Formulation Design and Optimization Methods for Cross-linked Polyethylene Insulation Materials for High-Voltage AC Cables

Abstract

A method for designing and optimizing the formulation of XLPE insulation material for high-voltage AC cables includes the following steps: Step 1: Determine the initial formulation elements, including the types of base resin, antioxidant, and crosslinking agent; Step 2: Test samples made by hot pressing of pure base resin, sequentially passing qualitative evaluation index I and quantitative evaluation index II to obtain a preferred base resin; Step 3: Prepare multiple blends of crosslinking agent / antioxidant / base resin, sequentially passing qualitative evaluation index II and quantitative evaluation index II to obtain an optimized crosslinking agent formulation; Step 4: Prepare multiple blends of crosslinking agent / antioxidant / base resin, sequentially passing qualitative evaluation index III and quantitative evaluation index III to obtain an optimized antioxidant formulation; Step 5: Verify electrical performance. If all test parameters meet the requirements, the material formulation is considered an optimized formulation; otherwise, return to the alternative samples for testing. This invention improves the key performance characteristics of high-voltage AC cable insulation materials that are of general concern.

Description

Technical Field

[0001] This invention belongs to the field of power transmission technology, and more specifically to the field of cross-linked polyethylene power cable insulation technology, and relates to the formulation design and optimization method of cross-linked polyethylene insulation material for high-voltage AC cables. Background Technology

[0002] For urban power supply, high-voltage overhead lines are not suitable. XLPE (Cross-linked polyethylene) insulated high-voltage cables have advantages such as small footprint, high safety, and less susceptibility to weather conditions. Therefore, XLPE insulated high-voltage AC cables have become one of the most important power transmission carriers. The high-voltage XLPE insulation material formula mainly consists of LDPE (Low Density Polyethylene) base resin, crosslinking agents, and antioxidants.

[0003] To ensure excellent insulation performance, XLPE insulation materials for voltage levels of 35kV and above typically use dicumyl peroxide (DCP) as a crosslinking agent, which initiates and completes the crosslinking reaction under heat. With the base resin remaining constant, the DCP content significantly affects the performance of high-voltage cable XLPE insulation materials. Insufficient DCP content can lead to inadequate crosslinking efficiency, potentially resulting in insufficient crosslinking and negatively impacting the material's mechanical properties and long-term temperature resistance. To achieve the performance indicators of internationally used XLPE cable insulation materials, the amount of peroxide added during the crosslinking process in domestic XLPE cable insulation materials is 1-1.5 times that of foreign materials. The DCP content in domestically produced 35kV XLPE insulation materials sometimes reaches as high as 2-2.2 phr. Excessive DCP addition can also cause a series of problems. First, it will lead to over-crosslinking of the material, resulting in a decrease in mechanical properties. Second, the increased amount of crosslinking byproducts such as acetophenone and cumyl alcohol will impose more stringent requirements on the degassing process of the cable insulation layer, thus significantly impacting the actual production efficiency of the cable and increasing production costs. Third, some crosslinking byproducts cannot be completely removed through the degassing process, resulting in micropores in the cable insulation material, which will significantly reduce the overall performance of the cable insulation layer. Therefore, reducing the amount of DCP added is an important way to improve XLPE cable insulation material while ensuring its excellent performance.

[0004] XLPE, as the main insulation material for cables, inevitably operates in high-temperature environments during production and use. Therefore, it is highly susceptible to aging under the influence of heat and oxygen, affecting its electrical, mechanical, and thermal stability, and reducing the cable's service life. Antioxidants, commonly used additives in polymer materials, can delay the oxidative aging of polymers and are an essential component of XLPE cable insulation materials. Different antioxidants and their contents have varying effects on the electrical, thermal, and mechanical properties of XLPE cable insulation materials, and their resistance to heat and oxygen aging also differs significantly. Furthermore, since the main mechanism of antioxidants is to scavenge active free radicals in the polymer, and the thermal crosslinking reaction of XLPE can only be completed under the action of highly active free radicals initiated by DCP, antioxidants also have a significant impact on the material's crosslinking behavior.

[0005] The formulation design of high-voltage cross-linked polyethylene (VLL) insulation materials mainly includes three parts: determining the LDPE base resin, designing the anti-aging formulation, and designing the cross-linking reaction formulation. Previously, designing a VLL insulation material formulation typically required comprehensive experiments on multiple LDPE resins, antioxidants, and cross-linking agents under multiple component conditions, comparing and analyzing the material properties of different formulations to obtain a suitable formulation. As mentioned earlier, the cross-linked material exhibits significant performance differences compared to the uncross-linked LDPE base resin. Furthermore, the cross-linking agent and antioxidant, two important formulation additives, are theoretically interdependent. Clearly, the formulation design of VLL insulation materials is a complex problem. Theoretically, orthogonal experimental design can be used to solve complex problems, offering advantages such as fewer experiments, better results, simplicity, ease of use, and high efficiency. However, the performance evaluation of insulation materials involves multiple aspects, including cross-linking performance, mechanical properties, electrical properties, and aging performance. The mechanisms of interaction between cross-linking agents, antioxidants, and macromolecules are complex, making orthogonal experimental design imprecise, unable to determine experimental patterns, and unable to obtain reliable results.

