Methods for detecting methane content in cable insulation

Abstract

This invention belongs to the field of cable testing technology, specifically relating to a method for detecting methane content in cable insulation. By using a standard methane gas cylinder of corresponding concentration to fill a clean headspace vial with methane gas, it reduces the amount of other gases remaining in the headspace vial after vacuuming. The remaining gas after vacuuming is methane, thereby improving the accuracy of standard gas sample preparation and thus improving measurement precision. Furthermore, the use of a headspace sampler avoids the inefficiency and additional variables of traditional manual sampling, significantly improving the detection efficiency and accuracy of methane content in the insulation. In addition, by taking samples from different locations within the insulation layer, the detection precision is improved, ensuring the accuracy of the test results.

Description

Technical Field

[0001] This invention belongs to the field of cable testing technology, specifically relating to a method for detecting methane content in cable insulation. Background Technology

[0002] In the traditional field of marine power transmission, high-voltage three-core submarine cables play a crucial role in transmitting large-capacity, long-distance power. The insulation layer of the core units in traditional three-core high-voltage submarine cables contains cross-linking byproducts (e.g., high-voltage cross-linked polyethylene, XLPE). These byproducts originate from the decomposition of peroxide cross-linking agents (such as methane and ethane), and their residue can lead to problems such as: the formation of micropores within the insulation layer, induced electric tree aging, localized electric field distortion, accelerated high-voltage breakdown risk, reduced long-term cable lifespan, 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 testing methods suffer from low accuracy and low 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 known technology. Summary of the Invention

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

[0006] To achieve the above objectives, the present invention provides a method for detecting methane content in cable insulation, comprising:

[0007] Take a cable of a predetermined length to be tested, and cut several samples from different positions of the insulation layer;

[0008] Several samples were placed in headspace bottles.

[0009] Remove the sample from the headspace vial, weigh it, and then reinsert it into the corresponding headspace vial.

[0010] After each empty bottle has been left to stand at room temperature, seal it.

[0011] Making a labeling gas head-empty bottle;

[0012] The sample and standard gas headspace vial were analyzed using a gas chromatograph-headspace sampler; the chromatograms were examined and the methane volume concentration f was recorded; the methane mass concentration inside the insulation layer was calculated.

[0013] The preparation of the standard gas headspace bottle includes:

[0014] Take a clean headspace vial and insert the tubing connected to the methane standard gas vial of the corresponding concentration into the bottom of the headspace vial; invert the headspace vial and continuously fill it with methane standard gas; after pulling out the tubing, seal the headspace vial;

[0015] After evacuating the headspace vial, a standard methane gas of the corresponding concentration is extracted and injected into the headspace vial to obtain a standard gas headspace vial.

[0016] In addition, several samples were cut from different locations of the insulation layer, including:

[0017] Configure magnetic fluid;

[0018] A predetermined amount of magnetic fluid is dropped onto the contaminated area of ​​the sample; the contaminated area is the area caused by continuous friction between the tool and the semiconductive shielding layer, resulting in debris from the semiconductive shielding layer covering the insulating layer.

[0019] After applying a magnetic field of preset strength for a preset time, the magnetofluid is removed.

[0020] Optionally, before sealing each headspace vial after allowing it to stand at room temperature, the process also includes:

[0021] Inject high-purity nitrogen gas into the bottle at a flow rate of 45-55 mL / min;

[0022] Simultaneously freeze the samples to -25~-35℃;

[0023] After purging with nitrogen for 5-15 minutes, add the 3Å molecular sieve and seal immediately.

[0024] Optionally, a cable to be tested of a preset length is taken, and several samples are cut from different positions of the insulation layer. The preset length of the cable to be tested is at least 0.5m.

[0025] Optionally, if the cable to be tested is a thick-insulated cable with an insulation layer thickness of not less than 20 mm, then cut 10 1 mm thin slices along the radial direction of the cable to be tested, discard the first 7 thin slices, retain the last 3 thin slices, and take the 3 thin slices corresponding to different positions of the insulation layer as samples. The 3 samples are respectively located in the area near the conductive layer, the central area of ​​the insulation layer, and the area near the metal layer.

[0026] Optionally, if the cable under test is a thin-insulated cable with an insulation layer thickness of less than 20 mm, the cable under test is cut along the axial direction to the metal layer. If the number of slices is odd, the median sample is taken; if the number of slices is even, two median samples are taken.

