A method for locating cable defects based on pulse waveform modulation

Abstract

This invention proposes a cable defect location method based on pulse waveform modulation. The method uses a measurement circuit containing a waveform generator to locate cable defects. When the minimum pulse width of the measurement pulse in the measurement circuit is limited, the method uses a nonlinear programming approach to adjust the pulse width, pulse rising edge, and pulse falling edge to obtain a complete cable impedance spectrum and maximize the minimum amplitude of the frequency point to compensate for missing frequency components. This invention can improve the accuracy of cable defect location and effectively suppress noise interference in the field.

Description

Technical Field

[0001] This invention relates to the field of high voltage technology, and in particular to a cable defect location method based on pulse waveform modulation, which can be used to locate cable defects when frequency points are missing when measuring broadband impedance spectrum using a signal generator and oscilloscope and applying frequency domain reflection method. Background Technology

[0002] Due to their excellent environmental performance, cross-linked polyethylene (XLPE) cables have been widely used in underground power transmission and distribution networks. However, the corrosive effects of humid environments and the aging of insulation materials can easily lead to potential defects inside the cables. These defects may gradually evolve into faults, thereby threatening the safety and stability of the entire power system. Therefore, the rapid and accurate location of potential defects and the timely detection of early anomalies in insulation condition are crucial for proactively preventing faults and avoiding power outages.

[0003] As a common cable inspection method, Time Domain Reflectometry (TDR) effectively locates open-circuit or short-circuit faults caused by impedance abrupt changes by injecting low-voltage pulses into the cable and observing the reflected signals. However, this method has limited ability to detect high-resistance faults and early defects with gradual impedance changes. To improve sensitivity to subtle impedance changes, Frequency Domain Reflectometry (FDR) was developed. This technique injects a linearly swept frequency signal into the cable and identifies defects by utilizing the signal's response characteristics in the frequency domain. Within this framework, the Broadband Impedance Spectrum (BIS) method locates defects by measuring the curve of the cable's input impedance as a function of frequency, because the impedance change caused by a defect is represented in this spectrum. If a suitable algorithm is used to extract defect information from the BIS, the specific location of the defect can be determined. Summary of the Invention

[0004] This invention proposes a cable defect location method based on pulse waveform modulation, which can improve the accuracy of cable defect location and effectively suppress noise interference on site.

[0005] The present invention adopts the following technical solution.

[0006] A cable defect location method based on pulse waveform modulation is disclosed. The method uses a measurement circuit containing a waveform generator to locate cable defects. When the minimum pulse width of the measurement pulse in the measurement circuit is limited, the method uses a nonlinear programming method to adjust the pulse width, pulse rise edge, and pulse fall edge to obtain a complete cable impedance spectrum without missing frequency points, and maximizes the minimum amplitude of the frequency points to compensate for missing frequency components. This results in an impedance spectrum with wider frequency coverage and better integrity for cable defect location.

[0007] The cable defect location method employs a measurement circuit that includes a waveform generator, a voltage signal acquisition device, a T-connector and connecting cable, and a switching device.

[0008] The waveform generator is used to generate pulse signals, forming an excitation signal that is injected into the cable system under test.

[0009] The voltage signal acquisition device is responsible for acquiring the voltage signal at the port of the cable system under test.

[0010] T-shaped connectors are used to connect voltage signal acquisition devices, waveform generators, and the cable system under test.

[0011] The connecting cable is a signal cable with known parameters added at the measurement end. This is because there must be an electrical connection between the waveform generator, the signal acquisition device, and the cable system under test. Here, a coaxial signal cable is used for connection, and the impact of this connection on the test cannot be ignored.

[0012] The switching device is responsible for connecting or disconnecting the cable system under test.

[0013] Step 1: Connect the measurement circuit, check whether the connection is electrically reliable, and record the length of the connecting signal cable as l0;

[0014] Step 2: The waveform generator acts as a pulse generator, outputting a square wave signal with a pulse width of w and a repetition frequency of f, i.e., a pulse signal. Turn on the voltage signal acquisition device to prepare to acquire the port voltage signal within a single cycle in steady state.

