Method for optimizing and consistency control of 750kv ac xlpe cable insulation thickness

Abstract

The present application relates to 750kV AC XLPE cable and its insulation thickness optimization. With thickness function t(r) / multi-part {t i} as the core, combined with electric-thermal coupling optimization and online thickness measurement-towing-cooling closed loop mapping, E max ≤α·E allow (α is the peak coefficient, preferably not more than 0.90), the eccentricity e is controlled within the specified threshold range(example not more than 3%, preferably not more than 2.5%), b / a is located in 1.9-2.4. Cooperate with VCV tower and CPC three-layer co-extrusion, low-degassing XLPE, 0.01mm measurement and control and N≥8 nodes, and through the center unit segmentation conductor and multi-stage tight pressing, self-centering rolling shaping, the roundness is improved to about 0.2mm; under the same diameter / accessory caliber, peak reduction E max , R th does not increase and saves materials, and the power frequency, impact and thermal cycle test verification is completed according to IEC comparable caliber.

Description

Technical Field

[0001] This invention relates to the field of power cable design and manufacturing technology, specifically to a functional / zonal optimization method for 750kV AC voltage-rated cross-linked polyethylene (XLPE) cables and their insulation thickness, as well as manufacturing consistency control. Background Technology

[0002] In ultra-high voltage AC transmission scenarios, single-core extruded cross-linked polyethylene (XLPE) cables are used in long-distance and hub-and-spoke engineering applications. Their design and evaluation must simultaneously consider multiple constraints such as electrical, thermal, and mechanical factors, and form a unified system object with terminals, connectors, and other accessories. In engineering practice, international standards for voltage dividers are typically used as the baseline: 30-150kV refers to IEC60840, and >150kV up to 500kV refers to IEC62067. When the voltage level is increased to 750kV, it exceeds the upper limit of IEC62067, and engineering practice often involves extended verification and consistency comparison within the existing standard framework.

[0003] For thick-insulated AC systems, the mainstream approach is to adopt a three-layer co-extrusion system of "inner semi-conductive shield - insulation layer - outer semi-conductive shield" on the structural side to obtain a smooth, continuous interface and homogenize the electric field; on the process side, VCV vertical and CCV horizontal continuous vulcanization lines are used in parallel, with the support of high-purity materials and clean manufacturing, online thickness measurement and eccentricity monitoring, degassing and thermal history management.

[0004] In public practice approaching the target level, representative approaches include: on the one hand, using a large cross-section, thick insulation homogenization design for 500-550kV levels, relying on mature accessory platforms and complete sets of test calibers to achieve system usability; on the other hand, focusing on manufacturing consistency, suppressing tolerances and eccentricities through vertical vulcanization, automatic head alignment, and process feedback, coupled with highly sensitive partial discharge detection and condition diagnosis. These approaches have a validated basis at the 500kV level, but directly extrapolating to higher levels requires a reassessment of the process window and validation criteria.

[0005] For the 750kV level, the increased thermal resistance and temperature rise gradient caused by thick insulation, the sensitivity of space charge to geometric eccentricity and interface micro-defects, the radial non-uniformity of crosslinking and crystallization during the extrusion-cooling stage, and the lack of a unified test caliber pose objective challenges for further engineering. Summary of the Invention

[0006] Technical problems to be solved Regarding electric-thermal field confinement, under comparable IEC / GB standards and unfavorable operating boundaries, the maximum radial field strength E of the insulation layer is controlled. max To an acceptable window, while avoiding thermal resistance R thThe unfavorable rise and the resulting imbalance in the temperature gradient.

[0007] Regarding the uniform pressure distribution between the attachment and the body, it suppresses the local field concentration caused by geometric eccentricity, interface micro-defects, and scale enlargement; under the constraint of the same outer diameter or attachment diameter, it reduces the variance of thickness distribution and material redundancy.

[0008] In terms of manufacturing consistency and closed-loop control, ensuring thickness distribution t(r) / {t i The manufacturability and reproducibility of the material ensure that the offline design and online measurement and control, traction and cooling process parameters are kept consistent, and that the eccentricity e and layer thickness tolerance are stably and verifiably controlled.

[0009] In addition, the conductor roundness and the contact quality of the semiconductive buffer layer-metal sheath interface need to be constrained to reduce the probability of partial discharge and thermo-electric coupling instability.

[0010] Technical solution This invention focuses on systematically optimizing "thickness function / zoning, online closed loop, accessory collaboration, and material and process purity" to form a set of technical features that can be directly mapped to production lines and type test calibers.