[0006] Currently, high-voltage AC cable insulation materials still hold a significant advantage internationally, and several domestic companies are also capable of independently producing 110kV and below AC cross-linked polyethylene insulation materials. However, domestic manufacturers currently lack a definitive and feasible method for designing material formulations for high-voltage AC cable insulation materials. There are two main approaches to determining material formulations: one is to obtain a preliminary formulation based on national standards through extensive, unsystematic, and disorganized testing; the other is to borrow formulations from commonly used XLPE insulation materials without optimizing the formulation based on the specific characteristics of the materials used. The former method, lacking a systematic approach, is clearly arbitrary and wastes considerable time, manpower, and resources. The latter method, due to differences in the brands and manufacturers of the base resin, antioxidants, and cross-linking agents used by manufacturers, results in significantly different material properties. Furthermore, if other commercially available materials are used, the impact of raw material differences is not considered, thus failing to yield an optimal material formulation.

[0007] Over the past decade, the advancement of urbanization has led to a large demand for cross-linked polyethylene (XLPE) insulation materials. Many material manufacturing or cable manufacturing companies are preparing to build new XLPE insulation material production lines. At this time, the lack of material formulation design will lead to an increase in R&D costs and R&D cycle. In addition, although the insulation material manufacturing companies that have already started production have applicable formulations, they still face the following three situations, which require formulation optimization: (1) The performance level of domestically manufactured materials has not yet reached the same level as international materials of the same grade, and there is still a strong demand for material formulation upgrades in the future. (2) When developing XLPE insulation materials with higher voltage levels, it is necessary to upgrade low-voltage materials to higher-voltage materials, which requires formulation optimization. (3) Due to changes in production processes or models faced by base resin manufacturers, when petrochemical companies adjust their production capacity, the supply of base resin materials may change, requiring the use of new grades of base resin for replacement. At this time, the performance of the material changes, and the formulation needs to be re-optimized.

[0008] In summary, the formulation design and optimization of cross-linked polyethylene (XLPE) insulation materials are crucial for improving material performance and product upgrades. However, a systematic approach to formulation design and optimization is currently lacking. Existing technologies often involve large-scale, unregulated testing, which is extremely detrimental to shortening the R&D cycle and improving the performance of insulation materials. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide a method for designing and optimizing cross-linked polyethylene insulation materials for high-voltage AC cables, thereby solving the problems existing in the formulation design and optimization of cross-linked polyethylene insulation materials for high-voltage AC cables: (1) lack of target and systematic approach; (2) long experimental cycle and large workload; (3) formulation design only refers to basic relevant standards, lacking evaluation and optimization of key performance.

[0010] This invention adopts the following technical solution. A method for designing and optimizing the formulation of XLPE insulation material for high-voltage AC cables, comprising the following steps:

[0011] Step 1: Determine the initial formulation elements, including the types of base resin, antioxidants, and crosslinking agents;

[0012] Step 2: Based on the base resin selected in Step 1, test the sample made by hot pressing of pure base resin, and optimize it through two levels of indicators: qualitative evaluation index I and quantitative evaluation index I; to obtain the optimized base resin.

[0013] Step 3: Prepare multiple blends of crosslinking agent / antioxidant / base resin, wherein the mass fractions of antioxidant and base resin remain constant in multiple blends, and the mass fractions of crosslinking agent are ordered in ascending order. Test the samples of each blend and optimize them through two levels of indicators: qualitative evaluation index II and quantitative evaluation index II; to obtain the optimized crosslinking agent formulation.

[0014] Step 4: Prepare multiple blends of crosslinking agent / antioxidant / base resin, wherein the mass fractions of crosslinking agent and base resin remain constant, and the mass fractions of antioxidant are ordered in ascending order. Test the samples of each blend and optimize them through two levels of indicators: qualitative evaluation index III and quantitative evaluation index III; to obtain the optimized antioxidant formulation.

[0015] Step 5: Electrical performance verification. If all test parameters meet the requirements, the material formula is considered an optimized formula; otherwise, return to the alternative test candidates for testing.

[0016] In steps 2, 3, or 4, qualitative evaluation refers to determining whether the test result parameters meet qualitative evaluation indicators I, II, or III. If they do, the material is considered qualified; if not, the material is discarded, and the qualitative evaluation of the next candidate material is initiated. This process continues until all candidate materials have completed the tests for qualitative evaluation indicators I, II, or III, and materials that meet the qualitative evaluation indicators are selected. Then, the quantitative evaluation indicators I, II, or III are initiated.

[0017] Preferably, the qualitative evaluation index I includes: rheological performance parameters measured by a rotational rheometer, tensile strength parameters and elongation at break parameters obtained by stress-strain testing, and dielectric loss tangent and relative permittivity parameters measured by a high-voltage Schering bridge.

[0018] Preferably, the quantitative evaluation index I includes: the two-parameter Weibull distribution parameters of the AC breakdown field strength measured based on the cylindrical electrode;

[0019] The characteristic breakdown strength is used as the first parameter 'a', and the shape parameter is used as the second parameter 'b'. Different resins are prioritized based on the size of 'a', with higher priority for larger 'a'. If 'a' is the same, the priority is further subdivided based on 'b', with higher priority for materials with the same 'a'.

[0020] All resins to be selected are prioritized and ranked. The resin with the highest priority proceeds to step 3 first; the resins with the next highest priority are considered as candidates for testing.

[0021] Preferably, in step 3, the sample made of the xphr crosslinking agent / 0.3phr antioxidant 300 / base resin blend is tested and optimized through two levels of indicators: qualitative evaluation index II; quantitative evaluation index II;

[0022] Where x is a user-defined sequence of variables: x1, x2, x3, x4...x n1 , represents the mass fraction of the crosslinking agent, x≥0, n1 represents the sequence length, i.e. the number of blend groups.

