阅读说明：本技术 一种电缆绝缘材料的热寿命评估方法 (Thermal life evaluation method of cable insulation material ) 是由 刘飞 江平开 于 2020-12-23 设计创作，主要内容包括：本发明公开了一种电缆绝缘材料的热寿命评估方法,采用在空气气氛和缓慢升温速率条件下进行TGA分析,较好地模拟了热老化箱中的常规热老化过程,克服了在普遍采用的氮气气氛和高升温速率条件下因环境含氧量差异和温度过高导致材料老化机理发生改变以及所得化学反应活化能过高的缺陷。此外,通过监测材料在加速热老化试验过程中的重量变化,很好地解决了热分析法测定化学反应活化能时转化率选择的难题,实现了活化能的可靠测定,结合一个高温点下的加速热老化试验结果,应用点斜法,能够对电缆绝缘材料的热寿命进行快速有效的评估。(The invention discloses a thermal life evaluation method of a cable insulating material, which better simulates the conventional thermal aging process in a thermal aging box by adopting TGA analysis under the conditions of air atmosphere and slow heating rate, and overcomes the defects of material aging mechanism change and over-high activation energy of the obtained chemical reaction caused by the difference of oxygen content and over-high temperature of the environment under the commonly adopted nitrogen atmosphere and high heating rate. In addition, the problem of conversion rate selection when the chemical reaction activation energy is measured by a thermal analysis method is well solved by monitoring the weight change of the material in the process of an accelerated thermal aging test, the reliable measurement of the activation energy is realized, and the thermal life of the cable insulation material can be quickly and effectively evaluated by combining the accelerated thermal aging test result at a high temperature point and applying a point-oblique method.)

1. A method for evaluating the thermal life of a cable insulation material, comprising the steps of:

the method comprises the following steps: selecting an aging temperature, carrying out an accelerated thermal aging test on the cable insulating material at the aging temperature and different aging times, simulating the thermal aging process of the cable insulating material, obtaining an aging curve based on elongation at break, formulating a service life standard, and calculating the thermal life of the cable insulating material at the aging temperature according to the service life standard;

step two: detecting the weight change of the cable insulating material in the temperature and time nodes for accelerating thermal aging to obtain an aging curve based on the weight loss percentage, and calculating the weight loss percentage of the cable insulating material in the service life termination standard according to the aging curve;

step three: carrying out thermal weight loss analysis on the cable insulating material at different heating rates under the air atmosphere to obtain a thermal weight loss curve, carrying out chemical reaction kinetic analysis on the thermal weight loss curve by applying an Ozawa method, and calculating chemical reaction activation energy corresponding to weight loss percentage when the insulating cable material fails;

step four: and (3) bringing the chemical reaction activation energy obtained by calculation in the step three into an Arrhenius equation:

LnL＝A+B/T

in the formula: and A is a constant, B is E/R, E is chemical reaction activation energy, R is a gas constant, a life curve is drawn by adopting a point-skew method according to an Arrhenius equation, and the thermal life of the cable insulation material at the working temperature is calculated according to the life curve.

2. The method for evaluating the thermal life of a cable insulation material according to claim 1, wherein the aging temperature in the first step is 120 to 150 ℃.

3. The method for evaluating the thermal life of a cable insulation material according to claim 1, wherein the end-of-life criterion in the first step is a 50% reduction in elongation at break of the cable insulation material.

4. The method for evaluating thermal life of cable insulation according to claim 1, wherein the detecting weight change of cable insulation in step two specifically comprises:

drying the cable insulation material to be subjected to accelerated thermal aging, and weighing the cable insulation material to obtain a first weight value;

carrying out accelerated thermal aging treatment on the cable insulating material, drying the cable insulating material after the accelerated thermal aging treatment is finished, and weighing the weight of the cable insulating material to obtain a second weight value;

the difference value between the first weight value and the second weight value is the weight change of the cable insulation material.

