A cable defect positioning method under frequency point incomplete condition

Abstract

The present application relates to the high voltage technical field, especially to a kind of cable defect positioning method under the condition of frequency point is not complete.The method includes: step one: determine the basic parameters of the cable section to be measured, test the impedance spectrum of the cable section to be measured;The parameters of the impedance spectrum given by the basic parameters are analyzed and compared, and the basic parameters of the cable section to be measured are updated iteratively until the difference between the two impedance spectrums meets the requirements;Step two: after determining the basic parameters of the cable section to be measured, regularly monitor the impedance spectrum of the cable section;Expand the impedance spectrum by applying the impedance spectrum of healthy cable and fault cable, and obtain the expanded impedance spectrum;Step three: integral transform is applied to obtain the cable diagnosis result, if no defect is detected, the basic parameters of the cable section to be measured are updated according to the current measurement result, and the impedance spectrum data of the healthy cable body is updated to the historical measurement result;If a defect is detected, output the cable defect diagnosis result.The present application improves the fault positioning accuracy under the condition of frequency point is not complete, compresses the cost of test diagnosis, and has good practicability.

Description

Technical Field

[0001] This invention relates to the field of high voltage technology, and in particular to a method for locating cable defects when frequency points are incomplete. Background Technology

[0002] Cables, as a crucial component of urban power distribution networks, 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, leading to insulation deterioration and posing a threat to the safe and stable operation of the cable system. Unlike severe faults such as short circuits or open circuits, soft faults caused by insulation deterioration do not lead to the immediate collapse of the cable system, but they can still become unstable factors threatening the stability of the power grid. Detecting the location of abnormally aging sections will provide guidance for cable inspection work.

[0003] Currently, methods for locating hidden defects in power cables are divided into traveling wave method and impedance method. Among them, the traveling wave method, based on traveling wave theory, has become an important method for high-precision defect location. The traveling wave method is further 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 probability of identifying minor faults, but it has high requirements for equipment and needs to collect a sufficient number of frequency points to achieve high-precision defect location. However, when the cable length is long enough, a sufficient number of frequency points are required to achieve high-precision location. However, there is an upper limit to the number of frequency points that can be collected by an impedance analyzer within a given bandwidth. When the bandwidth is specified, the spectral response is limited, which may lead to incomplete frequency points and reduced location accuracy. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for locating cable defects in the case of incomplete frequency points, which improves the fault location accuracy in the case of incomplete frequency points, reduces the cost of testing and diagnosis, and has good practicality.

[0005] This invention provides a method for locating cable defects when frequency points are incomplete, 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] By analyzing and comparing the impedance spectrum given by the basic parameters and the measured impedance spectrum, the basic parameters of the cable segment are updated and iterated 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, monitor the impedance spectrum of the cable segment periodically;

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

[0010] Step 3: Apply integral transform to obtain cable diagnosis results. If no defects are detected, update the basic parameters of the cable segment under test based on the current measurement results, and update the impedance spectrum data of the healthy cable to the historical measurement results.

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

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

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

[0014] In one specific embodiment of the present invention, in step two, when the bandwidth is limited to [f] l ,f h When the number of impedance spectrum data points collected by the equipment is N, the impedance spectrum is expanded to M by using the impedance spectra of healthy and faulty cables, where M > N.

[0015] M / N∈2 k

[0016] k is a positive integer.

[0017] In one specific embodiment of the present invention, the method for expanding the impedance spectrum using the impedance spectra of healthy cables and faulty cables in step two is as follows:

[0018]

[0019] The frequency points for impedance spectrum testing are set A.

[0020] Among them, Z test (f) is the impedance spectrum of a potentially faulty power cable that was actually tested, Z health (f) is the impedance spectrum calculated based on the basic parameters of a healthy cable, typically taken as the load at the end being open during testing.

[0021] but

[0022]

[0023] γ(f) and Z(f) are the propagation constant and characteristic impedance of a healthy cable at frequency f, respectively, both values ​​being 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 resistance, inductance, capacitance and conductance per unit length in the transmission line model of a healthy cable.

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

[0026]

[0027] wherein, x represents the position to be subjected to location analysis, a and b are boundaries of the interval to be located, which are not greater than the cable length l, and a < b; ζ is the threshold for fault location. When R(x)>ζ, the judgment result of the fault position is given; otherwise, 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 to the historical measurement result. Compared with the prior art, the cable defect location method of the present invention under the condition of incomplete frequency points has the following beneficial effects:

[0028] (1) By utilizing the correlation between historically retained healthy cable information, faulty cable information and historical healthy cable information, the spectrum of the faulty cable is expanded, an integrated cable impedance spectrum is obtained, and the fault location accuracy under the situation of incomplete frequency points is improved;

[0029] (2) Based on the expanded impedance spectrum, the equipment requirements for field applications are reduced, and compared with the existing cable location methods, under the condition of the same accuracy, the cost of testing and diagnosis is compressed;

[0030] (3) It provides a solution for application scenarios with incomplete frequency points, and improves the practicability of the test system. Description of Drawings

[0031] Figure 1 is an application scheme diagram of impedance spectrum expansion;

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

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

[0034] Figure 4 is an analysis result diagram of the original impedance spectrum and the expanded impedance spectrum. Detailed Description of the Embodiments

[0035] To further understand the present invention, the embodiments of the present invention are described below with reference to examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the present invention.

