CN112257232B - Life model establishing method based on ethanol in oil and transformer paper polymerization degree - Google Patents
Life model establishing method based on ethanol in oil and transformer paper polymerization degree Download PDFInfo
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Abstract
The invention discloses a life model building method based on ethanol in oil and transformer paper polymerization degree, which comprises the steps of firstly designing aging tests of oil paper insulation systems with different initial moisture contents and different aging temperatures; analyzing the change rule and the correlation of the ethanol content in the sample with the temperature, the moisture and the aging degree; establishing a relation model of the degree of polymerization of the ethanol and the insulating paper, and substituting a fitting model of simultaneous ethanol and aging time into a numerical value of the degree of polymerization at the end point of the service life to monitor the service life of the oil-paper insulating system under different initial moisture conditions and different temperatures in a laboratory; and fitting the kinetic model based on the ethanol with the accelerated thermal aging data, and establishing an oil paper insulation aging model at any temperature on the basis of a time-temperature superposition theory. The invention adopts ethanol as the characteristic index for the service life evaluation of the transformer insulation paper, and provides a theoretical basis for representing the overheat fault of the transformer and judging the insulation condition at the winding hot point due to the special sensitivity of the ethanol in the oil to high temperature.
Description
Technical Field
The invention relates to the technical field of electrical equipment fault diagnosis, in particular to a life model establishing method based on the polymerization degree of ethanol in oil and transformer paper.
Background
The oil-immersed transformer has high insulation strength and long service life, and is widely applied to high-voltage and ultra-high voltage power transmission and transformation systems. The oil paper insulation is used as a main insulation form of the transformer internal insulation, and the performance gradually ages and declines due to the action of multiple factors such as a thermal field, an electric field, moisture and the like in the long-term operation of equipment, so that the oil paper insulation becomes one of main causes of the transformer failure. The aging of the paper insulation is considered to be the key to determine the service life of the oil-immersed transformer, because the paper insulation is difficult to replace after aging and is the main cause of the reduction of the mechanical strength of the transformer.
The main component of the insulating paper is cellulose accounting for more than 90% of the insulating paperboard, and the insulating paper is a linear high molecular polymer formed by connecting beta-D glucose monomers through 1,4 glycosidic bonds. The microscopic process of the insulation paper aging is the process of breaking cellulose macromolecules, and in the process, chemical compounds such as carbon oxygen gas, furfural, low molecular alcohols and the like with certain micromolecular characteristics can be generated. And the aging characteristic products can be diffused into the insulating oil, and the evaluation on the aging state of the transformer paper insulation can be realized through the periodic sampling and online monitoring of the insulating oil. However, the method using carbon-oxygen gas is too single and has a great disadvantage in practical use because the carbon-oxygen gas is generated not only by the insulating paper but also by the long-term oxidation of the insulating oil, and there is no fixed proportional relationship between the two sources. The furfural compound dissolved in oil is proposed as an oil paper insulation aging characteristic parameter due to the relatively stable performance and the close correlation between the generation amount and the degradation degree of the insulation paper. However, the evaluation of the aging of the insulating paper by using furfural in oil also has certain limitations, such as the measured value on site is generally low; the mass concentration change of the furfural can only reflect the long-term working condition of the transformer and cannot reflect the short-term over-temperature state in time.
