A method and system for assessing local defects in cables

Abstract

This invention discloses a method and system for assessing local defects in cables, belonging to the field of cable defect assessment technology. It addresses the technical problem that existing time-frequency domain reflection methods cannot effectively characterize the severity of local defects in cables, thus failing to accurately assess the extent of damage. The method includes: converting the reflected signal collected during cable inspection to the time-frequency domain to obtain a time-frequency domain signal; extracting a reference signal from the time-frequency domain signal and comparing the reference signal with other time-frequency domain signals to obtain a defect location curve; determining the defect location of the cable under test based on the defect location curve; constructing a virtual attenuation curve of the incident signal based on the defect location; determining the instantaneous reflection coefficient based on the virtual attenuation curve; and calculating the percentage of impedance change at the defect location based on the instantaneous reflection coefficient to characterize the corresponding defect severity.

Description

Technical Field

[0001] This invention relates to the field of cable defect assessment technology, and in particular to a method and system for assessing local defects in cables. Background Technology

[0002] Cables, as a medium for transmitting electrical energy, play a vital role in urban power supply, electronic equipment signal transmission, and vehicle power supply. However, manufacturing processes, installation wear, and prolonged high-load operation can lead to localized electric field distortion points in cables, resulting in insulation deterioration defects or even permanent breakdown faults, causing power outages and significant economic losses.

[0003] To rapidly and non-destructively detect local defects in cables, various cable inspection methods have been developed, including insulation resistance testing, time-domain reflectometry, and frequency-domain reflectometry. However, insulation resistance testing can only assess the overall insulation degradation of the cable and cannot locate local defects; time-domain reflectometry can locate extreme faults such as open circuits and short circuits, but cannot locate subtle local defects; frequency-domain reflectometry can locate subtle local defects, but its sensitivity is insufficient due to its frequency-domain resolution capability.

[0004] A novel time-frequency domain reflection method has been proposed, combining time-domain and frequency-domain reflection methods. It possesses both time-domain and frequency-domain resolution capabilities and exhibits high sensitivity in defect location. However, the location curve generated by the time-frequency domain reflection method only contains defect location information, and the peak value corresponding to the defect location cannot effectively characterize the severity of the defect. Therefore, it cannot accurately assess the extent of damage from localized cable defects and cannot provide more precise data guidance for cable maintenance. Summary of the Invention

[0005] This invention provides a method and system for assessing local defects in cables, which addresses the following technical problem: existing time-frequency domain reflection methods cannot effectively characterize the severity of local defects in cables, thus failing to accurately assess the extent of damage caused by local defects.

[0006] The embodiments of the present invention adopt the following technical solutions:

[0007] On one hand, embodiments of the present invention provide a method for assessing local defects in cables, the method comprising:

[0008] The reflected signals collected during cable testing are converted to the time-frequency domain to obtain time-frequency domain signals;

[0009] A reference signal is extracted from the time-frequency domain signal, and the reference signal is compared with the remaining time-frequency domain signals to obtain the defect location curve;

[0010] Based on the defect location curve, the defect location of the cable under test is determined;

[0011] Based on the location of the defect, a virtual attenuation curve for the incident signal is constructed;

[0012] Based on the virtual attenuation curve, the instantaneous reflection coefficient is determined;

[0013] Based on the instantaneous reflection coefficient, the percentage change in impedance at the defect location is calculated to characterize the corresponding defect severity.

[0014] In one feasible implementation, the reflected signal collected during cable testing is converted to the time-frequency domain to obtain a time-frequency domain signal, specifically including:

[0015] An incident signal is injected into one end of the cable under test, and an open circuit or short circuit is applied to the other end of the cable under test; wherein, the incident signal is a time-frequency domain signal;

[0016] The reflected signal of the cable under test is collected at the signal injection end of the cable under test;

[0017] According to the time-frequency domain conversion formula, the reflected signal is converted to the time-frequency domain to obtain the time-frequency domain signal corresponding to the reflected signal.