[0036] The testing hardware is a precision impedance analyzer with a maximum sampling frequency of N. The impedance spectrum of the system under test is obtained by measuring the impedance spectrum at the beginning of the testing system, with N data points. The frequency range of the impedance spectrum is limited to f. l ~f h .

[0037] An embodiment of the present invention discloses a method for locating cable defects when frequency points are incomplete, such as... Figure 1 As shown, it includes the following steps:

[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 permittivity, resistivity, cable size, etc.

[0039] Then test the impedance spectrum of the cable segment under test;

[0040] By analyzing and comparing the impedance spectrum given by the basic parameters and the measured impedance spectrum, the basic parameters of the cable segment are updated iteratively until the difference between the two impedance spectra meets the requirements, i.e., within a given range f. l ~f h The normalized impedance spectrum error is less than 1e-3.

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

[0042] To ensure the equidistant spacing between spectral data points, it is necessary to guarantee that M / N∈2. k , where k is a positive integer. The method for expanding the spectrum is as follows, denoted as set A for the frequency points of the impedance spectrum test:

[0043]

[0044] Among them, Z test (f) is the impedance spectrum of a potentially faulty power cable that was actually tested, Z health (f) is the impedance spectrum calculated based on the basic parameters of a healthy cable. It is generally assumed that the load at the end of the cable is open during testing.

[0045]

[0046] γ(f) and Z(f) are the propagation constant and characteristic impedance of a healthy cable at frequency f, respectively, both values ​​being 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 respectively resistance, inductance, capacitance and conductance in the per-unit-length transmission line model of a healthy cable.

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

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

[0050]

[0051] Wherein, x represents the position to be subjected to location analysis, a and b are boundaries of the interval to be located, which are not greater than the cable length l and satisfy a < b. ζ is the threshold for fault location. When R(x) > δ, the judgment position of the fault location is given; otherwise, the basic parameters of the cable section to be tested are updated according to the current measurement result, and the impedance spectrum data of the healthy cable body is updated to the historical measurement result.

[0052] Using the above discrimination method, a test example applied to a 100 m long RG58 coaxial cable is given. An Agilent precision impedance analyzer Agilent 4294A is used as the measuring instrument, and the measured impedance spectrum is shown in Figure 2 . The impedance spectrum obtained by using the expanded impedance spectrum method is shown in Figure 3 . The integral transformation analysis is performed on the original impedance spectrum and the expanded impedance spectrum, and the obtained results are shown in Figure 4 .

[0053] According to Figure 4 , it can be seen that the positioning accuracy provided by the expanded impedance spectrum is higher. The method proposed in the patent can improve the positioning precision and accuracy by expanding the impedance spectrum when frequency points are incomplete.

[0054] The present invention analyzes the matching degree between a faulty cable and a healthy cable, maps to the frequency points corresponding to the faulty cable according to the extension of the theoretical calculation result of the healthy cable, obtains an expanded frequency domain spectrum through the mapped extension to expand frequency points, applies the expanded spectrum to the integral transformation in the frequency reflection method, finally locates the position of hidden defects, and finally realizes the positioning of hidden defects in power cables under the situation of incomplete frequency points caused by equipment limitation.

[0055] The description of the above embodiments is only used to help understand the method of the present invention and its core idea. It should be pointed out that for those skilled 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 those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those 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 invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for locating cable defects when frequency points are incomplete, 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; By analyzing and comparing the impedance spectrum given by the basic parameters and the measured impedance spectrum, the basic parameters of the cable segment are updated and iterated 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, monitor the impedance spectrum of the cable segment periodically; The impedance spectrum is expanded by applying the impedance spectra of healthy cables and faulty cables to obtain the expanded impedance spectrum; With bandwidth limited to [f] l ,f h When the number of impedance spectrum data points collected by the equipment is N, the impedance spectrum is expanded to M by using the impedance spectra of healthy and faulty cables, where M > N. ； The method for expanding the impedance spectrum using the impedance spectra of healthy and faulty cables is as follows: The test frequencies for the impedance spectrum are set A. in, It is the impedance spectrum of the faulty power cable that was actually tested. The impedance spectrum is calculated based on the basic parameters of a healthy cable, with the load at the end of the cable open during the test. but and These are the propagation constant and characteristic impedance of a healthy cable at frequency f, respectively, and both values ​​are related to frequency f. It is the cable length; writing ; writing ; in, , , , These represent the resistance, inductance, capacitance, and conductance per unit length in a healthy cable transmission line model. Step 3: Using the expanded impedance spectrum as the object of integral transformation, the fault location analysis results are given according to the following formula: wherein, represents the position to be positioned and analyzed, a and b are the boundaries of the interval to be positioned, and shall not be greater than the cable length , and a < b; the threshold for positioning a fault is , when R(x) > , the judgment position of the fault location is given; otherwise, the basic parameters of the cable section to be tested are updated according to the current measurement result, and the impedance spectrum data of the healthy cable body is updated to the historical measurement result; [f l ,f h [This is to limit bandwidth.] 2. The cable defect location method under incomplete frequency point conditions according to claim 1, characterized in that, The basic parameters of the cable segment include conductivity, relative permittivity, resistivity, and cable size.

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

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