CN108680613B - Method for evaluating moisture content in insulating paper by using initial slope of complex dielectric constant - Google Patents

Abstract

A method for evaluating the moisture content in insulating paper by utilizing the initial slope of the complex permittivity, the method uses the absolute value |k| of the rate of change of the real part of the complex permittivity at f= ^10-4 Hz as the frequency representing the degree of moisture in the oil paper. Domain feature parameters. The variation law of complex permittivity and |k| of oil-paper insulation samples with different moisture contents was experimentally studied, and the research results showed that with the increase of the moisture content M of insulating paper, the real part of the complex permittivity of oil-paper samples was ε′ The curve shifts to the high frequency band, and the variation range of the real part of the complex permittivity in the low frequency band increases with the increase of the moisture content. By fitting the measured ε′‑f curve, it is found that |k| The excellent exponential function relationship can be used for quantitative evaluation of the dampness degree of oil-paper insulation. The method proposed in the invention has the characteristics of easier operation and no damage to the internal insulation structure of the transformer, and is more suitable for the evaluation of the dampness state of the transformer insulation.

Description

A method for evaluating the moisture content in insulating paper using the initial slope of complex permittivity

technical field

The invention relates to the field of insulation state diagnosis, detection and life evaluation of oil-paper-insulated electrical equipment, in particular to a method for evaluating the moisture content in insulating paper by using the initial slope of complex dielectric constant.

Background technique

As the core equipment in the power system, the oil-immersed transformer is related to the balance and stability of the entire power grid, so it is very important to ensure the safe operation of the transformer. Oil-paper insulation is the main structure of insulation in oil-immersed transformers. The degree of dampness of insulating paper is closely related to the overall performance of oil-paper insulation. Studies have shown that the increase of moisture content will promote the thermal aging process of insulating paper, while the thermal aging process of insulating paper is accelerated. During the aging process, it will gradually crack to generate moisture and absorb moisture from the air, which accelerates the thermal aging cracking process of the insulating paper, resulting in worse and worse insulation performance of the oil-paper, making the operating life of the insulating paper rapidly reduced. Therefore, it is very important to quantitatively evaluate the moisture content of oil-paper insulation. In the process of sampling, the traditional method for detecting moisture content in insulating paper is likely to cause damage to the insulating structure, and the unbalanced distribution of moisture between the cardboards will also lead to inaccurate test results. Frequency Domain Spectroscopy (FDS), as a non-destructive electrical measurement technology, can effectively obtain the data of oil-paper insulation complex permittivity changing with frequency. It has strong anti-interference ability and is more suitable for oil-immersed type. On-site diagnosis and assessment of transformer insulation condition.

The dielectric constant is the response of the medium to the electric field under the applied electric field, and its part represents the macroscopic polarization degree, which can effectively reflect the magnitude and change state of the polarization intensity. The dielectric response of the dielectric to the applied electric field depends largely on The frequency of the electric field, and when the frequency of the applied electric field is high, the polarization relaxation process will not occur instantaneously, but will be delayed to establish, so there is a certain phase difference with the applied electric field, thus, the dielectric The constant ε*(ω) is a complex function of the frequency of the applied electric field: ε*(ω)=ε′(ω)-iε″(ω), where the real part of the complex permittivity ε′(ω) represents the magnitude of the polarization , the imaginary part ε″(ω) reflects the various relaxation polarization losses caused by the inability of the various steering polarizations inside the material to keep up with the high-frequency electric field changes at high frequencies. At present, scholars at home and abroad have carried out research on the quantitative relationship between frequency domain characteristic quantities such as complex permittivity and the moisture content of oil-paper insulation based on FDS testing technology. The results show that the increase of moisture content will lead to changes in the internal polarization and relaxation process of the insulation. Therefore, monitoring the change of the complex permittivity of the oil-paper insulation and studying the change law of the real part and the imaginary part respectively can more effectively analyze the change of the oil-paper insulation state.

