A hybrid AC / DC overlay DC cable and its manufacturing method

Abstract

This invention discloses a hybrid AC / DC superimposed DC cable and its manufacturing method, comprising a cable core and a protective layer. The protective layer covers the cable core. The cable core includes a core wire and an insulating sleeve. The core wire includes a conductor and a semi-conductive strip. The insulating sleeve includes a conductor shielding layer, a modified insulation layer, and an insulating shielding layer. The protective layer includes a buffer layer, a metal sheath layer, and an outer sheath layer. The cable adopts a concentric and coaxial layered structure, consisting of, from the inside out: conductor, semi-conductive strip, conductor shielding layer, modified insulation layer, insulating shielding layer, buffer layer, metal sheath layer, and outer sheath layer. This invention addresses the design of superimposed voltages, clarifying the dynamic transition law of dielectric constant and conductivity control, significantly improving the operational stability under composite electric fields, and simultaneously suppressing space charge: the modified material significantly reduces space charge accumulation, slows down insulation degradation, and extends the service life of the cable under complex electric fields.

Description

Technical Field

[0001] This invention relates to the field of cable technology, and in particular to an AC / DC hybrid superimposed DC cable and its preparation method. Background Technology

[0002] With the rapid development of flexible direct current transmission (VSC-HVDC) and new energy grid connection technologies, direct current transmission has become an important direction.

[0003] Traditional DC cables are primarily designed for pure DC electric fields and use cross-linked polyethylene (XLPE) as insulation material. However, in actual operation, due to the converter topology and system operating mode, DC lines often have AC components or ripple voltage superimposed, forming a "mixed AC / DC" operating condition, which may present the following problems:

[0004] Electric field distortion: Under a combined electric field, the electric field distribution pattern within the cable insulation layer changes (from dielectric constant control to conductivity control), easily leading to severe electric field distortion; Space charge accumulation: Under AC / DC superposition conditions, traditional XLPE materials exhibit severe space charge injection and accumulation, resulting in localized stress concentration and accelerated insulation aging; Poor matching: The conductivity of the insulation layer and the semi-conductive shielding layer of existing cables is insufficiently matched under thermo-electric coupling, making them prone to electric field reversal during polarity reversal or thermal cycling, affecting operational reliability. Summary of the Invention

[0005] The purpose of this invention is to provide an AC / DC hybrid superimposed DC cable and its preparation method to overcome the above-mentioned shortcomings in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: an AC / DC hybrid superimposed DC cable, comprising a cable core and a protective layer, wherein the protective layer covers the outside of the cable core;

[0007] The cable core includes a core wire and an insulating sleeve. The core wire includes a conductor and a semi-conductive tape. The insulating sleeve includes a conductor shielding layer, a modified insulation layer, and an insulating shielding layer. The protective layer includes a buffer layer, a metal sheath layer, and an outer sheath layer.

[0008] As a further description of the above technical solution: the cable adopts a concentric and coaxial layered structure, which consists of the following layers from the inside out: conductor, semi-conductive strip, conductor shielding layer, modified insulation layer, insulation shielding layer, buffer layer, metal sheath layer and outer sheath layer.

[0009] As a further description of the above technical solution: the conductor shielding layer, the modified insulation layer and the insulation shielding layer are formed by a three-layer co-extrusion process, and the modified insulation layer covers the conductor shielding layer, and the insulation shielding layer covers the modified insulation layer.

[0010] As a further description of the above technical solution: the modified insulation layer (122) is made of special DC-grade modified cross-linked polyethylene. Through cross-linking structure regulation, nano-doping and deep trap monomer modification to optimize the material trap energy level, the space charge is effectively suppressed in the long-term operating temperature range of 90℃, and the space charge density is reduced by more than 30% compared with conventional AC XLPE.

[0011] As a further description of the above technical solution: the core wire is precisely designed with conductivity and temperature coefficient to form a reasonable conductivity gradient with the modified insulation layer in the full temperature range of 25℃-90℃, which is suitable for thermo-electric coupling and polarity reversal conditions, and avoids electric field reversal and stress concentration.

