Method for detecting methane content in cable insulation layer

Abstract

The invention belongs to the technical field of cable detection, and particularly relates to a method for detecting the content of methane in a cable insulation layer, methane gas inflation is carried out on a clean headspace bottle by using a methane standard gas bottle with a corresponding concentration, remaining of other gases after the headspace bottle is vacuumized is reduced, the remaining gas after the headspace bottle is vacuumized is methane, and the methane content in the cable insulation layer is reduced. Therefore, the accuracy of standard gas sample preparation is improved, and the measurement precision is improved; by adopting the headspace sample injector, the low efficiency and additional variables of traditional manual sample injection are avoided, and the detection efficiency and accuracy of the methane content in the insulating layer are greatly improved; in addition, different positions of the insulating layer are taken as to-be-detected sample wafers, so that the detection precision is improved, and the accuracy of a detection result is ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of cable detection, and in particular relates to a method for detecting methane content in a cable insulation layer. Background Art

[0002] In traditional offshore power transmission, high-voltage three-core submarine cables play a key role, enabling high-capacity, long-distance power transmission. The insulation layer of the cell units within traditional three-core high-voltage submarine cables contains cross-linked byproducts (e.g., high-voltage cross-linked polyethylene, XLPE). These byproducts originate from the decomposition of peroxide cross-linking agents (e.g., methane and ethane). These residues can lead to the formation of micropores within the insulation layer, inducing electrical dendrites, localized electric field distortion, accelerated high-voltage breakdown, reduced long-term cable life, and difficulties in cable joint fabrication.

[0003] Therefore, the methane content in the cable insulation layer needs to be tested before the cable leaves the factory to ensure the stability of the cable during subsequent use. However, existing detection methods have problems such as low detection accuracy and low detection efficiency.

[0004] It should be noted that this part of the present invention only provides background technology related to the present invention and does not necessarily constitute prior art or public known technology. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for detecting methane content in a cable insulation layer, so as to solve the problems of low accuracy and low detection efficiency of existing detection methods.

[0006] In order to achieve the above object, the present invention provides a method for detecting methane content in a cable insulation layer, comprising: Take a preset length of the cable to be tested and cut several samples from different positions of the insulation layer; Place several samples into headspace bottles respectively; Take out the sample from the headspace bottle and weigh it. After completion, put it back into the corresponding headspace bottle; Each headspace bottle was allowed to stand at room temperature and then sealed; Make a standard gas headspace bottle; Use a gas chromatograph-headspace sampler to detect the sample and the standard gas headspace bottle; view the chromatogram and record the methane volume concentration f; obtain the methane mass concentration in the insulation layer by calculation; Wherein, the preparation of the standard gas headspace bottle comprises: Take a clean headspace bottle and insert the hose connected to the methane standard gas bottle of corresponding concentration into the bottom of the headspace bottle; turn the headspace bottle upside down and continue to fill the bottle with methane standard gas; pull out the hose and seal the headspace bottle; After the headspace bottle is vacuumed, methane standard gas of corresponding concentration is extracted and injected into the headspace bottle to obtain a standard gas headspace bottle; and cutting a number of samples from different positions of the insulation layer, including: Configure magnetic fluid; dripping a preset amount of magnetic fluid onto the dirty area of ​​the sample; the dirty area is the area where the semi-conductive shielding layer debris covers the insulating layer due to continuous friction between the tool and the semi-conductive shielding layer; After applying a magnetic field of a preset intensity for a preset time, the magnetic fluid is removed.

[0007] Optionally, before each headspace bottle is allowed to stand at room temperature and sealed, the following steps are further included: Inject high-purity nitrogen into the bottle at a flow rate of 45-55 mL / min; Synchronously freeze the samples to -25~-35℃; After nitrogen purge for 5-15 min, 3Å molecular sieves were added and the tube was sealed immediately.

[0008] Optionally, taking a cable to be tested of a preset length and cutting a plurality of samples from different positions of the insulation layer also includes: the preset length of the cable to be tested is at least 0.5 m.

[0009] Optionally, if the cable to be tested is a thick-insulated cable with an insulation layer thickness of not less than 20 mm, the cable to be tested is cut into 10 1 mm thin slices along the radial direction, and the first 7 slices are discarded, and the last 3 slices are retained. The 3 slices are taken from different positions of the insulation layer corresponding to the 3 slices as samples, and the 3 samples are located in the area close to the conductive layer, the center area of ​​the insulation layer, and the area close to the metal layer, respectively.

