A precise method for collecting micro-flow gas using double-ended alternating heating of land cables

Abstract

The present invention relates to a land cable double-end alternating heating type micro-flow gas precision collection method. By pre-installing the head-end and tail-end gas extraction valves at the terminal sealing caps at both ends of the cable, a pressurizing device is used to inject inert gas into the cable to increase the internal pressure, causing the original gas to migrate axially to the head-end to form a gas enrichment area; then the head-end gas extraction valve and the adjacent cable segment are locally heated, and the enriched gas is extracted and collected at a micro-flow rate of 10-20mL / min. After the head-end gas extraction is completed, the pressurization and heating process is repeated at the tail-end to achieve tail-end gas collection; this method effectively reduces gas retention and the impact of pressure difference on long-distance cables by alternating gas extraction at both ends, combined with local heating and micro-flow control, to avoid cable deformation and damage, and improve gas collection accuracy. This method solves the problem in the prior art that the cable lacks effective detection means after degassing and cannot confirm whether the internal residual gas concentration meets the standard, improves the quality and reliability of cable degassing, and ensures product quality.

Description

Technical Field

[0001] The present invention relates to the technical field of cable degassing, and in particular to a land cable double-end alternating heating method for precise micro-flow gas collection. Background Art

[0002] In current cross-linked polyethylene (XLPE) cable production, while the degassing process effectively removes byproduct gases (such as low-molecular hydrocarbons like methane and ethane) from the cable after cross-linking, there is a lack of efficient extraction and quantitative detection technology for residual gases within semi-finished or finished cables after degassing. This makes it impossible to verify whether the degassing effect of semi-finished or finished cables meets the required standards. Furthermore, current domestic and international standards (such as GB and IEC) have not yet clearly defined concentration limits for gas impurities within cables after degassing or installation, resulting in a lack of basis for quality control in the production process and a lack of guidance. Exceeding the specified residual gas concentration can cause insulation degradation, partial discharge, or even breakdown failure. Existing detection systems are unable to assess the risk of residual gas before shipment, making long-term operational reliability difficult to ensure. Furthermore, the standards do not cover the potential impact of gas migration or accumulation after installation, further exacerbating safety hazards during operation. Therefore, there is an urgent need to establish technical methods for the efficient extraction and detection of gases within semi-finished or finished cables, and to promote the development of relevant limit standards, in order to improve the quality verification system for the degassing process and enhance the operational reliability of cables throughout their lifecycle. Summary of the Invention

[0003] This application aims to address the problem that the residual gas concentration of semi-finished or finished cables after degassing cannot be effectively detected due to the lag in degassing effect verification and the lack of standards in the existing technology, and proposes a land cable double-end alternating heating micro-flow gas precision collection method. This method pre-installs gas extraction valves at both ends of the semi-finished or finished cable after degassing, uses high-pressure gas transmission at one end to evenly distribute the gas inside the cable to reduce the gas migration resistance, and simultaneously implements local heating at the gas extraction end to reduce the pressure difference and inhibit gas retention, and cooperates with micro-flow gas collection to control the pressure difference balance; the synergistic effect of the three can effectively reduce gas adsorption, compensate for long-distance transmission pressure loss, avoid cable structure deformation and damage, significantly improve gas collection accuracy, and realize direct quantitative detection of residual gases such as methane and ethane inside the semi-finished or finished cable after degassing. By measuring the gas concentration, the adequacy of degassing can be accurately determined, and quality risks such as local discharge or insulation breakdown during operation caused by residual gas can be effectively prevented. The technical solutions provided by this application are as follows:

[0004] On the one hand, the present application provides a land cable double-end alternating heating method for precise micro-flow gas collection, comprising the following steps:

[0005] Pre-installed gas extraction valve: pre-install the first-end gas extraction valve and the tail-end gas extraction valve on the terminal sealing caps at both ends of the cable;

[0006] Pressurization: Inert gas is injected into the cable through a pressurizing device to increase the pressure inside the cable. At the same time, the pressure difference is used to cause the original gas inside the cable to migrate along the cable axis toward the gas extraction valve at the head end, and form a gas enrichment area in the head end area;

[0007] Heating: using a heating device to locally heat the head-end air extraction valve and the adjacent cable segment;

[0008] Gas extraction: connect the gas extraction valve at the head end to the vacuum pump to remove the air from the pipeline, and then extract the enriched gas at a flow rate of 10-20 mL / min and collect it into the gas collection device;

[0009] Reverse gas extraction: After completing gas extraction at the head end of the cable, close the gas extraction valve at the head end and switch to the tail end of the cable. Repeat pressurization and local heating to allow the gas to be enriched and collected directionally at the gas extraction valve at the tail end.

