Power cable local defect degree and type evaluation method based on impedance spectroscopy

Abstract

The invention provides a power cable local defect degree and type evaluation method based on an impedance spectroscopy method. The power cable local defect degree and type evaluation method comprises the following steps: acquiring an attenuation coefficient, a phase shift coefficient and characteristic impedance of a healthy cable in a frequency range; calculating the impedance spectrum of the to-be-measured cable, and measuring the impedance spectrum of the to-be-measured cable through an impedance analyzer; drawing a defect positioning function curve by adopting an improved orthogonal integral algorithm; obtaining a calculation peak value; calculating the reflection coefficient and the characteristic impedance of each defect section; and judging the defect degree and type, calculating the ratio of the characteristic impedance of the defect section to the characteristic impedance difference of the healthy cable, and obtaining the degree quantitative evaluation of the defect section. By calculating the severity of the defect section and judging the defect type, quantitative calculation and comparison of the defect degrees of various types of cables are realized, a unified defect degree evaluation method and a type judgment method are formulated for various types of cables, the anti-interference capability is relatively high, and the reliability is high. And a new method and thought are provided for cable defect state evaluation based on the impedance spectrum.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cable monitoring, in particular to a method for evaluating the degree and type of local defects of power cables based on impedance spectrum method. BACKGROUND

[0002] In the actual operation and inspection process of cables, the information of the defect position of the cable is the standard and basis for formulating the cable repair strategy. The quantitative evaluation of the defect severity and the judgment of the type of the defect position of the cable can timely discover potential problems, take targeted measures to repair the defects, delay the aging process of the cable, thereby prolonging the service life of the cable, further optimizing the allocation of maintenance resources, and improving the efficiency and effect of power operation and maintenance.

[0003] In the prior art, after measuring the frequency domain impedance of the cable to obtain the defect positioning function curve, the peak values of each defect section identified in the curve are normalized to the first end or the last end of the cable, so as to realize the comparison of the defect degree of each defect section in the cable relative to the first end or the last end. This technology can only obtain the relative defect degree of the defect section of the cable relative to the first end or the last end of the cable, and cannot obtain the actual absolute defect degree of the defect section, and further cannot be compared with other cables. Therefore, this technology can only realize the qualitative comparison of the defect degree of each defect section of the same cable, and cannot obtain the actual size of the defect degree, so it cannot realize the unified judgment of the defect degree of various cables, and further cannot formulate a general defect degree evaluation method, so the application range is narrow, and the combination with intelligent data systems is also limited. SUMMARY

[0004] In view of the deficiencies of the prior art, the present application aims to provide a general evaluation method for the degree and type of local defects of power cables based on impedance spectrum method, to solve the problems in the background art. The present application provides a method for calculating the characteristic impedance of the local defect section based on impedance spectrum technology, to further obtain the absolute size of the degree of the defect section and judge the defect type. The quantitative calculation and comparison of the defect degree of various types of cables are realized, a general defect degree evaluation method and type judgment method are formulated for various types of cables, the application range is wide, and a new method and idea are provided for the defect state evaluation of cables based on impedance spectrum.