[0023] Preferably, the qualitative evaluation index II includes two parameters: gel content and thermal elongation, which characterize the degree of crosslinking, as well as tensile strength and elongation at break parameters obtained based on stress-strain testing.

[0024] Preferably, the quantitative evaluation index II includes: cross-linking gas production characteristic parameters. All materials are prioritized, with higher priority given to those with better gas production characteristics and smaller degassing residue. When degassing residue is equal, the smaller the x-value, the higher the priority. The priority ranking of all materials to be optimized is as follows: x1, x2, x3, x4…x n1 n1 represents the sequence length. The sequence with the highest priority will proceed to step 4 first, while the sequence with the next highest priority will be considered as a candidate for testing.

[0025] Preferably, in step 4, antioxidants are used alone or in combination, based on their mechanism of action and synergistic effect.

[0026] Preferably, the qualitative evaluation index III includes two parameters: gel content and thermal elongation, which characterize the degree of crosslinking, and tensile strength and elongation at break parameters obtained from air aging performance tests before and after aging.

[0027] Preferably, the quantitative evaluation index III includes: prioritizing all components based on the characteristic parameters of the cross-linking reaction kinetic curve; the further to the right the peak point of the cross-linking reaction kinetic curve is, the higher the priority; when the cross-linking reaction kinetic curves overlap, the higher the gel content in the qualitative evaluation index III, the higher the priority; when the cross-linking reaction kinetic curves overlap and the gel content in the qualitative evaluation index III is equal, the higher the elongation at break in the qualitative evaluation index III, the higher the priority; prioritizing all components to be optimized, with the one ranked first proceeding to step 5 first.

[0028] The beneficial effects of this invention are that, compared with the prior art, as mentioned above, this invention puts forward specific requirements for the key performance of the base resin and provides clear guiding methods. It fully limits the impurity content, gel point content and polar groups of the base resin by electrical properties, avoids the blind selection of base resin, avoids the failure of the formulation due to the lack of assessment of key performance, avoids the waste of subsequent experimental work, and greatly improves the success rate of formulation design.

[0029] Based on the inventors' long-term experimental research, the additional effects of antioxidants must be considered during the optimization of crosslinking agent formulations. In this invention, the crosslinking agent formulation optimization process uses 0.3 phr antioxidant 300 as a co-oxidant. The special molecular structure of this antioxidant gives it the functions of both a primary antioxidant and a secondary antioxidant, making it functionally representative and compatible. As a result, the crosslinking agent optimization process is more universal, providing a more reliable structural basis for subsequent antioxidant formulation optimization, thereby reducing potential waste of manpower, material resources, and time.

[0030] The three-level quantitative evaluation indexes designed in this invention have optimized the formulations of the base resin, crosslinking agent, and anti-aging agent, respectively. By prioritizing the weights, the formulation design is tilted towards high electrical resistance, easy degassing, anti-scorching, and anti-aging properties. This improves the key performance that is of general concern in the research and development of high-voltage AC cable insulation materials, which is conducive to the research and development of AC crosslinked polyethylene insulation materials with higher voltage levels, and also conducive to the iterative upgrading of AC crosslinked polyethylene insulation materials with fixed grades. Attached Figure Description

[0031] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0032] The present application will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and should not be construed as limiting the scope of protection of the present application.

[0033] like Figure 1As shown in the figure, Embodiment 1 of the present invention provides a method for the formulation design and optimization of XLPE insulation materials for high-voltage AC cables, including the following steps:

[0034] Step 1: Determine the initial formulation elements, including the types of base resin, antioxidant, and crosslinking agent.

[0035] Specifically, it includes determining the grades of the base resins to be optimized, determining the grades of antioxidants, and determining the grade of crosslinking agent DCP. The resins are denoted as L1, L2, L3..., the antioxidants are denoted as AO1, AO2, AO3..., and the crosslinking agents are denoted as D1, D2, D3,....

[0036] Step 2: Optimize the base resin. Based on the base resin selected in Step 1, test the specimens made by hot pressing the pure base resin, and optimize them through two levels of indicators in sequence: qualitative evaluation indicator I; quantitative evaluation indicator I.

[0037] Qualitative evaluation means judging whether the test result parameters meet qualitative evaluation indicator I. If they meet, it is regarded as qualified; if not, the material is discarded, and the qualitative evaluation of the next alternative resin material is carried out until the qualitative evaluation indicator I tests of all alternative resin materials are completed, and the resin materials that meet qualitative evaluation indicator I are screened out, and then enter quantitative evaluation indicator I.

[0038] In a preferred but non-limiting embodiment of the present invention, qualitative evaluation indicator I includes: rheological property parameters measured based on a rotational rheometer, tensile strength parameters and elongation at break parameters obtained based on a stress-strain test, and dielectric loss tangent and relative permittivity parameters measured based on a high-voltage Schering bridge. Meeting the above indicators simultaneously is regarded as qualified and enters quantitative evaluation indicator I.

[0039] It should be noted that the above qualitative evaluation indicator I is introduced as a preferred embodiment in the technical solution of the present invention. Those skilled in the art can set more stringent qualitative evaluation indicator I according to design requirements. For example, but not limited to, adding more judgment conditions in qualitative evaluation indicator I, or raising the passing standards of each specific indicator, etc. Similarly, it can be understood that in the subsequent technical solutions of the present invention, all qualitative evaluation indicators and quantitative evaluation indicators are introduced as preferred embodiments in the technical solution of the present invention, and those skilled in the art can make adaptive adjustments to them.