[0011] At the structural design level, a radial thickness function t(r) or at least two segmented thicknesses {t} is used. i A coaxial three-layer co-extrusion system with} as its core. By establishing an electro-thermal coupled field model, E max / E allow Insulation thermal resistance R th Incorporating indicators such as thickness variance into a unified objective function, and constraining the boundary consistency of eccentricity e, temperature rise ΔT, and the uniform pressure geometry of the attachments, thereby obtaining a result that satisfies E max ≤α·E allow The optimal thickness allocation scheme is determined. As an optional implementation, the thickness ratio of the three zones can be set to 1.00:(0.97±0.02):(0.94±0.02), and the outer / inner radius ratio b / a can be in the range of 1.9-2.4. This allows for maintaining necessary thickness margin in the near-conductor region and reducing redundancy in the outer layer, thereby balancing electric field uniformity, thermal resistance, and the volume V of the insulating material. ins (or its equivalent material usage).

[0012] At the method design level, the optimization results are discretized into no fewer than 8 control nodes, forming a parameterized bridge for consistency between "design-manufacturing-verification". The production line is equipped with dual-channel (X-ray / infrared) online thickness measurement, with sampling at least 200Hz, establishing a closed loop of "thickness measurement data - eccentricity calculation - controller - traction / cooling actuator". The control strategy assigns higher weight to the near-conductor region and implements stricter tolerance constraints (preferably ±0.3mm) on the inner layer to suppress interference with the e-coating. max Sensitive conductor-side eccentricity error. Through this mapping, the online deviation can be converted into real-time linkage adjustment of traction speed, cooling curve and extrusion amount, so that e stably meets the factory judgment of no more than 3%, and preferably converges to the engineering target of no more than 2.5%.

[0013] At the system and manufacturing levels, collaborative boundaries are established in materials, die head and crosslinking processes, conductors and interface engineering. The insulating material uses a combination of a low-degassing peroxide crosslinking system and high-molecular-weight multi-branched linear polyethylene resin. Near-Class 100 ultra-clean environments and full-process purification management are implemented in key feeding stages to reduce the impact of byproducts, micropores, and foreign particles on space charge and partial discharge threshold. The three-layer co-extrusion uses an automatic centering (CPC) die head, combined with VCV vertical crosslinking and rapid locking "super crosslinking" to reduce gravity sagging and initial deformation. On the conductor side, central unit segmentation and multi-stage segmented compression, combined with self-centering rolling pressure rollers for shaping, improve roundness accuracy to approximately 0.2mm, reducing micro-protrusions on the electric field initiation surface. The accessory transition uses equalization geometry as the boundary input, limiting t(r) / {t i The feasible domain of the semiconductive buffer layer is defined; a composite system is adopted and the dispersion and pre-tightening of conductive fillers are optimized to control the contact resistance with the corrugated aluminum sheath to no more than 8Ω, thereby suppressing the risk of local overheating and discharge caused by interfacial thermo-electric coupling. The above system features, as subordinate or preferred solutions, provide manufacturability and long-term reliability support for the main structure-method line.

[0014] In terms of verification and criteria, the standards are aligned with IEC comparable standards and internal controls with higher sensitivity are introduced. Type approval and qualification items are implemented according to the IEC 62067 / 60840 framework, employing a multi-stress test combination (power frequency, shock superposition, thermal cycling, and partial discharge). A standardized enterprise standard for partial discharge sensitivity and voltage trajectory is established: the voltage is increased to 1.75×U0, held for 10 seconds, then decreased to 1.5×U0 for PD measurement (PD≤5pC). An A / B comparison using the "equal thickness baseline - optimized scheme" is used to output E. max / E allow R th The statistical change in the volume of insulating material Vins (or its equivalent material usage) and the e-distribution ensures a closed loop of evidence for design-manufacturing-verification.

[0015] Beneficial effects Under the synergistic effect of the above-mentioned technical features of "thickness function / partitioning - online closed loop - accessory collaboration - material and process purity", the effect of this solution in engineering applications is reflected in the synchronous convergence of electric field and thermal field, manufacturing consistency and system-level compatibility; the following effects are for illustration and are not intended to limit the claims.

