Variable Voltage Dielectric Response Test Method for Evaluating the Moisture Status of Oil-Paper Insulation

Abstract

The present invention discloses a variable voltage dielectric response test method for evaluating the damp state of oil-paper insulation, and uses the nonlinear coefficient under low-frequency sinusoidal excitation as a characteristic parameter reflecting the damp state. Under laboratory conditions, the nonlinear coefficients of oil-paper insulation samples with different moisture contents at different temperatures are measured under low-frequency excitation (recommended to be less than 1Hz), and the functional relationship between the moisture content and the nonlinear coefficient at different temperatures is obtained by fitting; then, low-frequency excitation voltage signals of different amplitudes are applied to the oil-paper composite insulation to be tested, and the response current signal is collected to obtain the nonlinear coefficient of the transformer to be tested, and the functional relationship between the moisture content and the nonlinear coefficient at the test temperature corresponding to the excitation frequency is substituted to quantitatively evaluate the damp degree of the oil-paper composite insulation; thereby achieving the purpose of effectively evaluating the damp state of the oil-paper composite insulation in the case of unknown structural parameters.

Description

Technical Field

[0001] The invention relates to the field of moisture evaluation of oil-paper insulation, and in particular to a variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation. Background Art

[0002] The moisture of oil-paper insulation is an important factor affecting the operational reliability of oil-immersed electrical equipment. In the absence of hanging covers for sampling, obtaining the moisture content of oil-paper insulation inside the equipment through non-destructive testing methods is a major demand of the power industry. Dielectric response testing is a non-destructive testing method that is currently widely used. The obtained frequency domain dielectric spectrum (FDS) has the advantage of being less affected by noise and is often used to evaluate the moisture content of oil-paper insulation.

[0003] This method first measures the FDS curves of oil-immersed paperboard with different moisture contents in the laboratory and forms an offline database. Then, considering the oil-paper insulation structure, the measured oil-paper insulation FDS is converted into insulation paper FDS. Taking the transformer main insulation as an example, the oil channel, partition and brace are usually equivalent to the XY model to obtain the structural parameters X and Y values. X represents the ratio of the total thickness of the partition to the thickness of the main insulation, and Y represents the ratio of the brace to the average circumference between the windings, such as Figure 1 shown.

[0004] Then, combined with the XY model, the FDS of the overall test result of the transformer is equivalently converted into the FDS of its insulation paperboard such as braces and partitions by equations (1)-(3), and finally matched with the FDS curve in the offline database to realize the evaluation of the moisture state of the transformer oil-paper composite insulation.

[0005]

[0006]

[0007]

[0008] Where σ(T) represents the DC conductivity of the insulating oil at temperature T, and the value of σ at 20°C is 5×10 -11 S / cm; ε tot * (ω) indicates that the oil-paper meets the complex dielectric constant of insulation; ε pap * (ω) represents the complex dielectric constant of insulating paper; ε oil * (v) represents the complex dielectric constant of insulating oil; ε0 represents the dielectric constant of vacuum, ε0 = 8.85 × 10 -12 F / m; ε pap "(ω) represents the imaginary part of the insulation paper complex capacitance, ε pap ’(ω) represents the real part of the complex capacitance of the insulating paper; tanδ pap (ω) represents the dielectric loss of insulating paper.

[0009] After measuring the complex dielectric constant of oil-paper composite insulation, the FDS of insulation paper can be obtained by equations (1)-(3).

[0010] Research has shown that FDS can be used to directly extract features or model extracted features to reflect the moisture state of insulating paper, and this testing technology has the advantages of strong anti-interference ability and non-destructiveness to the tested samples. In recent years, it has received widespread attention in the field of insulation state evaluation and diagnosis of oil-filled equipment, and commercial insulation diagnostic analyzers based on FDS have been launched.