[0018] In one feasible implementation, the reflected signal is converted to the time-frequency domain according to the time-frequency domain conversion formula to obtain the time-frequency domain signal corresponding to the reflected signal, specifically including:

[0019] according to The offset Z(k) of the discrete time interval is obtained; where k is the sequence order of the integration time τ, Δτ is the interval of the discrete integration time sequence, and τ min Let τ be the minimum value, Δt be the discrete time interval, and |int be the integer operation;

[0020] Substitute Z(k) In this process, the time-frequency domain signal TFD(n,m) is obtained;

[0021] Where n is the time series of the incident signal, m is the frequency series of the reflected signal, K is the upper limit of k, R(k) is the value of the k-th rectangular window sequence, r is the reflected signal sequence, * is the complex conjugate operator, and f min Δf is the minimum value of the frequency sequence, and Δf is the interval of the frequency sequence.

[0022] In one feasible implementation, a reference signal is extracted from the time-frequency domain signal, and the reference signal is compared with the remaining time-frequency domain signals to obtain a defect location curve, specifically including:

[0023] The time-frequency domain signal corresponding to the reflected signal collected at the beginning of the cable under test is extracted and used as the reference signal;

[0024] The reference signal is compared with the remaining part of the time-frequency domain signal to obtain the comparison result, and the defect location curve is determined based on the propagation distance of the time-frequency domain signal.

[0025] In one feasible implementation, the reference signal is compared with the remaining portion of the time-frequency domain signal to obtain a comparison result, and the defect location curve is determined based on the propagation distance of the time-frequency domain signal, specifically including:

[0026] According to the comparison formula, the reference signal is compared with the other signals in the time-frequency domain signal sequence to obtain the comparison result;

[0027] The comparison formula is as follows:

[0028]

[0029] Where C(n) is the alignment result, n is the time series of the incident signal, m is the frequency series of the reflected signal, and T is the time series of the incident signal. s Let Δt be the duration of the incident signal, Δt be the discrete time interval, M be the total number of frequency sequences, and TFD(p,m) be the time-frequency domain signal of the p-th time sequence. s (p,m) is the reference signal, | int This is a rounding operation;

[0030] according to Determine the propagation distance variation value x(n) of the time-frequency domain signal in the time-frequency domain signal sequence; where t0 is the initial value of the signal time, and v is the wave speed of the signal propagating in the cable;

[0031] The defect location curve is determined by taking the propagation distance x(n) of each time-frequency domain signal as the abscissa and the comparison result C(n) between each time-frequency domain signal and the reference signal as the ordinate.

[0032] In one feasible implementation, determining the defect location of the cable under test based on the defect location curve specifically includes:

[0033] In the defect location curve, the horizontal coordinates corresponding to the two ends of the cable under test are located respectively;

[0034] Obtain all peak values ​​of the curve within the range of both ends of the cable under test, and find the target peak value that is greater than a preset threshold among all peak values;

[0035] The horizontal coordinate value corresponding to the target peak value is determined as the defect location of the cable under test;

[0036] The number of the target peak values ​​is determined as the number of defects in the cable under test.

[0037] In one feasible implementation, a virtual attenuation curve of the incident signal is constructed based on the defect location, specifically including:

[0038] according to Construct the virtual attenuation curve V of the incident signal a (n);

[0039] Where n is the time series of the incident signal, n' represents the propagation time series of time series n in the cable under test, max represents taking the maximum value of the contour sequence of all virtual attenuation signals corresponding to n', j is an imaginary number, Δτ is the interval of the discrete integration time series, k is the sequence order of the integration time τ, K is the upper limit of k, t0 is the initial value of the signal time, Δt is the interval of the discrete time, and τ min It is the minimum value of τ;

[0040] Where s(n,n') is the virtual decay signal corresponding to the change of n' in the time series n, defined as:

[0041]

[0042] Where M is the total number of frequency sequences, Δf is the interval of the frequency sequences, N' is the total number of n', and f min is the minimum value of the frequency sequence, γ' and γ″ are the real and imaginary parts of the cable propagation coefficient, respectively, and s(n') is the incident signal.

[0043] In one feasible implementation, determining the instantaneous reflection coefficient based on the virtual attenuation curve specifically includes:

[0044] The virtual attenuation signal at the defect location is compensated for by coefficients to obtain the instantaneous reflection coefficient at each defect location:

[0045]

[0046] Where, n 0.25 1 Let n be the sequence position of the first target peak in the defect location curve, and n is the position of the peak. 0.25 1 Greater than 1; n 0.25 i Let r(n) be the sequence position of the i-th target peak, and r(n) be the reflected signal. a (n) is the virtual attenuation curve of the incident signal, R c (n 0.25 1 R is the instantaneous reflection coefficient of the first target peak. c (n 0.25 i) represents the instantaneous reflection coefficient of the i-th target peak.

