Preparation method of ultra-high voltage ac-dc unequal diameter cable flexible joint

Abstract

The application discloses a preparation method of an ultra-high voltage AC / DC unequal-diameter cable soft joint, mainly including two core steps of inner semi-conductive shielding layer recovery and main insulation layer recovery. In the inner semi-conductive shielding layer recovery process, the conductor connection area is first wrapped with a shielding tape, and then is heated and shaped by using an inner shielding variable-diameter mold; the transition slope of the inner cavity of the mold is accurately matched with the transition slope of the conductor welding part, so that the interface contact characteristics are optimized and effective strength compensation is realized. In the main insulation layer recovery process, the extrusion crosslinking process is accurately divided into four stages of starting and filling, transition and forming, stable production, ending and deceleration, and the extrusion speed, heating temperature and mold pressure of each stage are controlled accurately in sections. The smooth filling, uniform coating and dense forming of the insulation material in the variable-diameter mold cavity are ensured, and the thermal degradation and structure defects of the material are effectively inhibited.

Description

Technical Field

[0001] This invention relates to the field of cable technology, and in particular to a method for preparing flexible joints for ultra-high voltage AC / DC cables of unequal diameter. Background Technology

[0002] In offshore wind power flexible DC projects, to adapt to complex marine terrain, segmented construction, and maintenance and replacement needs, it is often necessary to connect submarine cables of different diameters, such as connecting main cables and branch cables or replacing old cables with new ones. If large-section submarine cables are selected based solely on the landing point with the lowest current carrying capacity, investment costs will increase significantly. However, differential diameter connection technology allows for differentiated cable selection between the offshore and landing sections, providing an economical and effective solution. Currently, differential diameter connections mainly exist in three forms: rigid joints, flexible joints, and intelligent joints. Among them, flexible joints exhibit significant advantages due to their smooth conductor welding transition, reverse stress cone structure design, and the flexibility of fabrication both in factories and on-site. However, existing flexible joint manufacturing technologies, especially in the crucial process of extrusion cross-linking of the main insulation layer, generally lack precise stage division and multi-parameter coordinated control of the extrusion process. This makes it difficult to ensure uniform filling, dense molding, and surface quality of the insulation material at the differential diameter structure, thus restricting the long-term operational reliability of the joint. Therefore, developing a method that can precisely control the entire extrusion cross-linking process is crucial for improving the performance of flexible joints for ultra-high voltage AC / DC cables with unequal diameters. Summary of the Invention

[0003] This invention addresses the problems of uneven insulation layer filling, poor interface contact, and easy generation of internal defects caused by the unequal diameter structure in the fabrication of ultra-high voltage AC / DC cable flexible joints. By employing a precise diameter-recovery process for the inner semiconductive shielding layer and a four-stage refined control of the main insulation extrusion cross-linking process, optimized contact and strength compensation at the shielding layer-conductor interface are achieved. This ensures uniform filling, dense molding, and complete sealing of the insulation material in the unequal diameter region, improving the joint's interface quality, insulation integrity, and long-term operational reliability. The technical solution provided by this invention is as follows:

[0004] On one hand, the present invention provides a method for preparing an ultra-high voltage AC / DC unequal diameter cable flexible joint, comprising the following steps:

[0005] The steps for restoring the inner semiconductive shielding layer are as follows: a shielding tape is wrapped around the conductor connection area, and then the inner shielding tape is heated and shaped using an inner shielding tape diameter-changing mold. The transition slope of the inner cavity of the inner shielding tape diameter-changing mold is consistent with the transition slope after conductor welding, and the shielding tape is formed and cured at 160~180℃ through its integrated heating module; wherein the thickness of the inner semiconductive shielding layer after the shielding tape is wrapped is not less than 1.05 times the thickness of the cable body shielding layer.

[0006] Main insulation layer restoration steps: Extrusion cross-linking using an insulation reducing die, including the following stages:

[0007] During the start-up and filling phase, the extruder is fed at an initial rate of 3-4 kg / min, and then the initial rate is increased to a stable rate of 4-5 kg / min with a linear slope of 0.2 kg / min. Simultaneously, the die temperature is controlled to be 5-15°C higher than the barrel melt temperature, and the die pressure is stabilized at 1.1-1.5 MPa. The initial rate is 50%-80% of the stable rate.

