CN112067653A - Method for evaluating appropriate degassing time of XLPE (cross linked polyethylene) flat plate sample or cable - Google Patents

Method for evaluating appropriate degassing time of XLPE (cross linked polyethylene) flat plate sample or cable Download PDF

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CN112067653A
CN112067653A CN202010934565.2A CN202010934565A CN112067653A CN 112067653 A CN112067653 A CN 112067653A CN 202010934565 A CN202010934565 A CN 202010934565A CN 112067653 A CN112067653 A CN 112067653A
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xlpe
degassing
weight loss
cable
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CN112067653B (en
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陈向荣
孟繁博
戴超
张梦甜
丰如男
张永明
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Ningbo Orient Wires & Cables Co ltd
Zhejiang University ZJU
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Zhejiang University ZJU
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Abstract

The invention discloses a method for evaluating the suitable degassing time of an XLPE flat plate sample or a cable, which comprises the following steps: (1) preparing/selecting a sample; (2) sample pretreatment; (3) degassing treatment; (4) constructing an evaluation model; (5) the degassing time was evaluated. According to the method, the weight change rule of the samples is obtained through the degassing weight loss rates of the multiple samples with the same specification, the weight loss change rate parameter is calculated by taking the average weight loss rate as a reference, and the proper degassing time is further obtained. The method can greatly reduce the time of heat treatment of the XLPE sample by researchers, improve the scientific research efficiency, reduce the degassing time cost of cable manufacturers, and provide a scientific and effective analysis means for the analysis of the XLPE material and the production of cables in the aspect of avoiding the influence of crosslinking byproducts.

Description

Method for evaluating appropriate degassing time of XLPE (cross linked polyethylene) flat plate sample or cable
Technical Field
The invention belongs to the technical field of high voltage and insulation, and particularly relates to a method for evaluating the appropriate degassing time of an XLPE (cross linked polyethylene) flat plate sample or cable.
Background
With the continuous acceleration of urbanization process and the continuous and rapid development of economy in China, the demand of power consumption is increasing sharply, and the existing power grid infrastructure is urgently needed to be upgraded and modified to adapt to the rapidly increasing demand of power transmission. Meanwhile, with the increasing demand of offshore wind power and island power supply, the power cable has become one of key power devices for building urban underground energy comprehensive channels and realizing long-distance large-capacity power transmission and large-scale utilization of new energy power. At present, China is investing heavily in infrastructure construction to meet the development requirements of new energy, and upgrading and deploying of energy infrastructure provides sufficient power for development of high-voltage cables and simultaneously increases tiles for growth of the high-voltage cable market. Cross-linked Polyethylene (XLPE) is widely applied to extrusion power cable insulation due to excellent electrical, thermal and mechanical properties, and compared with the traditional oil paper insulated cable, the XLPE insulated cable has the advantages of simple manufacturing process, large transmission capacity, convenience in maintenance, low cost and the like. In recent 20 years, with continuous breakthrough of XLPE cable insulation material technology and continuous progress of power transmission technology, XLPE insulated high-voltage cables have become one of the key power devices for domestic and foreign engineering applications, such as cross-sea power transmission, asynchronous grid interconnection, and the like.
At present, Dicumyl peroxide (DCP) is generally adopted as a polyethylene crosslinking agent in the production process of cables, crosslinking reaction of polyethylene occurs to generate XLPE under the conditions of high temperature and high pressure, but in the process of thermal decomposition of DCP, various byproducts such as methane, acetophenone, cumyl alcohol and the like can be generated, introduction of these impurity molecules can directly influence the dielectric property and material property of the cables, accumulation of space charges inside the materials can be easily caused under the action of an electric field, electric field distortion can be caused, the aging process of the cables can be accelerated, partial discharge can be caused in severe cases, and even insulation breakdown can be caused. Meanwhile, in the operation process of the cable, residual byproducts in the material can be heated and gradually volatilized and are accumulated in the cable sheath, and the excessive byproduct accumulation can increase the internal pressure of the sheath, so that the deformation is generated, and the safe and stable operation of a cable system is influenced. Thus, the removal of by-products is particularly important for the production process of the cable. At present, cable manufacturers mainly perform degassing treatment by placing cable insulation wire cores in special degassing chambers at 65-75 ℃, degassing time is different due to different voltage grades and conductor specification and sizes of cables, and according to XLPE insulation thickness, degassing processes are few days and more dozens of days. The current knowledge of the degassing process remains largely empirical, and there is no clear specification and evaluation method for the degassing time.
Compared with full-size solid cables, the sheet sample prepared by the flat plate hot pressing method is widely applied to laboratory research, the hot pressing conditions of the sheet preparation are basically unified at present, the influence of the hot pressing conditions on the degassing time of the flat plate sample can be ignored under the unified hot pressing conditions, but the thickness of the sample formed by the flat plate hot pressing method is not equal from 0.1 to several millimeters, and the method is widely applied to the dielectric property test of XLPE materials and the research of the modification of novel cable materials. However, the degassing treatment corresponding to the thin sheet sample prepared by the method is still not standard, in the process of preparing the existing sample, researchers generally adopt a heat treatment mode to degas for a period of time, namely, the degassing work of the XLPE material is determined to be finished, but the current degassing mode mainly avoids the influence of crosslinking byproducts by fixed or ultra-long degassing time (such as 2 days and even longer), however, the components of the materials produced by different cable material manufacturers are different, the fixed degassing time is not reasonable and effective, the longer degassing time not only reduces the working efficiency of the researchers, but also introduces a factor of thermal aging due to long-time degassing, and further loses the significance of researching the characteristics of the materials. Therefore, a quantitative and effective method for evaluating suitable degassing time is needed to determine the degassing time so as to solve the degassing problem of the conventional XLPE cable material.
