A Synchronous Testing Device for Time-Frequency Response of Oil-Paper Insulation and an Aging State Evaluation Method

Abstract

An oil-paper insulation time-frequency domain response synchronous test device and an aging state evaluation method, belonging to the technical field of insulation material aging detection. The present invention aims to solve the problem that the existing technology cannot achieve synchronous measurement of time / frequency domain dielectric response. The solution of the present invention: The host computer outputs a frequency domain excitation instruction to the signal generator, and the generated AC voltage signal is amplified by the power amplifier and applied to the test specimen via the AC side protection resistor. The responsive AC signal is collected after passing through the DC blocking capacitor; the host computer outputs a time domain excitation instruction to the high-voltage DC power supply, and the generated DC voltage signal is synchronously applied to the test specimen via the DC side protection resistor. The responsive DC signal is collected after passing through the high-voltage silicon stack; the host computer calculates the change curve of the AC side impedance modulus with frequency and the change curve of the DC side impedance modulus with time according to the synchronously collected AC signal and DC signal, and then obtains the DP value of the test specimen to realize the quantitative evaluation of the aging degree of the transformer.

Description

Technical Field

[0001] The present invention relates to the technical field of insulation material aging detection, and particularly relates to a synchronous test device for the time-frequency domain response of oil-paper insulation and an aging state evaluation method. Background Art

[0002] Oil-immersed power equipment and its components (transformers, instrument transformers, high-voltage bushings) are key equipment in the power system, and their stable operation is crucial. Under long-term complex operating conditions, the internal insulation of oil-immersed power equipment and its components will age. Therefore, it is crucial to accurately obtain the degree of insulation aging during regular inspections and formulate corresponding maintenance and operation strategies accordingly.

[0003] The power operation and maintenance departments in China regularly carry out equipment maintenance plans in spring and autumn, and conduct off-line inspections on equipment according to the DL / T596-2021 Power Equipment Preventive Test Regulations, such as partial discharge, power frequency withstand voltage, power frequency dielectric loss, insulation resistance, dissolved gas analysis in oil, infrared imaging temperature, winding DC resistance test, etc. The operating status of the equipment is reflected through the above test results. According to the feedback of a large number of on-site test evaluation results, the above methods are more accurate in evaluating problems such as concentrated insulation defects or developed latent faults in the equipment, but still cannot quantitatively analyze problems such as insulation aging, and can only make qualitative comparisons based on the factory parameter values or the test values (power frequency dielectric loss, insulation resistance) between the three phases of the current power equipment.

[0004] In order to further quantitatively obtain the degree of insulation aging, time-frequency domain equipment / technologies have gradually been used by the power industry and testing institutions from preliminary university scientific research. The existing time-frequency domain dielectric response test technology needs to be tested independently. The test period of the time-domain dielectric response is 40 minutes (only the polarization process), and the test period of the frequency-domain dielectric response (0.001 Hz to 1000 Hz) is 35 minutes. For on-site testers, due to the limited equipment maintenance cycle, if the two test methods are carried out in sequence, the equipment detection cycle will be increased. At the same time, the detection parameters considered in the traditional two test methods are different. In the time domain, the polarization and depolarization currents are mainly analyzed (related to the insulation structure size of the test equipment), and in the frequency domain, the dielectric loss factor under broadband excitation is mainly considered (independent of the insulation structure size of the test equipment). The response parameters obtained by the two test methods are not directly comparable. There is a scheme in the prior art to apply an AC-DC superimposed electric field to the test sample, but it cannot achieve synchronous measurement of the time / frequency domain dielectric response, and at the same time, the consistency of the results obtained by the two test methods is not considered, making the evaluation results of the oil-paper insulation aging state too one-sided and incomparable. There is an urgent need for a new technical solution to solve the above problems. Summary of the Invention

[0005] Aiming at the problem that the existing technology cannot achieve synchronous measurement of time / frequency domain dielectric response, the present invention provides a synchronous test device for time-frequency domain response of oil-paper insulation and an aging state evaluation method.