[0040] The specific requirements of qualitative evaluation indicator I are as follows:

[0041]

[0042] Quantitative evaluation index I includes: a two-parameter Weibull distribution parameter based on the AC breakdown field strength measured by a cylindrical electrode. Tests are conducted on different resins to be selected, using the characteristic breakdown strength (scale parameter) as the first parameter 'a' and the shape parameter as the second parameter 'b'. Resins are prioritized based on the value of 'a'. A larger 'a' indicates higher priority. If 'a' is the same, the priority is further subdivided based on 'b', with smaller 'b' values ​​indicating higher priority for materials with the same 'a'. All resins to be selected are prioritized, with the highest-ranked resin proceeding to step 3 first. Resins with lower priority are considered as candidates for testing.

[0043] It is understandable that in engineering practice, the types of base resin, antioxidant and crosslinking agent selected in step 1 can be one or more, which does not affect the implementation of subsequent steps. For example, but not limited to, if the base resin selected in step 1 includes only one type, the quantitative evaluation and sorting process in step 2 will be skipped by default and proceed directly to the subsequent steps.

[0044] It is worth noting that the specific indicators of existing high-voltage cross-linked polyethylene insulation materials are mainly based on industry-standard specifications. However, these standards only specify requirements for a limited number of properties. Key characteristics directly dependent on the material formulation, such as rheological properties, gas generation characteristics, and cross-linking kinetics curves, are not defined in the standards. Specifically, rheological properties determine the material's extrusion processing characteristics and the uniformity of the insulation structure; gas generation characteristics determine the time and effectiveness of degassing after cross-linking, ultimately affecting the material's long-term operational stability; and the cross-linking kinetics curve determines the material's resistance to scorching and limits the maximum extrusion length of cable insulation.

[0045] Furthermore, existing industry standards only specify requirements for the performance of cross-linked materials, without outlining specific requirements for the base resin. There is also a lack of guiding methods. Since the impurity content and gel point of the base resin play a decisive role in the electrical properties of cross-linked polyethylene insulation materials, this leads to a lack of direction in the initial material formulation design. One improvement of this invention over existing technologies is that it specifies requirements for these key base resin properties. This avoids formulation failures caused by insufficient assessment of key performance characteristics, significantly improving the success rate of formulation design.

[0046] More specifically, one of the outstanding substantive features and significant beneficial effects of this invention lies in the fact that, in the qualitative evaluation index I of this invention, the rheological properties measured by a rotational rheometer are used to qualitatively evaluate the base resin. This is more specific than conventional base resin performance indicators (density, melt index, etc.), and fully considers the requirements of the base resin for cable insulation material manufacturing and cable insulation layer manufacturing processes. Viscosity at low shear frequency is used to limit the zero-shear viscosity of the material, avoiding core separation after extrusion due to insufficient zero-shear viscosity. Viscosity at high shear frequency is used to limit the extrusion processing performance of the material, avoiding problems such as excessive or insufficient extrusion amount and poor extrusion quality. The limitation of tensile strength and elongation at break is to avoid the problem that the tensile strength and elongation at break cannot meet the requirements after cross-linking, and further limits the macromolecular structure of the base resin.

[0047] In the qualitative index I of this invention, the Weibull distribution parameter of the breakdown field strength further specifies the purity of the material, avoiding the presence of large-sized impurities or gel points. From a variety of candidate resins, those with higher purity are preferentially selected based on electrical performance. At the same time, the dielectric loss tangent value further restricts the purity of the material's molecular structure, avoiding the presence of a large number of polar groups in the macromolecules. This ensures that the base resin has sufficient margin in the dielectric loss tangent, preventing the problem of the dielectric loss tangent exceeding the requirements after adding polar molecules such as antioxidants and crosslinking agents to the base resin.

[0048] Step 3: Optimize the crosslinking agent formulation. Prepare multiple blends of crosslinking agent / antioxidant / base resin, where the mass fractions of antioxidant and base resin remain constant, and the mass fractions of crosslinking agent are ordered in ascending order. Test the samples of each blend and optimize them through two levels of indicators: qualitative evaluation index II and quantitative evaluation index II.

[0049] In a preferred but non-limiting embodiment of the present invention, the sample prepared from the blend of x phr crosslinking agent / 0.3 phr antioxidant 300 / base resin is tested in two steps, and the optimal evaluation is performed using two levels of indicators: qualitative evaluation indicator II; and quantitative evaluation indicator II. Wherein, x is a user-defined variable sequence: x1, x2, x3, x4…x n1 , representing the mass fraction of the crosslinking agent, x≥0, n1 represents the sequence length, i.e., the number of blend groups. A preferred but non-limiting embodiment is that each x value forms an arithmetic sequence, with the smallest arithmetic progression not less than 0.01 phr and not greater than 0.2 phr.

[0050] It should be noted that since there are already a large variety of antioxidants, when adopting the technical solution of combining a primary antioxidant and a secondary antioxidant, the types of candidate formulations are even more complex. As one of the improvements of the present invention over the prior art, the consideration of selecting antioxidant 300 and the beneficial technical effects that can be achieved at least lie in that the selection of the antioxidant has an impact on the optimization process of the DCP content and subsequent processes. Therefore, when optimizing the DCP content, the antioxidant formulation used in combination should be representative and feasible, so as to reduce the workload and simplify the formulation optimization process while ensuring the reliability of the optimized result of the DCP content.

