220KV polypropylene insulation direct current cable

Abstract

The invention relates to the technical field of electric wires and cables, and discloses a 220KV polypropylene insulated direct current cable which is formed by compounding polypropylene matrix resin, a maleic anhydride grafted propylene-alpha-olefin copolymer, core-shell structure semiconductor nanoparticles with amino silane functionalized surfaces and an aromatic ketone charge transport regulator, wherein the nanoparticles are anchored to the copolymer through covalent bonds to construct a multi-scale space charge regulation and control system. By adopting the technical scheme, excellent flexibility, low-temperature brittleness resistance and mechanical strength can be realized, space charge accumulation can be effectively inhibited, the electric field distribution uniformity and stability can be improved, and the reliability of an insulation system can be ensured.

Description

Technical Field

[0001] This invention belongs to the field of wire and cable technology, and relates to a 220KV polypropylene insulated DC cable. Background Technology

[0002] High-voltage direct current (HVDC) transmission technology, as a crucial component of modern power transmission, is increasingly widely used globally due to its unique advantages in high-capacity, long-distance transmission, asynchronous grid connection, and renewable energy integration. Among these technologies, DC cables, as key equipment in HVDC systems, have their core performance determined by their insulation materials. While traditional cross-linked polyethylene (XLPE) materials are mature in AC cables, their inherent space charge accumulation and dielectric loss characteristics pose significant challenges in HVDC cable applications under high-voltage conditions.

[0003] In comparison, polypropylene, due to its excellent electrical properties, especially its low dielectric constant and loss tangent, as well as its good thermal properties and environmental friendliness, is considered an ideal alternative to DC cable insulation materials for voltage levels of 220KV and above, and has broad application prospects.

[0004] However, pure polypropylene has significant limitations in practical applications, mainly manifested in its high rigidity, insufficient toughness, and prominent low-temperature brittleness. This makes it prone to cracking and damage during cable manufacturing, laying, and long-term operation, especially under complex terrain and extreme climatic conditions, where its reliability fails to meet stringent requirements. To overcome these shortcomings, those skilled in the art have actively explored ways to improve the overall performance of polypropylene through material modification.

[0005] Existing technological approaches primarily focus on multiphase composite design, aiming to improve mechanical properties while maintaining or even optimizing electrical properties. A commonly adopted strategy is elastomer blending, such as adding an appropriate amount of EPDM rubber to a polypropylene matrix. Studies have shown that this significantly improves the flexibility of polypropylene, increasing it by approximately 40%, while simultaneously enhancing the material's resistance to torsion, effectively alleviating the problem of excessive rigidity in pure polypropylene.

[0006] To further optimize the macroscopic properties of the blend system, a stepped cooling process is typically employed to improve the dispersion of EPDM rubber in the polypropylene matrix, thereby obtaining a more uniform composite structure. Building upon this, nanofiller synergistic technology has also been introduced, such as combining nano-magnesium oxide with EPDM rubber in polypropylene systems. This synergistic effect, while maintaining or even optimizing dielectric properties, can increase the flexural modulus of the material by approximately 25%, while simultaneously reducing the low-temperature embrittlement temperature to -40°C, significantly broadening the material's application range.

[0007] In addition, structural design optimization has become an important auxiliary means to further reduce the risks caused by mechanical stress concentration during cable laying and operation. For example, a three-layer co-extrusion process (including conductor shielding layer, insulation layer and insulation shielding layer) is adopted, and the modulus matching of each layer of materials is finely adjusted in order to effectively disperse stress during cable bending, stretching and other processes, and avoid the generation of local high stress areas.