SUMMARY OF THE INVENTION

In view of the above technical problems, in order to more accurately evaluate the damp state of the transformer oil-paper insulation by using the frequency domain dielectric characteristic quantity, the present invention proposes a method for evaluating the moisture content in the insulating paper by using the initial slope of the complex dielectric constant. The absolute value |k| of the rate of change of the real part of the dielectric constant at f=10 ^-4 Hz is used as a frequency domain characteristic parameter to characterize the degree of dampness of the oil paper. The variation law of complex permittivity and |k| of oil-paper insulation samples with different moisture contents was experimentally studied, and the research results showed that with the increase of the moisture content M of insulating paper, the real part of the complex permittivity of oil-paper samples was ε′ The curve shifts to the high frequency band, and the variation range of the real part of the complex permittivity in the low frequency band increases with the increase of the moisture content. By fitting the measured ε′-f curve, it is found that |k| The excellent exponential function relationship can be used to quantitatively evaluate the moisture level of oil-paper insulation. The method of the invention has the characteristics of easier operation and no damage to the internal insulation structure of the transformer, and is more suitable for evaluating the damp state of the transformer insulation.

The technical scheme adopted in the present invention is:

A method for evaluating the moisture content in insulating paper using the initial slope of the complex dielectric constant, comprising the following steps:

Step 1: Define the initial rate of change of the real part of the complex permittivity εˊ with frequency at the frequency point f=10 ^-4 Hz k _c is the initial slope of the real part of the complex permittivity, and the oil-immersed type is predicted by k _c The degree of dampness of the insulating paper in the transformer;

Step 2: Use the three-electrode device to simulate the internal insulation structure of the oil-immersed transformer, record the structure parameters, obtain the complex capacitance parameters of the insulating paper through the frequency domain dielectric spectrum measuring instrument, and obtain the complex capacitance parameters of the insulating paper through the calculation formula of the capacitance parameters, the dielectric constant and the insulation structure. Obtain the change curve of the real part εˊ of the complex permittivity with frequency, and calculate the absolute value of the initial change rate |k _c | at the frequency point f=10 ^-4 Hz;

Step 3: Five groups of insulating paper samples with different moisture contents were prepared in the laboratory by natural moisture absorption, the absolute value of the initial change rate of the real part εˊ of the complex permittivity was measured and calculated |k _c |, and the analysis |k _c |Quantitative relationship with the degree of dampness of insulating paper;

Step 4: The exponential fitting relation function expression is obtained by data fitting |k _c | and the moisture content M of the insulating paper _. The smaller the absolute value |k _c | is, the smaller the water content in the insulating paper is, and the better the insulation degree inside the transformer is. The moisture content of the insulating paper in the transformer can be evaluated quantitatively by |k _c |.

In the step 1, when the test frequency f=10 ^-4 Hz is defined, the absolute value of the initial rate of change of εˊ is |k _c |, and the initial slope |k _c |=dεˊ/df, because the value range of |k _c | In the low frequency band, its value depends on the establishment process of rapid polarization inside the insulating oil-paper, and the polarization relaxation process in the dielectric is closely related to its internal water molecule content, so |k _c | can be used for transformer oil-paper insulation. Assessing the degree of dampness.

In the step 1, the initial rate of change k _c of the frequency domain dielectric spectrum curve of the real part of the complex permittivity at the frequency point f=10 ^-4 Hz is proposed, and the frequency domain dielectric spectrum test is performed on the insulating oil paper sample, and the result is obtained. The first 7 measured data points of the frequency domain spectrum curve of the real part of the complex permittivity are obtained by curve fitting to obtain the linear relationship of the first 7 data points, and then the frequency f=10- ⁴ Hz is substituted into the initial rate of change k _c , k _c reflects the establishment process of rapid polarization inside the insulating oil-paper, which mainly depends on the water molecule content in the oil-paper insulating medium, and can be used to evaluate the overall moisture state of the oil-paper insulation of oil-immersed transformers. In the step 2, a three-electrode device was used in the laboratory to simulate the internal insulation structure of the oil-immersed transformer, and the measurement was carried out at room temperature (about 30°C); in the preparation process of the insulating paper moisture test sample, the experimental material was A cellulose kraft insulating paper disc with a thickness of about 0.5mm and a radius of 65mm and 25# Merigao transformer insulating oil, expose the dried insulating paper to the air to absorb moisture naturally, and use a precision electronic scale every half an hour to measure Once weighed, five groups of insulating paper samples with moisture content M of 0.7%, 1.3%, 2.5%, 3.4% and 4.2% were prepared by calculating the mass of the insulating paper discs before and after moisture absorption. Sheets of insulating paper are stacked together.