[0006] In one aspect of the present invention, a synchronous test device for time-frequency domain response of oil-paper insulation is provided, which includes a signal generator 1, a power amplifier 2, an AC side protection resistor 3, a DC side protection resistor 4, a high-voltage DC power supply 5, an AC micro-current acquisition module 6, a DC-blocking capacitor 7, a high-voltage silicon stack 9, a DC micro-current acquisition module 10 and a host computer 11;

[0007] The host computer 11 outputs a frequency-domain excitation command to the signal generator 1. The AC voltage signal generated by the signal generator 1 is amplified by the power amplifier 2. The amplified AC voltage signal is applied to the specimen to be tested 8 via the AC side protection resistor 3. The AC signal responded by the specimen to be tested 8 is collected by the AC micro-current acquisition module 6 after passing through the DC-blocking capacitor 7 and fed back to the host computer 11;

[0008] The host computer 11 outputs a time-domain excitation command to the high-voltage DC power supply 5. The DC voltage signal generated by the high-voltage DC power supply 5 is synchronously applied to the specimen to be tested 8 via the DC side protection resistor 4. The DC signal responded by the specimen to be tested 8 is collected by the DC micro-current acquisition module 10 after passing through the high-voltage silicon stack 9 and fed back to the host computer 11;

[0009] The host computer 11 calculates the change curve of the AC side impedance modulus with frequency and the change curve of the DC side impedance modulus with time according to the synchronously collected AC signal and DC signal, and obtains the DP value of the specimen to be tested 8 according to the change curve of the AC side impedance modulus with frequency and the change curve of the DC side impedance modulus with time to realize the quantitative evaluation of the aging degree of the transformer.

[0010] Preferably, the frequency-domain excitation command is an excitation signal for setting the sweep frequency range and the AC voltage amplitude. The sweep frequency range is 0.001 to 1000 Hz, the range of the AC voltage amplitude is 0 to 5 V, and the peak-to-peak voltage output after the excitation signal passes through the power amplifier (2) is 0 to 2000 V; the time-domain excitation command is for setting the DC voltage amplitude, and the range of the DC voltage amplitude is 0 to 2000 V.

[0011] Preferably, the AC side protection resistor (3) and the DC side protection resistor (4) are used to prevent damage to the AC voltage source and the DC voltage source after the specimen to be tested (8) is broken down. The AC side protection resistor (3) and the DC side protection resistor (4) adopt a first set value.

[0012] Preferably, the DC-blocking capacitor (7) is used to prevent the DC side voltage from being applied to the AC signal source; the high-voltage silicon stack (9) adopts a product with a second set value withstand voltage level, which is used to prevent the AC voltage from being applied to the DC high-voltage side.

[0013] In another aspect of the present invention, there is provided an aging state evaluation method for the time-frequency domain response synchronous test of oil-paper insulation, which is implemented based on the above-mentioned time-frequency domain response synchronous test device for oil-paper insulation. The method includes the following steps:

[0014] Step 1: Set the frequency domain excitation command including the sweep frequency range and the AC voltage amplitude, the time domain excitation command including the DC voltage amplitude on the DC side, and the test temperature;

[0015] Step 2: Conduct the time-frequency domain response synchronous test experiment at each frequency point within the sweep frequency range on the AC side. During the experiment at each frequency point, collect the AC signal of the test object response on the AC side and synchronously collect the DC signal of the test object response on the DC side;

[0016] Step 3: The upper computer obtains the AC impedance at the set test temperature based on the collected AC signal and the AC voltage amplitude applied to the test object, and draws the curve of the AC impedance versus the frequency change; the upper computer obtains the DC impedance at the set test temperature based on the collected DC signal and the DC voltage amplitude applied to the test object, and draws the curve of the DC impedance versus the time change;

[0017] Step 4: Perform Fourier transforms on the obtained curves of the AC impedance versus the frequency change and the DC impedance versus the time change respectively to generate two corresponding transformed impedance curves. Calculate the mean value of the transformed impedance modulus in the transformed impedance curve and the original impedance modulus in the test curve and perform normalization processing to obtain the curve of the AC side normalized impedance modulus versus the frequency change and the curve of the DC side normalized impedance module versus the time change;