[0008] However, as high-voltage direct current (HVDC) transmission technology develops towards higher voltage levels and longer service life, and as cable application environments become increasingly complex, some inherent characteristics of the aforementioned multiphase composite design at the principle level have gradually revealed deep contradictions and limitations in addressing new challenges, especially in application scenarios such as 220kV DC cables where insulation reliability requirements are extremely high. Although existing technologies have made significant progress in macroscopic mechanical properties through elastomer blending and nanofiller synergy, and claim to maintain dielectric properties to a certain extent, this maintenance is often limited to conventional dielectric parameters such as dielectric constant and dielectric loss, and has not completely solved, and may even exacerbate, the most critical challenge of HVDC insulation materials—the accumulation and control of space charge. The reason for this is that, regardless of how well the dispersion of the introduced EPDM elastomer and nano-magnesium oxide filler is optimized, a large number of microscopic heterogeneous interfaces inevitably form between them and the polypropylene matrix. Due to the differences in their local physicochemical properties (such as defect states, electron affinity, molecular chain packing density, etc.) compared to the bulk polypropylene, these interfaces are highly susceptible to becoming active centers for charge injection, capture, and release under long-term high DC electric fields.

[0009] While blends of EPDM and polypropylene improve flexibility, at the molecular level, traps of varying depths can form between the amorphous regions of EPDM and the crystalline regions of polypropylene, as well as at their phase boundaries, leading to a significant accumulation of space charge in these regions. The introduction of nano-magnesium oxide, while improving mechanical properties and low-temperature brittleness, introduces numerous nano-polymer interfaces due to the large specific surface area and surface defects of the nanoparticles when combined with the polymer matrix. These nano-interfaces have been shown to possess highly complex charge-trapping properties, potentially providing shallow traps to facilitate charge migration or forming deep traps to capture space charge for extended periods.

[0010] Under high-voltage direct current (HVDC) electric fields, especially in complex operating conditions such as temperature gradients, electric field reversals, or long-term operation, the space charge accumulated at heterogeneous interfaces can significantly distort the electric field distribution inside the cable. This can lead to local electric field strengths far exceeding design values, and may even trigger partial discharge or accelerate material aging, thereby severely weakening the long-term dielectric breakdown strength and service life of the material. This electric field distortion problem caused by space charge accumulation is one of the core risks of DC cable insulation failure, and it is difficult to fundamentally solve through simple structural optimization methods such as modulus matching, because it is the result of the interaction between the material's internal microstructure and the electric field.

[0011] Therefore, how to fundamentally suppress the accumulation of space charge at the heterogeneous interface introduced by the multiphase composite system while ensuring the excellent mechanical properties and low-temperature toughness of polypropylene, and improve the electric field stability and insulation reliability of the material under long-term high-voltage DC operation conditions, has become a key challenge and a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0012] This invention provides a 220KV polypropylene insulated DC cable with excellent flexibility, low-temperature brittleness resistance and mechanical strength.

[0013] To achieve the above-mentioned objectives, this invention provides a 220KV polypropylene insulated DC cable, wherein the cable is composed of the following components in parts by weight through a specific preparation process: 100 parts of polypropylene matrix resin; 5-20 parts of maleic anhydride-grafted propylene-α-olefin copolymer; 0.5-5 parts of core-shell structured semiconductor nanoparticles with aminosilane functionalized surfaces; and 0.01-0.5 parts of aromatic ketone charge transport modifier.

[0014] Furthermore, the polypropylene matrix resin is isotactic polypropylene with an isotacticity of not less than 97%, and its melt index is 1.5~3.0 g / 10min under conditions of 230℃ and 2.16 kg load. The polypropylene matrix resin, as the main body of the insulating material, constitutes the continuous phase of the material and provides basic electrical insulation and thermomechanical properties.

[0015] Furthermore, the maleic anhydride-grafted propylene-α-olefin copolymer serves to simultaneously toughen and modify the polypropylene matrix and provide chemical reaction sites for the interfacial anchoring of subsequent functional nanoparticles. Specifically, the propylene-α-olefin copolymer backbone in the copolymer is composed of propylene units and α-olefin units, wherein the α-olefin is 1-octene, and its molar content in the copolymer is 10%~25%.

[0016] The maleic anhydride functional groups grafted onto the main chain of the copolymer have a grafting rate of 0.5% to 2.0% by weight. The introduction of the maleic anhydride-grafted propylene-α-olefin copolymer forms a micro-dispersed phase in the polypropylene matrix through its flexible segments, effectively absorbing and dissipating external impact energy, thereby significantly improving the flexibility and impact resistance of the material. At the same time, the polar maleic anhydride groups it contains provide the necessary reactive sites for subsequent chemical bonding with surface-functionalized nanoparticles.