Disclosure of Invention
The invention aims to solve the problem that the proper degassing time of an XLPE flat plate sample or cable insulation is difficult to estimate due to the influences of thickness and degassing temperature, and provides a method for judging the proper degassing time of the flat plate sample and the XLPE cable, which is used for estimating the efficient and proper degassing treatment time for dielectric property test and insulating material modification of an XLPE cable material.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention firstly discloses a method for evaluating the appropriate degassing time of an XLPE (cross linked polyethylene) flat plate sample based on a flat plate hot pressing method, which comprises the following steps of:
step 1: weighing XLPE cable material particles, and preparing an XLPE flat plate sample;
step 2: cutting an XLPE flat plate sample, preparing a degassed sample to be tested, measuring the average thickness of the sample to be tested and weighing the initial weight;
and step 3: placing the sample into a drying oven, setting the degassing temperature, carrying out degassing treatment, taking out the sample at each set sampling time, and weighing;
and 4, step 4: after the weight of the sample is stable, stopping the experiment, calculating the weight loss rate in the degassing process of the sample, and constructing an appropriate degassing time model of the XLPE flat plate sample as follows:
Figure BDA0002671467290000031
wherein n is the number of terms of the set expansion formula and is based on Fick's second law (namely formula (7) or (8)), the number of terms of the general solution is obtained by solving through a variable separation method, n is a positive integer, the value of n represents the degree of the degassing model approaching the weight loss rate trend of the sample, the larger the value of n is, the more the model (14) approaches the weight loss rate trend of the sample, but the corresponding calculated amount can be increased, the calculation speed and precision are considered, n is generally set to be 1-4(ii) a In embodiments of the invention, n is taken to be 2 or 1; a. thei，τiIs the weight loss rate change constant of the expansion of the ith term; d is the average molecular diffusion coefficient, which is a temperature-dependent function; t is the suitable degassing time of the plate sample, daThe average thickness of the test piece is,tsfor a preset sample weight loss rate change threshold, for a flat plate sample,ts0.1% is recommended; t is0In order to set delay time which represents the delay time for reaching a preset sample weight loss rate change threshold value in the degassing process, T is taken in the invention0Is 60 in min;
and 5: and (4) obtaining the proper degassing time of the flat plate sample to be evaluated by using the model obtained in the step (4) according to the thickness and the degassing temperature of the sample to be evaluated.
Preferably, the step 2 is: trimming the overflow part of the edge of the sample to obtain a degassed sample to be tested, or preparing the degassed sample to be tested into a set shape; measuring the thickness of the sample at different positions by using a 0.001mm thickness meter or a higher-precision device, and calculating the average thickness d of the sample according to the formula (1)ave，
Figure BDA0002671467290000032
In the formula diJ is the number of measurements for each measurement of the thickness of the sample;
immediately thereafter, the current sample weight is weighed using an electronic scale with an accuracy of at least 0.1 mg.
In step 4, the steps for constructing the appropriate degassing time model of the XLPE flat plate sample are as follows:
firstly, the degassing temperature T in the step 3) is obtained2The molecular diffusion coefficients of acetophenone and cumyl alcohol in XLPE, either temperature T1The lower diffusion coefficient can be obtained according to arrhenius equation (4),
Figure BDA0002671467290000033
in the formula, D1Is the temperature T1Diffusion coefficient of lower, D2Is the temperature T2The diffusion coefficient is given by E, the activation energy of the material and R, a constant;
further, according to the temperature T1The average molecular diffusion coefficient D at this temperature was calculated according to formula (5):
Figure BDA0002671467290000041
in the formula DaIs acetophenone molecule at temperature T1Molecular diffusion coefficient ofcIs cumyl alcohol at a temperature T1Lower molecular diffusion coefficient;
further, according to the formula (6), the average weight loss rate W of the sample at each sampling time is calculatedaveWherein the maximum weight loss rate is selected as the upper error limit EupMinimum weight loss rate as lower limit of error Elow，
Figure BDA0002671467290000042
In the formula, i is the number of patterns.
