CN111596181B - Transformer solid insulation aging evaluation method based on bubble escape temperature - Google Patents

Transformer solid insulation aging evaluation method based on bubble escape temperature Download PDF

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CN111596181B
CN111596181B CN202010481836.3A CN202010481836A CN111596181B CN 111596181 B CN111596181 B CN 111596181B CN 202010481836 A CN202010481836 A CN 202010481836A CN 111596181 B CN111596181 B CN 111596181B
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bubble
temperature
time
moisture content
insulating
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CN111596181A (en
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赵涛
晁倪杰
刘云鹏
佟以鑫
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North China Electric Power University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/12—Testing dielectric strength or breakdown voltage ; Testing or monitoring effectiveness or level of insulation, e.g. of a cable or of an apparatus, for example using partial discharge measurements; Electrostatic testing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N25/00—Investigating or analyzing materials by the use of thermal means
    • G01N25/20—Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/20—Design optimisation, verification or simulation
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2111/00—Details relating to CAD techniques
    • G06F2111/10—Numerical modelling
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2119/00—Details relating to the type or aim of the analysis or the optimisation
    • G06F2119/04—Ageing analysis or optimisation against ageing
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2119/00—Details relating to the type or aim of the analysis or the optimisation
    • G06F2119/08—Thermal analysis or thermal optimisation

Abstract

The invention discloses a transformer solid insulation aging evaluation method based on bubble escape temperature, which comprises the following steps of: establishing an insulating thermal bubble escaping temperature numerical model of an insulating paperboard; converting the moisture content of the insulating oil into the moisture content of the insulating paperboard based on an Oommen equilibrium curve; substituting the moisture content of the insulating paper board into the model to obtain a comparison table of the bubble escaping temperature and the insulating aging degree under the moisture content of the insulating paper board; carrying out a temperature rise test on the transformer, and observing and recording the actually measured temperature of the bubbles escaping from the insulating paper board; and finding out the corresponding insulation aging degree according to a comparison table of the bubble escaping temperature and the insulation aging degree. The transformer solid insulation aging evaluation method based on the bubble escape temperature provided by the invention is simple and practical, and the aging state of the main insulating cellulose of the transformer is estimated based on the relation between the insulation aging degree and the bubble escape temperature under the moisture content of the insulating paper board.

Description

Transformer solid insulation aging evaluation method based on bubble escape temperature
Technical Field
The invention relates to the technical field of electrical equipment insulation, in particular to a transformer solid insulation aging evaluation method based on bubble escape temperature.
Background
The power transformer is the main equipment in the power grid, and the insulation condition and the health level of the power transformer are directly related to the safety and stability of the power grid. At present, the insulation structure of the power transformer is mainly a composite insulation structure formed by oil, paper, paperboard and other insulation materials. For aged or undermaintained transformers, the moisture content of the oil-paper insulation will gradually increaseIncreasing gradually. One of the operational risks of these transformers is that at high temperatures the moisture in the insulating paper or cardboard evaporates into water vapour and escapes from the insulating paper in the form of bubbles. And, at present, with CO in the oil2Compared with the concentration criterion of CO and furfural, the polymerization degree is still the most direct and effective criterion for representing the solid insulation life. However, most old transformers are not preset with solid insulation samples for testing, and the polymerization degree measurement needs a hanging core (cover) to take a paper/board sample of a transformer body and to be properly reinforced, so that the transformer belongs to destructive tests and is not easy to detect regularly. Studies have shown that the degree of polymerization directly affects the size of the voids between the cardboard fibers. Aging results in a decrease in the average degree of polymerization and an increase in the average interfiber porosity, thereby resulting in a decrease in the initial bubble escape temperature of the paper oil insulation. Therefore, the aging state of the transformer body can be inferred by observing the bubble escape temperature only by mastering the relation between the bubble escape temperature and the average pore size among the fibers, and the method has practical significance for the evaluation of the insulation aging of the transformer.
Disclosure of Invention
The invention aims to provide a transformer solid insulation aging evaluation method based on bubble escape temperature, which is simple and practical and has important significance for insulation aging evaluation of a power transformer, and the aging state of main insulation cellulose of the transformer is estimated based on the relation between the insulation aging degree and the bubble escape temperature under the moisture content of an insulation paperboard under the condition that a transformer body is not damaged.