[0010] Optionally, if the cable to be tested is a thin-insulated cable with an insulation layer thickness of less than 20 mm, the cable to be tested is cut axially to the metal layer. If the number of slices is odd, a median sample is taken; if the number of slices is even, two median inner samples are taken.

[0011] Optionally, before the sample is tested using a gas chromatograph-headspace sampler, the following steps may be included: Check the water level of the hydrogen generator. If it is lower than the lower water level, add sufficient distilled water to the white bottle at the back. If the light is not on, turn on the headspace sampler, main unit, and fully automatic air source and hydrogen generator switches in sequence. The order cannot be reversed.

[0012] Optionally, check and adjust the gas pressure entering the gas chromatograph to 0.4-0.5 MPa.

[0013] Optionally, when a gas chromatograph-headspace sampler is used to detect the sample, the detection parameters of the gas chromatograph include: SS injection port temperature: 180~220℃; SS inlet pressure: 7~7.8 psi; Total flow rate: 40~60ml / min; Septum purge flow rate: 2~4ml / min; Injection mode: split; split ratio (8-12):1; split flow rate: 30-50 ml / min; Column flow rate: 3~5ml / min; Column pressure: 7~7.8psi; Average linear velocity of chromatographic column: 45~50cm / s; Column box temperature: 180~200℃; Column oven equilibration time: 0.4~0.6min.

[0014] Optionally, the parameters of the headspace sampler of the gas chromatograph are: Headspace sampler heating box: 170~190℃; Headspace sampler loop: 180~190℃; Headspace sampler transfer line: 180~200℃; Headspace sample bottle equilibration time: 25~35min; Injection duration: 0.4~0.6min; GC cycle: 14-16 min; Headspace bottle shaking times: 18 times / min; The shaking acceleration is 60cm / s 2 ; Headspace filling pressure: 20~30psi; Filling pressure balance time: 0.05~0.15min.

[0015] Optionally, when a gas chromatograph-headspace sampler is used to detect the sample, the method further includes: Optionally, when a gas chromatograph-headspace sampler is used to detect the methane content of the sample in the headspace bottle, the method further includes: The methane mass concentration C of each sample was calculated using the following formula:

[0016] Where P is the air pressure when the bottle is sealed, f is the methane volume concentration, m is the mass of the sample, and t is the temperature when the bottle is sealed.

[0017] The present invention has at least the following beneficial effects: The present invention provides a method for detecting methane content in a cable insulation layer. The method uses methane standard gas of corresponding concentration to inflate a clean headspace bottle with methane gas, thereby reducing the amount of other gases remaining in the headspace bottle after vacuuming. The residual gas after vacuuming is methane, thereby improving the accuracy of standard gas sample preparation and further improving measurement accuracy. The use of a headspace sampler avoids the inefficiency and additional variables of traditional manual sampling, greatly improving the detection efficiency and accuracy of the methane content in the insulation layer. The sampling of different positions of the insulation layer as samples to be tested is conducive to improving the detection accuracy and ensuring the accuracy of the test results.

[0018] By using magnetic nanofluid technology, a magnetic field is applied to control the flow of magnetic fluid on the sample surface, which then absorbs debris and removes it with a scraper. This helps to ensure the cleanliness of the sample surface and further improve the detection accuracy of methane content.

[0019] Furthermore, by introducing high-purity nitrogen into the headspace bottle and simultaneously cooling it to the corresponding temperature, the production of ethane can be suppressed, thereby reducing the impact of ethane on the accuracy of methane concentration detection, which is conducive to improving the final methane detection accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 This is a flow chart of a method for detecting methane content in a cable insulation layer provided by an embodiment of the present invention; Figure 2 It is a specific flowchart of step S500 provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0022] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0023] Those skilled in the art will understand that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art in the art to which this invention belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and, unless specifically defined as such, will not be interpreted in an idealized or overly formal sense.

[0024] It should be further understood that the wording "comprising" used in the description of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof. The wording "and / or" used herein includes all or any unit and all combinations of one or more associated listed items.

[0025] like Figure 1 As shown, an embodiment of the present invention provides a method for detecting methane content in a cable insulation layer, comprising the following steps S100 to S500: S100, taking a cable to be tested of a preset length, and cutting a plurality of samples at different positions of the insulation layer.