[0011] The traditional FDS test method requires a lot of time to establish an offline database of insulation paperboard samples with different moisture contents, and the oil-paper insulation equipment has a complex structure. Although the XY model is believed to be able to link the actual insulation structure FDS with the insulation paperboard sample FDS in the offline database, on the one hand, the effectiveness of the model conversion lacks systematic verification, which may have a significant impact on the evaluation structure. On the other hand, the structural parameters X and Y are usually difficult to obtain. These factors greatly limit the method's evaluation of the moisture content of oil-paper insulation with different structures. Therefore, it is necessary to propose a new method for evaluating the moisture content of oil-paper composite insulation to achieve accurate evaluation of the moisture content of oil-paper composite insulation when the insulation structure parameters X and Y are unknown. Summary of the invention

[0012] In order to solve the deficiencies in the prior art, the object of the present invention is to provide a variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation.

[0013] The present invention adopts the following technical solution.

[0014] A variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation, which is less affected by the insulation structure and can be extended to the evaluation of the moisture state of oil-paper composite insulation with different insulation structures, comprises the steps of:

[0015] (1) Build a test system for measuring nonlinear coefficients;

[0016] (2) Prepare samples with different moisture contents, test the nonlinear coefficient of low-frequency sinusoidal excitation voltage at different temperatures, and fit the functional relationship between the moisture content of insulation paperboard and the nonlinear coefficient at different temperatures;

[0017] (3) Determine the nonlinear coefficient of the transformer under low-frequency sinusoidal excitation voltage and record the on-site test temperature;

[0018] (4) Combined with the on-site test temperature, the functional relationship between the moisture content and the nonlinear coefficient at the corresponding temperature is selected to calculate the moisture content of the oil-paper insulation.

[0019] Furthermore, in step 1, the test system can output a low-frequency sinusoidal excitation voltage signal of 0 to ±2000V, measure a response current signal with an amplitude of 1pA to 1mA, and synchronously collect the excitation voltage signal and the response current signal.

[0020] Furthermore, in step 2, insulating paperboard samples with different moisture contents mc are prepared by natural moisture absorption method, and the moisture content range of the samples includes the moisture content of the oil-paper composite insulation to be tested.

[0021] Furthermore, it is recommended that the test frequency be less than 1 Hz and the minimum excitation voltage differ from the maximum excitation voltage by 20 times.

[0022] Furthermore, in step 2, the specific steps are:

[0023] (2.1) For a sample with a certain moisture content mc, n different amplitudes {U mk |k=1,2,…n} sinusoidal voltage as excitation, and obtain the amplitude of the fundamental component of the corresponding response current {I mk |k=1,2,…n};

[0024] (2.2) With the minimum excitation voltage amplitude U m1 And its corresponding response current fundamental component amplitude I m1 As the reference value, the n measured values ​​U m and I m Converted into dimensionless per-unit voltage U* and per-unit current I*, as follows:

[0025]

[0026]

[0027] (2.3) Take U * and I * As the horizontal and vertical coordinates to form the U of the oil-impregnated paperboard * -I * Curve, introduce formula (3) to quantify the nonlinear degree coefficient of the insulating paperboard sample to be tested;

[0028] I * =(U * ) α (6)

[0029] Among them, α is defined as the nonlinear coefficient;

[0030] (2.4) Continue to test insulation paperboard samples with different moisture contents mc to obtain the nonlinear coefficient;

[0031] (2.5) The functional relationship between the moisture content of the insulation paperboard sample and the nonlinear coefficient is obtained by fitting.

[0032] Furthermore, at different temperatures, the nonlinear coefficients of oil-paper insulation samples with different moisture contents under low-frequency sinusoidal excitation were measured, and the functional relationship between the moisture content and the nonlinear coefficient at different temperatures was fitted.

[0033] Further, in step 3, the nonlinear coefficient of the actual oil-paper composite insulation under the low-frequency sinusoidal excitation voltage is tested, specifically including:

[0034] Sequentially collect the excitation voltage and response current at different amplitudes and extract the fundamental wave;

[0035] With minimum excitation voltage amplitude U m1 And its corresponding response current fundamental component amplitude I m1 As the reference value, the measured U m and I m It is converted into dimensionless per-unit voltage U* and per-unit current I*, and then the nonlinear coefficient of the insulation to be tested under low-frequency sinusoidal excitation is calculated.

