Water-tree-resistant modified insulation process for cross-linked polyethylene aerial insulated cable

Abstract

The invention relates to the technical field of cross-linked polyethylene overhead insulated power cables, and discloses a water-tree-resistant modified insulation process for a cross-linked polyethylene overhead insulated cable, which comprises the following steps: mixing a cross-linked polyethylene matrix and a polar modifier to generate a modified melt, introducing the modified melt into an extrusion flow channel provided with a convergence section, and carrying out extrusion molding to obtain the cross-linked polyethylene overhead insulated power cable. The extrusion resistance generated by the change rate of the convergence angle of the convergence section is utilized to induce the polar modifier to generate a layered grain orientation structure in the matrix, the layered grain orientation structure is frozen through gradient cooling, the cross-linking reaction heat is utilized to drive the polar modifier and the matrix to generate molecular chain segment interpenetration, and an interface cross-linked network is formed on a spherocrystal interface. According to the method, localized distribution of the polar modifier is realized by utilizing geometric constraint of an extrusion flow field, a continuous physical barrier is constructed in a matrix spherocrystal gap, a migration path of water molecules is blocked, a micro space for inducing water tree initiation is eliminated, and the aging resistance and charge purity of a cable insulation system are synergistically enhanced.

Description

A water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables Technical Field

[0001] This invention belongs to the technical field of cross-linked polyethylene overhead insulated power cables, and particularly relates to a water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables. Background Technology

[0002] Currently, cross-linked polyethylene (XLPE) possesses excellent dielectric properties and mechanical strength. Modification with polar additives is a common method to ensure the long-term operational stability of medium- and high-voltage overhead insulated cables. Especially in coastal and rainy areas with high humidity, overhead cables are exposed to atmospheric circulation and alternating humidity environments for extended periods. Microscopic cracks on the insulation surface easily attract condensate, and water treeing aging induced by moisture penetration has become a key factor limiting the service life of overhead lines. In existing insulation extrusion processes, the modifying component mainly enters the extruder through physical blending with the base material, utilizing its polar groups to construct charge traps to inhibit water treeing aging induced by moisture penetration. However, this process is often... Due to the randomness of melt thermal motion, the modified components in conventional extrusion molding are spatially discrete. During the process of melt leaving the die and cooling down to solidify, due to the lack of an effective physical induction mechanism, a large amount of polar modifier remains inside the crystalline region of polyethylene, rather than directionally accumulating in amorphous regions such as spherulite interstices where moisture can easily migrate. If the modifier content is simply increased, the polar groups in the crystalline region will destroy the lattice regularity, leading to an increase in dielectric loss and a decrease in thermal stability of the insulation layer. If a low content distribution is maintained, it is impossible to build a continuous physical barrier in the amorphous region, causing the anti-water tree performance to degrade more rapidly with the extension of cable operating time.

[0003] To address this contradiction, the industry has attempted to stabilize component distribution through chemical grafting or the addition of compatibility agents. However, in continuous production processes spanning several kilometers, the consistency of chemical reactions is highly susceptible to temperature fluctuations and shear history differences across different sections of the extruder. This makes it difficult to control the uniformity of the longitudinal structure of the insulation layer and fails to fundamentally alter the topological shrinkage tendency of the components at the microscopic level. While auxiliary hardware structures improve the operating environment, they cannot compensate for defects in the material's microstructure. Chinese invention patent CN120933708B discloses a cross-linked polyethylene insulated control cable, which uses a calcium chloride drying block inside the insulation shell in conjunction with a mechanical anti-loosening device to suppress moisture intrusion from the external environment. This solution is a passive defense, increasing the mechanical complexity and maintenance costs of cable accessories. It cannot intervene in the spontaneous topological shrinkage of modified components during extrusion molding and is difficult to construct a continuous molecular-level barrier within the microscopic spherulitic interstices of the insulation matrix.

[0004] Therefore, how to utilize the stress field distribution characteristics in the extrusion process to drive the modified components to undergo directional evolution and construct a layered, localized distribution structure is the technical problem to be solved by this invention. Summary of the Invention

[0005] This invention provides a water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables, comprising the following steps:

[0006] Step S1: The cross-linked polyethylene matrix is ​​melt-mixed with a polar modifier to generate a modified melt;

[0007] Step S2: The modified melt is introduced into an extrusion channel with a convergence section. The extrusion resistance is set by the rate of change of the convergence angle of the convergence section. A non-uniform shear strain field is formed in the extrusion channel by the extrusion resistance. The non-uniform shear strain field is used to limit the shear deformation of the polar modifier in the cross-linked polyethylene matrix, so as to generate a layered orientation structure in the cross-linked polyethylene matrix.

[0008] Step S3: Adjust the outlet temperature and traction rate of the extrusion channel, perform gradient cooling on the modified melt leaving the die, and according to the temperature drop gradient between the outlet temperature and the cooling medium, perform physical confinement on the polar modifier before the molecular chain elastic retraction occurs, and freeze the layered orientation structure in the spherulite gaps of the cross-linked polyethylene matrix.

[0009] In step S4, during the crosslinking heating stage, the heat of the crosslinking reaction drives the polar groups contained in the polar modifier to migrate and localize to the amorphous region of the crosslinked polyethylene matrix. The heat of the crosslinking reaction also induces the polar modifier and the crosslinked polyethylene matrix to perform interpenetration of molecular chain segments on both sides of the interface, anchoring the polar modifier in a frozen state to the spherulite interface of the crosslinked polyethylene matrix, generating an interfacial crosslinking network, and restricting the polar groups from entering the interior of the grains of the crosslinked polyethylene matrix.

