CN102998323B - Method for evaluating aging degree of crosslinked polyethylene cable based on nuclear magnetic resonance - Google Patents

Abstract

The invention relates to a method for evaluating the aging degree of cross-linked polyethylene cables based on nuclear magnetic resonance. The molecular structure of cross-linked polyethylene cables under different aging degrees is tested and compared by a nuclear magnetic resonance analyzer, and the aging degree and temperature of cross-linked polyethylene are obtained. The proportional relationship between time, so as to obtain the aging state and remaining life of the XLPE cable to be evaluated according to the proportional relationship. The invention realizes the detection of the aging performance of the insulating material from a microcosmic perspective.

Description

A method for evaluating the aging degree of XLPE cables based on nuclear magnetic resonance

technical field

The invention relates to a method for evaluating the aging degree of cross-linked polyethylene cables based on the principle of nuclear magnetic resonance, in particular to the relationship between the microscopic properties of cross-linked polyethylene cables and their aging degrees.

Background technique

Cross-linked polyethylene (XLPE) cables are very important equipment in power transmission and transformation. They have a bottleneck effect on the safe and stable transmission of power loads. The design life of XLPE cables is about 30 years. Some domestic power companies invested in them early The cables used are close to the service life. Due to irregular construction, weather and man-made reasons, the cable operating environment has deteriorated, such as cable accumulation, water accumulation, mud, garbage filling, etc. Studies have shown that changes in the operating environment in the cable tunnel will cause the temperature of the cable to rise and the aging of the cable to increase. In addition to affecting the life of the cable itself, it will also affect the stability of the grid operation. Due to the destruction of the cable operating environment and the overlapping of cables, the heat dissipation channels of many cables are damaged, so that the cables are prone to local overheating and other faults during operation, including: local overheating, insulation damp and aging, and mechanical damage caused by irregular construction .

Due to the harsh operating environment, in order to ensure the safe operation of cables, underground cables in many cities are operated at low loads, resulting in low cable utilization efficiency and high operating costs. In order to improve the economy of cable operation and the reliability of power supply, the State Grid has begun to conduct asset life assessment. Therefore, on the premise of ensuring the safe operation of the cable, improving the operating efficiency and utilization of the cable and reducing the operating cost of the cable are problems that the power company needs to solve urgently.

The current thermal aging life evaluation methods such as conventional method, differential scanning calorimetry (DSC), Weibull distribution model method and other methods. However, the conventional method and the Weibull distribution model method only consider the change of a certain performance parameter of the insulating material, which is somewhat different from the actual situation. The DSC method cannot theoretically establish the relationship between the information provided by the method and the functional failure of the material. Especially due to the complexity of XLPE cable laying and operating environment, it will be affected by factors such as electricity, heat, chemistry and water in actual operation, leading to its aging. At present, mainly from a macro perspective, according to a certain XLPE A performance change is used to evaluate its operating status and service life, and the impact of its aging factors cannot be fully considered, resulting in a certain difference between the evaluation result and the actual situation.

Contents of the invention

The purpose of the present invention is to overcome the deficiency of evaluating the aging degree of cross-linked polyethylene cables based on a certain macro parameter, and provide a method for evaluating the aging degree of cross-linked polyethylene cables according to the microscopic molecular structure of insulating materials. When the insulating material ages, its molecular structure or polymer sequence will change, which will cause the microstructure of the material to change. The molecular structure diagram is shown in Figure 1 under normal conditions. Since the main composition of cross-linked polyethylene is C-H bond, when When aging occurs, the molecular structure changes. As a method for detecting the microstructure of substances, nuclear magnetic resonance can better distinguish the chemical composition of polymer materials and different structural sequences in the same composition. To characterize the aging of XLPE, it is convenient to evaluate the aging state of XLPE cables.

The characteristics of the NMR spectrum (such as the width, shape and area of the spectral line) can characterize the nature and environment of the atomic nucleus and determine the molecular structure. After NMR analysis and inversion of the sample, we can obtain chemical shift (relaxation time), peak area and other parameters, which can be used to determine the change of molecular structure. During the aging process of insulating materials, due to the degradation and deterioration of macromolecules, the molecular structure of the material will change. First, the molecular structure of the insulating material under normal conditions is tested by a nuclear magnetic resonance analyzer, and the molecular structure of the insulating material during the aging process is tested. , the relationship between the aging degree of cross-linked polyethylene and its microscopic properties can be obtained, and the aging degree can be evaluated more accurately.