In the described step 2, the measuring instrument selects the IDAX-300 dielectric response tester produced by Megger Company in the United States, and tests are carried out at room temperature. 65mm cellulose kraft insulating paper disc and 25# Merigo transformer insulating oil, expose the dried insulating paper to the air to absorb moisture naturally, and weigh it with a precision electronic scale every half an hour. Five groups of insulating paper samples with moisture content M of 0.7%, 1.3%, 2.5%, 3.4%, and 4.2% were prepared from the mass of insulating paper discs before and after the tide. made;

Test equipment parameter setting method: set the maximum value of AC power supply voltage U _max = 200V, the highest test frequency f _H = 4kHz, the lowest test frequency f _L = 0.1mHz, then data measurement can be carried out. Among them, U _max is the common setting value for the frequency domain dielectric spectrum test of oil-paper insulation. Selecting the test frequency band as 0.1mHz-4kHz can make the measurement range wider, obtain more information, and make the measurement results more reliable.

In the step 3, since the oil-paper insulation is a conventional dielectric, the alternating voltage applied to the oil-paper insulation causes the polarization relaxation phenomenon to occur inside. When the number of polar molecules in the medium increases, the polarization strength increases, which will inevitably cause With the increase of the dielectric constant, the slow polarization and fast polarization in the low frequency band can have enough time to establish. However, after the oil-paper insulation is damp, the number of water molecules increases sharply. Since the internal polarization is mostly caused by water and belongs to a rapid polarization process, the initial change rate of the complex permittivity of the oil-paper insulation is strongly affected by the water polarization, resulting in a decrease in ε′. faster rate. It can be seen from the above analysis that the change of water content has a very obvious influence on the real part of the complex permittivity ε′. The functional relationship between the water content M and k _c is obtained by comparing the experimental data and the results.

|k _c |=2.177×10 ⁴ ·exp(0.4372M)

The value of M can be obtained by substituting the measured k _c of the oil-paper insulation of the transformer into the above formula, so that the damp state of the oil-paper insulation can be preliminarily predicted and evaluated.

The present invention is a method for evaluating the moisture content in insulating paper by utilizing the initial slope of the complex dielectric constant, and the technical effects are as follows:

1. The damp state of the paper insulation in the oil-paper insulation can be analyzed without hanging the core, damaging the insulating material, and without sampling, providing a reliable basis for accurately evaluating the insulation performance of the oil-paper insulation power equipment.

2. The initial slope of the complex permittivity |kc| proposed by the present invention can effectively and reliably evaluate the moisture level of the internal paper insulation of the transformer, and the operation is simpler and will not cause damage to the insulation structure of the equipment. It is a better transformer. Diagnosis method of insulation damp state.

Description of drawings

FIG. 1 is a schematic diagram of the operation flow of the present invention.

Figure 2 is the measured frequency domain spectrogram of the real part of the complex permittivity.

3 is a schematic diagram of a three-electrode device;

In Figure 3: 1-measurement electrode, 2-high voltage electrode, 3-protection electrode, 4-transformer oil, 5-insulating paper sample.

Figure 4 is a fitting curve diagram of M and the absolute value of the initial slope value |k _c | of the real part of the complex permittivity of different moisture samples.

Detailed ways

Principle analysis:

A method for evaluating the moisture content in insulating paper using the initial slope of the complex permittivity, combining experiments and theory to study the relationship between the absolute value of the initial slope of the complex permittivity real part frequency domain dielectric spectrum curve and the moisture content of the paper insulation in the oil-paper insulation The influence law and numerical relationship. The alternating voltage applied to the dielectric causes polarization relaxation phenomenon inside the dielectric. When the number of polar molecules in the dielectric increases, the polarization strength increases, which will inevitably lead to an increase in the dielectric constant and slow polarization in the low frequency band. and fast polarization can have sufficient time to establish. However, after the insulation is damp, the number of water molecules increases sharply. Since the internal polarization is mostly caused by moisture and belongs to a rapid polarization process, the initial change rate of the complex permittivity of oil-paper insulation is strongly affected by the moisture polarization, resulting in a decrease in ε′. The rate is faster. As a strong polar molecule, the increase in the number of water molecules increases the overall polarization of the oil paper. Since |k _c | is the initial value of the change rate of the real part of the complex permittivity ε′, that is, the polarization The instantaneous change rate of ε′, which is mainly affected by the rapid polarization process, that is, the influence of water molecules on the dipole polarization, so with the increase of water content, the initial value of the change rate of ε′ |k _c | will be gradually increase. The functional relationship between |k _c | and M of oil-paper samples with different water contents M can be obtained by fitting, which can be used to quantitatively evaluate the degree of dampness of oil-paper insulation.