5. The method for evaluating thermal life of a cable insulation material according to claim 1, wherein the rate of temperature rise in step three is not more than 5K/min.

6. The method for evaluating the thermal life of the cable insulation material according to claim 5, wherein the thermal weight loss curve is subjected to chemical reaction kinetic analysis by using an Ozawa method in the third step, and the chemical reaction activation energy corresponding to the weight loss percentage is obtained when the insulation cable material fails, according to the following equation:

in the formula: r is a gas constant;

and (3) selecting the weight loss percentage of the cable insulation material when the cable insulation material fails, drawing a relation curve of lg beta-1/T, and solving the chemical reaction activation energy E through the slope of a linear fitting straight line.

Technical Field

The invention belongs to the technical field of insulating material diagnosis, and particularly relates to a thermal life evaluation method of a cable insulating material.

Background

In actual operation, the cable can bear the comprehensive action of multiple stresses such as electricity, heat, machinery, environment and the like, and the insulation of the cable can be gradually aged and degraded until the insulation fails. Many early-laid medium and low voltage power cables have been in service for about 30 years, reaching or even exceeding the design life. Extensive replacement involves a lot of manpower, material and financial resources, and further service may cause cable failure to jeopardize the grid security with greater losses, and power companies therefore have to strike a balance between economy and reliability of power supply, which requires predicting the residual life of the cable to determine a reasonable time for cable replacement. Additionally, cable or material manufacturers also need to make life assessments of the cables or materials to ensure that the new cables reach the design life.

At present, an internationally recognized thermal aging model is a phenomenological model based on an Arrhenius equation, a relationship between temperature and thermal life is established, the life evaluation must be combined with an accelerated thermal aging test, and the life at a working temperature is extrapolated through the life at a high temperature. Conventional thermal aging testing methods generally select at least three temperatures for thermal exposure testing, apply statistical methods to verify whether the logarithm of the lifetime is linear with the reciprocal of the absolute temperature, and then predict the lifetime by extrapolation of a thermal lifetime map. However, the conventional heat aging test method takes too long, and the average value of the end time at the lowest test temperature is not less than 5000 hours according to the International electrotechnical Commission Standard IEC 60216.

In order to improve the efficiency of the accelerated thermal life test and reduce the cost, a method for rapidly evaluating the insulation aging life is being explored at home and abroad, and a thermal analysis method is generally introduced into the evaluation of the thermal life. According to an Arrhenius diagram obtained by an analytical method, namely a logarithm-1/T diagram of the reaction rate, the activation energy of the reaction is obtained, so that the slope of the life curve is calculated, and the life curve is drawn by combining the accelerated aging test result of a high-temperature point. However, the current thermal analysis method (such as the TGA method) usually adopts a nitrogen atmosphere and has a high temperature rise rate, so that the aging mechanism of the material is changed, and the obtained reaction activation energy is often too high. In addition, there is no reliable basis for the choice of conversion (or percent weight loss) when calculating activation energy using a method that circumvents the reaction mechanism, such as the Ozawa method. Therefore, there is a need to improve the existing thermal analysis methods and to provide a more reliable method for rapidly assessing the thermal aging life.

Disclosure of Invention

The invention aims to provide a thermal life evaluation method of a cable insulating material, which can improve the test efficiency, reduce the test cost and provide support for determining a cable replacement strategy and researching and developing a new cable material.