[0008] During the transition and forming stage, the extruder is controlled to run at a stable rate of 4~5 kg / min, so that the insulating material passes through the diameter-changing area of ​​the insulating reducing die and covers the surface of the cable joint.

[0009] During the stable production phase, the extruder is maintained at a stable rate of 4~5 kg / min to complete the insulation extrusion of the main body of the cable connector.

[0010] During the finishing and deceleration phase, the extruder is controlled to reduce the steady rate at a linear slope of 0.15 kg / min until it comes to a complete stop;

[0011] During the transition and forming stage and the stable production stage, the extruder barrel temperature is 140~170℃ and the die pressure is 1.2~1.6MPa. At the same time, the insulating variable diameter die is subjected to zoned temperature control, and the temperature control temperature of the narrow section or variable diameter area is higher than that of the wide section area.

[0012] Optionally, the temperature control temperature of the narrow cross-section or variable diameter region of the insulating reducing mold is 5-10°C higher than the temperature control temperature of the wide cross-section region.

[0013] Optionally, during the stable production phase, the die head pressure is monitored in real time and its fluctuation range is maintained within a preset threshold; at the same time, process parameters such as extrusion temperature, die head pressure, and extrusion speed are continuously recorded.

[0014] Optionally, after the closing and deceleration phase, the method further includes:

[0015] Cleaning stage: Control the screw of the extruder to rotate at a speed of 8~12 rpm and maintain a back pressure of 15~25 MPa.

[0016] Optionally, the transition slope after the conductors are welded is obtained by cutting slits at the ends of two conductors of different diameters. The slit angle of the conductor with a smaller cross-section is 40°~50°, and the slit angle of the conductor with a larger cross-section is 25°~35°. The two conductors are then joined together to form an X-shaped interface. After welding and trimming, a transition slope of 1°~3° is formed.

[0017] Optionally, the inner screen diameter-changing mold in the inner semiconductive shielding layer restoration step includes:

[0018] The mold body has an inner cavity transition slope that matches the transition slope after conductor welding;

[0019] A heating module integrated on the mold body;

[0020] And a temperature sensor connected to the heating module.

[0021] Optionally, the transition length of the insulating reverse stress cone is controlled at 150~200 mm, and the angle of the differential insulation transition zone is controlled at 10°~15°.

[0022] Optionally, the method further includes a crosslinking vulcanization step: after extrusion of the main insulation layer recovery step, the insulation layer is crosslinked vulcanized at a temperature of 210~220℃ and a pressure of 1.2~1.6MPa, and the temperature-pressure curve of the process is recorded.

[0023] Optionally, the method further includes an outer semiconductive shielding layer recovery step: after completing the cross-linking vulcanization step, a semiconductive shielding tape is wrapped around the insulation layer, and the wrapped semiconductive shielding tape is heat-treated at a temperature controlled between 180 and 210°C.

[0024] On the other hand, the present invention also provides an ultra-high voltage AC / DC unequal diameter cable flexible joint, which is prepared by the method described above.

[0025] By adopting the above technical solution, the method for preparing ultra-high voltage AC / DC unequal diameter cable flexible joints provided by the present invention has the following beneficial effects:

[0026] 1. In the inner semiconductive shielding layer restoration step of this invention, an inner shielding diameter-changing mold matching the transition slope after conductor welding is used to achieve a geometrically continuous transition between the shielding tape wrapping and the conductor surface. Simultaneously, the thickness of the inner semiconductive shielding layer after wrapping is controlled to be no less than 1.05 times the thickness of the cable body shielding layer, ensuring sufficient thickness margin for the shielding functional layer. Furthermore, the shielding tape is heated to an optimized temperature range of 160~180℃ using a heating module integrated into the mold for thermosetting. This effectively ensures the quality of the interface contact between the restored inner semiconductive shielding layer and the conductor. Through precise control of the thickness margin, the continuity between the shielding layer and the body is maintained, and reliable strength compensation is provided for any potential attenuation of shielding performance during the restoration process, thereby ensuring the uniformity of the electric field distribution and interface stability throughout the connection area.