A method for evaluating the moisture content in insulating paper using the initial slope of the complex dielectric constant, comprising the following steps:

1), define the initial change rate of the real part εˊ of the complex permittivity with frequency at the frequency point f=10 ^-4 Hz k _c is the initial slope of the real part of the complex permittivity, and the oil-immersed type is predicted by k _c The degree of dampness of the insulating paper in the transformer; it has been verified by experiments that k _c has an obvious gradient relationship with the moisture content M of the paper insulation in the oil-paper insulation of the transformer. Assess the degree of dampness.

2) Use a three-electrode device to simulate the internal insulation structure of the oil-immersed transformer, record the structural parameters, obtain the complex capacitance parameters of the insulating paper through the frequency domain dielectric spectrum measuring instrument, and obtain the complex capacitance parameters of the insulating paper through the calculation formula of the capacitance parameters, the dielectric constant and the insulation structure. Obtain the change curve of the real part εˊ of the complex permittivity with frequency, and calculate the absolute value of the initial change rate |k _c | at the frequency point f=10 ^-4 Hz;

3) In the laboratory, five groups of insulating paper samples with different moisture contents were prepared by natural moisture absorption, and the absolute value of the initial change rate of the real part εˊ of the complex permittivity was measured and calculated |k _c |, and analyzed |k _c Quantitative relationship between | and the degree of dampness of insulating paper; according to the analysis of experimental data, the alternating voltage applied to the dielectric causes polarization relaxation phenomenon inside the dielectric. When the number of polar molecules in the dielectric increases, the polarization strength increases. It is bound to cause an increase in the dielectric constant, and both the slow polarization and the fast polarization in the low frequency band can have enough time to establish. However, after the insulation is damp, the number of water molecules increases sharply. Since the internal polarization is mostly caused by moisture and belongs to a rapid polarization process, the initial change rate of the complex permittivity of oil-paper insulation is strongly affected by the moisture polarization, resulting in a decrease in ε′. The rate is faster. As a strong polar molecule, the increase in the number of water molecules increases the overall polarization of the oil paper. Since |k _c | is the initial value of the change rate of the real part of the complex permittivity ε′, that is, the polarization The instantaneous change rate of ε′, which is mainly affected by the rapid polarization process, that is, the influence of water molecules on the dipole polarization, so with the increase of water content, the initial value of the change rate of ε′ |k _c | will be gradually increase. The functional relationship between |k _c | and M of oil-paper samples with different water contents M is obtained by data fitting |k _c |=21770·exp(0.4372M), which can be used for quantitative evaluation of oil-paper insulation dampness.

As shown in Figure 2, it is the measured frequency domain spectrum of the real part of the complex permittivity of the present invention, ε' is the real part of the complex permittivity, f is the frequency, M is the moisture content in the insulating paper, the frequency domain medium The electric spectrum lines show the variation trend of the real part ε' of the complex permittivity with frequency. The absolute value of the slope dε′/df of each dielectric line at f=10 ⁻⁴ Hz is the initial slope |k _c |.

Preparation of moisture test samples: In order to facilitate the determination of the moisture content of the samples, the insulating paper of each group of samples was fully dried and then weighed to obtain the gross weight of each group of samples, and then exposed to the air (at room temperature) to absorb moisture naturally. Use a precision electronic scale to weigh once every half an hour, and divide the increased weight after moisture absorption by the mass after moisture absorption to get its moisture content. After the moisture content of the test product is recorded, the FDS test is performed.

Build the experimental platform: use a three-electrode device to simulate the internal insulation structure of the oil-immersed transformer, as shown in Figure 3, the measuring instrument IDAX-300 dielectric response tester produced by Megger Company in the United States, the measured voltage peak U _max is 200V, and the highest test Frequency f _H = 4 kHz, minimum test frequency f _L = 0.1 mHz. In order to prevent the interference of external temperature, the experiments were carried out in an incubator at 30 °C. Before the test, the samples were allowed to stand in the incubator for 6 hours to fully preheat and ensure that the overall test sample reached 30 °C.