[0036] Furthermore, the excitation voltage and response current at different amplitudes are collected and the fundamental wave is extracted. The specific steps are as follows:

[0037] The synchronous acquisition circuit simultaneously acquires the excitation voltage signal and the response current signal, and performs Fourier transform on the acquired discrete excitation voltage and response current waveforms:

[0038]

[0039] Where x(n) is a uniform discrete sampling sequence, and X(k) is a discrete Fourier transform sequence;

[0040] After mapping the time domain signal to the frequency domain, the frequency domain spectrum containing only the fundamental component is inverse Fourier transformed to obtain the time domain fundamental component of the excitation voltage and response current:

[0041]

[0042] Among them, x F (k) is the discrete Fourier transform sequence containing only the fundamental component, X F (n) is a uniform discrete sampling sequence containing only the fundamental component;

[0043] Finally, according to the known amplification factor, the excitation and response amplitudes under each test voltage are restored to obtain the fundamental components of the excitation and response.

[0044] The beneficial effect of the present invention lies in that, compared with the prior art, the present invention measures the nonlinear coefficient under low-frequency sinusoidal excitation for oil-paper insulation samples with different moisture contents at different temperatures, and obtains the functional relationship between the moisture content and the nonlinear coefficient at different temperatures by fitting; then, the same excitation voltage signals with different amplitudes are applied to the oil-paper composite insulation to be tested, and the response current signal is collected to obtain the nonlinear coefficient of the transformer to be tested, and the functional relationship between the moisture content and the nonlinear coefficient at the test temperature is substituted to quantitatively evaluate the moisture degree of the oil-paper composite insulation; compared with the traditional FDS moisture state evaluation method, the moisture state of the oil-paper composite insulation can be evaluated when the structural parameters are unknown. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is the equivalent XY model of transformer main insulation;

[0046] Figure 2 Flow chart of the variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation;

[0047] Figure 3 It is the schematic diagram of the nonlinear coefficient test system;

[0048] Figure 4 These are oil-paper insulation samples with different structures;

[0049] Figure 5 FDS curves of oil-paper insulation samples with different structures;

[0050] Figure 6 When U is 0.01Hz * -I * Characteristic curve;

[0051] Figure 7 This is a wiring diagram for three-phase dual-winding transformer tests. DETAILED DESCRIPTION

[0052] The present application is further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present application.

[0053] The present invention is mainly based on the fact that oil-paper insulation of different structures exhibits nonlinear characteristics after being damp under low-frequency excitation. Under laboratory conditions, the nonlinear coefficient of oil-paper insulation samples with different moisture contents under low-frequency sinusoidal excitation (the recommended test frequency is less than 1Hz, and the minimum excitation voltage and the maximum excitation voltage differ by 20 times) are measured at different temperatures, and the functional relationship between the moisture content and the nonlinear coefficient at different temperatures is fitted; then, low-frequency sinusoidal excitation voltages of different amplitudes are applied to the oil-paper composite insulation to be tested, and the response current signal is collected to obtain the nonlinear coefficient of the transformer to be tested, and the functional relationship between the moisture content and the nonlinear coefficient at the test temperature is substituted into the formula to quantitatively evaluate the degree of dampness of the oil-paper composite insulation; thereby achieving the purpose of effectively evaluating the dampness state of the oil-paper composite insulation when the structural parameters are unknown. The method of the present invention is less affected by the insulation structure and can be extended to the evaluation of the dampness state of oil-paper composite insulation with different insulation structures.

[0054] like Figure 2 As shown, the variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation according to the present invention includes specific implementation steps:

[0055] (1) Build a test system for measuring nonlinear coefficients;

[0056] The test system for determining the nonlinear coefficient needs to send out high and low voltage sinusoidal excitation voltage signals and measure weak current signals at the same time. The present invention builds a test system that can output a sinusoidal excitation voltage amplitude range of 0-±2000V, a sinusoidal excitation frequency that can be as low as 0.1mHz, and a measurable response current signal amplitude range of 1pA to 1mA. The test system can synchronously collect the excitation voltage signal and the response current signal. The test system includes a signal generator, an amplification circuit, an attenuation circuit, a synchronous acquisition circuit, and a filtering circuit, etc. The schematic diagram is shown in FIG. Figure 3 shown.

