High-flame-retardant insulating medium-voltage cable

Abstract

The invention relates to the technical field of cables, and provides a high-flame-retardant insulating medium-voltage cable which sequentially comprises a conductor, an inner shielding layer, an insulating layer, an outer shielding layer, a fire-resistant wrapping layer, an armor layer and a sheath layer from inside to outside, the conductor is a copper conductor, the insulating layer is a crosslinked polyethylene insulating layer, and the outer shielding layer is an outer shielding layer. The outer shielding layer comprises an insulation shielding layer and a copper strip shielding layer arranged on the outer side of the insulation shielding layer, and the armor layer is a steel strip armor layer; the raw materials of the crosslinked polyethylene insulating layer comprise polyethylene, a water-tree-resistant master batch, a crosslinking agent and a processing aid, the raw materials of the water-tree-resistant master batch comprise polyethylene, a copolymer and inorganic porous aggregates, and the copolymer is poly (butadiene-vinylpyridine) and / or poly (vinylphosphonic acid). According to the technical scheme, the problem that a water tree is prone to occurring in a crosslinked polyethylene insulated medium-voltage cable in the related technology is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of cables, and in particular to a highly flame-retardant insulated medium-voltage cable. Background Art

[0002] Cables used to transmit and distribute high-power electrical energy are called power cables. Power cables are categorized by voltage level into low-voltage cables, medium-voltage cables, high-voltage cables, and ultra-high-voltage cables. Cables with voltage levels of 1kV and below are generally called low-voltage cables, cables with voltage levels of 6-35kV are called medium-voltage cables, cables with voltage levels of 110kV are called high-voltage cables, and cables with voltage levels of 220-500kV are called ultra-high-voltage cables.

[0003] Currently, cross-linked polyethylene (XLPE) is the primary insulation material for cables. With the widespread adoption of XLPE-insulated cables, water trees are common in most medium-voltage cables with extended service life. This is particularly prevalent in cables installed in humid environments in the south. Water trees primarily arise from the fact that XLPE is a semi-crystalline compound, with moisture, impurities, and cross-linking byproducts primarily concentrated in the amorphous regions. During cable production, when XLPE is rapidly cooled from high temperatures, water vapor and other gases diffuse through the insulation, forming micropores. However, the moisture in the polyethylene cannot be completely eliminated. Under the influence of the cross-linking electric field, the repeatedly alternating induction force causes fatigue in the XLPE surrounding the micropores, forming water trees and making the medium-voltage cable susceptible to failure. Summary of the Invention

[0004] The present invention provides a highly flame-retardant insulated medium-voltage cable, which solves the problem of water treeing in cross-linked polyethylene insulated medium-voltage cables in the related art.

[0005] The technical solutions of the present invention are as follows: A highly flame-retardant insulated medium-voltage cable comprises, from the inside out, a conductor, an inner shielding layer, an insulating layer, an outer shielding layer, a fire-resistant wrapping layer, an armor layer, and a sheath layer. The conductor is a copper conductor, the insulating layer is a cross-linked polyethylene insulating layer, the outer shielding layer comprises an insulating shielding layer and a copper tape shielding layer arranged outside the insulating shielding layer, and the armor layer is a steel tape armor layer. The raw materials of the cross-linked polyethylene insulating layer include polyethylene, an anti-water tree masterbatch, a cross-linking agent, and a processing aid. The raw materials of the anti-water tree masterbatch include polyethylene, a copolymer, and an inorganic porous granular material. The copolymer is poly(butadiene-vinyl pyridine) and / or poly(vinyl phosphonic acid).

[0006] In the high flame retardant insulated medium voltage cable of the present invention, a copper conductor with excellent conductivity and stability is used as the core conductive structure, which can effectively reduce the power loss during the transmission process and ensure stable and efficient power transmission. The arrangement of the inner shielding layer and the outer shielding layer, especially the outer shielding structure combining the insulating shielding layer with the copper tape shielding layer, can evenly distribute the electric field, suppress the local discharge phenomenon, reduce the risk of insulation aging and breakdown caused by electric field distortion, and improve the reliability and safety of cable operation. In addition, the fire-resistant wrapping layer gives the cable a certain fire resistance; the steel tape armor layer enhances the mechanical strength of the cable, so that it has good pressure resistance and impact resistance, can effectively resist external mechanical damage, protect the internal structure from damage, and prevent leakage accidents caused by damage to the insulation layer due to external mechanical force.