[0018] Step 5: According to the curve of the AC side normalized impedance modulus versus the frequency change and the curve of the DC side normalized impedance module versus the time change, and in combination with the following formula, obtain the DP value:

[0019]

[0020] where DP is the degree of polymerization of the oil-impregnated paperboard; A(T) is the fitting parameter on the AC side, B(T) is the fitting parameter on the DC side, which is related to the test temperature T; Z ac (f n ) is the normalized impedance modulus on the AC side at the nth test frequency point f n , n = 1, 2,,,, N, N is the number of test frequency points, Z ac (f n ) is in ohms; Z dc (t m ) is the normalized impedance modulus on the DC side at the mth test time point t m , m = 1, 2,…, M, M is the number of test time points, Z dc (t m)The unit is ohm.

[0021] Preferably, the AC side fitting parameter A(T) and the DC side fitting parameter B(T) are obtained through preliminary tests. The specific process includes the following steps:

[0022] Step 1: Set the test temperature range and interval; prepare P groups of test samples 8 with known DP values.

[0023] Step 2: Within the set test temperature range, extract Q test temperatures according to the set interval, and conduct time-frequency domain response synchronous test experiments on the P groups of test samples 8 with DP values one by one at each test temperature. The implementation steps of the time-frequency domain response synchronous test experiment are from Step 1 to Step 5; an overdetermined system of equations is obtained through the experiment:

[0024]

[0025] In the overdetermined system of equations, DP1, DP2... DP P are the DP values of the P groups of test samples 8, and the P DP values are not equal.

[0026] A(T i ) and B(T i ) are the AC parameters and DC parameters at the i-th test temperature T i within the test range, where i = 1, 2,..., Q; A(T i ) and B(T i ) are obtained by automatically optimizing through the above overdetermined system of equations.

[0027] Step 4: Fit the Q groups of AC parameter test data A(T1), A(T2),..., A(T Q ) at different test temperatures into the AC side fitting parameter A(T):

[0028] A(T) = 3.45e 0.025T / 4.35+0.448

[0029] Step 5: Fit the Q groups of DC parameter test data B(T1), B(T2),..., B(T Q ) at different test temperatures into the DC side fitting parameter B(T):

[0030] B(T) = 0.255e 0.01T+0.468 .

[0031] Preferably, the test temperature T range is from 10 degrees Celsius to 90 degrees Celsius, and the test temperature T interval is the third set value.

[0032] Preferably, a DC impedance vs. time curve is plotted, including: dividing the time spent on testing the specimen to be tested into m test time periods, where the equivalent impedance value for each test period is the average of the impedance at the first test point and the impedance at the last test point in that test period, and plotting a DC impedance vs. time curve based on the impedance values of each test period and the time spent on testing the specimen to be tested.

[0033] Advantages of the present invention:

[0034] 1. A time / frequency domain synchronous detection device and an aging degree evaluation method for the aging state detection of oil-immersed power equipment are provided. Instead of collecting the results of applying an AC-DC superimposed electric field to the specimen to be tested, the present invention collects two types of data, AC and DC, in real time for the synchronous measurement of time / frequency domain dielectric response.

[0035] 2. By performing a single test, the time-domain impedance characteristics and frequency-domain impedance characteristics of oil-paper insulation can be obtained simultaneously. The influence of environmental noise and large differences in impedance orders of magnitude on the evaluation results is eliminated by averaging and normalizing the two test results.

[0036] 3. By calculating the value of DP, quantitative analysis of the aging degree of oil-paper insulation can be directly achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a schematic structural diagram of a time / frequency domain response synchronous test device for oil-paper insulation according to the present invention;

[0038] Figure 2 is a flowchart of an aging state evaluation method for time / frequency domain response synchronous test of oil-paper insulation;

[0039] Figure 3 is the impedance curve on the AC side;

[0040] Figure 4 is the impedance curve on the DC side;

[0041] Figure 5 is the curve of the fitting parameter A(T) on the AC side and the fitting parameter B(T) on the DC side. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] Hereinafter, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0043] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0044] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but it is not intended to limit the present invention.