[0012] A method for preparing an AC / DC hybrid superimposed DC cable, used to prepare the AC / DC hybrid superimposed DC cable according to any one of claims 1-5, the cable preparation method comprising:

[0013] S1: Conductor processing: The conductor raw material is drawn and compressed to obtain a shaped conductor;

[0014] S2: Preparation of the modified insulating layer:

[0015] Ultra-clean polyethylene base material, nano-modifier, deep-trap monomer, and functional additives are added to a high-speed mixer according to the specified ratio and premixed at a constant temperature to obtain a uniform premix.

[0016] The nano-modifiers are: one or two of nano-silica, nano-alumina, nano-magnesium oxide, nano-titanium dioxide, organo-modified montmorillonite, and nano-zinc oxide, which are synergistically doped.

[0017] The deep trap monomer is one of the following: maleic anhydride (MAH), glycidyl acrylate (GMA), methyl methacrylate (MMA), and 4-vinylpyridine (4-VP);

[0018] The premixed material is added to a twin-screw extruder, melt-blended, extruded and granulated to obtain DC-XLPE modified granules. The DC-XLPE modified granules are then cross-linked at high temperature and cooled and shaped to obtain a modified insulation layer.

[0019] S3: A semiconductive strip is wrapped around the outside of the conductor to form the core wire. Then, the conductor shielding layer, the modified insulation layer obtained in S2, and the insulation shielding layer are added to the three-layer co-extrusion production line and co-extruded simultaneously with the conductor shielding and insulation shielding. After high-temperature cross-linking and cooling and shaping, the three-layer co-extrusion process is used to continuously extrude the insulation sleeve in one go to form an insulation sleeve that tightly wraps around the core wire to finally obtain the cable core. The insulation shielding layer uses a DC-specific semiconductive shielding material, which forms a synergistic conductivity gradient with the modified insulation layer in the range of 25℃-90℃ to prevent electric field reversal.

[0020] S4: Buffer layer covering: A buffer layer is extruded or wrapped around the outside of the insulating shielding layer to achieve mechanical buffering and water-blocking protection. The buffer layer is made of semi-conductive buffer water-blocking tape and has the functions of mechanical buffering, radial water blocking and electric field equalization.

[0021] S5: Metal sheath layer forming: The metal sheath layer is formed using a corrugated aluminum sleeve and is treated with anti-corrosion, combining radial water blocking, mechanical protection and electric field shielding functions to form a metal shielding and sheath structure;

[0022] S6: Outer sheath extrusion: The outer sheath and outer electrode are extruded outside the metal sheath to complete the overall encapsulation of the cable. The outer sheath is made of weather-resistant high-density polyethylene (HDPE), which has excellent resistance to environmental stress cracking, UV resistance and long-term outdoor operation stability.

[0023] As a further description of the above technical solution: In step S2, the density of the ultra-clean polyethylene base material is 0.918-0.930 g / cm³, and the melt flow rate is 0.15-0.35 g / 10 min;

[0024] The amount of nano-modifier added is 1.0%-3.0% of the polyethylene base material mass, the amount of deep trap monomer added is 0.3%-1.0%, and the amount of functional additive added is 0.4%-0.8%; the speed of the high-speed mixer is 1200-1500 rpm, the mixing time is 8-15 minutes, and the mixing temperature is 45-65℃.

[0025] As a further description of the above technical solution: In step S2, the temperatures of each zone of the twin-screw extruder are: Zone 1 135-145℃, Zone 2 150-160℃, Zone 3 165-175℃, Zone 4 170-180℃, the die head temperature is 175-185℃, the screw speed is 200-300 rpm, and the modified DC-XLPE granules are obtained after water cooling, air drying and pelletizing.

[0026] As a further description of the above technical solution: In step S3, the temperature of the three-layer co-extrusion die head is controlled in zones: the temperature of the conductor shielding layer is 160-170℃, the temperature of the modified insulation layer is 175-185℃, and the temperature of the insulation shielding layer is 170-180℃.

[0027] The cross-linking tube temperature is 240-260℃ and the cross-linking pressure is 0.8-1.2MPa, resulting in an insulating sleeve with uniform thickness and suppressed space charge.