In order to solve the problems, the technical scheme of the invention is as follows:

a method for evaluating the thermal life of a cable insulation material comprises the following steps:

the method comprises the following steps: selecting an aging temperature, carrying out an accelerated thermal aging test on the cable insulating material at the aging temperature and different aging times, simulating the thermal aging process of the cable insulating material, obtaining an aging curve based on elongation at break, formulating a service life standard, and calculating the thermal life of the cable insulating material at the aging temperature according to the service life standard;

step two: detecting the weight change of the cable insulating material in the temperature and time nodes for accelerating thermal aging to obtain an aging curve based on the weight loss percentage, and calculating the weight loss percentage of the cable insulating material in the service life termination standard according to the aging curve;

step three: carrying out thermal weight loss analysis on the cable insulating material at different heating rates under the air atmosphere to obtain a thermal weight loss curve, carrying out chemical reaction kinetic analysis on the thermal weight loss curve by applying an Ozawa method, and calculating chemical reaction activation energy corresponding to weight loss percentage when the insulating cable material fails;

step four: and (3) bringing the chemical reaction activation energy obtained by calculation in the step three into an Arrhenius equation:

LnL＝A+B/T

in the formula: and A is a constant, B is E/R, E is chemical reaction activation energy, R is a gas constant, a life curve is drawn by adopting a point-skew method according to an Arrhenius equation, and the thermal life of the cable insulation material at the working temperature is calculated according to the life curve.

Preferably, the aging temperature in the first step is 120-150 ℃.

Preferably, the end-of-life criterion in step one is a 50% reduction in the elongation at break of the cable insulation.

Preferably, the detecting the weight change of the cable insulating material in the second step specifically includes:

drying the cable insulation material to be subjected to accelerated thermal aging, and weighing the cable insulation material to obtain a first weight value;

carrying out accelerated thermal aging treatment on the cable insulating material, drying the cable insulating material after the accelerated thermal aging treatment is finished, and weighing the weight of the cable insulating material to obtain a second weight value;

the difference value between the first weight value and the second weight value is the weight change of the cable insulation material.

Preferably, the temperature rise rate in the third step is not more than 5K/min.

Preferably, in step three, an Ozawa method is applied to perform chemical reaction kinetic analysis on the thermogravimetry curve, and the chemical reaction activation energy corresponding to the weight loss percentage is obtained when the insulated cable material fails, according to the following equation:

in the formula: r is a gas constant;

and (3) selecting the weight loss percentage of the cable insulation material when the cable insulation material fails, drawing a relation curve of lg beta-1/T, and solving the chemical reaction activation energy E through the slope of a linear fitting straight line.

Due to the adoption of the technical scheme, compared with the prior art, the invention has the following advantages and positive effects:

1) the invention provides a method for evaluating the thermal life of a cable insulating material, which selects an aging temperature, carrying out accelerated thermal aging test on the cable insulating material, measuring the elongation at break and the weight of the cable insulating material before thermal aging and at the end of different aging periods, obtaining an aging curve based on the elongation at break and an aging curve based on the weight loss percentage at an aging temperature, establishing a service life termination standard, calculating the thermal life of the cable insulating material at the aging temperature, and the weight loss percentage of the sample at the end of the service life is calculated, then, in the air atmosphere, a slow temperature rise mode is adopted, and (3) carrying out Thermal Gravimetric Analysis (TGA) on the cable insulating material, measuring thermal gravimetric curves at a plurality of heating rates, carrying out chemical reaction kinetic analysis on the thermal gravimetric curves by applying an Ozawa method, and obtaining the chemical reaction activation energy corresponding to the weight loss percentage. And finally, drawing a life curve by a point-skew method according to an Arrhenius equation, and further extrapolating to obtain the thermal life of the insulating material at the working temperature. The TGA analysis is carried out under the conditions of air atmosphere and slow heating rate, so that the conventional thermal aging process in a thermal aging box is well simulated, and the defects that the aging mechanism of the material is changed and the activation energy of the obtained chemical reaction is overhigh due to the difference of oxygen content in the environment and overhigh temperature under the commonly adopted conditions of nitrogen atmosphere and high heating rate are overcome. In addition, the difficulty of conversion rate selection when the chemical reaction activation energy is determined by a thermal analysis method is well solved by monitoring the weight change of the material in the process of an accelerated thermal aging test. The invention provides a more reliable and rapid method for evaluating the thermal life of a cable insulation material.