[0057] (2) The functional relationship between the moisture content of the insulation paperboard and the nonlinear coefficient was measured;

[0058] Samples with different moisture contents were prepared under laboratory conditions. The nonlinear coefficient with an amplitude of 100V-2000V under low-frequency sinusoidal excitation voltage was tested at different temperatures. The functional relationship between the moisture content of the insulation paperboard and the nonlinear coefficient at different temperatures was fitted.

[0059] In order to realize the moisture assessment method for oil-immersed transformers, it is first necessary to determine the nonlinear coefficient of insulating cardboard samples with different moisture contents under laboratory conditions, and to prepare insulating cardboard samples with different moisture contents mc by the natural moisture absorption method.

[0060] For example, at room temperature 20°C, for a sample with a certain moisture content mc, under low-frequency sinusoidal excitation (recommended to be less than 1Hz), n different amplitudes {U mk|k=1,2,…n} sinusoidal voltage as excitation, and obtain the amplitude of the fundamental component of the corresponding response current {I mk |k=1,2,…n}, with the minimum excitation voltage amplitude U m1 And its corresponding response current fundamental component amplitude I m1 As the reference value, the n measured values ​​U m and I m Converted into dimensionless per-unit voltage U* and per-unit current I*, as shown in equations (4)-(5).

[0061]

[0062]

[0063] U * and I * As the horizontal and vertical coordinates to form the U of the oil-impregnated paperboard * -I * Then, equation (6) is introduced to quantify the nonlinear coefficient of the insulation to be tested.

[0064] I * =(U * ) α (6)

[0065] Among them, α is defined as the nonlinear coefficient, which represents the degree of nonlinearity between U* and I*. When α=1, the insulating medium is a linear system, otherwise it is a nonlinear system. The larger the value of |α-1|, the greater the degree of nonlinearity of the insulating medium.

[0066] At room temperature of 20°C, the insulation cardboard samples with different moisture contents mc were tested to obtain the nonlinear coefficient; the functional relationship between the moisture content of the insulation cardboard at room temperature of 20°C and the nonlinear coefficient was fitted.

[0067] Similarly, at different temperatures, the nonlinear coefficients of oil-paper insulation samples with different moisture contents under low-frequency sinusoidal excitation were measured, and the functional relationship between moisture content and the nonlinear coefficient at different temperatures was fitted.

[0068] (3) Test the nonlinear coefficient of the actual oil-paper composite insulation under low-frequency sinusoidal excitation (recommended to be less than 1 Hz) and record the on-site test temperature;

[0069] Since the test object is affected by external noise and other factors, in order to avoid the influence of waveform distortion on nonlinear characteristics, the fundamental wave of the collected discrete excitation voltage and response current waveforms is extracted. First, the discrete waveform is Fourier transformed, as shown in formula (7).

[0070]

[0071] Where x(n) is the uniform discrete sampling sequence and X(k) is the discrete Fourier transform sequence.

[0072] After mapping the time domain signal to the frequency domain, the frequency domain spectrum containing only the fundamental component is inverse Fourier transformed to obtain the time domain fundamental component of the excitation voltage and response current, as shown in equation (8).

[0073]

[0074] Among them, x F (k) is the discrete Fourier transform sequence containing only the fundamental component, X F (n) is a uniform discrete sampling sequence containing only the fundamental component. Finally, according to the known attenuation and amplification factors, the excitation and response amplitudes under each test voltage are restored to obtain the fundamental components of the excitation and response, thus realizing the fundamental extraction of the excitation voltage and response current.

[0075] Since oil-paper insulation usually exhibits linear characteristics under low voltage excitation, the lowest test voltage and its response current are selected as the reference values.