[0045] Specific Embodiment 1: The following will be described in conjunction with Figure 1 This embodiment will be described. The time-frequency domain response synchronous test device for oil-paper insulation in this embodiment includes a signal generator 1, a power amplifier 2, an AC side protection resistor 3, a DC side protection resistor 4, a high-voltage DC power supply 5, an AC micro-current acquisition module 6, a DC-blocking capacitor 7, a high-voltage silicon stack 9, a DC micro-current acquisition module 10, and a host computer 11;

[0046] The host computer 11 outputs a frequency-domain excitation command to the signal generator 1. The AC voltage signal generated by the signal generator 1 is amplified by the power amplifier 2. The amplified AC voltage signal is applied to the test specimen 8 via the AC side protection resistor 3. The AC signal responded by the test specimen 8 is collected by the AC micro-current acquisition module 6 after passing through the DC-blocking capacitor 7 and fed back to the host computer 11;

[0047] The host computer 11 outputs a time-domain excitation command to the high-voltage DC power supply 5. The DC voltage signal generated by the high-voltage DC power supply 5 is synchronously applied to the test specimen 8 via the DC side protection resistor 4. The DC signal responded by the test specimen 8 is collected by the DC micro-current acquisition module 10 after passing through the high-voltage silicon stack 9 and fed back to the host computer 11;

[0048] The host computer 11 calculates the change curve of the AC side impedance modulus with frequency and the change curve of the DC side impedance modulus with time according to the synchronously collected AC signal and DC signal, and obtains the DP value of the test specimen 8 according to the change curve of the AC side impedance modulus with frequency and the change curve of the DC side impedance modulus with time to realize the quantitative evaluation of the aging degree of the transformer.

[0049] The frequency-domain excitation command is an excitation signal for setting the sweep frequency range and the AC voltage amplitude. The sweep frequency range is 0.001 to 1000 Hz, the range of the AC voltage amplitude is 0 to 5 V, and the peak-to-peak voltage output after the excitation signal passes through the power amplifier (2) is 0 to 2000 V; the time-domain excitation command is for setting the DC voltage amplitude, and the range of the DC voltage amplitude is 0 to 2000 V.

[0050] The test device of this embodiment provides two parallel circuits, an AC-side test circuit and a DC-side test circuit. The AC-side test circuit is as follows: The AC voltage signal generated by the signal generator 1 is amplified by the power amplifier 2, and then applied to the specimen under test 8 via the AC-side protection resistor 3. The AC signal responded by the specimen under test 8 is collected by the AC micro-current acquisition module 6 after passing through the DC-blocking capacitor 7. The DC-side test circuit is as follows: The DC voltage signal generated by the high-voltage DC power supply 5 is synchronously applied to the specimen under test 8 via the DC-side protection resistor 4. The DC signal responded by the specimen under test 8 is collected by the DC micro-current acquisition module 10 after passing through the high-voltage silicon stack 9. In the time-frequency domain synchronous sampling system, the DC-blocking capacitor 7 and the high-voltage silicon stack 9 are used to achieve the synchronous application of AC and DC voltages on the specimen under test 8, and the DC micro-current signal and the AC micro-current signal are respectively collected in the two circuits.

[0051] The AC-side protection resistor 3 and the DC-side protection resistor 4 are used to prevent damage to the AC voltage source and the DC voltage source after the specimen under test 8 is broken down. The AC-side protection resistor 3 and the DC-side protection resistor 4 adopt the first set value, and the first set value is a 1-megohm resistor.

[0052] The DC-blocking capacitor 7 is used to prevent the DC-side voltage from being applied to the AC signal source; the high-voltage silicon stack 9 adopts a product with a second set value of withstand voltage level, which is used to prevent the AC voltage from being applied to the DC high-voltage side. The second set value is 20,000 volts.

[0053] Specific embodiment two: The following is combined with Figures 1 to 5 to illustrate this embodiment. The aging state evaluation method for synchronous test of the time-frequency response of oil-paper insulation described in this embodiment is realized based on the synchronous test device for the time-frequency response of oil-paper insulation described in Embodiment 1. This method includes the following steps:

[0054] Step 1: Set the frequency-domain excitation instruction including the frequency sweep range and the AC voltage amplitude, the time-domain excitation instruction including the DC voltage amplitude on the DC side, and the test temperature.