[0027] 2. This invention systematically improves process quality and product performance by precisely dividing the extrusion recovery process of the main insulation layer into four key stages: start-up and filling, transition and molding, stable production, and finishing and deceleration. In the start-up and filling stage, a slow start-up and stepped temperature control method is used to ensure the molten material fills the mold cavity smoothly and at low pressure, effectively avoiding initial pressure shocks and material degradation. In the transition and molding stage, precise control of the temperature of the dies with varying diameters and the extrusion speed ensures uniform material coverage of the cable joint, eliminating defects such as bubbles and flow marks. In the stable production stage, maintaining constant parameters and introducing statistical process control for trend monitoring ensures the consistency and accuracy of the entire joint's geometric dimensions. In the finishing and deceleration stage, a gradual deceleration using a ramp-down method allows the melt to fully compensate for the shrinkage at the end of the die under pressure, avoiding shrinkage cavities and tailing phenomena. This staged control method comprehensively improves the integrity, uniformity, and long-term operational reliability of the main insulation layer recovery. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 A schematic diagram of a flexible connector provided in an embodiment of the present invention;

[0030] Figure 2 The embodiments of the present invention provide Figure 1 A magnified view of part A;

[0031] Figure 3 A schematic diagram of an insulating reverse stress cone provided in an embodiment of the present invention. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0033] The term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the invention. In the description of the invention, it should be understood that the terms "upper," "lower," "top," "bottom," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" and "second" may explicitly or implicitly include one or more of that feature. Moreover, the terms "first," "second," etc., are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein.

[0034] When a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included. For example, a specified range from “1 to 10” should be considered to include any and all subranges between the minimum value 1 and the maximum value 10. Exemplary subranges of the range 1 to 10 include, but are not limited to, 1 to 6.1, 3.5 to 7.8, 5.5 to 10, etc.

[0035] The method for preparing an ultra-high voltage AC / DC unequal diameter cable flexible joint provided in this invention specifically includes the following steps:

[0036] In the inner semiconductive shielding layer restoration step, shielding tape is wrapped around the conductor connection area. After wrapping, an inner shielding variable diameter mold with an inner cavity transition slope consistent with the transition slope after conductor welding (preferably controlled within the range of 1°~3°) is used for heating and shaping. This ensures that the shielding layer achieves geometric continuity and uniform transition in the length direction. The shielding tape is heated to 160~180℃ by a heating module integrated on the inner shielding variable diameter mold to complete thermosetting. This ensures that the shielding material is fully cross-linked and formed, and avoids overheating damage. The thickness of the inner semiconductive shielding layer after wrapping is not less than 1.05 times the thickness of the cable body shielding layer. Through precise control of the thickness margin, the continuity between the shielding layer and the body is maintained, and reliable strength compensation is provided for the possible attenuation of shielding performance during the restoration process. This ensures the uniformity of electric field distribution and interface stability of the entire connection area.

[0037] Main insulation layer restoration steps: Extrusion cross-linking using an insulation reducing die, including the following stages:

[0038] During the start-up and filling phase, the extruder is fed at an initial rate of 3–4 kg / min, followed by a linear increase in rate from the initial rate to a stable rate of 4–5 kg / min at a rate of 0.2 kg / min. Simultaneously, the die temperature is controlled to be 5–15°C higher than the barrel melt temperature, and the die pressure is stabilized at 1.1–1.5 MPa. The initial rate is 50%–80% of the stable rate. The control objective of this start-up and filling phase (from the start of extrusion to the material reaching the narrowest point of the die) is to smoothly and with low pressure push the molten material in the die channel into the die cavity, avoiding initial pressure shock and material thermal degradation. Precise control of the melt flow behavior within the die channel is achieved by employing a "slow start" and "ramp-up" extrusion speed. Specifically, the extruder is fed at an initial rate of 3-4 kg / min, gradually increasing to a stable rate of 4-5 kg / min with a linear slope of 0.2 kg / min. This aims to match the filling space requirements of cable cross-section variations, prioritizing low-pressure, stable filling of narrow cross-section areas before gradually accelerating filling of wider cross-section areas. This effectively suppresses the "spraying" phenomenon caused by initial pressure impact and the risk of pressure sensor overload. Simultaneously, by setting the die temperature 5-15°C higher than the barrel melt temperature and stabilizing the die pressure within the range of 1.1-1.5 MPa, the flow resistance caused by geometric abrupt changes in the contraction zone of the variable-diameter flow channel is effectively compensated, reducing melt viscosity and promoting gradual material distribution and uniform filling within the die cavity. Furthermore, the barrel employs zoned temperature control: the feeding section is controlled at 120-130°C to ensure solid conveying efficiency, while the melting and homogenization sections are stabilized at 135-145°C to ensure sufficient material plasticization and prevent thermal degradation. This stage relies on real-time monitoring of extrusion pressure as feedback, and through the synergistic effect of speed gradient increase and temperature-pressure parameters, it jointly ensures the stability of melt flow and interface forming quality in the varistor mold.

[0039] During the transition and forming stage, the extruder is controlled to operate at a stable rate of 4~5 kg / min, allowing the insulating material to pass through the diameter-changing zone of the insulating reducing die and cover the surface of the cable joint. The control objective of this transition and forming stage (when the material passes through the reducing section and begins to form the shape of the joint) is to achieve a stable and defect-free extrusion process, ensuring that the insulating material uniformly covers the cable joint and eliminates air bubbles and flow marks. By controlling the extruder to operate at a stable rate of 4~5 kg / min (the core speed of the process), while ensuring production efficiency, problems such as melt fracture (manifested as sharkskin or bamboo-like defects at the die orifice), surface roughness due to insufficient material relaxation, and excessive die pressure caused by excessive speed are avoided.

[0040] During the stable production phase, the extruder continues to operate at a stable rate of 4-5 kg / min to complete the insulation extrusion of the main body of the cable connector. The control objective of this stable production phase (extrusion of the main body of the connector) is to maintain the stability of the process parameters established in the transition and forming phases, ensuring that the insulation layer of the entire connector has highly consistent physical properties and precise geometric dimensions. The core of this phase is to maintain constant key parameters such as extrusion speed, barrel temperature, die temperature, and die pressure through an automated control system, and effectively resist external interference such as voltage fluctuations. At the same time, the system continuously records process data such as temperature, pressure, and speed, and uses statistical process control (SPC) tools to monitor and analyze parameter trends in real time. When a continuous and slow upward trend in die pressure is detected within the allowable range, an early warning mechanism is immediately triggered, prompting maintenance operations such as die cleaning or filter replacement, thereby achieving proactive prevention and quality control of potential faults.

[0041] During the finishing and deceleration phase, the extruder is controlled to reduce the stable rate at a linear slope of 0.15 kg / min until it comes to a complete stop. The control objective of this finishing and deceleration phase (from the forming of the joint end to the cessation of extrusion) is to achieve a perfect finish and avoid "tailing" or "shrinkage" phenomena in the flow channel of the die with different diameters, while preparing for the preparation of the next joint. By using "ramp descent" control for the extrusion speed, the running speed is gradually reduced near the end, and finally stopped completely at an extremely low speed, so that the melt can fully fill the gap formed by the shrinkage at the end of the die under pressure. At the same time, after the extrusion stops, the screw is controlled to rotate at a low speed for a short time and maintain an appropriate back pressure to "clean" the residual material in the barrel, ensuring a uniform melt state for the next start-up.