[0007] The cross-linked polyethylene insulation layer in this highly flame-retardant insulated medium-voltage cable is made from polyethylene as the base material, supplemented with an anti-water tree masterbatch, a cross-linking agent, and a processing aid. The anti-water tree masterbatch effectively inhibits the growth of water trees, preventing insulation degradation caused by moisture intrusion and extending the cable's service life in humid environments. The cross-linking agent promotes cross-linking reactions in the polyethylene molecules, forming a three-dimensional network structure that ensures the insulation layer's heat resistance, mechanical strength, and chemical stability. The processing aid optimizes the raw material's processing properties, ensuring a uniform and dense insulation layer, further enhancing the cable's overall performance.

[0008] In the water-tree resistant masterbatch of the highly flame-retardant insulated medium-voltage cable of the present invention, the inorganic porous granular material may be any one or more conventional inorganic porous granular materials in the art, such as porous silica, molecular sieve, and the like.

[0009] As a further technical solution, the inorganic porous granules are polymer-coated inorganic porous granules, and the polymer in the polymer-coated inorganic porous granules is poly(butadiene-vinyl pyridine) and / or poly(vinyl phosphonic acid).

[0010] In the prior art, coupling agents are generally used to modify the surface of inorganic porous particles. However, since the surface of the inorganic porous particles modified by the coupling agent is hydrophobic, the effect of inhibiting water trees is limited. In the cross-linked polyethylene insulation layer of the highly flame-retardant insulated medium-voltage cable of the present invention, poly(butadiene-vinyl pyridine) and / or poly(vinyl phosphonic acid) are used to coat the inorganic porous particles, which further inhibits the formation of water trees in the cross-linked polyethylene insulated medium-voltage cable compared to the prior art, while also improving the flame retardancy.

[0011] As a further technical solution, the method for preparing the polymer-coated inorganic porous granules includes the following steps: dissolving the polymer, adding the inorganic porous granules, mixing evenly, and drying to obtain the polymer-coated inorganic porous granules.

[0012] As a further technical solution, the uniform mixing is achieved by ultrasonic means.

[0013] As a further technical solution, the power of the ultrasound is 200~400W, for example, it can be 200W, 210W, 220W, 230W, 240W, 250W, 260W, 270W, 280W, 290W, 300W, 310W, 320W, 330W, 340W, 350W, 360W, 370W, 380W, 390W or 400W, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0014] As a further technical solution, the frequency of the ultrasound is 30~50kHz, for example, it can be 30kHz, 31kHz, 32kHz, 33kHz, 34kHz, 35kHz, 36kHz, 37kHz, 38kHz, 39kHz, 40kHz, 41kHz, 42kHz, 43kHz, 44kHz, 45kHz, 46kHz, 47kHz, 48kHz, 49kHz or 50kHz, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0015] As a further technical solution, the ultrasound time is 0.5~2h, for example, it can be 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2h, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0016] As a further technical solution, the high molecular polymer coated inorganic porous granules are first coated with poly(vinylphosphonic acid) and then coated with poly(butadiene-vinylpyridine).

[0017] The cross-linked polyethylene insulation layer of the highly flame-retardant insulated medium-voltage cable of the present invention utilizes inorganic porous granules that are first coated with poly(vinyl phosphonic acid) and then with poly(butadiene-vinyl pyridine). This further suppresses water treeing in the cross-linked polyethylene insulated medium-voltage cable while also further improving flame retardancy. This is because poly(vinyl phosphonic acid) has a higher polarity than poly(butadiene-vinyl pyridine), and its addition to the polyethylene base material can affect the water treeing effect. Therefore, the present invention utilizes poly(vinyl phosphonic acid) with a higher polarity as an inner layer, first coating the inorganic porous granules, and then further coating them with poly(butadiene-vinyl pyridine) with a relatively lower polarity as an outer layer. This further suppresses water treeing in the cross-linked polyethylene insulated medium-voltage cable, and the combined use of poly(butadiene-vinyl pyridine) and poly(vinyl phosphonic acid) further enhances flame retardancy.