[0076] With minimum excitation voltage amplitude U m1 And its corresponding response current fundamental component amplitude I m1 As the reference value, the n measured U m and I m Converted into dimensionless per-unit voltage U* and per-unit current I*, the nonlinear coefficient of the transformer under 0.01 Hz sinusoidal excitation is obtained by equations (4)-(6).

[0077] (4) Combined with the actual test temperature, the functional relationship between the moisture content at the corresponding temperature and the nonlinear coefficient fitting is selected to calculate the moisture content of the oil-paper composite insulation and quantitatively evaluate the moisture state;

[0078] Combined with the on-site test temperature, the corresponding moisture content and the functional relationship obtained by fitting the nonlinear coefficient are selected to calculate the moisture content of the oil-paper composite insulation. In this way, the moisture state of the oil-paper composite insulation can be evaluated when the insulation structure parameters are unknown.

[0079] Compared with the existing FDS-based method for assessing the moisture status of oil-immersed transformers, the objective improvement effect of this patent is illustrated by taking oil-paper insulation samples with different insulation structures as test objects. Under laboratory conditions, a circular insulating cardboard with a thickness of 1mm and a diameter of 141mm is used to simulate the partition of the transformer main insulation. On top of it is stacked a ring-shaped insulating cardboard with a thickness of 1mm, an inner diameter of 100mm or 80mm, and an outer diameter of 141mm to simulate the strut structure. The middle of the ring is filled with insulating oil to simulate an oil channel of the same thickness. By changing the number of strut layers and the inner diameter, main insulation models with different X and Y values ​​can be obtained. Figure 4As shown, models of four structures are given, hereinafter referred to as structures a, b, c and d.

[0080] The natural moisture absorption method was used to make each insulation sample reach a moisture content of different gradients, and the average moisture content of the ring and circular cardboard was taken as the moisture content value of the structural sample, as shown in Table 1.

[0081] Table 1

[0082] Structure a Structure b Structure c Structure mc1 1.06％ 1.05％ 1.02％ 1.09％ mc2 2.03％ 2.06％ 2.05％ 2.05％ mc3 3.06％ 3.03％ 3.08％ 3.05％ mc4 4.07％ 4.05％ 4.07％ 4.12％

[0083] The traditional FDS-based diagnostic method measures the overall frequency domain complex dielectric parameter curve ε of the transformer insulation. tot * (ω), the complex dielectric parameter ε of the solid insulation is calculated based on the structural X and Y parameters and equations (1)-(3). pap * (ω), and then compare it with the standard curve in the database to evaluate its moisture state. The measured FDS curves of samples with different structures are as follows Figure 5 shown.

[0084] like Figure 6 As shown in Figure 2, even if the solid insulation has similar water content, the dielectric parameter tanδ obtained by analysis is p (ω) is greatly affected by the insulation structure. In the low frequency band, the difference between the dielectric loss of samples with structures b, c and d and that of the single-layer paperboard sample with structure a gradually increases. The reason for this phenomenon is the polarization and loss caused by the interface charge of the composite material. Since the change of the insulation structure will affect the FDS curve, if the FDS curve is used to match the standard curve in the database, a large evaluation error will occur.

[0085] Next, the patented method was applied to evaluate the moisture state of oil-paper composite insulation with different structures. At room temperature, 1 mm insulation paperboard samples with moisture contents mc of 1.04%, 2.04%, 3.02% and 4.11% were prepared by natural moisture absorption method. Excitation voltages with a frequency of 0.01 Hz and amplitudes of 100 V, 200 V, 400 V, 600 V, 800 V, 1000 V, 1200 V, 1400 V, 1600 V, 1800 V and 2000 V were applied in sequence. The moisture content of insulation paperboard samples U was obtained from equations (4)-(6): * -I * Characteristic curves, such as Figure 6 shown.

[0086] The functional relationship between the moisture content of the insulation paperboard and the nonlinear coefficient at room temperature 20°C is fitted, as shown in formula (9).

