizing are: vacuum degree of 0.08-0.09 MPa, drying temperature of 90-100℃, drying time of 4-6 h, and passing through a 100-300 mesh sieve.

[0024] Preferably, the submicron-sized filler comprises modified zinc oxide.

[0025] The method for preparing the modified zinc oxide includes the following steps: A1. Referring to the preparation method of modified magnesium oxide, zinc oxide is modified with a silane coupling agent, and the amount of silane coupling agent is 1.5%~2.0% of the mass of zinc oxide to obtain silanized zinc oxide. A2. Redisperse silanized zinc oxide in anhydrous ethanol, add stearic acid, reflux and stir to react, separate, wash 1-2 times with anhydrous ethanol, dry and pulverize to obtain the product.

[0026] Preferably, the average particle size of the zinc oxide is 300-800 nm.

[0027] In some alternative solutions, the zinc oxide is sourced from Bohuas Nanotechnology (Ningbo) Co., Ltd.

[0028] Preferably, the solid-liquid ratio of the silanized zinc oxide and anhydrous ethanol is 1 g: (5-10) mL.

[0029] Preferably, the amount of stearic acid added is 1.0% to 1.5% of the mass of zinc oxide.

[0030] Preferably, in step A2, the specific parameters for the reflux stirring reaction are: temperature of 70~75℃, stirring speed of 300~400 r / min, and reaction time of 1-1.5 h.

[0031] Preferably, in step A2, the specific parameters for drying and pulverizing are: vacuum degree of 0.08-0.09 MPa, drying temperature of 75-85℃, drying time of 6-8h, and passing through a 100-300 mesh sieve.

[0032] Preferably, the nanoscale filler comprises modified montmorillonite.

[0033] The preparation method of the modified montmorillonite includes the following steps: B1. Disperse sodium-based montmorillonite in deionized water and stir at high speed to obtain a montmorillonite suspension. B2. Add a cationic intercalating agent to the montmorillonite suspension, stir and react for 3-4 hours while maintaining the rotation speed and temperature, then centrifuge and wash the filter cake repeatedly with deionized water until no white precipitate is formed when 0.1 mol / L AgNO3 solution is added to the filtrate, thus obtaining the intercalated montmorillonite wet filter cake. B3. Disperse the wet filter cake of intercalated montmorillonite in an aqueous ethanol solution, add the edge silanizing agent, disperse by ultrasonication, reflux and stir to react, separate, wash with anhydrous ethanol 2-3 times, dry and pulverize to obtain the final product.

[0034] Preferably, the sodium-based montmorillonite has an interlayer spacing of 1.2-1.4 nm and an ion exchange capacity of 110-130 meq / 100g.

[0035] In some alternative solutions, the sodium-based montmorillonite is sourced from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd.

[0036] Preferably, in step B1, the temperature of the deionized water is 75-80℃.

[0037] Preferably, the specific conditions for high-speed stirring are: a rotation speed of 800~1000 r / min and a dispersion time of 25-35 min.

[0038] Preferably, the montmorillonite suspension contains 3%-5% montmorillonite solids.

[0039] Preferably, in step B2, the cationic intercalating agent includes one or two of hexadecyltrimethylammonium bromide and octadecyltrimethylammonium chloride, and the amount added is 20%-40% of the mass of sodium montmorillonite.

[0040] Preferably, in step B3, the volume ratio of ethanol to deionized water in the ethanol-water solution is 9:1, the pH value is adjusted to 4.5~5.0 with acetic acid, and the solid-liquid ratio of the intercalated montmorillonite wet filter cake to the ethanol-water solution is 1g:(5-10)mL.

[0041] Preferably, in step B3, the edge silanizing agent includes one or more of KH550, KH570, and VTMS, and the amount added is 1.0%-1.5% of the mass of sodium montmorillonite.

[0042] Preferably, in step B3, the specific parameters for the reflux stirring reaction are: temperature of 75~80℃, stirring speed of 300~400 r / min, and reaction time of 1.5-2.5 h.