[0047] In one feasible implementation, the percentage change in impedance at the defect location is calculated based on the instantaneous reflection coefficient, specifically including:

[0048] according to Calculate the percentage impedance change ΔZ at the defect location; where R c This represents the instantaneous reflection coefficient at the current defect location.

[0049] On the other hand, embodiments of the present invention also provide a cable local defect assessment system, the system comprising:

[0050] A computer is used to convert the reflected signals collected during cable inspection into the time-frequency domain to obtain a time-frequency domain signal; extract a reference signal from the time-frequency domain signal and compare the reference signal with the remaining time-frequency domain signals to obtain a defect location curve; determine the defect location of the cable under test based on the defect location; construct a virtual attenuation curve of the incident signal based on the defect location; determine the instantaneous reflection coefficient based on the virtual attenuation curve; and calculate the percentage of impedance change at the defect location based on the instantaneous reflection coefficient to characterize the corresponding defect severity.

[0051] A signal generator is used to generate an incident signal and inject the incident signal into the beginning of the cable under test;

[0052] An oscilloscope is used to acquire the reflected signal from the cable under test back to the beginning of the cable and transmit it to the computer.

[0053] Compared with the prior art, the cable local defect assessment method and system provided in this embodiment of the invention have the following beneficial effects:

[0054] This invention, based on the detection of local defects in cables, proposes the concept of a virtual attenuation curve to characterize the transmission characteristics of the incident signal, providing a basis for compensation of the reflection coefficient. Furthermore, it proposes a time-frequency domain instantaneous reflection coefficient, based on which the impedance change at the defect location located by the time-frequency domain reflection method is evaluated, thus providing a standard for characterizing the degree of cable defects and offering more accurate data guidance for cable maintenance. Attached Figure Description

[0055] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings:

[0056] Figure 1 A flowchart of a cable local defect assessment method provided in an embodiment of the present invention;

[0057] Figure 2 A schematic diagram of a time-domain acquisition waveform provided in an embodiment of the present invention;

[0058] Figure 3 A schematic diagram of a defect location curve provided in an embodiment of the present invention;

[0059] Figure 4 A schematic diagram of an instantaneous reflection coefficient curve provided in an embodiment of the present invention;

[0060] Figure 5 This is a schematic diagram of a cable local defect assessment system provided in an embodiment of the present invention. Detailed Implementation

[0061] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this specification, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0062] This invention provides a method for assessing local defects in cables, such as... Figure 1 As shown, the method for assessing local defects in cables specifically includes steps S101-S106:

[0063] S101. Convert the reflected signal collected during the cable inspection process to the time-frequency domain to obtain the time-frequency domain signal.

[0064] Specifically, a time-frequency domain incident signal is first injected into one end of the cable under test, and the other end of the cable is either open-circuited or short-circuited. The discrete expression of the incident signal is:

[0065]

[0066] Where n is the time series of the incident signal, α is a factor controlling the duration of the incident signal, and β is a factor controlling the rate of rise of the incident signal frequency. The units of α and β are seconds. -2 , that is, seconds -2 In the expression for the incident signal, since α and β are directly multiplied by the square of time t, the units of α and β need to be the time power of -2. f0 is the center frequency of the incident signal, t0 is the initial value of the signal time, and Δt is the discrete time interval.

[0067] Furthermore, after the above-mentioned incident signal is injected into one end of the cable under test, the incident signal will be reflected at the signal injection point, the defect point and the other end of the cable. Therefore, the reflected signals of the three can be collected at the signal injection end.

[0068] Furthermore, according to the time-frequency domain conversion formula, the reflected signal is converted to the time-frequency domain to obtain the corresponding time-frequency domain signal. The specific conversion method is as follows:

[0069] according to The offset Z(k) of the discrete time interval is obtained; where k is the sequence order of the integration time τ, Δτ is the interval of the discrete integration time sequence, and τ min Let τ be the minimum value, Δt be the discrete time interval, and |int be the integer operation.

[0070] Substitute Z(k) In this process, the time-frequency domain signal TFD(n,m) is obtained; where n is the time sequence of the incident signal, m is the frequency sequence of the reflected signal, K is the upper limit of k, R(k) is the value of the k-th rectangular window sequence, r is the reflected signal sequence, * is the complex conjugate operator, and f min Δf is the minimum value of the frequency sequence, and Δf is the interval of the frequency sequence.