[0010] Complete the degassing of the semi-finished product or the finished product after winding into a coil.

[0011] In some specific embodiments, the head-end gas extraction valve or the tail-end gas extraction valve is a two-way valve, one end of the two-way valve is connected to the pressurizing device, and the other end is connected to the gas collecting device.

[0012] In some specific embodiments, during the pressurization step, the cable segment adjacent to the head-end air extraction valve or the tail-end air extraction valve is locally heated, and the heating range is the cable area extending 25-35 cm axially from the head-end air extraction valve or the tail-end air extraction valve.

[0013] In some specific embodiments, in the pressurizing step, the pressure of the injected inert gas is 0.1-0.6 MPa, and the duration is 8-12 min;

[0014] In the heating step, the heating temperature is 50-90°C and the duration is 15-25 minutes;

[0015] In the gas extraction step, the vacuum pump evacuates the air to a pressure less than 10 Pa.

[0016] In some specific embodiments, the heating device is a constant temperature circulating hot air gun, and its temperature control range is 50-90°C;

[0017] The pressurizing device includes an inert gas cylinder and a pressure controller, wherein the inert gas cylinder is used to inject inert gas into the interior of the cable, and the inert gas filled therein is nitrogen, and the purity of the nitrogen is not less than 99.999%; the pressure controller is used to adjust the injection pressure, and the adjustment range is 0.1-0.6MPa.

[0018] In some specific embodiments, the pressure difference between the air inlet of the head-end air extraction valve or the tail-end air extraction valve and the air outlet of the connecting end of the gas collection device is controlled within 0.3 MPa.

[0019] In some specific embodiments, when the cable diameter is less than 100 mm, the injection pressure is 70%-80% of the cable design pressure;

[0020] When the cable diameter is 100mm-200mm, the injection pressure is 60%-70% of the cable design pressure;

[0021] In the pressurizing step, when the calculated value of the injection pressure relative to the cable design pressure is greater than the adjustment range of the pressure controller, the maximum value of the adjustment range of the pressure controller is used as the injection pressure.

[0022] In some specific embodiments, the pressure resistance of the head-end air intake valve or the tail-end air intake valve is not less than 10 MPa;

[0023] The total residual content of impurity gases taken out of the tail-end gas extraction valve is less than 1.5 mg / g.

[0024] In some specific embodiments, in the heating step, the heating temperature of the polyethylene insulated cable is 55°C-65°C, and the heating temperature of the cross-linked polyethylene insulated cable is 75°C-85°C.

[0025] In some specific embodiments, the pressure difference control is achieved by adjusting at least one of the injection pressure, the heating temperature of the heating device, and the flow rate of the micro flow.

[0026] By adopting the above technical solution, the land cable double-end alternating heating micro-flow gas precision collection method provided by this application has the following beneficial effects:

[0027] 1. It can directly and quantitatively detect residual gases (methane, ethane, etc.) inside semi-finished or finished cables, filling the technical gap that cannot be verified after the degassing process. By measuring the gas concentration (e.g., methane ≤ 100ppm is the qualified threshold), the adequacy of degassing can be accurately judged, effectively preventing the risk of partial discharge or insulation breakdown during operation caused by gas residue, and improving the long-term reliability of the cable.

[0028] 2. The use of a portable gas extraction device can quickly complete the test in the factory or at the installation site (single operation ≤ 30 minutes), without relying on complex laboratory equipment. Compared with offline gas chromatography analysis, the cost is reduced by 70%, while avoiding cable transportation losses and improving detection efficiency and economy.

[0029] 3. The cable integrity is preserved through a non-destructive gas extraction design (no need to puncture the insulation layer). The synergistic effect of pressurization and local heating improves the detection rate of low-content gases (such as trace H2, methane, etc.). The pre-installed gas extraction valve supports long-term monitoring needs and is compatible with online analysis systems. It provides quantifiable acceptance criteria (such as gas concentration limits) for the degassing process, promoting the improvement of relevant testing clauses in national standards / IEC.

[0030] 4. Regular spot checks (such as before commissioning and after 5 years of operation) can also be used to detect cables with excessive gas residues at an early stage, avoid the expansion of faults, provide data support for cable operation and maintenance, and further ensure the safe and stable operation of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0032] Figure 1 A schematic diagram of the gas extraction device provided in an embodiment of the present application.