It was first reported in 2007 by Jalbert et al that methanol and ethanol oil-soluble byproducts were formed in large amounts during aging of 60-120 ℃ oilpaper insulation systems regardless of the type of insulation paper sample. Later, methanol was confirmed as a chemical marker of the degree of aging of transformer insulation paper by kinetic studies on the degradation of both types of insulation paper (standard wood kraft paper and heat upgraded insulation paper). And an optimized measurement method for measuring methanol and ethanol in the insulating oil with detection limit lower than ng g-1 is proposed in 2012. And methanol and ethanol are found to be more stable at different temperatures compared with other aging products in the research of pyrolysis products (more than 30), which has special significance for the research of the aging state of paper insulation. The successive research of the researchers has confirmed methanol as a characteristic marker for the early degradation degree of the insulating paper, and at the same time, a series of efforts have been made to confirm the generation mechanism and diagnostic advantage of ethanol. During the detection and analysis of the field transformer oil sample and the laboratory heat aged oil paper insulation sample, the generation concentration and the generation rate of methanol are generally higher than ethanol (ethanol content is higher than methanol in 10 percent of actual oil samples) when the temperature is lower than 210 ℃ (corresponding to the acid hydrolysis mechanism of the insulation paper), but the stability degree and the time for reaching the stability of the two are almost the same. Rodriguez-Celis et al, when studying cellulose degradation at high temperatures, suggested that the cellulose intermediate levoglucosan produced large amounts of ethanol, regardless of environmental variables. And indicates the specificity of ethanol for transformer paper insulation diagnostics, i.e., ethanol production may be associated with hot spots and thermal failures of transformer paper insulation. The characteristic provides a new idea and reliable basis for sensitively representing the paper insulation aging state of the transformer winding hot spot weak point. Therefore, it is essential to investigate the correlation between ethanol and paper insulation aging, to predict the transformer paper insulation life from the oil dissolution profile product ethanol.
Disclosure of Invention
The invention provides a life model establishing method based on ethanol in oil and the polymerization degree of transformer paper and application thereof, which are convenient for determining the aging state and the residual life of transformer paper insulation.
The technical scheme of the invention is that a life model building method based on ethanol in oil and transformer paper polymerization degree is characterized by comprising the following steps:
s1, preparing and pretreating materials of an insulating paper sample and an insulating oil sample, preparing the insulating paper samples with different initial moisture contents through a moisture absorption test, performing an accelerated thermal aging test, and periodically detecting the polymerization degree of the insulating paper and the ethanol content of the insulating paper sample;
s2, analyzing the experimental data obtained in the S1, selecting different aging times as references, obtaining a relation curve between the ethanol content in the oil and the polymerization degree under the corresponding aging time under the two initial water contents, and performing corresponding rule analysis;
s3, analyzing the experimental data obtained in the S1, respectively fitting curves of the ethanol content in the oil and the polymerization degree of the insulating paper under the conditions of two independent variables of different initial water contents and different aging temperatures, and jointly selecting a relational model of the ethanol and the polymerization degree through the regularity and the goodness of fit of a comparison curve and the influence degree of a single factor to obtain a fitting equation;
s4, taking the rule obtained in the S2 as a theoretical basis, performing curve fitting on the ethanol content and the aging time at different temperatures under different water contents, and performing exponential fitting formula on the fitting relation between the ethanol content and the time in the oil through comparison of the fitting goodness and selection of an optimal fitting model;
s5, substituting the polymerization degree at the estimated service life end point into the fitting equation in the S3 to obtain the ethanol content in the oil at the insulation service life end points with different initial water contents and different temperatures; correspondingly substituting the calculated ethanol content into the exponential fitting formula obtained in S4 to obtain oil paper insulation life models in different aging states;
and S6, extrapolating the oil paper insulation life model based on the ethanol characteristic parameter obtained in the accelerated thermal aging test in the step S5 to the actual temperature, and calculating the polymerization degree degradation rule at the operating temperature. A time-temperature superposition method is adopted as a theoretical basis, and an ethanol life equation at different temperatures is extrapolated to an actual life model through three steps of main curve construction, time-temperature translation factor calculation, activation energy calculation and main curve fitting, namely an oil paper insulation aging model based on the ethanol life equation at any temperature is established.
Further, in S1, during pretreatment, the insulating paper and the insulating oil are respectively placed in a vacuum oil immersion box, and vacuum drying is carried out for 36-72 hours at the temperature of 80-90 ℃.
Further, when the insulation paper samples with different initial moisture contents are prepared through a moisture absorption test in the S1, the moisture absorption test is realized by utilizing a humidifier and a precise electronic balance, so that a moisture absorption formula of the expected moisture of the paperboard can be calculated conveniently:
m＝m 0 ×(1-a％)/(1-b％)
wherein m is the weight of the desired moisture content of the board, m 0 The weight of the pretreated paperboard is a% of the initial moisture content and b% is the expected moisture contentWater content.