In order to achieve the purpose, the invention provides the following scheme:
a transformer solid insulation aging evaluation method based on bubble escape temperature comprises the following steps:
s1, establishing an insulation thermotropic bubble escaping temperature numerical model of the insulation paperboard, and determining the relationship between the insulation aging degree and the bubble escaping temperature under different moisture contents of the insulation paperboard;
s2, sampling and measuring the moisture content of the insulating oil in the transformer, and converting the moisture content of the insulating oil into the moisture content of the insulating paperboard based on an Oommen balance curve;
s3, substituting the moisture content of the insulating paperboard in the step S2 into the model in the step S1 to obtain a comparison table of the bubble escaping temperature and the insulation aging degree under the moisture content of the insulating paperboard;
s4, performing a temperature rise test on the transformer, and observing and recording the actually measured temperature of the bubbles escaping from the insulating paper board;
s5, using the measured temperature of the bubble escaping in step S4, the corresponding insulation aging degree is found by referring to the comparison table of the bubble escaping temperature and the insulation aging degree in step S3.
Optionally, in step S1, a numerical model of insulating thermal bubble escape temperature of the insulating paperboard is established, and a relationship between the insulating aging degree and the bubble escape temperature under different moisture contents of the insulating paperboard is determined, which specifically includes:
s101, establishing a bubble state equation consisting of ideal gas state equations of water vapor and other gases, wherein the bubble state equation comprises the following steps:
Pg(t)·V(t)＝ng·R·T(t) (1)
Pw(t)·V(t)＝nw(t)·R·T(t) (2)
wherein V (T) represents the bubble volume at time T, R represents an ideal gas constant, T represents time, T (T) represents the temperature at time T, the temperature T at each time can be obtained from the temperature rise gradient, Pw(t) represents the partial pressure of water vapor in the bubbles at time t, Pg(t) represents the partial pressure of the other gases at time t, ngAmount of substance representing initial state of bubble, nw(t) represents the amount of water vapor at time t;
s102, adding the formula (1) and the formula (2) to obtain:
(Pg(t)+Pw(t))·V(t)＝(ng+nw(t))·R·T(t) (3)
the volume V (t) can be obtained by shifting the term in the formula (3):
V(t)＝(ng+nw(t))·R·T(t)/(Pg(t)+Pw(t))
wherein, because the development process of the bubbles is similar to the process of isobaric expansion,
then, Pg(t)+Pw(t)＝Pin＝Pout，PinRepresenting the internal pressure of the bubble, PoutIndicating the external pressure of the bubble, then,
V(t)＝(ng+nw(t))·R·T(t)/Pout (4)
s103, replacing the formula (4) into the formula (2), and calculating Pw(t) and verified by the following formula:
Pw(t)＝min(Pw(t),Ps(t)) (5)
wherein, Ps(t) represents the saturated vapor pressure at the water surface temperature at time t;
s104, verifying the Pw(t) substituting the radius formula (6) at the lower side to obtain the bubble state at the moment:
Figure GDA0003547203420000031
wherein R represents an ideal gas constant, PoilDenotes oil pressure, PatmRepresenting atmospheric pressure, δ representing the surface tension of the oil, r (t) representing the bubble radius at time t, the time step of t being 1 second, then r (t-1) representing the bubble radius at time t-1, i.e. at the end of the previous time step calculation;
s105, judging whether bubbles escape or not according to a bubble escape criterion based on the bubble state, if so, exiting the cycle to output the current temperature, otherwise, starting to repeat the step S102, and finally outputting the current temperature which is the bubble escape temperature;
s106, based on the model, drawing a fitting curve between the average pore size among the fibers and the bubble escaping temperature under any moisture content of the insulating paperboard, taking the average pore radius among the fibers as 4/6/8/10/12 micrometers respectively, taking the pore length as four times of the radius, substituting the average pore size among the fibers into the moisture content of any insulating paperboard, drawing the fitting curve of the bubble escaping temperature under the moisture content relative to the average pore size among the fibers, determining the relation between the insulating paperboard and the escaping temperature according to the corresponding relation between the average pore size among the fibers and the aging degree, and taking the relation as a comparison table, wherein the more serious the aging degree of the fibers of the insulating paperboard is, the lower the bubble escaping temperature is.