[0027] Optionally, before analyzing the sample using a gas chromatograph-headspace sampler, the following steps are also included:

[0028] Check the water level in the hydrogen generator. If it is below the lower water level, add sufficient distilled water to the white bottle at the back. If the light does not turn on, turn on the headspace sampler, main unit, and fully automatic air source and hydrogen generator switches in sequence. The order must not be reversed.

[0029] Optionally, check and adjust the gas pressure entering the gas chromatograph to 0.4-0.5 MPa.

[0030] Optionally, when using a gas chromatograph-headspace sampler to detect the sample, the gas chromatograph detection parameters include:

[0031] SS injection port temperature: 180~220℃;

[0032] SS inlet pressure: 7~7.8 psi;

[0033] Total flow rate: 40~60ml / min;

[0034] Septum purge flow rate: 2~4 ml / min;

[0035] Injection mode: split; split ratio: (8~12):1; split flow rate: 30~50 ml / min;

[0036] Column flow rate: 3~5 ml / min;

[0037] Column pressure: 7~7.8 psi;

[0038] Average linear velocity of the chromatographic column: 45~50 cm / s;

[0039] Column oven temperature: 180~200℃;

[0040] Column box balancing time: 0.4~0.6 min.

[0041] Optionally, the parameters of the headspace sampler of the gas chromatograph are:

[0042] Headspace sampler heating chamber: 170~190℃;

[0043] Headspace sampler quantitative loop: 180~190℃;

[0044] Headspace sampler transfer line: 180~200℃;

[0045] Headspace vial equilibration time: 25~35 min;

[0046] Injection duration: 0.4~0.6 min;

[0047] GC cycle: 14~16 min;

[0048] Headspace bottle shaking frequency: 18 times / min;

[0049] The shaking acceleration is 60 cm / s² 2 ;

[0050] Headspace filling pressure: 20~30psi;

[0051] Filling pressure equalization time: 0.05~0.15min.

[0052] Optionally, when using a gas chromatograph-headspace sampler to detect the sample, the method further includes: Optionally, when using a gas chromatograph-headspace sampler to detect the methane content of the sample in the headspace vial, the method further includes:

[0053] The methane mass concentration C of each sample was calculated using the following formula:

[0054]

[0055] Where P is the gas pressure at the time of sealing, f is the methane volume concentration, m is the mass of the sample, and t is the temperature at the time of sealing.

[0056] The present invention has at least the following beneficial effects:

[0057] The method for detecting methane content in cable insulation provided by this invention utilizes standard methane gas of corresponding concentration to purge a clean headspace vial with methane. This reduces the amount of other gases remaining in the headspace vial after vacuuming, ensuring that the remaining gas after vacuuming is methane. This improves the accuracy of standard gas sample preparation and consequently, the measurement precision. Furthermore, the use of a headspace sampler avoids the inefficiency and additional variables of traditional manual sampling, significantly improving the detection efficiency and accuracy of methane content in the insulation. By taking samples from different locations within the insulation as test specimens, the detection precision is further enhanced, ensuring the accuracy of the test results.

[0058] By employing magnetic nanofluid technology, a magnetic field is applied to control the flow of the magnetic fluid on the sample surface, which then adsorbs debris and removes it with a scraper. This helps to ensure the cleanliness of the sample surface and further improves the detection accuracy of methane content.

[0059] Furthermore, by introducing high-purity nitrogen into the headspace vial and simultaneously cooling it to the appropriate temperature, the generation of ethane can be suppressed, thereby reducing the impact of ethane on the accuracy of methane concentration detection and improving the final accuracy of methane detection. Attached Figure Description

[0060] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0061] Figure 1 This is a flowchart of a method for detecting methane content in cable insulation provided by an embodiment of the present invention;

[0062] Figure 2 This is a schematic diagram of the specific process of step S500 provided in the embodiment of the present invention. Detailed Implementation

[0063] 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.

[0064] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless specifically defined as herein.

[0065] It should be further understood that the term "comprising" as used in this specification means the presence of the stated 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 term "and / or" as used herein includes all or any unit and all combinations of one or more associated listed items.

[0066] like Figure 1 As shown, this embodiment of the invention provides a method for detecting methane content in cable insulation, including the following steps S100~S500:

[0067] S100: Take a cable of the preset length to be tested and cut several samples from different positions of the insulation layer.

