Cable defect positioning method under incomplete frequency point condition

Abstract

The invention relates to the technical field of high voltage, in particular to a cable defect positioning method under the condition of incomplete frequency points. The method comprises the following steps: step 1, determining basic parameters of a to-be-tested cable section, and testing an impedance spectrum of the to-be-tested cable section; analyzing and comparing parameters of the impedance spectrum and the actually measured impedance spectrum given by combining the basic parameters, and updating the basic parameters of the iterative cable section until the difference between the two impedance spectrums meets the requirement; 2, after the basic parameters of the cable section to be measured are determined, the impedance spectrum of the cable section is monitored regularly; expanding the impedance spectrum by applying the impedance spectrums of the healthy cable and the fault cable to obtain an expanded impedance spectrum; step 3, applying integral transformation to obtain a cable diagnosis result, if no defect is detected, updating the basic parameters of the cable section to be detected according to the current measurement result, and updating the impedance spectrum data of the healthy cable body as a historical measurement result; and if the defect is detected, outputting a cable defect diagnosis result. According to the method, the fault positioning precision under the condition of incomplete frequency points is improved, the test and diagnosis cost is reduced, and the practicability is good.

Description

Technical Field

[0001] The present invention relates to the technical field of high voltage, and in particular to a cable defect location method under the condition of incomplete frequency points. Background Art

[0002] As an important part of the urban distribution network, cables maintain the safety of the urban power grid. Cables laid in cable trenches or buried directly in the ground may be affected by long-term abnormal operating environments such as mechanical damage and chemical damage, causing the degradation of the cable insulation layer, which poses a hidden danger to the safe and stable operation of the cable system. Unlike serious damage such as short circuits or open circuits, soft faults caused by insulation degradation cannot cause the immediate collapse of the cable system, but they will still become an unstable factor that threatens the stability of the power grid. If the location of the abnormal aging section can be detected, it will provide a guiding direction for the inspection of the cable.

[0003] Nowadays, the methods used to locate hidden defects in power cables are divided into traveling wave method and impedance method. Among them, the traveling wave method has become an important method for high-precision positioning of hidden defects because it is based on traveling wave theory. The traveling wave method is divided into time domain reflection method, frequency domain reflection method and time-frequency reflection method. Among them, the frequency domain reflection method has a higher possibility of identifying minor faults, but it has high requirements on equipment and needs to collect enough frequency points to achieve high-precision defect positioning. However, when the cable length is long enough, in order to achieve high-precision positioning, sufficient frequency points are required. However, there is an upper limit to the frequency points collected within a given bandwidth using an impedance analyzer. When the bandwidth is specified, the regularity of the spectrum response is limited, which may lead to incomplete frequency points and reduce positioning accuracy. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a cable defect locating method under the condition of incomplete frequency points, which improves the fault locating accuracy under the condition of incomplete frequency points, reduces the cost of test diagnosis, and has good practicality.

[0005] The present invention provides a cable defect location method under the condition of incomplete frequency points, comprising:

[0006] Step 1: Determine the basic parameters of the cable segment to be tested and test the impedance spectrum of the cable segment to be tested;

[0007] Analyze and compare the parameters of the impedance spectrum given by combining basic parameters with the parameters of the measured impedance spectrum, and update the basic parameters of the cable segment iteratively until the difference between the two impedance spectra meets the requirements;

[0008] Step 2: After determining the basic parameters of the cable segment to be tested, regularly monitor the impedance spectrum of the cable segment;

[0009] The impedance spectrum is expanded by using the impedance spectra of healthy cables and faulty cables to obtain the expanded impedance spectrum;

[0010] Step 3: Apply integral transformation to obtain the cable diagnosis result. If no defect is detected, the basic parameters of the cable segment to be tested are updated according to the current measurement result, and the impedance spectrum data of the healthy cable body is updated as the historical measurement result;

[0011] If a defect is detected, the cable defect diagnosis result is output.

[0012] In a specific embodiment of the present invention, the basic parameters of the cable segment include conductivity, relative dielectric constant, resistivity, and cable size.

[0013] In a specific embodiment of the present invention, in step 1, the condition that the difference between the two impedance spectra meets the requirement is: within a given range f l ~f h The normalized impedance spectrum error is less than 1e-3.

[0014] In a specific embodiment of the present invention, in step 2, when the bandwidth is limited to [f l ,f h ], the number of impedance spectrum data points collected by the equipment is N, and the impedance spectrum is expanded by applying the impedance spectrum of the healthy cable and the faulty cable to expand the number of data points of the impedance spectrum to M, M>N,

[0015] M / N∈2 k

[0016] k is a positive integer.

[0017] In a specific embodiment of the present invention, in step 2, the method of using the impedance spectra of healthy cables and faulty cables to expand the impedance spectrum is:

[0018]

[0019] The frequency points of the impedance spectrum test are set A.