[0010] Preferably, in step S2, the shear rate of the non-uniform shear strain field is controlled at... to The convergence angle of the convergence section is set to 15° to 45°. The process adjusts the convergence angle according to the real-time viscosity fluctuation of the modified melt to adjust the extrusion resistance, maintain the constant shear work of the polar modifier in the amorphous region, control the geometric deviation of the layered orientation structure in the axial and circumferential directions of the cable, and reduce the molecular chain movement space of the polar modifier during pressure release.

[0011] Preferably, in step S3, the cooling rate of the gradient cooling is set to 20°C / min to 50°C / min; the radial thickness of the layered orientation structure in the amorphous region is controlled to be 50nm to 200nm; the layered orientation structure forms a continuous physical barrier inside the spherulitic gaps of the cross-linked polyethylene matrix, blocking the migration path of water molecules and eliminating the microscopic physical space inside the cross-linked polyethylene matrix used to induce water tree germination.

[0012] Preferably, in step S4, the exothermic peak temperature of the crosslinking reaction is controlled between 160°C and 180°C. The thermal activation energy provided by the exothermic peak temperature drives the polar groups to migrate directionally to the weak amorphous region interface. The interface crosslinking network transforms the heterogeneous physical interface into a chemical and mechanical composite network, improving the bonding strength between the layered orientation structure and the crosslinked polyethylene matrix, and limiting the interface delamination caused by electrothermal cycling stress.

[0013] Preferably, the polar modifier includes maleic anhydride-grafted polyethylene, ethylene-vinyl acetate copolymer, and organosilane coupling agent. The amount of the polar modifier added is 1% to 5% of the mass of the cross-linked polyethylene matrix. The polar modifier performs spontaneous orientation arrangement under the action of extrusion resistance according to the difference in molecular chain polarity, thereby enhancing the microscopic bonding force between the layered orientation structure and the cross-linked polyethylene matrix.

[0014] Preferably, in step S2, the convergence angle of the convergence section is adjusted according to the amount of viscosity reduction caused by frictional heating of the modified melt to generate a shear rate increment. This shear rate increment is used to offset the stress loss in the extrusion channel, maintain the shear strain work of the polar modifier in the deformation zone constant, and ensure the uniform distribution of the interfacial crosslinking network along the axial length of the cable.

[0015] Preferably, the method further includes step S5: setting the flow rate ratio between the insulating layer and the adjacent functional layer at the outlet temperature, and using the damping structure provided at the edge of the extrusion channel to suppress the shear stress concentration of the modified melt at the interface, limiting the geometric distortion of the layer orientation structure at the co-extrusion interface between the insulating layer and the adjacent functional layer, and maintaining the physical integrity of the co-extrusion interface.

[0016] Preferably, this gradient cooling is achieved by passing a cooling rate threshold. Real-time control is implemented, and the cooling rate is not lower than the cooling rate threshold. : ,in, The minimum cooling rate required to maintain the frozen state of this layered orientation structure, For material characteristic coefficients, Let be the shear rate of this non-uniform shear strain field. This refers to the dynamic viscosity of the modified melt at the outlet temperature. The relaxation time for the elastic retraction of the molecular chains of this polar modifier is defined as the time required for this process. By adjusting the cooling rate, the shear deformation is ensured to be converted into the penetration force between the molecular chains.

[0017] Preferably, the molecular weight distribution index of the polar modifier is controlled between 2.5 and 4.0. This molecular weight distribution index is used to match the rheological response time of the cross-linked polyethylene matrix under the non-uniform shear strain field, ensuring that the polar modifier and the cross-linked polyethylene matrix undergo synchronous deformation when passing through the convergence section, thereby improving the microstructure distribution density of the interfacial cross-linked network.

[0018] Preferably, step S4 is followed by step S6: performing online heat treatment on the formed cable insulation layer, maintaining the ambient temperature of the cable insulation layer at 60°C to 80°C for 4 to 8 hours, releasing the internal stress remaining during the gradient cooling process, and fixing the localized distribution state of the polar modifier at the spherulite interface.

[0019] Compared with existing technologies, the water-tree resistant modified insulation process for cross-linked polyethylene insulated cables of this invention has the following advantages:

[0020] 1. In cross-linked polyethylene insulated cables, the melt flows through a non-uniformly damped channel. Under the action of radial shear stress gradient, the polar modified components undergo tensile deformation along the flow direction, realizing spatial reorganization from randomly distributed granular to oriented lamellar, constructing a continuous physical barrier network. This physically seals the migration path of moisture in the amorphous region, eliminates the physical space for water tree formation, and reduces the risk of insulation performance degradation during long-term operation.

[0021] 2. By combining the gradient cooling process after extrusion, the volume repulsion force generated by matrix crystallization is used to directionally compress the polar components in the lamellar state to the forming spherulite interface, so that the polar groups are locked in the weak amorphous region, while the charge purity is maintained inside the grain. This localized distribution of components achieves the synergy between aging protection performance and low insulation loss level, and avoids the polar groups from making a negative contribution to the overall insulation performance.