[0017] As a core technical feature of this invention, the core-shell structured semiconductor nanoparticles with aminosilane-functionalized surfaces constitute a localized space charge discharge network within the material. Specifically, the nanoparticles have a precise core-shell structure. The core is zinc oxide nanocrystals with a wurtzite structure and an average particle size of 20-50 nm. The zinc oxide core exhibits nonlinear conductivity, meaning its conductivity increases rapidly and nonlinearly with increasing applied electric field strength. The shell is an amorphous silica insulating layer uniformly coating the surface of the zinc oxide core, with a thickness of 2-5 nm.

[0018] Under normal operating electric field strength, the insulating shell effectively prevents carrier tunneling between adjacent zinc oxide cores by utilizing its wide bandgap characteristics, thereby maintaining the high bulk resistivity of the entire material. The surface functionalization of the nanoparticles refers to the chemical bonding of γ-aminopropyltriethoxysilane molecules to the outer surface of the silica shell. The γ-aminopropyltriethoxysilane molecules undergo hydrolytic condensation with the hydroxyl groups on the silica surface through their triethoxysilane ends, forming stable Si-O-Si covalent bonds; the amino functional group (-NH2) at the other end is exposed, allowing it to undergo ring-opening amidation with the maleic anhydride groups in the maleic anhydride-grafted propylene-α-olefin copolymer, forming stable amide bonds.

[0019] Through this design, the core-shell structured nanoparticles are firmly anchored to the interface of the elastomeric phase via covalent bonds, constructing a structurally stable toughened body-charge control center composite structure with extremely low interface defect density.

[0020] Furthermore, the aromatic ketone charge transport modifier is benzophenone with a chemical purity of not less than 99.5%. The benzophenone molecules are uniformly dispersed in the amorphous region of the polypropylene matrix in small molecule form. Its function is to introduce shallow trap levels into the polymer's band gap by utilizing the conjugated π-electron system present in its molecular structure. These shallow traps can capture injected charge carriers, but can also release them relatively quickly under the action of an electric field and thermal agitation, i.e., the so-called capture-detrapping mechanism. This mechanism promotes the mobility of charge carriers throughout the insulator, preventing the formation of high-density space charge packets in certain regions (such as near electrodes), thereby achieving a homogenization of the volume charge distribution on a macroscopic scale and synergistically suppressing local distortions of the electric field.

[0021] This invention also provides a method for preparing the aforementioned 220KV polypropylene insulated DC cable, which includes the following continuous and controlled steps aimed at constructing a pre-defined, hierarchical microscopic functional structure within the cable: The first step involved the preparation and surface functionalization of core-shell semiconductor nanoparticles. Firstly, silica-coated zinc oxide core-shell nanoparticles were prepared using a sol-gel method. Specifically, zinc oxide nanocrystals with an average particle size of 20–50 nm were dispersed in a mixed solvent of ethanol and deionized water, with a certain amount of ammonia added as a catalyst. The mixture was ultrasonically dispersed for 30 min to form a homogeneous suspension. Subsequently, an ethanol solution of tetraethyl orthosilicate was added dropwise at a constant rate under continuous mechanical stirring, and the reaction was carried out in a 40°C water bath for 12 h. After the reaction, the silica-coated zinc oxide core-shell nanoparticles were obtained by centrifugation, washing, and drying. Transmission electron microscopy confirmed that the silica shell thickness reached 2–5 nm.

[0022] Subsequently, surface aminosilane functionalization was performed. The obtained silicon oxide-coated zinc oxide nanoparticles were placed in anhydrous toluene and ultrasonically dispersed. Then, γ-aminopropyltriethoxysilane was added at a stoichiometric ratio of 1.5:1 (silane: theoretical hydroxyl number of particle surface area). Under a nitrogen atmosphere, the mixture was refluxed at 110°C for 24 hours. After the reaction, the nanoparticles were centrifuged and washed several times with toluene to remove unreacted silane. Finally, they were dried in a vacuum oven at 80°C for 12 hours to obtain the core-shell semiconductor nanoparticles with aminosilane functionalized surfaces.