Further, after the insulation paper samples with different initial moisture contents are prepared, repeatedly verifying the moisture contents of the paper samples by using a Karl Fischer moisture tester to finally obtain the actually measured initial moisture content of the insulation paper; and then respectively soaking the two paper samples with different initial moisture mass fractions in dry insulating oil for vacuum oil immersion treatment, and finally respectively putting the paper samples into an aging box with target temperature for accelerated thermal aging experiments.
Further, in S2, at the initial moisture content, the relationship between the ethanol content in the oil and the degree of polymerization at the corresponding aging time satisfies the following formula:
wherein y is the ethanol content in the insulating oil, and the unit is mug -1 (ii) a x is the average degree of polymerization DP of the insulating paper v ；A 1 And y 0 Is a constant obtained by substituting experimental data of an accelerated thermal aging test into the above formula.
According to S1, two controllable factor variables of temperature and moisture can be known. At present, the ethanol index is generated more from the thermal cracking reaction of the levoglucosan. However, it has been found that pyrolysis is a degradation mode in which the amount of by-products is low. In response to the massive production of ethanol at high temperatures, not only a process of pyrolysis, but also a specific reaction mechanism has yet to be deeply explored.
Furthermore, in S3, two independent variables of different initial water content and different aging temperature exist, and one variable is respectively taken as a main research object to explore the relation between the polymerization degree of the ethanol and the insulation paper under the factor; namely, the relation between the ethanol and the polymerization degree is obtained in S2 under different initial water contents, the relation between the ethanol and the polymerization degree is compared with the relation between the ethanol and the polymerization degree obtained in S3 under different temperatures, after comprehensive comparison, the ethanol content and the polymerization degree in the oil under different water contents and different aging temperatures are fitted, fitting parameters are obtained, the fitting result is good, the ethanol generation rule accords with a theoretical model, wherein the established mathematical expression of the relation between the ethanol and the polymerization degree is as follows:
in the formula, the corresponding relation between ethanol and polymerization degree at different temperatures when the initial moisture content of the insulation paper is 0.8% is shown;
the following formula is the correlation between ethanol and degree of polymerization at different temperatures when the initial moisture content of the insulation paper is 2.3%:
further, in S4, two common fitting curves are used to perform fitting analysis on the experimental data, i.e. linear fitting and exponential fitting, and through comparison of goodness of fit and selection of an optimal fitting model, the S4 exponential fitting formula is such that the fitting relationship between ethanol content in oil and time better conforms to the exponential expression:
in the formula, y is the ethanol content in the insulating oil, and x is the aging time t; d. e and f are constants obtained by substituting experimental data of the accelerated thermal aging test into the above formula.
Establishing a parameter equation after fitting the ethanol content and the aging time at different aging temperatures under different initial moisture contents, wherein when the initial moisture content is 0.8%, the corresponding relationship between the ethanol content and the aging time is as follows:
when the initial moisture content is 2.3%, the relationship model between the two is as the formula:
further, the formula obtained in S3 is equal to the formula obtained in S4 under the corresponding moisture and temperature conditions, and the fitting mathematical expression of the degree of polymerization and time corresponding to the experimental data is established as:
DP＝-aln(bt+c)
the formula is combined with a time-temperature superposition theory, a life model equation for evaluating the aging of the transformer insulation paper by ethanol characteristic products at different temperatures and under different moisture is established, the ethanol life equation at different temperatures is extrapolated to an actual life model, and an oil-paper insulation aging model of the ethanol-based life equation at any temperature is established.
