Cable local defect degree evaluation method based on impedance spectroscopy

Abstract

The invention provides a cable local defect degree evaluation method based on an impedance spectroscopy method. The cable local defect degree evaluation method comprises the following steps: acquiring an attenuation coefficient alpha (f) and a phase shift coefficient beta (f) of a healthy cable with the same model as a to-be-detected cable; obtaining an impedance spectrum when the tail end of the to-be-detected cable is open-circuited and short- Drawing a positioning curve F (L); calculating a characteristic parameter # imgabs0 # for representing the defect degree, and carrying out normalization processing on peak values of all positions of a positioning curve; calculating a defect degree characterization parameter delta; and comparing the table defect degree characterization quantity. According to the evaluation method provided by the invention, calculation and comparison of the defect degrees of various types of cables can be realized, a unified defect degree evaluation method is formulated for various types of cables, the application range is relatively wide, the anti-interference capability is relatively high, and a reliable basis is provided for cable defect state evaluation based on impedance spectroscopy.

Description

Technical Field

[0001] The present invention relates to the technical field of power operation and maintenance, and particularly to a method for evaluating the degree of local defects of a cable based on the impedance spectroscopy method. Background Technique

[0002] Under the action of various internal and external stresses, the cable is prone to different types of local defects. If not repaired, it will further cause the defects to develop into faults, affecting the power supply reliability of the power system. In the actual operation and inspection process of the cable, the quantitative evaluation and calculation of the severity of the cable defects can timely discover potential problems, take targeted measures to repair the defects, delay the cable aging process, thereby extending the service life of the cable, and further optimizing the allocation of maintenance resources, improving the efficiency and effect of power operation and maintenance.

[0003] In the existing evaluation methods for the severity of cable defects, by measuring the frequency-domain node impedance data of the cable under test, calculating the attenuation coefficient and phase shift coefficient of the cable under test, further calculating the frequency-domain distribution average value of the attenuation coefficient and the cable wave velocity, then performing a positioning conversion process on the cable frequency-domain dielectric impedance data to obtain the corresponding original cable positioning curve, and determining the specified defect peak and the defect peak coordinates corresponding to the specified defect peak in the original cable positioning curve. Based on the frequency-domain distribution average value of the attenuation coefficient and the defect peak coordinates, calculating the attenuation law function corresponding to the original cable positioning curve, and compensating the attenuation of the original positioning curve according to the attenuation law corresponding to the attenuation law function. Determining the cable end positioning peak in the compensated positioning curve, and normalizing the compensated positioning curve based on the cable end positioning peak, so as to determine the degree of cable defects of the cable under test through the amplitude of the processed defect peak.

[0004] However, in the above-mentioned existing solutions, although the degree of cable defects can be judged, the judgment of the degree of cable defects is limited to the cable under test itself at present, and it is impossible to achieve a unified judgment of the degree of various cable defects, and the applicable range is relatively narrow. Moreover, the existing technology can only qualitatively compare the degree of defects of the cable under test, and there is no unified calculation and evaluation method for the degree of defects, which limits its combination with the intelligent data system. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a method for evaluating the degree of local defects of a cable based on the impedance spectroscopy method, so as to solve the problems raised in the above background technology. The present invention can realize the calculation and comparison of the degree of defects of various types of cables, formulate a unified evaluation method for the degree of defects of various types of cables, has a wide applicable range and strong anti-interference ability, and provides a new method and idea for the evaluation of the cable defect state based on the impedance spectrum.