[0087]

[0088] In the formula, α represents the nonlinear coefficient in formula (3), and mc represents the moisture content to be determined.

[0089] The same sinusoidal excitation signal is applied to different insulation structures a, b, c and d, and the nonlinear coefficient α is obtained from equations (4)-(6). Substituting it into equation (9) gives the estimated moisture content mc, as shown in Table 2.

[0090] Table 2

[0091]

[0092]

[0093] As shown in Table 2, the patented method can effectively evaluate the moisture content of oil-paper composite insulation with different insulation structures. Compared with the traditional FDS moisture state evaluation method, the moisture state of oil-paper composite insulation can be evaluated when the structural parameters are unknown.

[0094] As an implementation case, the above scheme is used to evaluate the moisture status of a normally retired three-phase double-winding transformer to illustrate the objective improvement effect of this patent. The implementation plan of each step is given in detail below.

[0095] (1) Design the test system;

[0096] The test system consists of a data acquisition module, a high-voltage amplifier module, a weak current amplifier module, and a host computer. It can output 0 to ±2000V, with a minimum test frequency of 0.1mHz of sinusoidal excitation, and can measure response current signals with an amplitude range of 1pA to 1mA.

[0097] (2) The relationship between the moisture content of the insulation paperboard and the nonlinear coefficient function was measured;

[0098] In this case, insulating paperboard samples with different moisture contents were prepared at room temperature (20°C), and excitation voltages with a frequency of 0.01 Hz and amplitudes of 100 V, 200 V, 400 V, 600 V, 800 V, 1000 V, 1200 V, 1400 V, 1600 V, 1800 V, and 2000 V were applied in sequence. The relationship between the moisture content and the nonlinear coefficient in this frequency and amplitude range is obtained, as shown in formula (9).

[0099] (3) Connect wires in the specified manner;

[0100] For normal retirement of three-phase double-winding transformers, S9-200 / 10 shall Figure 7 The transformer was connected in a wired manner, with the high-voltage winding and the low-voltage winding short-circuited respectively, and the transformer casing and the test system were grounded together, so that the transformer casing played a shielding role. Pressure was applied to the high-voltage winding bushing end to obtain the response of the low-voltage winding bushing end. The transformer was put into operation in 2001 and retired normally in 2021.

[0101] (4) Obtaining the fundamental components of excitation and response;

[0102] A low-frequency sinusoidal excitation signal with the same frequency and amplitude range is applied to the retired transformer in turn, and the attenuated excitation voltage signal and the amplified response current signal are collected synchronously. The signal is transformed into the frequency domain by equation (7), and after the fundamental component is extracted, it is transformed into the time domain by equation (8). The waveform is then restored according to the attenuation and amplification factors to obtain the amplitude of the fundamental component of the excitation and response.

[0103] (5) Fitting nonlinear coefficients;

[0104] The nonlinear coefficient at 0.01 Hz is obtained from equations (4)-(6). Under sinusoidal excitation at 0.01 Hz with an amplitude of 100 V to 2000 V, the nonlinear coefficient of the retired transformer is 1.217.

[0105] (6) Quantitative assessment of moisture status;

[0106] The transformer test temperature is room temperature (20°C), so substituting the nonlinear coefficient α into equation (9), it can be estimated that the moisture content of the transformer is 3.46%.

[0107] The beneficial effect of the present invention is that, compared with the prior art, the present invention can quantitatively evaluate the moisture degree of oil-paper composite insulation; compared with the traditional FDS moisture status evaluation method, the moisture status of oil-paper composite insulation can be evaluated when the structural parameters are unknown.