[0026] Specifically, first obtain a cable to be tested with a preset length of at least 0.5 m.

[0027] When slicing is required, saw off a sample section of about 50mm from the middle, remove all protective layers except the insulating layer, and clean the surface copper chips.

[0028] (a) For cables with thick insulation (i.e., insulation layer diameter greater than or equal to 20mm), use a radial slicer to cut the cable radially into ten 1mm thin slices. Discard the first seven slices, retaining the last three. Samples are taken from each of these three slices at different locations on the insulation layer: near the conductive layer, in the center of the insulation layer, and near the metal layer. Specifically, use a punching machine with a die cutter capable of cutting small rectangular pieces to remove three samples each representing the inner, middle, and outer layers from the test sheet. Each sample must be complete, excluding the inner and outer shields. The inner layer sample must be in close contact with the conductive layer, the outer layer sample must be in contact with the insulation layer, and the middle layer sample must be measured using a steel ruler, midway between the inner and outer layers. These samples are then placed in three 20ml headspace vials, marked with the layers, and left unsealed. Testing is performed near the inner and outer shields, as well as in the middle, to determine the methane content at different locations on the insulation layer.

[0029] (b) For thin-insulated cables (insulation diameter less than 20 mm), use an axially cut cable slicer to slice the cable from the outside inward to the conductor shield at a thickness of 1 mm. After removing all thin slices with shielding, select the innermost slice, outermost slice, and middle slice of the remaining portion (select the median slice when the remaining portion is an odd number, and select the slice closer to the inner layer of the two median slices when the remaining portion is an even number). Specifically, use a punching machine with a die knife that can cut small rectangular pieces, and take three samples along the center axis of the slice (obtained by measuring with a steel ruler). After obtaining, place them separately in three 20 ml headspace bottles that have been marked for layering. Do not seal them. By taking three samples and placing them in the headspace bottles with the layering marks first, the residual methane and other residues released by the samples will dilute the gas residue in the headspace bottles, thereby improving measurement accuracy.

[0030] Furthermore, due to the continuous friction between the tool and the semi-conductive shielding layer, the semi-conductive shielding layer debris covers the insulating layer, resulting in inaccurate methane quantification. Therefore, when cutting several samples at different positions of the insulating layer, magnetic nanofluid technology can be used to solve the above problem. Specifically, the following steps are included: (1) Preparing a magnetic fluid; the magnetic fluid is prepared by mixing ferroferric oxide particles of 8-12 nm (optionally 10 nm) with perfluoropolyether oil in a ratio of 1:95-105 (preferably 1:100).

[0031] (2) Add a preset amount (0.9-1.1 μL, for example, 1 μL) of magnetic fluid to the dirt area of ​​the sample; the dirt area is the area where the semi-conductive shielding layer debris covers the insulating layer due to continuous friction between the tool and the semi-conductive shielding layer.

[0032] (3) After applying a magnetic field of a preset intensity for a preset time, remove the magnetic fluid with a polytetrafluoroethylene scraper. The intensity of the magnetic field is 0.4-0.6T, preferably 0.5T, and the application time is 25-35s, preferably 30s.

[0033] Because the semiconductive shielding layer contains debris such as carbon black, the carbon black is selectively adsorbed by the magnetic nanoparticles (affinity energy >40kJ / mol). By applying a magnetic field and controlling the flow of the magnetic fluid across the sample surface, the debris is adsorbed and removed with a scraper. Because the surface energy of the crosslinked byproduct (33mN / m) is lower than that of the magnetic fluid (16mN / m), the magnetic fluid does not wet the sample and remains on the sample, making it difficult to remove. Furthermore, the perfluoropolyether oil does not dissolve hydrocarbons, preventing methane from escaping and improving detection accuracy.

[0034] S200, placing several samples into headspace bottles respectively.

[0035] S300, taking the sample out of the headspace bottle and weighing it, then putting it back into the corresponding headspace bottle and recording the sample weight, sample tray number and degassing time.

[0036] S400: Place each standard gas headspace bottle on the table and let it stand until it reaches room temperature. Then, clamp the standard gas headspace bottle with a clamp and fix the bottle cap on the headspace bottle mouth to complete the sealing.

[0037] S500, making standard gas headspace bottles.