Optionally, in step S101, n isgAnd nwThe calculation equation for (t) is:
according to the polymerization condition of the insulating paperboard cellulose, the radius and the length of the micro-tube are given, the amount of other gas substances in the initial state is vacuum, and the initial state of the air bubble is calculated as follows:
Figure GDA0003547203420000032
wherein d represents the microtubule radius, L represents the microtubule length, T0Denotes room temperature, P0133Pa represents vacuum pressure, R represents ideal gas constant;
quantity n of substance of water vapor at time twThe expression of (t) is:
Figure GDA0003547203420000041
wherein k is1·k2＝0.441，k31.584; t represents time; pw(t-1) represents the partial pressure of water vapor in the bubbles at the end of a time step before time t; ps(t) represents the saturation vapor pressure at the water surface temperature at time t, r (t-1) represents the bubble radius at the time step immediately before time t, MwRepresenting the molar mass of water and Z representing the moisture content of the board.
Optionally, in step S105, the bubble escape criterion includes two items: firstly, the radius of the bubble is more than or equal to that of the micro-tube; second, the partial pressure of water vapor in the bubble is greater than the external pressure of the bubble, and if either of the partial pressure and the external pressure of the bubble meet, the criterion is met.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects: the invention provides a transformer solid insulation aging evaluation method based on bubble escape temperature, which establishes a numerical model of the oiled paper insulation bubble escape temperature, and draws a fitting curve between the average pore size (aging degree) between fibers and the bubble escape temperature under any insulation paper board moisture content as a comparison table, thereby evaluating the aging degree of an insulation body in a table look-up manner on the premise of not damaging the insulation body; the method comprises the steps of establishing a numerical model of the escape temperature of thermally induced bubbles in the oiled paper insulation based on certain physical research and mathematical derivation, converting the moisture content of the paperboard through actually measured moisture content in the oil, and drawing a fitting curve between the fiber pores of the paperboard and the escape temperature of the bubbles under any moisture content of the paperboard by using the numerical model; the actual initial escaping temperature of the bubbles is observed and recorded in the transformer temperature rise test, so that the aging degree of the insulation body is evaluated in a table look-up mode on the premise of not damaging the insulation body.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.
FIG. 1 is a scanning electron microscope image of the insulating paperboard fibers of the present invention;
FIG. 2 is a schematic view of a microtube model and a bubble embryo in a cellulose structure;
FIG. 3 is a saturated vapor pressure fit curve compared to experimental data;
FIG. 4 is an Oommen oil paper moisture balance curve;
FIG. 5 is a comparison chart plotting examples of paperboard moisture levels of 2/3/4/5/6%;
FIG. 6 is a flow chart of the aging evaluation method of the solid insulation of the transformer based on the bubble escaping temperature according to the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
The invention aims to provide a transformer solid insulation aging evaluation method based on bubble escape temperature, which is simple and practical and has important significance for insulation aging evaluation of a power transformer, and the aging state of main insulation cellulose of the transformer is estimated based on the relation between the insulation aging degree and the bubble escape temperature under the moisture content of an insulation paperboard under the condition that a transformer body is not damaged.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
As shown in fig. 6, the method for evaluating the aging of the solid insulation of the transformer based on the bubble escaping temperature provided by the invention comprises the following steps:
s1, establishing an insulation thermotropic bubble escaping temperature numerical model of the insulation paperboard, and determining the relationship between the insulation aging degree and the bubble escaping temperature under different moisture contents of the insulation paperboard;
s2, sampling and measuring the moisture content of the insulating oil in the transformer, and converting the moisture content of the insulating oil into the moisture content of the insulating paperboard based on an Oommen balance curve;
s3, substituting the moisture content of the insulating paperboard in the step S2 into the model in the step S1 to obtain a comparison table of the bubble escaping temperature and the insulation aging degree under the moisture content of the insulating paperboard;
s4, performing a temperature rise test on the transformer, and observing and recording the actually measured temperature of the bubbles escaping from the insulating paper board;
s5, the measured temperature of bubble escape in step S4 is used to refer to the comparison table of bubble escape temperature and insulation aging degree in step S3, and the corresponding insulation aging degree is found.