[0022] 3. The geometric impedance of the mold convergence section is used to compensate for the melt viscosity fluctuation. The shear rate increment generated by adjusting the convergence angle is used to offset the stress loss caused by frictional heat generation. The shear strain work of the modified component in the deformation zone is kept constant, ensuring that the geometric shape of the modified layer does not deviate nonlinearly with the temperature fluctuation during the production process. This ensures the consistency of the modified network in the circumferential orientation and axial length during the production of long-length cables. Attached Figure Description

[0023] Figure 1 is a flow chart of the anti-water tree insulation modification process of the present invention, which involves flow field induction and interface anchoring.

[0024] Figure 2 is a block diagram of the insulating extrusion system integrating flow field control and gradient cooling functions of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0026] It should be noted that all directional and positional terms used in this invention, such as: up, down, left, right, front, back, vertical, horizontal, inner, outer, top, bottom, transverse, longitudinal, center, etc., are only used to explain the relative positional relationship and connection between components in a specific state (as shown in the accompanying drawings). They are only for the convenience of describing this invention and do not require that this invention be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention. In addition, the descriptions of "first," "second," etc., in this invention are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated.

[0027] In the description of this invention, unless otherwise explicitly specified and limited, the terms installation, connection, and linking should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections; they can refer to direct connections or indirect connections through an intermediate medium; they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances.

[0028] In the description of this specification, references to the terms "an embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples," 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 present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example, and the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0029] This invention provides a water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables. By physically intervening in the rheological state of the melt during extrusion, a specific spatial distribution of polar modifiers in the insulation matrix is ​​achieved. The process encompasses stages such as modified melt generation, flow-induced lamination and reorganization, gradient cooling morphological freezing, and cross-linking-driven interface anchoring. Through the synergy of flow channel geometric impedance and thermodynamic boundary conditions, a continuous lamination-like physical barrier is constructed in the amorphous region of the overhead insulated cable insulation matrix. This addresses the issue of modified components remaining within the crystalline region of polyethylene due to randomized thermal motion in conventional extrusion processes. To address the issues of increased insulation loss and the inability to effectively intercept moisture migration, this process, through steps S1 and S2, mixes a cross-linked polyethylene matrix and a polar modifier in a co-rotating twin-screw extruder to generate a modified melt. The material temperature in the mixing zone is controlled within a high-viscosity range of 125°C to 132°C, utilizing the shear traction force of the matrix under high viscosity to act on the molecular chains. The generated modified melt is then introduced into an extrusion channel with a convergence section. Within this channel, the rate of change of the convergence angle, varying from 15° to 45°, generates extrusion resistance, causing the modified melt to experience [extrusion resistance] as it passes through this region. to The non-uniform shear rate gradient of the stress field induces the polar modifier to undergo tensile deformation along the flow direction, causing its geometry to change from a microsphere shape to a layered oriented structure.

[0030] The modified melt experiences pressure release the instant it leaves the die. To suppress the spontaneous topological retraction of the polar modifier molecular chains into a point-like distribution, process step S3 is executed; by adjusting the outlet temperature and traction rate of the extrusion channel, a cooling rate of [missing information] is applied to the modified melt leaving the die. to Gradient cooling; in this process, the minimum cooling rate required to maintain the frozen state is determined based on the material characteristic parameters. The specific calculation formula is as follows: ,in, Minimum cooling rate, in units of ; These are the material characteristic coefficients; The shear rate of the non-uniform shear strain field is expressed in units of . ; The dynamic viscosity of the modified melt at the outlet temperature is expressed in units of... ; The relaxation time of the polar modifier during the elastic recoil of the molecular chain is expressed in units of 1. By controlling the actual cooling rate to be greater than or equal to Physical confinement was performed before the polar modifier recoiled, fixing a lamellar-oriented structure with a radial thickness of 50 nm to 200 nm within the spherulitic intergranular spaces of the cross-linked polyethylene matrix; to determine the minimum cooling rate required to maintain the lamellar-oriented structure. The frequency of the mixture of polar modifier and cross-linked polyethylene matrix melt was scanned at 130℃ using a rotational rheometer, and the storage modulus was determined accordingly. With loss modulus angular frequency at intersection The reciprocal determines the elastic recovery relaxation time of the polar modifier molecular chain. The material characteristic coefficients were determined by combining the observation results of the geometric morphology of the layered structure of the sample group. The value is taken as the critical cooling rate during the collapse of the layered orientation structure in the formula. Input parameters to calculate material characteristic coefficients The minimum cooling rate threshold is stored in the minimum cooling rate threshold calculation module, which outputs cooling rate control commands to adjust the cooling water mist flow rate so that the actual cooling rate is not lower than the cooling rate threshold. Physical confinement is performed before the polar modifier molecular chain retracts.