[0033] The following is a supplementary description of the accompanying drawings:

[0034] 1-head end air intake valve; 2-tail end air intake valve; 3-pressurization device; 4-gas collection device. DETAILED DESCRIPTION

[0035] The following will be combined with the accompanying drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative work are within the scope of protection of this application.

[0036] References to "one embodiment" or "embodiment" herein refer to specific features, structures, or characteristics that may be included in at least one implementation of the present application. Throughout the description of this application, it should be understood that the terms "upper," "lower," "top," and "bottom," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely for ease of description and simplification. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation, and are therefore not to be construed as limiting the present application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly specifying the number of the technical features indicated. Thus, a feature designated "first" or "second" may explicitly or implicitly include one or more of such features. Furthermore, the terms "first," "second," etc. are used to distinguish similar objects and are not necessarily used to describe a specific order or sequential sequence. It should be understood that such terms are interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.

[0037] When a numerical range is disclosed herein, the above range is considered to be continuous and includes the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be merged. In other words, unless otherwise indicated, all ranges disclosed herein should be understood to include any and all subranges included therein. For example, a specified range from "1 to 10" should be considered to include any and all subranges between a minimum of 1 and a maximum of 10. Exemplary subranges of the range 1 to 10 include, but are not limited to, 1 to 6.1, 3.5 to 7.8, 5.5 to 10, etc.

[0038] See also Figure 1 , a schematic diagram of the gas extraction device provided in an embodiment of the present application, the cable is tightly wound on the cable drum, and its two axial ends are respectively defined as the head end (the starting section of the cable) and the tail end (the end of the cable), wherein the head end is provided with a head end gas extraction valve 1, and the tail end is provided with a tail end gas extraction valve 2; both valves adopt a two-way connecting structure design, one end of which is connected to the pressurizing device 3 through a closed pipe to inject high-pressure gas into the cable, and the other end is connected to the gas collecting device 4, which is used to collect gas samples escaping from the cable during the pressurization process.

[0039] The embodiment of the present application provides a land cable double-end alternating heating method for precise micro-flow gas collection, comprising the following steps:

[0040] Pre-installed gas valves: Pre-install the first-end gas valve 1 and the tail-end gas valve 2 at the terminal sealing caps at both ends of the cable; Pre-install pressure-resistant gas valves (such as Swagelok series, made of 316 stainless steel, with a pressure resistance of ≥10 MPa) at the terminal sealing caps at both ends of the cable, and verify their air tightness (leakage rate <1×10 -6 Pa·m 3 / s), while retaining the original sealing performance of the cable, it achieves pressure-resistant integration of the gas valve and cable terminal. Furthermore, explosion-proof boxes that comply with ATEX certification standards are installed in the installation areas of the head-end gas valve 1 and the tail-end gas valve 2. By enclosing the gas valve and its connecting pipes in an explosion-proof enclosure, the risk of flammable gases (such as hydrogen H2) that may exist within the cable can be effectively isolated and suppressed. In the event of trace gas leakage, the pressure relief structure and flame-retardant materials inside the explosion-proof box quickly block the combustion chain reaction, preventing electric sparks or high-temperature surfaces from igniting the flammable gas, thereby eliminating the potential explosion hazard.

[0041] Pressurization: Inert gas is injected into the cable through the pressurizing device 3 to increase the pressure inside the cable. At the same time, the pressure difference is used to cause the original gas inside the cable to migrate along the cable axis toward the head end gas valve 1, and form a gas enrichment area in the head end area. In order to solve the technical problems that semi-finished or finished cables after degassing are prone to local compression deformation and many bending parts after being wound into coils, resulting in uneven distribution of residual gas, and due to the squeezing between cables, the wound cables have many deformations and bending parts, making it difficult to accurately measure, the present invention adopts a pressurization technical solution. By injecting inert gas into the cable, a certain pressure is formed inside the cable. By pressurizing the inside of the cable, the cable is made fuller and closer to the unwound state of the cable, improving the problems of large gas discharge resistance caused by mutual squeezing and deformation of the cables during winding, and local dead zones where gas cannot flow. It effectively overcomes the gas migration resistance caused by the large residual dead volume of gas inside the cable, the adsorption characteristics of porous materials and the tortuous path, and improves gas fluidity. The pressurization process can not only overcome the physical adsorption force of the material surface on the gas, but also fully squeeze out the gas retained in dead volume areas such as tiny gaps and connections, thereby achieving uniform distribution and thorough expulsion of the residual gas inside the cable, ensuring that the detection process can obtain more representative gas samples, so that the measurement results truly reflect the overall residual gas concentration level inside the cable, and improving the detection accuracy and reliability.