[0055] The host computer can set the frequency bandwidth and voltage amplitude of the excitation signal through the program. The test frequency range is 0.001 to 1000 Hz, the voltage output range is 0 to 5 V, and the effective value of the AC voltage output after passing through the power amplifier is in the range of 0 to 2000 V; the output voltage range of the high-voltage DC source is 0 to 2000 V.

[0056] Step 2: Conduct time-frequency response synchronous test experiments at each frequency point within the frequency sweep range on the AC side. During the experiment at each frequency point, collect the AC signal responded by the specimen under test on the AC side and synchronously collect the DC signal responded by the specimen under test on the DC side.

[0057] In this step, synchronous acquisition is performed. Through the DC-blocking capacitor 7 and the high-voltage silicon stack 9, the AC and DC response currents are used to achieve impedance synchronous acquisition under high-voltage AC and DC excitations. The DC-blocking capacitor 7 is used to prevent the DC-side voltage from being applied to the AC signal source; the high-voltage silicon stack 9 uses a product with a withstand voltage level of 20,000 volts to prevent the AC voltage from being applied to the DC high-voltage side.

[0058] Step 3: The host computer obtains the AC impedance at the set test temperature based on the collected AC signal and the AC voltage amplitude applied to the test sample, and plots the curve of the AC impedance versus the frequency change; the host computer obtains the DC impedance at the set test temperature based on the collected DC signal and the DC voltage amplitude applied to the test sample, and plots the curve of the DC impedance versus the time change.

[0059] To plot the curve of the DC impedance versus the time change, it includes: dividing the time spent on testing the test sample into m test time periods. The equivalent impedance value of each test period is the average of the impedance at the starting test point and the impedance at the ending test point in this test period. The curve of the DC impedance versus the time change is plotted based on the impedance values of each test period and the time spent on testing the test sample.

[0060] In this step, based on the sampled AC and DC response current signals and the applied AC and DC excitation amplitudes, the AC and DC impedances are calculated, and the curves of the impedance versus the frequency and time are obtained.

[0061] Step 4: Both the curve of the AC impedance versus the frequency change and the curve of the DC impedance versus the time change obtained from the test are used as test curves. The test curves are subjected to Fourier transform to generate transformed impedance curves. The mean value of the transformed impedance modulus in the transformed impedance curve and the original impedance modulus in the test curve is calculated and normalized to obtain the curve of the AC-side normalized impedance modulus versus the frequency change and the curve of the DC-side normalized impedance module versus the time change.

[0062] The order of magnitude spans of the impedance data obtained on the DC and AC sides are relatively large, and there are differences in the data after Fourier transform. Therefore, the DC impedance data, AC impedance data, AC-transformed impedance data, and DC-transformed impedance data obtained from the test need to be averaged and normalized to eliminate the influence of environmental noise and data order of magnitude on the evaluation results.

[0063] Step 5: According to the curve of the AC-side normalized impedance modulus versus the frequency change and the curve of the DC-side normalized impedance module versus the time change, and combined with the following formula, the DP value is obtained:

[0064]

[0065] Wherein, DP is the degree of polymerization of oil-impregnated paperboard; A(T) is the fitting parameter on the AC side, and B(T) is the fitting parameter on the DC side, which are related to the test temperature T; Z ac (f n ) is the normalized impedance modulus value on the AC side at the nth test frequency point f n , n = 1, 2,,,, N, N is the number of test frequency points, Z ac (f n ) is in ohms; Z dc (t m ) is the normalized impedance modulus value on the DC side at the mth test time point t m , m = 1, 2,…, M, M is the number of test time points, Z dc (t m ) is in ohms.