The technical scheme adopted in the present invention is a method for evaluating the aging degree of XLPE cables based on nuclear magnetic resonance, comprising the following steps:

1) Using nuclear magnetic resonance to obtain the molecular structure of normal XLPE cables

Take the insulation layer material of normal XLPE cable, sample along the axial direction of the insulation layer, and cut into particles of uniform size, weigh 1.2 grams as a normal sample, and use a nuclear magnetic resonance analyzer to analyze the normal sample to obtain normal The peak area SP ₀ and the longitudinal relaxation time RTL ₀ of the cross-linked polyethylene cable; the parameters of the nuclear magnetic resonance analyzer are set as follows: the center frequency is 22MHz, the offset frequency is 584.347825 KHz, the pulse sequence repetition time is 2000us, and the number of repeated sampling is 16 .

2) Aging test and sampling of normal XLPE cables

Take a normal XLPE cable, cut it into several short cables, each short cable is 15-20 cm long, and use an air heat aging box to perform aging tests on the short cables at three temperatures: 90°C, 110°C and 130°C , sampling 7 times at each temperature, and the time interval of each sampling is proportional; the short cables after the aging test were sampled along the axial direction of the insulating layer, and cut into particles of uniform size, and weighed 1.2 grams as Aging samples.

3) NMR analysis of the characteristic quantities of aged XLPE cables

Analyze each aging sample in step 2) with a nuclear magnetic resonance analyzer, and obtain the relationship SP _T,t between the peak area of the XLPE cable and the aging temperature and aging time, and the relationship between the longitudinal relaxation time and the aging temperature and aging time In the relationship RTL _T,t , the subscripts T and t denote the aging temperature and aging time, respectively.

4) Take the cross-linked polyethylene cable to be evaluated, take samples along the axial direction of the insulating layer, cut them into particles of uniform size, weigh 1.2 grams as the sample to be tested, and analyze the sample to be tested with a nuclear magnetic resonance analyzer to obtain The peak area SP _Obj and the longitudinal relaxation time RTL _Obj of the XLPE cable are to be evaluated.

5) In the relationship between peak area and aging temperature and aging time SP _T,t in step 3) and the relationship between longitudinal relaxation time and aging temperature and aging time RTL _T,t , find out the relationship between the cross-linked polyethylene to be evaluated The peak area SP _Obj and longitudinal relaxation time RTL _Obj of the cable have the cable aging temperature and aging time under the same peak area and longitudinal relaxation time, and the aging temperature and aging time are recorded as T _S and t _S respectively.

6) Use the air heat aging box to age the normal XLPE cable at the aging temperature T _S obtained in step 5) until the insulation fails to meet the standard. The aging time when the cable is aged until the insulation fails to meet the standard is t _Se .

7) The time that the XLPE cable to be evaluated can still be used under the original load and operating environment is:

In the formula: It is the time for the XLPE cable to be evaluated to be used under the original load and operating environment; It is the time that the XLPE cable to be evaluated has been used before being tested.

The positive effect produced by the present invention due to the adoption of the above method is very significant, that is, the molecular structure of the cross-linked polyethylene cable under different aging degrees is tested and compared by a nuclear magnetic resonance analyzer, and the relationship between the aging degree of the insulating material and temperature and time is obtained. The proportional relationship realizes the evaluation of the aging state and remaining life of XLPE cables.

Description of drawings

Figure 1 is the molecular structure of XLPE cable;

Fig. 2 is the NMR inversion spectrogram;

Figure 3 is a graph showing the relationship between the peak area of the sample and the thermal aging time at 90°C;

Figure 4 is a graph showing the relationship between sample peak area and aging time at 110°C;

Figure 5 is a graph showing the relationship between sample peak area and aging time at 130°C;

Fig. 6 is the variation amplitude diagram of peak area relative to normal cross-linked polyethylene under different temperatures;

Figure 7 is a graph showing the relationship between the longitudinal relaxation time of peak 1 and the aging time at different temperatures;

Fig. 8 is a graph showing the relationship between the longitudinal relaxation time of peak 2 and the aging time at different temperatures.

Detailed ways

The method of the present invention refers to the following steps.

1) Using nuclear magnetic resonance to obtain the molecular structure of normal XLPE cables

Take the insulating layer material (cross-linked polyethylene) of the normal (new factory) cable sample, sample along the axial direction of the insulating layer, cut into particles of uniform size, and weigh 1.2g as a normal sample.