[0043] Preferably, in step B3, the specific parameters for drying and pulverizing are: vacuum degree of 0.08-0.09 MPa, drying temperature of 75-85℃, drying time of 8-10 h, and passing through a 300-500 mesh sieve.

[0044] By selecting fillers with three average particle sizes—micrometer, submicrometer, and nanometer—and synergistically compounding them according to weight ratio, the amount of space charge injected can be significantly reduced, electric field distortion can be effectively suppressed, and the charge suppression capability over a wide temperature range can be greatly improved. This solves the problems of narrow trap energy level distribution and insufficient charge suppression capability over a wide temperature range in existing single-scale fillers. The deep traps of micrometer-scale oxides preferentially capture high-energy charge carriers injected from the conductor, the medium-depth traps of submicrometer-scale zinc oxide intercept moderately energetic charge carriers and slow down their migration rate, and the shallow traps and two-dimensional tortuous channels between nanometer-scale montmorillonite sheets further capture low-energy charge carriers and extend their migration path. The three scales of fillers are evenly distributed in space and complementary in energy level, forming a continuous trap energy level spectrum from deep to shallow and a three-dimensional retardation network. This achieves hierarchical capture and synergistic suppression of charge carriers with different energies, avoiding charge accumulation imbalance caused by overload release of a single deep trap or insufficient capture of a single shallow trap.

[0045] By modifying three fillers with different average particle sizes, the fillers can be uniformly dispersed in a low-density polyethylene matrix, significantly reducing interfacial defects and simultaneously improving interfacial bonding strength. This effectively solves the problems of local electric field concentration caused by filler agglomeration and leakage current channels caused by interfacial micropores. Silane and micron-sized oxide surfaces form chemical anchoring through Si-OM covalent bonds, ensuring that the coating layer does not fall off under high-shear processing. The inner silane chemical bonds of submicron zinc oxide and the outer stearic acid long-chain physical entanglement structure establish a "chemical anchoring-physical compatibility" synergistic interface, which ensures structural stability and enhances compatibility with low-density polyethylene. The hexadecyltrimethylammonium bromide intercalation of nano-montmorillonite expands the sheets to achieve physical exfoliation, and the silane edge grafting forms chemical anchoring points to prevent sheet recombination. The functional zoning of intercalation and anchoring synergistically keeps the sheets in a persistently dispersed state in the matrix. The three modification strategies are precisely designed for the surface chemical properties of different fillers, jointly constructing a low-defect, highly stable multi-scale synergistic interface system between inorganic fillers and organic matrix. Furthermore, the modification treatment can improve the dispersion stability of the filler and enhance its charge suppression function, enabling the insulating material to maintain low conductivity temperature sensitivity and high long-term operational reliability across a wide temperature range from -2℃ seawater to +70℃ under full load. The modified particles collaboratively construct a denser carrier retardation network in three-dimensional space, while the stable interface structure reduces the interface activation channels for carriers at high temperatures, thereby synergistically reducing the temperature sensitivity of conductivity and suppressing the risk of thermo-electric positive feedback runaway.

[0046] Preferably, the amount of the functional additive added is 1.5%-2.5% of the mass of low-density polyethylene.

[0047] Preferably, the functional additives include crosslinking agents and antioxidants in a mass ratio of (8-12):1.

[0048] Preferably, the crosslinking agent comprises dicumyl peroxide.

[0049] Preferably, the antioxidant includes one or more of antioxidant 1010 and antioxidant 168.

[0050] The second aspect of this invention provides a method for preparing the insulation material of the high-voltage DC submarine cable, comprising the following steps: mixing low-density polyethylene and antioxidant at a low speed of 30-50 r / min for 3-5 min, then transferring the mixture to a mixer or twin-screw extruder, heating it to 120-130°C to melt the low-density polyethylene, adding filler, and melt-blending to obtain a composite base material; cooling the composite base material to below 100°C, adding a crosslinking agent, mixing at a low speed of 30-50 r / min for 3-5 min, and then extruding and granulating to obtain the final product.