[0005] In order to achieve the above-mentioned purpose, the present application is realized by the following technical scheme: a method for evaluating the degree and type of local defects of power cables based on impedance spectrum method, comprising the following steps: S1, obtaining the attenuation coefficient α ( f ), the phase shift coefficient β ( f ) and the characteristic impedance Zc ( f The attenuation coefficient of a healthy cable within a specific frequency range is obtained by measuring its impedance spectrum under open and short-circuit conditions at the ends. α ( f Phase shift coefficient β ( f and characteristic impedance Z c ( f ); S2. Obtain the impedance spectrum of the cable under test; measure the impedance spectrum of the cable under test using an impedance analyzer. The impedance spectrum includes the impedance amplitude spectrum and the impedance phase spectrum. S3. Use an improved orthogonal integral algorithm to plot the defect location function curve; construct an orthogonal function system {sin(2 βnd ) / e -2αnd , n =1, 2, 3…}, with sampling distance d As the computational step size, let the position variables in the orthogonal kernel function... nd Start from 0 and increment by step. d Traverse the space and calculate different... nd The defect location function F is obtained by using the discrete integral values ​​of the lower orthogonal kernel function and the phase spectrum of the impedance of the cable under test. x The peaks of the defect location function curve reflect the number, location, and range of local defect segments. S4. Obtain the calculated peak value. C Its value is the peak value of each defect segment in the defect location function curve and the peak value of the curve. The ratio, x The calculated peak value of each peak in the defect location function curve is obtained by assigning the value of the abscissa corresponding to the peak value of the curve. C ; S5. Calculate the reflection coefficient and characteristic impedance of each defect segment; S6. Determine the degree and type of defect; Based on the characteristic impedance of the defective section of the cable under test and the characteristic impedance of the healthy cable calculated above, obtain a quantitative assessment of the degree of defect of the cable under test; When the characteristic impedance of the defective section is less than that of the healthy cable, it is determined to be a local damage defect, otherwise it is determined to be a local aging defect.

[0006] Furthermore, the attenuation coefficient of the healthy cable is obtained in S1. α ( f Phase shift coefficient β ( f and characteristic impedance Z c ( f The methods include the following: According to the transmission line theory, the open-circuit impedance and the short-circuit impedance of the healthy cable are: ; wherein Z op is the open-circuit impedance, Z sc is the short-circuit impedance.

[0007] According to the open-circuit impedance and the short-circuit impedance formulas of the cable, the propagation coefficient γ( f ) and the characteristic impedance Z c ( f ) of the cable are respectively: ; wherein α is the attenuation coefficient, β is the phase shift coefficient, l is the total length of the selected healthy cable.

[0008] Further, the method for measuring the impedance spectrum in step S2 is: The cable impedance spectrum is measured by using a precision impedance analyzer ， In the test of the cable impedance spectrum, the cable measurement end is connected with the impedance analyzer port, the other end of the cable is controlled to be open-circuited, short-circuited or connected with a load, for a low-voltage multi-phase cable, two phases of the cable are used to form a loop for measurement; for a coaxial cable, the core wire and the metal shielding layer of the cable are used to form a loop for measurement.

[0009] Further, the improved orthogonal integral algorithm in step S3 is specifically: for a function system { f i ( x ), i =1,2…}, if the orthogonality shown in the following formula is met: ; that is, the product integral value of two same functions is not equal to 0 and the product integral value of two different functions is always 0, then the function system { f i ( x ), i =1, 2…} is called an orthogonal function system.

[0010] Further, the calculation method of the peak value C in step S4 is: The peak values i and of the first and last ends of the defect section at the th position are directly read from the defect positioning function curve, and the corresponding abscissa x i_sand x i_e , the calculation peak value corresponding to each peak value is obtained by the following calculation; ; In the formula and respectively, the first and last segment calculation peak value of the defect segment, is the phase shift coefficient of the healthy cable, and are the start and end frequencies of the impedance spectrum measurement.

[0011] Further, in step S5, the first defect segment head calculation peak value C 1_s , that is, the reflection coefficient of the first defect segment Г 1, the second defect segment head calculation peak value C 2_s , and the ratio of the first defect segment end calculation peak value C 1_e The absolute value of the ratio of the first defect segment end calculation peak value Г 2 / Г 1, the third defect segment head calculation peak value C 3_s , and the ratio of the second defect segment end calculation peak value C 2_e The absolute value of the ratio of the second defect segment end calculation peak value Г 3 / Г 2, and so on, to obtain the reflection coefficient of the defect segment in the cable i The ratio of the reflection coefficient of the defect segment in the cable i -1 Г i / Г i-1 , the reflection coefficient of each defect segment in the cable is obtained by recursion, and the characteristic impedance of each defect segment is calculated according to the reflection coefficient.

[0012] Further, it further includes a characteristic impedance calculation method: The calculation relationship of the reflection coefficient and the characteristic impedance of each defect segment is: ; In the formula is the reflection coefficient of the defect segment in the cable i ; is the characteristic impedance of the defect segment in the cable i ; Z c The characteristic impedance of each defect segment can be calculated by the above formula.