[0006] To achieve the above object, the present invention is implemented by the following technical solutions: A method for evaluating the degree of local defects of a cable based on impedance spectroscopy, comprising the following steps:

[0007] Step 1: By measuring the impedance spectra at the open - circuit and short - circuit ends of a healthy cable of the same model as the cable to be measured, obtain the attenuation coefficient α(f) and phase - shift coefficient β(f) of the healthy cable within the frequency range;

[0008] Step 2: Measure the impedance spectra at the open - circuit and short - circuit ends of the cable to be measured through an impedance analyzer. The impedance spectra include impedance amplitude spectra and impedance phase spectra;

[0009] Step 3: Use the orthogonal integral algorithm to draw the positioning curve F(L) of the cable to be measured;

[0010] Step 4: Construct new impedance phase spectra of the healthy cable and the cable to be measured, and calculate the mathematical fitting formula of the envelope line on the new impedance phase spectra changing with frequency. Calculate the characteristic parameters representing the degree of defects according to the mathematical fitting formula

[0011] Step 5: Take the peak value F(L end ) at the end in the positioning curve of the cable to be measured as the reference peak, and normalize the remaining characteristic peaks F(L j );

[0012] Step 6: Calculate the defect - degree characterization parameter δ;

[0013] Step 7: Compare the magnitudes of the defect - degree characterization quantities. A lower value corresponds to a higher degree of defects.

[0014] Further, in Step 3, construct an orthogonal function system {sin(2βnd), n = 1, 2, 3...}, use the sampling distance d as the calculation step size, let the position variable nd in the orthogonal kernel function start from 0 and traverse in space with a step size of d, calculate the discrete integral values of the orthogonal kernel function and the impedance phase spectrum of the cable to be measured at different nd to obtain the positioning curve F(L). The peak value of the positioning curve is used to reflect the quantity, position, and range information of local defects.

[0015] Further, the process of obtaining the attenuation coefficient α(f) and phase - shift coefficient β(f) of the healthy cable in Step 1 includes: According to the transmission - line theory, the open - circuit impedance and short - circuit impedance at the end of the healthy cable are:

[0016]

[0017] Among them, Z op is the open - circuit impedance, Z sc is the short - circuit impedance. From formulas (1) - (2), it can be seen that the propagation coefficient γ of the cable is

[0018]

[0019] α = Re(γ) (4)

[0020] β = Im(γ) (5)

[0021] In the actual application process, only the open - circuit impedance Z at the head end of a healthy cable with any known length is tested op and the short - circuit impedance Z sc , and then the attenuation coefficient α(f) and phase - shift coefficient β(f) of the cable can be calculated by using formulas (3) - (5).

[0022] Furthermore, the method for measuring the impedance spectrum in the second step includes: using a precision impedance analyzer, connecting the cable measurement end to the impedance analyzer port when testing the cable impedance spectrum, controlling the other end of the cable to be open - circuited, short - circuited or connected to a load. For a low - voltage multi - phase cable, two of its phases are used to form a loop for measurement; for a coaxial cable, the cable core wire and the metal shielding layer are used to form a loop for measurement.

[0023] Furthermore, the orthogonal integration algorithm in the third step includes:

[0024]

[0025] The selected orthogonal kernel function is {sin(2βnd), n = 1, 2, 3...}, where the position variable nd in the orthogonal kernel function is a discrete processing of the full length l of the cable to be measured. During the discretization process, d is used as the discretization step size, and n is used as the index value for loop calculation. Discretization in space is achieved by taking 0, d, 2d,..., nd,..., l in space. The algorithm operation process is shown in formula (7):

[0026]

[0027] where F(nd) is the amplitude characteristic of the positioning curve, which is discrete data; M is the number of frequency points of the actually measured impedance phase spectrum. Ideally, M → ∞, but in reality, due to the bandwidth and sampling limitations of the measurement instrument, it is a finite value. For example, if the impedance analyzer measures the impedance at 1600 equally spaced frequency points within the set frequency bandwidth, then M is 1600 at this time; arg(Z in ) i is the impedance phase value at the i - th measured frequency point; β i is the phase - shift coefficient value of the healthy cable at the i - th frequency point; n is the algorithm traversal variable, which is a natural number starting from 0 to the final value N; d is the calculation step size.

[0028] Further, calculate the discrete integral values of the impedance phase spectrum and the orthogonal kernel functions {sin(2βnd), n = 1, 2, 3...} at different nds according to Equation (7). After calculation, the positioning curves at different positions nd can be obtained, and taking the absolute value thereof gives the amplitude characteristics.