[0042] During the transition and molding stages, as well as the stable production stage, the extruder barrel temperature is maintained at 140~170℃, and the die pressure at 1.2~1.6MPa. This ensures that the cross-linked polyethylene (XLPE) insulation material reaches its optimal melt state and rheological properties. While ensuring the material fully fills the die gaps and effectively eliminates internal air bubbles, it strictly avoids molecular chain degradation or premature cross-linking reactions caused by excessive temperature, ultimately forming a dense, defect-free insulation layer microstructure. Simultaneously, zoned temperature control is implemented for the insulation die with varying diameters. The temperature control temperature for the narrow cross-section or variable diameter region is higher than that for the wide cross-section region. Preferably, the temperature control temperature for the narrow cross-section or variable diameter region is set 5~10℃ higher than that for the wide cross-section region, based on a coupled analysis of the thermal conductivity characteristics of the insulation material and the cross-sectional geometry. Because the insulation material accumulates thicker in the narrow diameter region, increasing the heat capacity, and the melt is more prone to heat loss due to increased shear and flow rate changes when flowing through the variable diameter region, without local temperature rise compensation, the material in this region is prone to problems such as insufficient melting and poor interface fusion. By applying a directional temperature rise of 5~10℃ to this region, the material fluidity can be effectively improved, the diffusion and entanglement of interfacial molecular chains can be promoted, and the thick-walled region and the thin-walled region can be simultaneously and fully cross-linked and cured.

[0043] Optionally, during the stable production phase, the die pressure is monitored in real time and its fluctuation range is maintained within a preset threshold (0.3 MPa). Simultaneously, process parameters such as extrusion temperature, die pressure, and extrusion speed are continuously recorded. Statistical process control (SPC) tools are used to monitor and analyze the trends of these parameters in real time. For example, even if the die pressure value remains within the normal range allowed by the process, a slow, continuous upward trend (such as exceeding a preset threshold slope or cumulative deviation) is considered a critical warning signal, indicating potential abnormalities such as die scorching, internal material carbonization and accumulation, or filter blockage. Timely maintenance measures (such as cleaning the die flow channel, checking and replacing the filter, or adjusting process parameters) are required. Early identification and intervention of parameter trends effectively prevent potential quality defects.

[0044] Optionally, after the termination and deceleration phases, the method further includes:

[0045] Cleaning stage: Control the screw of the extruder to rotate at a speed of 8~12 rpm (preferably 10 rpm) and maintain a back pressure of 15~25MPa. In this stage, after the main extrusion stops, the screw is rotated at a set low speed for a short time while maintaining a stable back pressure. This applies a controllable shearing action to the remaining material in the barrel, achieving the "cleaning" function. That is, it effectively removes residual melt, reduces the residence time of material in the high-temperature environment, avoids carbonization and accumulation, and at the same time keeps the melt state in the barrel uniform, preparing stable and reliable initial melt conditions for the next start-up.

[0046] Optionally, the transition slope after conductor welding is obtained by cutting slits at the ends of two conductors of unequal diameter. The slit angle for the smaller cross-section conductor is 40°~50°, and the slit angle for the larger cross-section conductor is 25°~35°. The two are then joined together to form an X-type interface. After welding and finishing, a transition slope of 1°~3° is formed. For specific structure, please refer to [link / reference]. Figure 1 and Figure 2 Specifically, the ends of two conductors with unequal diameters are first machined into slits at specific angles. The slit angle for the smaller cross-section conductor is 40°~50°, and the slit angle for the larger cross-section conductor is 25°~35°. This angle design satisfies the need for a larger heat dissipation area when welding the larger cross-section conductor and provides a smoother geometric transition. The two slits are then joined together to form an X-type interface (which, compared to the traditional V-type interface, effectively disperses thermal stress and avoids uneven welding). Silver-based welding wire is used in conjunction with TIG (tungsten inert gas) welding. Preheating is performed before welding to overcome the problem of insufficient penetration caused by the high thermal conductivity of copper conductors. The welding current is adjusted according to the cable cross-section (e.g., 2500mm²). 2 The recommended cross-section is 1000~1500A, and high-purity argon gas (purity ≥99.99%) is introduced at a rate of 20~25L / min as a protective gas. To protect adjacent insulation materials, a differentiated cooling water mold system is set up on both sides of the conductor. A higher flow rate (preferably ≥8L / min) is used on the larger cross-section side, and a lower flow rate (preferably 5~7L / min) is used on the smaller cross-section side. The difference in welding heat input on both sides is balanced by adjusting the heat dissipation efficiency. After welding, the diameter tolerance of the welded area is controlled within ±0.2mm after precision finishing, ultimately forming an optimized transition slope of 1°~3°. This slope design not only improves the electric field distribution and enhances the stability of insulation extrusion, but more importantly, it makes the temperature distribution of the unequal diameter transition zone more uniform. Given the sensitivity of the volume resistivity of DC cable insulation materials to temperature changes, this measure can effectively suppress electric field distortion caused by temperature differences and improve the reliability of the insulation system.