[0018] As a further technical solution, the method for preparing the high molecular polymer coated inorganic porous granules comprises the following steps: S1, dissolving poly(vinylphosphonic acid) in a solvent, adding the inorganic porous granules, mixing evenly, and drying to obtain pretreated inorganic porous granules; S2. Dissolve poly (butadiene-vinyl pyridine) in a solvent, add the pretreated inorganic porous granules, mix evenly, and dry to obtain high molecular polymer coated inorganic porous granules.

[0019] As a further technical solution, in step S1, the solvent is water; in step S2, the solvent is dichloromethane; and the mixing in step S1 and step S2 is independently performed by ultrasonication.

[0020] As a further technical solution, the power of the ultrasound is 200~400W, for example, it can be 200W, 210W, 220W, 230W, 240W, 250W, 260W, 270W, 280W, 290W, 300W, 310W, 320W, 330W, 340W, 350W, 360W, 370W, 380W, 390W or 400W, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0021] As a further technical solution, the frequency of the ultrasound is 30~50kHz, for example, it can be 30kHz, 31kHz, 32kHz, 33kHz, 34kHz, 35kHz, 36kHz, 37kHz, 38kHz, 39kHz, 40kHz, 41kHz, 42kHz, 43kHz, 44kHz, 45kHz, 46kHz, 47kHz, 48kHz, 49kHz or 50kHz, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0022] As a further technical solution, the ultrasound time is 0.5~2h, for example, it can be 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2h, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

[0023] As a further technical solution, the raw materials of the high molecular polymer-coated inorganic porous granules include high molecular polymer and inorganic porous granules in a mass ratio of 2.5 to 6:60.

[0024] As a further technical solution, the mass ratio of polyethylene, copolymer and inorganic porous pellets in the raw materials of the anti-water tree masterbatch is 100:3~7:15~25, for example, it can be 100:3:15, 100:3:20, 100:3:25, 100:5:15, 100:5:20, 100:5:25, 100:7:15, 100:7:20 or 100:7:25, but is not limited to the listed values. Other values not listed within this numerical range are also applicable.

[0025] As a further technical solution, the mass ratio of polyethylene, anti-water tree masterbatch, crosslinking agent and processing aid in the raw materials of the cross-linked polyethylene insulation layer is 100:15~25:1.5~2.5:0.3~10, for example, it can be 100:15:1.5:0.3, 100:15:2:0.5, 100:20:2.5:0.8, 100:15:1.5:1, 100:15:1.5:3, 100:15:1.5:5, 100:15:1.5:8, 100:15:1.5:10, 100:15:2:1, 100:15:2:5, 100:15:2:10, 100:15:2.5:1, 100:15:2.5:5, 100:15:2.5:10, 100:20:1.5:5, 100:20:1.5:10, 100:25:1.5:1 or 100:25:2.5:10, but is not limited to the listed values, and other values not listed within the numerical range are also applicable.

[0026] As a further technical solution, the processing aid includes one or more of an antioxidant, a cross-linking aid, and a lubricant.

[0027] In the cross-linked polyethylene insulation layer of the highly flame-retardant insulated medium-voltage cable of the present invention, the processing aid may be any one or more aids in the art that can improve the processing performance of the material, such as an antioxidant, a cross-linking aid, a lubricant, etc., preferably an antioxidant and / or a lubricant, the antioxidant is preferably one or more of antioxidant 1010, antioxidant 168, antioxidant 1076, and antioxidant 300, and the lubricant is preferably polyethylene wax.

[0028] As a further technical solution, the cross-linking agent is peroxide.

[0029] In the cross-linked polyethylene insulation layer of the highly flame-retardant insulated medium-voltage cable of the present invention, the cross-linking agent can be any one or more conventional cross-linking agents in the art, preferably a peroxide, and more preferably one or more of dicumyl peroxide (DCP), benzoyl peroxide (BPO), and di-tert-butyl peroxide isopropylbenzene (BIPB).