[0051] Preferably, the specific parameters for the melt blending are: temperature of 140-150℃, rotation speed of 50-80 r / min, and time of 15-20 min.

[0052] Preferably, the specific parameters for the extrusion granulation are: extrusion temperature of 120-135℃.

[0053] The third aspect of this invention provides the application of the aforementioned high-voltage DC submarine cable insulation material, which is used in the preparation of high-voltage DC submarine cables. When the high-voltage DC submarine cable insulation material is used, it needs to be extruded into sheet or tubular samples by a micro single-screw extruder at an extrusion temperature of 120-140°C. After extrusion, it is placed at 175-185°C for crosslinking for 15-25 minutes.

[0054] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. This invention proposes an insulation material for high-voltage DC submarine cables. By selecting fillers with three average particle sizes of micron, submicron, and nano, the three sizes of fillers construct a wide-distribution trap energy level in the matrix, effectively suppressing space charge injection and accumulation under DC electric field. Targeted surface modification of the three fillers can significantly improve filler dispersion and interfacial compatibility. The prepared insulation material has significantly reduced space charge accumulation and controllable electric field distortion rate over a wide temperature range, making it suitable for high-voltage DC submarine cables.

[0055] 2. By selecting fillers with three levels of average particle size (micron, submicron, and nano) and synergistically compounding them according to weight ratio, this invention can significantly reduce the amount of space charge injection, effectively suppress electric field distortion, and greatly improve the wide-temperature-range charge suppression capability, thus solving the problems of narrow trap energy level distribution and insufficient wide-temperature-range charge suppression capability of existing single-scale fillers.

[0056] 3. This invention modifies three fillers with different average particle sizes, enabling them to be uniformly dispersed in a low-density polyethylene matrix. This significantly reduces interfacial defects, simultaneously improves interfacial bonding strength, effectively solves the problems of local electric field concentration caused by filler agglomeration and leakage current channels caused by interfacial micro-voids, improves filler dispersion stability, and enhances charge suppression function. This allows the insulating material to maintain low conductivity temperature sensitivity and high long-term operational reliability in a wide temperature range from -2℃ seawater to +70℃ full-load high temperature. Detailed Implementation

[0057] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0058] All raw materials used in this invention are commercially available, as detailed below: The low-density polyethylene has a melt mass flow rate of 1.8-2.2 g / 10 min at 190°C and a DC volume resistivity of >10 PΩcm at 23°C. It is from Borealis, Borlink™ LS4258DCS.

[0059] Magnesium oxide and silicon dioxide, with an average particle size of 3-15 μm, are from Bohuas Nanotechnology (Ningbo) Co., Ltd.

[0060] Zinc oxide and titanium dioxide, with an average particle size of 300-800nm, Bohuas Nanotechnology (Ningbo) Co., Ltd.

[0061] Sodium-based montmorillonite, with an interlayer spacing of 1.2-1.4 nm and an ion exchange capacity of 110-130 meq / 100g, is from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd.

[0062] Example 1 This embodiment provides an insulation material for high-voltage direct current submarine cables, the raw materials for which are low-density polyethylene, fillers, and functional additives.

[0063] The amount of filler added is 2% of the mass of low-density polyethylene.

[0064] The filler is a micron-sized filler, a submicron-sized filler, and a nano-sized filler, with a mass ratio of 10:7:4.

[0065] The micron-sized filler is modified magnesium oxide.

[0066] The preparation method of the modified magnesium oxide includes the following steps: micron-sized magnesium oxide is added to an ethanol aqueous solution, ultrasonically dispersed, a silane coupling agent is added, the mixture is refluxed and stirred to react, separated, washed three times with anhydrous ethanol, dried and pulverized to obtain the modified magnesium oxide.