[0028] This invention provides a hybrid AC / DC superimposed DC cable and its manufacturing method. It offers the following advantages: For superimposed voltage design, the dynamic transition law of dielectric constant and conductivity control is clarified, significantly improving operational stability under composite electric fields. Simultaneously, space charge is suppressed: the modified material significantly reduces space charge accumulation, slows insulation degradation, and extends the cable's service life under complex electric fields. Through the synergistic matching of conductivity between the insulation and shielding layers, the problems of polarity reversal and electric field reversal under thermal cycling conditions are solved, improving operational reliability.

[0029] It should be understood that the foregoing general description and the following detailed description are exemplary and illustrative only, and are not intended to limit this disclosure.

[0030] This application provides an overview of various implementations or examples of the technology described in this disclosure, and is not a full disclosure of the entire scope or all features of the disclosed technology. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the cross-sectional structure of a hybrid AC / DC superimposed DC cable proposed in this invention.

[0032] Legend:

[0033] 100. Cable core; 200. Protective layer; 110. Core wire; 120. Insulating sleeve; 201. Buffer layer; 202. Metallic sheath layer; 203. Outer sheath layer; 111. Conductor; 112. Semi-conductive tape; 121. Conductor shielding layer; 122. Modified insulation layer; 123. Insulating shielding layer. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0035] In some existing applications, with the rapid development of flexible DC transmission (VSC-HVDC) and new energy grid connection technologies, DC transmission has become an important direction. Traditional DC cables are mainly designed for pure DC electric fields and use cross-linked polyethylene (XLPE) as insulation material. In actual operation, due to the influence of converter topology and system operation mode, DC lines often have AC components or ripple voltage superimposed, forming an "AC-DC mixed superposition" condition, which may lead to the following problems:

[0036] Electric field distortion: Under a combined electric field, the electric field distribution pattern in the cable insulation layer changes (from being controlled by the dielectric constant to being controlled by the conductivity), which can easily lead to severe electric field distortion.

[0037] Space charge accumulation: Under AC / DC superposition conditions, traditional XLPE materials suffer from severe space charge injection and accumulation, leading to localized stress concentration and accelerating insulation aging.

[0038] Poor matching: The conductivity of the insulation layer and the semi-conductive shielding layer of the existing cable is not well matched under the effect of thermo-electric coupling. The electric field is prone to reversal when the polarity is reversed or thermal cycling occurs, which affects the reliability of operation.

[0039] To change the above situation, this invention develops a DC-modified cross-linked polyethylene (DC-XLPE) insulation material with low space charge accumulation characteristics and synergistically optimizes the conductivity-temperature characteristics of the semi-conductive shielding layer, thereby constructing an insulation structure that can withstand a certain proportion of superimposed power frequency AC voltage on the basis of DC voltage for a long time.

[0040] In one embodiment of this application, an AC / DC hybrid superimposed DC cable includes a cable core and a protective layer. The protective layer covers the cable core. The cable core includes a core wire and an insulating sleeve. The core wire includes a conductor and a semi-conductive strip. The insulating sleeve includes a conductor shielding layer, a modified insulation layer, and an insulating shielding layer. The protective layer includes a buffer layer, a metal sheath layer, and an outer sheath layer. The cable adopts a concentric and coaxial layered structure, consisting of, from the inside out: conductor, semi-conductive strip, conductor shielding layer, modified insulation layer, insulating shielding layer, buffer layer, metal sheath layer, and outer sheath layer. The insulation shielding layer is formed using a three-layer co-extrusion process, and the modified insulation layer covers the conductor shielding layer, while the insulation shielding layer covers the modified insulation layer. For the design of superimposed voltages, the dynamic transition law of dielectric constant and conductivity control is clarified, significantly improving the operational stability under composite electric fields. Simultaneously, space charge is suppressed: the modified material significantly reduces space charge accumulation, slows down insulation degradation, and extends the cable's service life under complex electric fields. Through the synergistic matching of conductivity between the insulation and shielding layers, the problems of polarity reversal and electric field reversal under thermal cycling conditions are solved, improving operational reliability.