[0020] Among them, Z test (f) is the impedance spectrum of the power cable that may have a fault, which is actually tested. health (f) is the impedance spectrum calculated based on the basic parameters of a healthy cable. Generally, the end load is disconnected during the test.

[0021] but

[0022]

[0023] γ(f) and Z(f) are the propagation constant and characteristic impedance of a healthy cable at frequency f, respectively, and both values ​​are related to frequency f; l is the cable length; γ(f) is written as Z(f) writing

[0024] Wherein, R(f), L(f), C(f), and G(f) are respectively the resistance, inductance, capacitance, and conductance in the transmission line model per unit length of a healthy cable.

[0025] In a specific embodiment of the present invention, in step three, the expanded impedance spectrum is used as the object of integral transformation, and the fault location analysis result is given according to the following formula:

[0026]

[0027] Wherein, x represents the position to be analyzed for location, a and b are the boundaries of the interval to be located, which shall not be greater than the cable length l, and a < b; the threshold for locating the fault is ζ. When R(x) > ζ, the determined position of the fault location is given. Otherwise, according to the current measurement results, the basic parameters of the cable section to be measured are updated, and the impedance spectrum data of the healthy cable body is updated to the historical measurement results. Compared with the prior art, the cable defect location method in the case of incomplete frequency points of the present invention has the following beneficial effects:

[0028] (1) By utilizing the association between the historical healthy cable information, the fault cable information, and the historical healthy cable information, the spectrum of the fault cable is expanded, and a fused cable impedance spectrum is obtained, improving the fault location accuracy in the case of incomplete frequency points;

[0029] (2) Based on the expanded impedance spectrum, the equipment requirements for on-site application are reduced. Compared with the existing cable location methods, the test diagnosis cost is compressed under the same accuracy;

[0030] (3) A solution is provided for the application scenario with incomplete frequency points, improving the practicability of the test system. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a diagram of the impedance spectrum expansion application scheme;

[0032] Figure 2 It is the original measured impedance spectrum diagram;

[0033] Figure 3 It is the expanded measured impedance spectrum diagram;

[0034] Figure 4 It is the analysis result diagram of the original impedance spectrum and the expanded impedance spectrum. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] In order to further understand the present invention, the implementation scheme of the present invention will be described below in conjunction with embodiments. However, it should be understood that these descriptions are only for further explaining the features and advantages of the present invention, rather than limiting the present invention.

[0036] The test hardware is a precision impedance analyzer with a maximum sampling frequency of N. By measuring the impedance spectrum of the system to be tested at the head end of the test system, an impedance spectrum with N data points is obtained. The limited impedance spectrum frequency range is f l ~f h .

[0037] The embodiment of the present invention discloses a method for locating cable defects under the condition of incomplete frequency points, such as Figure 1 As shown, the following steps are included:

[0038] Step 1: Determine the basic parameters of the cable segment to be tested. The specific method is to preliminarily determine the basic parameters of the cable by testing the structural parameters and material parameters. The basic parameters of the cable include conductivity, relative dielectric constant, resistivity, cable size, etc.

[0039] Then test the impedance spectrum of the cable segment to be tested;

[0040] Analyze and compare the parameters of the impedance spectrum given by combining the basic parameters and the measured impedance spectrum, and update the basic parameters of the cable segment iteratively until the difference between the two impedance spectra meets the requirements, that is, within the given range f l ~f h The normalized impedance spectrum error is less than 1e-3.

[0041] Step 2: After determining the basic parameters of the cable segment to be tested, regularly monitor the impedance spectrum of the cable segment. At this time, due to equipment limitations, the impedance spectrum of the cable segment is limited to [f l ,f h ], the number of impedance spectrum data points collected by the equipment is N, and the impedance spectrum is expanded by using the impedance spectra of healthy cables and faulty cables to expand the number of data points of the impedance spectrum to M, where M>N.

[0042] In order to ensure the equidistant nature of the spectrum data points, it is necessary to ensure that M / N∈2 k , k is a positive integer. The spectrum expansion method is as follows, and the frequency points of the impedance spectrum test are set A:

[0043]

[0044] Among them, Z test (f) is the impedance spectrum of the power cable that may have a fault, which is actually tested. health (f) is the impedance spectrum calculated based on the basic parameters of a healthy cable. Generally, the end load is disconnected during the test.

[0045]

[0046] γ(f) and Z(f) are the propagation constant and characteristic impedance of a healthy cable at frequency f, respectively. Both values ​​are related to frequency f; l is the cable length. γ(f) can be written as Z(f) can be written as

[0047] wherein, R(f), L(f), C(f), and G(f) are the resistance, inductance, capacitance, and conductance of the transmission line model per unit length of the healthy cable, respectively.

[0048] The expanded impedance spectrum is obtained by the above method.