[0051] Antioxidant 300 is a commonly used additive in XLPE insulation. It combines the functions of a primary antioxidant and a secondary antioxidant. At the same time, the antioxidant formulation with 0.3 phr of antioxidant 300 is a mature solution proven feasible by engineering experience. The combination with 0.3 phr of antioxidant 300 aims to simplify the design process and obtain a reliable DCP content. In contrast, if any antioxidant formulation of the candidate formulations is used in combination to optimize the DCP content, since the candidate formulations are likely to be unrepresentative and even unable to meet the usage conditions, the results obtained are likely to be non-universal, causing great deviations in the design of the DCP content and the further optimization design of the antioxidant formulation.

[0052] In a further preferred embodiment, the qualitative evaluation index II includes: two parameters, namely the gel content and the heat elongation rate, which characterize the degree of crosslinking, and the tensile strength parameter and the elongation at break parameter obtained based on the stress-strain test. The test methods and specific parameter requirements of the qualitative evaluation index only need to meet the relevant general standards of the crosslinked polyethylene insulation material for the target voltage class. Meeting the above indicators simultaneously is regarded as qualified. If not, the formulation is abandoned and the qualitative evaluation of the next alternative crosslinking agent formulation material is entered. Until the tests of all candidate formulations for the qualitative evaluation index II are completed, the formulations that meet the qualitative evaluation index II are screened and enter the quantitative evaluation index II.

[0053] The quantitative evaluation index II includes: the crosslinking gas generation characteristic parameter. The materials of all components are ranked in order of priority. The one with better gas generation characteristics has a smaller residual gas content after degassing and a higher priority. When the residual gas content after degassing is equal, the one with a smaller x value has a higher priority. The priority ranking of all materials to be optimized for components is completed: x1, x2, x3, x4... x n1 , where n1 represents the sequence length. The one ranked first is preferentially entered into step 4. Those ranked second are used as alternative candidates for testing.

[0054] In a further preferred embodiment, the qualitative index II crosslinking gas production characteristic test method is as follows: Approximately 5-10g of the test formulation material is weighed using a precision balance. Before the reaction, the material is accurately weighed, hot-pressed and crosslinked in a flat vulcanizing machine, and then placed in a vacuum oven at -0.1MPa and 80℃ for degassing treatment for 72 hours. The gas production amount at different degassing times is obtained. First, the theoretical gas production amount is calculated, which is the percentage of the added DCP in the blend mass. The gas production amount is the percentage of material loss after crosslinking and degassing relative to the initial total mass of the blend. The degassing balance is expressed by the following formula.

[0055]

[0056] Another prominent substantive feature of this invention and one of its significant beneficial effects is that, in the step of optimizing the crosslinking agent formulation, the qualitative evaluation index II is used to limit the crosslinking performance and mechanical properties of the material, thereby limiting the lower limit of the crosslinking agent dosage. At the same time, a quantitative evaluation index II for crosslinking gas production is established to optimize the formulation with low gas production or high degassing efficiency. In effect, this can be regarded as exploring the upper limit of the crosslinking agent dosage, thereby solving many problems caused by the large amount of crosslinking agent in the material through this formulation optimization step.

[0057] Step 4: Optimize the antioxidant formulation. Prepare multiple blends of crosslinking agent / antioxidant / base resin, where the mass fractions of crosslinking agent and base resin remain constant, and the mass fractions of antioxidant are ordered in ascending order. Test the samples of each blend and select the best one by passing two levels of indicators: qualitative evaluation indicator III and quantitative evaluation indicator III.

[0058] In a preferred but non-limiting embodiment of the present invention, antioxidants, based on their mechanism of action and synergistic effects, can be used alone or in combination. Taking the combination of two antioxidants as an example, the specific steps are as follows: y phr antioxidant AO1 / z phr antioxidant AO2 / x n phr crosslinking agent / base resin blend, wherein, x n Let y be the mass fraction of the crosslinking agent obtained in step 3, and y be a custom variable sequence of the first antioxidant: y1, y2, y3, ... y n2 z is a user-defined sequence of variables for the second type of antioxidant: z1, z2, z3, ... z n3 y and z represent the mass fractions of the crosslinking agent, both y and z are greater than or equal to 0, and samples prepared with y and z not both being 0 are tested. n2 and n3 represent the sequence length, which is optimized through two levels of indicators: qualitative evaluation index III; and quantitative evaluation index III. In a further preferred but non-limiting embodiment, y and z are arithmetic sequences, with the minimum arithmetic progression value not less than 0.01 phr and not greater than 0.3 phr.

[0059] In a further preferred embodiment, the qualitative evaluation index III includes: two parameters, namely the gel content and the heat elongation rate, which characterize the degree of crosslinking, and the tensile strength parameters and the elongation at break parameters before and after aging obtained based on the air aging performance test. Meeting the above indicators simultaneously is regarded as qualified. If not, the formula is abandoned and the qualitative evaluation of the next alternative antioxidant formula material is carried out. Until the tests of all the alternative formulas for the qualitative evaluation index III are completed, the formula that meets the qualitative evaluation index III is selected and enters the quantitative evaluation index III.

[0060] The quantitative evaluation index III includes: the characteristic parameters based on the crosslinking reaction kinetic curve. The materials of all components are ranked in order of priority. The one with the peak point of the crosslinking reaction kinetic curve more to the right (with time as the horizontal axis) has a higher priority. When the crosslinking reaction kinetic curves coincide, the one with a higher gel content in the qualitative evaluation index III has a higher priority. When the crosslinking reaction kinetic curves coincide and the gel contents in the qualitative evaluation index III are equal, the one with a higher elongation at break in the qualitative evaluation index III has a higher priority. After ranking all the component materials to be optimized in order of priority, the one ranked first enters step 5 preferentially. Those ranked second are used as alternatives for testing.