[0038] In some embodiments, as Figure 2 As shown in the figure, the production of the standard gas headspace bottle in S500 specifically includes: S510: Take a clean headspace bottle and insert the hose connected to the methane standard gas cylinder of the corresponding concentration into the bottom of the headspace bottle. Invert the headspace bottle, cover the bottle mouth with the bottle cap, open the cylinder valve, and continuously fill the bottle with methane standard gas. The methane standard gas needs to fill the standard gas cylinder completely, and generally requires continuous ventilation for at least 1 minute.

[0039] S520, quickly pull out the hose and seal the headspace bottle, then close the gas cylinder valve.

[0040] For S530, use a flat-tip needle to connect to the vacuum pump and then insert into the headspace bottle.

[0041] It should be noted that the insertion position should not be in the center or edge of the bottle cap to reduce the probability of insufficient airtightness.

[0042] S540, turn on the vacuum pump, continue pumping for a preset time, and then pull out the needle.

[0043] Optionally, the specific pumping time is about 40 minutes.

[0044] The S550 uses a syringe to draw 20ml of methane standard gas of the desired concentration from a reservoir bag after multiple aspirations. This is then injected into a vacuum-filled headspace vial. If the syringe is essentially completely aspirated, the standard gas preparation is complete. If not, re-preparation is required. This method avoids the inefficiencies and additional variables of traditional manual sampling, significantly improving detection efficiency and accuracy.

[0045] In this embodiment, by using a methane standard gas cylinder of corresponding concentration to inflate the clean headspace bottle with methane gas, it is helpful to reduce the retention of other gases in the headspace bottle after vacuuming. The residual gas after vacuuming is methane, thereby improving the accuracy of standard gas sampling and further improving the measurement accuracy.

[0046] It should be noted that after the standard gas bottle is placed on the table, you should not directly touch the standard gas headspace bottle with your hands to avoid affecting the final test results.

[0047] S600, using a gas chromatograph-headspace sampler to detect the sample and the standard gas headspace bottle; viewing the chromatogram, recording the methane volume concentration f; and calculating the methane mass concentration in the insulation.

[0048] Optionally, before sealing the headspace bottle in step S400, the method further includes: (1) Inject high-purity nitrogen into the bottle at a flow rate of 45-55 mL / min (preferably 50 mL / min).

[0049] (2) Synchronously freeze the sample to -25-35°C (preferably -30°C) to reduce the ethane vapor pressure by 90%, thereby inhibiting the production of ethane and reducing the impact of ethane.

[0050] (3) After nitrogen purging for 5-15 minutes (preferably 10 minutes), add 3Å molecular sieves and immediately seal. The pore size of the molecular sieve is 0.3-0.33 nm.

[0051] By adopting the above steps (1) to (3), the ethane removal rate can be greater than 80%. Because the ethane kinetic diameter is 0.40 to 0.47 nm, and the methane kinetic diameter is about 0.38 nm, methane has a stronger ability to pass through the molecular sieve. By applying pressure with nitrogen, the effect of the molecular sieve is further reduced to improve the measurement accuracy.

[0052] Optionally, before the sample is detected using a gas chromatograph-headspace sampler, the method further includes: Check the water level of the hydrogen generator. If it is lower than the lower water level, add sufficient distilled water to the white bottle at the back. If the light is not on, turn on the headspace sampler, main unit, and fully automatic air source and hydrogen generator switches in sequence. Note that the order cannot be reversed.

[0053] Check and adjust the gas pressure entering the gas chromatograph, which is required to be 0.4-0.5Mpa.

[0054] In step S600, when a gas chromatograph-headspace sampler is used to detect a sample, the detection parameters of the gas chromatograph include: SS injection port temperature: 180~220℃; SS inlet pressure: 7~7.8psi; SS inlet refers to the split / splitless inlet.

[0055] Total flow rate: 40~60ml / min; Septum purge flow rate: 2~4ml / min; The injection mode is split, and the specific split ratio is (8~12):1; split flow rate: 30~50ml / min; using this stable split flow rate ensures sharp and symmetrical peak shape, facilitates accurate integration, avoids column overload (peak broadening, tailing), reduces peak area variation caused by injection volume fluctuations, and improves measurement accuracy.

[0056] Chromatographic column selection: Agilent 19095P-QO4: HP-PLOT Q (-60℃-270℃ 30m×530μm×40μm), column flow rate: 3~5ml / min; reduce the flow rate to ensure high reproducibility of retention times. Too low a flow rate will broaden the peaks (reduced sensitivity); too high a flow rate will increase mass transfer resistance, peak tailing or reduced resolution.