Since the thickness of the flat sample is much smaller than the length and width of the sample, the diffusion of the crosslinking by-products in the planar direction can be ignored, and only the diffusion in the direction perpendicular to the plane can be considered, so that the degassing time is independent of the shape of the sample and depends only on the thickness d of the sample; let the region of the crosslinking by-products in the initial state be-d/2<x<d/2, x represents the distance from the midpoint of the sample thickness, and when the initial time t is 0, the concentration function C (x, 0) is C0D/2, provided that during the diffusion of the crosslinking by-products, the by-products dissipate immediately after reaching the surface of the material>x or x>When d/2, C (x, t) ═ 0, as described above, based on fick's second law (7), equation (8) can be established:
Figure BDA0002671467290000043
Figure BDA0002671467290000044
where erf is the error function, CoutConcentration of the crosslinking by-products for diffusion into the air, see formula (9)
Figure BDA0002671467290000045
Assuming that the volume of the sample is V and the average density of the XLPE material is rho, the weight loss ratio is further formulated as formula (10)
Figure BDA0002671467290000046
Assuming that the sample weight is stable, there is no crosslinking by-product in the material, at which time ts，C(x，ts) 0, and then equations (11) and (12) are obtained,
M＝ρV+C0V (11)
Figure BDA0002671467290000051
in the formula WtsIs the weight loss rate when the sample is stable;
based on the formulas (8), (9), (10), (11) and (12), the average weight loss rate W is introducedaveEquation (6), construction of weight loss ratio and average molecular diffusion Rate D and average thickness D of sampleaThe approximate relation (13);
Figure BDA0002671467290000052
in the formula, n is a set expansion term number, and the value of n represents the degree of the model approaching the weight loss rate trend of the sample; a. thei，τiIs the weight loss rate change constant, y, of the expansion of the ith term0Is a constant;
further, based on the weight loss rate change parameter of the formula (13), constructing an XLPE flat plate sample proper degassing time model (14),
Figure BDA0002671467290000053
the invention also discloses a method for evaluating the appropriate degassing time of the XLPE cable insulation, which comprises the following steps:
step 1: selecting a cable to be degassed;
step 2: measuring the insulation evaluation thickness of the cable and weighing the initial weight;
and step 3: placing the cable into a degassing chamber for degassing treatment, taking out the cable at a set sampling time, and weighing;
and 4, step 4: after the weight of the cable is stable, stopping the experiment, calculating the weight loss rate in the cable degassing process, and constructing an XLPE cable suitable degassing time model as follows:
Figure BDA0002671467290000054
wherein n is the number of terms of the set expansion; a. thei，τiIs the weight loss rate change constant of the expansion of the ith term; d is the average molecular diffusion coefficient, which is a temperature-dependent function; t is the proper degassing time of the XLPE cable, and d is the thickness of cable insulation;tsis a preset sample weight loss rate change threshold value, T0The set delay time represents the delay time for reaching the preset sample weight loss rate change threshold value in the degassing process. In the invention, T is taken0Is 60 in min;
and 5: and (4) obtaining the appropriate degassing time of the XLPE cable to be evaluated by using the model obtained in the step (4) according to the insulation thickness and the degassing temperature of the XLPE cable to be evaluated.
Furthermore, because the length of the cable is far greater than the thickness of the body, the diffusion of crosslinking byproducts in the cable along the axial direction can be ignored, only the diffusion of crosslinking byproducts in the radial direction of the cable is considered, and one-dimensional symmetry can be establishedModel, see equation (7). Assuming that the area of crosslinking by-products is 0 in the initial state of the cable<x<d, d is the thickness of the insulating layer of the XLPE cable, 0 is the side of the XLPE insulation that is connected to the inner semiconductive layer, the concentration function C (x, 0) ═ C when the initial time t equals 00. Assuming that during the diffusion of the crosslinking by-products, the diffusion of the by-products into the core conductor is neglected and the by-products only diffuse to the outside and dissipate immediately after reaching the material surface, x>d, C (x, t) ═ 0, and equation (16) can be established
Figure BDA0002671467290000061
Where erf is the error function, see equation (9).
Further, assuming that the volume of the XLPE cable insulation is V and the average density of the XLPE material is ρ, the weight loss ratio can be further formulated as formula (10). Assuming that the sample weight is stable, there is no crosslinking by-product in the material, at which time tsThen C (x, t)s) 0, and then the formulas (11) and (12) are obtained.
Further, based on the formulas (16), (9), (10), (11) and (12), an average weight loss ratio W is introducedaveThe formula (6) can construct a relational expression (17) of the weight loss rate, the average molecular diffusion rate D and the sample thickness D, and further can construct a degassing mathematical model formula (18), so that the proper degassing time T of the cable can be ensured. For electric cabletsThe value of (A) can be determined according to actual requirements.
Compared with the prior art, the invention has the following beneficial technical effects: the invention provides a method for insulating proper degassing time of an XLPE (cross linked polyethylene) flat plate sample and a cable, which is characterized in that for the flat plate sample or the cable to be evaluated, only a plurality of parallel test samples are selected to obtain the degassing weight loss rate at any degassing temperature, the weight change rule of the sample is obtained, the weight loss change rate parameter is calculated by taking the average weight loss rate as the reference, a proper degassing time evaluation model is constructed, and then the proper degassing time of the flat plate sample or the cable to be evaluated at any degassing temperature and any thickness can be evaluated according to the model. The method can accurately evaluate the suitable degassing time of various different XLPE formula materials, has simple test method, convenient operation and strong universality, and has higher accuracy by comparing with an actual experimental method. The method can greatly reduce the time of heat treatment of the XLPE sample by researchers, improve the scientific research efficiency, reduce the degassing time cost of cable manufacturers, and provide a scientific and effective analysis means for the analysis of the XLPE material and the production of cables in the aspect of avoiding the influence of crosslinking byproducts.
Drawings
FIG. 1 is a flow chart of a method for evaluating the suitable degassing time of XLPE plate samples.
FIG. 2 is a schematic drawing of a sample prepared by XLPE plate hot pressing.