[0023] The second step is the preparation of the interfacial functional synergistic masterbatch. This step aims to achieve covalent bonding between the functional nanoparticles and the toughened elastomer, forming a uniformly dispersed masterbatch. The surface-functionalized nanoparticles obtained in the first step and the maleic anhydride-grafted propylene-α-olefin copolymer are added together in a co-rotating twin-screw extruder for reactive melt blending according to a preset final formulation ratio. The barrel temperature of the twin-screw extruder is set in segments from the feed port to the die exit as follows: 160℃, 180℃, 200℃, 210℃, 210℃, 205℃.

[0024] The screw speed was set to 300 r / min. Under this high-temperature molten state, the amino groups on the surface of the nanoparticles and the maleic anhydride groups on the copolymer underwent a complete amidation reaction. The extrudate was water-cooled and pelletized to obtain the interfacial functional synergistic masterbatch. In this masterbatch, the core-shell nanoparticles were covalently anchored in the elastomer matrix.

[0025] The third step is the final composite preparation of the insulation material. 100 parts of polypropylene matrix resin, the equivalent amount of the interfacial functional synergistic masterbatch calculated according to the final formula, and 0.01-0.5 parts of benzophenone are precisely fed into a high-shear twin-screw extruder via a loss-in-weight feeder for melt blending. The barrel temperature of the extruder is set in segments: 180℃, 195℃, 210℃, 220℃, 220℃, and 215℃. The screw speed is set to 400 r / min to ensure that all components achieve uniform dispersion at the molecular or nanoscale level within the polypropylene matrix. The extrudate is cooled and pelletized to obtain the 220KV polypropylene insulated DC cable granules described in this invention.

[0026] The inherent mechanism of action and non-obvious beneficial effects of the technical solution described in this invention are as follows: Firstly, through covalent bonding between maleic anhydride graft copolymers and aminosilane functionalized nanoparticles, a chemically stable and seamlessly transitioning interfacial functional layer is formed between the polypropylene matrix and the toughening phase. This functional layer fundamentally eliminates the numerous micropores and physical defects caused by poor compatibility in traditional blend systems. These defects are precisely the main sites leading to deep trap formation and stable space charge capture.

[0027] Secondly, the localized nonlinear conductivity network composed of core-shell semiconductor nanoparticles exhibits a passive intelligent response characteristic. Under the average electric field strength during normal cable operation, the high potential barrier of the silicon dioxide insulating shell effectively suppresses charge transport, and the material behaves as an excellent insulator. However, when excessive accumulation of space charge in local areas occurs due to factors such as electrode injection and temperature gradients, causing the instantaneous electric field strength in that area to exceed a certain threshold electric field, the probability of charge carriers tunneling through the silicon dioxide thin layer increases sharply, allowing them to enter the zinc oxide core with higher conductivity.

[0028] Through tunneling between adjacent nanoparticles, a temporary, micrometer-scale charge discharge path is formed, rapidly neutralizing or dispersing excess charge in the region, thereby restoring the local electric field strength to a safe level. This process is analogous to implanting billions of miniature surge arresters or Zener diodes inside the insulator, which can automatically and passively smooth out peaks and valleys and reduce electric field distortion without affecting the overall insulation performance of the material under normal operating conditions.

[0029] Third, the uniformly dispersed benzophenone molecules act as an active charge transport modifier, complementing the aforementioned passive response network. The shallow trap network provided by benzophenone, distributed throughout the matrix, begins to function in the early stages of space charge accumulation. By enhancing the effective mobility of charge carriers, it promotes the uniform redistribution of bulk charge throughout the insulating layer, macroscopically preventing the formation of large-scale charge packets, reducing the pressure on the localized nonlinear conductivity network, and further improving the stability of the entire system.