[0016] 1. Synchronous convergence of electric and thermal fields By implementing controlled thickening in the near-conductor region and slight thinning in the outer layer, a thickness function / zoning design is employed, combined with high conductor roundness and smooth three-layer interface, to suppress the amplification effect of microscopic field strength. Under the same outer diameter or accessory aperture constraint, the ratio of maximum radial field strength to allowable peak field strength (E) is minimized. max / E allow Compared to a uniformly thick baseline, this can reduce (for example, approximately 8-15%) (the example window α is preferably no greater than 0.90). For example, in the 435 / 750kV-1×3000mm shown in Table 1... 2 t nom Under a uniform thickness baseline of 38.0 mm (see Table 2), baseline E max =17.1kV / mm, corresponding to E max / E allow =0.90 (where E allow The point values ​​are based on Table 5, and consistency verification is performed under the comparable standards for power frequency, impact, and partial discharge listed in Table 3; after optimization, this ratio can be further reduced (see example). Figure 2 As a publicly available example of optimized output, with a geometric diameter of a=36.85mm and b=74.85mm, the thicknesses of the three sections are t1=13.06mm, t2=12.66mm, and t3=12.28mm, and the dividing radius is r. β1 =49.91mm, r β2 =62.57mm, corresponding to simulation output E max ≈15.8kV / mm, R th ≈0.32 K·m / W, V ins ≈1.33×10 -2 m 3 / m (all are example diameters for easy reproduction and verification). Under the same geometric diameter, the thickness of the three sections can also be selected as t1=13.20mm, t2=12.66mm, and t3=12.14mm, with the dividing radius taken as r. β1 =50.05mm, r β2 =62.71mm, corresponding to simulation output E max ≈15.7kV / mm, R th ≈0.32 K·m / W, V ins ≈1.33×10-2 m 3 / m (all are example specifications for easy reproduction and verification). Where E max Take the radial field strength at r=a, and compare it with E in Table 2. max Same caliber, V ins Press V ins =π(b 2 -a 2 Calculate R th The boundary conditions and thermal parameters are described in the following table. Figure 3 With Appendix A, R th Press R th =ln(b / a) / (2πλ) is used for calculation, where λ is the thermal conductivity of the insulating material, taken from the company's material data sheet or obtained by testing according to current national / industry standards (e.g., steady-state method or transient method). Simultaneously, the insulation thermal resistance R... th Without increasing or with a controlled increase (e.g., no more than 5%), the operating hotspot temperature difference and temperature rise gradient are kept within an acceptable range. This effect stems from the targeted redistribution of equipotential line concentration areas by t(r), and the coupling constraint of thickness variance and temperature rise ΔT. The mechanism is clear and the criteria are measurable.

[0017] Manufacturing consistency and eccentricity robustness A parameterized mapping and real-time closed loop were established for the “design node-online thickness measurement-traction / cooling” process. The inner layer t1 was set with a stricter thickness tolerance (preferably ±0.3mm) and a higher control weight, which enabled rapid correction of transient disturbances during cabling and cross-linking. The three sigma of the eccentricity e was significantly reduced, and the engineering target was optimized to be stable to no more than 2.5%. Correspondingly, the partial discharge PD was tested according to the stricter enterprise PD trajectory (increased to 1.75×U0 and held for 10s, then decreased to 1.5×U0 for testing). The partial discharge PD still met the sensitivity threshold of no more than 5pC, verifying the contribution of geometric and interface quality improvements to electrical stability.

[0018] System-level compatibility and long-term reliability Through the constrained coupling of "equalizing geometry-thickness function feasible region," the deviation of equipotential lines in the accessory-body transition zone is reduced. The semi-conductive buffer layer-metal sheath interface employs a composite system and optimized contact structure, controlling the contact resistance to no more than 8Ω, thus mitigating localized overheating and discharge triggering caused by thermo-electric coupling. On the material side, a low-degassing crosslinking system and high-molecular-weight multi-branched resin are used, along with purification and degassing to suppress space charge traps. These improvements maintain stable performance in comparable IEC standards for power frequency, impact, and thermal cycling tests, establishing a replicable engineering path and evidence system for the 750kV level. Attached Figure Description

[0019] The accompanying drawings are for illustrative purposes only and are not drawn to scale. They do not constitute a limitation on the scope of protection of this application. The terminology and parameters are consistent with those in the specification and claims.

[0020] Figure 1 This diagram illustrates the coaxial structure and insulation partitioning of a single-core extruded cable. The conductor 1, conductor shielding layer 2, and insulation layer 3 are labeled sequentially (determined by the thickness function t(r) or partition thickness {t}). i [Definition], insulation shielding layer 4, metal sheath 5, and outer sheath 6; indicate the inner radius a, outer radius b (b / a within the design window of 1.9-2.4), and boundary radius r. β1 With r β2 The thicknesses t1, t2, and t3 are used to illustrate the monotonically non-increasing distribution of the material from the inside out, characterized by a "thicker inside, thinner outside" shape, to suppress peak electric field strength while also considering thermal resistance. Typical conductor parameters (S≈3000mm) are listed in the figure captions if necessary. 2 d c ≈73.7mm).

[0021] Figure 2 t(r) / {t i A comparison of the radial distribution of E(r) with the optimized scheme and the uniform thickness baseline. The dashed line represents the uniform thickness baseline, and the solid line represents the optimized scheme; the upper part shows the thickness distribution curve (labeled r). β1 r β2 The lower part shows the electric field intensity E(r) curve under the same geometric and boundary conditions, and labels E. max E is indicated by a dashed line. allow And the target coefficient α (for example, α is preferably no greater than 0.90), which is used to visually reflect the peak reduction effect under the same outer diameter / accessory diameter constraints.