[0047] Optionally, the inner screen diameter-changing mold for the inner semiconductive shielding layer restoration step includes:

[0048] The mold body has a preset transition angle in its inner cavity that matches the transition slope after the conductor is welded, ensuring the geometric continuity of the shielding layer and the conductor interface and avoiding electric field distortion caused by angle deviation.

[0049] The heating module integrated into the mold body achieves thermosetting of the shielding strip within an optimized temperature range of 160~180℃ through precise temperature control, ensuring material fluidity and interfacial bonding strength.

[0050] In addition, a temperature sensor connected to the heating module monitors the mold temperature changes in real time to ensure the stability of the heating process and prevent local overheating or insufficient temperature from affecting the shielding layer performance. This inner screen variable diameter mold structure, through geometric matching design, precise temperature control, and process monitoring, collaboratively ensures the recovery quality of the inner semiconductive shielding layer, effectively compensates for the shielding strength attenuation that may be caused by the recovery process, and maintains the uniformity of the interface electric field.

[0051] Optionally, the transition length of the insulating reverse stress cone is controlled at 150~200 mm, and the angle of the transition zone between different diameter insulating sections is controlled at 10°~15°. Please refer to the specific structure. Figure 3 If the transition section is too long or the angle is too large, the local electric field strength will be too high at the point of diameter reduction, increasing the risk of breakdown and local overheating. The fundamental reason is that the abrupt change in geometry due to diameter reduction disrupts the uniform distribution of the electric field at the break in the insulation shield layer. Conversely, if the transition section is too short or the angle is too small, the transition of the diameter reduction joint will be insufficient, increasing local stress and current density gradient, affecting the long-term reliability of the diameter reduction joint. This is because, according to electromagnetic field theory, the electric field strength (E) on the conductor surface is inversely proportional to the radius of curvature (ρ) (E ∝ 1 / ρ). The smaller the radius of curvature (the sharper the geometry), the higher the electric field strength. At the diameter reduction joint, the cable insulation diameter undergoes a step change, forming a geometric defect with a very small radius of curvature. When current (or electric field lines) flows from the large-diameter cable to the small-diameter cable, it will be rapidly "crowded" in this abrupt change region, resulting in a sharp increase in electric field line density and a significant peak in electric field strength. The function of the stress cone is to smooth the electric field lines and reduce the concentrated electric field at the break point of the shielding layer by providing a gradual change in dielectric constant and capacitance distribution. However, under different diameter conditions, if the angle of the stress cone is not designed properly (such as the slope is too steep), the angle between its own curved surface and the cable insulation surface may form a new electric field concentration point, especially on the side with a larger diameter. If the radius of curvature is not properly matched, a smooth transition of the electric field will not be achieved. Therefore, the transition length of the insulated reverse stress cone is controlled at approximately 150~200mm, and the angle of the transition zone between different diameters is controlled at approximately 10°~15°. This angle range has been optimized, and its core lies in precisely controlling the capacitance gradient and electric field line shape of the reducing joint: the stress cone introduces a gradually increasing additional capacitance at the shielding layer break to compensate for the capacitance abrupt change caused by the disappearance of the shielding layer, thus achieving electric field rebalancing; a smaller angle (a gentler slope) can form a smoother capacitance transition zone, allowing the electric field lines to diverge from the conductor to the outer grounding shielding layer in a gentler manner, avoiding sharp bends and significantly reducing the maximum electric field strength; at the same time, within a limited space, the smaller the angle, the smoother the transition, but a longer transition length is required, so a reasonable balance must be struck between the angle and the length combination; especially in reducing joints, the slope of the stress cone should be coordinated with the curvature change of the cable insulation surface, and a smaller angle should be used on the side with the larger diameter to ensure a smooth transition with the insulation surface, avoid the generation of new electric field concentration points, ensure the uniformity of electric field distribution from the source, and improve the electrical stability and long-term operational reliability of the joint.