[0030] The working principle and beneficial effects of the present invention are: The insulating layer of the highly flame-retardant insulated medium-voltage cable provided by the present invention uses polyethylene, a copolymer, and inorganic porous granules as an anti-water tree masterbatch. The copolymer is poly(butadiene-vinyl pyridine) and / or poly(vinyl phosphonic acid). By adding poly(butadiene-vinyl pyridine) and / or poly(vinyl phosphonic acid) to the anti-water tree masterbatch, moisture accumulation is prevented, and the formation of water trees in the cross-linked polyethylene insulated medium-voltage cable is suppressed, thereby greatly ensuring the safety of power grid operation. DETAILED DESCRIPTION

[0031] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0032] The parameters of each raw material in the following examples and comparative examples are as follows: Polyethylene is LDPE Shanghai Petrochemical ZJ2600; The particle size of porous silica is 500 nm and the density is 2.66 g / cm 3 , with a specific surface area of 126.53m 2 / g, the preparation method is gas phase method; Poly(butadiene-vinylpyridine) is polybutadiene-b-poly-4-vinylpyridine with a weight average molecular weight of 20,000; Poly(vinylphosphonic acid) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. with the product number P486975.

[0033] Example 1 A highly flame-retardant insulated medium-voltage cable, comprising, from the inside to the outside, a copper conductor, an inner shielding layer, a cross-linked polyethylene insulation layer, an insulating shielding layer, a copper tape shielding layer, a fire-resistant wrapping layer, a steel tape armoring layer, and a sheathing layer; The cross-linked polyethylene insulation layer is prepared by the following method: S1, after blending 100 parts of polyethylene, 3 parts of poly(butadiene-vinyl pyridine) and 15 parts of porous silica, extrusion and granulation were performed to obtain an anti-water tree masterbatch; S2. Evenly mix 100 parts of polyethylene, 25 parts of anti-water tree masterbatch, 2.5 parts of dicumyl peroxide, and 0.3 parts of antioxidant 1010, and extrude the mixture onto the outer surface of the inner shielding layer to obtain a cross-linked polyethylene insulation layer.

[0034] Example 2 A highly flame-retardant insulated medium-voltage cable, comprising, from the inside to the outside, a copper conductor, an inner shielding layer, a cross-linked polyethylene insulation layer, an insulating shielding layer, a copper tape shielding layer, a fire-resistant wrapping layer, a steel tape armoring layer, and a sheathing layer; The cross-linked polyethylene insulation layer is prepared by the following method: S1. 100 parts of polyethylene, 5 parts of poly(butadiene-vinyl pyridine), 2 parts of poly(vinyl phosphonic acid) and 25 parts of porous silica are blended and then extruded and granulated to obtain an anti-water tree masterbatch; S2. Evenly mix 100 parts of polyethylene, 15 parts of anti-water tree masterbatch, 1.5 parts of benzoyl peroxide, 0.5 parts of antioxidant 168, and 2 parts of polyethylene wax, and extrude the mixture onto the outer surface of the inner shielding layer to obtain a cross-linked polyethylene insulation layer.

[0035] Example 3 A highly flame-retardant insulated medium-voltage cable, comprising, from the inside to the outside, a copper conductor, an inner shielding layer, a cross-linked polyethylene insulation layer, an insulating shielding layer, a copper tape shielding layer, a fire-resistant wrapping layer, a steel tape armoring layer, and a sheathing layer; The cross-linked polyethylene insulation layer is prepared by the following method: S1, after blending 100 parts of polyethylene, 3 parts of poly(vinylphosphonic acid) and 20 parts of porous silica, extrusion and granulation were performed to obtain an anti-water tree masterbatch; S2. Evenly mix 100 parts of polyethylene, 20 parts of anti-water tree masterbatch, 2 parts of dicumyl peroxide, 0.2 parts of antioxidant 300, and 1 part of polyethylene wax, and extrude the mixture onto the outer surface of the inner shielding layer to obtain a cross-linked polyethylene insulation layer.

[0036] Example 4 The only difference from Example 1 is that the porous silica is replaced by an equal amount of high molecular weight polymer-coated inorganic porous granules, and the high molecular weight polymer-coated inorganic porous granules are prepared by the following method: 5 parts of poly(butadiene-vinylpyridine) were dissolved in 100 parts of water, 60 parts of porous silica were added, and ultrasonic treatment was performed at a power of 300 W and a frequency of 40 kHz for 1.5 hours, followed by drying to obtain high molecular polymer-coated inorganic porous granules.