[0041] like Figure 1As shown in the preferred embodiment, an AC / DC hybrid superimposed DC cable includes a cable core 100 and a protective layer 200. The protective layer 200 covers the cable core 100. The cable core 100 includes a core wire 110 and an insulating sleeve 120. The core wire 110 includes a conductor 111 and a semi-conductive strip 112. The insulating sleeve 120 includes a conductor shielding layer 121, a modified insulation layer 122, and an insulating shielding layer 123. The protective layer 200 includes a buffer layer 201, a metal sheath layer 202, and an outer sheath layer 203. The cable adopts a concentric and coaxial layered structure, which, from the inside out, consists of: conductor 111, semi-conductive strip 112, conductor shielding layer 121, modified insulation layer 122, insulating shielding layer 123, buffer layer 201, metal sheath layer 202, and outer sheath layer 203.

[0042] This design ensures a uniform and stable electric field distribution. The concentric and coaxial symmetrical structure allows for a uniform radial distribution of the electric field within the insulation layer, effectively reducing local electric field concentration and adapting to complex AC / DC mixed superposition electric field conditions. Secondly, the tightly bonded interface and absence of air gaps and defects, combined with the layered structure that wraps layer by layer from the inside out, along with the three-layer co-extrusion process, ensures that there are no gaps or misalignments between the conductor 111 shield, modified insulation, and insulation shield, significantly reducing the risk of partial discharge and improving insulation reliability. The electrical performance transitions step by step with good matching. From the conductor 111 to the shield layer and then to the outer sheath, the electrical parameters transition step by step. Combined with the conductivity gradient matching design, it can suppress the accumulation of space charge and avoid electric field reversal during thermal-electric coupling and polarity reversal. The dual protection of mechanical protection and electrical shielding, the buffer layer 201, the metal sheath layer 202, and the outer sheath layer 203, provide mechanical buffering, metal shielding, corrosion and water resistance, and external physical protection in sequence, improving the structural stability of the cable during laying and long-term operation. The overall structure is optimized for AC / DC superposition scenarios and can withstand 10% to 15% of the rated AC component superposition for a long time. It has low electric field distortion rate, slow insulation aging, and longer service life.

[0043] As a preferred technical solution in this embodiment, the conductor shielding layer 121, the modified insulation layer 122, and the insulation shielding layer 123 are formed by a three-layer co-extrusion process, with the modified insulation layer 122 covering the conductor shielding layer 121 and the insulation shielding layer 123 covering the modified insulation layer 122. The three-layer co-extrusion process ensures that the three layers are extruded simultaneously without gaps or defects at the interlayer interface, resulting in a tight bond between the layers. This eliminates interlayer gaps and impurities, significantly reduces the probability of partial discharge, and improves the stability of the insulation system. The electric field distribution is more uniform, with a continuous transition in the three-layer structure and a smooth interface electric field without abrupt changes, which can effectively suppress distortion and stress concentration under AC / DC mixed electric fields. The tight interface reduces charge injection channels, and combined with the characteristics of the modified insulation layer 122, it further reduces space charge accumulation and delays insulation aging. The process results in high production efficiency, good quality consistency, and one-time molding that eliminates multiple extrusion processes. It also leads to high product dimensional accuracy, strong batch stability, and suitability for large-scale industrial manufacturing. It is suitable for high temperature and polarity reversal conditions with high interface bonding strength. It does not separate or delaminate under complex conditions such as thermo-electric coupling, load fluctuation, and polarity reversal, making it more reliable in long-term operation.

[0044] As a preferred technical solution in this embodiment, the modified insulation layer 122 uses a special DC-grade modified cross-linked polyethylene. Through cross-linking structure regulation and anti-charge agent doping to optimize the material trap energy level, it effectively suppresses space charge within the long-term operating temperature range of 90℃, reducing the space charge density by more than 30% compared to conventional AC XLPE. It significantly suppresses space charge accumulation through material trap energy level optimization, greatly reducing charge injection and accumulation, avoiding local electric field distortion, and fundamentally alleviating insulation degradation. It is suitable for complex AC / DC mixed superposition electric fields, maintaining stable electrical performance even under conditions such as DC superposition with AC, ripple, and polarity reversal, without severe electric field concentration. It can operate stably at the rated operating temperature of 90℃ for extended periods, meeting the requirements of high-load conditions such as DC transmission and offshore wind power. This extends cable lifespan by reducing space charge density by more than 30%, significantly slowing down insulation aging and improving the reliability of the cable throughout its entire life cycle. It also improves system operational safety by reducing the risk of partial discharge and insulation breakdown, making it suitable for 150kV high-voltage AC / DC mixed transmission scenarios and ensuring grid stability.