[0030] In summary, this invention systematically integrates and designs toughening modification, interface structure optimization, and space charge management strategies based on nonlinear conductivity and carrier transport regulation. The resulting 220KV polypropylene insulated DC cable maintains the inherent advantages of polypropylene, such as low dielectric loss and high thermal performance, while solving the problems of insufficient mechanical toughness and low-temperature brittleness. This promotes the electric field stability of the cable insulation under long-term and complex operating conditions, thereby improving its operational reliability and service life. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0032] The present invention provides a manufacturing process for a 220KV polypropylene insulated DC cable, as follows: I. Preparation and Surface Functionalization of Core-Shell Semiconductor Nanoparticles Core-shell structured nanoparticles were prepared using a sol-gel method. 2.0 g of zinc oxide nanocrystals with an average particle size of 40 nm were dispersed in 200 mL of an ethanol-water mixture (ethanol to water volume ratio of 4:1). 2.0 mL of ammonia (25 wt%) was added as a catalyst, and the mixture was ultrasonically dispersed for 30 min to form a homogeneous suspension. Subsequently, under continuous mechanical stirring, an ethanol solution of tetraethyl orthosilicate (5.0 mL of tetraethyl orthosilicate dissolved in 20 mL of anhydrous ethanol) was added dropwise at a constant rate of 0.5 mL / min, and the reaction was carried out in a 40 °C water bath for 12 h. After the reaction was completed, the nanoparticles were obtained by centrifugation, washing, and drying. Surface amino functionalization was performed on the particles: 2.0 g of the obtained particles were dispersed in 500 mL of anhydrous toluene, with a mass-volume ratio of particles to toluene of 1 g: 250 mL. An excess of 6.0 mL of γ-aminopropyltriethoxysilane was added, and the mixture was refluxed at 110 °C under nitrogen protection for 24 h to allow silane molecules to be firmly grafted onto the surface of the SiO2 shell through Si-O-Si covalent bonds. Finally, after centrifugation, washing, and vacuum drying, functionalized nanoparticles rich in amino groups were obtained. II. Preparation of Interfacial Functional Synergistic Masterbatch The aforementioned functionalized nanoparticles were combined with a maleic anhydride-grafted propylene-octene copolymer (ExxonMobil's Vistamaxx™ 6202) at a maleic anhydride grafting rate of 1.0%, and fed into a co-rotating twin-screw extruder for reactive melt blending according to the final formulation ratio. The extruder temperature was set at 160°C, 180°C, 200°C, 210°C, 210°C, and 205°C from the feed port to the die head, with a screw speed of 300 rpm. Under high-temperature shear, the amino groups on the nanoparticle surface and the maleic anhydride groups on the copolymer underwent a ring-opening amidation reaction, forming stable amide bonds, thereby covalently anchoring the nanoparticles to the elastomer phase. The extrudate was water-cooled and pelletized to obtain a masterbatch with uniformly dispersed nanoparticles and interfacial chemical bonding, exhibiting synergistic interfacial functionalities.

[0033] III. Composite Blending and Granulation of the Final Insulating Material One hundred parts of high isotactic polypropylene matrix resin (Borealis' Bormed™ HD945CF), along with a pre-calculated proportion of interfacial functional synergistic masterbatch, toughening copolymer, nanoparticles, and benzophenone charge modifier, were precisely fed into another high-shear twin-screw extruder via a loss-in-weight feeder. The extruder temperatures were set to 180°C, 195°C, 210°C, 220°C, 220°C, and 215°C, and the screw speed was increased to 400 rpm to ensure that all components, including the small benzophenone molecules, achieved uniform nanoscale dispersion within the polypropylene matrix. After melt blending, the material was extruded, cooled, and pelletized to obtain the final cable insulation material granules.

[0034] Based on the technical solution described in the patent, six sets of examples were designed by selecting key component variables: the amount of maleic anhydride-grafted propylene-α-olefin copolymer, the amount of core-shell nanoparticles, and the amount of benzophenone, covering the weight range of each component. The specific formulations are as follows:

[0035] Polypropylene matrix resin: isotacticity 97%, melt index 2.0 g / 10 min (230℃ / 2.16 kg); Maleic anhydride-grafted propylene-α-olefin copolymer: 18% 1-octene molar content, 1.2% maleic anhydride grafting rate; Core-shell nanoparticles: zinc oxide core with a particle size of 30 nm, silica shell with a thickness of 3 nm, and surface functionalized with aminosilane; Benzophenone: 99.5% purity.