[0068] Specifically, first select a cable to be tested with a preset length of at least 0.5m.

[0069] When slicing is required, cut a sample section of about 50mm from the middle, remove all protective layers except the insulation layer, and clean the surface copper shavings.

[0070] (a) For cables with thick insulation (i.e., insulation diameter greater than or equal to 20 mm), a radial slicer is used to cut 10 1 mm thin slices radially from the cable under test. The first 7 slices are discarded, and the last 3 slices are retained. These 3 slices are taken from different positions of the insulation layer as samples. These 3 samples are located in the area near the conductive layer, the central area of ​​the insulation layer, and the area near the metal layer, respectively. Specifically, a punching machine is used with a die cutter that can cut small rectangular pieces to remove 3 samples each representing the inner, middle, and outer layers from the test piece. Each slice must be intact and does not include the inner and outer shields. The inner layer sample must be in close contact with the conductor layer, the outer layer sample must be in contact with the insulation layer, and the middle layer sample must be measured at the midpoint between the inner and outer layers using a steel ruler. After obtaining the samples, they are placed into three 20 ml headspace vials with layer markings, without sealing them. By testing in the areas near the inner and outer shields and in the middle area, the methane content at different insulation layer positions can be obtained.

[0071] (b) For cables with thin insulation (insulation layer diameter less than 20mm), an axially cutting cable slicer is used to cut the cable from the outside to the conductor shield with a thickness of 1mm. After removing all the thin slices with shielding, the innermost slice, the outermost slice, and the middle slice of the remaining portion are selected (if the remaining portion is odd, the middle slice is selected; if the remaining portion is even, the slice closer to the inner layer of the two middle slices is selected). Specifically, using a punching machine with a die cutter that can cut small rectangular pieces, three samples are taken along the central axis of the slice (measured with a steel ruler). After obtaining the samples, they are placed into three 20ml headspace vials with layered markings. Do not seal the vials. By placing the three samples into the headspace vials first, the residual methane and other gases released from the samples are diluted to dilute the residual gas in the headspace vials, thus improving the measurement accuracy.

[0072] Furthermore, due to the continuous friction between the cutting tool and the semi-conductive shielding layer, debris from the semi-conductive shielding layer covers the insulating layer, leading to inaccurate methane quantification. Therefore, when cutting several samples from different locations on the insulating layer, magnetic nanofluid technology can be used to solve the above problem. Specifically, this includes the following steps:

[0073] (1) Configuring the magnetic fluid; the magnetic fluid is prepared by mixing 8-12nm (optionally 10nm) iron oxide particles with perfluoropolyether oil in a ratio of 1:95-105 (preferably 1:100).

[0074] (2) Add a preset amount (0.9-1.1 μL, for example 1 μL) of magnetorheological fluid to the contaminated area of ​​the sample; the contaminated area is the area caused by the continuous friction between the cutting tool and the semiconductive shielding layer, resulting in the semiconductive shielding layer debris covering the insulating layer.

[0075] (3) After applying a magnetic field of a preset strength for a preset time, remove the magnetic fluid with a polytetrafluoroethylene scraper. The magnetic field strength is 0.4-0.6T, preferably 0.5T, and the application time is 25-35s, preferably 30s.

[0076] Because the semiconductive shielding layer contains carbon black and other debris, the carbon black is selectively adsorbed by magnetic nanoparticles (affinity energy > 40 kJ / mol). By applying a magnetic field to control the flow of the magnetofluid on the sample surface, the debris is adsorbed and removed by a scraper. Since the surface energy of the crosslinking byproducts (33 mN / m) is lower than that of the magnetofluid (16 mN / m), the magnetofluid does not wet the sample and will not remain there. Furthermore, the perfluoropolyether oil does not dissolve hydrocarbons, thus protecting methane from leakage and improving detection accuracy.

[0077] S200, place several samples into headspace vials.

[0078] S300: Remove the sample from the headspace vial and weigh it. After that, put it back into the corresponding headspace vial and record the sample weight, sample tray number and degassing time.

[0079] S400: After placing each headspace gas cylinder on the table and letting it stand until it reaches room temperature, use a clamp to hold the headspace gas cylinder and fix the cap to the headspace gas cylinder opening to complete the seal.

[0080] S500, for making standard gas crown empty bottles.