[0066] The fitting parameter A(T) on the AC side and the fitting parameter B(T) on the DC side are obtained through preliminary tests. The specific process includes the following steps:

[0067] Step 1: Set the test temperature range and interval; prepare P groups of test samples (8) with known DP values;

[0068] Step 2: In the set test temperature range, extract Q test temperatures according to the set interval, and conduct time-frequency domain response synchronous test experiments on the P groups of test samples 8 with DP values at each test temperature one by one. The implementation steps of the time-frequency domain response synchronous test experiment are steps one to five; an overdetermined system of equations is obtained through the experiment:

[0069]

[0070] In the overdetermined system of equations, DP1, DP2……DP P are the DP values of the P groups of test samples 8, and the P DP values are not equal;

[0071] A(T i ) and B(T i ) are the AC parameters and DC parameters at the ith test temperature T i in the test range, i = 1, 2,…, Q; A(T i ) and B(T i ) are obtained by automatically optimizing through the above overdetermined system of equations;

[0072] Step 4: Fit the experimental data of the AC parameters A(T1), A(T2),..., A(T Q ) at Q different test temperatures to the fitting parameter A(T) on the AC side:

[0073] A(T) = 3.45e 0.025T / 4.35+0.448

[0074] The test temperature T ranges from 10 degrees Celsius to 90 degrees Celsius, and the test temperature T interval is the third set value, which is 10 degrees Celsius.

[0075] Step 5: Fit the DC parameter test data B(T1), B(T2),..., B(T Q ) at Q different test temperatures to the DC-side fitting parameter B(T):

[0076] B(T) = 0.255e 0.01T+0.468 .

[0077] See the fitting curve in Figure 5 as shown.

[0078] The two parameters A(T) and B(T) obtained after fitting can be directly applied to the time-frequency domain response synchronization test experiment at any test temperature to obtain the DP value, so as to realize the quantitative evaluation of the aging degree of the sample to be tested 8:

[0079] The specific test includes the following steps: In the interface of the upper computer, select the test frequency range, AC voltage amplitude, and DC voltage amplitude. After starting the test, the signal generator will receive the instructions from the upper computer and send out corresponding sine excitation signals of different frequency bands. In the embodiment, the applied frequency values are 1000, 470, 220, 110, 70, 40, 20, 10, 4.6, 2.2, 1, 0.46, 0.22, 0.1, 0.046, 0.022, 0.01, 0.0046, 0.0022, 0.0001 Hz. After passing through the power amplifier, a voltage with an effective value of 2000 V is output, and the output DC voltage is 2000 V. The AC and DC voltages are applied to the sample to be tested. On both the AC side and the DC side, an I / V conversion sampling unit is used to detect the micro current on the AC side and the DC side through the automatic transimpedance adjustment method. According to the applied voltage amplitude and the current values obtained at different test frequencies and different times, the change curves of the impedance modulus with frequency and time are calculated. Similarly, the above tests are carried out on oil-paper insulation materials with different aging degrees at different test temperatures. Finally, the functional relationship between the insulation aging degree and the test temperature and the impedance modulus (different frequencies, different times) is established, and the quantitative evaluation of the transformer aging degree can be realized through the DP value.

[0080] Application example

[0081] A retired 35 kV oil-immersed transformer that has been used for 10 years is selected as the test sample for testing. The time-frequency domain synchronization test is carried out on the transformer in a test environment of 21 degrees Celsius. The comparison results between the time-frequency domain test results and the calculation results obtained after Fourier transform are as Figure 3 , 4As shown. Through the calculation by the method of the present invention, the degree of polymerization (DP) value of the internal insulation of the 35 kV oil-immersed transformer is 698. According to DL / T 984-2018 "Guide for Judging the Aging of Insulation of Oil-Immersed Transformers", when the degree of polymerization value of the internal insulation paperboard of the transformer is greater than 500, the insulation state of the transformer is good and it is in the middle and early stage of aging.

[0082] Although the present invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed, as long as they do not depart from the spirit and scope of the present invention as defined by the appended claims. It should be understood that different dependent claims and the features described herein can be combined in a manner different from that described in the original claims. It should also be understood that the features described in connection with a single embodiment can be used in other described embodiments.