[0036] Two comparative examples were selected to compare the shortcomings of existing technologies and the advantages of this invention:

[0037] Key performance tests were conducted on the above embodiments and comparative examples, and the results are as follows:

[0038] Data comparison and analysis: Flexibility and impact resistance: Elongation at break (350%~510%) and impact strength (20~32kJ / m) of Examples 1-6 2 Both were significantly better than Comparative Example 1 (pure polypropylene, 150% / 8kJ / m³). 2 This indicates that the introduction of maleic anhydride graft copolymer effectively solved the problem of excessive rigidity in polypropylene. Among them, Example 2 (20 parts of maleic anhydride graft copolymer) showed the best toughness, verifying the positive effect of increasing the amount of toughening component on mechanical properties.

[0039] Low-temperature embrittlement resistance: The low-temperature embrittlement temperature (-50~-60℃) of the embodiment is much lower than that of Comparative Example 1 (-20℃) and Comparative Example 2 (-45℃), indicating that the present invention significantly improves the crack resistance of the material at extreme low temperatures through molecular design, meeting the laying requirements of complex environments.

[0040] Space charge suppression: The space charge density of the embodiment (2.0~5.5C / m) 3 This is only for comparative example 1 (15.8C / m) 3 13%~35% of ), Comparative Example 2 (10.2C / m 3 The core reason for the 20%~54% of the charge density is that the nonlinear conductivity network formed by the core-shell nanoparticles can passively dissipate local excess charge (as shown in Example 4, when the amount of nanoparticles is 5 parts, the charge density is the lowest); the shallow trap network of benzophenone promotes uniform charge distribution (the effect is significant when benzophenone is 0.5 parts in Example 6).

[0041] Breakdown field strength: The DC breakdown field strength of the examples (68~80kV / mm) is significantly higher than that of Comparative Example 1 (55kV / mm) and Comparative Example 2 (62kV / mm). In particular, the breakdown field strength of Example 4 (5 parts of nanoparticles) reaches 80kV / mm, which verifies the effect of space charge regulation on improving insulation reliability.

[0042] Dielectric loss: The dielectric loss of the examples (0.0022~0.0030) is close to that of pure polypropylene and lower than that of Comparative Example 2 (0.0035), indicating that the present invention optimizes performance without sacrificing the inherent low loss advantage of polypropylene.

[0043] Maleic anhydride graft copolymer: It not only toughens through flexible segments, but also anchors nanoparticles to the interface through amide bonds, eliminating the interface defects of traditional blends (comparing Example 3 and Comparative Example 2, Example 3 has a lower space charge density under the same amount of toughening agent).

[0044] Core-shell nanoparticles: The silica shell ensures high insulation (volume resistivity ≥10¹ under normal electric field). 6 Ω The nonlinear conductivity of the zinc oxide core triggers charge discharge under local high electric fields (Example 4 has the highest amount of nanoparticles and the best breakdown field strength).

[0045] Benzyl ketone: It actively regulates charge migration through a shallow trap-detrapping mechanism, forming a synergy with the passive release of nanoparticles (Example 6 has the highest amount of benzoyl ketone and a lower space charge density than Example 1).

[0046] This invention improves the flexibility of polypropylene (elongation at break increases by 133%~240%) and low-temperature brittleness resistance (embrittlement temperature decreases by 25~40℃), while solving the problem of space charge accumulation in traditional composite systems (charge density decreases by 56%-87%). Ultimately, it achieves a 24%~45% increase in DC breakdown field strength, fully meeting the stringent insulation requirements of 220KV high-voltage DC cables.

[0047] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A 220KV polypropylene insulated DC cable, characterized in that, Includes the following components in parts by weight: 100 parts of polypropylene matrix resin; 5-20 parts of maleic anhydride-grafted propylene-α-olefin copolymer; 0.5 to 5 parts of core-shell semiconductor nanoparticles with aminosilane-functionalized surfaces; as well as 0.01 to 0.5 parts of aromatic ketone charge transport modifiers.