[0067] The average particle size of the micron-sized magnesium oxide is 5 μm.

[0068] In the ethanol-water solution, the volume ratio of ethanol to deionized water is 9:1, and the pH is adjusted to 5.0 with acetic acid.

[0069] The solid-liquid ratio of the micron-sized magnesium oxide and the aqueous ethanol solution is 1 g: 10 mL.

[0070] The specific parameters for ultrasonic dispersion are: ultrasonic power of 200W, ultrasonic frequency of 40kHz, and time of 35min.

[0071] The parameters for ultrasonic dispersion described in this embodiment are all consistent.

[0072] The silane coupling agent is hexadecyltrimethoxysilane.

[0073] The amount of the silane coupling agent added is 1.2% of the mass of micron-sized magnesium oxide.

[0074] The specific parameters for the reflux stirring reaction are: temperature 70℃, stirring speed 350 r / min, and reaction time 2.5 h.

[0075] The specific parameters for drying and pulverizing are: vacuum degree of 0.08MPa, drying temperature of 95℃, drying time of 5h, and passing through a 200-mesh sieve.

[0076] The submicron-sized filler is modified zinc oxide.

[0077] The preparation method of the modified zinc oxide includes the following steps: A1. The method for modifying magnesium oxide in Example 1 is the same, except that the amount of silane is changed to 1.8% of the mass of zinc oxide to obtain silanized zinc oxide; A2. Redisperse silanized zinc oxide in anhydrous ethanol, add stearic acid, reflux and stir to react, separate, wash twice with anhydrous ethanol, dry and pulverize to obtain the product.

[0078] The average particle size of the zinc oxide is 500 nm.

[0079] The solid-liquid ratio of the silanized zinc oxide and anhydrous ethanol is 1 g: 10 mL.

[0080] The amount of stearic acid added is 1.2% of the mass of zinc oxide.

[0081] In step A2, the specific parameters for the reflux stirring reaction are: temperature 75℃, stirring speed 350 r / min, and reaction time 1h.

[0082] In step A2, the specific parameters for drying and pulverizing are: vacuum degree of 0.08 MPa, drying temperature of 80℃, drying time of 7 h, and passing through a 200-mesh sieve.

[0083] The nanoscale filler is modified montmorillonite.

[0084] The preparation method of the modified montmorillonite includes the following steps: B1. Disperse sodium-based montmorillonite in deionized water and stir at high speed to obtain a montmorillonite suspension. B2. Add a cationic intercalating agent to the montmorillonite suspension, stir and react for 3.5 hours while maintaining the rotation speed and temperature, then centrifuge and wash the filter cake repeatedly with deionized water until no white precipitate is formed when 0.1 mol / L AgNO3 solution is added to the filtrate, thus obtaining the intercalated montmorillonite wet filter cake. B3. Disperse the intercalated montmorillonite wet filter cake in an ethanol-water solution, add the edge silanizing reagent, ultrasonically disperse, reflux and stir to react, separate, wash three times with anhydrous ethanol, dry and pulverize to obtain the final product.

[0085] In step B1, the temperature of the deionized water is 80°C.

[0086] The specific conditions for the high-speed stirring are: a rotation speed of 900 r / min and a dispersion time of 30 min.

[0087] The montmorillonite suspension contains 4% montmorillonite solids.

[0088] In step B2, the cationic intercalating agent is hexadecyltrimethylammonium bromide, and the amount added is 30% of the mass of sodium montmorillonite.

[0089] In step B3, the volume ratio of ethanol to deionized water in the ethanol-water solution is 9:1, the pH is adjusted to 5.0 with acetic acid, and the solid-liquid ratio of the intercalated montmorillonite wet filter cake to the ethanol-water solution is 1g:10mL.

[0090] In step B3, the edge silanizing agent is KH570, and the amount added is 1.2% of the mass of sodium montmorillonite.