[0013] Further, in step S6, the idea of calculating the defect degree quantization value is: Defect degree quantization calculation: By comparing the characteristic impedance of the healthy cable Z c The severity of each defect is compared, and the parameter E i The severity quantization value of the defect segment at the first i E i The calculation is as follows: ; E i The greater the value of the defect degree quantization value, the more severe the defect.

[0014] Further, it also includes defect type judgment: when the characteristic impedance of the defect segment is less than the characteristic impedance of the healthy cable, it is determined as a local damage defect, and when the characteristic impedance of the defect segment is greater than the characteristic impedance of the healthy cable, it is determined as a local aging defect.

[0015] Advantages of the present application: The power cable local defect degree and type evaluation method based on the impedance spectrum method can obtain the reflection coefficient and characteristic impedance of the defect segment on the basis of the cable defect positioning function, and further realize the quantitative evaluation of the actual absolute defect degree of the to-be-tested cable. This evaluation can be used not only for the healthy segment of the to-be-tested cable, but also for the comparison of the same type of healthy cable, so that the defect degree comparison of different cables of the same type can be realized. Further, the unified judgment of the defect degree of multiple cables of the same type can be realized, a general defect degree evaluation method can be developed, the application range can be expanded, and an intelligent diagnosis system can be developed.

[0016] The power cable local defect degree and type evaluation method based on the impedance spectrum method can obtain the characteristic impedance of the defect segment cable according to the frequency variation characteristics of the cable input impedance, analyze the influence mechanism of the change of the cable characteristic impedance on the input impedance spectrum when the cable has a local defect, and effectively extract the absolute value of the feature quantity related to the defect degree of the cable, thereby providing a reliable basis for evaluating the defect degree of the cable.

[0017] The power cable local defect degree and type evaluation method based on the impedance spectrum method has a simple measurement circuit and does not damage the cable. The diagnosis result of the method can be used to preferentially select the defect with a deeper defect degree for repair work, which has a positive significance for the prevention of cable faults and the reduction of power failure accidents. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The measurement wiring diagram provided in the present application is shown in the figure.​ Figure 2 The defect positioning function obtained in the embodiment of the present application is plotted into a defect positioning function curve; Figure 3 A flow chart for calculating the reflection coefficient of each defect segment in the embodiment of the present application is shown in FIG. 4; Figure 4 An experimental cable structure diagram in which the degrees of two defects are different in the embodiment of the present application is shown in FIG. 5; Figure 5 A measured impedance spectrum of a cable in which the degrees of two defects are different in the embodiment of the present application is shown in FIG. 6; Figure 6 A graph of the attenuation coefficient and the phase shift coefficient of a healthy cable in the embodiment of the present application is shown in FIG. 7; Figure 7 A graph of the defect positioning function curve in the embodiment of the present application is shown in FIG. 8. DETAILED DESCRIPTION

[0019] In order to make the technical means, creative features, purposes and effects achieved by the present application easy to understand, the present application is further described below in combination with specific embodiments.

[0020] Please refer to Figures 1 to 7 , the present application provides the following technical solutions: Embodiment 1: A power cable partial defect degree and type evaluation method based on impedance spectrum method mainly includes the following steps: S1, obtaining the attenuation coefficient α ( f ), the phase shift coefficient β ( f ) and the characteristic impedance Z c ( f ) of a healthy cable. By measuring the impedance spectrum of the healthy cable when the end is open and short-circuited, the attenuation coefficient α ( f ), the phase shift coefficient β ( f ) and the characteristic impedance Z c ( f ) of the healthy cable in the frequency range are obtained.

[0021] S2, obtaining the impedance spectrum of a to-be-tested cable. The impedance spectrum of the to-be-tested cable is measured by an impedance analyzer, and the impedance spectrum includes an impedance amplitude spectrum and an impedance phase spectrum.