[0061] Another prominent substantial feature of the present invention and one of the significant beneficial effects brought by it is that, according to the dependence of these key performances on the material formula (antioxidant or crosslinking agent), the performance test link is positioned at the optimal position in the screening method process, greatly reducing the number of repeated tests and unnecessary duplicate tests. Thus, not only the overall workload is reduced, but also the efficiency of formula design and optimization is improved. For example, the crosslinking reaction kinetic curve depends on both the antioxidant and the crosslinking agent. However, in the present invention, the crosslinking reaction kinetic curve is used as the content of the quantitative evaluation index III and is not considered in the quantitative evaluation index II, thereby reducing the workload.

[0062] Step 5: Electrical performance verification. When all the test result parameters meet the requirements, the material formula is regarded as the optimized formula; otherwise, return to test the alternatives.

[0063] In a preferred but non-limiting embodiment of the present invention, the electrical performance verification is a qualitative evaluation, specifically including: conductivity, dielectric loss factor, relative permittivity, and AC breakdown strength.

[0064] In a further preferred embodiment, when all the above parameters meet the requirements, the material formulation is considered an optimized formulation. If any parameter fails to meet the requirements, the formulation is abandoned, and the process returns to quantitative evaluation index III. Electrical performance verification tests are then conducted according to priority. During the verification test, parameters that were not met by the previous formulation are tested first, followed by other parameters. If all formulations in qualitative evaluation index III fail the electrical performance verification, the process returns to quantitative evaluation index II and continues testing according to priority. If none of the formulations in quantitative evaluation index II yield a final formulation, the process returns to quantitative evaluation index I. If none of the formulations in quantitative evaluation index I yield a final formulation, the initial formulation elements are redefined, and the formulation design and optimization are repeated step by step. The electrical performance parameter requirements described in step 5 are the targets for this formulation design and optimization. They can be based on custom requirements or industry standards.

[0065] One of the more prominent substantive features of this invention and the significant beneficial effects it brings is that comprehensive electrical performance testing serves only as the final verification of each formulation, thus avoiding repeated and low-reference-value electrical performance testing.

[0066] The beneficial effects of this invention are that, compared with the prior art, as mentioned above, this invention puts forward specific requirements for the key performance of the base resin and provides clear guiding methods. It fully limits the impurity content, gel point content and polar groups of the base resin by electrical properties, avoids the blind selection of base resin, avoids the failure of the formulation due to the lack of assessment of key performance, avoids the waste of subsequent experimental work, and greatly improves the success rate of formulation design.

[0067] Based on the inventors' long-term experimental research, the additional effects of antioxidants must be considered during the optimization of crosslinking agent formulations. In this invention, the crosslinking agent formulation optimization process uses 0.3 phr antioxidant 300 as a co-oxidant. The special molecular structure of this antioxidant gives it the functions of both a primary antioxidant and a secondary antioxidant, making it functionally representative and compatible. As a result, the crosslinking agent optimization process is more universal, providing a more reliable structural basis for subsequent antioxidant formulation optimization, thereby reducing potential waste of manpower, material resources, and time.

[0068] The three-level quantitative evaluation indexes designed in this invention have optimized the formulations of the base resin, crosslinking agent, and anti-aging agent, respectively. By prioritizing the weights, the formulation design is tilted towards high electrical resistance, easy degassing, anti-scorching, and anti-aging properties. This improves the key performance that is of general concern in the research and development of high-voltage AC cable insulation materials, which is conducive to the research and development of AC crosslinked polyethylene insulation materials with higher voltage levels, and also conducive to the iterative upgrading of AC crosslinked polyethylene insulation materials with fixed grades.

[0069] To more clearly illustrate the implementation steps of the present invention and the beneficial technical effects that can be achieved, a specific example is described below.

[0070] A method for designing and optimizing the formulation of XLPE insulation material for high-voltage AC cables includes the following steps:

[0071] Step 1: Determine the initial formulation elements, including one type of base resin LDPE, denoted as L1, two types of antioxidants, namely antioxidant 1010 and antioxidant 1035, denoted as AO1 and AO2, and the type of crosslinking agent is DCP, denoted as D1.

[0072] Step 2: Select the base resin. This example does not contain multiple resins, so only the qualitative evaluation index I step is performed, and the quantitative evaluation index I step is not required.

[0073] The specific test results of qualitative evaluation index I for L1 resin in this example are as follows:

[0074]

[0075] If all qualitative evaluation indicators I are met, skip the step of quantitative evaluation indicator I and proceed to step 3.

[0076] Step 3: Optimize the content of crosslinking agent D1.

[0077] LDPE granules were added to a mixer at 110℃ and 50 r / min and mixed until completely melted. Antioxidant 300 was added to the mixer and mixed for 5 min, followed by the addition of a certain amount of DCP and mixing for 3 min to prepare crosslinkable polyethylene. A certain amount of crosslinkable polyethylene was hot-pressed in a flat vulcanizing machine at 110℃ for 15 min to form a crosslinking agent. The mixture was then transferred to a flat vulcanizing machine at 175℃ and pressurized to 15 MPa for 30 min. After cooling, the mixture was transferred to a water-cooled flat vulcanizing machine under the same pressure to prepare XLPE samples of different specifications. Specifically, the samples prepared from the blend of x phr crosslinking agent / 0.3 phr antioxidant 300 / base resin were tested in two steps. x is a user-defined variable sequence: x1 = 1.6, x2 = 1.7, x3 = 1.8, x4 = 1.9.