[0022] Figure 3 This is for the simulation of electro-thermal coupling, including boundary and mesh convergence. The left figure shows the two-dimensional axisymmetric geometry and boundary: conductor potential V, insulation shield grounded; thermal boundary uses the upper limit of conductor temperature T. c,max With ambient temperature T a It can also list convection / radiation parameters and aperture. The right figure shows the convergence curve: the horizontal axis is the number of grid cells or the smallest cell size, and the vertical axis is E. max The relative rate of change with ΔT is used to show a convergence threshold of no more than 2% to demonstrate the stability and reproducibility of the simulation results.

[0023] Figure 4This is a block diagram of the online thickness measurement closed-loop control system. Thickness data is collected from the online thickness sensor 7 (dual-channel: X-ray / infrared), output to the controller 8 via the eccentricity calculation module, and the controller issues adjustment commands to the traction / cooling actuator 9 according to the strategy of "the weight of the near-conductor region is higher than that of the outer layer". The diagram shows key parameters such as sampling frequency (≥200Hz), number of control nodes (N≥8), and inner layer t1 preferred tolerance (±0.3mm) to illustrate the consistency mapping of design-manufacturing-verification and real-time closed loop.

[0024] Figure 5 The accompanying diagrams show a comparison of equipotential lines / isotherms in the transition zone of the main body (equal thickness baseline and optimized scheme). (a) shows the equal thickness baseline, and (b) shows the optimized scheme. The curves are distinguished by line type. The uniformity of the equipotential lines in the transition zone is compared under the same attachment geometry and boundary conditions, with engineering thresholds noted (maximum deviation of equipotential lines not exceeding 5%). If necessary, the enterprise-specific caliber for partial discharge detection is indicated in the figure captions (increase to 1.75×U0 and hold for 10s, decrease to 1.5×U0 for measurement, PD≤5pC), to demonstrate the correlation between boundary consistency and local high-field suppression.

[0025] Explanation of reference numerals in the attached figures 1-Conductor; 2-Conductor shielding layer; 3-Insulating layer [t(r) / {t] i}〕;4-Insulating shielding layer;5-Metal sheath;6-Outer sheath;7-Online thickness sensor;8-Controller;9-Traction / cooling actuator。

[0026] Symbol and parameter descriptions a - Inner radius of insulation; b - Outer radius of insulation; b / a - Ratio of outer to inner radius of insulation; Boundary radius: r β1 ,r β2 / mm: depends on b / a and {t i} Proportion; t1, t2, t3 - zone thickness; t(r) - radial thickness function; E(r) - radial electric field intensity; E max - Maximum radial electric field strength at the interface between the conductor shielding layer and the insulation layer; E allow - Permissible peak field strength (point values ​​are based on Table 5); α - Peak coefficient; e - Eccentricity; N - Number of control nodes; ΔT - Temperature rise or conductor-ambient temperature difference; ΔT allow - Permissible temperature rise threshold; T c,max - Upper limit of conductor temperature; T a - Ambient temperature. Detailed Implementation

[0027] Terms and Symbols For ease of understanding, the following standardized terms and symbols are agreed upon: U0 represents the rated voltage of the cable relative to ground (U0≈435kV, approximately equal to Ur / √3); Eallow This indicates the permissible peak field strength (its point value can be determined based on the material's AC breakdown margin statistics and combined with the company's internal control coefficients; see Table 5 for examples, and consistency verification should be performed under the comparable calibers of power frequency, impact, and partial discharge listed in Table 3); E max The maximum radial electric field intensity is represented at the interface between the conductor shielding layer and the insulating layer (r=a); α is the peak value coefficient; t(r) is a function of the radial thickness of the insulating layer; {t i} represents the set of partition thicknesses; n is the number of partitions; Var[t(r)] is the thickness variance index; e is the eccentricity, e max The eccentricity control threshold; ΔT is the temperature rise or the conductor-ambient temperature difference. allow The allowable temperature rise threshold; V ins This refers to the volume of the insulation material or an equivalent index related to the amount of insulation material used; b / a is the ratio of the outer radius to the inner radius of the insulation; R th The insulation thermal resistance (calculated per unit length using a steady-state radial thermal conduction equivalent model; boundary conditions are detailed below) is... Figure 3 ); λ is the thermal conductivity of the insulating material, taken from the company's material data sheet or obtained by testing according to current national / industry standards; d c The outer diameter of the conductor shield.

[0028] Design Boundaries and Parameter Specifications This implementation method clearly defines the boundary conditions for consistency between "design-manufacturing-verification" during the project initiation phase. Geometrically, it assumes a large cross-section conductor configuration, typically taking the conductor shield outer diameter as approximately d. c ≈73.7mm (cross-section S≈3000mm) 2 ), nominal insulation thickness t nom =38.0mm, with a structural ratio b / a within the manufacturable range of 1.9-2.4 (preferably 2.0-2.3; typical value approximately 2.03). Based on the coaxial structure diameter and rated phase voltage U0, the maximum electric field strength E at the conductor shielding interface is... max It is approximately 17.1 kV / mm, which can be used as a reference value for equal-thickness baselines.