[0071] S102. Extract the reference signal from the time-frequency domain signal and compare the reference signal with the remaining time-frequency domain signals to obtain the defect location curve.

[0072] Specifically, after converting the reflected signal to the time-frequency domain, the time-frequency domain signal corresponding to the reflected signal collected at the beginning of the cable under test is extracted as a reference signal. Then, the reference signal is compared with the remaining parts of the time-frequency domain signal to obtain the comparison result. Finally, the defect location curve is determined based on the propagation distance of the time-frequency domain signal.

[0073] Furthermore, the specific method for determining the defect location curve is as follows:

[0074] First, according to the comparison formula, the reference signal is compared with the remaining signals in the time-frequency domain signal sequence to obtain the comparison result; wherein, the comparison formula is:

[0075]

[0076] Where C(n) is the alignment result, n is the time series of the incident signal, m is the frequency series of the reflected signal, and T is the time series of the incident signal. s Let Δt be the duration of the incident signal, Δt be the discrete time interval, M be the total number of frequency sequences, and TFD(p,m) be the time-frequency domain signal of the p-th time sequence. s (p,m) is the reference signal, i.e., the time-frequency domain signal corresponding to the reflected signal returned from the beginning of the cable under test. int This is for rounding up.

[0077] Secondly, the final defect location curve uses the signal propagation distance as the independent variable, and the sequence expression of the signal propagation distance with respect to n is as follows: The propagation distance change value x(n) of the time-frequency domain signal in the time-frequency domain signal sequence is obtained from the above formula; where t0 is the initial value of the signal time and v is the wave speed of the signal propagating in the cable.

[0078] Finally, the defect location curve is determined by using the propagation distance x(n) of the time-frequency domain signal as the abscissa and the comparison result C(n) between the time-frequency domain signal and the reference signal as the ordinate.

[0079] S103. Based on the defect location curve, determine the defect location of the cable under test.

[0080] Specifically, in the defect location curve, the horizontal coordinates corresponding to both ends of the cable under test are located. Then, all peak values ​​of the curve within the range of both ends of the cable under test are obtained, and a target peak value greater than a preset threshold is found among all the peak values.

[0081] Furthermore, the horizontal coordinate value corresponding to the target peak value is determined as the defect location of the cable under test, and the number of target peak values ​​is determined as the number of defects in the cable under test.

[0082] In one embodiment, assuming the cable under test is 10 meters long, after obtaining the defect location curve, the peak value corresponding to the x-coordinate = 0m is determined as the signal injection end of the cable under test, and the peak value closest to the x-coordinate = 10m is determined as the other end of the cable under test. Within the x-coordinate range between the two ends of the cable under test, peak values ​​exceeding a preset threshold (e.g., 0.25) are searched. The x-coordinate corresponding to the peak values ​​that meet the requirements is the distance between the defect location and the signal injection end, and the number of peak values ​​that meet the requirements is the number of defects in the cable under test.

[0083] S104. Based on the defect location, construct a virtual attenuation curve for the incident signal.

[0084] Specifically, the aforementioned defect location curve serves to locate the position and number of defects. To assess the severity of each defect, this invention proposes a defect severity characterization method that determines the percentage of impedance change at the defect location using the instantaneous reflection coefficient. Obtaining the instantaneous reflection coefficient requires first constructing an instantaneous curve of the incident signal propagating in the cable under test and the abrupt change at the reflection point. The virtual attenuation curve of the incident signal is defined as follows:

[0085]

[0086] Where n is the time series of the incident signal, n' represents the propagation time series of time series n in the cable under test, max represents taking the maximum value of the contour sequence of all virtual attenuation signals corresponding to n', j is an imaginary number, Δτ is the interval of the discrete integration time series, k is the sequence order of the integration time τ, K is the upper limit of k, t0 is the initial value of the signal time, Δt is the interval of the discrete time, and τ min It is the minimum value of τ.

[0087] Where s(n,n') is the virtual decay signal corresponding to the change of n' in the time series n, defined as:

[0088]

[0089] Where M is the total number of frequency sequences, Δf is the interval of the frequency sequences, N' is the total number of n', and f min is the minimum value of the frequency sequence, γ' and γ″ are the real and imaginary parts of the cable propagation coefficient, respectively, and s(n') is the incident signal.