[0029] Further, the calculation method of the characteristic parameter characterizing the defect degree in Step 4 includes: respectively performing data fitting on the variation of the upper envelope line of the new phase impedance spectra of the healthy cable and the cable to be measured with respect to the frequency f:

[0030] y h (f) = M h (f) (8)

[0031] y b (f) = M b (f) (9)

[0032] In the above formula, y h (f) and y b (f) are respectively the numerical values of the upper envelope lines of the new phase impedance spectra of the healthy cable and the cable to be measured, which are quantities varying with the frequency; M h and M b represent the mathematical relationships between y h (f), y b (f) and the frequency f.

[0033] Further, the characteristic parameter characterizing the defect degree is calculated as:

[0034]

[0035] In the formula, α(f) is the attenuation coefficient of the healthy cable obtained in Step 1, l is the length of the cable to be measured, is the overall difference between the cable to be measured and the healthy cable, characterizing the overall defect degree of the cable to be measured. The higher the frequency, the more accurate the calculation of the characteristic parameter of the defect degree. Take the

[0036] Further, the process of normalizing the peaks at each position of the positioning curve in Step 5 includes: obtaining the characteristic parameter of the defect degree of the cable to be measured, the cable defect positioning curve F(L), and the local defect position of the cable. The peaks at each position of the positioning curve can be normalized. Taking the peak F(L end ) at the end of the positioning curve of the cable to be measured as the reference peak, normalize the remaining characteristic peaks F(L j ), and the calculation is as shown in Equation (11):

[0037]

[0038] where C j is the attenuation compensation coefficient of the peak value of the positioning curve after normalization processing at the j-th defect, and through C j the ratio of the defect degree of each defect of the cable to be measured can be obtained. Combining with the quantitative degree values of the defects at each location of the cable to be measured can be obtained. l is the total length of the cable to be measured, and l j is the defect position at the j-th location from the measurement end of the cable to be measured. F(l j ) and F(l) are the amplitudes of the positioning curves at the positions of l j and l respectively. M is the number of frequency points of the actually measured impedance phase spectrum. α i and β i are the attenuation coefficient value and the phase shift coefficient value of the healthy cable of the same type as the cable to be measured at the i-th frequency point respectively.

[0039] Furthermore, when there are n defects in the cable, the attenuation compensation coefficients C 1 , C 2 ... C n of the n defects are calculated respectively. Then, the defect degree characterization quantity δ j of the j-th defect is calculated as follows:

[0040]

[0041] where Δl j is the defect length of each defect and can be obtained in the calculation of step three.

[0042] Advantages of the present invention:

[0043] 1. This patent proposes a method for evaluating local defects of cables based on the impedance spectrum method. By analyzing the mathematical model of the cable, characteristic parameters representing the overall differences between the cable to be measured and the healthy cable are derived from the mathematical model. Further analyzing the positioning curve and combining the number of defects of the cable to be measured, the quantitative values of the defect degrees at each location of the cable to be measured can be calculated. This method realizes the effective discrimination of the defect degrees of different types of cables, provides a unified calculation method for the degree judgment of local defects in various types of cables, breaks through the dependence of the existing method on the specific type and operating conditions of the measured object, and has significant system universality. By constructing a defect quantitative evaluation model, the quantitative analysis of defect parameters and the accurate judgment of the severity are realized, providing a unified characterization benchmark for the quantitative diagnosis of the insulation deterioration degree of cables.