[0045] As a preferred technical solution in this embodiment, the core wire 110 is precisely designed with high conductivity and temperature coefficient to form a reasonable conductivity gradient with the modified insulation layer 122 within a full temperature range of 25℃-90℃. This adapts to thermo-electric coupling and polarity reversal conditions, avoiding electric field reversal and stress concentration. The electric field remains uniform across the entire temperature range. Within the operating temperature range of 25℃ to 90℃, the conductivity matching between the core wire 110 and the modified insulation layer 122 is stable, with no sudden changes or distortions in the electric field. This effectively resists polarity reversal impacts, preventing severe reversal and overshoot of the interface electric field during polarity reversal, significantly improving the operational safety of the DC cable. It adapts to complex thermo-electric coupling conditions, maintaining good matching even under the combined action of temperature and electric field, preventing local stress concentration and delaying insulation aging. It reduces the risk of space charge and interface breakdown. The reasonable conductivity gradient can suppress charge accumulation at the interface, reducing the risk of partial discharge and breakdown caused by interface defects. It improves long-term operational reliability, ensuring stable operation of the cable in a mixed AC / DC environment from an electrical matching perspective, extending its service life.

[0046] A method for preparing an AC / DC hybrid superimposed DC cable, used to prepare an AC / DC hybrid superimposed DC cable as described in the above embodiments, includes the following steps:

[0047] S1: Conductor 111 processing: The conductor 111 raw material is drawn and compressed to obtain the shaped conductor 111;

[0048] S2: Preparation of modified insulating layer 122:

[0049] Ultra-clean polyethylene base material, nano-modifier, deep-trap monomer, and functional additives are added to a high-speed mixer according to the specified ratio and premixed at a constant temperature to obtain a uniform premix.

[0050] The nano-modifiers are: one or two of nano-silica, nano-alumina, nano-magnesium oxide, nano-titanium dioxide, organo-modified montmorillonite, and nano-zinc oxide, which are synergistically doped.

[0051] The deep trap monomer is one of the following: maleic anhydride (MAH), glycidyl acrylate (GMA), methyl methacrylate (MMA), and 4-vinylpyridine (4-VP);

[0052] The premixed material is added to a twin-screw extruder, melt-blended, extruded and granulated to obtain DC-XLPE modified granules. The DC-XLPE modified granules are then cross-linked at high temperature and cooled and shaped to obtain modified insulation layer 122.

[0053] S3: A semiconducting strip 112 is wrapped around the outside of the conductor 111 to form the core wire 110. Then, the conductor shielding layer 121, the modified insulation layer 122 obtained in S2, and the insulation shielding layer 123 are added to the three-layer co-extrusion production line and co-extruded simultaneously with the conductor shielding and insulation shielding. After high-temperature cross-linking and cooling shaping, the three-layer co-extrusion process is used to continuously extrude the insulation sleeve 120 in one go, which tightly covers the outside of the core wire 110, and finally the cable core 100 is obtained. The insulation shielding layer 123 uses a DC-specific semiconducting shielding material, which forms a synergistic conductivity gradient with the modified insulation layer 122 in the range of 25℃-90℃ to prevent electric field reversal.

[0054] S4: Buffer layer 201 covering: Buffer layer 201 is extruded or wrapped around the outside of insulating shielding layer 123 to achieve mechanical buffering and water-blocking protection. The buffer layer 201 is made of semi-conductive buffer water-blocking tape and has the functions of mechanical buffering, radial water blocking and electric field equalization.

[0055] S5: Metal sheath layer 202 forming: The metal sheath layer 202 is formed by corrugated aluminum sleeve and is treated with anti-corrosion, and has the functions of radial water blocking, mechanical protection and electric field shielding, forming a metal shielding and protective layer structure;

[0056] S6: Outer sheath layer 203 extrusion: The outer sheath layer 203 and the outer electrode are extruded outside the metal sheath to complete the overall encapsulation of the cable. The outer sheath layer 203 is made of weather-resistant high-density polyethylene (HDPE), which has excellent resistance to environmental stress cracking, UV resistance and long-term outdoor operation stability.