[0090] This invention derives an attenuation signal expression s(n,n') based on the attenuation sequence n' after the incident signal is injected into the cable under test, characterizing the transmission characteristics of the incident signal and providing a calculation basis for reflection coefficient compensation. Furthermore, it proposes a time-frequency domain instantaneous reflection coefficient and, based on this coefficient, calculates the percentage impedance change of defects located by the time-frequency domain reflection method in the cable, thus achieving an assessment of the degree of cable defects and solving a technical problem that was not addressed in the prior art.

[0091] S105. Determine the instantaneous reflection coefficient based on the virtual attenuation curve.

[0092] Specifically, since the incident signal will be reflected at the defect location, i.e., at the impedance discontinuity, the virtual attenuation signal after the defect location needs to be compensated for with instantaneous reflection coefficient. Based on the above virtual attenuation curve, the instantaneous reflection coefficient is calculated using the following formula:

[0093]

[0094] Where, n 0.251 Let n be the sequence position of the first target peak in the defect location curve, and n is the position of the peak. 0.25 1 Greater than 1; n 0.25 i Let r(n) be the sequence position of the i-th target peak, and r(n) be the reflected signal. a (n) is the virtual attenuation curve of the incident signal, R c (n 0.25 1 R is the instantaneous reflection coefficient of the first target peak. c (n 0.25 i ) represents the instantaneous reflection coefficient of the i-th target peak.

[0095] S106. Calculate the percentage change in impedance at the defect location based on the instantaneous reflection coefficient to characterize the corresponding defect degree.

[0096] Specifically, after obtaining the instantaneous reflection coefficient, this invention defines a formula for calculating the percentage change in impedance at the defect, used to characterize the severity of the defect: Among them, R c This represents the instantaneous reflection coefficient at the current defect location.

[0097] Based on the percentage change in defect impedance in the above formula, the degree of change of the defect relative to the normal cable portion can be characterized, thereby enabling the assessment of cable defects.

[0098] To verify the effectiveness of the above solution, a specific embodiment is used to implement and verify the above technical solution:

[0099] In this embodiment, a 200m long cable is used as the test object, and a defect with an impedance change of 22.2% is set at 100m. The incident signal parameter α is 1.2 × 10⁻⁶. 14 s -2 β is 9.8 × 10 13 s -2 f0 is 5MHz.

[0100] Figure 2 and Figure 3 This shows the time-frequency domain waveform of the measured reflected signal and the defect location curve. Figure 3The location results show three peak values ​​exceeding 0.25, representing the cable start, defect location, and cable end, respectively. The peak at 0m is the peak value at the cable start, and the peak value at 200m is the peak value at the cable end. There is only one peak value between these two, which is identified as the defect location. Reading the coordinates, the defect location is at 99.6m, with an absolute error of 0.4m and a peak value of 0.9764. The peak value at the cable end is 0.9281. Although the defect location was identified, the defect severity (i.e., the peak value) at that location is not significantly different from the peak values ​​at the start and end. Therefore, the peak value of the location curve obtained based on the comparison function cannot effectively characterize the defect severity.

[0101] According to the instantaneous reflection coefficient scheme proposed in this invention, it can be concluded that... Figure 4 The instantaneous reflection coefficient curve in the image. Based on the defect location curve, the location result is 99.6m, which can be obtained... Figure 4 The instantaneous reflection coefficient at the corresponding position was found to be 0.09925, and the calculated impedance change was 22.04%, which differed from the actual defect impedance change by 0.16%, thus verifying the effectiveness of the method proposed in this invention.

[0102] In addition, embodiments of the present invention also provide a cable local defect assessment system, such as... Figure 5 As shown, the cable local defect assessment system specifically includes:

[0103] A computer is used to convert the reflected signals collected during cable inspection into the time-frequency domain to obtain a time-frequency domain signal; extract a reference signal from the time-frequency domain signal and compare the reference signal with the remaining time-frequency domain signals to obtain a defect location curve; determine the defect location of the cable under test based on the defect location; construct a virtual attenuation curve of the incident signal based on the defect location; determine the instantaneous reflection coefficient based on the virtual attenuation curve; and calculate the percentage of impedance change at the defect location based on the instantaneous reflection coefficient to characterize the corresponding defect severity.

[0104] A signal generator is used to generate an incident signal and inject the incident signal into the beginning of the cable under test;

[0105] An oscilloscope is used to acquire the reflected signal from the cable under test back to the beginning of the cable and transmit it to the computer.