Fig. 4 is the fitting curve of the absolute value of the initial slope of the real part of the complex permittivity of the oil-paper insulation system under different M measured by the present invention and M. As can be seen from the figure, the rise of the moisture content M in the insulating paper will drive the initial The absolute value of the slope |k _c | increases at the same time.

Due to the sharp increase in the number of water molecules, since the internal polarization is mostly caused by water and belongs to a fast polarization process, the initial change rate of the complex permittivity of oil-paper insulation is strongly affected by the water polarization, resulting in a faster decline rate of ε′. Therefore, the absolute value of the initial slope of the complex permittivity |k _c | also increases, and its relationship with M is discussed below.

The initial slope is a function of moisture content as follows:

|k _c |=2.177×10 ⁴ ·exp(0.4372M);

In the formula, |k _c | is the absolute value of the initial slope of the frequency domain dielectric spectrum curve of the real part of the complex permittivity, and M is the moisture content of the paper insulation in the oil-paper insulation structure (in percent);

In summary, the fitting curves and |k| of the FDS initial segment of the moisture samples ε' with different M are obtained, as shown in Table 1 below:

It can be seen from the data in Table 1 above that |k _c | increases gradually with the increase of the water content M. This is because the water molecule is a strong polar molecule, and the increase of its number increases the overall polarization strength of the oil paper. , since |k _c | is the initial value of the rate of change of the real part of the complex permittivity ε′, that is, the rate of change of ε′ at the moment of polarization, this value is mainly affected by the rapid polarization process, that is, the polarization of water molecules to the dipole Therefore, as the moisture content increases, the initial value of the rate of change of ε′ |k _c | increases gradually.

The real part and imaginary part of the relative permittivity of the oil-paper insulation sample are very sensitive to the moisture content of the insulating paperboard. With the increase of the moisture content, the ε′ and ε″ gradually increase, and the ε′-f curve tends to move in the high frequency band , and the decreasing range of the real part ε' of the complex permittivity increases gradually in the frequency band of f=10- ⁴ ~ 10- ² Hz.

In the present invention, the absolute value of the slope |k _c | of the FDS curve of the real part ε' of the complex permittivity at f= ^10-4 Hz is used as the characteristic parameter in the frequency domain to quantitatively evaluate the damp degree of the oil-paper insulation. The f curve is fitted and calculated, and |k _c | is obtained. There is a good exponential function relationship between this value and M. Another set of experiments verifies that the characteristic parameter |k _c | proposed by the present invention is used in evaluating the moisture level of oil-paper insulation. The effectiveness of |k _c | can be used in the field testing of the moisture level of transformer oil-paper insulation. The research results of the present invention provide a new research idea for further proposing to use the initial slope characteristic quantity to diagnose the damp state of the insulation.

Claims (6)

1. A method for evaluating the moisture content in insulating paper by using the initial slope of complex dielectric constant is characterized by comprising the following steps:

step 1: defining the real part of the complex dielectric constant as the frequency variation curve at the frequency point f as 10^-4Initial rate of change k in Hz_cIs the initial slope of the real part of the complex permittivity, by k_cPredicting the moisture degree of insulating paper in the oil-immersed transformer;

step 2: the method comprises the steps of simulating an internal insulation structure of the oil-immersed transformer by adopting a three-electrode device, recording structural parameters, obtaining complex capacitance parameters of insulation paper through a frequency domain dielectric spectrum measuring instrument, obtaining a change curve of a complex dielectric constant real part 'along with frequency through a calculation formula of the capacitance parameters, the dielectric constant and the insulation structure, and calculating the change curve of the complex dielectric constant real part' along with the frequency at a frequency pointf＝10^-4Absolute value of initial rate of change in Hz_c|；

And step 3: five groups of insulation paper samples with different water contents are prepared in a laboratory through a natural moisture absorption method, and the absolute value | k of the initial change rate of the real part' of the complex dielectric constant is measured and calculated_cAnalysis of | k_cThe quantitative relation between the moisture degree of the insulating paper and the moisture degree of the insulating paper;

and 4, step 4: data fitting | k_cObtaining an exponential fitting relation function expression of | and the moisture content M of the insulating paper, wherein the function relation expression is | k_c21770 exp (0.4372M), which explains the absolute value of the initial rate of change | k_cThe smaller the | is, the smaller the water content in the insulating paper is, the better the insulation degree in the transformer is, and the | k is used_cAnd quantitatively evaluating the water content of the transformer insulation paper.