[0091] In step B3, the specific parameters for the reflux stirring reaction are: temperature 80℃, stirring speed 350 r / min, and reaction time 2h.

[0092] In step B3, the specific parameters for drying and pulverizing are: vacuum degree of 0.08 MPa, drying temperature of 80℃, drying time of 9h, and passing through a 400-mesh sieve.

[0093] The amount of the functional additive added is 2% of the mass of low-density polyethylene.

[0094] The functional additives are crosslinking agents and antioxidants in a mass ratio of 10:1.

[0095] The crosslinking agent is dicumyl peroxide.

[0096] The antioxidants are antioxidant 1010 and antioxidant 168 in a mass ratio of 2:1.

[0097] The preparation method of the insulation material for the high-voltage DC submarine cable includes the following steps: low-density polyethylene and antioxidant are mixed at a low speed of 40 r / min for 4 min, then transferred to a mixer or twin-screw extruder, heated to 125°C to melt the low-density polyethylene, then filler is added, and the composite base material is obtained by melt blending; the composite base material is cooled to 95°C, a crosslinking agent is added, and the mixture is mixed at a low speed of 40 r / min for 3 min, then extruded and granulated to obtain the final product.

[0098] The specific parameters for the melt blending are: temperature 145℃, rotation speed 60r / min, and time 20min.

[0099] The specific parameters for the extrusion granulation are: extrusion temperature of 130℃.

[0100] Example 2 This embodiment provides an insulation material for a high-voltage direct current submarine cable. The specific implementation method is the same as that in Embodiment 1, except that the mass ratio of the micron-level filler, submicron-level filler and nano-level filler is 4:3:2.

[0101] Comparative Example 1 The only difference between this comparative example and Example 1 is: The filler is a micron-sized filler, a submicron-sized filler, and a nano-sized filler, with a mass ratio of 1:1:1.

[0102] Comparative Example 2 The only difference between this comparative example and Example 2 is: Micron-sized fillers are replaced with modified magnesium oxide with modified silica, and the particle sizes of silica and magnesium oxide are the same.

[0103] Comparative Example 3 The only difference between this comparative example and Example 1 is: Modified titanium dioxide was used instead of modified zinc oxide for submicron-sized fillers, and the particle sizes of titanium dioxide and zinc oxide were the same.

[0104] Comparative Example 4 The only difference between this comparative example and Example 1 is: The average particle size of the micron-sized magnesium oxide is 15 μm.

[0105] Comparative Example 5 The only difference between this comparative example and Example 1 is: The three filler modification methods are consistent and are the same as the preparation method of modified magnesium oxide in Example 1.

[0106] Comparative Example 6 The only difference between this comparative example and Example 1 is: In the preparation method of the modified zinc oxide, stearic acid is used for modification first, followed by silane coupling agent modification.

[0107] Comparative Example 7 The only difference between this comparative example and Example 1 is: In the preparation method of the modified montmorillonite, the anionic surfactant sodium dodecylbenzenesulfonate is used instead of the cationic intercalating agent.

[0108] Performance testing The insulating materials prepared in the examples and comparative examples were used to prepare experimental samples for testing their performance. The preparation method of the experimental samples was as follows: the insulating material was extruded into sheets with a thickness of 0.5~1mm and a diameter of 40mm using a micro single-screw extruder at an extrusion temperature of 130℃. After extrusion, the sheets were crosslinked at 180℃ for 20min to obtain the experimental samples.

[0109] Space charge suppression capability: The experimental sample thickness was 0.5 mm. The space charge distribution was determined using the PEA method according to JB / T 12928-2016, and the electric field distortion rate was calculated by inversion. The PEA test used aluminum electrodes. The experimental sample was pre-treated with a short circuit at 60℃ for 24 hours to eliminate residual charge. A DC electric field of 40 kV / mm was applied at 70℃ for a total duration of 120 min, with space charge distribution data collected every 5 min. The pulse voltage was 200 V, and the pulse width was 5 ns. The space charge distribution was decoupled using a deconvolution algorithm, and the electric field distribution was obtained by numerical integration of the Poisson equation. The electric field distortion rate = E max -E avg / E avg ×100%, where E avg E represents the average electric field strength (applied voltage ÷ insulation layer thickness). max This represents the maximum value of the measured electric field distribution.