[0042] Heating: A heating device is used to locally heat the gas extraction valve 1 at the head end and the adjacent cable segment. To address the issue of uneven heating of semi-finished or finished cables after degassing due to winding, a targeted heating method is used, with a temperature range of 50-80°C, to heat the sealing cap and the adjacent 25-35cm cable segment. This temperature is monitored in real time using an infrared thermal imager, creating a localized high-temperature zone at the gas extraction end. This effectively reduces the gas adsorption rate of the insulation material and reduces the amount of residual gas, thereby improving detection accuracy. This localized heating solution also avoids the energy waste and structural damage risk associated with heating the entire cable. The semi-finished or finished cable after degassing is long. After one end is supplied with gas and transported for a long time, a pressure drop will occur, which will easily lead to a large pressure difference between the air inlet and outlet ends. The large pressure difference increases the possibility of cable bending and deformation, resulting in an increased risk of damaging the semi-finished or finished cable. In addition, the bending increases the dead zone for gas discharge and increases gas retention, affecting measurement accuracy. In order to solve the problem of detection deviation caused by pressure drop during long-distance gas extraction, the local pressure is increased by the thermal expansion effect generated by local heating at the outlet end, which not only compensates for the pressure loss caused by long-distance transportation, but also reduces the risk of cable bending and deformation at the air extraction end caused by excessive pressure difference, avoiding the problem of detection deviation caused by pressure drop during long-distance gas extraction. Cable deformation causes expansion of gas discharge dead zones and gas retention; in addition, large pressure differences lead to faster airflow speeds. Semi-finished or finished cables are long and arranged in a winding manner with many curved sections. When high-pressure airflow passes over the curved sections at high speed, local low-pressure vortices are formed, which may also increase the trapped gas and affect the measurement accuracy. Local heating causes the gas to expand due to heat, increasing the local pressure, reducing the adsorbed gas and compensating for the pressure loss during long-distance transportation. That is, local heating improves the pressure balance and effectively suppresses the low-pressure vortex effect generated by the high-pressure airflow at the curved parts of the cable, further reducing the probability of gas retention, reducing damage to the cable and improving measurement accuracy.

[0043] Gas extraction: Connect the gas extraction valve 1 at the head end to the vacuum pump to remove the air in the pipeline, and then extract the enriched gas at a micro-flow rate of 10-20mL / min and collect it into the gas collection device 4. Specifically, the gas collection device 4 can be an air bag or a gas chromatograph sampling bottle. The dynamic gas extraction method is used to improve the gas collection efficiency and detection accuracy by precisely controlling the gas extraction parameters. First, connect the gas extraction valve to the vacuum pump and evacuate to <10Pa to completely remove the air in the pipeline, and then slowly extract the enriched gas at a micro-flow rate of 10-20mL / min, and detect the gas extraction flow rate in real time by a mass flow meter. This flow rate design can effectively avoid gas mixing and maintain a stable pressure gradient. Based on the gas density difference formed by the temperature difference between the head and tail ends (high-density gas at the nitrogen inlet at the cold end and low-density gas at the gas outlet at the hot end), a natural convection driving system pointing to the gas outlet is constructed. Micro-flow control further enhances this convection effect and promotes the directional migration of residual gas to the gas outlet. Precise flow control not only ensures sufficient driving force to overcome the resistance of long-distance transportation, but also avoids the problem of gas retention caused by excessive pressure difference fluctuations, and effectively prevents cable structural damage caused by sudden pressure changes. By dynamically balancing the local pressure at the gas outlet, it not only compensates for the pressure loss during long-distance transportation, but also significantly reduces the retention of gas inside the cable, thereby realizing an efficient and low-loss gas collection process.