[0022] S3, plotting a defect positioning function curve by using an improved orthogonal integral algorithm. An orthogonal function system {sin(2 βnd ) / e -2αnd , n =1, 2, 3…} is constructed, and the sampling distanced As the calculation step, let the position variable in the orthogonal kernel function nd From 0 to step d In space, the different nd The lower orthogonal kernel function and the discrete integral value of the measured cable impedance phase spectrum to obtain the defect positioning function F( x ) The peak value of the defect positioning function curve can reflect the number, position, range and other information of the local defect.

[0023] S4, calculate the peak value C Its value is the ratio of the curve peak value corresponding to each defect segment in the defect positioning function curve and The value of the abscissa corresponding to the curve peak value is obtained x The calculation peak value of each peak value in the defect positioning function curve C .

[0024] S5, calculate the reflection coefficient and characteristic impedance of each defect. The first defect segment head calculation peak value C 1_s The reflection coefficient of the first defect segment Г 1, the second defect segment head calculation peak value C 2_s The absolute value of the ratio of the first defect segment end calculation peak value C 1_e The ratio of the reflection coefficient of the second defect segment to the reflection coefficient of the first defect segment Г 2 / Г 1, the third defect segment head calculation peak value C 3_s The absolute value of the ratio of the second defect segment end calculation peak value C 2_e The ratio of the reflection coefficient of the third defect segment to the reflection coefficient of the second defect segment Г 3 / Г 2, and so on, to obtain the ratio of the reflection coefficient of the first defect segment to the reflection coefficient of the first defect segment in the cable i -1 i Г i / Г i-1 , the reflection coefficient of each defect segment in the cable is obtained by recursive method, and the characteristic impedance of each defect segment is calculated according to the reflection coefficient.

[0025] ​S6, judging the defect degree and type. According to the characteristic impedance of the defect section of the cable to be measured and the characteristic impedance of the healthy cable obtained by the foregoing calculation, the degree quantitative evaluation of the defect section of the cable to be measured is obtained; when the characteristic impedance of the defect section is less than the characteristic impedance of the healthy cable, it is determined as a local damage defect, and vice versa, it is determined as a local aging defect.

[0026] The method for obtaining the attenuation coefficient of the healthy cable in S1 is as follows: α f , the phase shift coefficient β f and the characteristic impedance Z c f are as follows: According to the transmission line theory, the open-circuit impedance and the short-circuit impedance of the healthy cable at the end are: . Among them Z op is the open-circuit impedance, Z sc is the short-circuit impedance. According to formulas (1)-(2), the propagation coefficient γ of the cable is: . In practical application, only the open-circuit impedance Z op and the short-circuit impedance Z sc of the first end of the healthy cable with any known length need to be tested, and then the attenuation coefficient α f , the phase shift coefficient β f and the characteristic impedance Z c f can be calculated by using formulas (3)-(6).

[0027] The method for measuring the impedance spectrum in step S2 is as follows: The measurement of the cable impedance spectrum adopts a precision impedance analyzer ， When the cable impedance spectrum is tested, the cable measurement end is connected with the port of the impedance analyzer, and 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 to measure; for a coaxial cable, the loop is formed by the core wire and the metal shielding layer of the cable. The measurement connection diagram is shown in Figure 1 .

[0028] The input impedance spectrum of the cable to be measured is measured by the above method.

