A method for assessing the degree of local defects in cables based on impedance spectroscopy.

Abstract

The application provides a cable partial defect degree evaluation method based on impedance spectrum method, comprising the following steps: obtaining the attenuation coefficient alpha (f) and the phase shift coefficient beta (f) of a healthy cable of the same model as the cable to be measured; obtaining the impedance spectrum of the cable to be measured when the end of the cable is open and short; drawing a positioning curve F (L); calculating the peak value of the positioning curve of the characteristic parameter representing the defect degree; calculating the defect degree characteristic parameter delta; and comparing the size of the defect degree characteristic parameter. The evaluation method provided by the application can realize the calculation and comparison of the defect degree of various types of cables, and can provide a unified defect degree evaluation method for various types of cables, has a wide application range, has strong anti-interference ability, and provides a reliable basis for the cable defect state evaluation based on the impedance spectrum.

Description

Technical Field

[0001] This invention relates to the field of power operation and maintenance technology, specifically to a method for assessing the degree of local defects in cables based on impedance spectroscopy. Background Technology

[0002] Cables are prone to developing various types of localized defects under different internal and external stresses. If left unchecked, these defects can escalate into malfunctions, impacting the reliability of the power supply system. During actual cable operation and maintenance, quantitative assessment of the severity of defects can promptly identify potential problems, enabling targeted repairs, slowing down cable aging, extending cable lifespan, optimizing maintenance resource allocation, and improving the efficiency and effectiveness of power system operation and maintenance.

[0003] In existing methods for assessing the severity of cable defects, the attenuation coefficient and phase shift coefficient of the cable under test are calculated by measuring its frequency domain node impedance data. The average frequency domain distribution of the attenuation coefficient and cable wave velocity is then calculated. Next, the cable's frequency domain dielectric impedance data is processed for location transformation to obtain the corresponding original cable location curve. A specified defect peak and its corresponding coordinates are then identified within this curve. Based on the average frequency domain distribution of the attenuation coefficient and the defect peak coordinates, an attenuation law function corresponding to the original cable location curve is calculated. Attenuation compensation is then applied to the original location curve based on the attenuation law function. The cable end location peak is then identified within the compensated location curve. Finally, the compensated location curve is normalized based on this cable end location peak, and the degree of cable defect is determined by the amplitude of the processed defect peak.

[0004] However, while the existing solutions mentioned above can determine the degree of cable defects, this assessment is limited to the cable being tested at that moment and cannot achieve a unified assessment of the degree of defects in various cables, thus limiting their applicability. Furthermore, current technologies can only qualitatively compare the degree of defects in the tested cables and lack a unified method for calculating and evaluating defect levels, restricting their application and integration with intelligent data systems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for assessing the degree of local defects in cables based on impedance spectroscopy, thereby solving the problems mentioned in the background. This invention can calculate and compare the degree of defects in various types of cables, establish a unified method for assessing the degree of defects in various types of cables, has a wide range of applications, and strong anti-interference capabilities, providing a new method and approach for assessing the state of cable defects based on impedance spectroscopy.

[0006] To achieve the above objectives, the present invention provides a method for assessing the degree of local defects in cables based on impedance spectroscopy, comprising the following steps:

[0007] Step 1: By measuring the impedance spectrum of a healthy cable of the same model as the cable under test when the end is open-circuited and short-circuited, the attenuation coefficient α(f) and phase shift coefficient β(f) of the healthy cable in the frequency range are obtained.

[0008] Step 2: Measure the impedance spectrum of the cable under test when it is open-circuited and short-circuited using an impedance analyzer. The impedance spectrum includes the impedance amplitude spectrum and the impedance phase spectrum.

[0009] Step 3: Use the orthogonal integration algorithm to plot the positioning curve F(L) of the cable under test;

[0010] Step 4: Construct new impedance phase spectra for the healthy cable and the cable under test, and calculate the mathematical fitting formula for the envelope of the new impedance phase spectrum as a function of frequency. Based on the mathematical fitting formula, calculate the characteristic parameters characterizing the degree of defect.

[0011] Step 5: Use the peak value F(L) at the end of the positioning curve of the cable under test. end Using the baseline peak as the reference peak, the remaining characteristic peaks F(L) are... j Normalization was performed.

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

[0013] Step 7: Compare the magnitudes of the defect severity indicators in the table; lower values ​​correspond to higher defect severity.