[0044] Reverse gas extraction: After completing gas extraction at the head end of the cable, close the head end gas extraction valve 1, switch to the tail end of the cable, and repeat pressurization and local heating to allow the gas to be directionally enriched and collected at the tail end gas extraction valve 2. After completing gas extraction at the head end of the cable and closing the head end gas extraction valve 1, switch the gas extraction device to the tail end of the cable, and inject inert gas into the cable again through the pressurization device 3, using the pressure difference to drive the residual gas inside the cable to migrate and enrich toward the tail end; then use the heating device to locally heat the tail end gas extraction valve 2 and its adjacent cable section to further promote gas release and directional aggregation; finally, connect a vacuum pump to extract the enriched gas at the tail end at a micro flow rate (10-20mL / min) and collect it in the gas collection device 4. This process achieves symmetrical operation of alternating gas extraction at both ends of the cable. Through the standardized process of repeated pressurization-heating-gas extraction at the head and tail ends, it eliminates the possible deviation of single-end gas extraction and ensures the comparability and consistency of gas collection data at both ends. If the gas concentration difference between the head and tail ends is significant, it prompts the need to optimize the pressurization or heating parameters to eliminate measurement errors, further ensuring the reliability and traceability of the degassing quality.

[0045] The above method realizes accurate degassing of the semi-finished product or finished product after degassing and the cable after winding into a coil.

[0046] In some specific embodiments, the head-end gas extraction valve 1 or the tail-end gas extraction valve 2 is a two-way valve, one end of which is connected to the pressurizing device 3, and the other end is connected to the gas collection device 4. The use of a two-way valve design realizes a high degree of integration of the pressurized gas injection and vacuum gas extraction processes. The same valve can complete the two-way flow function of inert gas injection and enriched gas extraction, avoiding the air tightness risk and operational complexity caused by frequent valve switching in traditional processes; the two-way sealing characteristics of the valve ensure that the system air tightness remains stable during the conversion of pressurization and gas extraction processes, effectively blocking the reverse infiltration of pipeline air, and improving the purity of gas collection and the accuracy of detection data; eliminating the operational bottleneck of one-way valve switching, making the gas extraction processes at the head end and tail end of the cable completely symmetrical and unified, not only simplifying the process flow, but also ensuring the reproducibility of the degassing process and data comparability.

[0047] In some specific embodiments, during the pressurization step, the cable segment adjacent to the head-end gas valve 1 or the tail-end gas valve 2 is locally heated. The heating range is the cable area extending 25-35 cm axially from the head-end gas valve 1 or the tail-end gas valve 2. Local heating causes the cable material in the target area to expand due to heat, effectively reducing gas adsorption and retention in the insulation material and accelerating the migration rate of gas molecules from the material interior to the valve-enriched area. At the same time, the precise heating range of 25-35 cm avoids the energy waste and structural damage risk caused by heating the entire cable, while ensuring the continuity of the temperature gradient between the gas-enriched area and the gas valve, promoting directional gas flow.

[0048] In some specific embodiments, in the pressurization step, the pressure of the injected inert gas is 0.1-0.6 MPa, and the duration is 8-12 min; preferably, the pressure of the injected inert gas is 0.3 MPa, and the duration is 10 min, forcing the internal gas to move toward the target end, forming an efficient gas migration driving force system, which not only avoids the risk of cable structure damage caused by excessive pressure, but also overcomes the problem of insufficient gas migration power caused by too low pressure.

[0049] In the heating step, the heating temperature is 50-90°C and the duration is 15-25 minutes; preferably, the heating temperature is 60°C and the duration is 20 minutes, which reduces the adsorption rate of the gas in the insulating material and forms an efficient gas desorption process window, thereby avoiding the risk of thermal damage to the cable insulation material caused by high temperature and overcoming the problem of insufficient gas desorption efficiency under low temperature conditions.

[0050] During the gas extraction step, the vacuum pump evacuates the air to a pressure of less than 10Pa, which can effectively remove residual air and trace moisture in the pipeline, reduce background gas interference, and ensure that the gas samples collected during the subsequent pressurization process only come from the residual gas inside the cable, thereby improving the accuracy of gas composition detection and the reliability of degassing effect verification.

[0051] In some specific embodiments, the heating device is a constant-temperature circulating hot air gun with a temperature control range of 50-90°C. This constant-temperature circulating hot air gun achieves uniform heat transfer through forced convection, avoiding the localized overheating that can occur with traditional heating methods. This ensures a uniform and stable temperature distribution on the cable surface, effectively preventing thermal stress damage to the material caused by excessive temperature gradients. The wide temperature range of 50-90°C allows precise matching of the optimal desorption temperature for different insulation material properties (such as polyethylene and cross-linked polyethylene), ensuring that gas molecules receive sufficient desorption activation energy while preventing the insulation material from exceeding its thermal deformation temperature threshold.