[0029] The improved orthogonal integral algorithm in step S3 is specifically as follows:​​​​​​ The fundamental principle of orthogonality is: for a function system { f i ( x ), i =1, 2…}, if they satisfy the orthogonality shown in equation (7), 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 i ( x ), i =1, 2…} is an orthogonal function system. As can be seen from equation (7), the orthogonality of the orthogonal function system is essentially a kind of screening property. ; The chosen orthogonal kernel function is {sin(2 βnd ) / e -2αnd , n =1, 2, 3…}. Position variables in orthogonal kernel functions. nd It is the total length of the cable under test l A discrete processing step. The discretization process uses... d As a walking distance, with n As the index value for loop calculations, it is set to 0 in space. d , 2 d ,…, nd ,…, l Spatial discretization is achieved. The algorithm operation process is shown in equation (7), which calculates the impedance phase spectrum and different nd The orthogonal kernel function under the following conditions {sin(2 βnd ) / e -2αnd , n The discrete integral values ​​of {=1, 2, 3…} can be calculated to obtain the values ​​at different positions. nd The absolute value of the defect location function is the amplitude characteristic. ; Among them, F( nd ) represents the amplitude characteristics of the defect location function; M The number of frequency points of the actual measured impedance phase spectrum, under ideal conditions. M →∞, but in practice, it is limited to a finite value due to the bandwidth and sampling limitations of the measuring instrument. For example, if an impedance analyzer measures the impedance at 1600 equally spaced frequency points within a set frequency bandwidth, then at this point... M 1600; arg( Z in ) i For the measured number i Impedance phase value at each frequency point; β i For the first iThe phase shift coefficient value of a healthy cable at each frequency point; α i For the first i Attenuation coefficient of healthy cable at a specific frequency point; n The variable to iterate over in the algorithm is 0, starting from the final value. N _n_ natural numbers; d To calculate the step size.

[0030] The calculated defect location function is plotted as a defect location function curve as shown in Figure 2.

[0031] As shown in Figure 2, the horizontal axis of the defect location function curve represents the distance from the cable's beginning, and the vertical axis represents the amplitude characteristic of the defect location function. This cable contains three peak values. The first two peak values ​​correspond to two defective segments within the cable, and the third peak value corresponds to the cable's end. The figure shows that the distances from the beginning of the two defects are 29.97m and 60.39mm, respectively, and the lengths of the two defective segments are... and .

[0032] Calculate the peak value in step S4 C The calculation method is as follows: Read the first [value] directly from the defect location function curve graph. i Peak values ​​at the beginning and end of the defective segment and and the x-axis corresponding to the peak value x i_s and x i_e The calculated peak values ​​corresponding to each peak value are obtained through the following calculations; ; In the formula and The peak values ​​were calculated for the first and last segments of the defective segment, respectively. The phase shift coefficient of a healthy cable. and These are the start and stop frequencies for impedance spectrum measurement, respectively.

[0033] In step S5, the approach to calculating the reflection coefficient and characteristic impedance of each defect segment is as follows: The flowchart for calculating the reflection coefficient of each defect segment is shown in Figure 3.

[0034] Characteristic impedance calculation method: The known relationship between the reflection coefficient and characteristic impedance of each segment is as follows: ; In the formula For the first i The reflection coefficient of the defective section; the characteristic impedance of the first i defect section; Z c The characteristic impedance of the healthy cable can be calculated by formula (11), and the characteristic impedance of each defect section can be calculated.

[0035] In step S6, the idea of calculating the defect degree quantization value is as follows: Defect degree quantization calculation: by comparing with the characteristic impedance of the healthy cable Z c The comparison can obtain the severity of each defect section, and the parameter E i indicates the severity quantization value of the first i defect section, E i The calculation is as follows: ; E i The greater the value of the defect severity quantization value, the more serious the defect, and the quantitative calculation of the defect degree in the cable is realized.

[0036] Defect type judgment: When the characteristic impedance of the defect section is less than the characteristic impedance of the healthy cable, it is determined as a local damage defect, and when the characteristic impedance of the defect section is greater than the characteristic impedance of the healthy cable, it is determined as a local aging defect.

[0037] Example 2: This embodiment is used to verify the effectiveness of the above technical content. In this experiment, radio frequency coaxial cables are used as experimental objects, and two local defects with different local defect degrees are set. In the experiment, a 88 m long radio frequency coaxial cable with a model of SYV50-3 and a characteristic impedance of 50 Ω is used to simulate a local defect 1, a radio frequency coaxial cable with a model of RVSP 2*0.5 and a characteristic impedance of 120 Ω is used to simulate a local defect 2, and a radio frequency coaxial cable with a model of SYV75-5 and a characteristic impedance of 75 Ω is used to simulate a healthy cable section. The entire cable structure is shown in FIG. 4, and the measured impedance spectrum is shown in FIG. 5. The upper curve in FIG. 5 is an impedance amplitude spectrum, and the lower curve is an impedance phase spectrum.