[0057] As a preferred technical solution in this embodiment, in step S2, the density of the ultra-clean polyethylene base material is 0.918-0.930 g / cm³, and the melt flow rate is 0.15-0.35 g / 10 min;

[0058] The amount of nano-modifier added is 1.0%-3.0% of the polyethylene base material mass, the amount of deep trap monomer added is 0.3%-1.0%, and the amount of functional additive added is 0.4%-0.8%; the speed of the high-speed mixer is 1200-1500 rpm, the mixing time is 8-15 minutes, and the mixing temperature is 45-65℃.

[0059] As a preferred technical solution in this embodiment, in step S2, the temperatures of each zone of the twin-screw extruder are: zone 1 135-145℃, zone 2 150-160℃, zone 3 165-175℃, zone 4 170-180℃, the die head temperature is 175-185℃, and the screw speed is 200-300 rpm. After water cooling, air drying, and pelletizing, DC-XLPE modified granules are obtained.

[0060] As a preferred technical solution in this embodiment, in step S3, the temperature of the three-layer co-extrusion die head is controlled in zones: the temperature of conductor shielding layer 121 is 160-170℃, the temperature of modified insulation layer 122 is 175-185℃, and the temperature of insulation shielding layer 123 is 170-180℃.

[0061] The cross-linking tube temperature is 240-260℃ and the cross-linking pressure is 0.8-1.2MPa, resulting in an insulating sleeve with uniform thickness and suppressed space charge.

[0062] The cable manufactured using the above method can operate stably under the condition of DC voltage superimposed with 10% to 15% of the rated AC voltage, and the electric field distortion rate is controlled within ±15%. It is suitable for scenarios such as flexible DC transmission, offshore wind power DC transmission, and urban DC distribution networks.

[0063] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0064] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A hybrid AC / DC overlay DC cable, comprising a cable core (100) and a protective layer (200), characterized in that: The protective layer (200) covers the outside of the cable core (100); The cable core (100) includes a core wire (110) and an insulating sleeve (120). The core wire (110) includes a conductor (111) and a semi-conductive strip (112). The insulating sleeve (120) includes a conductor shielding layer (121), a modified insulation layer (122), and an insulating shielding layer (123). The protective layer (200) includes a buffer layer (201), a metal sheath layer (202), and an outer sheath layer (203).

2. The AC / DC hybrid superimposed DC cable according to claim 1, characterized in that, The cable adopts a concentric and coaxial layered structure, which consists of the following layers from the inside out: conductor (111), semi-conductive strip (112), conductor shielding layer (121), modified insulation layer (122), insulation shielding layer (123), buffer layer (201), metal sheath layer (202) and outer sheath layer (203).

3. The AC / DC hybrid superimposed DC cable according to claim 2, characterized in that, The conductor shielding layer (121), the modified insulation layer (122), and the insulation shielding layer (123) are formed by a three-layer co-extrusion process, and the modified insulation layer (122) covers the conductor shielding layer (121), and the insulation shielding layer (123) covers the modified insulation layer (122).

4. The AC / DC hybrid superimposed DC cable according to claim 2, characterized in that, The modified insulation layer (122) is made of special DC-grade modified cross-linked polyethylene. Through cross-linking structure regulation, nano-doping and deep trap monomer modification to optimize the material trap energy level, the space charge is effectively suppressed in the long-term operating temperature range of 90℃, and the space charge density is reduced by more than 30% compared with conventional AC XLPE.

5. The AC / DC hybrid superimposed DC cable according to claim 4, characterized in that, The core wire (110) is precisely designed with high conductivity and temperature coefficient to form a reasonable conductivity gradient with the modified insulation layer (122) in the full temperature range of 25℃-90℃, which is suitable for thermo-electric coupling and polarity reversal conditions, and avoids electric field reversal and stress concentration.