[0029] In terms of materials, a relative permittivity is adopted. Not greater than 2.35, dielectric loss tangent tanδ not greater than 5×10 -4 An XLPE system with a gel content ≥90% (50Hz, 20°C), wherein one preferred material is... Typically, the tanδ is located in the range of 2.11–2.23, and the tanδ is typically located in the range of (2.1–2.6) × 10⁻⁶. -5This meets the requirements of ultra-clean extruded insulation for low loss and high resistivity. In terms of process control, a dual-channel online thickness measurement and eccentricity monitoring system with a resolution of 0.01mm is introduced, with a sampling frequency of no less than 200Hz. The number of control nodes is set to N≥8 to match the subsequent discretization of the thickness function.

[0030] Regarding quality objectives, the factory-determined eccentricity value should not exceed 3%, and the engineering convergence target is preferably not greater than 2.5%. The partial discharge test voltage can be given as a multiple of the rated voltage U0 relative to ground, preferably measured at approximately 1.5 × U0 (approximately 650 kV), and a voltage trajectory of rising to 1.75 × U0, holding for 10 seconds, and then decreasing to 1.5 × U0 can be used, with a sensitivity criterion not exceeding 5 pC. Type and qualification tests can be organized according to comparable IEC / GB standards, such as a power frequency withstand voltage of 2U0 for 60 minutes; lightning impulse and switching impulse tests are determined according to the highest system voltage Um used. These standards constitute the common boundaries and acceptance windows for all embodiments.

[0031] For ease of understanding and reproduction, the following provides the structural dimensions, baseline electric field distribution, and comparable test voltage aperture of a preferred engineering embodiment for reference (see Tables 1 to 3).

[0032] Table 1 ZC-YJLW03 435 / 750kV-1×3000mm 2 Cable structure dimensions (example data) Table 2 435 / 750kV-1×3000mm 2 Cable insulation electric field strength distribution under rated phase voltage U0 (example data) (where E max Take the position r=a, E min (At point r=b) Table 3. Comparable Diameter Test Voltage Reference for 435 / 750kV Cable Systems (Example Data) Structural Example A: Weighted Thickness Allocation in Two Zones like Figure 1 As shown, the single-core extruded cable in this embodiment adopts a coaxial structure and performs partitioned modeling of the insulation thickness.

[0033] This embodiment aims to reduce peak electric field strength without significantly increasing thermal resistance. The insulating layer is divided into an inner layer t1 and an outer layer t2, with a boundary radius... ,in Located between 1.6 and 2.0; the preferred layer thickness ratio is t1:t2 = 1.00:0.92-0.98. Controlled thickening of the near-conductor region is used to reduce E. max The far conductor region is slightly thinned to offset the increase in thermal resistance caused by the increase in thickness, and the overall outer diameter is maintained while keeping the accessory diameter unchanged.

[0034] In terms of manufacturing mapping, t1 is set as a "high-weight layer," assigning stricter thickness tolerances (preferably ±0.3mm) and higher node weights in online control. When online thickness measurement detects t1 deviating from the threshold, the control system prioritizes adjusting the traction speed and cooling curve to bring thickness fluctuations back to the set range within a single cycle. With this allocation strategy, finite element verification can achieve E under the most unfavorable boundary conditions. max / E allow The target is ≤α, and ΔR th Maintain within a tolerance range of 0-5%.

[0035] Structural Example B: Three-zone thickness and transition optimization In scenarios requiring a finer-grained balance between electric and thermal fields, a three-part structure {t1, t2, t3} and a boundary radius r are adopted. β1 ,r β2 The thickness ratio is set at 1.00:0.97±0.02:0.94±0.02, and the geometric ratio b / a remains in the range of 1.9-2.4. The obtained optimal solution t*(r) is discretized into N≥8 control points with equidistant or non-equidistant nodes, and mapped to a parameter table of extrusion amount, traction speed segment, and cooling curve; the near-conductor nodes are given higher weights to obtain a faster control response when disturbances occur.

[0036] The conductor structure employs a central unit segmentation followed by multi-stage, segmented compaction and self-centering rolling roller shaping, achieving a roundness accuracy consistently around 0.2mm to suppress microscopic field concentration at the initial surface. Electro-thermal coupling simulations and A / B comparisons conducted using this scheme show that, under the same accessory aperture and outer diameter constraints, E... max / E allow The target of ≤0.90 can be satisfied in parallel with no increase in thermal resistance.