Further, when a fitted mathematical expression of the polymerization degree and the time is combined with a time-temperature superposition theory, the construction of a main curve and the calculation of a time-temperature translation factor are firstly carried out, and the translation factor alpha at any temperature in the calculation is calculated T Is defined as the following expression:
in the formula, t ref Translating a point of the fitted curve at the temperature T to a corresponding time point of a part of the main curve; t is t T Fitting the corresponding numerical point time on the curve under the temperature T;
secondly, calculating the activation energy, and calculating the time-temperature translation factor alpha at different temperatures when the heat aging mechanism of the same insulating material is not changed T All satisfy the Arrhenius equation;
in the formula, E a Is the apparent activation energy of the insulating material, kJ/mol; r is gas constant, 8.314; t is a unit of ref And T are both absolute temperature, K;
thirdly, fitting the main curve; and finally, based on an ethanol life equation, an extrapolated life model at any aging temperature is as follows:
the invention also relates to the method for evaluating the service life model, which is used for obtaining the residual service life of the paper insulation at any temperature of the model under the condition of evaluating two moisture contents.
The invention has the following beneficial effects:
because the generation of ethanol can be related to hot spots and thermal faults of transformer paper insulation, a new idea and a reliable basis are provided for sensitively representing the aging state of the paper insulation at the hot spot weakness of the transformer winding. To better explain this behavior, a typical oilpaper insulation system aging test was designed at different initial moisture contents and different temperatures. By sampling and measuring the ethanol content in the oil and the polymerization degree of the insulating paper in the accelerated thermal aging process, the ethanol generation rule is analyzed, and an ethanol life equation under the combined action of temperature and moisture is established. On the basis of researching a relation model between ethanol content and aging time, the service life of the oil paper insulation system is predicted. And the high-temperature accelerated aging data is extrapolated to the operating temperature, and a thermal aging life evaluation model based on ethanol under laboratory conditions is improved.
Drawings
FIG. 1 is a flow chart of an experimental process of an embodiment of the present invention.
FIG. 2 is a graph of the relationship between ethanol in oil and the degree of polymerization for different moisture contents and different aging temperatures for the preparation of the examples of the present invention.
FIG. 3 is a graph showing the fitted relationship between ethanol and the degree of polymerization of the insulating paper at different aging temperatures and with different water contents according to the embodiment of the present invention.
FIG. 4 is an analytical graph fitted to ethanol content, DPv and aging time for various temperatures and moisture levels for an embodiment of the invention.
FIG. 5 is a graph of experimental fit analysis of ethanol content versus aging time at different temperatures and different moisture contents for an embodiment of the present invention.
FIG. 6 is a graph showing the change of polymerization degree with time at different temperatures under the same moisture content based on the equation of ethanol lifetime.
FIG. 7 is a main graph of the water content of the sample of the present invention after translation at different temperatures.
Detailed Description
The present invention will be further explained with reference to the following examples and drawings, but the present invention should not be construed as being limited thereto.
Example (b):
the method for establishing the service life model based on the polymerization degrees of ethanol in oil and transformer paper comprises the following steps of:
(1) The method comprises the steps of preparing and preprocessing materials of an insulating paper sample and an insulating oil sample, preparing the insulating paper samples with different initial moisture contents through a moisture absorption test, and detecting the polymerization degree of the insulating paper and the ethanol content of the insulating paper sample regularly and respectively by means of an accelerated thermal aging test. The insulating paper and the insulating oil are respectively placed in a vacuum oil immersion tank for vacuum drying at 90 ℃ for 48 hours.
Moisture is the "first killer" that affects the insulation life of paper, as Fabre and Pichon suggest that the moisture content of the paper is proportional to its aging rate, lundgaard et al have found that 4% moisture content of paper can shorten the life of a transformer by 40 times. It is generally considered that the paper has a water content of less than 1% in a dry state. Therefore, the insulating paper sample which is fully dried in the pretreatment process is taken out under the comprehensive consideration of material selection and laboratory conditions, the time is strictly controlled, a humidifier is adopted to enable the surface of the insulating paper to be uniformly moistened, and the insulating paper sample is placed on a high-precision balance to be weighed. A moisture absorption formula facilitating calculation of the desired moisture of the board:
m＝m 0 ×(1-a％)/(1-b％)
wherein m is the weight of the board at the desired moisture, m 0 For the weight of the pretreated board, a% is the initial moisture content and b% is the expected moisture content.