[0078] The selection is carried out using two levels of indicators: qualitative evaluation indicator II; and quantitative evaluation indicator II.

[0079] Qualitative evaluation index II includes two parameters characterizing the degree of crosslinking: gel content and thermal elongation; and tensile strength and elongation at break parameters obtained from stress-strain testing. The specific testing methods for these qualitative evaluation indexes are based on standard JB / T10437-2004. The requirements are: gel content ≥ 82%, elongation at load ≤ 80%, tensile strength ≥ 20 MPa, and elongation at break ≥ 500%.

[0080]

[0081] After testing, when x3 = 1.8 and x4 = 1.9, the above indicators are met simultaneously, which is considered qualified and enters the quantitative evaluation indicator II.

[0082] Qualitative Index II Crosslinking Gas Generation Characteristics Test Method: Weigh approximately 5g of the test formulation material using a precision balance. Weigh it accurately before the reaction. After hot pressing and crosslinking reaction in a flat vulcanizing machine, place it in a vacuum oven at -0.1MPa and 80℃ for degassing treatment for 72h. Weigh it accurately again to obtain the gas generation, theoretical gas generation, and degassing residue.

[0083]

[0084] Step 4: Optimize the antioxidant formulation.

[0085] The two antioxidants can be used alone or in combination, specifically in two steps: Y PHR antioxidant AO1 / Z PHR antioxidant AO2 / X. n The samples prepared from the PHR crosslinking agent / base resin blend were tested. The sample preparation method was the same as in step 3 above. y is a custom variable sequence: y1 = 0.3, y2 = 0.15, y3 = 0, and z is a custom variable sequence: z1 = 0, z2 = 0.15, z3 = 0.3. The optimal values ​​were selected through two levels of indicators: qualitative evaluation indicator III and quantitative evaluation indicator III.

[0086] Qualitative evaluation index III includes two parameters characterizing the degree of crosslinking: gel content and thermal elongation, and tensile strength and elongation at break parameters obtained before and after aging based on air aging performance tests. The specific test methods are based on standard JB / T10437-2004. Requirements include a gel content ≥82%, elongation at break ≤80%, tensile strength ≥20 MPa before aging, elongation at break ≥500%, and changes in tensile strength and elongation at break after aging both less than ±20%.

[0087]

[0088] Materials with y2 = 0.15, z2 = 0.15, and y3 = 0, z3 = 0.3 simultaneously meet the above indicators and are considered qualified, proceeding to quantitative evaluation index III. The cross-linking reaction kinetic curves show that material (y2 = 0.15, z2 = 0.15) has a high priority. Material (y3 = 0, z3 = 0.3) has a lower priority and is considered an alternative for testing.

[0089] Step 5: Electrical performance verification.

[0090] Electrical performance verification is a qualitative evaluation, specifically including: conductivity, dielectric loss factor, relative permittivity, and AC breakdown strength. The test methods are specifically based on standard JB / T 10437-2004, and the requirements are as follows:

[0091]

[0092] All parameters of the material (y2 = 0.15, z2 = 0.15) meet the requirements, resulting in the final formulation: the base resin is L1, the crosslinking agent is D1 with a content of 1.8 phr, and the antioxidants are antioxidants AO1 and AO2 used together, each with a content of 0.15 phr.

[0093] The beneficial effects of this invention are that, compared with the prior art, as mentioned above, this invention puts forward specific requirements for the key performance of the base resin and provides clear guiding methods. It fully limits the impurity content, gel point content and polar groups of the base resin by electrical properties, avoids the blind selection of base resin, avoids the failure of the formulation due to the lack of assessment of key performance, avoids the waste of subsequent experimental work, and greatly improves the success rate of formulation design.

[0094] Based on the inventors' long-term experimental research, the additional effects of antioxidants must be considered during the optimization of crosslinking agent formulations. In this invention, the crosslinking agent formulation optimization process uses 0.3 phr antioxidant 300 as a co-oxidant. The special molecular structure of this antioxidant gives it the functions of both a primary antioxidant and a secondary antioxidant, making it functionally representative and compatible. As a result, the crosslinking agent optimization process is more universal, providing a more reliable structural basis for subsequent antioxidant formulation optimization, thereby reducing potential waste of manpower, material resources, and time.

[0095] This invention employs three levels of quantitative evaluation indicators to optimize the formulations of the base resin, crosslinking agent, and anti-aging agent, respectively. By prioritizing these indicators with appropriate weights, the formulation design is tilted towards high electrical resistance, easy degassing, scorch resistance, and anti-aging properties. This improves key performance characteristics commonly valued in the research and development of high-voltage AC cable insulation materials, facilitating the development of AC crosslinked polyethylene insulation materials for higher voltage levels, and also promoting the iterative upgrading of fixed-grade AC crosslinked polyethylene insulation materials. The applicant has provided a detailed description of the embodiments of this invention in conjunction with the accompanying drawings. However, those skilled in the art should understand that the above embodiments are merely preferred implementations of this invention. The detailed description is only intended to help readers better understand the spirit of this invention and is not intended to limit the scope of protection of this invention. On the contrary, any improvements or modifications made based on the inventive spirit of this invention should fall within the scope of protection of this invention.