[0037] Structural Example C: Parameterized Implementation of a Continuous Function t(r) like Figure 2 As shown, the parameterized implementation of the continuous function t(r) can be used to obtain the electric field distribution and thickness assignment of the target.

[0038] When it is necessary to maintain both peak clipping and thermal resistance under control over a wider operating range, the insulation thickness is expressed as a continuous function t(r) = t0 + β·r. n(n is between 0.8 and 1.6) or piecewise spline function. The discretization process constructs a control set according to the node allocation principle of "high density in the near-conductor region and low density in the outer layer"; thickness measurement data is acquired at a fixed sampling frequency on the production line side, and the data is processed by the eccentric calculation module and then input into the controller. The controller performs weighted adjustments on the traction and cooling actuators to realize the process reproduction of the thickness function.

[0039] This embodiment achieves boundary consistency with the attached equalization geometry through feasible region constraints, ensuring that the equipotential lines in the transition region deviate within the engineering threshold.

[0040] Materials and Interface Implementation The insulating material employs a combination of a low-degassing peroxide crosslinking system and high-molecular-weight multi-branched linear polyethylene resin. This approach reduces crosslinking byproducts and optimizes crystal morphology to suppress space charge and micropore formation. To achieve cleanliness and interface quality, the critical feeding process is conducted in a near-Class 100 clean environment. The three-layer co-extrusion die head undergoes surface treatment and flow channel optimization to create a smooth and continuous interface between the inner and outer semiconductive shielding and the insulating layer, effectively reducing the risk of spike field strength caused by interface micro-protrusions and air gaps.

[0041] Table 4 Electrical properties of different ultra-clean XLPE insulation materials (50Hz, example data) Table 5. AC breakdown statistics of XLPE-1 insulation samples (example data; from company test data, for E...) allow Point value basis) Note: Table 5 presents example statistical results under the company's material and sample treatment conditions, used to illustrate the breakdown margin of the material system; the technical effects of this application are expressed in terms of E... max / E allow R th The consistency criterion with manufacturing is the main evaluation criterion.

[0042] Note: In the model cable margin test, based on the statistical data, the average breakdown field strength of the imported material sample was about 51 kV / mm, and the average breakdown field strength of the self-made material sample was about 45 kV / mm (see Table 5); the results are used to illustrate that the selected material system has a high electrical margin, and are not intended to limit the claims.

[0043] The crosslinking process preferably uses a VCV vertical line, coupled with an automatic centering (CPC) die head and a fast-locking "super crosslinking" window to reduce eccentric drift caused by gravity sagging and initial deformation. This material-interface-crosslinking path serves as the manufacturability support for the main solution and provides parameter windows and alternative options in subordinate implementations.

[0044] Conductor and Cable Assembly Implementation The conductor is divided into central units and a segmented combination compression mold is introduced during the cabling process. Through multi-level, segmented shaping, the fan-shaped strands are gradually brought closer to the target profile. With the help of self-centering rolling pressure rollers, dynamic elastic adjustment is achieved to avoid excessive deformation in a single step, which would introduce unnecessary internal stress.

[0045] This process improves the conductor roundness accuracy from approximately 0.4 mm to approximately 0.2 mm, significantly reducing the influence of conductor surface geometric undulations on the initial equipotential surface and suppressing the occurrence of microscopic field strength concentration points from the source.

[0046] Online thickness measurement and closed-loop control implementation method like Figure 4 As shown, the online thickness measurement, eccentricity calculation, controller, and traction / cooling actuator constitute a closed-loop control system.

[0047] The online thickness measurement system adopts a dual-channel architecture and achieves a resolution on the order of 0.01 mm. The data enters the eccentric calculation module and is then sent to the industrial controller. The controller generates adjustment commands for the traction and cooling actuators based on the control law that "the weight of the near-conductor region is higher than that of the outer layer".

[0048] Control nodes are configured with N≥8, and inner nodes are set with tighter tolerance bands. When thickness or eccentricity exceeds the limits, an abnormal handling sequence is triggered: priority is given to slowing down to steady state, adjusting the cooling curve, and verifying the extrusion flow rate. If necessary, a short-term line stop is performed for reset. This closed loop uses the production cycle time as the clock reference, ensuring that the thickness field and eccentricity remain repeatable and statistically stable under large-size structures.

[0049] Production line and process window implementation The production line adopts a serial process of "conductor drawing and stranding - three-layer co-extrusion - cross-linking - cooling - semi-conductive buffer layer wrapping - metal sheath - outer sheath", with three-layer co-extrusion being the core process. The cross-linking section achieves a stable cross-linking degree gradient through high-pressure nitrogen circulation and a dual closed-loop system of temperature and pressure; the cooling section is segmented according to the radial temperature gradient to avoid residual stress caused by thermal history; the metal sheath can be a corrugated aluminum sheath or a flat aluminum sheath, and the subsequent outer sheath can be integrally formed with the metal sheath.