[0044] 2. The method for evaluating the degree of local defects in a cable based on impedance spectroscopy can accurately locate the defect position in the cable and, on this basis, synchronously achieve an accurate diagnosis of the severity of the defect without applying any high voltage during the entire measurement process, ensuring no damage to the cable itself. Using its diagnostic technology, the degree of defects inside the cable can be accurately quantified and evaluated. According to the diagnostic results provided by this method, maintenance personnel can arrange maintenance operations more scientifically and reasonably, giving priority to repairing those parts with deeper defect degrees and greater potential risks, thereby effectively preventing the occurrence of cable faults, significantly reducing power outages caused by cable faults, and having a positive significance for improving the stability of the power system, ensuring power supply safety, and optimizing the allocation of maintenance resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a flowchart of a method for evaluating the degree of local defects in a cable based on impedance spectroscopy according to the present invention;

[0046] Figure 2 is the impedance spectrum measurement circuit of the present invention;

[0047] Figure 3 is the positioning curve graph in the embodiment of the present invention;

[0048] Figure 4 is the impedance phase spectrum and upper envelope graph of a cable with two defects in the embodiment of the present invention;

[0049] Figure 5 is the schematic diagram of the structure of an experimental cable with two different degrees of defects in the embodiment of the present invention;

[0050] Figure 6 is the measured impedance spectrum of a cable with two different degrees of defects in the embodiment of the present invention;

[0051] Figure 7 is the attenuation coefficient and phase shift coefficient of SYV75-5 cable in the embodiment of the present invention;

[0052] Figure 8 is the positioning curve graph in the embodiment of the present invention; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0053] To make the technical means, creative features, achieved purposes, and functions of the present invention easy to understand, the present invention will be further described below in conjunction with specific embodiments.

[0054] Please refer to Figures 1 to 8, the present invention provides the following technical solution: A method for evaluating the degree of local defects of a cable based on impedance spectroscopy. By performing refined analysis on the input impedance of the cable, it is clear that when the degree of cable defects is different, the degree of change in its distribution parameters (R, L, C, G) is also different, which will cause different degrees of change in the propagation coefficient of the cable, further resulting in the deviation of the phase characteristic of the input impedance from the design value. According to the frequency-varying characteristics of the cable input impedance, analyzing the influence mechanism of the change in the cable propagation coefficient on the input impedance spectrum when there are local defects in the cable can effectively extract characteristic quantities related to the degree of cable defects, providing a reliable basis for evaluating the degree of cable defects. At the same time, through the analysis of the characteristic quantities, a quantitative evaluation method for the degree of cable defects is proposed. This method includes the following steps:

[0055] S1. By measuring the impedance spectra at the open and short circuits of the end of a healthy cable of the same model as the cable to be tested, obtain the attenuation coefficient α(f) and phase shift coefficient β(f) of the healthy cable within the frequency range;

[0056] S2. Measure the impedance spectra at the open and short circuits of the end of the cable to be tested through an impedance analyzer. The impedance spectra include impedance amplitude spectra and impedance phase spectra;

[0057] S3. Use the orthogonal integral algorithm to draw the positioning curve F(L);

[0058] S4. Construct new impedance phase spectra of the healthy cable and the cable to be tested, and calculate the mathematical fitting formula of the envelope line changing with frequency on the new impedance phase spectra. Calculate the characteristic parameter representing the degree of defect according to the mathematical fitting formula

[0059] S5. Take the peak value F(L end ) at the end in the positioning curve of the cable to be tested as the reference peak, and normalize the remaining characteristic peaks F(L j );

[0060] S6. Calculate the characteristic parameter δ representing the degree of defect;

[0061] S7. Compare the magnitudes of the characteristic quantities representing the degree of defect. A lower value corresponds to a higher degree of defect.

[0062] In this embodiment, the method for obtaining the attenuation coefficient α(f) and phase shift coefficient β(f) of the healthy cable in S1 is as follows:

[0063] According to the transmission line theory, the open-circuit impedance and short-circuit impedance at the end of the healthy cable are:

[0064]

[0065]

[0066] Among them, Zop is the open - circuit impedance, Z sc is the short - circuit impedance. As can be seen from formulas (1) to (2), the propagation coefficient γ of the cable is

[0067]

[0068] α = Re(γ) (4)

[0069] β = Im(γ) (5)

[0070] In practical applications, only the open - circuit impedance Z op and the short - circuit impedance Z sc at the head end of a healthy cable with any known length need to be measured, and then the attenuation coefficient α(f) and phase - shift coefficient β(f) of the cable can be calculated using formulas (3) to (5).