[0081] In some embodiments, such as Figure 2 As shown, the specific steps for preparing a standard gas head-empty bottle in S500 include:

[0082] S510: Take a clean headspace bottle and insert the tubing connected to the methane standard gas bottle of the corresponding concentration into the bottom of the headspace bottle; invert the headspace bottle, loosely cover the bottle opening with the cap, open the gas cylinder valve, and continuously fill the bottle with methane standard gas. The methane standard gas needs to completely fill the standard gas bottle, generally requiring continuous venting for at least 1 minute.

[0083] S520, quickly pull out the hose and seal the headspace cylinder, then close the cylinder valve.

[0084] S530, after connecting the vacuum pump using a flat-head pin, is inserted into the headspace bottle.

[0085] It is important to note that the insertion point should not be located in the exact center or edge of the bottle cap, in order to reduce the probability of insufficient airtightness.

[0086] S540, turn on the vacuum pump, continue pumping for a preset time, then remove the needle.

[0087] Optionally, the specific evacuation time is approximately 40 minutes.

[0088] S550 involves using a syringe to repeatedly aspirate 20 ml of methane standard gas of the specified concentration from the gas storage bag and injecting it into a vacuum-sealed headspace vial. If the gas in the syringe is largely aspirated naturally, the standard gas preparation is complete; otherwise, it needs to be prepared again. This method avoids the inefficiency and additional variables of traditional manual injection, greatly improving detection efficiency and accuracy.

[0089] In this embodiment, by using a standard methane gas cylinder of the corresponding concentration to fill the clean headspace bottle with methane gas, it is beneficial to reduce the amount of other gases remaining in the headspace bottle after vacuuming. The remaining gas after vacuuming is methane, thereby improving the accuracy of standard gas sample preparation and thus improving measurement accuracy.

[0090] It is important to note that after the standard gas cylinder is placed on the table, do not touch the empty standard gas cylinder directly with your hands, as this may affect the final test results.

[0091] S600 uses a gas chromatograph-headspace sampler to detect the sample and standard gas headspace bottle; examines the chromatogram and records the methane volume concentration f; and calculates the methane mass concentration inside the insulation.

[0092] Optionally, prior to headspace bottle sealing in step S400, the following steps are also included:

[0093] (1) Inject high-purity nitrogen into the bottle at a flow rate of 45-55 mL / min (preferably 50 mL / min).

[0094] (2) Simultaneously freeze the sample to -25-35℃ (preferably -30℃) to reduce the vapor pressure of ethane by 90%, thereby inhibiting the production of ethane and reducing the influence of ethane.

[0095] (3) After purging with nitrogen for 5-15 min (preferably 10 min), add 3 Å molecular sieve and seal immediately. The molecular sieve has a pore size of 0.3-0.33 nm.

[0096] By using the above steps (1) to (3), the ethane removal rate can be greater than 80%, because the kinetic diameter of ethane is 0.40 to 0.47 nm, and the kinetic diameter of methane is about 0.38 nm. Methane has a stronger ability to pass through molecular sieves. By applying pressure with nitrogen, the effect of molecular sieves can further reduce the influence of ethane on methane content and improve measurement accuracy.

[0097] Optionally, before detecting the sample using a gas chromatograph-headspace sampler, the following steps are also included:

[0098] Check the water level in the hydrogen generator. If it is below the lower water level, add sufficient distilled water to the white bottle at the back. If the light does not turn on, turn on the headspace sampler, main unit, and fully automatic air source and hydrogen generator switches in sequence. Note that the order should not be reversed.

[0099] Check and adjust the gas pressure entering the gas chromatograph; it should be 0.4-0.5 MPa.

[0100] In step S600, when the sample is detected using a gas chromatograph-headspace sampler, the detection parameters of the gas chromatograph include:

[0101] SS injection port temperature: 180~220℃;

[0102] SS inlet pressure: 7~7.8psi; where SS inlet refers to split / splitless inlet.

[0103] Total flow rate: 40~60ml / min;

[0104] Septum purge flow rate: 2~4 ml / min;

[0105] The injection mode is split, with a specific split ratio of (8~12):1; the split flow rate is 30~50 ml / min; using this stable split flow rate ensures sharp and symmetrical peaks, which is conducive to accurate integration, avoids column overload (peak broadening, tailing), reduces peak area variation caused by injection volume fluctuations, and improves measurement accuracy.