[0038] According to step S1, the open circuit short circuit impedance spectrum of the healthy cable of the same type is measured, and the attenuation coefficient and the phase shift coefficient of the SYV75-5 cable in [1 MHz, 120 MHz] are obtained as shown in FIG. 6.

[0039] According to step S3, the defect positioning function curve is obtained by processing the impedance phase spectrum as shown in FIG. 7.

[0040] As can be seen from Fig. 7, the defect locating function curve F( x ) has three peaks, corresponding to the defect point 1, the defect point 2 and the cable end, corresponding to the positions of 17.9 m, 48.5 m and 87.9 m, the lengths of the two defects are and , and the total length of the cable is 87.9 m.

[0041] According to step S4, the calculation of the peak is as follows: ; According to step S5, the defect segment reflection coefficient and the characteristic impedance are calculated as follows: ; Finally, the characteristic impedances of the two defect segments are as follows: ; According to step S6, the defect degree quantization value can be calculated as shown in the table: At the same time, according to the calculation result of the characteristic impedance, it can be known that the characteristic impedance of the first defect segment of the cable is smaller than that of the healthy cable, which is a local damage defect, and the characteristic impedance of the second defect segment of the cable is larger than that of the healthy cable, which is a local aging defect, and the difference between the calculation result of the characteristic impedance and the actual value is not more than 3%, which indicates the rationality of the method for judging the defect type.

[0042] The above shows and describes the basic principles and main features of the present application and the advantages of the present application, and it is obvious for those skilled in the art that the present application is not limited to the details of the above exemplary embodiments, and the present application can be realized in other specific forms without departing from the spirit or essential characteristics of the present application.

[0043] In addition, it should be understood that although the present specification is described in terms of embodiments, each embodiment does not contain only one independent technical solution, and the description manner of the specification is only for the sake of clarity, and 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 which can be understood by those skilled in the art.

Claims

1. A method for assessing the degree and type of local defects in power cables based on impedance spectroscopy, characterized in that... It includes the following steps: S1. By measuring the impedance spectrum of the same healthy cable segment or a healthy cable of the same type as the cable under test under open and short circuit conditions at the end, the attenuation coefficient of the healthy cable within the frequency range is obtained. α ( f Phase shift coefficient β ( f and characteristic impedance Z c ( f ); S2. Measure the impedance spectrum of the cable under test using an impedance analyzer. The impedance spectrum includes the impedance amplitude spectrum and the impedance phase spectrum. S3. Construct an orthogonal function system {sin(2 βnd ) / e -2αnd , n =1, 2, 3…}, with sampling distance d As the computational step size, let the position variables in the orthogonal kernel function... nd Start from 0 and increment by step. d Traverse the space and calculate different... nd The defect location function F is obtained by using the discrete integral values ​​of the lower orthogonal kernel function and the phase spectrum of the impedance of the cable under test. x The peaks of the defect location function curve reflect the number, location, and range of local defect segments. S4. The peak value of each defect segment in the defect location function curve and The ratio is the calculated peak value of each peak in the defect location function curve. C ,in x This represents the value of the horizontal axis corresponding to the peak of the curve; S5. Calculate the reflection coefficient and characteristic impedance of each defect segment; S6. Based on the characteristic impedance of the defective section of the cable under test and the characteristic impedance of the healthy cable obtained from the above calculation, a quantitative assessment of the degree of defect of the cable under test is obtained; when the characteristic impedance of the defective section is less than the characteristic impedance of the healthy cable, it is judged as a local damage defect, otherwise it is judged as a local aging defect.

2. The method for evaluating the degree and type of local defects in power cables based on impedance spectroscopy as described in claim 1, characterized in that, Obtain the attenuation coefficient of the healthy cable in S1. α ( f Phase shift coefficient β ( f and characteristic impedance Z c ( f The methods include the following: According to transmission line theory, the open-circuit impedance and short-circuit impedance at the end of a healthy cable are: ； ； in Z op It is the open-circuit impedance. Z sc It is the short-circuit impedance; From the formulas for the open-circuit impedance and short-circuit impedance of a cable, we can know the propagation coefficient of the cable. γ ( f and characteristic impedance Z c ( f They are respectively: ； In the formula α The attenuation coefficient is... β The phase shift coefficient, l The total length of the selected healthy cable.