[0108] The applicant of the present invention has made a detailed explanation and description of the implementation examples of the present invention in conjunction with the drawings in the specification. However, those skilled in the art should understand that the above implementation examples are only preferred implementation schemes of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, but not to limit the scope of protection of the present invention. On the contrary, any improvements or modifications based on the inventive spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. A variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation, characterized in that: The method is less affected by the insulation structure and can be extended to the evaluation of the moisture state of oil-paper insulation with different insulation structures. The method comprises the steps of: (1) Build a test system for measuring nonlinear coefficients; (2) Prepare samples with different moisture contents, test the nonlinear coefficient of low-frequency sinusoidal excitation voltage at different temperatures, and fit the functional relationship between the moisture content of insulation paperboard and the nonlinear coefficient at different temperatures; Step (2) specifically includes: (2.1) For a sample with a certain moisture content mc, k different amplitudes U are used in turn under low-frequency sinusoidal excitation. mk The sinusoidal voltage is used as the excitation to obtain the amplitude of the fundamental component of the corresponding response current I mk , where k = 1, 2, ... n; (2.2) With the minimum excitation voltage amplitude U m1 And its corresponding response current fundamental component amplitude I m1 As the reference value, the n measured voltage amplitudes U m and the current amplitude I m Converted into dimensionless per-unit voltage U* and per-unit current I*, as follows: (2.3) Take U * and I * As the horizontal and vertical coordinates to form the U of the oil-impregnated paperboard * -I * Curve, introduce formula (6) to quantify the nonlinear coefficient of the insulation paperboard sample to be tested; AND * =(U * ) α (6) Among them, α is defined as the nonlinear coefficient; (2.4) Continue to test insulation paperboard samples with different moisture contents mc to obtain the nonlinear coefficient; (2.5) The functional relationship between the moisture content of the insulation paperboard sample and the nonlinear coefficient is obtained by fitting; (3) Determine the nonlinear coefficient of actual oil-paper insulation under low-frequency sinusoidal excitation voltage and record the on-site test temperature; (4) Combined with the on-site test temperature, the functional relationship between the moisture content and the nonlinear coefficient at the corresponding temperature is selected to calculate the moisture content of the oil-paper insulation.

2. The variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation according to claim 1 is characterized in that: In step (1), the test system can output a low-frequency sinusoidal excitation voltage signal of 0 to ±2000V, measure a response current signal with an amplitude of 1pA to 1mA, and synchronously collect the excitation voltage signal and the response current signal.

3. The variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation according to claim 2 is characterized in that: In step (2), the natural moisture absorption method is used to prepare insulating paperboard samples with different moisture contents mc, and the moisture content range of the samples includes the moisture content of the oil-paper insulation to be tested.

4. The variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation according to claim 1 is characterized in that: The test frequency is less than 1Hz, and the minimum excitation voltage differs from the maximum excitation voltage by 20 times.

5. The variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation according to claim 4 is characterized in that: At different temperatures, the nonlinear coefficients of oil-paper insulation samples with different moisture contents under low-frequency sinusoidal excitation were measured, and the functional relationship between moisture content and nonlinear coefficient at different temperatures was fitted.

6. The variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation according to claim 1, characterized in that: In step (3), the nonlinear coefficient of the actual oil-paper insulation under the low-frequency sinusoidal excitation voltage is tested, which specifically includes: Sequentially collect the excitation voltage and response current at different amplitudes and extract the fundamental wave; With minimum excitation voltage amplitude U m1 And its corresponding response current fundamental component amplitude I m1 As the reference value, the measured U m and I m It is converted into dimensionless per-unit voltage U* and per-unit current I*, and then the nonlinear coefficient of the insulation to be tested under low-frequency sinusoidal excitation is calculated.

7. The variable voltage dielectric response test method for evaluating the moisture state of oil-paper insulation according to claim 6 is characterized in that: Collect the excitation voltage and response current at different amplitudes and extract the fundamental wave. The specific steps are as follows: Synchronously collecting the attenuated excitation voltage signal and the amplified response current signal, and performing Fourier transform on the collected discrete excitation voltage and response current waveforms; After mapping the time domain signal to the frequency domain, the frequency domain spectrum containing only the fundamental wave component is subjected to inverse Fourier transform to obtain the time domain fundamental wave components of the excitation voltage and the response current; Finally, according to the known attenuation and amplification factors, the excitation voltage and response current amplitudes under each excitation voltage are restored to obtain the fundamental components of the excitation voltage and response current.

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