In step S1, a numerical model of the insulating thermal bubble escape temperature of the insulating paperboard is established, and the specific analysis is as follows:
in view of the porous nature of the insulating paperboard fibers, the present invention assumes that the insulating paper or paperboard is composed of a cluster of cylindrical capillaries, and assumes that the diameter of the capillaries is constant along the length direction, on the basis of which the escape temperature of the air bubbles in the oil-impregnated insulating paper is analyzed and calculated.
The composition of the gas in the bubbles in the actual oil paper insulation system is divided into two categories: water vapor and other gases (mainly including nitrogen, oxygen, carbon monoxide, carbon dioxide, various low molecular hydrocarbons). The water vapor is generated by the evaporation of water in the insulating paper fibers heated by the windings; other gases are mainly air (N) remaining in the insulating paper during vacuum (less than 133Pa) drying2,O2) And the gas generated by the insulation aging of the oiled paper. This determines their significant difference in the amount of substance n of water vapour during the temperature risewWill rise significantly with the temperature rise process; and the amount of other gaseous substances ngNo change occurs. Considering that the insulating oil is subjected to drying and degassing treatment and the aging of the oil and paper in the temperature rise process is not considered, n is increased along with the increase of the air temperaturegWithout a significant increase, and bubble expansion is considered to be an isobaric expansion process, the partial pressure P of water vapour in the bubbleswWill be obviously improved, and the partial pressure P of other gases is obviously increasedgObviously reduces the diffusion of other gases into the oil, ngDoes not significantly decrease, and assumes n is increasing temperature based on the above analysisgIs a constant value.
In general, various actual gases closely follow the ideal gas equation of state at one or more atmospheres.
In step S1, establishing a numerical model of insulation thermal bubble escape temperature of the insulation paperboard, and determining a relationship between insulation aging degree and bubble escape temperature under different moisture contents of the insulation paperboard, specifically including:
s101, establishing a bubble state equation consisting of ideal gas state equations of water vapor and other gases, wherein the bubble state equation comprises the following steps:
Pg(t)·V(t)＝ng·R·T(t) (1)
Pw(t)·V(t)＝nw(t)·R·T(t) (2)
wherein V (T) represents the bubble volume at time T, R represents an ideal gas constant, T represents time in units of s, T (T) represents the temperature at time T, the temperature T at each time can be determined from the temperature rise gradient, Pw(t) represents the partial pressure of water vapour in the bubbles at time t in Pa, Pg(t) represents the partial pressure of the other gases at time t in Pa, ngThe amount of a substance representing the initial state of bubbles in mol, nw(t) represents the amount of water vapour in mol at time t;
s102, adding the formula (1) and the formula (2) to obtain:
(Pg(t)+Pw(t))·V(t)＝(ng+nw(t))·R·T(t) (3)
the volume V (t) can be obtained by shifting the term in the formula (3):
V(t)＝(ng+nw(t))·R·T(t)/(Pg(t)+Pw(t))
wherein, because the development process of the bubbles is similar to the process of isobaric expansion,
then, Pg(t)+Pw(t)＝Pin＝Pout，PinRepresenting the internal pressure of the bubble, PoutWhich represents the external pressure, then,
V(t)＝(ng+nw(t))·R·T(t)/Pout (4)
s103, replacing the formula (4) into the formula (2), and calculating Pw(t) and verified by the following formula:
Pw(t)＝min(Pw(t),Ps(t)) (5)
wherein, Ps(t) indicates saturation at the water surface temperature at time tAnd a vapor pressure; the evaporation process is stopped when the partial pressure of the water vapor reaches the saturated vapor pressure, that is, the time of the evaporation rate action may be less than a time step, so the calculation needs to be checked, and the saturated vapor pressure at the corresponding temperature at the moment is compared with the saturated vapor pressure at the moment, and the two are smaller than the partial pressure of the water vapor at the moment.