[0014] Furthermore, in step three, an orthogonal function system {sin(2βnd), n=1,2,3…} is constructed. 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. The discrete integral values ​​of the phase spectrum of the orthogonal kernel function and the impedance of the cable under test are calculated under different nd values ​​to obtain the positioning curve F(L). The peak value of the positioning curve is used to reflect the number, location, and range information of local defects.

[0015] Furthermore, the process of obtaining the attenuation coefficient α(f) and phase shift coefficient β(f) of the healthy cable in step one includes: From transmission line theory, the open-circuit impedance and short-circuit impedance at the end of the healthy cable are:

[0016]

[0017] Z op It is the open-circuit impedance, Z sc It is the short-circuit impedance. From formulas (1) to (2), we know that the propagation coefficient γ of the cable is...

[0018]

[0019] α=Re(γ) (4)

[0020] β=Im(γ) (5)

[0021] In practical applications, only the open-circuit impedance Z at the beginning of a healthy cable of any known length is tested. op and short-circuit impedance Z sc Then, the attenuation coefficient α(f) and phase shift coefficient β(f) of the cable can be calculated using formulas (3) to (5).

[0022] Furthermore, the method for measuring the impedance spectrum in step two includes: using a precision impedance analyzer, connecting the cable measurement end to the impedance analyzer port when testing the cable impedance spectrum, and controlling the other end of the cable to be open-circuited, short-circuited, or connected to a load. For low-voltage multiphase cables, the measurement is performed by forming a circuit with two phases of the cable; for coaxial cables, the measurement is performed by forming a circuit with the cable core and the metal shielding layer.

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

[0024]

[0025] 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 under test. In the discretization process, d is used as the discretization step size and n is used as the index value of the loop calculation. The spatial discretization is achieved by taking 0,d,2d,…,nd,…,l. The algorithm operation process is shown in Equation (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 actual measured impedance phase spectrum. Ideally, M→∞, but in reality, it is a finite value limited by the bandwidth and sampling of the measuring instrument. For example, if the impedance analyzer measures the impedance of 1600 equally spaced frequency points within the set frequency bandwidth, then M is 1600; arg(Z in ) i β represents the impedance phase value at the measured i-th frequency point; 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 and ending at the final value N; d is the calculation step size.

[0028] Furthermore, according to equation (7), the discrete integral values ​​of the impedance phase spectrum and the orthogonal kernel function {sin(2βnd), n=1,2,3…} under different nd are calculated. After calculation, the positioning curves under different nd positions can be obtained, and the absolute value is taken as the amplitude characteristic.

[0029] Furthermore, the calculation method for the characteristic parameters characterizing the degree of defect in step four includes: performing data fitting on the change of the upper envelope of the new phase impedance spectrum of the healthy cable and the cable under test with frequency f, respectively.

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

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

[0032] In the above formula, y h (f), y b (f) represents the values ​​of the upper envelope of the new phase impedance spectrum of the healthy cable and the cable under test, respectively, which are quantities that vary with frequency; M h and M b Indicates y h (f), y b The mathematical relationship between (f) and frequency f.

[0033] Furthermore, characteristic parameters characterizing the degree of defect. The calculation method is as follows:

[0034]

[0035] In the formula, α(f) is the attenuation coefficient of the healthy cable obtained in step one, and l is the length of the cable to be tested. This represents the overall difference between the cable under test and a healthy cable, characterizing the overall defect level of the cable under test. Higher frequencies provide more accurate calculations of the characteristic parameters for defect level; the highest frequency measured is used.

[0036] Furthermore, the process of normalizing the peak values ​​at various points on the positioning curve in step five includes: obtaining the characteristic parameters of the defect degree of the cable under test through steps three and four. The cable defect location curve F(L) and the location of local defects in the cable can be used to normalize the peak values ​​at various points on the location curve. The peak value F(L) at the end of the cable's location curve is then used as the reference. end Using the baseline peak as the reference peak, the remaining characteristic peaks F(L) are... j After normalization, the calculation is shown in equation (11):

[0037]

[0038] In the formula C j The attenuation compensation coefficient for the peak value of the localization curve after normalization of the defect at point j is determined by C. j It can obtain the ratio of the defect severity of each defect in the cable under test, combined with... This allows us to obtain the quantitative values ​​of defects at various points in the cable under test. l is the total length of the cable under test. j It is the defect location at the j-th distance from the measuring end of the cable under test, F(l) j F(l) and F(l) are respectively l j The amplitude of the positioning curve at position l, M is the number of frequency points of the actual measured impedance phase spectrum, and α i and β i These are the attenuation coefficient and phase shift coefficient values ​​of a healthy cable of the same type as the cable under test at the i-th frequency point.