[0031] To reduce the risk of interfacial delamination of physically stacked layered structures due to differences in thermal expansion coefficients under long-term electrothermal cycling stress, step S4 utilizes the exothermic reaction generated during the crosslinking heating stage; by controlling the exothermic peak temperature of the crosslinking reaction at 160°C... Up to 180 Within the region, thermal pulses activate the movement of molecular chain segments on both sides of the interface, driving the frozen polar modifier to interpenetrate with the cross-linked polyethylene matrix, generating an interfacial cross-linked network. This transforms the heterogeneous physical interface into a chemical and mechanical composite network, increasing the bonding strength between the layered structure and the matrix. Polar groups are thus locked at the amorphous region interface, reducing the number of polar groups entering the grain interior, enhancing the aging resistance of the insulation system while maintaining charge purity. In the construction of the polar modifier system, maleic anhydride-grafted polyethylene serves as a reactive component, utilizing its anhydride groups to interact with the free radicals on the surface of the cross-linked polyethylene matrix. The components undergo a coupling reaction, with the ethylene-vinyl acetate copolymer acting as a polar carrier, utilizing its side chains to provide charge traps. The organosilane coupling agent acts as an interface modifier, forming chemical bonds through the functional groups at both ends of the molecular chain. During the melt mixing stage, the components undergo a self-assembly effect through polar attraction under shear heat at 130°C, forming a composite micro-region structure with maleic anhydride-grafted polyethylene as the shell and ethylene-vinyl acetate copolymer as the core. In the non-uniform shear strain field generated by the convergence section, the rheological response time of each component of this composite structure remains synchronized within 0.1s to 0.3s.

[0032] In the continuous production of long-length cables, the frictional heat generated by the extruder die head causes fluctuations in melt viscosity. To ensure the consistency of layer thickness in the circumferential direction, step S2 introduces geometric impedance adaptive compensation logic. The system monitors the viscosity changes of the modified melt through sensors, adjusts the convergence angle of the convergence section to generate an incremental shear rate, and uses this incremental rate to compensate for the stress loss caused by the decrease in viscosity, maintaining a constant shear strain work obtained by the polar modifier in the deformation zone. The convergence section is equipped with a hydraulically driven conical die core to perform axial displacement adjustment of the convergence angle. The die core moves along the central axis of the flow channel under the action of a hydraulic cylinder, and the angle is set in the range of 15° to 45° by changing the radial clearance between the conical surface and the die cavity wall. The central control system obtains the real-time melt pressure based on the pressure sensor at the inlet of the convergence section. Temperature measured by temperature sensor Calculate dynamic viscosity When frictional heating is detected, leading to a decrease in dynamic viscosity, the hydraulic system pushes the die core towards the extrusion die to reduce the radial flow cross-section and increase the extrusion resistance. This offsets the stress loss caused by the decrease in melt viscosity through an increase in shear rate, allowing the polar modifier to maintain shear strain work within the deformation zone. In addition to quantifying the indicators, step S5 sets the outlet flow rate ratio between the insulating layer and the adjacent functional layer at the co-extrusion interface, and uses a damping structure set at the edge of the extrusion channel to suppress the shear stress concentration of the modified melt at the interface, guiding the edge melt to generate radial deflection flow, and constructing a needle-like barrier structure perpendicular to the interface direction at the co-extrusion interface; this needle-like structure is connected to the transverse layer texture inside the insulating layer to form a three-dimensional moisture interception system; the polar modifiers used in the process include maleic anhydride-grafted polyethylene, ethylene-vinyl acetate copolymer, and organic... The silane coupling agent is added at a rate of 1% to 5% of the mass of the cross-linked polyethylene matrix; the molecular weight distribution index of the polar modifier is controlled in the range of 2.5 to 4.0 to match the rheological response time of the matrix under non-uniform shear strain field, so that the two undergo synchronous deformation when passing through the convergence section; after step S4 is completed, step S6 is performed to treat the formed cable insulation layer; the ambient temperature is maintained at 60℃ to 80℃ for 4h to 8h to release the residual internal stress generated in the gradient cooling stage and fix the distribution state of the polar modifier at the spherulite interface.

[0033] Example 1: In the continuous extrusion molding of insulation layers for medium and high voltage overhead insulated cables, the topological stability requirements of the molecular chains under different ambient temperature fields of overhead cables present a technical challenge of spontaneous retraction of modified components due to thermal motion. To solve this problem, a polar modifier containing 3% by weight and cross-linked polyethylene matrix is ​​fed into a co-rotating twin-screw extruder, and melt homogenization is performed at a material temperature of 130°C; the modified melt enters at a convergence angle of 30°C. ∘ The extrusion channel convergence section utilizes the geometric impedance of the convergence section to generate... The central shear rate; this non-uniform shear strain field drives the polar modifier to transform from a microsphere shape to a lamellar orientation structure parallel to the extrusion direction; the modified melt is cooled by a cooling system based on its dynamic viscosity. for shear rate for and the relaxation time of the elastic recoil of the polar modifier molecular chain. for According to the formula Determine the minimum cooling rate 35 ;in, Minimum cooling rate, in units of ; These are the material characteristic coefficients; The shear rate of the non-uniform shear strain field is expressed in units of . ; The dynamic viscosity of the modified melt at the outlet temperature is expressed in units of... ; The relaxation time of the polar modifier molecular chain during elastic recovery is expressed in units of 1. .

[0034] The system sets the actual cooling rate to 4. By utilizing the temperature field generated by gradient cooling to physically confine the polar modifier molecular chains before they retract, a radial thickness of 120 mm is achieved. The lamellar orientation structure is frozen within the amorphous region of the cross-linked polyethylene matrix; the frozen lamellar orientation structure enters the cross-linking channels, utilizing the heat generated by the cross-linking reaction to create a 175°C temperature inside the insulation layer. The exothermic peak value; this thermal pulse acts as the driving force for the thermal motion of molecular chain segments on both sides of the interface, causing the polar modifier and the cross-linked polyethylene matrix to interpenetrate with each other, generating an interfacial cross-linked network at the interface; this chemical anchoring mechanism locks the lamellar structure at the localized position of the spherulite interface, restricting the penetration of the modified components into the grain interior; the insulating layer at 20 The dielectric loss tangent is as follows: And after 30 After accelerated water tree aging tests, the length of the water trees decreased by 72% compared to the control group that did not use the layering and recombination process. The polar modifier is localized in the amorphous region and arranged in a layered pattern in the cross-linked polyethylene matrix to construct a continuous physical barrier system.