[0071] In this embodiment, the method for measuring the impedance spectrum in step S2 is as follows:

[0072] The measurement of the cable impedance spectrum uses a precision impedance analyzer. When testing the cable impedance spectrum, connect the cable measurement end to the impedance analyzer port, and control the other end of the cable to be open - circuited, short - circuited or connected to a load. For low - voltage multi - phase cables, measure with two of the cable phases forming a loop; for coaxial cables, measure with the cable core wire and the metal shielding layer forming a loop. The schematic diagram of the measurement wiring is as Figure 2 shown.

[0073] The input impedance spectrum of the cable to be measured is obtained by the above - mentioned method.

[0074] In this embodiment, the orthogonal integration algorithm in step S3 is specifically as follows:

[0075] The basic principle of orthogonality is: for a function system {f i (x), i = 1, 2…}, if it satisfies the orthogonality shown in formula (6), that is, when the integral value of the product of two identical functions is not equal to 0 while the integral value of the product of two different functions is always 0, then {f i (x), i = 1, 2…} is called an orthogonal function system. As can also be seen from formula (6), the orthogonality of the orthogonal function system is essentially a kind of screening property.

[0076]

[0077] The selected orthogonal kernel function is {sin(2βnd), n = 1, 2, 3...}. The position variable nd in the orthogonal kernel function is a discrete processing of the full length l of the cable to be measured. In the discretization process, d is used as the discretization step size, and n is used as the index value for loop calculation. Discretization in space is achieved by taking 0, d, 2d,..., nd,..., l in space. The algorithm operation process is shown in Equation (7). Calculate the discrete integral value of the impedance phase spectrum and the orthogonal kernel function {sin(2βnd), n = 1, 2, 3...} at different nd. After calculation, the positioning curve at different positions nd can be obtained, and taking its absolute value gives the amplitude characteristic.

[0078]

[0079] Among them, F(nd) is the amplitude characteristic of the positioning curve, which is discrete data; M is the number of frequency points of the actually measured impedance phase spectrum. Ideally, M → ∞, but in reality, it is a finite value due to the bandwidth and sampling limitations of the measuring instrument. For example, if an impedance analyzer measures the impedance of 1600 equally spaced frequency points within the set frequency bandwidth, then M is 1600 at this time; arg(Z in ) i is the impedance phase value at the i-th measured frequency point; β i is the healthy cable phase shift coefficient value at the i-th frequency point; n is the algorithm traversal variable, which is a natural number starting from 0 to the final value N; d is the calculation step size.

[0080] The calculated positioning function is plotted as a positioning curve as Figure 3 shown:

[0081] According to Figure 3 it can be seen that the abscissa value of the positioning curve is the distance from the cable head end, and the ordinate is the amplitude characteristic value of the positioning curve. This cable has 3 peaks. The first two peaks correspond to two defects in the cable, and the third peak corresponds to the cable end; it can be seen from the figure that the distances of the two defects from the head end are 24.77 meters and 59.82 meters respectively, and the lengths of the two defects are Δl 1 = 25.39 - 24.77 = 0.62 meters and Δl 2 = 60.46 - 59.82 = 0.6 meters.