Utilize the nuclear magnetic resonance analyzer to analyze the normal sample, the parameters of the nuclear magnetic resonance analyzer are set as follows: the center frequency (SF1) is 22MHz, the offset frequency (O1) is 584.347825 KHz, the pulse sequence repetition time (TR) is 2000us, and the number of repeated sampling (NS) is 16, and accompanying drawing 2 is the NMR inversion spectrum of the sample obtained under this parameter setting.

Through nuclear magnetic resonance analysis, the peak area SP ₀ and longitudinal relaxation time RTL ₀ of the normal cross-linked polyethylene cable are obtained, and the peak area SP ₀ and RTL ₀ are used as the basis for evaluating the aging degree of the cross-linked polyethylene cable.

2) Aging test and sampling of normal XLPE cables

According to the IEC6-811-1 standard, take a normal (new factory) XLPE cable, cut it into several short cables, and the length of each short cable is 15-20 cm. Aging tests were carried out on short cables at three temperature levels of 130°C, and samples were taken 7 times at each test temperature.

At different temperature levels, according to the temperature increase of 10 ° C, the life of the sample will be reduced by 1/2 to 1/3 to calculate the aging time at different temperature levels, and at the same temperature level, the time interval of sample sampling should be equal The relationship, that is, the aging time of each sample is an proportional relationship. For several short cables after aging, the sampling method is still to sample along the axial direction of the insulation layer, and cut into particles of uniform size, 1.2g for each sample. Samples with different aging degrees were sampled three times, and each NMR test was sampled three times, which can fully eliminate the influence of test errors.

3) Carry out nuclear magnetic resonance analysis on the characteristics of aging cables

Analyze each aging sample in step 2) with a nuclear magnetic resonance analyzer, and obtain the relationship SP _T,t between the peak area of the cross-linked polyethylene cable and the aging temperature and aging time, as shown in Figures 3-5, and the longitudinal relaxation The relationship between aging time and aging temperature and aging time RTL _{T, t} , the subscripts T and t represent the aging temperature and aging time respectively, as shown in Figures 7 to 8; for example, the peak area aged at 90°C for 120 hours is expressed as: SP _90,120 .

4) Take the cross-linked polyethylene cable to be evaluated, take samples along the axial direction of the insulating layer, cut them into particles of uniform size, weigh 1.2 grams as the sample to be tested, and analyze the sample to be tested with a nuclear magnetic resonance analyzer to obtain The peak area SP _Obj and the longitudinal relaxation time RTL _Obj of the XLPE cable are to be evaluated.

5) In the relationship between peak area and aging temperature and aging time SP _T,t in step 3) and the relationship between longitudinal relaxation time and aging temperature and aging time RTL _T,t , find out the relationship between the cross-linked polyethylene to be evaluated The peak area SP _Obj and longitudinal relaxation time RTL _Obj of the cable have the cable aging temperature and aging time under the same peak area and longitudinal relaxation time, and the aging temperature and aging time are recorded as T _S and t _S respectively.

6) Use T _S and t _S to age the normal (new factory) XLPE cables until the insulation fails to meet the standard. The aging time when the cable is aged until the insulation fails to meet the standard is t _Se .

7) Therefore, it can be concluded that the time that the XLPE cable to be evaluated can still be used under the original load and operating environment is:

In the formula: It is the time for the XLPE cable to be evaluated to be used under the original load and operating environment; It is the time that the XLPE cable to be evaluated has been used before being tested.

The application process of the technical solution of the present invention will be further described below by taking a 110kV XLPE cable as an example.

1) Get the peak area of normal cable as 6925.45 , the longitudinal relaxation time of peak 1 is 20.6663ms, and the longitudinal relaxation time of peak 2 is 144.278ms.

2) Aging the cable according to the IEC60811 standard at 90°C, 110°C, and 130°C, and obtaining the curve relationship between the peak area and the longitudinal relaxation time of each aged sample.

3) NMR analysis of the cable to be evaluated (2 years or 17200h), and its peak area is 5583.18 , the longitudinal relaxation time of peak 1 is 17.7387ms, and the longitudinal relaxation time of peak 2 is 127.5420ms.

4) In the obtained graph, find the same temperature point and time point as the peak area of the cable to be evaluated and the longitudinal relaxation time of different peaks, here is 130°C, 240h.

5) Obtain the same batch of new factory cables for aging at 130°C, and the time until insulation failure (insulation does not meet the standard) is 2000h.

6) The remaining life of the cable is 2000/240*17200-17200=14.40 years.