[0039] Furthermore, when there are n defects in the cable, the attenuation compensation coefficients C1, C2...C1 for each of the n defects are calculated. n Then the defect severity characterization δ at the j-th defect location is... j The calculation is as follows:

[0040]

[0041] In the formula Δl j The length of each defect can be obtained in the calculation in step three.

[0042] The beneficial effects of this invention are:

[0043] 1. This patent proposes a method for assessing local defects in cables based on impedance spectroscopy. This method analyzes the mathematical model of the cable, deriving characteristic parameters representing the overall differences between the tested cable and healthy cables. Further analysis of the location curve, combined with the number of defects in the tested cable, allows for the calculation of quantitative values ​​for the degree of defects at various locations within the cable. This method effectively distinguishes the degree of defects in different types of cables, providing a unified calculation method for judging the degree of local defects in various types of cables. It overcomes the dependence of existing methods on specific types of tested objects and operating conditions, exhibiting significant system universality. By constructing a quantitative defect assessment model, it achieves quantitative analysis of defect parameters and accurate judgment of severity, providing a unified characterization benchmark for the quantitative diagnosis of cable insulation degradation.

[0044] 2. This impedance spectroscopy-based method for assessing the degree of local cable defects, without causing any damage to the cable itself and without applying any high voltage during the entire measurement process, can not only accurately locate the defect within the cable but also simultaneously and accurately diagnose the severity of the defect. Using its diagnostic technology, the degree of internal cable defects can be accurately quantified. Based on the diagnostic results provided by this method, maintenance personnel can more scientifically and rationally arrange maintenance work, prioritizing the repair of areas with deeper defects and higher potential risks. This effectively prevents cable faults and significantly reduces power outages caused by cable faults, which is of positive significance for improving the stability of the power system, ensuring power supply safety, and optimizing the allocation of maintenance resources. Attached Figure Description

[0045] Figure 1 This is a flowchart of a method for assessing the degree of local defects in cables based on impedance spectroscopy, according to the present invention.

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

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

[0048] Figure 4 The impedance phase spectrum and upper envelope diagram of a cable containing two defects in an embodiment of the present invention are shown.

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

[0050] Figure 6 These are the measured impedance spectra of two cables with different defect levels in this embodiment of the invention.

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

[0052] Figure 8 This is a positioning curve diagram from an embodiment of the present invention; Detailed Implementation

[0053] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0054] Please see Figures 1 to 8This invention provides the following technical solution: a method for assessing the degree of local defects in cables based on impedance spectroscopy. This method, through refined analysis of the cable's input impedance, clarifies that the degree of change in the distributed parameters (R, L, C, G) varies with the degree of cable defect, leading to different degrees of change in the cable's propagation coefficient, and further causing the input impedance phase characteristics to deviate from the design value. Based on the frequency-varying characteristics of the cable's input impedance, the influence mechanism of the cable's propagation coefficient change on the input impedance spectrum when local defects exist is analyzed. This effectively extracts characteristic quantities related to the degree of cable defects, providing a reliable basis for assessing the degree of cable defects. Furthermore, through the analysis of these characteristic quantities, a quantitative assessment method for the degree of cable defects is proposed, which includes the following steps:

[0055] S1. By measuring the impedance spectrum of a healthy cable of the same model as the cable under test when the end is open-circuited and short-circuited, the attenuation coefficient α(f) and phase shift coefficient β(f) of the healthy cable in the frequency range are obtained.

[0056] S2. Measure the impedance spectrum of the cable under test when it is open-circuited and short-circuited at the end using an impedance analyzer. The impedance spectrum includes the impedance amplitude spectrum and the impedance phase spectrum.

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

[0058] S4. Construct new impedance phase spectra for both the healthy cable and the cable under test, and calculate the mathematical fitting formula for the envelope of the new impedance phase spectrum as a function of frequency. Based on the mathematical fitting formula, calculate the characteristic parameters representing the degree of defect.

[0059] S5. Use the peak value F(L) at the end of the positioning curve of the cable under test. end Using the baseline peak as the reference peak, the remaining characteristic peaks F(L) are... j Normalization was performed.