[0057] Chromatographic column pressure: 7~7.8psi; increasing the pressure is conducive to reasonably controlling the methane peak time.

[0058] Average linear velocity of the chromatographic column: 45~50cm / s; a relatively low average linear velocity of the chromatographic column allows methane to obtain the minimum plate height, the smallest peak width, and the largest peak height, thereby increasing sensitivity and improving measurement accuracy.

[0059] Column box temperature: 180~200℃; Column oven equilibration time: 0.4~0.6min.

[0060] In this example, gas chromatography detection was performed using a suitable column temperature, which helped maintain favorable kinetics and optimized diffusion and mass transfer of methane molecules within the column, resulting in symmetrical, sharp chromatographic peaks and improved measurement accuracy. Column equilibration time control ensured sufficient time for the entire column temperature to become uniform and stable, and for the baseline to stabilize, after a temperature change (especially at the end of a temperature programming event). This completely eliminated the negative effects of thermal hysteresis and baseline fluctuation on retention time reproducibility, peak shape, and integration accuracy.

[0061] Optionally, set the FID (Flame Ionization Detector) temperature to 280-320°C (preferably 300°C). Setting a higher temperature maintains a constant FID ionization efficiency and ensures that the methane response factor (peak area / mass) does not fluctuate with sample changes.

[0062] FID air flow rate: 380~420 ml / min (preferably 400 ml / min).

[0063] FID hydrogen gas flow rate: 25-35 ml / min (preferably 30 ml / min). Proper control of hydrogen and hydrogen gas flow rates optimizes flame ionization efficiency. However, too low a hydrogen flow rate results in inadequate methane ionization, resulting in decreased sensitivity. Too high a hydrogen flow rate increases flame turbulence and noise.

[0064] FID makeup gas (N2): 20-30 ml / min (preferably 25 ml / min). Set a higher purge flow rate to accelerate sample transfer and focus the chromatographic peak.

[0065] The carrier gas flow is calibrated by constant makeup gas plus fuel gas flow.

[0066] The minimum peak width of FID acquisition is 5Hz / 0.04min, which helps improve the integration accuracy of methane measurement and thus improve measurement accuracy.

[0067] Optionally, in step S600, the parameters of the head space sampler of the gas chromatograph are: Headspace sampler heating box: 170~190℃ (preferably 180℃); Headspace sampler loop: 180~190℃ (preferably 185℃); Headspace sampler transfer line: 180~200℃ (preferably 190℃); Headspace sample bottle equilibration time: 25~35min (preferably 30min); Injection duration: 0.4~0.6min (preferably 0.5min).

[0068] In this embodiment, the headspace sampler heating box temperature is high to strongly drive the release of methane from various matrices into the headspace gas phase, ensuring that the releasable methane is volatilized as completely as possible. By setting the headspace sampler heating box to 180°C, the headspace sampler quantitative loop to 185°C, and the headspace sampler transfer line to 190°C, the temperature is gradually increased to form a high temperature gradient along the entire sampling and transfer path, completely preventing condensation of the sample gas (especially the water vapor therein) at any link, ensuring that the sample gas representing the equilibrium concentration extracted from the headspace gas phase enters the GC chromatographic column without loss or change, and intact.

[0069] Furthermore, the GC cycle time is set to 14-16 minutes (preferably 15 minutes). By setting a longer cycle time, all samples can be analyzed under completely consistent system conditions. The GC cycle time refers to the total time required for the gas chromatograph to complete a complete analysis process after all pretreatments are completed.

[0070] The sample bottle was shaken at 16-20 times / min (preferably 18 times / min).

[0071] The shaking acceleration is 60cm / s 2 By increasing the shaking frequency and high shaking acceleration, the mass transfer dynamics are enhanced, forcing the adsorbed methane to desorb, thereby improving the accuracy of methane detection.

[0072] Headspace filling pressure: 20~30psi (preferably 25psi); the silicone rubber bottle pad is slightly permeable to methane, which increases the headspace filling pressure, prevents methane loss, and improves measurement accuracy.

[0073] Filling pressure equilibrium time: 0.05~0.15min (preferably 0.1min) to avoid gas composition not representing the true equilibrium state (uneven methane distribution), and the extraction mode is single extraction.

[0074] By optimizing the above parameters, the precision (reproducibility) and accuracy (truly reflecting the sample content) of the analysis results are guaranteed.