[0037] Example 5 The only difference from Example 1 is that the porous silica is replaced by an equal amount of high molecular weight polymer-coated inorganic porous granules, and the high molecular weight polymer-coated inorganic porous granules are prepared by the following method: A1. Dissolve 2.5 parts of poly(vinylphosphonic acid) in 100 parts of water, add 60 parts of porous silica, and ultrasonicate at a power of 300 W and a frequency of 50 kHz for 1 hour, and dry to obtain pretreated inorganic porous granules; A2. Dissolve 3.5 parts of poly(butadiene-vinylpyridine) in 100 parts of dichloromethane, add the above-mentioned pretreated inorganic porous granules, and ultrasonicate at a power of 300 W and a frequency of 50 kHz for 1 hour, and dry to obtain high molecular polymer-coated inorganic porous granules.

[0038] Example 6 The only difference from Example 1 is that the porous silica is replaced by an equal amount of high molecular weight polymer-coated inorganic porous granules, and the high molecular weight polymer-coated inorganic porous granules are prepared by the following method: A1. Dissolve 3.5 parts of poly(butadiene-vinyl pyridine) in 100 parts of dichloromethane, add 60 parts of porous silica, and ultrasonicate at a power of 300 W and a frequency of 50 kHz for 1 hour, and dry to obtain pretreated inorganic porous pellets; A2. Dissolve 2.5 parts of poly(vinylphosphonic acid) in 100 parts of water, add the above-mentioned pretreated inorganic porous granules, and ultrasonicate at a power of 300 W and a frequency of 50 kHz for 1 hour, and dry to obtain high molecular polymer-coated inorganic porous granules.

[0039] Example 7 The only difference from Example 1 is that the porous silica is replaced by an equal amount of high molecular weight polymer-coated inorganic porous granules, and the high molecular weight polymer-coated inorganic porous granules are prepared by the following method: A1. Dissolve 2.5 parts of poly(vinylphosphonic acid) in 100 parts of water, add 60 parts of porous silica, and ultrasonicate at a power of 300 W and a frequency of 50 kHz for 1 hour, and dry to obtain pretreated inorganic porous granules; A2. Dissolve 3.5 parts of poly(vinylphosphonic acid) in 100 parts of water, add the above-mentioned pretreated inorganic porous granules, and ultrasonicate at a power of 300 W and a frequency of 50 kHz for 1 hour, and dry to obtain high molecular polymer-coated inorganic porous granules.

[0040] Example 8 The only difference from Example 1 is that the porous silica is replaced by an equal amount of high molecular weight polymer-coated inorganic porous granules, and the high molecular weight polymer-coated inorganic porous granules are prepared by the following method: A1. Dissolve 2.5 parts of poly(butadiene-vinyl pyridine) in 100 parts of dichloromethane, add 60 parts of porous silica, and ultrasonicate at a power of 300 W and a frequency of 50 kHz for 1 hour, and dry to obtain pretreated inorganic porous pellets; A2. Dissolve 3.5 parts of poly(butadiene-vinylpyridine) in 100 parts of dichloromethane, add the above-mentioned pretreated inorganic porous granules, and ultrasonicate at a power of 300 W and a frequency of 50 kHz for 1 hour, and dry to obtain high molecular polymer-coated inorganic porous granules.

[0041] Comparative Example 1 A highly flame-retardant insulated medium-voltage cable, comprising, from the inside to the outside, a copper conductor, an inner shielding layer, a cross-linked polyethylene insulation layer, an insulating shielding layer, a copper tape shielding layer, a fire-resistant wrapping layer, a steel tape armoring layer, and a sheathing layer; The cross-linked polyethylene insulation layer is prepared by the following method: S1. Blend 100 parts of polyethylene and 15 parts of porous silica, and then extrude and granulate to obtain an anti-water tree masterbatch; S2. Evenly mix 100 parts of polyethylene, 25 parts of anti-water tree masterbatch, 2.5 parts of dicumyl peroxide, and 1 part of antioxidant 1010, and extrude the mixture onto the outer surface of the inner shielding layer to obtain a cross-linked polyethylene insulation layer.

[0042] Performance testing: (1) Water tree resistance: Referring to the method in ASTM D6097-01a "Standard Test Method for Relative Tolerance of Emissive Water Tree Growth of Solid Insulating Materials", the water tree resistance of the cross-linked polyethylene insulation layer is expressed by the water tree growth resistance RWTG: RWTG=L / WTL; Where: L is the distance between the needle and plate electrodes (L = 3.2 mm), WTL is the maximum length of the water tree; (2) Flame retardancy: The oxygen index of the cross-linked polyethylene insulation layer was tested according to the method in GB / T 2406.2-2009. The sample size was: length 100 mm × width 10 mm × thickness 4 mm. Ignition method: Method A top surface ignition method. The test results are recorded in Table 1, where --- indicates that no test was performed.