[0060] S6. Calculate the defect severity characterization parameter δ;

[0061] S7. Compare the magnitudes of the defect severity indicators in the table; lower values ​​correspond to higher defect severity.

[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 transmission line theory, the open-circuit impedance and short-circuit impedance at the end of a healthy cable are:

[0064]

[0065]

[0066] Zop It is the open-circuit impedance, Z sc It is the short-circuit impedance. 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, it is only necessary to test the open-circuit impedance Z at the beginning of a healthy cable of any known length. op and short-circuit impedance Z sc 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] Cable impedance spectrum is measured using a precision impedance analyzer. During the test, the cable measurement end is connected to the impedance analyzer port, and the other end of the cable is controlled to be open-circuited, short-circuited, or connected to a load. For low-voltage multiphase cables, the measurement is performed using two phases forming a circuit; for coaxial cables, the measurement is performed using the cable cores and the metallic shielding layer forming a circuit. A schematic diagram of the measurement wiring is shown below. Figure 2 As shown.

[0073] The input impedance spectrum of the cable under test was obtained by measuring using the method described above.

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

[0075] The fundamental principle of orthogonality is: for a function system {f} i If {f(x), i=1,2…} satisfies the orthogonality shown in equation (6), that is, the integral of the product of two identical functions is not equal to 0 while the integral of the product of two different functions is always 0, then {f} is called {f(x), i=1,2…}. i (x), i=1,2…} is an orthogonal function system. As can be seen from equation (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 total length l of the cable under test. In the discretization process, d is used as the discrete step size and n is used as the index value of the loop calculation. In space, 0, d, 2d, …, nd, …, l are taken to achieve spatial discretization. The algorithm operation process is shown in Equation (7). The discrete integral values ​​of the impedance phase spectrum and the orthogonal kernel function {sin(2βnd), n=1,2,3…} under different nd are calculated. After calculation, the positioning curve under different positions nd can be obtained. The absolute value of the curve is the amplitude characteristic.

[0078]

[0079] Where F(nd) is the amplitude characteristic of the positioning curve, which is discrete data; M is the number of frequency points of the actual measured impedance phase spectrum. Ideally, M→∞, but in reality, it is a finite value limited by the bandwidth and sampling of the measuring instrument. For example, if the impedance analyzer measures the impedance of 1600 equally spaced frequency points within the set frequency bandwidth, then M is 1600; arg(Z in ) i β represents the impedance phase value at the measured i-th frequency point; 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 and ending at the final value N; d is the calculation step size.

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

[0081] according to Figure 3 As can be seen, the horizontal axis of the positioning curve represents the distance from the cable's beginning, and the vertical axis represents the amplitude characteristic value of the positioning curve. This cable contains three peak values. The first two peak values ​​correspond to two defects in the cable, and the third peak value corresponds to the cable's end. The graph shows that the distances from the beginning of the two defects are 24.77 meters and 59.82 meters, respectively, and the lengths of the two defects are Δl1 = 25.39 - 24.77 = 0.62 meters and Δl2 = 60.46 - 59.82 = 0.6 meters, respectively.

[0082] In this embodiment, the method for calculating the characteristic parameters representing the degree of defect in step S4 is as follows:

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

[0084] Data fitting was performed on the upper envelope of the new impedance phase spectrum of the healthy cable and the cable under test as a function of 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) represents the values ​​of the upper envelope of the new impedance phase spectrum of the healthy cable and the cable under test, respectively, which are quantities that vary with frequency; M h and M b Indicates y h (f), y b The mathematical relationship between (f) and frequency f.

[0088] Characteristic parameters that characterize 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, and l is the length of the cable to be tested. The overall difference between the cable under test and a healthy cable characterizes the overall defect level of the cable under test. In equation (10), most parameters are frequency-dependent; the higher the frequency, the more accurate the calculation of the characteristic parameters of the defect level. The highest frequency measured is used. In this embodiment, the approach to normalizing the peak values ​​at various points on the positioning curve in step S5 is as follows:

[0091] The cable defect degree characteristic parameters are obtained through steps S3 and S4. The cable defect location curve F(L) and the location of local defects in the cable can be normalized by normalizing the peak values ​​at each point of the location curve, as shown in Equation (11):

[0092]

[0093] In the formula, l is the total length of the cable under test, l j It is the defect location at the j-th distance from the measuring end of the cable under test, F(l) j F(l) and F(l) are respectively l j The amplitude of the positioning curve at position l, M is the number of frequency points of the actual measured impedance phase spectrum, and α i and β i These are the attenuation coefficient and phase shift coefficient values ​​of a healthy cable of the same type as the cable under test at the i-th frequency point.