[0049] Step 3: Take the expanded impedance spectrum as the object of the integral transform, and give the location analysis result of the fault according to the following formula:

[0050]

[0051] wherein, x represents the position to be analyzed for location, a and b are the boundaries of the interval to be located, which shall not be greater than the cable length l, and a < b. The threshold for locating the fault is ζ. When R(x) > δ, the determined position of the fault location is given. Otherwise, according to the current measurement results, the basic parameters of the cable section to be measured are updated, and the impedance spectrum data of the healthy cable body is updated to the historical measurement results.

[0052] A test example of applying the above discrimination method to a 100m long RG58 coaxial cable. The Agilent 4294A precision impedance analyzer is used as the measuring instrument, and the measured impedance spectrum is as Figure 2 shown. By using the method of expanding the impedance spectrum, the obtained impedance spectrum is as Figure 3 shown. Integral transform analysis is performed on the original impedance spectrum and the expanded impedance spectrum, and the obtained results are as Figure 4 shown.

[0053] According to Figure 4 the results shown, it can be seen that the location accuracy given by using the expanded impedance spectrum is higher. The method proposed in the patent can improve the location accuracy and accuracy by expanding the impedance spectrum in the case of incomplete frequency points.

[0054] The present invention analyzes the matching degree between the faulty cable and the healthy cable, extends according to the theoretical calculation results of the healthy cable, maps to the corresponding frequency points of the faulty cable, expands the frequency points through the extended mapping, obtains the frequency domain expansion spectrum, uses the expanded spectrum for the integral transform in the frequency reflection method, and finally locates the position of the hidden defect, and finally realizes the location of the hidden defect of the power cable in the situation of incomplete frequency points caused by equipment limitations.

[0055] The description of the above embodiments is only used to help understand the method and its core idea of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0056] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A cable defect location method under incomplete frequency point conditions, characterized in that: include: Step 1: Determine the basic parameters of the cable segment to be tested and test the impedance spectrum of the cable segment to be tested; Analyze and compare the parameters of the impedance spectrum given by combining basic parameters with the parameters of the measured impedance spectrum, and update the basic parameters of the cable segment iteratively until the difference between the two impedance spectra meets the requirements; Step 2: After determining the basic parameters of the cable segment to be tested, regularly monitor the impedance spectrum of the cable segment; The impedance spectrum is expanded by using the impedance spectra of healthy cables and faulty cables to obtain the expanded impedance spectrum; Step 3: Apply integral transformation to obtain the cable diagnosis result. If no defect is detected, the basic parameters of the cable segment to be tested are updated according to the current measurement result, and the impedance spectrum data of the healthy cable body is updated as the historical measurement result; If a defect is detected, the cable defect diagnosis result is output.

2. The cable defect location method under incomplete frequency point condition according to claim 1, characterized in that: The basic parameters of the cable segment include conductivity, relative dielectric constant, resistivity, and cable size.

3. The cable defect location method under incomplete frequency point condition according to claim 1, characterized in that: In step 1, the condition that the difference between the two impedance spectra meets the requirement is: within a given range f l ~f h The normalized impedance spectrum error is less than 1e-3.

4. The cable defect location method under incomplete frequency point condition according to claim 1, characterized in that: In the step 2, when the bandwidth is limited to [f l ,f h ], the number of impedance spectrum data points collected by the equipment is N, and the impedance spectrum is expanded by applying the impedance spectrum of the healthy cable and the faulty cable to expand the number of data points of the impedance spectrum to M, M>N, M / N∈2 k k is a positive integer.

5. The cable defect location method under incomplete frequency point condition according to claim 4, characterized in that: In the step 2, the method of using the impedance spectra of healthy cables and faulty cables to expand the impedance spectrum is: The frequency points of the impedance spectrum test are set A. Among them, Z test (f) is the impedance spectrum of the power cable that may have a fault, which is actually tested. health (f) is the impedance spectrum calculated based on the basic parameters of a healthy cable. Generally, the end load is disconnected during the test. but γ(f) and Z(f) are the propagation constant and characteristic impedance of a healthy cable at frequency f, respectively, and both values ​​are related to frequency f; l is the cable length; γ(f) is written as Z(f) writing Among them, R(f), L(f), C(f), and G(f) are the resistance, inductance, capacitance, and conductance in the transmission line model of a healthy cable per unit length, respectively.

6. The cable defect location method under incomplete frequency point condition according to claim 1, characterized in that: In the step 3, the expanded impedance spectrum is used as the transformation object of the integral transformation, and the fault location analysis result is given according to the following formula: Among them, x represents the position for quasi-location analysis, a and b are the boundaries of the quasi-location interval, which shall not be greater than the cable length l, and a < b; the threshold for fault location is When is satisfied, the determined position of the fault location is given; otherwise, based on the current measurement results, the basic parameters of the cable section to be measured are updated, and the impedance spectrum data of the healthy cable body is updated to the historical measurement results.

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