FIG. 3 is a graph of the relationship between the degassing time at 70 ℃ and the weight loss rate of an XLPE sample.
FIG. 4 is a graph of the relationship between the degassing time at 60 ℃ and the weight loss rate of an XLPE sample.
FIG. 5 is a thermogravimetric analysis (TGA) graph of XLPE samples.
Detailed Description
The present invention will be described in more detail with reference to the following embodiments and the accompanying drawings.
Step 1, as shown in fig. 2, is a schematic diagram of sample preparation by XLPE flat plate hot pressing method. Preparing an XLPE sample by adopting a standard flat plate hot pressing method, wherein a stainless steel plate with 4 hollowed squares is used as a mold in the embodiment, the mold is 25 multiplied by 0.2mm, and the hollowed size is 10 multiplied by 0.2 mm; placing about 2.5g of XLPE particles into each square, uniformly dispersing the XLPE particles into a mold, covering the upper surface and the lower surface of the mold with PET, placing the mold into a flat plate clamp, and placing the mold and the flat plate clamp into a vulcanizing machine; prepressing was carried out at 120 degrees for 5 minutes, then vulcanization was carried out at 180 degrees for 15 minutes under a pressure of 20T, and thereafter the mold was cooled, and the specimens were taken out to prepare 8 specimens in total.
Since the thickness of the flat plate specimen is much smaller than the length and width of the specimen, the diffusion of the crosslinking by-products in the planar direction can be ignored, and only the diffusion in the perpendicular planar direction can be considered, so that the degassing time is independent of the shape of the specimen and depends only on the thickness of the specimen.
For XLPE cables, since the cable length is much greater than the thickness of the body, diffusion of the crosslinking by-products in the cable in the axial direction can also be ignored, only diffusion in the radial direction of the cable being considered.
And 2, cutting the XLPE sample into a square shape of 10 multiplied by 10mm, selecting different positions of the sample for testing by adopting a thickness meter of 0.001mm for multiple times, and calculating the average thickness of the sample according to the formula (1), wherein the average thickness of the sample is 0.294m as shown in the table 1.
Figure BDA0002671467290000071
In the formula diJ is the number of measurements for each measurement of the thickness of the sample;
the initial weight of the sample was then weighed using an electronic scale with an accuracy of 0.1mg, as shown in Table 2.
TABLE 1 average thickness of the samples
Figure BDA0002671467290000072
Figure BDA0002671467290000081
And 3, placing the sample into an oven, setting the temperature of the oven to be 70 ℃, taking out the sample according to a preset time interval, placing the sample into a dryer, cooling, weighing by using an electronic scale with the precision of 0.1mg, and recording the weight of the sample, wherein the weight is shown in table 2.
The degassing treatment of the sample in the step 3 is carried out in an oven, and the following conditions are met:
(1) only XLPE flat plate samples are ensured in the oven, and other samples cannot be mixed;
(2) the test piece is vertically hung in the middle of the oven, and the distance between a plurality of XLPE flat plate test samples is at least 20 mm;
(3) the volume of the XLPE flat plate sample is not more than 0.5 percent of the volume of the oven, and the temperature in the oven is constant.
For the flat plate sample, the time of each sampling in step 3 can also be determined according to formula (2) (the sampling time interval can also be set by itself, the sampling interval of this embodiment is shown in tables 2-4):
t＝ek-1 (2)
in the formula, t is the time of the kth sampling and is in min; k is the number of times of sampling and is a positive integer.
For the XLPE cable samples, the sampling interval in step 3 can also be determined according to equation (16) (see tables 6-8 for the sampling intervals in this example):
t＝ek+3 (16)
in the formula, t is the time of the kth sampling and is in min; k is the number of times of sampling and is a positive integer. The XLPE cable samples were much longer to degas than the plate samples, so the first sampling time was much more delayed than the plate samples.
TABLE 2 weight of sample in degassed condition at 70 deg.C
Figure BDA0002671467290000082
Figure BDA0002671467290000091
And 4, calculating the weight loss rate of the sample according to the formula (3) as shown in the table 3.
Figure BDA0002671467290000092
In the formula WjIs the weight loss ratio of the sample, MiIs the initial weight of the sample before degassing, GjIs the weight after degassing;
according to the formula (4), the acetophenone molecular diffusion coefficient at 70 ℃ is 2.7E-7m2The diffusion coefficient of cumyl alcohol molecules is 1.41E-7m2/h，
Figure BDA0002671467290000093
In the formula, D1Is the temperature T1Diffusion coefficient of lower, D2Is the temperature T2The diffusion coefficient is given by E, the activation energy of the material and R, a constant;
according to the formula (5), the molecular average diffusion coefficient was calculated to be 2.26E-7m2/h。
Figure BDA0002671467290000094
In the formula DaIs acetophenone molecule at temperature T1Molecular diffusion coefficient ofcIs cumyl alcohol at a temperature T1Lower molecular diffusion coefficient;
TABLE 3 weight loss ratio of sample under degassing condition at 70 deg.C
Figure BDA0002671467290000095
According to the formula (6), the average weight loss rate W of the sample at each sampling moment is calculatedaveWherein the maximum weight loss rate is selected as the upper error limit EupMinimum weight loss rate as lower limit of error ElowThe results are shown in Table 4, and the graph of the relationship between the degassing time and the weight loss of the sample is shown in FIG. 3.