For the accuracy of the experiment, the moisture content of the paper sample is repeatedly verified by using a Karl Fischer moisture tester, and the actually-measured initial moisture content of the insulating paper is finally obtained. And respectively soaking the two paper samples with different initial moisture mass fractions in dry insulating oil to carry out vacuum oil immersion treatment. And then respectively putting the materials into an aging box with a target temperature to perform an accelerated thermal aging experiment. In the experimental process, samples are taken at certain time intervals to measure the polymerization degree of the insulating paper and detect the content of ethanol, as shown in figure 1.
(2) And (2) analyzing the experimental data obtained in the step (1), selecting different aging times as references, obtaining a relation curve between the ethanol content in the oil and the polymerization degree under the corresponding aging time under two initial moisture contents, and performing corresponding rule analysis. I.e. the higher the temperature, the higher the ethanol content. As can be seen from fig. 2, since the aging temperature directly affects the ultimate degree of polymerization of cellulose, the higher the aging temperature, the higher the peak value of ethanol content in the insulating oil. However, at 110 ℃, even at the later stage of the aging of the insulating paper, the ethanol content in the oil is kept flat, and no obvious increasing trend exists, which is greatly different from the curve law at 130 ℃ and 150 ℃. The research shows that the ethanol is closely related to the cracking of the insulating paper at the hot spot of the transformer winding, and the experimental result shows the relevance between the ethanol generation amount and the abnormal aging at high temperature. From the analysis of the distribution characteristics between the oils, it was found that ethanol has a tendency to accelerate the formation with time, and it can be explained that the stronger the adsorption force of the insulating oil to ethanol is, the more the ethanol present in the paper starts to dynamically migrate into the oil. It was also attributed in previous studies to a change in the partition ratio of ethanol between oil-paper due to cumulative stacking of oxidation products in the insulating oil, thereby affecting the polarity of the oil sample. The relationship between the ethanol content in the oil and the degree of polymerization was found by fitting a curve to satisfy the following formula:
wherein y is the ethanol content in the insulating oil, and the unit is mug -1 (ii) a x is the average degree of polymerization of the insulating paperDPv；A 1 And y 0 Is a constant obtained by substituting experimental data of an accelerated thermal aging test into the above formula.
(3) Analyzing the experimental data obtained in the step (1), fitting the relation between ethanol and polymerization degree of the insulating paper under different water contents at different aging temperatures, and analyzing the internal mechanism. As can be seen from FIG. 3, the amount of moisture content in the paper oil insulation system has a significant driving effect on the direction of ethanol production by cellulose pyrolysis. At the same aging temperature, the water content promotes an increase in the ethanol content up to the limiting degree of polymerization. The figure also fully shows another possible production mode of the ethanol, namely, ethanol compounds are produced through the derivative after intermediate products are generated through a cellulose acid hydrolysis reaction mechanism. When the aging temperature is low, the influence of moisture on the cracking of the insulating paper to generate ethanol is larger. As the aging temperature increases, the effect of moisture on ethanol content gradually diminishes, which can be explained as pyrolysis begins to dominate the ethanol production. The individual effects and the combined effects of temperature, moisture and degree of aging on ethanol production are also well illustrated in FIG. 4.
The method aims to respectively fit curves of the ethanol content in the oil and the degree of polymerization of the insulating paper under two independent variable conditions of different initial water contents and different aging temperatures, and jointly select a relational model of the ethanol and the degree of polymerization by comparing the regularity and the goodness of fit of the curves in the step (2) and the step (3) and the influence degree of single factors. After comprehensive comparison, the ethanol content and the polymerization degree in the oil under different water contents and different aging temperatures are fitted to obtain fitting parameters, the fitting result is good, and the ethanol generation rule accords with a theoretical model. Wherein, the established mathematical expression of the relation between the ethanol and the polymerization degree is as follows:
in the formula, the correspondence between ethanol and polymerization degree at different temperatures is shown when the initial moisture content of the insulation paper is 0.8%.