[0082] In this embodiment, the calculation method of the characteristic parameter representing the defect degree in step S4 is:

[0083] Taking the impedance spectrum of the cable with two defects as an example, the upper envelope of the newly extracted impedance phase spectrum is as Figure 4 shown:

[0084] Data fitting is performed on the changes of the upper envelopes of the newly extracted impedance phase spectra of the healthy cable and the cable to be measured with respect to the frequency f:

[0085] y h (f) = M h (f) (8)

[0086] y b (f) = M b (f) (9)

[0087] In the above formula, y h (f), y b (f) are respectively the values of the upper envelopes of the new impedance phase spectra of the healthy cable and the cable under test, which are quantities varying with frequency; M h and M b represent the mathematical relationships between y h (f), y b (f) and the frequency f

[0088] Characteristic parameter for characterizing the degree of defect The calculation method is as follows:

[0089]

[0090] In the formula, α(f) is the attenuation coefficient of the healthy cable obtained in S1, l is the length of the cable under test, is the overall difference between the cable under test and the healthy cable, characterizing the overall defect degree of this cable under test. Most of the parameters in formula (10) are parameters varying with frequency. The higher the frequency, the more accurate the calculation of the characteristic parameter for the degree of defect. Take the highest frequency of the measurement In this embodiment, in step S5, the idea of normalizing the peaks at each location of the positioning curve is as follows:

[0091] Obtain the characteristic parameter of the cable defect degree The cable defect positioning curve F(L) and the local defect position of the cable. The peaks at each location of the positioning curve can be normalized, as shown in formula (11):

[0092]

[0093] In the formula, l is the total length of the cable under test, l j is the defect position at the jth location from the measurement end of the cable under test, F(l j ) and F(l) are respectively the amplitudes of the positioning curve at the positions of l j and l, M is the number of frequency points of the actually measured impedance phase spectrum, α i and β i are respectively the attenuation coefficient value and the phase shift coefficient value of the healthy cable of the same type as the cable under test at the ith frequency point

[0094] In this embodiment, in step S6, the idea of calculating the characterization parameter of the defect degree is as follows: when there are n defects in the cable, the attenuation compensation coefficients C 1 , C 2 ... C n of the n defects are calculated respectively. Then, the defect degree characterization quantity δ n of the j-th defect is calculated as follows:

[0095]

[0096] In the formula, Δl j is the defect length of each defect, which can be obtained in the calculation of S3.

[0097] This embodiment also sets up an experiment to verify the effectiveness of the above method. In this embodiment, a radio frequency coaxial cable is used as the experimental object, and two local defects with different local defect degrees are set. In the experiment, a radio frequency coaxial cable with a length of 0.1 m, a characteristic impedance of 25 Ω, and a model number of SFF25-1 and a radio frequency coaxial cable with a characteristic impedance of 50 Ω and a model number of SYV50-3 are used to simulate the local defect segments, while a radio frequency coaxial cable with a characteristic impedance of 75 Ω and a model number of SYV75-5 is used to simulate the healthy cable segment. The entire spliced cable is constructed as Figure 5 shown, and the measured impedance spectrum is as Figure 6 shown. Figure 6 The upper curve is the impedance amplitude spectrum, and the lower curve is the impedance phase spectrum.

[0098] According to step S1, the open-circuit and short-circuit impedance spectra at the end of the healthy cable of the same model are measured, and the attenuation coefficient and phase shift coefficient of the SYV75-5 cable in [1 MHz, 120 MHz] are obtained as Figure 7 shown.

[0099] According to step S3, by processing the impedance phase spectrum, a positioning curve is obtained as Figure 8 shown.

[0100] As can be seen from Figure 8 , there are three peaks in the positioning curve F(L), corresponding to the defect point 1, the defect point 2, and the cable end, corresponding to the positions of 34.76 m, 69.82 m, and 100.11 m. The distances of the two defects from the head end are 34.76 meters and 69.82 meters respectively, and the lengths of the two defects are Δl 1 = 35.38 - 34.76 = 0.62 meters and Δl 2 = 60.46 - 59.82 = 0.61 meters, and the total length of the cable is 100.11 meters.

[0101] According to step S4, the calculation process is as follows:

[0102] Calculate the envelope calculation expression for the healthy cable:

[0103]

[0104] Calculate the envelope calculation expression for the cable to be tested:

[0105]

[0106] Calculate the characteristic quantity:

[0107]

[0108] It is the value of the overall difference between the cable to be tested and the healthy cable. The larger this value is, the more serious the overall defect of the cable to be tested is.