[0050] The entire process involves a closed-loop inspection system with multiple checks from raw material intake, online extrusion, and semi-finished and finished products to ensure consistency between process quality and end-point inspection.

[0051] Annex Collaboration and Transition Zone Design like Figure 5 As shown in the appendix, the comparison of equipotential lines / isotherms in the transition zone of the body is used to evaluate boundary consistency.

[0052] Annex-ontology consistency is achieved by introducing equalizing geometric boundaries into the optimization solution. Annex geometric parameters are used as constraint inputs, limiting t(r) / {t...} i The feasible region of the transition section is defined, and the deviation of the equipotential lines is controlled within a set threshold. This boundary is synchronously mapped to manufacturing parameters and acceptance criteria to ensure that the electric field remains continuous and uniform under assembly and operating conditions.

[0053] Implementation of Semiconductor Buffer Layer and Sheath Interface The semi-conductive buffer layer employs a composite material system, and its volume resistivity stability under electric field and temperature is optimized through the type, particle size, and dispersion state of conductive fillers. Simultaneously, dense gold-plated copper wire is incorporated to improve electrical contact with the corrugated aluminum sheath. The winding process utilizes preload and overlap ratio to create a repeatable mechanical and electrical bond strength.

[0054] With this combination, the contact resistance at the buffer layer-sheath interface is reduced from approximately 20 Ω to no more than 8 Ω, thereby reducing the risk of discharge caused by interface current concentration and local overheating.

[0055] Experimental and Verification Implementation Methods like Figure 3 As shown, the boundary conditions and mesh convergence curves of the two-dimensional axisymmetric electro-thermal coupling model are used to prove the stability of the calculation results.

[0056] Simulation verification uses a two-dimensional axisymmetric model. Temperature-related material parameters can be obtained from the company's material data sheet or by testing according to current national / industry standards (including λ). The mesh is refined in the near-conductor and interface regions. The convergence criteria for the mesh and the solution are respectively based on E. max The threshold is a relative change of no more than 2% with respect to ΔT. Type approval and qualification are conducted simultaneously according to comparable IEC standards, using an A / B comparison with an "equal thickness baseline-optimized scheme," and the output E... max / E allow R th Volume V of insulating material ins The statistical change in the distribution of (or equivalent material usage) and e (where E max Take the position r=a, R th With V ins The calculation and extraction criteria are consistent with Table 2. Figure 3 (Consistent with Appendix A). For the power frequency withstand voltage, partial discharge, and impulse test voltages of the 435 / 750kV system, refer to the comparable diameters listed in Table 3. Simulation inputs include geometric dimensions a, b, and thickness distribution t(r) / {t i Material parameters With λ and electrical and thermal boundary conditions; post-processing extracts E according to r=a maxR is calculated according to the steady-state radial thermal conduction equivalent model. th At the same time, calculate the volume V of the insulating material per unit length. ins It is used for comparison with an equal-thickness baseline.

[0057] Partial discharge was measured using a trajectory of increasing to 1.75 × U0, holding for 10 seconds, and then decreasing to 1.5 × U0. The sensitivity criterion was no greater than 5 pC, and the test was repeated after thermal cycling. This verification path ensures that the design objectives are measurable, reproducible, and traceable. The test can be organized and implemented according to the comparable IEC / GB standards listed in Table 3, and the statistical results of partial discharge, indicators before and after thermal cycling, and geometric consistency should be recorded for E max R th Verify the effectiveness of manufacturing consistency control.

[0058] Alternative Implementation and Equivalent Solutions Without deviating from the inventive concept of "thickness function or partitioning - online closed loop - accessory coordination - material and process purity", piecewise linear or spline family functions can be used to replace the power function form t(r); the number of control nodes and sampling frequency can be adjusted while maintaining the principle of high weight in the near-conductor region; when production line conditions are limited, the eccentricity and layer thickness tolerance can be maintained under CCV line by strengthening the head alignment and cooling curve constraints; the specific materials and structures of the metal sheath, outer sheath, and semi-conductive buffer layer can also be conventionally replaced. The above alternative schemes constitute equivalent transformations to the main scheme.

[0059] Feasibility and Reproducibility Description To facilitate reproduction and examination verification, the "Input Parameter Freeze Table" shown in Appendix A can be referenced in mass production. This table is used for reproduction verification and consistency control, and does not constitute a limitation on the claims. It includes E allow α, b / a, e max , , tanδ, {t i} or t(r) parameter, {r βi The table includes parameters such as the number of nodes N and the sampling frequency. These parameters define the input boundaries of the embodiment and facilitate reproduction and verification. Material electrical and thermal parameters can be obtained from material data sheets or through testing according to current national / industry standards. The boundary conditions and index extraction criteria for electro-thermal coupling simulation are detailed in [link to table]. Figure 3 The test voltage trajectory and criterion are shown in Table 3. The above descriptions are merely some embodiments of this application, used for illustration and not limitation; all equivalent modifications within the scope of the claims should be covered within this application.