[0035] Example 2: In the experimental verification scenario simulating a cable production workshop environment, there are voltage harmonics in the power supply system and fluctuations in ambient temperature due to day and night alternations. These interfere with the melt viscosity stability during the extrusion process, inducing radial non-uniformity in the thickness of the modified layer. A cable extrusion simulation production line was used, which includes a co-rotating twin-screw extrusion unit, an extrusion channel with a convergence section, a water mist gradient cooling box, and a high-pressure dry crosslinking pipeline. The experimental data were obtained from a sensor network located at key workstations of the above equipment. The temperature sensor had a measurement accuracy of 0.1℃, and the sampling frequency of the pressure sensor was set to 50Hz. To simulate signal interference in a real industrial environment, Gaussian white noise with a signal-to-noise ratio of 20dB was introduced into the sensor data acquisition loop.

[0036] Cooling rate as a key process parameter The determination of this parameter involves a technical trade-off between balancing the freezing rate of the laminated structure and the release rate of residual internal stress within the insulation layer; the parameter cooling rate... The value is affected by the dynamic viscosity of the modified melt at the outlet of the extrusion channel. The shear rate generated by the convergence segment And the elastic recovery relaxation time of the molecular chain of polar modifiers The common constraints; based on the rheological characteristics of the materials, a cooling rate calculation model is established: ,in, The minimum cooling rate required to maintain the layered structure, in units of ; These are the material characteristic coefficients, which are determined through offline rheological calibration experiments; The shear rate of the non-uniform shear strain field is expressed in units of . ; The dynamic viscosity of the modified melt at the outlet temperature is expressed in units of... ; The relaxation time of the polar modifier during the elastic recoil of the molecular chain is expressed in units of 1. The system is based on real-time monitoring. Values ​​and presets Value, calculate the current operating condition. The threshold is 35.2 Based on this, the flow rate of the cooling water mist is adjusted to stabilize the actual cooling rate at 40.5. To achieve the goal of locking the deformation state of the polar modifier molecular chain before it shrinks back to a spherical shape, eight sample groups were set up to construct a multi-dimensional control system. Control group 1 used pure cross-linked polyethylene matrix; control group 2 added 3% polar modifier but did not set a convergence section in the flow channel; experimental groups 3 to 5 added 1%, 3%, and 5% polar modifier respectively and used the process of this invention; out-of-range group 6 added 0.5% polar modifier; out-of-range group 7 added 7% polar modifier; control group 8 used the process of this invention but turned off the exothermic in-situ repair function of cross-linking reaction in step S4; the performance test data of each group of samples are shown in Table 1.

[0037]

[0038] Experimental data confirmed that the performance indicators of experimental groups 3 to 5 showed a non-linear correlation with their component gradient; although control group 2 added modified components, due to the lack of induced flow field, polar groups generated local charge accumulation inside the spherulites, resulting in a decrease in the dielectric loss tangent value. Rise to Experimental group 4 utilized a non-uniform shear strain field to orient the polar modifier to the amorphous region interface, and combined this with gradient cooling to lock its radial thickness at 120 mm. This allows it to construct a continuous layered physical barrier in the amorphous region, causing the dielectric loss tangent to fall back to [value missing]. Furthermore, the length of the water tree decreased by 62% compared to control group 2; when the amount of polar modifier added was less than 1%, as shown in out-of-range group 6, the amorphous region could not form a layer density sufficient to cover the water migration path, resulting in a decrease in anti-water tree performance; when the amount added exceeded 5%, as shown in out-of-range group 7, the polar groups underwent supersaturation aggregation at the interface, resulting in a sharp increase in the dielectric loss tangent. This indicates that the insulation performance has entered the deterioration zone; control group 8, lacking the interfacial interpenetration step driven by the cross-linking reaction heat, only maintains a physical stacking state between the lamination structure and the matrix, resulting in micro-peeling during aging cycles, causing its dielectric loss to deteriorate to a certain level. The experimental results confirm that the present invention achieves a synergistic effect of water tree suppression efficiency and low insulation loss within a component ratio range of 1% to 5% through the synergistic effect of flow channel geometric impedance and gradient cooling.

[0039] Example 3: This example, in conjunction with Figures 1 and 2, describes a water-tree-resistant modified insulation process for cross-linked polyethylene overhead insulated cables. As shown in Figure 1, the process begins in step S1, where the cross-linked polyethylene matrix and a polar modifier are melt-mixed to generate a modified melt. In step S2, the modified melt is introduced into an extrusion channel with a convergence section. The extrusion resistance is set by the rate of change of the convergence angle of the convergence section, and a non-uniform shear strain field is formed in the extrusion channel to limit the shear deformation of the polar modifier and generate a lamellar orientation structure in the cross-linked polyethylene matrix. Next, step S3 is executed, where gradient cooling is performed by adjusting the outlet temperature and traction rate of the extrusion channel. The polar modifier is physically confined according to the temperature drop gradient, thereby freezing the lamellar orientation structure in the spherulite interstices of the cross-linked polyethylene matrix. Finally, in step S4, during the cross-linking heating stage, the heat of reaction is used to drive the polar groups to migrate to the amorphous region for localization and induce interpenetration of molecular chain segments to anchor the polar modifier at the spherulite interface to generate an interfacial cross-linking network, restricting the polar groups from entering the interior of the grains.