S104, verifying the Pw(t) substituting the radius formula (6) at the lower side to obtain the bubble state at the moment:
Figure GDA0003547203420000071
wherein R represents an ideal gas constant, PoilIndicates the oil pressure, PatmRepresenting atmospheric pressure, δ representing the surface tension of the oil, r (t) representing the bubble radius at time t, the time step of t being 1 second, then r (t-1) representing the bubble radius at time t-1, i.e. at the end of the previous time step calculation;
s105, judging whether bubbles escape or not according to a bubble escape criterion based on the bubble state, if so, exiting the cycle to output the current temperature, otherwise, starting to repeat the step S102, and finally outputting the current temperature which is the bubble escape temperature;
s106, based on the model, drawing a fitting curve between the average pore size among the fibers and the bubble escaping temperature under any moisture content of the insulating paperboard, taking the average pore radius among the fibers as 4/6/8/10/12 micrometers respectively, taking the pore length as four times of the radius, substituting the average pore size among the fibers into the moisture content of any insulating paperboard, drawing the fitting curve of the bubble escaping temperature under the moisture content relative to the average pore size among the fibers, determining the relation between the insulating paperboard and the escaping temperature according to the corresponding relation between the average pore size among the fibers and the aging degree, and taking the relation as a comparison table, wherein the more serious the aging degree of the fibers of the insulating paperboard is, the lower the bubble escaping temperature is.
Wherein in the step S101, ngAnd nwThe calculation equation for (t) is:
according to the polymerization condition of the insulating paperboard cellulose, the radius and the length of the micro-tube are given, the amount of other gas substances in the initial state is vacuum, and the initial state of the air bubble is calculated as follows:
Figure GDA0003547203420000081
wherein d represents the microtubule radius, L represents the microtubule length, T0Denotes room temperature, P0133Pa represents vacuum pressure, R represents ideal gas constant;
for the surface evaporation rate of moisture, the empirical formula currently widely accepted under surface gas quiescent conditions is: j ═ k1·(Ps-Pw/1000)1.22Wherein J is the evaporation rate in grams/(cubic meters per hour); k is a radical of1Is a coefficient related to experimental conditions; pwThe water surface vapor pressure; psThe unit of pressure is kilopascals, which is the saturated vapor pressure at the surface temperature of the water.
Since the temperature variation is a continuous function, the time step Δ t is divided by the difference instead of the differential, and the amount of evaporated water vapor species in a Δ t is calculated:
Figure GDA0003547203420000082
in the formula MwThe molar mass of water, S, is the contact area between the surface of the water and the air bubbles. Since the moisture in the microtube has an unshaped structure and has two existing forms of single-layer strong bond combination and multi-layer adsorption, the accurate theoretical derivation of S cannot be carried out, but obviously S is in positive correlation with the moisture content Z of the paperboard and also in positive correlation with the volume of bubbles. The experimental results show that the moisture content of the cardboard has a great influence on the development of the air bubbles, and in order to highlight the influence of the moisture content of the cardboard, the expression of S with respect to the radius r of the air bubbles and the moisture content Z (%) of the cardboard is set as follows:
Figure GDA0003547203420000091
wherein k is2、k3Are all coefficients.
From the above analysis, it can be seen that: quantity n of substance of water vapor at time twThe expression of (t) is:
Figure GDA0003547203420000092
wherein, finally, combining the test data, the invention sets the coefficient of the above formula as k1·k2＝0.441，k31.584; t represents time; pw(t-1) represents the partial pressure of water vapor in the bubbles at the end of a time step before time t; ps(t) represents the saturation vapor pressure at the water surface temperature at time t, r (t-1) represents the bubble radius at the time step immediately before time t, MwRepresenting the molar mass of water and Z representing the moisture content of the board.