[0106] 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 time, too low a flow rate will result in peak broadening (decreased sensitivity); too high a flow rate will increase mass transfer resistance, peak tailing, or decreased resolution.

[0107] Column pressure: 7~7.8 psi; increasing the pressure helps to properly control the methane elution time.

[0108] Average column velocity: 45~50cm / s; a relatively low average column velocity results in the minimum plate height, minimum peak width, and maximum peak height for methane, improving sensitivity and measurement accuracy.

[0109] Column oven temperature: 180~200℃;

[0110] Column box balancing time: 0.4~0.6 min.

[0111] In this embodiment, when using gas chromatography for detection, a suitable column temperature helps maintain a good kinetic process, resulting in more ideal diffusion and mass transfer of methane molecules within the column. This leads to symmetrical, sharp chromatographic peaks and improves measurement accuracy. Controlling the column equilibration time allows sufficient time for the entire column temperature to become uniform and stable after temperature changes (especially after the temperature program ends), ensuring a stable baseline. This completely eliminates the negative impact of thermal hysteresis and baseline fluctuations on retention time reproducibility, peak shape, and integration accuracy.

[0112] Optionally, the FID detector (Flame Ionization Detector) temperature is 280~320℃ (preferably 300℃). Setting a higher temperature keeps the FID ionization efficiency constant, ensuring that the methane response factor (peak area / mass) does not fluctuate with changes in the sample.

[0113] FID airflow rate: 380~420 ml / min (preferably 400 ml / min).

[0114] FID hydrogen fuel flow rate: 25~35 ml / min (preferably 30 ml / min). Proper control of hydrogen and hydrogen fuel flow rate optimizes flame ionization efficiency. Too low a hydrogen flow rate results in insufficient methane ionization, leading to decreased sensitivity; too high a hydrogen flow rate increases flame turbulence, resulting in increased noise.

[0115] FID purge gas (N2): 20~30 ml / min (preferably 25 ml / min). Set a larger purge flow rate to accelerate sample transport and focus the chromatographic peak.

[0116] The carrier gas flow rate is calibrated to a constant tail gas plus fuel gas flow rate.

[0117] The minimum peak width for FID acquisition is 5 Hz / 0.04 min. This is beneficial for improving the integration accuracy of methane measurements, thereby improving overall measurement accuracy.

[0118] Optionally, in step S600, the parameters of the headspace sampler of the gas chromatograph are:

[0119] Headspace sampler heating chamber: 170~190℃ (preferably 180℃);

[0120] Headspace sampler quantitative loop: 180~190℃ (preferably 185℃);

[0121] Headspace sampler transfer line: 180~200℃ (preferably 190℃);

[0122] Headspace vial equilibration time: 25-35 min (preferably 30 min);

[0123] Injection duration: 0.4~0.6 min (preferably 0.5 min).

[0124] In this embodiment, a high temperature is used in the headspace sampler heating chamber to forcefully drive the release of methane from various matrices into the headspace gas phase, ensuring that the releaseable methane evaporates as completely as possible. The headspace sampler heating chamber is set to 180°C, the headspace sampler quantitative loop is set to 185°C, and the headspace sampler transfer line is set to 190°C, gradually increasing the temperature to form a high-temperature gradient throughout the sampling and transfer path. This completely prevents the condensation of sample gas (especially water vapor) at any stage, ensuring that the sample gas representing the equilibrium concentration extracted from the headspace gas phase enters the GC column without loss, alteration, or damage.

[0125] Furthermore, the GC cycle time is 14-16 min (preferably 15 min); by setting a longer cycle time, it is ensured that all samples are analyzed under completely consistent system conditions. The GC cycle time refers to the total time required for the gas chromatograph to complete one full analytical cycle after all pretreatments are completed.

[0126] Shake the sample vial 16-20 times / min (preferably 18 times / min).

[0127] The shaking acceleration is 60 cm / s² 2 By increasing the number of shaking cycles and the shaking acceleration, the mass transfer force is enhanced, forcing the adsorbed methane to desorb and thus improving the accuracy of methane detection.

[0128] Headspace filling pressure: 20~30psi (preferably 25psi); the silicone rubber gasket has slight permeability to methane, increasing the headspace filling pressure prevents methane loss and improves measurement accuracy.