[0052] The pressurizing device 3 includes an inert gas cylinder and a pressure controller, wherein the inert gas cylinder is used to inject inert gas into the cable. The inert gas filled inside is nitrogen, and the purity of nitrogen is not less than 99.999%; the pressure controller is used to adjust the injection pressure, and the adjustment range is 0.1-0.6MPa. Using high-purity nitrogen (≥99.999%) as the pressurizing medium can effectively prevent the interference of impurity gases (such as oxygen and moisture) on the gas detection results inside the cable, ensuring the purity of the collected samples and the accuracy of the test data; the pressure controller provides a precise pressure adjustment range of 0.1-0.6MPa, which not only avoids the risk of cable structural damage caused by high-pressure injection, but also overcomes the problem of insufficient gas migration power caused by low-pressure injection. In particular, combined with the preferred working pressure of 0.3MPa, it achieves the best balance between gas replacement efficiency and cable safety; the combination of cylinder and pressure controller realizes closed-loop control of the inert gas injection process, can stably maintain the set pressure value, effectively prevent gas retention or reverse osmosis caused by pressure fluctuations, and create stable process conditions for subsequent gas enrichment and micro-flow extraction.

[0053] In some specific embodiments, the pressure difference between the air inlet of the head-end air extraction valve 1 or the tail-end air extraction valve 2 and the air outlet of the connection end of the gas collection device 4 is controlled within 0.3MPa. Controlling the pressure difference within the range of 0.3MPa can not only ensure sufficient driving force to promote the directional migration of gas from the inside of the cable to the collection device, but also avoid excessive pressure difference causing cable structure deformation or excessive gas flow rate causing local eddy current retention; the dynamic adjustment mechanism allows fine-tuning of various parameters according to real-time monitoring data, such as appropriately increasing the heating temperature to promote gas desorption, reducing the microflow rate to slow down the gas extraction speed to extend the contact time, and forming an optimal process combination; in particular, the pressure difference control range matches the process of forming the preferred heating temperature of 60°C and the microflow rate parameters of 10-20mL / min, which not only ensures that the gas molecules are efficiently desorbed from the inside of the insulating material and migrate to the valve area, but also maintains the system pressure stability, effectively preventing secondary adsorption or cable damage caused by pressure difference fluctuations, and improving the degassing efficiency and measurement accuracy.

[0054] In some specific embodiments, when the cable diameter is less than 100 mm, the injection pressure is 70%-80% of the cable design pressure;

[0055] When the cable diameter is 100mm-200mm, the injection pressure is 60%-70% of the cable design pressure;

[0056] In the pressurization step, when the calculated value of the injection pressure relative to the cable design pressure is greater than the adjustment range of the pressure controller, the maximum value of the adjustment range of the pressure controller is used as the injection pressure.

[0057] Specifically, when the cable diameter is less than 100mm, the injection pressure is set at 70%-80% of the cable design pressure. This higher pressure ratio is mainly based on the structural characteristics of small-diameter cables. When winding, the contact area between cables per unit volume is large, the extrusion dead zone is large, and the gaps are small. This leads to relatively high gas flow resistance, requiring higher pressure to overcome the resistance and ensure effective gas expulsion. When the cable diameter is 100-200mm, the injection pressure is set at 60%-70% of the cable design pressure. This is because although large-diameter cables have greater circumferential stress and relatively poor pressure-bearing performance, when winding, the contact area between cables is small, the extrusion dead zone is small, and the gaps are large, resulting in lower gas flow resistance. A relatively low pressure can be used while ensuring the expulsion effect to avoid overpressure damage to the insulation. In addition, when the calculated injection pressure exceeds the adjustment range of the pressure controller, the maximum adjustment value of the controller is used as the actual injection pressure to ensure process feasibility and stability.

[0058] In some specific embodiments, the pressure resistance of the head-end air extraction valve 1 or the tail-end air extraction valve 2 is not less than 10MPa. The high pressure resistance index (≥10MPa) provides sufficient safety margin for the pressurization process. Even if the internal pressure of the cable temporarily exceeds the design pressure due to process fluctuations or parameter adjustments (such as when a small-diameter cable uses a higher ratio of 70%-80% of the design pressure), the valve can still maintain structural integrity and effectively avoid the risk of leakage or failure due to overpressure; secondly, the high-pressure valve and the precise pressure controller (adjustment range 0.1-0.6MPa) form a redundant protection mechanism. Even if a transient pressure peak occurs during the dynamic adjustment process (such as pressure differential fluctuations caused by rapid gas injection or sudden temperature changes), It can also ensure that the system sealing is not affected, preventing gas reverse permeation or pressure runaway. In addition, the pressure resistance index forms a gradient match with the maximum withstand pressure specified by the cable manufacturer (the injection pressure is strictly lower than the upper limit of the cable withstand pressure), which not only meets the degassing process's requirements for pressure control accuracy, but also provides double safety protection for the cable insulation layer. Especially when a lower pressure ratio (60%-70% of the design pressure) is used for large-diameter cables (with large hoop stress and small flow resistance), the high-pressure valve can fully cover the process pressure range, avoiding process interruptions or safety accidents caused by insufficient valve pressure resistance.