[0110] Volume resistivity: The experimental sample thickness was 1 mm. Referring to GB / T 31838.2-2019, a three-electrode system was used, with stainless steel as the electrode material. The experimental sample was pretreated for 48 hours at 23±2℃ and 50±5%RH. Short-circuit discharge was performed for 2 hours before testing. A voltage of 1000V DC was applied, with a charging time of 60 seconds. Readings were taken after current fluctuations were <5%. For testing at 70℃, the sample was kept at a constant temperature in an oven for ≥30 minutes before being quickly transferred to the electrode system for volume resistivity testing. The testing time was <2 minutes to reduce temperature drift. Volume resistivity ρ v =R v ×S / d, where R v S is the volume resistivity (Ω), and S is the electrode area (cm²). 2 ), where d is the sample thickness (cm).

[0111] Conductivity temperature sensitivity: The experimental sample thickness was 1 mm. Referring to GB / T 31838.2-2019, the volume resistivity was measured at seven temperature points: -20℃, 0℃, 20℃, 40℃, 60℃, 70℃, and 90℃. Each temperature point was held at the same temperature for ≥30 min, and the test voltage was 1000V DC. The conductivity temperature sensitivity was calculated using lg(1 / ρ) v Plotting a graph against 1000 / T, the conductivity and activation energy E are obtained by determining the slope of the linear fit. a .

[0112] Seabed environmental adaptability: The experimental sample thickness was 1 mm. Referring to GB / T 2951.13-2008, the sample was immersed in a 3.5% NaCl solution (simulated seawater) at 70℃ for 336 hours. After removal, the surface was rinsed with deionized water, and the surface moisture was absorbed with filter paper. After recovery in a 23℃, 50%RH environment for 24 hours, the volume resistivity was measured. The volume resistivity change rate = (ρ... v测试后 -ρ v测试前 ) / ρ v测试前 ×100%.

[0113] The experimental results are shown in Table 1.

[0114] Table 1 Table 1 shows that the high-voltage DC submarine cable insulation materials prepared in Examples 1-2 have the best overall performance, with low electric field distortion rate and high volume resistivity, meeting the requirements of high-voltage DC. Simultaneously, the low activation energy indicates low temperature sensitivity and excellent seawater resistance. Comparative Example 1 has the same proportion of three-stage fillers, lacking a gradient distribution; Comparative Example 2 shows that the interface trap depth and charge suppression ability of silica are weaker than those of magnesium oxide; Comparative Example 3 shows that the high dielectric constant and semiconductor characteristics of titanium dioxide exacerbate electric field distortion; Comparative Example 4 shows that the magnesium oxide particles are too large, resulting in uneven dispersion and reduced trapping efficiency; Comparative Example 5 only modifies with silane, without zinc oxide stearic acid compatibilization and montmorillonite intercalation, resulting in poor dispersion and numerous interface defects; Comparative Example 6 shows that the zinc oxide modification order is reversed, with stearic acid coating first hindering silane chemical anchoring, making the coating layer easy to detach and reducing performance; Comparative Example 7 shows that the montmorillonite anionic intercalation, with mismatched intercalating agents, leads to small interlayer spacing, difficulty in peeling off nanosheets, and low charge trap density, resulting in insufficient performance of the prepared cable insulation materials.

[0115] Therefore, this invention proposes an insulation material for high-voltage DC submarine cables. By selecting fillers with three levels of average particle size (micron, submicron, and nanometer) and performing targeted surface modification on the three fillers, the prepared insulation material exhibits significantly reduced space charge accumulation and controllable electric field distortion rate over a wide temperature range.