[0106] As a possible implementation method, such as Figure 5 As shown, the process used in this system is as follows:

[0107] (1) The computer designs the incident signal parameters and imports the signal into the signal generator through the connecting line;

[0108] (2) The signal generator produces an actual signal and injects it into the test cable;

[0109] (3) The oscilloscope acquires the signal reflected back to the beginning of the test cable through the BNC connector and connecting cable, and transmits it to the computer;

[0110] (4) The computer converts the reflected signal to the time-frequency domain;

[0111] (5) The computer calculates the comparison function between the time-frequency domain reference signal and the rest; and determines the peak value with an amplitude greater than 0.25 within the range of both ends of the test cable as the defect location;

[0112] (6) The computer calculates the instantaneous reflection coefficient based on the number and location of the defects;

[0113] (7) The computer calculates the percentage of impedance change at the defect based on the reflection coefficient, thereby assessing the degree of defect.

[0114] It should be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0115] This specification can be described in the general context of computer-executable instructions that are executed by a computer, such as program modules. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform a specific task or implement a specific abstract data type. This specification can also be practiced in distributed computing environments, where tasks are performed by remote processing devices connected via a communication network. In distributed computing environments, program modules can reside in local and remote computer storage media, including storage devices.

[0116] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the embodiments of apparatus, devices, and non-volatile computer storage media are basically similar to the method embodiments, so the descriptions are relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0117] The foregoing has described specific embodiments of this specification. Other embodiments are within the scope of this invention. In some cases, the actions or steps described in this specification may be performed in a different order than those shown in the embodiments and still achieve the desired results. Furthermore, the processes depicted in the drawings do not necessarily require a specific or sequential order to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0118] The above description is merely one or more embodiments of this specification and is not intended to limit this specification. Various modifications and variations can be made to the one or more embodiments of this specification by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of one or more embodiments of this specification should be included within the scope of protection of this specification.

Claims

1. A method for assessing local defects in cables, characterized in that, The method includes: The reflected signals collected during cable testing are converted to the time-frequency domain to obtain time-frequency domain signals; A reference signal is extracted from the time-frequency domain signal, and the reference signal is compared with the remaining time-frequency domain signals to obtain the defect location curve; Based on the defect location curve, the defect location of the cable under test is determined; Based on the location of the defect, a virtual attenuation curve for the incident signal is constructed, specifically including: according to Construct a virtual attenuation curve for the incident signal. V a (n) ; in, n The time series of the incident signal, n’ Let n represent the propagation process of time series n in the cable under test, and max represent the propagation process of time series n in all... n’ The maximum value is taken from the contour sequence corresponding to the virtual attenuation signal, where j is an imaginary number. The intervals are discrete integral time series. k Integration time The sequence order, K for k The upper limit, t 0 represents the initial value of the signal time, and Δt represents the discrete time interval. min for The minimum value; in, s(n,n') For time series n n’ The changing virtual decay signal is defined as: ； in, M The total number of frequency sequences. Δf The interval of the frequency sequence, N’ for n’ The total number f min The minimum value of the frequency sequence. 'and Let be the real and imaginary parts of the cable propagation coefficient, respectively. s(n') For the incident signal; Based on the virtual attenuation curve, the instantaneous reflection coefficient is determined, specifically including: The virtual attenuation signal at the defect location is compensated for by coefficients to obtain the instantaneous reflection coefficient at each defect location: ； in, n 0.25 1 This refers to the sequence position of the first target peak in the defect location curve, and n 0.25 1 Greater than 1; n 0.25 i Let be the sequence position of the i-th target peak. r(n) For reflected signals, V a (n) This is a virtual attenuation curve for the incident signal. R c ( n 0.25 1 () represents the instantaneous reflection coefficient of the first target peak. R c ( n 0.25 i Let be the instantaneous reflection coefficient of the i-th target peak; Based on the instantaneous reflection coefficient, the percentage change in impedance at the defect location is calculated to characterize the corresponding defect severity.

2. The method for assessing local defects in cables according to claim 1, characterized in that, The reflected signals collected during cable testing are converted to the time-frequency domain to obtain time-frequency domain signals, specifically including: An incident signal is injected into one end of the cable under test, and an open circuit or short circuit is applied to the other end of the cable under test; wherein, the incident signal is a time-frequency domain signal; The reflected signal of the cable under test is collected at the signal injection end of the cable under test; According to the time-frequency domain conversion formula, the reflected signal is converted to the time-frequency domain to obtain the time-frequency domain signal corresponding to the reflected signal.