2. The 220KV polypropylene insulated DC cable according to claim 1, characterized in that, The polypropylene matrix resin is isotactic polypropylene with an isotacticity of not less than 97%, and its melt index is 1.5~3.0 g / 10min under the conditions of 230℃ and 2.16 kg load.

3. A 220KV polypropylene insulated DC cable according to claim 1, characterized in that, The propylene-α-olefin copolymer backbone in the maleic anhydride-grafted propylene-α-olefin copolymer is composed of propylene units and α-olefin units, wherein the α-olefin is 1-octene, and the molar content of the α-olefin in the copolymer is 10%~25%.

4. A 220KV polypropylene insulated DC cable according to claim 3, characterized in that, The maleic anhydride functional groups grafted onto the main chain of the copolymer have a grafting rate of 0.5% to 2.0% by weight.

5. A 220KV polypropylene insulated DC cable according to claim 1, characterized in that, The core of the core-shell semiconductor nanoparticles with aminosilane functionalization on the surface is zinc oxide nanocrystals with a wurtzite structure and an average particle size of 20~50nm.

6. A 220KV polypropylene insulated DC cable according to claim 5, characterized in that, The shell of the core-shell structured semiconductor nanoparticle is an amorphous silicon dioxide insulating layer uniformly coated on the surface of the zinc oxide core, and the thickness of the silicon dioxide shell layer is 2~5nm.

7. A 220KV polypropylene insulated DC cable according to claim 6, characterized in that, The surface functionalization of the nanoparticles refers to the chemical bonding of γ-aminopropyltriethoxysilane molecules to the outer surface of the silica shell.

8. A 220KV polypropylene insulated DC cable according to claim 7, characterized in that, The γ-aminopropyltriethoxysilane molecule undergoes a hydrolytic condensation reaction with the hydroxyl groups on the surface of the silica via its triethoxysilane end, forming a stable Si-O-Si covalent bond; the amino functional group at its other end is used to undergo a ring-opening amidation reaction with the maleic anhydride groups in the maleic anhydride-grafted propylene-α-olefin copolymer, forming a stable amide bond.

9. A method for preparing a 220kV polypropylene insulated DC cable, used to manufacture the 220kV polypropylene insulated DC cable according to any one of claims 1 to 8, characterized in that, Includes the following steps: The first step is the preparation and surface functionalization of core-shell structured semiconductor nanoparticles; The second step is the preparation of the interface functional synergistic masterbatch; and The third step is the final composite preparation of the insulating material.

10. The method for preparing a 220KV polypropylene insulated DC cable according to claim 9, characterized in that, The preparation and surface functionalization of the core-shell structured semiconductor nanoparticles in the first step specifically includes: Core-shell nanoparticles of zinc oxide coated with silica were prepared by a sol-gel method. Zinc oxide nanocrystals were dispersed in a mixed solvent of ethanol and deionized water, with a certain amount of ammonia added as a catalyst. The mixture was ultrasonically dispersed for 30 min to form a uniform suspension. Subsequently, an ethanol solution of tetraethyl orthosilicate was added dropwise at a constant rate of 0.5 mL / min under continuous mechanical stirring, and the reaction was carried out in a 40℃ water bath for 12 h. After the reaction, the core-shell nanoparticles of zinc oxide coated with silica were obtained by centrifugation, washing, and drying. Subsequently, surface aminosilane functionalization was performed. The obtained silicon oxide-coated zinc oxide nanoparticles were placed in anhydrous toluene, ultrasonically dispersed, and then γ-aminopropyltriethoxysilane with a stoichiometric ratio of 1.5:1 was added. Under nitrogen atmosphere protection, the mixture was heated to 110°C and refluxed for 24 hours. After the reaction was completed, the nanoparticles were centrifuged and washed several times with toluene to remove unreacted silane. Finally, they were dried in a vacuum oven at 80°C for 12 hours to obtain the core-shell structured semiconductor nanoparticles with aminosilane functionalization.

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