[0129] Filling pressure equilibrium time: 0.05~0.15min (preferably 0.1min) to avoid the gas composition not representing the true equilibrium state (uneven distribution of methane), and the extraction mode is single extraction.

[0130] By optimizing the above parameters, the precision (reproducibility) and accuracy (truly reflecting the sample content) of the analytical results are guaranteed.

[0131] By reviewing the chromatogram, record the methane volume concentration f (ppm). Carefully observe the methane peak shape for any significant abnormal fluctuations or deviations. If any are found, report them to the testing department to replace the standard gas. Finally, calculate the methane mass concentration C (unit: ppm) for each sample using the following formula:

[0132]

[0133] Where P is the gas pressure at the time of sealing, 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 at the time of sealing.

[0134] The method for detecting methane content in cable insulation provided in this invention utilizes a standard methane gas cylinder of corresponding concentration to fill a clean headspace vial with methane gas. This reduces the amount of other gases remaining in the headspace vial after vacuuming, ensuring that the remaining gas after vacuuming is methane. This improves the accuracy of standard gas sample preparation and consequently, the measurement precision. Furthermore, the use of a headspace sampler avoids the inefficiency and additional variables of traditional manual sampling, significantly improving the detection efficiency and accuracy of methane content in the insulation. By taking samples from different locations within the insulation as test specimens, the detection precision is further enhanced, ensuring the accuracy of the test results.

[0135] Furthermore, by employing magnetic nanofluid technology, a magnetic field is applied to control the flow of the magnetic fluid on the sample surface, which then adsorbs debris and removes it with a scraper. This helps to ensure the cleanliness of the sample surface and further improves the detection accuracy of methane content.

[0136] Furthermore, by introducing high-purity nitrogen into the headspace vial and simultaneously cooling it to the appropriate temperature, the generation of ethane can be suppressed, thereby reducing the impact of ethane on the accuracy of methane concentration detection and improving the final accuracy of methane detection.

[0137] The present invention will be further described below through specific embodiments and comparative examples:

[0138] Example 1:

[0139] The methane content in the inner, middle, and outer insulation layers of a certain type of cable was independently tested three times using the above method. The results are shown in Table 1 below:

[0140] Table 1 Comparison of methane content detection in insulation layers at different locations.

[0141]

[0142] Comparative Example 1:

[0143] The degree of degassing of this type of cable was tested using the traditional thermogravimetric analysis method. The method involved rapidly heating the cable to 175°C at a rate of 50°C / min and holding the temperature isothermally for 30 minutes. The weight changes of different insulation layers during this period were observed, and the results are shown in Table 2 below.

[0144] Table 2 Comparison of heating time and gas content in insulation layers at different locations

[0145]

[0146] As can be seen from the table, thermogravimetric analysis can only provide a rough quantitative analysis of the content of residual volatile gases in the insulation (at 175°C). However, it has very little reference value for the analysis of methane gas, which is the most harmful gas in cable systems. Both the content and the proportion between different layers are quite different from the actual situation.

[0147] Comparative Example 2:

[0148] Instead of using a headspace sampler, the traditional manual injection method of gas chromatography was used for injection. The results are shown in Table 3 below:

[0149] Table 3 Comparison of methane content detection in insulation layers at different locations

[0150]

[0151] 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.

[0152] Comparative Example 3:

[0153] Without using S400 pre-freezing followed by nitrogen purging, and by using molecular sieves to eliminate the influence of ethane, the results are shown in Table 4 below:

[0154] Table 4 Comparison of methane content detection in insulation layers at different locations

[0155]

[0156] As can be seen from Table 4, since the method was not used to eliminate the influence of ethane as much as possible, the overall methane quantification results of each layer increased slightly, which led to the deviation of the experimental results.