[0116] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An insulation material for high-voltage direct current submarine cables, characterized in that, The raw materials for its preparation include low-density polyethylene, fillers, and functional additives; the low-density polyethylene has a melt flow rate of 1.8-2.2 g / 10 min at 190°C and a DC volume resistivity of >10 PΩcm at 23°C; the amount of filler added is 1.6%-2.7% of the mass of low-density polyethylene.

2. The high-voltage DC submarine cable insulation material according to claim 1, characterized in that, The packing includes micron-sized packing, submicron-sized packing and nano-sized packing; the mass ratio of the micron-sized packing, submicron-sized packing and nano-sized packing is (8-12):(5-9):(3-6).

3. The high-voltage DC submarine cable insulation material according to claim 2, characterized in that, The micron-sized filler includes one or two of modified magnesium oxide and modified aluminum oxide; the submicron-sized filler includes modified zinc oxide; and the nano-sized filler includes modified montmorillonite.

4. The high-voltage DC submarine cable insulation material according to claim 3, characterized in that, The method for preparing the modified magnesium oxide includes the following steps: micron-sized magnesium oxide is added to an aqueous ethanol solution, ultrasonically dispersed, a silane coupling agent is added, the mixture is refluxed and stirred to react, separated, washed 2-3 times with anhydrous ethanol, dried and pulverized to obtain the final product.

5. The high-voltage DC submarine cable insulation material according to claim 4, characterized in that, The average particle size of the micron-sized magnesium oxide is 3-8 μm.

6. The high-voltage DC submarine cable insulation material according to claim 4, characterized in that, The method for preparing the modified zinc oxide includes the following steps: A1. Referring to the preparation method of modified magnesium oxide, zinc oxide is modified with a silane coupling agent, and the amount of silane coupling agent is 1.5%~2.0% of the mass of zinc oxide to obtain silanized zinc oxide. A2. Redisperse silanized zinc oxide in anhydrous ethanol, add stearic acid, reflux and stir to react, separate, wash 1-2 times with anhydrous ethanol, dry and pulverize to obtain the product.

7. The high-voltage DC submarine cable insulation material according to claim 6, characterized in that, The average particle size of the zinc oxide is 300-800 nm.

8. The high-voltage DC submarine cable insulation material according to claim 3, characterized in that, The preparation method of the modified montmorillonite includes the following steps: B1. Disperse sodium-based montmorillonite in deionized water and stir at high speed to obtain a montmorillonite suspension. B2. Add a cationic intercalating agent to the montmorillonite suspension, stir and react for 3-4 hours while maintaining the rotation speed and temperature, then centrifuge and wash the filter cake repeatedly with deionized water until no white precipitate is formed when 0.1 mol / L AgNO3 solution is added to the filtrate, thus obtaining the intercalated montmorillonite wet filter cake. B3. Disperse the wet filter cake of intercalated montmorillonite in an aqueous ethanol solution, add the edge silanizing agent, disperse by ultrasonication, reflux and stir to react, separate, wash with anhydrous ethanol 2-3 times, dry and pulverize to obtain the final product.

9. The high-voltage DC submarine cable insulation material according to claim 8, characterized in that, The sodium-based montmorillonite has an interlayer spacing of 1.2-1.4 nm and an ion exchange capacity of 110-130 meq / 100g.

10. A method for preparing insulation material for high-voltage direct current submarine cables according to any one of claims 1 to 9, characterized in that, The process includes the following steps: mixing low-density polyethylene and antioxidants at a low speed of 30-50 r / min for 3-5 min, then transferring the mixture to an internal mixer or twin-screw extruder, heating it to 120-130℃ to melt the low-density polyethylene, adding fillers, and melt-blending to obtain a composite base material; cooling the composite base material to below 100℃, adding a crosslinking agent, mixing at a low speed of 30-50 r / min for 3-5 min, and then extruding and granulating to obtain the final product.

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