Drawings

Fig. 1 is a flowchart illustrating steps of a method for evaluating thermal life of a cable insulation according to an embodiment of the present invention;

FIG. 2 is an aging curve based on elongation at break obtained in step one of FIG. 1;

FIG. 3 is an aging curve based on percent weight loss obtained in step two of FIG. 1;

FIG. 4 is a thermogravimetric plot at different temperature ramp rates obtained in step three of FIG. 1;

FIG. 5 is a graph showing the relationship lg β -1/T;

FIG. 6 is a thermal life graph of step four in FIG. 1, plotted by dot-slope method.

Detailed Description

The method for evaluating the thermal life of the cable insulation material according to the present invention is further described in detail with reference to the accompanying drawings and specific examples. Advantages and features of the present invention will become apparent from the following description and from the claims.

Referring to fig. 1, the present embodiment provides a method for evaluating thermal life of a cable insulation material, comprising the steps of:

the method comprises the following steps: selecting an aging temperature, wherein the aging temperature range is 120-150 ℃, preferably 140 ℃, performing an accelerated thermal aging test on the cable insulating material at the aging temperature and different aging times, simulating the thermal aging process of the cable insulating material, obtaining an aging curve based on elongation at break, and establishing a life termination standard, wherein in the embodiment, the life termination standard is that the elongation at break of the cable insulating material is reduced by 50%, and the thermal life of the cable insulating material at the aging temperature is calculated according to the life termination standard;

step two: detecting the weight change of the cable insulating material in the temperature and time nodes for accelerating thermal aging to obtain an aging curve based on the weight loss percentage, and calculating the weight loss percentage of the cable insulating material in the service life termination standard according to the aging curve;

in this embodiment, detecting the weight change of the cable insulation material specifically includes:

drying the cable insulation material to be subjected to accelerated thermal aging, and weighing the cable insulation material to obtain a first weight value;

carrying out accelerated thermal aging treatment on the cable insulating material, drying the cable insulating material after the accelerated thermal aging treatment is finished, and weighing the weight of the cable insulating material to obtain a second weight value;

the difference value of the first weight value and the second weight value is the weight change of the cable insulation material;

step three: performing thermogravimetric analysis on the cable insulating material at different heating rates under an air atmosphere to obtain a thermogravimetric curve, wherein in the embodiment, the heating rate is not more than 5K/min, performing chemical reaction kinetic analysis on the thermogravimetric curve by using an Ozawa method, and calculating chemical reaction activation energy corresponding to the weight loss percentage when the insulating cable material fails;

in this example, the activation energy of the chemical reaction was solved by the Ozawa method according to the following equation:

in the formula: r is a gas constant;

selecting the weight loss percentage of the cable insulation material when the cable insulation material fails, drawing a relation curve of lg beta-1/T, and solving chemical reaction activation energy E through the slope of a linear fitting straight line;

step four: and (3) bringing the chemical reaction activation energy obtained by calculation in the step three into an Arrhenius equation:

LnL＝A+B/T

in the formula: and A is a constant, B is E/R, E is chemical reaction activation energy, R is a gas constant, a life curve is drawn by adopting a point-skew method according to an Arrhenius equation, and the thermal life of the cable insulation material at the working temperature is calculated according to the life curve.

In this example, the method for evaluating the fast thermal life of the cable insulation material is specifically implemented as follows, firstly, the cable insulation material is processed into a standard dumbbell-shaped test sample, and the test sample is placed in an aging oven for accelerated thermal aging test, wherein the aging temperature and the aging time are shown in table 1.