Figure BDA0002671467290000101
TABLE 4 average, maximum and minimum weight loss rates of samples at 70 deg.C in degassed conditions
Time/min Wave/％ Eup/％ Elow/％
0 0 0 0
5 0.2938 0.3755 0.1505
15 0.3994 0.5455 0.2509
25 0.5421 0.7312 0.3663
50 0.6142 0.7905 0.4615
100 0.6425 0.8530 0.5123
225 0.6601 0.9012 0.5312
425 0.6834 0.9170 0.5446
745 0.6882 0.8814 0.5496
1495 0.6626 0.7905 0.5538
2195 0.6725 0.8893 0.5589
Selecting average weight loss rate WaveSubstituting the formula (7) to obtain the change parameter of the weight loss rate, taking the simplification degree and the precision requirement of calculation into consideration, taking 2 as n in the example, and taking the change parameter A of the weight loss rate as a parameter1Is-0.30, τ1Is 0.5, A2Is-2.324,. tau2Is 0.63, y0Is-4.95, wherein the number of the weight loss rate change rate parameters corresponds to the value of n, the calculation precision can be improved by increasing n, and further, the proper degassing time T is calculated according to the formula (14)>660min, the weight loss rate is about 0.68 percent.
Figure BDA0002671467290000102
n is the number of terms of the set expansion; a. thei，τiIs the weight loss rate change constant of the expansion of the ith term; d is the average molecular diffusion coefficient, which is a temperature-dependent function; t is the suitable degassing time of the plate sample, daThe average thickness of the test piece is,tsthe weight loss rate is a preset weight loss rate change threshold value of the sample; t is0For a set delay time, T is taken in the invention0Is 60 in min; when at T0Within the time of (3), if the result obtained by calculation on the right side of the formula is smaller than the sample weight loss rate change threshold value, the degassing can be considered to reach the set requirement, namely the degassing is finished.
For a flat plate of the sample,ts0.1% is recommended, under strict conditionstsTake 0. And can be adjusted according to the experiment requirement. In the case of an XLPE cable,tsthe value of (a) can be determined according to actual requirements, for example, 0.2% is selected.
In the method, the construction steps of the appropriate degassing time models of the XLPE flat plate sample and the XLPE cable are the same, and the XLPE flat plate sample is taken as an example and described as follows:
firstly, the degassing temperature T in the step 3) is obtained2The molecular diffusion coefficients of acetophenone and cumyl alcohol in XLPE, either temperature T1The lower diffusion coefficient can be obtained according to arrhenius equation (4),
Figure BDA0002671467290000111
in the formula, D1Is the temperature T1Diffusion coefficient of lower, D2Is the temperature T2The diffusion coefficient is given by E, the activation energy of the material and R, a constant;
according to temperature T1The average molecular diffusion coefficient D at this temperature was calculated according to formula (5):
Figure BDA0002671467290000112
in the formula DaIs acetophenone molecule at temperature T1Molecular diffusion coefficient ofcIs cumyl alcohol at a temperature T1Lower molecular diffusion coefficient;
according to the formula (6), the average weight loss rate W of the sample at each sampling moment is calculatedaveWherein the maximum weight loss rate is selected as the upper error limit EupMinimum weight loss rate as lower limit of error Elow，
Figure BDA0002671467290000113
Wherein i is the number of patterns;
since the thickness of the flat plate specimen is much smaller than the length and width of the specimen, the diffusion of the crosslinking by-product in the planar direction can be ignored, and only the diffusion in the vertical plane direction can be considered, so that the degassing time is independent of the shape of the specimen and depends only on the thickness of the specimen; let the region of the crosslinking by-products in the initial state be-d/2<x<d/2, x represents the distance from the midpoint of the sample thickness, and when the initial time t is 0, the concentration function C (x, 0) is C0D/2, provided that during the diffusion of the crosslinking by-products, the by-products dissipate immediately after reaching the surface of the material>x or x>When d/2, C (x, t) ═ 0, as described above, based on fick's second law (7), equation (8) can be established:
Figure BDA0002671467290000114
Figure BDA0002671467290000115
where erf is the error function, CoutConcentration of the crosslinking by-products for diffusion into the air, see formula (9)
Figure BDA0002671467290000116
Assuming that the volume of the sample is V and the average density of the XLPE material is rho, the weight loss ratio is further formulated as formula (10)
Figure BDA0002671467290000121
Assuming that the sample weight is stable, there is no crosslinking by-product in the material, at which time ts，C(x，ts) 0, and then equations (11) and (12) are obtained,
M＝ρV+C0V (11)
Figure BDA0002671467290000122
in the formula WtsIs the weight loss rate when the sample is stable;
based on the formulas (8), (9), (10), (11) and (12), the average weight loss rate W is introducedaveEquation (6), construction of weight loss ratio and average molecular diffusion Rate D and average thickness D of sampleaThe approximate relation (13);
Figure BDA0002671467290000123
in the formula, n is a set expansion term number, and the value of the expansion term number represents the degree of the model approaching the weight loss rate trend of the sample; a. thei，τiIs the weight loss rate change constant, y, of the expansion of the ith term0Is a constant;
further, based on the weight loss rate change parameter of the formula (13), constructing an XLPE flat plate sample proper degassing time model (14),
Figure BDA0002671467290000124
to further verify the effectiveness of the present invention, a square sample of 10 × 10mm is prepared according to the above process using the same material, and the sample is tested at different positions by using a 0.001mm thickness meter, and the average thickness of the sample is calculated according to the formula (1), as shown in table 5, the average thickness of the sample is 0.224 mm. The initial weight of the sample was then weighed using an electronic scale with an accuracy of 0.1mg, as shown in Table 6.