The following formula is a correlation between ethanol and degree of polymerization at different temperatures when the initial moisture content of the insulating paper is 2.3%:
(4) Based on the relation model of the degree of polymerization of the ethanol and the insulating paper, the degree of polymerization, the aging time and the content of the ethanol in the oil are organically related, and the aging degree of the insulating paper can be indirectly evaluated. The experimentally fitted curves of ethanol content at different temperatures and ageing time at different moisture contents are shown in figure 5, indicating that at the same initial moisture content, the effect of temperature on the rise in ethanol content in the oil can be found to be critical at the same ageing stage. Two fitting curves are adopted to carry out fitting analysis on experimental data, and through comparison of fitting goodness and selection of an optimal fitting model, the fitting relation of ethanol content in oil and time is more in accordance with an exponential expression:
in the formula, y is the ethanol content in the insulating oil, and x is the aging time t; d. e and f are constants obtained by substituting experimental data of the accelerated thermal aging test into the above formula.
Establishing a parameter equation after fitting the ethanol content and the aging time at different aging temperatures under different initial moisture contents, wherein when the initial moisture content is 0.8%, the corresponding relation between the ethanol content and the aging time is as follows:
when the initial moisture content is 2.3%, the relation model between the two is as the formula:
(5) Substituting the polymerization degree at the estimated service life end point into the ethanol content in the oil at the insulation service life end points with different initial water contents and different temperatures obtained in the step (3); and (5) correspondingly substituting the calculated ethanol content into the index fitting formula obtained in the step (4), so that the insulation life of the oil paper in different aging states can be obtained.
(6) And (4) extrapolating the oil paper insulation life model based on the ethanol characteristic parameter obtained in the step (5) under the accelerated thermal aging test to the actual temperature, and calculating the polymerization degree degradation rule at the operating temperature. A time-temperature superposition method is adopted as a theoretical basis, and an ethanol life equation at different temperatures is extrapolated to an actual life model through three steps of main curve construction, time-temperature translation factor calculation, activation energy calculation and main curve fitting, namely an oil paper insulation aging model based on the ethanol life model at any temperature is established. The microstructure parameter change rule of the insulating paper cellulose is influenced by temperature and dynamic load. The theoretical basis of the time-temperature superposition principle is to change the influence degree of time and temperature on parameters into equivalence, namely, the temperature rise and the observation time extension are equivalent to the change of microscopic molecular parameters. And (3) making the formula obtained in the step (3) equal to the formula obtained in the step (4) under the corresponding water and temperature conditions by using the fitted ethanol life equation, and establishing a fitted mathematical expression of the polymerization degree and the time corresponding to the experimental data as follows:
DP＝-aln(bt+c)
the formula is combined with a time-temperature superposition theory, a life model equation for evaluating the aging of the transformer insulation paper by ethanol characteristic products under different temperatures and moisture is established, and the specific steps for the accelerated high-temperature laboratory data extrapolation are as follows:
first, the time-dependent change curves of the polymerization degree at different temperatures under the same moisture content conditions are plotted on the same coordinate as shown in FIG. 6. The lowest experimental temperature of 110 ℃ was chosen as the reference temperature T in FIG. 6 ref The graph at the non-reference temperature is translated along the horizontal axis, the magnitude of the shift being referred to as the time-temperature translation factor α T So that it is plotted against a reference temperatureBecomes a smooth connected whole and finally becomes a main curve. The formation of the main curve after translation at different temperatures for the same moisture content is shown in FIG. 7. And calculating the structure of the main curve and the time-temperature translation factor, and calculating the translation factor alpha at any temperature T Is defined as the following expression:
in the formula, t ref Translating a point of the fitted curve at the temperature T to a corresponding time point of a part of the main curve; t is t T The corresponding time of the numerical point on the fitted curve at the temperature T is calculated.
Secondly, calculating the activation energy, and calculating the time-temperature translation factor alpha at different temperatures when the heat aging mechanism of the same insulating material is not changed T All satisfy the Arrhenius equation.
In the formula, E a Is the apparent activation energy of the insulating material, kJ/mol; r is gas constant, 8.314; t is a unit of ref And T are both absolute temperature, K.