[0040] As shown in Figure 2, the system includes a central control system and a series of sequentially connected processing units. The central control system integrates a real-time melt viscosity monitoring module, a convergence angle adaptive compensation module, and a minimum cooling rate threshold calculation module, which are used to process viscosity feedback, generate angle adjustment commands, and generate cooling rate control commands, respectively. The raw material melting and mixing unit includes a co-rotating twin-screw extruder and matrix and modifier inlets, which are responsible for conveying the modified melt to the subsequent units. The shear flow field induction unit receives the angle adjustment command from the central control system and has a non-uniform shear flow channel, a variable convergence angle die, and an edge damping structure inside, which are used to output the layered orientation melt gradient cooling and shaping unit in response to the cooling rate control command. The melt is transformed into a frozen state insulation layer through a temperature gradient control zone and a physical limiting freezing device. The interface crosslinking and anchoring unit has a crosslinking reaction heat driving zone and a molecular chain interpenetration generator, which transforms the frozen state insulation layer into a crosslinked insulated cable. Finally, the cable is finally processed by an online heat treatment unit containing a stress relief constant temperature chamber.

[0041] Example 4: In the industrial continuous production scenario of insulation extrusion for medium and high voltage overhead insulated cables, there is a technical challenge of the difference in molecular weight distribution of cross-linked polyethylene matrix in different batches, which leads to the drift of melt rheological response characteristics and thus induces inconsistent deformation of the lamination orientation structure; the system executes material characteristic coefficients The offline calibration program is applied to each batch of newly received raw materials. Using a capillary rheometer with a measurement accuracy of at least 1%, the shear viscosity curve of the modified melt is measured within a temperature range of 130℃ to 150℃. The aspect ratio change rate of the polar modifier under steady-state shear is calculated, and the obtained data is input into a least-squares fitting model to determine the material characteristic coefficients corresponding to that batch of raw materials. The value is 0.85. During continuous operation of the extruder, to address viscosity fluctuations caused by frictional heat generated at the extruder die, the system executes a geometric impedance adaptive compensation procedure. The logical path of this procedure is as follows: real-time melt pressure is obtained using pressure and temperature sensors installed at the inlet of the convergence section. With temperature According to real-time parameters and Calculate the current dynamic viscosity of the melt. and compare it with the preset reference viscosity. The viscosity deviation value was obtained by comparison. If the deviation value If the value continuously exceeds the baseline value by 5%, the control unit will adjust the settings according to the mapping relationship. Calculate the compensation adjustment amount for the convergence angle. ,in, This is the convergence angle adjustment amount, in units of... , The compensation sensitivity coefficient, in units of , This is the viscosity deviation value, in units of... In a typical operating scenario, when real-time melt pressure and temperature are monitored, the calculated dynamic viscosity of the melt is 5000. Dropped to 4700 At that time, the system determined the viscosity deviation value to be 300. The drive adjustment mechanism increases the convergence angle of the convergence section in steps from 30.0° to 32.5°. This increase in extrusion resistance offsets the shear stress loss caused by the decrease in viscosity, maintaining a constant equivalent shear work obtained by the modified component within the deformation zone. This ensures that the layer thickness is consistent throughout the entire axial length of the cable.

[0042] At the co-extrusion interface between the cable insulation layer and the adjacent functional layer, to suppress the lateral initiation of water treeing at the interface, process step S5 adjusts the output frequency of the inverters of the insulation layer extruder and the functional layer extruder, setting the outlet flow rate ratio of the insulation layer and the functional layer at the co-extrusion point within the range of 0.98 to 1.02; simultaneously, a tooth depth of 2.0 is utilized at the edge of the flow channel. The multi-stage damping structure generates radial flow splitting velocity, causing the modified melt at the edge to deflect at an angle of 45° to 60° relative to the extrusion axis. This constructs a needle-like barrier structure penetrating the spherulite interstices at the interface. Its generation logic is based on a lookup table mapping between the tooth depth of the multi-stage damping structure and the melt Reynolds number: when the melt Reynolds number is detected to be between 1500 and 2500, the control unit drives the edge damping teeth towards the center of the flow channel via a hydraulic actuator, fixing the effective tooth depth at 2.0 mm and generating a 45° radial deflection flow splitting. The flow rate ratio adjustment corresponds to the frequency output of the extruder's inverter. (The insulation layer...) For every 1Hz increase in extruder frequency, the flow rate ratio increases by 0.02. By controlling the frequency difference within 5Hz, the flow rate ratio is kept within the target range of 0.98 to 1.02, achieving a distribution density of 120 to 150 pins per square millimeter. The resulting cable insulation system, characterized by computed tomography (CT) microscopy, shows that the polar modifier has a lamination orientation degree of no less than 0.95 in the amorphous region, and the average distribution density of pin-like structures at the interface is 120 to 150 per square millimeter. This process enables the localized arrangement of the polar modifier in complex flow field environments, allowing the insulation layer to withstand... During the high field strength aging test, the fluctuation range of its dielectric properties is less than 3%, thus completing the physical constraints and steady-state quality control of the insulation process.