[0059] The total residual content of impurity gas taken from the tail-end gas extraction valve 2 is less than 1.5 mg / g. This can be used to determine that the maximum gas collection of the residual gas was achieved during the first gas extraction at one end, thereby verifying the accuracy of the data. This result can verify that the first gas extraction process has achieved the maximum gas collection of the residual gas inside the cable, thereby ensuring the accuracy of the test data. Through the two-way gas extraction process design, after gas extraction at the head end, the gas extraction at the tail end can still control the gas residue to an extremely low level of less than 1.5 mg / g. This data shows that the first gas extraction has effectively driven out and collected most of the adsorbed gas inside the cable, confirming that the synergistic effect of pressurization, local heating and micro-flow extraction can efficiently complete gas enrichment and extraction; the comparability of the residual gas content at the tail end with the head end data further eliminates the possibility of local adsorption or incomplete gas extraction, provides a reliable basis for the evaluation of the degassing effect, and ensures the accuracy of the test results of the residual gas content inside the cable and the reliability of the process.

[0060] In some specific embodiments, during the heating step, the heating temperature for polyethylene insulated cables is 55°C-65°C, while the heating temperature for cross-linked polyethylene insulated cables is 75°C-85°C. During the heating step, differentiated temperature control strategies are adopted for cables with different insulation material properties. The heating temperature for polyethylene insulated cables is set at 55°C-65°C, while the heating temperature for cross-linked polyethylene insulated cables is set at 75°C-85°C. This temperature-grading design is based on the differences in the thermodynamic properties of the two types of insulation materials. Polyethylene (PE) molecular chains have a linear structure and a low glass transition temperature. Within the 55°C-65°C range, moderate thermal expansion is achieved to reduce gas adsorption and retention, while also avoiding the risk of material softening or thermal degradation caused by excessive molecular chain movement due to high temperatures. Cross-linked polyethylene (XLPE) forms a three-dimensional network skeleton due to its chemical crosslinking structure, which significantly increases its glass transition temperature. Therefore, a higher temperature range (75°C-85°C) is required to effectively promote the desorption and migration of gas molecules from the dense cross-linked network. This temperature range ensures the thermal stability of the cross-linked structure while maximizing gas desorption efficiency. Through material adaptability temperature control, the gas removal efficiency is improved and the physical and chemical properties of the insulating material are ensured not to be damaged.

[0061] In some specific embodiments, the pressure difference control is achieved by adjusting at least one of the injection pressure, the heating temperature of the heating device, and the flow rate of the micro flow. In terms of pressure control, the driving force for the migration of gas molecules inside the cable from the insulating material to the gas extraction valve is directly changed by precisely adjusting the inert gas injection pressure (range 0.1-0.6MPa). Under the premise of ensuring that the pressure is always lower than the cable's design tolerance value (such as a safety threshold of 70%-80% of the design pressure for small-diameter cables and 60%-70% for large-diameter cables), the balance between the gas migration rate and the safety of the cable structure is optimized. In terms of thermodynamic regulation, the heating temperature is set in a gradient manner (55℃-65℃ for polyethylene insulated cables and 75℃-85℃ for cross-linked polyethylene insulated cables). The temperature increase is used to promote the thermal expansion effect of the insulating material to reduce gas adsorption and retention, while increasing the thermal activity of gas molecules to accelerate the desorption process, forming a complementary and synergistic mechanism with pressure regulation. In terms of fluid dynamics control, by dynamically maintaining the micro-flow rate in the range of 10-20mL / min, pressure difference fluctuations and secondary gas adsorption caused by high-speed airflow are avoided, and low-speed extraction is prevented from prolonging the degassing cycle, thereby achieving an optimal balance between enrichment efficiency and extraction stability. The multivariable coupling control system constructed by the collaborative construction of three parameters can perform real-time process optimization for variables such as cable diameter (such as ≤100mm or 100-200mm), insulation material type (PE / XLPE), and winding status. While ensuring degassing efficiency (total impurity gas residual content ≤1.5mg / g), it effectively avoids the risk of overvoltage damage to the insulation layer and uneven gas distribution, providing quantifiable and repeatable technical guarantees for high-precision residual gas detection.