At the same time, in order to further illustrate the feasibility of this method, according to the internationally recognized empirical formula, that is, the relationship between the life and temperature of the cable within a certain operating temperature range mostly conforms to the following empirical formula:

Where R is the gas constant, which is 8.314, and the activation energy is a characteristic quantity that characterizes the aging degree of insulating materials, and will be obtained through experimental data Compared with the test obtained by the IEC60811 standard elongation at break For comparison, when the elongation at break test of the cable insulation layer is carried out according to the IEC60811 standard, the test samples are also taken from the same batch of samples in step 2), and the activation energy is 130.181kJ/mol, and the activation energy obtained by NMR method 128.35kJ/mol is similar, and then the cable insulation failure test can be used to solve the variable a, that is, the life formula obtained by the nuclear magnetic resonance principle is similar to the life formula obtained by the IEC60811 standard, which shows that this method is accurate in evaluating the life of XLPE cables .

The method for evaluating the aging degree of cross-linked polyethylene cables based on the nuclear magnetic resonance principle provided by the invention can be applied to the aging evaluation of high-voltage insulating materials, and realizes the detection of the aging performance of insulating materials from a microscopic perspective. Especially in the case of rapid development of UHV power grids in my country and serious aging of high-voltage insulating materials, the invention provides an effective method for evaluating insulating materials.

Claims (2)

1., based on a twisted polyethylene cable degree of aging appraisal procedure for nuclear magnetic resonance, it is characterized in that, comprise the following steps:

1) nuclear magnetic resonance is utilized to obtain the molecular structure of normal twisted polyethylene cable

Get the insulating layer material of normal twisted polyethylene cable, along the sampling of insulation course axial direction, and be cut into particle of uniform size, take 1.2 grams as normal specimens, utilize magnetic nuclear resonance analyzer to analyze normal specimens, obtain the peak area SP of normal twisted polyethylene cable ₀and longitudinal relaxation time RTL ₀;

2) aging test carried out to normal twisted polyethylene cable and sample

Get normal twisted polyethylene cable, be cut into some sections of stub cables, every section of stub cable grows 15 ~ 20 centimetres, utilize air thermal aging oven under 90 DEG C, 110 DEG C and 130 DEG C of three temperature, carry out aging test to stub cable, sample 7 times at each temperature, the time interval of every sub-sampling is geometric ratio relation; By the stub cable after aging test respectively along the sampling of insulation course axial direction, and be cut into particle of uniform size, take 1.2 grams as aged samples;

3) nuclear magnetic resonance spectroscopy is carried out to aging twisted polyethylene cable characteristic quantity

Utilize magnetic nuclear resonance analyzer to step 2) in each aged samples analyze, obtain the peak area of twisted polyethylene cable and the relation SP of aging temperature and digestion time _t,t, and the relation RTL of longitudinal relaxation time and aging temperature and digestion time _t,t, subscript T and t represents aging temperature and digestion time respectively;

4) twisted polyethylene cable to be assessed is got, along the sampling of insulation course axial direction, and be cut into particle of uniform size, take 1.2 grams as testing sample, utilize magnetic nuclear resonance analyzer to analyze testing sample, obtain the peak area SP of twisted polyethylene cable to be assessed _objand longitudinal relaxation time RTL _obj;

5) the relation SP of the peak area in step 3) and aging temperature and digestion time _t,tand the relation RTL of longitudinal relaxation time and aging temperature and digestion time _t,tin, find out the peak area SP with twisted polyethylene cable to be assessed _objand longitudinal relaxation time RTL _objthere is the cable aging temperature under identical peak area and longitudinal relaxation time and digestion time, remember that this aging temperature and digestion time are T respectively _s, t _s;

6) the aging temperature T that air thermal aging oven obtains in step 5) is utilized _sunder, carry out aging to normal twisted polyethylene cable, until insulation not up to standard, note cable be aged to insulation not up to standard time digestion time be t _se;

7) time that twisted polyethylene cable to be assessed can also use under original load and running environment is:

In formula: for the time that twisted polyethylene cable to be assessed can also use under original load and running environment; for the time that twisted polyethylene cable to be assessed had used before being detected.

2. a kind of twisted polyethylene cable degree of aging appraisal procedure based on nuclear magnetic resonance according to claim 1, it is characterized in that: magnetic nuclear resonance analyzer optimum configurations is as follows: centre frequency is 22MHz, deviation frequency is 584.347825 KHz, the pulse train repetition time is 2000us, and repeated sampling number of times is 16.