[0043] Example 5: In the scenario of switching specifications for medium and high voltage overhead cables, the pressure reference in the flow channel shifts due to the change in conductor diameter. Before the extrusion stage, the system performs a pre-calibration procedure that includes displacement response and pressure feedback. During the no-load stage when the extruder maintains a constant speed, the drive convergence section adjustment mechanism generates a triangular wave cyclic displacement with an amplitude of 2.0°. Simultaneously collect the melt pressure changes monitored by the flow channel sensor and convert them into equivalent viscosity changes. The compensation sensitivity coefficient was calculated using the least squares method. The calculation formula is as follows: ,in, The compensation sensitivity coefficient, in units of ; The preset variation in the convergence angle during the calibration process is given in units of 1. ; This is the equivalent viscosity change derived from pressure sensor data, in units of... ; obtained coefficients Stored in the control unit's memory, it serves as the execution reference for subsequent adaptive adjustment procedures, anchoring the control parameters to the geometric characteristics of the physical flow channel.

[0044] Complete the compensation sensitivity coefficient After calibration, the system initiates online monitoring of the production line, using online rheological monitoring instruments installed in the die cooling zone to collect dynamic modulus data of the modified melt, and inputs this data into the material characteristic coefficients. The corrected model; when the detected deviation of the rheological response time from the reference batch of raw materials exceeds 3%, the system automatically adjusts the minimum cooling rate according to the direction of the deviation. The control threshold is used to offset the physical interference caused by the difference in molecular weight distribution of raw materials on the lamination freezing process, maintain the lamination thickness of polar modifier in cross-linked polyethylene matrix, and after calibration and dynamic correction, the production line offsets the viscosity decay caused by temperature rise in real time during operation, so that the average radial spacing of the water tree interception network inside the insulation layer is stabilized in the range of 1.5 to 2.0 times the spherulite size, and the cable insulation layer presents the preset physical distribution state.

[0045] Example 6: In the scenario of calibrating the geometric parameters of the extrusion channel for large-diameter high-voltage overhead insulated cables, due to the increase in the thickness of the cable insulation layer, nonlinear distortion of the radial shear stress gradient occurs at the die orifice. Before operation, the system performs a calibration process including multi-level damping structure geometry depth calibration. A fluid dynamics simulation procedure; this procedure uses the average Reynolds number of the insulating layer melt. As input, a discretized flow field model with different damping tooth depths is constructed in the control unit, and the ratio of the radial deflection velocity to the axial mainstream velocity at the interface is calculated. The calculation formula is as follows: ,in, The flow rate ratio; This is the flow channel geometry correction factor, determined through flow channel resistance experiments; The tooth depth of a multi-stage damping structure, in units of ; The Reynolds number is the number of the melt flow; the obtained tooth depth parameters are stored in the control unit, and the damping structure controls the interface stress distribution by adjusting the tooth profile displacement at the flow channel edge.

[0046] The system determines the ratio of maleic anhydride-grafted polyethylene to ethylene-vinyl acetate copolymer in the polar modifier by executing an interface trap energy level calibration procedure based on thermally stimulated current spectral lines. Test samples containing different component ratios are prepared in a laboratory environment. Depolarization current data of the samples are collected in the temperature range of 30℃ to 90℃ using a high-voltage electrometer. A peak-splitting fitting model is used to extract the trap energy level depth corresponding to the amorphous region interface. ,in, Energy level depth, unit: Its calculations refer to the energy level mapping relationship derived from the Arrhenius equation; if the detected If the measured value is within the preset range of 0.85 eV to 1.15 eV, it is determined that the component ratio meets the kinetic conditions for constructing a layered physical barrier in the amorphous region. Under this logic, when the mass ratio of maleic anhydride-grafted polyethylene to ethylene-vinyl acetate copolymer is adjusted to 1.2:1.0, the charge trapping ability of the amorphous region interface is stable, and the volume resistivity drift rate is less than 5% in a 60°C water bath environment. This result serves as the basis for the execution of the polar modifier ratio input during the production process, maintaining the physical effectiveness of the moisture interception system under different environmental humidity conditions.

[0047] To determine the time parameters for the online heat treatment in step S6, the system executes a residual stress relaxation calibration procedure based on dynamic thermomechanical analysis, and inputs the cumulative strain energy density of the insulating matrix during the gradient cooling stage into the control unit. And according to the formula Calculate the required heat treatment time, where, Heat treatment time, in units of ; It is the stress relaxation constant; Strain energy density, in units of ; The ambient temperature for online heat treatment is expressed in °C. The reference room temperature is expressed in °C. The heat treatment time determined by this calculation model reduces the residual mechanical stress inside the insulation layer while ensuring the geometric stability of the layered orientation structure.

[0048] The embodiments of this application have been described above with reference to the accompanying drawings. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. This application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of this application without departing from the spirit of this application and the scope of protection of this invention, and all of these forms are within the protection scope of this application.