And thirdly, fitting the main curve. The specific treatment method comprises the following steps: fitting the main curve by using a polymerization degree and time correlation equation derived based on an ethanol life equation to obtain fitting parameters a, b and c at a reference temperature, so that the time t used when the insulating paper is aged to a certain polymerization degree at the reference temperature can be obtained Tref . Substituting the time-temperature translation factor formula into the time-temperature translation factor at a certain temperature to calculate the time t for the insulation paper to age to a certain polymerization degree at the certain temperature T . Such as the limiting degree of polymerization DP of the insulating paper EOL The residual life L of the insulation sample at any temperature can be deduced by counting as the end point of the life T . And the warning content of the ethanol at the end point of the service life under the combination of the moisture and the temperature can be obtained by the equation system obtained in the step (3)C wT If L is to be T And C wT The method has one-to-one correspondence, and provides a certain theoretical basis for determining the ethanol threshold value of the aging characteristic product of the insulation system in the future. Thus, based on the ethanol lifetime equation, the extrapolated lifetime model at any aging temperature is:
through the steps, the residual life at any temperature under two moisture contents is verified, the fitting goodness reaches 0.961 and 0.973 respectively, and the fitting goodness is good.
Claims (8)
1. A life model building method based on the polymerization degree of ethanol in oil and transformer paper is characterized by comprising the following steps:
s1, preparing and pretreating materials of an insulating paper sample and an insulating oil sample, preparing the insulating paper samples with different initial moisture contents through a moisture absorption test, performing an accelerated thermal aging test, and periodically detecting the polymerization degree of the insulating paper and the ethanol content of the insulating paper sample;
s2, analyzing the experimental data obtained in the S1, selecting different aging times as references, obtaining a relation curve between the ethanol content in the oil and the polymerization degree under the corresponding aging time under the two initial water contents, and performing corresponding rule analysis;
s3, analyzing the experimental data obtained in the S1, respectively fitting curves of the ethanol content in the oil and the polymerization degree of the insulating paper under the conditions of two independent variables of different initial water contents and different aging temperatures, and jointly selecting a relational model of the ethanol and the polymerization degree through the regularity and the goodness of fit of a comparison curve and the influence degree of a single factor to obtain a fitting equation;
s4, taking the rule obtained in the S2 as a theoretical basis, carrying out curve fitting on the ethanol content and the aging time at different temperatures under different water contents, and adopting an exponential fitting formula for the fitting relation between the ethanol content and the time in the oil through comparison of the fitting goodness and selection of an optimal fitting model;
s5, substituting the polymerization degree at the estimated service life end point into the fitting equation in the S3 to obtain the ethanol content in the oil at the insulation service life end points with different initial water contents and different temperatures; then correspondingly substituting the calculated ethanol content into the exponential fitting formula obtained in S4 to obtain oil paper insulation life models in different aging states;
s6, extrapolating the oil paper insulation life model based on the ethanol characteristic parameter obtained in the step S5 under the accelerated thermal aging test to the actual temperature for calculating the polymerization degree degradation rule at the operating temperature; the method comprises the following steps of adopting a time-temperature superposition method as a theoretical basis, and extrapolating an ethanol life equation at different temperatures to an actual life model through three steps of main curve construction, time-temperature translation factor calculation, activation energy calculation and main curve fitting, namely establishing an oil paper insulation aging model based on the ethanol life equation at any temperature;
specifically, the formula obtained in S3 is equal to the formula obtained in S4 under the corresponding moisture and temperature conditions, and the fitting mathematical expression of the degree of polymerization and time corresponding to the experimental data is established as:
DP＝-aln(bt+c)
combining the formula with a time-temperature superposition theory, establishing a life model equation for evaluating the aging of the transformer insulation paper by ethanol characteristic products at different temperatures and under different moisture, extrapolating the ethanol life equation at different temperatures to an actual life model, and establishing an oil-paper insulation aging model at any temperature based on the ethanol life equation;
combining a fitting mathematical expression of the polymerization degree and the time with a time-temperature superposition theory, firstly constructing a main curve and calculating a time-temperature translation factor, and calculating a translation factor alpha at any temperature in the calculation T Is defined as the following expression:
in the formula, t ref Translating a point of the fitted curve at the temperature T to a corresponding time point of a part of the main curve; t is t T Fitting the corresponding numerical point time on the curve under the temperature T;
secondly, calculating activation energy, and calculating the time-temperature translation factor alpha at different temperatures when the heat aging mechanism of the same insulating material is not changed T All satisfy the Arrhenius equation;
in the formula, E a Is the apparent activation energy of the insulating material, kJ/mol; r is gas constant, 8.314; t is ref And T are both absolute temperature, K;
thirdly, fitting the main curve; and finally, based on an ethanol life equation, an extrapolated life model at any aging temperature is as follows:
2. the method of claim 1, wherein: in the S1, during pretreatment, the insulating paper and the insulating oil are respectively placed in a vacuum oil immersion box, and vacuum drying is carried out for 36-72 h at the temperature of 80-110 ℃.