6. A method for preparing an AC / DC hybrid superimposed DC cable, used to prepare the AC / DC hybrid superimposed DC cable according to any one of claims 1-5, characterized in that, Cable manufacturing methods include: S1: Conductor (111) processing: The conductor (111) raw material is drawn and compressed to obtain the shaped conductor (111); S2: Preparation of the modified insulating layer (122): Ultra-clean polyethylene base material, nano-modifier, deep-trap monomer, and functional additives are added to a high-speed mixer according to the specified ratio and premixed at a constant temperature to obtain a uniform premix. The nano-modifiers are: one or two of nano-silica, nano-alumina, nano-magnesium oxide, nano-titanium dioxide, organo-modified montmorillonite, and nano-zinc oxide, which are synergistically doped. The deep trap monomer is one of the following: maleic anhydride (MAH), glycidyl acrylate (GMA), methyl methacrylate (MMA), and 4-vinylpyridine (4-VP); The premixed material was added to a twin-screw extruder, melt-blended, extruded and granulated to obtain DC-XLPE modified granules. The DC-XLPE modified granules were cross-linked at high temperature and cooled and shaped to obtain a modified insulation layer (122). S3: A semiconducting strip (112) is wrapped around the conductor (111) to form a core wire (110). Then, the conductor shielding layer (121), the modified insulation layer (122) obtained in S2, and the insulation shielding layer (123) are added to the three-layer co-extrusion production line and co-extruded simultaneously with the conductor shielding and insulation shielding. After high-temperature cross-linking and cooling, the three-layer co-extrusion process is used to continuously extrude the insulation sleeve (120) in one go, which tightly covers the core wire (110) to finally obtain the cable core (100). The insulation shielding layer (123) uses DC special semiconducting shielding material, which forms a synergistic conductivity gradient with the modified insulation layer (122) in the range of 25℃-90℃ to prevent electric field reversal. S4: Buffer layer (201) covering: The buffer layer (201) is extruded or wrapped around the outside of the insulating shielding layer (123) to achieve mechanical buffering and water blocking protection. The buffer layer (201) is made of semi-conductive buffer water blocking tape and has the functions of mechanical buffering, radial water blocking and electric field equalization. S5: Metal sheath layer (202) forming: The metal sheath layer (202) is formed by corrugated aluminum sleeve and is treated with anti-corrosion, and has the functions of radial water blocking, mechanical protection and electric field shielding, forming a metal shielding and sheath structure; S6: Outer sheath layer (203) extrusion: The outer sheath layer (203) and the outer electrode are extruded outside the metal sheath to complete the overall encapsulation of the cable. The outer sheath layer (203) is made of weather-resistant high-density polyethylene, which has excellent resistance to environmental stress cracking, UV resistance and long-term outdoor operation stability.

7. The method for preparing a hybrid AC / DC superimposed DC cable according to claim 6, characterized in that, In step S2, the density of the ultra-clean polyethylene base material is 0.918-0.930 g / cm³, and the melt flow rate is 0.15-0.35 g / 10 min; The amount of nano-modifier added is 1.0%-3.0% of the polyethylene base material mass, the amount of deep trap monomer added is 0.3%-1.0%, and the amount of functional additive added is 0.4%-0.8%; the speed of the high-speed mixer is 1200-1500 rpm, the mixing time is 8-15 minutes, and the mixing temperature is 45-65℃.

8. The method for preparing a hybrid AC / DC superimposed DC cable according to claim 6, characterized in that, In step S2, the temperatures of each zone of the twin-screw extruder are: zone 1 135-145℃, zone 2 150-160℃, zone 3 165-175℃, zone 4 170-180℃, the die head temperature is 175-185℃, and the screw speed is 200-300 rpm. After water cooling, air drying, and pelletizing, DC-XLPE modified pellets are obtained.

9. The method for preparing a hybrid AC / DC superimposed DC cable according to claim 6, characterized in that, In step S3, the temperature of the three-layer co-extrusion die head is controlled in zones: the temperature of the conductor shielding layer (121) is 160-170℃, the temperature of the modified insulation layer (122) is 175-185℃, and the temperature of the insulation shielding layer (123) is 170-180℃. The cross-linking tube temperature is 240-260℃ and the cross-linking pressure is 0.8-1.2MPa, resulting in an insulating sleeve with uniform thickness and suppressed space charge.

Images