TABLE 1 aging conditions

Aging temperature	Aging period 1	Aging period 2	Aging period 3	Aging period 4	Aging period 5
						140℃	96h	72h	72h	48h	24h

A tensile chamber test was performed on a set of samples not subjected to the accelerated thermal aging treatment and on sets of samples subjected to the aging cycle treatment described above, each set containing five samples, according to ASTM D638-2003, on an electronic tensile tester (model Instron Series IX4465), to obtain an aging curve based on elongation at break, as shown in FIG. 2. The thermal life of the cable insulation material at 140 ℃ was determined from the aging curve to be 329.5 hours, using the 50% reduction in initial elongation at break as the end-of-life criterion.

Each set of samples was weighed a total of two times before and after aging, cooled to room temperature before weighing and dried to obtain an aging curve based on percent weight loss, as shown in fig. 3. The percent weight loss of the cable insulation at end of life was found to be 5.0% from the aging curve.

In the air atmosphere, thermal weight loss analysis is carried out on four samples at four heating rates, wherein the four heating rates are respectively 0.5 ℃/min, 2 ℃/min, 3.5 ℃/min and 5 ℃/min, the test temperature ranges are all 50-600 ℃, and the thermal weight loss curve is shown in fig. 4.

And (3) carrying out chemical reaction kinetic analysis on the four thermogravimetric curves to solve the activation energy, wherein the method can avoid the reaction mechanism. Specifically, the activation energy is solved by adopting an Ozawa method, and the method is based on an equation:

the weight loss percentage of the material when the material fails is selected to be 5.0%, the relationship of lg beta to 1/T is drawn, as shown in figure 5, the activation energy E is 133.4kJ/mol which is obtained by linear fitting of the slope of a straight line, and the value is equivalent to the cable insulation equivalent activation energy obtained by conventional thermal life assessment, which indicates that the activation energy is reliably obtained by an improved thermal analysis method.

According to an arrhenius equation LnL + a + B/T (where a is a constant, B is E/R is a constant, E is reaction activation energy, and R is a gas constant) of thermal life evaluation, a life curve is drawn by a point-and-slope method, as shown in fig. 6, it is known that the working temperature of the cable insulation material is 80 ℃, and the thermal life is 27.7 years by extrapolation, and the life evaluation result is reasonable for a new cable insulation material, thus proving that the method of the present invention is effective.

The embodiment provides a thermal life evaluation method of a cable insulating material, which includes the steps of selecting an aging temperature, conducting an accelerated thermal aging test on the cable insulating material, measuring the elongation at break and the weight of the cable insulating material before thermal aging and at the end of different aging periods, obtaining an aging curve based on the elongation at break and an aging curve based on the weight loss percentage at the aging temperature, establishing a life termination standard, obtaining the thermal life of the cable insulating material at the aging temperature, calculating the weight loss percentage of a sample at the end of the life, conducting thermal weight loss analysis (TGA) on the cable insulating material in a slow heating mode under the air atmosphere, measuring thermal weight loss curves at a plurality of heating rates, conducting chemical reaction kinetic analysis on the thermal weight loss curves by applying an Ozawa method, and obtaining chemical reaction activation energy corresponding to the weight loss percentages. And finally, drawing a life curve by a point-skew method according to an Arrhenius equation, and further extrapolating to obtain the thermal life of the insulating material at the working temperature. The TGA analysis is carried out under the conditions of air atmosphere and slow heating rate, so that the conventional thermal aging process in a thermal aging box is well simulated, and the defects that the aging mechanism of the material is changed and the activation energy of the obtained chemical reaction is overhigh due to the difference of oxygen content in the environment and overhigh temperature under the commonly adopted conditions of nitrogen atmosphere and high heating rate are overcome. In addition, the difficulty of conversion rate selection when the chemical reaction activation energy is determined by a thermal analysis method is well solved by monitoring the weight change of the material in the process of an accelerated thermal aging test. The invention provides a more reliable and rapid method for evaluating the thermal life of a cable insulation material.

The embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to the above embodiments. Even if various changes are made to the present invention, it is still within the scope of the present invention if they fall within the scope of the claims of the present invention and their equivalents.