2. The method for estimating the moisture content in the insulating paper by using the initial slope of the complex dielectric constant as claimed in claim 1, wherein: in step 1, a test frequency f is defined as 10^-4At Hz,' the absolute value of the initial rate of change is | k_cInitial slope | k_cD'/df due to | k_cThe value range of | is in a low frequency band, the value of | depends on the establishment process of the rapid polarization in the insulating oil paper, and the polarization relaxation process in the dielectric is closely related to the content of water molecules in the dielectric, so that | k is obtained_cAnd l, evaluating the moisture degree of the oil paper insulation of the transformer.

3. The method for estimating the moisture content in the insulating paper by using the initial slope of the complex dielectric constant as claimed in claim 1, wherein: in the step 1, a frequency domain dielectric spectrum curve of the real part of the complex dielectric constant is provided at a frequency point f to 10^-4Initial rate of change k in Hz_cPerforming frequency domain dielectric spectrum test on the insulating oil paper sample to obtain the first 7 measured data points of the frequency domain spectrum curve of the real part of the complex dielectric constant, obtaining a linear relation of the first 7 data points through curve fitting, and then setting the frequency f to be 10^-4Substituting Hz to obtain the initial change rate k_c，k_cReflects the insulating oilpaperThe method is characterized by comprising an internal rapid polarization establishing process, wherein the polarization establishing process mainly depends on the content of water molecules in an oil paper insulating medium, and can be used for evaluating the moisture state of the oil paper insulating whole body of the oil-immersed transformer.

4. The method for estimating the moisture content in the insulating paper by using the initial slope of the complex dielectric constant as claimed in claim 1, wherein: in the step 2, a three-electrode device is used for simulating an internal insulation structure of the oil-immersed transformer in a laboratory, and measurement is carried out at room temperature; in the preparation process of the insulating paper moisture test sample, the experimental materials adopt a cellulose kraft insulating paper wafer with the thickness of 0.5mm and the radius of 65mm and 25# American high transformer insulating oil, the dried insulating paper is exposed in the air to absorb moisture naturally, a precision electronic scale is used for weighing once every half hour, five groups of insulating paper test samples with the moisture contents M of 0.7%, 1.3%, 2.5%, 3.4% and 4.2% are prepared by calculating the mass of the insulating paper wafer before and after moisture absorption, and each group of test samples are formed by overlapping five insulating paper wafers.

5. The method for estimating the moisture content in the insulating paper by using the initial slope of the complex dielectric constant as claimed in claim 1, wherein: in the step 2, the measuring instrument selects an IDAX-300 dielectric response tester produced by Megger company of America, the testing is carried out at room temperature, in the preparation process of the insulation paper moisture test sample, the test materials adopt a cellulose kraft insulation paper wafer with the thickness of about 0.5mm and the radius of 65mm and 25# American high transformer insulation oil, the dried insulation paper is exposed in the air to absorb moisture naturally, weighing is carried out once by a precision electronic scale every half hour, five groups of insulation paper test samples with the moisture content M of 0.7%, 1.3%, 2.5%, 3.4% and 4.2% are prepared by calculating the mass of the insulation paper wafer before and after moisture absorption, and each group of test samples are formed by overlapping five insulation paper wafers;

the method for setting the parameters of the test equipment comprises the following steps: setting the maximum value U of the AC supply voltage_max200V, highest test frequency f_H4kHz, lowest test frequency f_LData measurements were taken at 0.1 mHz.

6. The method for estimating the moisture content in the insulating paper by using the initial slope of the complex dielectric constant as claimed in claim 1, wherein: in the step 4, the water contents M and k_cThe functional relationship between the two is:

|k_c|＝2.177×10⁴·exp(0.4372M)

k for insulating actual measurement transformer oilpaper_cSubstituting the formula to obtain the value of M, thereby preliminarily predicting and evaluating the moisture state of the oil paper insulation.

Images