3. The method for assessing local defects in cables according to claim 2, characterized in that, According to the time-frequency domain conversion formula, the reflected signal is converted to the time-frequency domain to obtain the time-frequency domain signal corresponding to the reflected signal, specifically including: according to This yields the offset of the discrete time interval. Z(k) ;in, k Integration time The sequence order, The intervals are discrete integral time series. min for The minimum value, where Δt is the discrete time interval, | int This is a rounding operation; Will Z(k) Substitution In the process, the time-frequency domain signal is obtained. TFD(n,m) ; in, n The time series of the incident signal, m The frequency sequence of the reflected signal. K for k The upper limit, R(k) For the first k The values ​​of a rectangular window sequence, r is the reflected signal sequence, and * is the complex conjugate operator. f min The minimum value of the frequency sequence. Δf The interval of the frequency sequence.

4. The method for assessing local defects in cables according to claim 1, characterized in that, Extracting a reference signal from the time-frequency domain signal and comparing the reference signal with the remaining time-frequency domain signals to obtain a defect location curve, specifically includes: The time-frequency domain signal corresponding to the reflected signal collected at the beginning of the cable under test is extracted and used as the reference signal; The reference signal is compared with the remaining part of the time-frequency domain signal to obtain the comparison result, and the defect location curve is determined based on the propagation distance of the time-frequency domain signal.

5. The method for assessing local defects in cables according to claim 4, characterized in that, The reference signal is compared with the remaining portion of the time-frequency domain signal to obtain the comparison result. Based on the propagation distance of the time-frequency domain signal, the defect location curve is determined, specifically including: According to the comparison formula, the reference signal is compared with the other signals in the time-frequency domain signal sequence to obtain the comparison result; The comparison formula is as follows: ； in, C(n) For comparison results, n The time series of the incident signal, m The frequency sequence of the reflected signal. T s Let be the duration of the incident signal, and Δt be the interval between discrete time steps. M The total number of frequency sequences. TFD(p,m) Let p be the time-frequency domain signal of the p-th time series. TFD s (p,m) For reference signal, | int This is a rounding operation; according to Determine the propagation distance variation of the time-frequency domain signal in the time-frequency domain signal sequence. x (n) ;in, t 0 represents the initial value of the signal time, and v represents the wave speed at which the signal propagates in the cable; The propagation distance of each time-frequency domain signal x(n) The x-axis represents the comparison results between each time-frequency domain signal and the reference signal. C(n) Using the vertical axis as the ordinate, the defect location curve is determined.

6. The method for assessing local defects in cables according to claim 1, characterized in that, Based on the defect location curve, the location of the defect in the cable under test is determined, specifically including: In the defect location curve, the horizontal coordinates corresponding to the two ends of the cable under test are located respectively; Obtain all peak values ​​of the curve within the range of both ends of the cable under test, and find the target peak value that is greater than a preset threshold among all peak values; The horizontal coordinate value corresponding to the target peak value is determined as the defect location of the cable under test; The number of the target peak values ​​is determined as the number of defects in the cable under test.

7. The method for assessing local defects in cables according to claim 1, characterized in that, Based on the instantaneous reflection coefficient, the percentage change in impedance at the defect location is calculated, specifically including: according to Calculate the percentage change in impedance at the defect location. ;in, R c This represents the instantaneous reflection coefficient at the current defect location.

8. A cable local defect assessment system, employing a cable local defect assessment method as described in any one of claims 1-7, characterized in that, The system includes: A computer is used to convert the reflected signals collected during cable inspection into the time-frequency domain to obtain a time-frequency domain signal; extract a reference signal from the time-frequency domain signal and compare the reference signal with the remaining time-frequency domain signals to obtain a defect location curve; determine the defect location of the cable under test based on the defect location; construct a virtual attenuation curve of the incident signal based on the defect location; determine the instantaneous reflection coefficient based on the virtual attenuation curve; and calculate the percentage of impedance change at the defect location based on the instantaneous reflection coefficient to characterize the corresponding defect severity. A signal generator is used to generate an incident signal and inject the incident signal into the beginning of the cable under test; An oscilloscope is used to acquire the reflected signal from the cable under test back to the beginning of the cable and transmit it to the computer.

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