TABLE 5 average thickness of the samples
Categories Average thickness/mm Categories Average thickness/mm
Sample No. 1 0.215 Sample No. 5 0.23
Sample No. 2 0.215 Sample No. 6 0.23
Sample No. 3 0.235 Sample 7 0.22
Sample No. 4 0.23 Sample 8 0.216
Further, the sample was placed in an oven, the oven temperature was set to 60 ℃ according to the above conditions, the sample was taken out at a time interval similar to the formula (2), placed in a desiccator, cooled, weighed with an electronic scale of 0.1mg precision, and the sample weight was recorded as shown in table 6.
TABLE 6 weight of sample in 60 ℃ degassing condition
Figure BDA0002671467290000131
Further, the weight loss ratio of the sample was calculated according to the formula (3) as shown in table 7.
Figure BDA0002671467290000132
In the formula WjIs the weight loss ratio of the sample, MiIs the initial weight of the sample before degassing, GjIs the weight after degassing;
according to the formula (4), the acetophenone molecular diffusion coefficient at 60 ℃ is 6.6E-8m2H, a diffusion coefficient of cumyl alcohol molecules of 7.9E-8m2Calculated molecular average diffusion coefficient of 7.3E-8m according to equation (5)2/h。
TABLE 7 weight loss ratio of sample under 60 deg.C degassing condition
Figure BDA0002671467290000133
According to the formula (6), the average weight loss rate W of the sample at each sampling moment is calculatedaveWherein the maximum weight loss rate is selected as the upper error limit EupMinimum weight loss rate as lower limit of error ElowThe results are shown in Table 8, and the graph of the relationship between the degassing time and the weight loss of the sample is shown in FIG. 4.
TABLE 8 average, maximum and minimum weight loss of samples at 60 deg.C in degassed conditions
Time/min Wave/％ Eup/％ Elow/％
0 0 0 0
5 0.3271 0.4205 0.2641
15 0.3933 0.4665 0.3494
25 0.4687 0.5454 0.3813
40 0.4868 0.5520 0.4173
60 0.5126 0.5651 0.4401
80 0.5051 0.5780 0.4442
130 0.5223 0.5784 0.4492
190 0.5023 0.5788 0.4520
450 0.5240 0.5782 0.4596
650 0.5004 0.5738 0.4526
940 0.5039 0.5698 0.4553
1180 0.5213 0.5696 0.4590
1800 0.5033 0.5436 0.4515
Based on the change parameter of the weight loss rate at 70 ℃, the change parameter tau of the weight loss rate of the sample at 60 ℃ can be obtained by combining the diffusion coefficient D at 60 ℃ and the thickness D of the sample1Is 0.28, τ2Is 0.35, the other weight variables are consistent at 70 ℃, and the proper degassing time T at 60 ℃ is calculated>690min at which time the weight loss was about 0.51%, which is somewhat different from the results at 70 c, mainly due to changes in sample thickness and partial volatilization of the crosslinking by-products during sample preparation.
To further demonstrate the effectiveness of the present invention, thermogravimetric analysis (TGA) was used to analyze the weight loss of the same batch of samples degassed at 70 ℃. A thermogravimetric analysis experiment was performed using a Pyris 1TGA instrument manufactured by Perkin-Elmer, USA, and 17.54mg of a sample that was not degassed was weighed, heated to 180 ℃ at 50 ℃/min at room temperature, kept at a constant temperature for 30min, then cooled to room temperature at 20 ℃/min, and kept at the constant temperature for 20 min. The results are shown in fig. 5, and it can be seen from fig. 5 that the overall weight of the sample is reduced by about 0.6%, which is substantially consistent with the weight loss at the appropriate degassing time evaluated in accordance with the present invention. The weight and volume of the samples prepared in thermogravimetric analysis are much less than the method described in the present invention, and the small amount of volatilization of the by-products produces a small deviation in the results, but this result also further demonstrates the effectiveness of the present invention.

Claims (10)

1. A method for evaluating the appropriate degassing time of an XLPE flat plate sample based on a flat plate hot pressing method is characterized by comprising the following steps of:
step 1: weighing XLPE cable material particles, and preparing an XLPE flat plate sample;
step 2: cutting an XLPE flat plate sample, preparing a degassed sample to be tested, measuring the average thickness of the sample to be tested and weighing the initial weight;
and step 3: placing the sample into a drying oven, setting the degassing temperature, carrying out degassing treatment, taking out the sample at each set sampling time, and weighing;
and 4, step 4: after the weight of the sample is stable, stopping the experiment, calculating the weight loss rate in the degassing process of the sample, and constructing an appropriate degassing time model of the XLPE flat plate sample as follows:
Figure FDA0002671467280000011
wherein n is the number of terms of the set expansion; a. thei，τiIs the weight loss rate change constant of the expansion of the ith term; d is the average molecular diffusion coefficient, which is a temperature-dependent function; t is the suitable degassing time of the flat plate sample, T0For a set delay time, daThe average thickness of the test piece is,tsthe weight loss rate is a preset weight loss rate change threshold value of the sample;
and 5: and (4) obtaining the proper degassing time of the flat plate sample to be evaluated by using the model obtained in the step (4) according to the thickness and the degassing temperature of the sample to be evaluated.