[0062] The above are only optional embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A land cable double-end alternating heating method for precise micro-flow gas collection, characterized in that: The following steps are involved: Pre-installed gas extraction valves: pre-install the first-end gas extraction valve (1) and the tail-end gas extraction valve (2) at the terminal sealing caps at both ends of the cable; Pressurization: Inert gas is injected into the cable through the pressurization device (3) to increase the pressure inside the cable, and at the same time, the pressure difference is used to cause the original gas inside the cable to migrate along the cable axis toward the head end gas valve (1), and form a gas enrichment area in the head end area; Heating: using a heating device to locally heat the head-end air extraction valve (1) and the adjacent cable segment; Gas extraction: connect the head-end gas extraction valve (1) to a vacuum pump to remove air from the pipeline, and then extract the enriched gas at a flow rate of 10-20 mL / min and collect it into the gas collection device (4); Reverse gas extraction: After completing gas extraction at the cable head end, close the head end gas extraction valve (1), switch to the cable tail end, and repeat pressurization and local heating to allow the gas to be directionally enriched and collected at the tail end gas extraction valve (2); Complete the degassing of the semi-finished product or the finished product after winding into a coil.

2. The land cable double-end alternating heating micro-flow gas precise collection method according to claim 1 is characterized in that: During the pressurization step, the cable section adjacent to the head-end air extraction valve (1) or the tail-end air extraction valve (2) is locally heated, and the heating range is the cable area extending 25-35 cm axially from the head-end air extraction valve (1) or the tail-end air extraction valve (2).

3. The land cable double-end alternating heating micro-flow gas precision collection method according to claim 1 is characterized in that: In the pressurization step, the pressure of the injected inert gas is 0.1-0.6 MPa and the duration is 8-12 minutes; In the heating step, the heating temperature is 50-90°C and the duration is 15-25 minutes; In the gas extraction step, the vacuum pump evacuates the air to a pressure less than 10 Pa.

4. The land cable double-end alternating heating micro-flow gas precise collection method according to claim 1 is characterized in that: The heating device is a constant temperature circulating hot air gun, and its temperature control range is 50-90°C; The pressurizing device (3) comprises an inert gas cylinder and a pressure controller, wherein the inert gas cylinder is used to inject inert gas into the interior of the cable, wherein the inert gas filled therein is nitrogen, and the purity of the nitrogen is not less than 99.999%; and the pressure controller is used to adjust the injection pressure, and the adjustment range is 0.1-0.6MPa.

5. The land cable double-end alternating heating micro-flow gas precise collection method according to claim 4 is characterized in that: The pressure difference between the air inlet of the head-end air extraction valve (1) or the tail-end air extraction valve (2) and the air outlet of the connection end of the gas collection device (4) is controlled within 0.3 MPa, so as to ensure sufficient driving force to promote the directional migration of gas from the interior of the cable to the gas collection device (4).

6. The land cable double-end alternating heating micro-flow gas precise collection method according to claim 4 is characterized in that: When the cable diameter is less than 100 mm, the injection pressure is 70%-80% of the cable design pressure; When the cable diameter is 100mm-200mm, the injection pressure is 60%-70% of the cable design pressure; In the pressurizing step, when the calculated value of the injection pressure relative to the cable design pressure is greater than the adjustment range of the pressure controller, the maximum value of the adjustment range of the pressure controller is used as the injection pressure.

7. The land cable double-end alternating heating micro-flow gas precise collection method according to claim 1 is characterized in that: The pressure resistance of the head-end air extraction valve (1) or the tail-end air extraction valve (2) is not less than 10 MPa; The total residual content of impurity gases taken out of the tail-end gas extraction valve (2) is less than 1.5 mg / g.

8. The land cable double-end alternating heating micro-flow gas precise collection method according to claim 3 is characterized in that: In the heating step, the heating temperature of the polyethylene insulated cable is 55°C-65°C, and the heating temperature of the cross-linked polyethylene insulated cable is 75°C-85°C.

9. The land cable double-end alternating heating micro-flow gas precise collection method according to claim 5, characterized in that: The pressure difference control is achieved by adjusting at least one of the injection pressure, the heating temperature of the heating device, and the flow rate of the micro flow.

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