In step S102, in the process that the air bubbles in the oil paper insulation gradually expand from the air bubble blank and finally escape, the pressure on the air bubble boundary is the key for determining the expansion of the air bubbles, so that the solution condition of the equation can be obtained:
Figure GDA0003547203420000093
pressure P inside the bubbleinFrom partial pressure P of water vapour in the bubbleswAnd partial pressure P of other gasesgSum composition, external pressure PoutFrom atmospheric pressure PatmOil pressure PoilAnd taking into account the additional pressure due to the surface tension delta of the oil
Figure GDA0003547203420000094
And (4) forming. The bubble is approximated to a sphere with radius r, the bubble volume is small at the initial moment, the bubble expansion is caused by the evaporation of heated moisture, the development speed of the process is slow, the development process of the bubble is approximated to an isobaric expansion process, and then P is obtainedg(t)+Pw(t)＝Pin＝Pout。
The bubble state equations (1), (2) and the boundary pressure condition (9) are arranged to obtain:
Figure GDA0003547203420000095
in order to obtain the corresponding temperature and bubble radius at any moment, the temperature rise process is divided into a plurality of time step lengths delta t, and delta t is small enough. Thus r in the additional pressure term in the formula can be replaced by the radius at the end of the last time step r (t-1), and a simplified equation for calculating r can be obtained by root-shifting equation (10):
Figure GDA0003547203420000101
on the basis of the simplification, the following formula (7) is obtained by adding the shifting terms of the two formulas (1) and (2), the denominator is substituted by the pressure condition formula (3) on the boundary of the air bubble, and the numerator is used for ng、nw(t) substitution to obtain volume V (t):
V(t)＝(ng+nw(t))·R·T(t)/(Pg(t)+Pw(t))
the model is divided into two cases for the criterion of bubble escape: the criterion is that in the estimation of the bubble effect onset temperature, the bubble radius is considered equal to the micropipe radius as a criterion for bubble escape. In addition, there are two ways of vaporizing liquid water. One is evaporation, a process of gasification that occurs calmly at the surface of the liquid, and the other is boiling, which occurs vigorously inside the liquid. Boiling means that a large number of bubbles filled with saturated water vapor are generated inside the liquid. In this case, the water molecules can directly generate bubbles and escape from the microtubule even without contacting the bubble embryo. The condition for its occurrence in the calculation of the invention is the pressure P of the water vapour in the bubbleswGreater than or equal to the external pressure P of the bubblesout. This means that the liquid water rapidly vaporizes and the bubbles expand violently and escape, so that Pw>PoutIs another criterion for bubble escape.
According to the model, a fitting curve between the average pore size between fibers and the bubble escaping temperature under any moisture content of the insulating paper/board is drawn to be used as a comparison table. The average radius of the pores between the fibers is 4/6/8/10/12 micrometers, the length of the pores is four times of the radius, the average radius is substituted into any paperboard moisture content, and a fitting curve of the bubble escaping temperature under the moisture content with respect to the average pore size between the fibers can be drawn, and the fitting curve is used as a comparison table. In the comparison table, the abscissa is the aging degree of the paperboard fibers, the five grades are divided, the higher the shift is, the more serious the aging degree of the paperboard fibers is, and the ordinate is the bubble escaping temperature. Through a temperature rise test, the initial escaping temperature of the bubbles is recorded, and the insulation aging state of the transformer body can be evaluated by looking up a table.
In one embodiment, as shown in fig. 1, the insulation paper, the insulation paperboard, etc. are made of natural plant fibers, and the main components thereof are cellulose, hemicellulose and lignin. Taking common kraft paper commonly used in transformers as an example, the chemical components of the kraft paper include about 90% of cellulose, 6-7% of hemicellulose and 3-4% of lignin. The cellulose has a molecular chain of about 1 μm, and the cellulose is regularly and closely aggregated in parallel to form microfibrils, and amorphous substances such as hemicellulose and lignin exist among the microfibrils. The collection of the above substances forms cells: the cell surface is a cell membrane made of cellulose, called the primary wall. This is followed by a secondary wall, which is a cell wall composed of microfibrils and amorphous material, that accounts for the majority of the fiber. The center of the cell is the portion of the cell cavity called the "hollow lumen". The cells are then held together by lignin to form fibers. Porosity is therefore an inherent property of insulating paper.
As shown in fig. 2, the present invention builds a fiber micro-tube model with reference to a capillary bundle model. The insulating paper made by interlacing the fibers causes air holes among the fibers because of incomplete bonding among the fibers, and the fibers also have cell cavities and air holes among the layered structures of cell walls. The fiber microtubule model describes the porous structure of the fiber concisely and vividly, and establishes a foundation for researching the state of a bubble embryo in the fiber structure.
As shown in fig. 3, many scholars have verified the saturated vapor pressure of water. In order to facilitate the calculation and use of the model, Gaussian curve fitting is carried out on corresponding data of the temperature and the saturated vapor pressure in T epsilon [273,453] K to obtain a functional relation:
Ps＝a·exp(-((T-b)/c)2)
coefficient of the formulaRespectively a is 4.606 × 106，b＝590.7，c＝111.3，PsRepresents the saturated vapor pressure (Pa), and T represents the temperature (K).