2. The method for evaluating the suitable degassing time of an XLPE plate specimen based on plate hot pressing method according to claim 1, wherein the step 1 is: weighing XLPE cable material particles, uniformly dispersing the XLPE cable material particles in a mould, covering the upper surface and the lower surface of the mould by using PET, putting the mould in a flat plate clamp, and putting the mould and the flat plate clamp into a vulcanizing machine; and setting the prepressing time and the vulcanizing time and the temperature of a vulcanizing machine, cooling the mold after the sample preparation is finished, and taking out the XLPE flat plate sample.
3. The method for evaluating the suitable degassing time of an XLPE plate specimen based on plate hot pressing method according to claim 1, wherein the step 2 is: trimming the overflow part of the edge of the sample to obtain a degassed sample to be tested, or preparing the degassed sample to be tested into a set shape; selecting different positions of the sample for multiple times by using a 0.001mm thickness meter or a higher-precision deviceMeasuring the thickness, and calculating the average thickness d of the sample according to the formula (1)ave，
Figure FDA0002671467280000012
In the formula diJ is the number of measurements for each measurement of the thickness of the sample;
immediately thereafter, the current sample weight is weighed using an electronic scale with an accuracy of at least 0.1 mg.
4. The method for evaluating the suitable degassing time of XLPE flat plate samples based on flat plate hot pressing method according to claim 1, characterized in that the degassing treatment of the samples in step 3 is performed in an oven and the following conditions are satisfied:
(1) only XLPE flat plate samples are ensured in the oven, and other samples cannot be mixed;
(2) the test piece is vertically hung in the middle of the oven, and the distance between a plurality of XLPE flat plate test samples is at least 20 mm;
(3) the volume of the XLPE flat plate sample is not more than 0.5 percent of the volume of the oven, and the temperature in the oven is constant.
5. The method for evaluating the degassing time of an XLPE plate specimen based on plate hot pressing method as claimed in claim 1, wherein the sampling time in step 3 is determined by the predetermined time or according to the formula (2):
t＝ek-1 (2)
in the formula, t is the time of the kth sampling and is in min; k is the number of times of sampling and is a positive integer.
6. The method for evaluating the degassing time of an XLPE flat plate sample based on the flat plate hot pressing method as claimed in claim 1, wherein in step 3, the sample is placed in a dryer at the ambient temperature again each time the sample is taken out of the oven and weighed, and the sample is weighed after the temperature is returned to the ambient temperature, and the weight of the sample is measured in mg to an accuracy of 0.1mg or more.
7. The method for evaluating the suitable degassing time of the XLPE flat plate sample based on the flat plate hot pressing method according to claim 1, wherein in the step 4, the weight loss rate of the sample during degassing is calculated according to the formula (3):
Figure FDA0002671467280000021
in the formula WjIs the weight loss ratio of the sample, MiIs the initial weight of the sample before degassing, GjIs the weight after degassing;
in step 4, the steps for constructing the appropriate degassing time model of the XLPE flat plate sample are as follows:
firstly, the degassing temperature T in the step 3) is obtained2The molecular diffusion coefficients of acetophenone and cumyl alcohol in XLPE, either temperature T1The lower diffusion coefficient can be obtained according to arrhenius equation (4),
Figure FDA0002671467280000022
in the formula, D1Is the temperature T1Diffusion coefficient of lower, D2Is the temperature T2The diffusion coefficient is given by E, the activation energy of the material and R, a constant;
according to temperature T1The average molecular diffusion coefficient D at this temperature was calculated according to formula (5):
Figure FDA0002671467280000031
in the formula DaIs acetophenone molecule at temperature T1Molecular diffusion coefficient ofcIs cumyl alcohol at a temperature T1Lower molecular diffusion coefficient;
according to the formula (6), the average weight loss rate W of the sample at each sampling moment is calculatedaveWherein the maximum weight loss rate is selected as the upper error limit EupMinimum weight loss rate as lower limit of error Elow，
Figure FDA0002671467280000032
Wherein i is the number of patterns;
since the thickness of the flat plate sample is far smaller than the length and width of the sample, the diffusion of the crosslinking by-product along the plane direction is ignored, and only the diffusion condition in the direction vertical to the plane is considered, so that the degassing time is independent of the shape of the sample and only depends on the thickness d of the sample; let the region of the crosslinking by-products in the initial state be-d/2<x<d/2, x represents the distance from the midpoint of the sample thickness, and when the initial time t is 0, the concentration function C (x, 0) is C0D/2, provided that during the diffusion of the crosslinking by-products, the by-products dissipate immediately after reaching the surface of the material>x or x>When d/2, C (x, t) ═ 0, as described above, based on fick's second law (7), equation (8) can be established:
Figure FDA0002671467280000033
Figure FDA0002671467280000034
where erf is the error function, CoutConcentration of the crosslinking by-products for diffusion into the air, see formula (9)
Figure FDA0002671467280000035
Assuming that the volume of the sample is V and the average density of the XLPE material is rho, the weight loss ratio is further formulated as formula (10)
Figure FDA0002671467280000036