[0094] In this embodiment, the approach to calculating the characterization parameters of the defect severity in step S6 is as follows: when there are n defects in the cable, calculate the attenuation compensation coefficients C1, C2...C at each of the n defects.n Then the defect severity characterization δ at the j-th defect location is... n The calculation is as follows:

[0095]

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

[0097] This embodiment also includes an experiment to verify the effectiveness of the above method. In this embodiment, an RF coaxial cable is used as the experimental object, and two local defects with different degrees of local defects are set up. In the experiment, a 0.1m long RF coaxial cable with a characteristic impedance of 25Ω (SFF25-1) and a 50Ω (SYV50-3) is used to simulate the local defect segment, while a 75Ω (SYV75-5) RF coaxial cable is used to simulate the healthy segment of the cable. The entire spliced ​​cable structure is as follows: Figure 5 As shown, the measured impedance spectrum is as follows: Figure 6 As shown, Figure 6 The upper curve is the impedance amplitude spectrum, and the lower curve is the impedance phase spectrum.

[0098] Based on step S1, the open-circuit and short-circuit impedance spectra of the same type of healthy cable were measured, and the attenuation coefficient and phase shift coefficient of the SYV75-5 cable in [1MHz, 120MHz] were obtained as follows: Figure 7 As shown.

[0099] Based on step S3, the positioning curve is obtained by processing the impedance phase spectrum as shown below. Figure 8 As shown.

[0100] from Figure 8 As can be seen, the positioning curve F(L) has three peaks, corresponding to defect point 1, defect point 2, and the end of the cable, respectively, at positions of 34.76m, 69.82m, and 100.11m. The distances of the two defects from the beginning of the cable are 34.76m and 69.82m, respectively, and the lengths of the two defects are Δl1=35.38-34.76=0.62m and Δl2=60.46-59.82=0.61m, respectively. The total length of the cable is 100.11m.

[0101] Based on step S4, the calculation process is as follows:

[0102] The expression for calculating the envelope on a healthy cable is as follows:

[0103]

[0104] The expression for calculating the envelope of the cable under test is as follows:

[0105]

[0106] Calculate characteristic quantities:

[0107]

[0108] This is the numerical value representing the overall difference between the cable under test and a healthy cable. The larger this value, the more severe the overall defect of the cable under test.

[0109] According to step S5, the peak values ​​of the positioning curve are normalized at each point:

[0110] First defect calculation:

[0111]

[0112] Second defect calculation:

[0113]

[0114] Based on step S6, calculate the defect severity characterization result:

[0115]

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

[0117] The foregoing has shown and described the basic principles and main features of the present invention and its advantages. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0118] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider 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 a partial defect of a cable based on impedance spectroscopy, characterized by, The method comprises the following steps: Step one, obtaining the attenuation coefficient α(f) and phase shift coefficient β(f) of the healthy cable in the frequency range by measuring the impedance spectrum of the healthy cable of the same model as the to-be-tested cable when the end of the healthy cable is open-circuit and short-circuit; Step two, measuring the impedance spectrum of the to-be-tested cable when the end of the to-be-tested cable is open-circuit and short-circuit by means of an impedance analyzer, and the impedance spectrum comprises an impedance amplitude spectrum and an impedance phase spectrum; Step three, drawing a positioning curve F(L) by means of a quadrature integral algorithm; Step four, constructing a new impedance phase spectrum of the healthy cable and the cable to be tested, and calculating a mathematical fitting formula of envelope curve varying with frequency on the new impedance phase spectrum, and calculating a characteristic parameter representing the defect degree according to the mathematical fitting formula ; Step five, take the peak value F(L end ) at the end of the positioning curve of the cable to be tested as the reference peak, and normalize the remaining characteristic peaks F(L j ). Step six, calculating a defect degree characteristic parameter δ; Step seven, comparing the size of the defect degree characteristic parameter δ, and a lower value corresponds to a higher defect degree; The flow of the normalization processing of the peak values of the positioning curve in the fifth step includes: obtaining the characteristic parameters of the defect degree of the cable to be tested by the third step and the fourth step The cable defect positioning curve F(L) and the local defect position of the cable, and the attenuation compensation coefficients of different defects are calculated, as shown in formula (11): ； In the formula C j The attenuation compensation coefficient for the peak value of the localization curve after normalization of the defect at point j is determined by C. j Obtain the ratio of the defect severity of each defect in the cable under test, and combine it with φ(f max This yields the quantitative values ​​of defects at various points in the cable under test, where l is the total length of the cable under test. j It is the defect location at the j-th distance from the measuring end of the cable under test, F(l) j F(l) and F(l) are respectively l j The amplitude of the positioning curve at position l, M is the number of frequency points of the actual measured impedance phase spectrum, and α i and β i Let C1, C2, ..., Cn be the attenuation coefficient and phase shift coefficient of a healthy cable of the same type as the cable under test at the i-th frequency point. When there are n defects in the cable, calculate the attenuation compensation coefficients C1, C2, ..., Cn for the j-th defect. n Then the defect severity characterization quantity at the j-th defect The calculation is as follows: ； wherein is the defect length of each defect, obtained in the calculation of step three.