[0043] Table 1 Test results of water tree resistance and flame retardancy of cross-linked polyethylene insulation layer

[0044] Table 1 shows that the water tree growth resistance of the cross-linked polyethylene insulation layers obtained in Examples 1-8 is higher than that of Comparative Example 1, demonstrating that the addition of poly(butadiene-vinyl pyridine) and / or poly(vinyl phosphonic acid) to the anti-water tree masterbatch can suppress water tree formation in cross-linked polyethylene insulated medium-voltage cables. Furthermore, the oxygen index of the cross-linked polyethylene insulation layer is above 27.1, demonstrating high flame retardancy and providing excellent flame retardancy for the insulated medium-voltage cable.

[0045] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A highly flame-retardant insulated medium-voltage cable, characterized in that: From the inside to the outside, it includes a conductor, an inner shielding layer, an insulating layer, an outer shielding layer, a fire-resistant wrapping layer, an armor layer and a sheath layer. The conductor is a copper conductor, the insulating layer is a cross-linked polyethylene insulating layer, the outer shielding layer includes an insulating shielding layer and a copper tape shielding layer arranged on the outside of the insulating shielding layer, and the armor layer is a steel tape armor layer. The raw materials of the cross-linked polyethylene insulating layer include polyethylene, anti-water tree masterbatch, a cross-linking agent and a processing aid. The raw materials of the anti-water tree masterbatch include polyethylene, a copolymer and an inorganic porous granular material. The copolymer is poly(butadiene-vinyl pyridine) and / or poly(vinyl phosphonic acid).

2. A highly flame-retardant insulated medium voltage cable according to claim 1, characterized in that: The inorganic porous granules are high molecular polymer coated inorganic porous granules, and the high molecular polymer in the high molecular polymer coated inorganic porous granules is poly(butadiene-vinyl pyridine) and / or poly(vinyl phosphonic acid).

3. A highly flame-retardant insulated medium voltage cable according to claim 2, characterized in that: The method for preparing the high molecular polymer coated inorganic porous granules comprises the following steps: dissolving the high molecular polymer, adding the inorganic porous granules, mixing evenly, and drying to obtain the high molecular polymer coated inorganic porous granules.

4. A highly flame-retardant insulated medium voltage cable according to claim 2, characterized in that: The high molecular polymer coated inorganic porous granules are first coated with poly(vinylphosphonic acid) and then coated with poly(butadiene-vinylpyridine).

5. A highly flame-retardant insulated medium voltage cable according to claim 4, characterized in that: The method for preparing the high molecular polymer coated inorganic porous granules comprises the following steps: S1, dissolving poly(vinylphosphonic acid) in a solvent, adding the inorganic porous granules, mixing evenly, and drying to obtain pretreated inorganic porous granules; S2. Dissolve poly (butadiene-vinyl pyridine) in a solvent, add the pretreated inorganic porous granules, mix evenly, and dry to obtain high molecular polymer coated inorganic porous granules.

6. A highly flame-retardant insulated medium voltage cable according to claim 5, characterized in that: In step S1, the solvent is water; in step S2, the solvent is dichloromethane; and the mixing in step S1 and step S2 is performed independently by ultrasonication.

7. The highly flame-retardant insulated medium voltage cable according to claim 2, characterized in that: The raw materials of the high molecular polymer-coated inorganic porous granules include the high molecular polymer and the inorganic porous granules in a mass ratio of 2.5 to 6:

60.

8. The highly flame-retardant insulated medium voltage cable according to claim 1, characterized in that: The mass ratio of polyethylene, copolymer and inorganic porous granules in the raw materials of the anti-water tree masterbatch is 100:3~7:15~25; The mass ratio of polyethylene, anti-water tree masterbatch, crosslinking agent and processing aid in the raw materials of the cross-linked polyethylene insulation layer is 100:15-25:1.5-2.5:0.3-10.

9. The highly flame-retardant insulated medium voltage cable according to claim 1, characterized in that: The processing aids include one or more of antioxidants, cross-linking aids, and lubricants.

10. The highly flame-retardant insulated medium voltage cable according to claim 1, characterized in that: The cross-linking agent is peroxide.

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