[0157] 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 cable insulation, characterized in that, include: Take a cable of a predetermined length to be tested, and cut several samples from different positions of the insulation layer; Several samples were placed in headspace bottles. Remove the sample from the headspace vial, weigh it, and then reinsert it into the corresponding headspace vial. After each empty bottle has been left to stand at room temperature, seal it. Making a labeling gas head-empty bottle; Gas chromatograph-headspace sampler was used to analyze the sample slides and standard gas headspace bottles; Examine the chromatogram and record the methane volume concentration f; calculate the methane mass concentration inside the insulation layer. The preparation of the standard gas headspace bottle includes: Take a clean headspace vial and insert the tubing connected to the methane standard gas vial of the corresponding concentration into the bottom of the headspace vial; invert the headspace vial and continuously fill it with methane standard gas; after pulling out the tubing, seal the headspace vial; After evacuating the headspace vial, a standard methane gas of the corresponding concentration is extracted and injected into the headspace vial to obtain a standard gas headspace vial. In addition, several samples were cut from different locations of the insulation layer, including: Configuring a magnetic fluid; wherein the magnetic fluid is composed of 8-12nm iron oxide particles and perfluoropolyether oil in a ratio of 1:95-105; A predetermined amount of magnetic fluid is dropped onto the contaminated area of ​​the sample; the contaminated area is the area caused by continuous friction between the tool and the semiconductive shielding layer, resulting in debris from the semiconductive shielding layer covering the insulating layer. After applying a magnetic field of preset strength for a preset time, the magnetofluid is removed. Before sealing each headspace bottle after it has been allowed to stand at room temperature, the following steps are also included: Inject high-purity nitrogen gas into the bottle at a flow rate of 45-55 mL / min; Simultaneously freeze the samples to -25~-35℃; After purging with nitrogen for 5-15 minutes, add a 3Å molecular sieve and seal immediately; the molecular sieve has a pore size of 0.3-0.33 nm.

2. The method for detecting methane content in cable insulation according to claim 1, characterized in that, The test involves taking a cable of a predetermined length and cutting several samples from different locations of the insulation layer. The predetermined length of the cable to be tested is at least 0.5m.

3. The method for detecting methane content in cable insulation according to claim 2, characterized in that, If the cable to be tested is a thick-insulated cable with an insulation layer thickness of not less than 20mm, then cut 10 thin slices of 1mm each along the radial direction of the cable to be tested, discard the first 7 thin slices, keep the last 3 thin slices, and take the 3 thin slices corresponding to different positions of the insulation layer as samples. The 3 samples are respectively located in the area near the conductive layer, the central area of ​​the insulation layer, and the area near the metal layer.

4. The method for detecting methane content in cable insulation according to claim 2, characterized in that, If the cable under test is a thin-insulated cable with an insulation layer thickness of less than 20mm, then the cable under test is cut along the axial direction to the metal layer. If the number of slices is odd, the median sample is taken; if the number of slices is even, two median-to-inner samples are taken.

5. The method for detecting methane content in cable insulation according to claim 1, characterized in that, Before using a gas chromatograph-headspace sampler to analyze the sample, the following steps are also included: Check the water level in the hydrogen generator. If it is below the lower water level, add sufficient distilled water to the white bottle at the back. If the light does not turn on, turn on the headspace sampler, main unit, and fully automatic air source and hydrogen generator switches in sequence. The order must not be reversed.

6. The method for detecting methane content in cable insulation according to claim 5, characterized in that, Check and adjust the gas pressure entering the gas chromatograph to 0.4-0.5 MPa.

7. The method for detecting methane content in cable insulation according to claim 1, characterized in that, When using a gas chromatograph-headspace sampler to analyze a sample, the gas chromatograph's 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: 2~4 ml / min; Injection mode: split; split ratio: (8~12):1; split flow rate: 30~50 ml / min; Column flow rate: 3~5 ml / min; Column pressure: 7~7.8 psi; Average linear velocity of the chromatographic column: 45~50 cm / s; Column oven temperature: 180~200℃; Column box balancing time: 0.4~0.6 min.

8. The method for detecting methane content in cable insulation according to claim 7, characterized in that, The parameters of the headspace sampler for a gas chromatograph are: Headspace sampler heating chamber: 170~190℃; Headspace sampler quantitative loop: 180~190℃; Headspace sampler transfer line: 180~200℃; Headspace vial equilibration time: 25~35 min; Injection duration: 0.4~0.6 min; GC cycle: 14~16 min; Headspace bottle shaking frequency: 18 times / min; The shaking acceleration is 60 cm / s² 2 ; Headspace filling pressure: 20~30psi; Filling pressure equalization time: 0.05~0.15min.

9. The method for detecting methane content in cable insulation according to claim 1, characterized in that, When using a gas chromatograph-headspace sampler to analyze a sample, the following is also included: The methane mass concentration C of each sample was calculated using the following formula: Where P is the gas pressure at the time of sealing, f is the methane volume concentration, m is the mass of the sample, and t is the temperature at the time of sealing.

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