[0109] Normalize the peak values at each location of the positioning curve according to step S5:

[0110] Calculation of the first defect:

[0111]

[0112] Calculation of the second defect:

[0113]

[0114] According to step S6, calculate the result of the characteristic quantity representing the defect degree:

[0115]

[0116] According to step S7, the calculated characteristic value of defect point 1 is 2.3556, and the calculated characteristic value of defect point 2 is 2.4226. Obviously, from the experimental setup, it can be found that the defect degree of the first defect is higher, and the calculation result of the calculated characteristic value also shows that defect 1 has a higher defect degree, which is consistent with the experimental expectation and verifies the effectiveness of the method provided by the present invention.

[0117] The above shows and describes the basic principles, main features and advantages of the present invention. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0118] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for evaluating the degree of local defects in cables based on impedance spectroscopy, characterized in that: The following steps are involved: Step 1: By measuring the impedance spectrum of a healthy cable of the same model as the cable to be tested when the end is open-circuited and short-circuited, the attenuation coefficient α(f) and phase shift coefficient β(f) of the healthy cable within the frequency range are obtained; Step 2: Using an impedance analyzer to measure the impedance spectrum of the cable under test when the end is open-circuited or short-circuited, the impedance spectrum includes an impedance amplitude spectrum and an impedance phase spectrum; Step 3: Draw the positioning curve F(L) using the orthogonal integration algorithm; Step 4: construct new impedance phase spectra of healthy cables and cables to be tested, and calculate the mathematical fitting formula of the envelope of the new impedance phase spectrum with frequency. According to the mathematical fitting formula, the characteristic parameters representing the degree of defects are calculated. Step 5: Use the peak value F(L) at the end of the cable positioning curve to be tested. end ) as the reference peak, and the remaining characteristic peaks F(L j ) is normalized; Step 6: Calculate the defect degree characterization parameter δ; Step 7: Compare the defect levels in the table. Lower values ​​correspond to higher defect levels.

2. According to claim 1, a method for evaluating the degree of local defects of a cable based on impedance spectroscopy is characterized in that: In step three, an orthogonal function system {sin(2βnd), n=1,2,3…} is constructed, and the sampling distance d is used as the calculation step size. The position variable nd in the orthogonal kernel function is traversed in space starting from 0 with a step size d, and the discrete integral value of the orthogonal kernel function and the impedance phase spectrum of the cable to be tested under different nd is calculated to obtain a positioning curve F(L). The peak value of the positioning curve is used to reflect the number, position, and range information of local defects.

3. A cable local defect degree assessment method based on impedance spectroscopy according to claim 1, characterized in that: The process of obtaining the attenuation coefficient α(f) and the phase shift coefficient β(f) of the healthy cable in step 1 includes: According to the transmission line theory, the end open circuit impedance and short circuit impedance of the healthy cable are: Where Z op is the open circuit impedance, Z sc is the short-circuit impedance. From formulas (1) to (2), we can see that the propagation coefficient γ of the cable is: α=Re(γ)(4) β=Im(γ)(5) In actual application, only the open circuit impedance Z at the head end of a healthy cable of any known length is tested. op and short-circuit impedance Z sc , and then use formulas (3) to (5) to calculate the attenuation coefficient α(f) and phase shift coefficient β(f) of the cable.

4. A method for evaluating the degree of local defects of cables based on impedance spectroscopy according to claim 2, characterized in that: The method for measuring the impedance spectrum in step 2 includes: using a precision impedance analyzer, connecting the cable measurement end to the impedance analyzer port when testing the cable impedance spectrum, controlling the other end of the cable to be open-circuited, short-circuited or connected to a load, and for low-voltage multi-phase cables, measuring with two phases of the cable forming a loop; for coaxial cables, measuring with the cable core wire and the metal shielding layer forming a loop.