3. The method for evaluating the degree and type of local defects in power cables based on impedance spectroscopy as described in claim 1, characterized in that, The method for measuring the impedance spectrum in step S2 is as follows: The cable impedance spectrum was measured using a precision impedance analyzer. ， When testing the impedance spectrum of a cable, connect the measuring end of the cable to the port of the impedance analyzer, and control 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.

4. The method for evaluating the degree and type of local defects in power cables based on impedance spectroscopy as described in claim 1, characterized in that, The improved orthogonal integration algorithm in step S3 is as follows: For a function system { f i ( x ), i =1, 2…}, if they satisfy the orthogonality shown in the following formula: ； That is, when 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 it is said that { f i ( x ), i =1, 2…} is an orthogonal function system.

5. The method for evaluating the degree and type of local defects in power cables based on impedance spectroscopy as described in claim 1, characterized in that, Calculate the peak value in step S4 C The calculation method is as follows: The peak values ​​at the beginning and end of the i-th defect segment are directly read from the defect location function curve. and and the x-axis corresponding to the peak value x i_s and x i_e The calculated peak values ​​corresponding to each peak are obtained through the following calculations: ； In the formula and The peak values ​​were calculated for the first and last segments of the defective segment, respectively. The phase shift coefficient of a healthy cable. and These are the start and stop frequencies for impedance spectrum measurement, respectively.

6. The method for evaluating the degree and type of local defects in power cables based on impedance spectroscopy as described in claim 1, characterized in that: In step S5, the peak value at the beginning of the first defect segment is calculated. C 1_s That is, the reflection coefficient of the first defect segment. Г 1. Calculated peak value at the beginning of the second defect segment C 2_s The calculated peak value at the end of the first defect segment C 1_e The absolute value of the ratio is the ratio of the reflection coefficient of the second defect segment to the reflection coefficient of the first defect segment. Г 2 / Г 1. Calculated peak value at the beginning of the third defect segment C 3_s The calculated peak value at the end of the second defect segment C 2_e The absolute value of the ratio is the ratio of the reflection coefficient of the third defect segment to the reflection coefficient of the second defect segment. Г 3 / Г 2. And so on, to obtain the first [section / section] in the cable. i The reflection coefficient of the defective section is the same as that of the first section. i The ratio of the reflection coefficient of the defect segment at -1 Г i / Г i-1 The reflection coefficient of each defective section in the cable is obtained by recursion, and the characteristic impedance of each defective section is calculated based on the reflection coefficient.

7. The method for evaluating the degree and type of local defects in power cables based on impedance spectroscopy as described in claim 6, characterized in that, The characteristic impedance is calculated as follows: The known relationship between the reflection coefficient and characteristic impedance of each defect segment is as follows: ； In the formula For the first i The reflection coefficient of the defective section; For the first i The characteristic impedance of the defective section; Z c The characteristic impedance of a healthy cable can be calculated using the above formula to obtain the characteristic impedance of each defective section.

8. The method for evaluating the degree and type of local defects in power cables based on impedance spectroscopy as described in claim 1, characterized in that, The approach to calculating the quantification value of defect severity in step S6 is as follows: Defect severity quantification calculation: By comparing with the characteristic impedance of a healthy cable Z c The severity of each defect is compared, and parameters are defined. E i Indicates the first i The quantification value of the severity of the defect segment. E i The calculation is as follows: ； E i The larger the value, the more severe the defect.

9. The method for evaluating the degree and type of local defects in power cables based on impedance spectroscopy as described in claim 8, characterized in that, It also includes the determination of defect type: When the characteristic impedance of the defective section is less than that of the healthy cable, it is determined to be a localized damage defect; when the characteristic impedance of the defective section is greater than that of the healthy cable, it is determined to be a localized aging defect.

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