Claims

1. A water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables, characterized in that, Includes the following steps: Step S1: The cross-linked polyethylene matrix and the polar modifier are melt-mixed to generate a modified melt. Step S2: The modified melt is introduced into an extrusion channel with a convergent section. The extrusion resistance is set using the rate of change of the convergence angle of the convergent section. This extrusion resistance forms a non-uniform shear strain field within the extrusion channel, and this non-uniform shear strain field limits the shear deformation of the polar modifier in the cross-linked polyethylene matrix, thereby generating a layered orientation structure in the cross-linked polyethylene matrix. Step S3: The outlet temperature and traction rate of the extrusion channel are adjusted to perform gradient cooling on the modified melt leaving the die. The temperature decrease between the outlet temperature and the cooling medium is considered. In step S4, before the polar modifier undergoes elastic retraction of its molecular chains, physical confinement is applied to the polar modifier, freezing the layered orientation structure within the spherulitic intergranular spaces of the cross-linked polyethylene matrix. In the cross-linking heating stage, the heat of the cross-linking reaction drives the polar groups contained in the polar modifier to migrate and localize to the amorphous region of the cross-linked polyethylene matrix. The heat of the cross-linking reaction also induces the interpenetration of molecular chain segments between the polar modifier and the cross-linked polyethylene matrix on both sides of the interface, anchoring the frozen polar modifier at the spherulitic interface of the cross-linked polyethylene matrix, generating an interfacial cross-linking network, and restricting the polar groups from entering the interior of the cross-linked polyethylene matrix grains.

2. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 1, characterized in that, In step S2, the shear rate of the non-uniform shear strain field is controlled at... to The convergence angle of the convergence section is set to 15° to 45°. The process adjusts the convergence angle according to the real-time viscosity fluctuation of the modified melt to adjust the extrusion resistance, maintain the constant shear work of the polar modifier in the amorphous region, control the geometric deviation of the layered orientation structure in the axial and circumferential directions of the cable, and reduce the molecular chain movement space of the polar modifier during pressure release.

3. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 1, characterized in that, In step S3, the cooling rate of the gradient cooling is set to 20℃ / min to 50℃ / min; the radial thickness of the layered orientation structure in the amorphous region is controlled to be 50nm to 200nm; and the layered orientation structure forms a continuous physical barrier inside the spherulitic interstices of the cross-linked polyethylene matrix.

4. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 1, characterized in that, In step S4, the exothermic peak temperature of the crosslinking reaction is controlled between 160°C and 180°C. The thermal activation energy provided by the exothermic peak temperature drives the polar group to migrate directionally to the weak amorphous region interface. This interfacial crosslinking network transforms the heterogeneous physical interface into a chemical and mechanical composite network, enhancing the bonding strength between the layered orientation structure and the crosslinked polyethylene matrix, and limiting interfacial delamination caused by electrothermal cycling stress.

5. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 1, characterized in that, The polar modifier includes maleic anhydride-grafted polyethylene, ethylene-vinyl acetate copolymer, and organosilane coupling agent. The amount of the polar modifier added is 1% to 5% of the mass of the cross-linked polyethylene matrix. The polar modifier performs spontaneous orientation arrangement under the action of extrusion resistance based on the difference in molecular chain polarity, thereby enhancing the microscopic bonding force between the layered orientation structure and the cross-linked polyethylene matrix.

6. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 1, characterized in that, In step S2, based on the decrease in viscosity of the modified melt due to frictional heating, the convergence angle of the convergence section is adjusted to generate an incremental shear rate. This incremental shear rate is used to offset the stress loss in the extrusion channel, maintain the constant shear strain work of the polar modifier in the deformation zone, and ensure the uniform distribution of the interfacial crosslinking network along the axial length of the cable.

7. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 1, characterized in that, The method also includes step S5: setting the flow rate ratio between the insulating layer and the adjacent functional layer at the outlet temperature, and using the damping structure provided at the edge of the extrusion channel to suppress the shear stress concentration of the modified melt at the interface, limiting the geometric distortion of the layer orientation structure at the co-extrusion interface between the insulating layer and the adjacent functional layer, and maintaining the physical integrity of the co-extrusion interface.

8. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 3, characterized in that, This gradient cooling is achieved through a cooling rate threshold. Real-time control is implemented, and the cooling rate is not lower than the cooling rate threshold. ： ,in, The minimum cooling rate required to maintain the frozen state of this layered orientation structure, For material characteristic coefficients, Let be the shear rate of this non-uniform shear strain field. This refers to the dynamic viscosity of the modified melt at the outlet temperature. The relaxation time for the elastic retraction of the molecular chains of this polar modifier is defined as the time required for this process. By adjusting the cooling rate, the shear deformation is ensured to be converted into the penetration force between the molecular chains.

9. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 1, characterized in that, The molecular weight distribution index of the polar modifier is controlled between 2.5 and 4.

0. This molecular weight distribution index is used to match the rheological response time of the cross-linked polyethylene matrix under the non-uniform shear strain field, ensuring that the polar modifier and the cross-linked polyethylene matrix undergo synchronous deformation when passing through the convergence section, thereby improving the microstructure distribution density of the interfacial cross-linked network.

10. The water-tree resistant modified insulation process for cross-linked polyethylene overhead insulated cables according to claim 1, characterized in that, Step S4 is followed by step S6: performing online heat treatment on the formed cable insulation layer, maintaining the ambient temperature of the cable insulation layer at 60°C. The mixture is kept at 80°C for 4 to 8 hours to release the residual internal stress during the gradient cooling process and fix the localized distribution of the polar modifier at the spherulite interface.

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