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

3. The method for evaluating the degree of partial defect of a cable based on impedance spectroscopy according to claim 1, characterized in that, In step one, the process of obtaining the attenuation coefficient α(f) and the phase shift coefficient β(f) of the healthy cable comprises the following steps: according to the transmission line theory, the end open-circuit impedance and the end short-circuit impedance of the healthy cable are as follows: ； ； where Z op is the open circuit impedance, Z sc is the short circuit impedance, and the propagation coefficient γ of the cable is given by equations (1) and (2): ； ； ； In practice, only the open circuit impedance Z of the first end of a healthy cable of any known length is tested op and the short circuit impedance Z sc The attenuation coefficient a(f) and the phase shift coefficient β(f) of the cable can then be calculated using equations (3) to (5).

4. The method for evaluating the degree of partial defect of a cable based on impedance spectroscopy according to claim 2, characterized in that, In step two, the method for measuring the impedance spectrum comprises the following steps: when the cable impedance spectrum is tested, the cable measurement end is connected with the port of the impedance analyzer, the other end of the cable is controlled to be open-circuit, short-circuit or connected with a load, for a low-voltage multi-phase cable, the loop is formed by two phases of the cable, and for a coaxial cable, the loop is formed by the core wire and the metal shielding layer of the cable.

5. The method for evaluating the degree of partial defect of a cable based on impedance spectroscopy according to claim 1, characterized in that, The quadrature integral algorithm in step three comprises the following steps: ； 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 to-be-tested cable, in the discretization process, d is taken as a discrete step, n is taken as an index value for cyclic calculation, and the space is discretized by taking 0, d, 2d, …, nd, …, l, and the algorithm operation process is as shown in formula (7): ； Wherein, F(nd) is the amplitude characteristic of the positioning curve; M is the actual measured impedance phase spectrum frequency point number, ideally M→∞, but actually limited to a finite value by the bandwidth and sampling of the measuring instrument, the impedance analyzer measures 1600 frequency points of impedance at equal intervals in the set frequency bandwidth, so M is 1600 at this time; arg(Z in ) i is the measured impedance phase value at the 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.

6. The method for evaluating the degree of partial defect of a cable based on impedance spectroscopy according to claim 5, characterized in that: According to formula (7), the discrete integral values of the impedance phase spectrum and the orthogonal kernel function {sin(2βnd), n=1, 2, 3…} under different nd are calculated, and after the calculation, the positioning curve under different positions nd is obtained, and the absolute value is the amplitude characteristic.

7. The method for evaluating the degree of partial defect of a cable based on impedance spectroscopy according to claim 1, characterized in that, In step four, the calculation method of the characteristic parameter for representing the defect degree of the to-be-tested cable comprises the following steps: the upper envelope lines of the new impedance phase spectrum of the healthy cable and the to-be-tested cable are fitted respectively: ； ； In the above formula are the values of the envelope of the new impedance phase spectrum of the healthy cable and the cable under test, respectively, as a function of frequency; M h and M b denotes and the mathematical relationship of the frequency f.

8. The method for evaluating the degree of partial defect of a cable based on impedance spectroscopy according to claim 7, characterized in that: Characteristic variable representing the degree of defect The calculation method is: ； In the formula, a(f) is the attenuation coefficient of the healthy cable obtained in step one, l is the length of the cable to be measured, and φ(f) is the overall difference between the cable to be measured and the healthy cable, representing 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. The highest frequency of measurement is taken as .

Images