Assuming that the sample weight is stable, there is no crosslinking by-product in the material, at which time ts，C(x，ts) 0, and then equations (11) and (12) are obtained,
M＝ρV+C0V (11)
Figure FDA0002671467280000041
in the formula WtsIs the weight loss rate when the sample is stable;
based on the formulas (8), (9), (10), (11) and (12), the average weight loss rate W is introducedaveEquation (6), construction of weight loss ratio and average molecular diffusion rate D and average specimen thickness DaThe approximate relation (13);
Figure FDA0002671467280000042
in the formula, n is a set expansion term number, and the value of the expansion term number represents the degree of the model approaching the weight loss rate trend of the sample; a. thei，τiIs the weight loss rate change constant, y, of the expansion of the ith term0Is a constant;
constructing an XLPE flat plate sample proper degassing time model (14) based on the weight loss rate change parameter of the formula (13),
Figure FDA0002671467280000043
8. a method for evaluating the suitable degassing time of XLPE cable insulation, characterized in that it comprises the following steps:
step 1: selecting a cable to be degassed;
step 2: measuring the insulation evaluation thickness of the cable and weighing the initial weight;
and step 3: placing the cable into a degassing chamber for degassing treatment, taking out the cable at a set sampling time, and weighing;
and 4, step 4: after the weight of the cable is stable, stopping the experiment, calculating the weight loss rate in the cable degassing process, and constructing an XLPE cable suitable degassing time model as follows:
Figure FDA0002671467280000044
wherein n is the number of terms of the set expansion; a. thei，τiIs the weight loss rate change constant of the expansion of the ith term; d is the average molecular diffusion coefficient, which is a temperature-dependent function; t is the proper degassing time of the XLPE cable, and d is the thickness of cable insulation;tsis a preset sample weight loss rate change threshold value, T0Is a set delay time;
and 5: and (4) obtaining the appropriate degassing time of the XLPE cable to be evaluated by using the model obtained in the step (4) according to the insulation thickness and the degassing temperature of the XLPE cable to be evaluated.
9. A method for evaluating the suitable degassing time of XLPE cables according to claim 8, characterized in that: the sampling time in the step 3 is determined according to a preset selection basis or according to a formula (15):
t＝ek+3 (15)
in the formula, t is the time of the kth sampling and is in min; k is the number of times of sampling and is a positive integer;
and 4, when the cable is taken out of the degassing chamber and weighed, putting the cable at the ambient temperature again, and weighing the cable after the cable is restored to the ambient temperature.
10. A method for adapting the degassing time of XLPE cables according to claim 8, characterized in that: in step 4, the method for evaluating the suitable degassing time of the sample mainly comprises the following steps:
firstly, obtaining the molecular diffusion coefficients of acetophenone and cumyl alcohol in XLPE at the degassing treatment temperature in the step 3), obtaining the diffusion coefficient at any temperature according to an Allen-ius formula, and calculating the average molecular diffusion coefficient D at any temperature;
calculating the average weight loss rate W of the sample at each sampling momentave；
Figure FDA0002671467280000051
Wherein i is the number of patterns;
because the length of the cable is far greater than the thickness of the body, the diffusion of crosslinking byproducts in the cable along the axial direction can be ignored, only the diffusion of the crosslinking byproducts in the radial direction of the cable is considered, and then a one-dimensional symmetrical model is established, which is shown in a formula (7)
Figure FDA0002671467280000052
Assuming that the area of crosslinking by-products is 0 in the initial state of the cable<x<d, d is the thickness of the insulating layer of the XLPE cable, 0 is the side of the XLPE insulation that is connected to the inner semiconductive layer, the concentration function C (x, 0) ═ C when the initial time t equals 00Assuming that during the diffusion of the crosslinking by-products, the diffusion of the by-products into the core conductor is neglected and the by-products diffuse only to the outside and dissipate immediately after reaching the surface of the material, x>When d, C (x, t) ═ 0, then equation (16) is established
Figure FDA0002671467280000053
Where erf is an error function, see equation (9)
Figure FDA0002671467280000054
Assuming that the volume of the XLPE cable insulation is V and the average density of the XLPE material is rho, the weight loss rate can be further arranged into a formula (10);
Figure FDA0002671467280000061
assuming that the sample weight is stable, there is no crosslinking by-product in the material, at which time tsThen C (x, t)s) 0, and then obtaining equations (11) and (12);
M＝ρV+C0V (11)
Figure FDA0002671467280000062
based on the equations (16), (9), (10), (11) and (12), the average weight loss rate W is introducedaveEquation (6) can construct a relational expression (17) between the weight loss rate and the average molecular diffusion rate D and the sample thickness D,
Figure FDA0002671467280000063
in the formula
n is a set expansion term number, and the value of the expansion term number represents the degree of the model approaching the weight loss rate trend of the sample; a. thei，τiIs the weight loss rate change constant, y, of the expansion of the ith term0Is a constant;
constructing a degassing mathematical model formula (18) based on the weight loss rate change parameters of the formula (17), and determining the appropriate degassing time T of the XLPE cable;
Figure FDA0002671467280000064
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