[0052] Optionally, the method further includes a cross-linking vulcanization step: after extrusion to restore the main insulation layer, the insulation layer is cross-linked and vulcanized at a temperature of 210~220℃ and a pressure of 1.2~1.6MPa, and the temperature-pressure curve of this process is recorded. Through precise thermal-pressure coupling control, the cross-linking reaction is ensured to proceed fully and uniformly, achieving the target degree of cross-linking. Simultaneously, continuous pressure effectively suppresses the generation of volatile byproduct bubbles, ensuring the microscopic density of the material. The temperature-pressure time-series curve is fully recorded during this process, forming traceable quality control data. This cross-linking vulcanization process, combined with the prior mold optimization design and segmented extrusion control, jointly ensures that the insulation layer achieves the required electrical insulation strength and mechanical-physical properties, providing dual protection for the long-term stable operation of the cable joint.

[0053] Optionally, the method further includes an external semiconductive shielding layer restoration step: after completing the cross-linking vulcanization step, a semiconductive shielding tape is wrapped around the insulation layer, and the wrapped semiconductive shielding tape is heat-treated at a temperature controlled between 180 and 210°C. This is to restore the equipotential of the outer surface of the insulation layer, uniformly dissipate surface leakage current and capacitive current, and prevent surface discharge.

[0054] Optionally, the method also includes a welding pretreatment step. First, a heating and straightening process is performed on the cable: the cable body is fixed with angle steel, and the cable surface is uniformly heated to 85-90°C and maintained for 24 hours using a heating band to eliminate internal stress. Then, the outer sheath and metal sheath are stripped, exposing 100-120mm working sections at each end of the conductor. The water-blocking material inside the conductor is thoroughly cleaned. Alcohol is used to wipe the water-blocking strip, while acetone solvent combined with ultrasonic cleaning technology is used for the water-blocking adhesive. Compared to the traditional manual alcohol layering wiping process, this method increases the cleaning efficiency from 5 meters / hour to 20 meters / hour, and reduces the residual adhesive content from ≥5% to ≤1%, significantly improving cleanliness. After cleaning, the stranded structure of the conductor is precisely restored according to its original pitch, laying the foundation for subsequent welding processes.

[0055] Optionally, the method also includes a cable joint process design optimization process based on numerical simulation. First, the initial process dimensions of the joint are determined based on the cable body structure, including the conductor connection length and the angle and length of the inverse stress cone. Then, a three-dimensional finite element model containing the conductor, insulation layer, and sheath layer is established using multiphysics simulation software (such as COMSOL or ANSYS). In the model, the conductivity of the insulation material (such as cross-linked polyethylene) is set as a bivariate function dependent on both electric field strength and temperature, and an electro-thermal bidirectional coupled solver is used for analysis: a field strength-dependent nonlinear conductivity constitutive relation is set in the current module, and a conductor Joule heat source and outer convection heat dissipation boundary conditions are applied in the solid heat transfer module to accurately simulate the temperature field distribution of the joint during actual operation. Finally, the key geometric parameters of the inverse stress cone are optimized synergistically through parametric scanning. The angle θ (preferably a scanning range of 10°~45°, which can be subdivided into gradients such as 10°~15° and 15°~20°) mainly controls the distribution of the electric field concentration region, while the length L (preferably a scanning range of 100~250 mm, which can be subdivided into gradients such as 100~150 mm and 150~200 mm) dominates the attenuation gradient of the axial electric field. Finally, the optimal combination of geometric parameters that makes the electric field distribution most uniform and the peak field strength the lowest is obtained.