[0060] Appendix A Input Parameter Freeze Table (Reproduction Verification Standard) This appendix serves to solidify the input criteria and boundary conditions for the consistency of the "design-manufacturing-verification" process in this application. The parameters in the table are given in the form of frozen values / ranges, representing the parameter caliber of the preferred embodiments of this application; they can be equivalently adjusted according to the type verification data and operational boundaries of specific projects, and do not constitute a limitation on the claims. The parameters in the table are used to define the input boundaries of the embodiments and facilitate reproduction and verification; the electrical and thermal parameters of the materials can be obtained from material data sheets or by testing according to current national / industry standards. The boundary conditions and index extraction criteria for electro-thermal coupling simulation are detailed in [link to relevant documentation]. Figure 3 The test voltage trajectory and criterion are shown in Table 3.

[0061]

Claims

1. A cross-linked polyethylene cable for a 750kV AC voltage rating, comprising a conductor, a conductor shielding layer, an insulation layer, an insulation shielding layer, a metal sheath, and an outer sheath arranged coaxially in sequence, characterized in that: The radial thickness of the insulating layer is determined by a continuous thickness function t(r) or a set of partitioned thicknesses {t}. i } is defined, wherein the set of partition thicknesses {t} i } (i=1...n, n≥2) corresponds to the thickness of n concentric partitions from the near conductor side outwards, and satisfies t1≥t2≥...≥t n This is to make the insulating layer relatively thicker near the conductor and gradually thinner away from the conductor; the inner radius of the insulating layer is a, and the outer radius is b.

2. The cable according to claim 1, wherein n ≥ 3, and at least one dividing radius r is provided between the partitions. βi Used for t1, t2, ..., t n The partition thickness is defined radially discretely.

3. The cable according to claim 2, wherein when n=3, the partition thickness satisfies t1:t2:t3 = 1.00:(0.97±0.02):(0.94±0.02).

4. The cable according to claim 1, wherein the geometric ratio b / a is located in the range of 1.9-2.4, preferably in the range of 2.0-2.

3.

5. The cable according to claim 1, wherein the conductor shielding layer, the insulation layer, and the insulation shielding layer are formed by three-layer co-extrusion to create a smooth, continuous interface, and the relative permittivity of the insulation material is... Not greater than 2.35, dielectric loss tangent tanδ not greater than 5×10 -4 (50Hz, 20°C), volume resistivity not less than 1.0×10⁻⁶ 14 Ω·m, gel content not less than 90%.

6. A method for optimizing and manufacturing the insulation thickness of the cable as described in claim 1, characterized in that, include: S1, establish an electro-thermal coupling model of the coaxial structure, with t(r) or {t i } represents design variables; S2, constructing with (E max / E allow ), R th Var[t(r)] and the volume V of the insulating material ins To optimize the objective function of the index, the constraint e ≤ e is applied. max ΔT ≤ ΔT allow b / a is located at 1.9-2.4 (preferably 2.0-2.3) and the consistency of the pressure equalization boundary and manufacturing feasibility constraints of the accessories; S3, solve for the thickness distribution and discretize it into several control nodes to establish the parameter mapping between thickness distribution and online thickness measurement-traction-cooling; S4. Based on the power frequency, impact, thermal cycling and partial discharge test results of comparable IEC / GB standards, the thickness allocation and process control thresholds are verified, and a set of consistent parameters for mass production is formed.

7. The method according to claim 6, wherein the partial discharge is performed according to the following voltage trajectory: the voltage is increased to 1.75×U0, held for 10s, and then decreased to 1.5×U0 for measurement, with a sensitivity criterion of no more than 5pC.

8. The method according to claim 6, wherein the online thickness measurement adopts a dual-channel combination of X-ray and infrared, with a resolution of 0.01 mm, a sampling frequency of not less than 200 Hz, and the control node N is 8-16, and the thickness tolerance of the near-conductor side node is set to be stricter than that of the outer layer.

9. A production and control system for implementing the method of claim 6, comprising a dual-channel online thickness sensor, an eccentricity calculation module, an industrial controller, and a traction / cooling actuator, wherein the controller performs real-time closed-loop adjustment of the traction speed and cooling curve based on thickness measurement and eccentricity data, and maps the control node deviation of thickness allocation to the adjustment amount of traction speed and cooling parameters.

10. The system of claim 9, wherein the controller is adjusted according to a strategy of "higher weight for near-conductor regions than for outer layers", and the peripheral equalization geometry parameters are used as boundary inputs to limit {t}. i r βi The feasible region of}.

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