[0075] By examining the chromatogram, record the methane volume concentration f (ppm), and pay attention to the methane peak shape to see if there is any obvious abnormal fluctuation or deviation. If so, report it to the testing technology department to replace the standard gas. Finally, use the following formula to calculate the methane mass concentration C (unit: ppm) of each sample:

[0076] Where P is the air pressure when the bottle is sealed, f is the methane volume concentration recorded by the gas chromatograph (unit: ppm), m is the mass of the sample, and t is the temperature when the bottle is sealed.

[0077] The method for detecting methane content in a cable insulation layer provided by an embodiment of the present invention uses a methane standard gas cylinder of corresponding concentration to inflate a clean headspace bottle with methane gas, which is beneficial to reducing the retention of other gases in the headspace bottle after vacuuming. The residual gas after vacuuming is methane, thereby improving the accuracy of standard gas sampling and further improving measurement accuracy; and the use of a headspace sampler avoids the inefficiency and additional variables of traditional manual sampling, greatly improving the detection efficiency and accuracy of the methane content in the insulation layer; by taking different positions of the insulation layer as samples to be tested, it is beneficial to improve the detection accuracy and ensure the accuracy of the test results.

[0078] Furthermore, magnetic nanofluid technology is used to control the flow of magnetic fluid on the sample surface by applying a magnetic field, which then absorbs debris and removes it with a scraper. This is beneficial to ensuring the cleanliness of the sample surface and further improving the detection accuracy of methane content.

[0079] Furthermore, by introducing high-purity nitrogen into the headspace bottle and simultaneously cooling it to the corresponding temperature, the production of ethane can be suppressed, thereby reducing the impact of ethane on the accuracy of methane concentration detection, which is conducive to improving the final methane detection accuracy.

[0080] The present invention will be further described below through specific examples and comparative examples: Example 1: The above method was used to conduct three independent tests on the methane content in the inner, middle and outer insulation of a certain type of cable. The results are shown in Table 1 below: Table 1 Comparison of methane content detection in insulation layers at different locations

[0081] Comparative Example 1: The traditional thermogravimetric analysis method was used to test the degassing degree of the cable. The method was to rapidly heat the temperature to 175°C at a rate of 50°C / min, and maintain the temperature for 30 minutes. The weight change of the insulation in different layers during this period was observed. The results are shown in Table 2 below: Table 2 Comparison of heating time and gas content in the insulation layer at different positions

[0082] As can be seen from the table, the thermogravimetric analysis method can only perform a rough quantitative analysis of the residual volatile gas content in the insulation (at 175°C). However, the analysis of methane gas, which is the most harmful to the cable system, has extremely low reference value. Both the content and the ratio between different layers are greatly different from the actual situation.

[0083] Comparative Example 2: Instead of using a headspace sampler, the traditional gas chromatography manual injection method was used for injection. The results are shown in Table 3 below: Table 3 Comparison of methane content detection in insulation layers at different locations

[0084] As can be seen from Table 3, the traditional manual injection method is greatly affected by temperature, air pressure, and injection speed, and is difficult to quantify and estimate, resulting in poor repeatability of the test results, and the values ​​are significantly greater than those obtained by the method in Example 1.

[0085] Comparative Example 3: The S400 was not pre-frozen and then filled with nitrogen, and molecular sieves were used to eliminate the influence of ethane. The results are shown in Table 4 below: Table 4 Comparison of methane content detection in insulation layers at different locations

[0086] As can be seen from Table 4, due to not using this method to eliminate the influence of ethane as much as possible, the methane quantitative results of each layer have a slight increase as a whole, which leads to deviation in the test results.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for detecting methane content in a cable insulation layer, characterized in that: include: Take a preset length of the cable to be tested and cut several samples from different positions of the insulation layer; Place several samples into headspace bottles respectively; Take out the sample from the headspace bottle and weigh it. After completion, put it back into the corresponding headspace bottle; Each headspace bottle was allowed to stand at room temperature and then sealed; Make a standard gas headspace bottle; Use gas chromatograph-headspace sampler to detect samples and standard gas headspace bottles; Check the chromatogram and record the methane volume concentration f; obtain the methane mass concentration in the insulating layer by calculation; Wherein, the preparation of the standard gas headspace bottle comprises: Take a clean headspace bottle and insert the hose connected to the methane standard gas bottle of corresponding concentration into the bottom of the headspace bottle; turn the headspace bottle upside down and continue to fill the bottle with methane standard gas; pull out the hose and seal the headspace bottle; After the headspace bottle is vacuumed, methane standard gas of corresponding concentration is extracted and injected into the headspace bottle to obtain a standard gas headspace bottle; and cutting a number of samples from different positions of the insulation layer, including: Configure magnetic fluid; dripping a preset amount of magnetic fluid onto the dirty area of ​​the sample; the dirty area is the area where the semi-conductive shielding layer debris covers the insulating layer due to continuous friction between the tool and the semi-conductive shielding layer; After applying a magnetic field of a preset intensity for a preset time, the magnetic fluid is removed.