The specific algorithm step for calculating the evolution of the bubbles in the temperature rise process comprises the following steps: first, relevant parameters are set and an initial state is calculated. By the formula
Figure GDA0003547203420000111
Calculating ngThe value of (c). Since the partial pressure of water vapor in the bubble in the initial state is equal to the saturated vapor pressure at the corresponding temperature, the partial pressure P of water vapor can be obtained by calculating the saturated vapor pressure at the initial temperature by the formulaw. Substituting the data obtained above into the formula:
Figure GDA0003547203420000112
and solving the one-dimensional cubic equation, and keeping a real number solution, namely an initial value r (0), so that the initial value calculation is completed. Then utilizing according to the initial state
Figure GDA0003547203420000113
And (3) calculating the quantity of the water vapor substances in the bubbles at the next moment, knowing that the quantity of the other gas substances is a fixed value, and according to an ideal gas state equation: p (t) V (t) n (t) R.T (t) and P equal to the pressure inside and outside the bubbleg+Pw＝Poil+Patm+PeSolving the current bubble volume, solving the partial pressure of the water vapor by replacing an ideal gas state equation, verifying, and substituting the verified result into a bubble radius formula:
Figure GDA0003547203420000121
and then, judging whether bubbles escape by using two criteria, if so, exiting the cycle to output the current temperature, and otherwise, starting to perform the related calculation of the next time step.
As shown in fig. 4, the graph is an Oommen moisture balance curve commonly used in the profession, which expresses the balance relationship between the moisture in the oil and the moisture content between the paper boards at a certain temperature, and can be used for the conversion of the moisture content between the oil and the moisture content between the paper boards.
As shown in fig. 5, the graph is a comparison table drawn by taking the moisture content of the paperboard as an example of 2/3/4/5/6%, in practical application, firstly, the comparison table is drawn by using the model calculation result under the measured moisture content, and then the aging evaluation of the insulation body can be carried out according to the observed and recorded bubble escape temperature.
The invention provides a transformer solid insulation aging evaluation method based on bubble escape temperature, which establishes a numerical model of the oiled paper insulation bubble escape temperature, and draws a fitting curve between the average pore size (aging degree) between fibers and the bubble escape temperature under any insulation paper board moisture content as a comparison table, thereby evaluating the aging degree of an insulation body in a table look-up manner on the premise of not damaging the insulation body; the method comprises the steps of establishing a numerical model of the escape temperature of thermally induced bubbles in the oiled paper insulation based on certain physical research and mathematical derivation, converting the moisture content of the paperboard through actually measured moisture content in the oil, and drawing a fitting curve between the fiber pores of the paperboard and the escape temperature of the bubbles under any moisture content of the paperboard by using the numerical model; the actual initial escaping temperature of the bubbles is observed and recorded in the transformer temperature rise test, so that the aging degree of the insulation body is evaluated in a table look-up mode on the premise of not damaging the insulation body.
The principle and the embodiment of the present invention are explained by applying specific examples, and the above description of the embodiments is only used to help understanding the method and the core idea of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.