Claims

1. A method for designing and optimizing the formulation of XLPE insulation material for high-voltage AC cables, characterized in that, Includes the following steps: Step 1: Determine the initial formulation elements, including the types of base resin, antioxidants, and crosslinking agents; Step 2: Based on the base resin selected in Step 1, test the sample made by hot pressing of pure base resin, and optimize it through two levels of indicators: qualitative evaluation index I and quantitative evaluation index I; to obtain the optimized base resin. Step 3: Prepare multiple blends of crosslinking agent / antioxidant / base resin, wherein the mass fractions of antioxidant and base resin remain constant in multiple blends, and the mass fractions of crosslinking agent are ordered in ascending order. Test the samples of each blend and optimize them through two levels of indicators: qualitative evaluation index II and quantitative evaluation index II; to obtain the optimized crosslinking agent formulation. Step 4: Prepare multiple blends of crosslinking agent / antioxidant / base resin, wherein the mass fractions of crosslinking agent and base resin remain constant, and the mass fractions of antioxidant are ordered in ascending order. Test the samples of each blend and optimize them through two levels of indicators: qualitative evaluation index III and quantitative evaluation index III; to obtain the optimized antioxidant formulation. Step 5: Electrical performance verification. If all test parameters meet the requirements, the material formula is considered an optimized formula. Otherwise, return to the alternative test candidates for testing.

2. The method for formula design and optimization of XLPE insulation material for high-voltage AC cables according to claim 1, characterized in that: In steps 2, 3, or 4, qualitative evaluation refers to determining whether the test result parameters meet qualitative evaluation indicators I, II, or III. If they do, the material is considered qualified; if not, the material is discarded, and the qualitative evaluation of the next candidate material is initiated. This process continues until all candidate materials have completed the tests for qualitative evaluation indicators I, II, or III, and materials that meet the qualitative evaluation indicators are selected. Then, the quantitative evaluation indicators I, II, or III are initiated.

3. The method for formula design and optimization of XLPE insulation material for high-voltage AC cables according to claim 2, characterized in that: Qualitative evaluation index I includes: rheological performance parameters measured by a rotational rheometer, tensile strength parameters and elongation at break parameters obtained by stress-strain testing, and dielectric loss tangent and relative permittivity parameters measured by a high-voltage Schering bridge.

4. The method for designing and optimizing the formulation of XLPE insulation material for high-voltage AC cables according to claim 2, characterized in that: Quantitative evaluation index I includes: the two-parameter Weibull distribution parameters of the AC breakdown field strength measured based on the cylindrical electrode; The characteristic breakdown strength is used as the first parameter 'a', and the shape parameter is used as the second parameter 'b'. Different resins are prioritized based on the size of 'a', with higher priority for larger 'a'. If 'a' is the same, the priority is further subdivided based on 'b', with higher priority for materials with the same 'a'. All resins to be selected are prioritized and ranked. The resin with the highest priority proceeds to step 3 first; the resins with the next highest priority are considered as candidates for testing.

5. The method for formula design and optimization of XLPE insulation material for high-voltage AC cables according to claim 2, characterized in that: In step 3, the samples prepared by the blend of x phr crosslinking agent / 0.3 phr antioxidant 300 / base resin were tested and optimized through two levels of indicators: qualitative evaluation index II; quantitative evaluation index II. Where x is a user-defined sequence of variables: x1, x2, x3, x4...x n1 , represents the mass fraction of the crosslinking agent, x≥0, n1 represents the sequence length, i.e. the number of blend groups.

6. The method for designing and optimizing the formulation of XLPE insulation material for high-voltage AC cables according to claim 5, characterized in that: Qualitative evaluation index II includes two parameters characterizing the degree of crosslinking: gel content and thermal elongation, as well as tensile strength and elongation at break parameters obtained based on stress-strain tests.

7. The method for formula design and optimization of XLPE insulation material for high-voltage AC cables according to claim 5, characterized in that: Quantitative evaluation index II includes: cross-linking gas production characteristic parameters. All materials are prioritized, with higher priority given to those with better gas production characteristics and smaller degassing residue. When degassing residue is equal, lower x-values ​​have higher priority. The priority ranking of all materials to be optimized is as follows: x1, x2, x3, x4…x n1 n1 represents the sequence length. The sequence with the highest priority will proceed to step 4 first, while the sequence with the next highest priority will be considered as a candidate for testing.

8. The method for formula design and optimization of XLPE insulation material for high-voltage AC cables according to claim 2, characterized in that: In step 4, antioxidants are used alone or in combination, based on their mechanism of action and synergistic effects.

9. The method for designing and optimizing the formulation of XLPE insulation material for high-voltage AC cables according to claim 8, characterized in that: Qualitative evaluation index III includes two parameters: gel content and thermal elongation, which characterize the degree of crosslinking, and tensile strength and elongation at break parameters obtained before and after aging based on air aging performance tests.

10. The method for designing and optimizing the formulation of XLPE insulation material for high-voltage AC cables according to claim 8, characterized in that: Quantitative evaluation index III includes: prioritizing all components based on the characteristic parameters of the cross-linking reaction kinetic curve. The further to the right the peak point of the cross-linking reaction kinetic curve is, the higher the priority. When the cross-linking reaction kinetic curves overlap, the higher the gel content in qualitative evaluation index III, the higher the priority. When the cross-linking reaction kinetic curves overlap and the gel content in qualitative evaluation index III is equal, the higher the elongation at break in qualitative evaluation index III, the higher the priority. Prioritize all components to be optimized, and the one ranked first will proceed to step 5 first.

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