2. The method for detecting methane content in a cable insulation layer according to claim 1, characterized in that: Before each headspace bottle is allowed to stand at room temperature and sealed, the following steps are also included: Inject high-purity nitrogen into the bottle at a flow rate of 45-55 mL / min; Synchronously freeze the samples to -25~-35℃; After nitrogen purge for 5-15 min, 3Å molecular sieves were added and the tube was sealed immediately.

3. The method for detecting methane content in a cable insulation layer according to claim 1, characterized in that: Taking a cable to be tested of a preset length and cutting a plurality of samples from different positions of the insulation layer also includes: the preset length of the cable to be tested is at least 0.5m.

4. The method for detecting methane content in a cable insulation layer according to claim 3, characterized in that: If the cable to be tested is a thick-insulated cable with an insulation layer thickness of not less than 20 mm, cut the cable to be tested into 10 1 mm thin slices along the radial direction, discard the first 7 slices, and retain the last 3 slices. Take the 3 slices from different positions of the insulation layer as samples, and the 3 samples are located in the area close to the conductive layer, the center area of ​​the insulation layer, and the area close to the metal layer respectively.

5. The method for detecting methane content in a cable insulation layer according to claim 3, characterized in that: If the cable to be tested is a thin-insulated cable with an insulation layer thickness of less than 20 mm, cut the cable to be tested axially to the metal layer. If the number of slices is odd, take the middle sample; if the number of slices is even, take two middle-center samples.

6. The method for detecting methane content in a cable insulation layer according to claim 1, characterized in that: Before using gas chromatograph-headspace sampler to test the sample, it also includes: Check the water level of the hydrogen generator. If it is lower than the lower water level, add sufficient distilled water to the white bottle at the back. If the light is not on, turn on the headspace sampler, main unit, and fully automatic air source and hydrogen generator switches in sequence. The order cannot be reversed.

7. The method for detecting methane content in a cable insulation layer according to claim 6, characterized in that: Check and adjust the gas pressure entering the gas chromatograph to 0.4-0.5Mpa.

8. The method for detecting methane content in a cable insulation layer according to claim 1, characterized in that: When using a gas chromatograph-headspace sampler to test samples, the gas chromatograph detection parameters include: SS injection port temperature: 180~220℃; SS inlet pressure: 7~7.8 psi; Total flow rate: 40~60ml / min; Septum purge flow rate: 2~4ml / min; Injection mode: split; split ratio (8-12):1; split flow rate: 30-50 ml / min; Column flow rate: 3~5ml / min; Column pressure: 7~7.8psi; Average linear velocity of chromatographic column: 45~50cm / s; Column box temperature: 180~200℃; Column oven equilibration time: 0.4~0.6min.

9. The method for detecting methane content in a cable insulation layer according to claim 8, characterized in that: The parameters of the headspace sampler of the gas chromatograph are: Headspace sampler heating box: 170~190℃; Headspace sampler loop: 180~190℃; Headspace sampler transfer line: 180~200℃; Headspace sample bottle equilibration time: 25~35min; Injection duration: 0.4~0.6min; GC cycle: 14-16 min; Headspace bottle shaking times: 18 times / min; The shaking acceleration is 60cm / s 2 ; Headspace filling pressure: 20~30psi; Filling pressure balance time: 0.05~0.15min.

10. The method for detecting methane content in a cable insulation layer according to claim 1, characterized in that: When using a gas chromatograph-headspace sampler to test the sample, it also includes: The methane mass concentration C of each sample was calculated using the following formula: Where P is the air pressure when the bottle is sealed, f is the methane volume concentration, m is the mass of the sample, and t is the temperature when the bottle is sealed.

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