3. The method of claim 1, wherein: when the insulation paper samples with different initial moisture contents are prepared through the moisture absorption test in the S1, the moisture absorption test is realized by utilizing a humidifier and a precise electronic balance, so that a moisture absorption formula of expected moisture of the paperboard can be conveniently calculated:
m＝m 0 ×(1-a％)/(1-b％)
wherein m is the weight of the desired moisture content of the board, m 0 For the weight of the pretreated board, a% is the initial moisture content and b% is the expected moisture content.
4. The method of claim 3, wherein: after the insulation paper samples with different initial moisture contents are prepared, repeatedly verifying the moisture contents of the paper samples by using a Karl Fischer moisture tester to finally obtain the actually-measured initial moisture content of the insulation paper; and respectively soaking the two paper samples with different initial moisture mass fractions in dry insulating oil for vacuum oil immersion treatment, and finally respectively putting the paper samples into an aging box with a target temperature for accelerated thermal aging experiments.
5. The method of claim 3, wherein: in S2, under the initial moisture content, the relation curve between the ethanol content in the oil and the polymerization degree under the corresponding aging time satisfies the following formula:
wherein y is the content of ethanol in the insulating oil, and the unit is mug -1 (ii) a x is the average degree of polymerization DP of the insulating paper v ，A 1 And y 0 Is a constant obtained by substituting experimental data of an accelerated thermal aging test into the above formula.
6. The method of claim 1, wherein: in S3, two independent variables of different initial water contents and different aging temperatures exist, and one variable is respectively used as a main research object to research the relation between the polymerization degree of the ethanol and the insulation paper under the factor; namely, in S2, the relation between the ethanol and the polymerization degree is obtained under different initial moisture contents, and in S3, the relation between the ethanol and the polymerization degree is obtained under different temperatures for comparison, the ethanol content and the polymerization degree in the oil under different moisture contents and different aging temperatures are fitted, fitting parameters are obtained, the fitting result is good, the ethanol generation rule accords with a theoretical model, wherein the established mathematical expression of the relation between the ethanol and the polymerization degree is as follows:
in the formula, the corresponding relation between ethanol and polymerization degree at different temperatures is shown when the initial moisture content of the insulating paper is 0.8%;
the following formula is the correlation between ethanol and degree of polymerization at different temperatures when the initial moisture content of the insulation paper is 2.3%:
7. the method of claim 1, wherein: in S4, two common fitting curves are adopted to carry out fitting analysis on experimental data, namely a linear fitting mode and an exponential fitting mode, and through comparison of the fitting goodness and selection of an optimal fitting model, an S4 exponential fitting formula is that the fitting relation of ethanol content in oil and time is more in accordance with an exponential expression:
in the formula, y is the ethanol content in the insulating oil, and x is the aging time t; d. e and f are constants obtained by substituting experimental data of the accelerated thermal aging test into the above formula;
establishing a parameter equation after fitting the ethanol content and the aging time at different aging temperatures under different initial moisture contents, wherein when the initial moisture content is 0.8%, the corresponding relationship between the ethanol content and the aging time is as follows:
when the initial moisture content is 2.3%, the relationship model between the two is as the formula:
8. the method according to any one of claims 1 to 7, wherein the residual life of the insulating paper at any temperature is evaluated at two moisture contents.
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