Claims (3)

1. A transformer solid insulation aging evaluation method based on bubble escape temperature is characterized by comprising the following steps:
s1, establishing an insulation thermotropic bubble escaping temperature numerical model of the insulation paperboard, and determining the relationship between the insulation aging degree and the bubble escaping temperature under different moisture contents of the insulation paperboard; the method specifically comprises the following steps:
s101, establishing a bubble state equation consisting of ideal gas state equations of water vapor and other gases, wherein the bubble state equation comprises the following steps:
Pg(t)·V(t)＝ng·R·T(t) (1)
Pw(t)·V(t)＝nw(t)·R·T(t) (2)
wherein V (T) represents the bubble volume at time T, R represents an ideal gas constant, T represents time, T (T) represents the temperature at time T, the temperature T at each time can be obtained from the temperature rise gradient, Pw(t) represents the partial pressure of water vapor in the bubbles at time t, Pg(t) represents the partial pressure of the other gases at time t, ngAmount of substance representing initial state of bubble, nw(t) represents the amount of water vapor at time t;
s102, adding the formula (1) and the formula (2) to obtain:
(Pg(t)+Pw(t))·V(t)＝(ng+nw(t))·R·T(t) (3)
the volume V (t) can be obtained by shifting the term in the formula (3):
V(t)＝(ng+nw(t))·R·T(t)/(Pg(t)+Pw(t))
wherein, because the development process of the bubbles is similar to the process of isobaric expansion,
then, Pg(t)+Pw(t)＝Pin＝Pout，PinRepresenting the internal pressure of the bubble, PoutIndicating the pressure outside the bubble, then,
V(t)＝(ng+nw(t))·R·T(t)/Pout (4)
s103, replacing the formula (4) into the formula (2), and calculating Pw(t) and verified by the following formula:
Pw(t)＝min(Pw(t),Ps(t)) (5)
wherein, Ps(t) represents the saturated vapor pressure at the water surface temperature at time t;
s104, verifying the Pw(t) substituting the radius formula (6) at the lower side to obtain the bubble state at the moment:
Figure FDA0003547203410000021
wherein R represents an ideal gas constant, PoilIndicates the oil pressure, PatmRepresents atmospheric pressure, δ represents the surface tension of the oil, r (t) represents the bubble radius at time t, the time step of t is 1 second, then r (t-1) represents the bubble radius at the time step preceding time t;
s105, based on the bubble state, judging whether bubbles escape according to a bubble escape criterion, if so, exiting the cycle to output the current temperature, otherwise, starting to repeat the step S102, and finally outputting the current temperature which is the bubble escape temperature;
s106, based on the model, drawing a fitting curve between the average pore size among the fibers and the bubble escape temperature under any moisture content of the insulating paperboard, taking the average pore radius among the fibers as 4/6/8/10/12 micrometers respectively, taking the pore length as four times of the radius, substituting the average pore size among the fibers into the moisture content of any insulating paperboard, drawing the fitting curve of the bubble escape temperature under the moisture content relative to the average pore size among the fibers, determining the relation between the insulating paperboard and the escape temperature according to the corresponding relation between the average pore size among the fibers and the aging degree, and taking the relation as a comparison table, wherein the more serious the aging degree of the fibers of the insulating paperboard is, the lower the bubble escape temperature is;
s2, sampling and measuring the moisture content of the insulating oil in the transformer, and converting the moisture content of the insulating oil into the moisture content of the insulating paperboard based on an Oommen balance curve;
s3, substituting the moisture content of the insulating paperboard in the step S2 into the model in the step S1 to obtain a comparison table of the bubble escaping temperature and the insulation aging degree under the moisture content of the insulating paperboard;
s4, performing a temperature rise test on the transformer, and observing and recording the actually measured temperature of the bubbles escaping from the insulating paper board;
s5, using the measured temperature of the bubble escaping in step S4, the corresponding insulation aging degree is found by referring to the comparison table of the bubble escaping temperature and the insulation aging degree in step S3.
2. The method for evaluating the aging of the solid insulation of the transformer based on the temperature of the escaping bubbles according to claim 1, wherein n is the same as n in step S101gAnd nwThe calculation equation for (t) is:
according to the polymerization condition of the insulating paperboard cellulose, the radius and the length of the micro-tube are given, the amount of other gas substances in the initial state is vacuum, and the initial state of the air bubbles is calculated as follows:
Figure FDA0003547203410000031
wherein d represents the microtubule radius, L represents the microtubule length, T0Denotes room temperature, P0133Pa represents vacuum pressure, R represents ideal gas constant;
quantity n of substance of water vapor at time twThe expression of (t) is:
Figure FDA0003547203410000032
wherein k is1·k2＝0.441，k31.584; t represents time; pw(t-1) represents the partial pressure of water vapor in the bubbles at the end of a time step before time t; ps(t) represents the saturation vapor pressure at the water surface temperature at time t, r (t-1) represents the bubble radius at the time step immediately before time t, MwRepresenting the molar mass of water and Z representing the moisture content of the board.
3. The method for evaluating aging of solid insulation of transformer based on bubble evolution temperature according to claim 2, wherein in step S105, the bubble evolution criterion includes two items: firstly, the radius of the bubble is more than or equal to that of the microtube; second, the partial pressure of water vapor in the bubble is greater than the external pressure of the bubble, and if either of the partial pressure and the external pressure of the bubble meet, the criterion is met.
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