Claims

1. An aging state evaluation method for synchronous testing of time-frequency domain response of oil-paper insulation, characterized in that, The method includes the following steps: Step 1: Set a frequency-domain excitation instruction including a sweep frequency range and an AC voltage amplitude, a time-domain excitation instruction including a DC voltage amplitude on the DC side, and test the temperature; Step 2: On the AC side, perform a time-frequency domain response synchronous test experiment at each frequency point within the sweep frequency range. During the experiment at each frequency point, collect the AC signal of the AC-side test specimen response and synchronously collect the DC signal of the DC-side test specimen response; Step 3: The host computer obtains the AC impedance at the set test temperature according to the collected AC signal and the AC voltage amplitude applied to the test specimen, and draws a curve of the AC impedance versus the frequency change; the host computer obtains the DC impedance at the set test temperature according to the collected DC signal and the DC voltage amplitude applied to the test specimen, and draws a curve of the DC impedance versus the time change; Step 4: Take both the obtained curve of the AC impedance versus the frequency change and the curve of the DC impedance versus the time change as test curves, perform a Fourier transform on the test curves to generate a transformed impedance curve, calculate the mean value of the transformed impedance modulus in the transformed impedance curve and the original impedance modulus in the test curve and perform a normalization process to obtain a curve of the AC-side normalized impedance modulus versus the frequency change and a curve of the DC-side normalized impedance module versus the time change; Step 5: According to the curve of the AC-side normalized impedance modulus versus the frequency change and the curve of the DC-side normalized impedance module versus the time change, and in combination with the following formula, obtain the DP value: Wherein, DP is the degree of polymerization of oil-impregnated paperboard; A(T) is the fitting parameter on the AC side, and B(T) is the fitting parameter on the DC side, which are related to the test temperature T; Z ac (f n ) is the normalized impedance modulus value on the AC side at the nth test frequency point f n , n = 1, 2,,,, N, and N is the number of test frequency points. Z ac (f n ) is in ohms; Z dc (t m ) is the normalized impedance modulus value on the DC side at the mth test time point t m , m = 1, 2,..., M, and M is the number of test time points. Z dc (t m ) is in ohms; The AC-side fitting parameter A(T) and the DC-side fitting parameter B(T) are obtained through a preliminary experiment. The specific process includes the following steps: Step 1: Set a test temperature range and interval; prepare P sets of test specimens (8) with known DP values; Step 2: Within the set test temperature range, extract Q test temperatures according to the set interval, and perform a time-frequency domain response synchronous test experiment on the P sets of test specimens (8) with DP values at each test temperature one by one. The implementation steps of the time-frequency domain response synchronous test experiment are Steps 1 to 5; an overdetermined system of equations is obtained through the experiment: In the overdetermined system of equations, DP1, DP2... DP P are the DP values of P groups of samples to be tested (8), and the P DP values are not equal; A(T i ) and B(T i ) are the AC parameters and DC parameters at the i-th test temperature T i within the test range, where i = 1, 2, …, Q; A(T i ) and B(T i ) are obtained by automatically optimizing through the above overdetermined equations; Step 4: Fit the AC parameter test data A(T1), A(T2),..., A(T Q ) at Q different test temperatures into the AC side fitting parameter A(T): A(T) = 3.45e 0.025T / 4.35+0.448 Step 5. Fit the DC parameter test data B(T1), B(T2),..., B(T Q ) at Q different test temperatures into the DC side fitting parameter B(T): B(T) = 0.255e 0.01T+0.468 .

2. The aging state evaluation method for synchronous test of time-frequency domain response of oil-paper insulation according to claim 1, characterized in that The test temperature T range is from 10 degrees Celsius to 90 degrees Celsius, and the test temperature T interval is the third set value.

3. The aging state evaluation method for the time-frequency domain response synchronous test of oil-paper insulation according to claim 1, characterized in that Drawing the curve of the DC impedance versus the time change includes: dividing the time spent on testing the test specimen into m test time periods, and the equivalent impedance value of each test time period is the average of the impedance at the starting test point and the impedance at the ending test point in this test time period. Draw the curve of the DC impedance versus the time change according to the impedance values of each test time period and the time spent on testing the test specimen.

Images