5. A cable local defect degree assessment method based on impedance spectroscopy according to claim 1, characterized in that: The orthogonal integration algorithm in step 3 includes: The selected orthogonal kernel function is {sin(2βnd),n=1,2,3…}. The position variable nd in the orthogonal kernel function is a discrete processing of the total length l of the cable to be tested. In the discretization process, d is used as the discrete step length, n is used as the index value of the loop calculation, and 0, d, 2d,…, nd,…, l is taken in space to realize spatial discretization. The algorithm operation process is shown in formula (7): Where, F(nd) is the amplitude characteristic of the positioning curve; M is the number of frequency points of the impedance phase spectrum actually measured. Ideally, M→∞, but in reality it is limited to a finite value by the bandwidth and sampling of the measuring instrument. For example, if the impedance analyzer measures the impedance of 1600 frequency points at equal intervals within the set frequency bandwidth, then M is 1600; arg(Z in ) i is the impedance phase value at the measured i-th frequency point; β i is the phase shift coefficient value of the healthy cable at the i-th frequency point; n is the algorithm traversal variable, which is a natural number starting from 0 to the final value N; d is the calculation step size.

6. A method for evaluating the degree of local defects of cables based on impedance spectroscopy according to claim 5, characterized in that: According to formula (7), the discrete integral value of the impedance phase spectrum and the orthogonal kernel function {sin(2βnd),n=1,2,3...} under different nd is calculated. After calculation, the positioning curve under different positions nd can be obtained, and its absolute value is the amplitude characteristic.

7. A method for evaluating the degree of local defects of cables based on impedance spectroscopy according to claim 1, characterized in that: The calculation method of the characteristic parameter characterizing the degree of defect of the cable to be tested in step 4 includes: performing data fitting on the upper envelope of the new impedance phase spectrum of the healthy cable and the cable to be tested respectively: y h (f)=M h (f)(8) y b (f)=M b (f)(9) In the above formula, y h (f), y b (f) are the envelope values ​​of the new impedance phase spectrum of the healthy cable and the cable under test, respectively, and are the quantities that change with frequency; M h and M b Represents y h (f), y b (f) and the mathematical relationship of frequency f.

8. A method for evaluating the degree of local defects of cables based on impedance spectroscopy according to claim 7, characterized in that: Characteristic parameters that characterize the degree of defects The calculation method is: Where α(f) is the attenuation coefficient of the healthy cable obtained in step 1, l is the length of the cable to be tested, The overall difference between the tested cable and the healthy cable represents the overall defect degree of the tested cable. The higher the frequency, the more accurate the calculation of the characteristic parameters of the defect degree. The highest frequency of the measurement is taken.

9. A method for evaluating the degree of local defects of cables based on impedance spectroscopy according to claim 7, characterized in that: The process of normalizing the peak values ​​at each location of the positioning curve in step 5 includes: obtaining characteristic parameters of the degree of defects of the cable to be tested through steps 3 and 4 The cable defect location curve F(L) and the local defect position of the cable can be used to calculate the attenuation compensation coefficient of different defects, as shown in formula (11): Where C j is the attenuation compensation coefficient of the peak value of the positioning curve after normalization of the j-th defect, which is calculated by C j The ratio of the defect levels of each defect in the tested cable can be obtained, combined with Then the quantitative degree of defects at each location of the tested cable can be obtained, where l is the total length of the tested cable, and l j is the defect position at the jth distance from the measuring end of the cable to be tested, F(l j ) and F(l) are l j and the amplitude of the positioning curve at position l, M is the actual measured impedance phase spectrum frequency point number, α i and β i are respectively the attenuation coefficient value and phase shift coefficient value of a healthy cable of the same model as the cable to be tested at the i-th frequency point.

10. A method for evaluating the degree of local defects of cables based on impedance spectroscopy according to claim 9, characterized in that: When there are n defects in the cable, calculate the attenuation compensation coefficients C1, C2, ..., C of the defects at j respectively. n , then the defect level characterization value δ of the jth defect j The calculation is as follows: Where Δl j is the defect length of each defect, which can be obtained from the calculation in step 3.

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