[0056] Please refer to Figure 1 The present invention also provides an ultra-high voltage AC / DC unequal diameter cable flexible joint, which is prepared by the above method.

[0057] The above are merely optional 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 method for preparing an ultra-high voltage AC / DC unequal diameter cable flexible joint, characterized in that, Includes the following steps: The steps for restoring the inner semiconductive shielding layer are as follows: a shielding tape is wrapped around the conductor connection area, and then the inner shielding tape is heated and shaped using an inner shielding tape diameter-changing mold. The transition slope of the inner cavity of the inner shielding tape diameter-changing mold is consistent with the transition slope after conductor welding, and the shielding tape is formed and cured at 160~180℃ through its integrated heating module; wherein the thickness of the inner semiconductive shielding layer after the shielding tape is wrapped is not less than 1.05 times the thickness of the cable body shielding layer. Main insulation layer restoration steps: Extrusion cross-linking is performed using an insulation reducing die. Includes the following stages: During the start-up and filling phase, the extruder is controlled to feed material at an initial rate of 3-4 kg / min, and then the initial rate is increased to a stable rate of 4-5 kg / min with a linear slope of 0.2 kg / min. The die head temperature is synchronously controlled to be 5-15°C higher than the barrel melt temperature, and the die head pressure is stabilized at 1.1-1.5 MPa; wherein the initial rate is 50%-80% of the stable rate; During the transition and forming stage, the extruder is controlled to run at a stable rate of 4~5 kg / min, so that the insulating material passes through the diameter-changing area of ​​the insulating reducing die and covers the surface of the cable joint. During the stable production phase, the extruder is maintained at a stable rate of 4~5 kg / min to complete the insulation extrusion of the main body of the cable connector. During the finishing and deceleration phase, the extruder is controlled to reduce the steady rate at a linear slope of 0.15 kg / min until it comes to a complete stop; Cleaning stage: Control the screw of the extruder to rotate at a speed of 8~12 rpm and maintain a back pressure of 15~25 MPa; During the transition and forming stage and the stable production stage, the extruder barrel temperature is 140~170℃ and the die pressure is 1.2~1.6MPa. At the same time, the insulating variable diameter die is subjected to zoned temperature control, and the temperature control temperature of the narrow section or variable diameter area is 5~10℃ higher than that of the wide section area. The transition slope after the conductors are welded is obtained by cutting slits at the ends of two conductors of different diameters. The slit angle of the conductor with a smaller cross-section is 40°~50°, and the slit angle of the conductor with a larger cross-section is 25°~35°. The two are then joined together to form an X-shaped interface. After welding and trimming, a transition slope of 1°~3° is formed.

2. The method according to claim 1, characterized in that, During the stable production phase, the die head pressure is monitored in real time and its fluctuation range is maintained within a preset threshold; at the same time, process parameters such as extrusion temperature, die head pressure, and extrusion speed are continuously recorded.

3. The method according to claim 1, characterized in that, The inner screen diameter-changing mold for the inner semiconductive shielding layer restoration step includes: The mold body has an inner cavity transition slope that matches the transition slope after conductor welding; A heating module integrated on the mold body; And a temperature sensor connected to the heating module.

4. The method according to claim 1, characterized in that, The transition length of the insulating reverse stress cone is controlled at 150~200 mm, and the angle of the transition zone of the differential insulation is controlled at 10°~15°.

5. The method according to claim 1, characterized in that, The method further includes a cross-linking vulcanization step: after extrusion of the main insulation layer recovery step, the insulation layer is cross-linked vulcanized at a temperature of 210~220℃ and a pressure of 1.2~1.6MPa, and the temperature-pressure curve of the process is recorded.

6. The method according to claim 5, characterized in that, The method further includes an external semiconductive shielding layer recovery step: after completing the cross-linking vulcanization step, a semiconductive shielding tape is wrapped around the insulation layer, and the wrapped semiconductive shielding tape is heat-treated at a temperature controlled between 180 and 210°C.

7. An ultra-high voltage AC / DC unequal diameter cable expansion joint, characterized in that, Prepared by the method described in any one of claims 1 to 6.

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