[0015] Step 3: Connect switch K, collect the voltage signal at this time, record it as Uc, and the sampling rate is F;

[0016] Step 4: Disconnect switch K, collect the voltage signal at this time, record it as Uo, and the sampling rate is F;

[0017] Step 5: Using impedance spectrum analysis algorithms, based on the pulse waveform-controlled broadband impedance spectrum measurement method for cables, under the condition of limited minimum pulse width, the missing frequency components are compensated by adjusting the pulse width and rising and falling edges, thereby obtaining an impedance spectrum with wider frequency coverage and better integrity for defect location.

[0018] In steps two, three, and four, pulses are injected into the cable under test, and the pulse signal of the cable's pulse response is measured.

[0019] Step five includes the following steps;

[0020] Step 1: Determine the frequency range of the broadband impedance spectrum of the cable to be measured, as well as the adjustment range of the pulse width, rising edge, and falling edge of the pulse generator. Use nonlinear programming to change the pulse width, rising edge, and falling edge of the pulse so that the final generated pulse has no missing frequency points within the specified frequency range, and the amplitude of the smallest frequency component in the frequency band reaches the maximum.

[0021] Step 2: Inject the pulse calculated in Step 1 into the cable under test, and use the broadband impedance spectrum measurement method based on pulse voltage to calculate the impedance spectrum Z of the cable where there are no missing frequency points.

[0022] Step 3: Perform a fast Fourier transform on the impedance spectrum to obtain... The defect location results for the cable are given according to the following formula:

[0023]

[0024] Where x is the distance of the cable defect from the cable start-up point, and v is the speed at which the signal propagates in the cable, we have

[0025]

[0026] Where c is the speed of light, ε r is the relative permittivity of the cable insulation.

[0027] In step 1, for a pulse signal U1(t) with amplitude E, pulse width τ, and period T1, ignoring rising and falling edges, it can be expressed by the formula:

[0028]

[0029] Where a0 is the DC component of the pulse, ak is the amplitude of the kth harmonic of the pulse, and ω1 is the fundamental frequency of the pulse, satisfying:

[0030]

[0031]

[0032]

[0033] When satisfied ,Right now When ak=0, it means that the frequency point is missing and the amplitude is 0.

[0034] Without considering rise and fall edges, it is generally believed that the smaller the pulse width, the higher the upper limit of the measurable frequency. When the cable length is short, to maintain the accuracy of defect detection, it is often necessary to increase the upper limit of the impedance spectrum measurement frequency. However, an excessively high upper limit frequency may result in some frequency points not being effectively acquired, leading to incomplete spectral data. If the frequency information is incomplete, the impedance spectrum will be unable to fully represent the defect characteristics, thus affecting the accuracy of defect location.

[0035] In step 1, for a pulse signal U2(t) with amplitude E, pulse width τ, rising edge t1, falling edge t2, and period T1, it can be expressed by the formula:

[0036]

[0037] in

[0038]

[0039]

[0040] .

[0042] In step 1, when the spectrum range to be measured is large and the minimum pulse width is limited, the amplitude of each frequency component is not zero by changing the pulse width, rising edge and falling edge.

[0043] In step 1, by adjusting the pulse width and rising and falling edge parameters of the pulse, the missing frequency components in the broadband impedance spectrum of the cable are effectively supplemented, thereby constructing more complete impedance spectrum data to improve the accuracy of cable defect location.

[0044] In step 1, the pulse width, rising edge, and falling edge of the pulse are optimized by nonlinear programming to maximize the minimum amplitude of the frequency point, so as to effectively suppress noise interference at the cable defect location operation site.

[0045] In the measurement circuit, the internal resistance of the waveform generator needs to be matched with the impedance between the connecting signal cable, and the input impedance of the voltage signal acquisition device is set at the MΩ level, i.e., approximately open circuit.

[0046] The advantages of this invention compared to the prior art are:

[0047] 1. By adjusting the pulse width and rising and falling edge parameters, the missing frequency components in the broadband impedance spectrum of the cable are effectively supplemented, thereby constructing more complete impedance spectrum data and improving the accuracy of cable defect location.

[0048] 2. By optimizing the pulse width, rising edge, and falling edge of the pulse through nonlinear programming, the minimum amplitude of the frequency point can be maximized, which can effectively suppress noise interference in the field. Attached Figure Description

[0049] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0050] Appendix Figure 1 This is a schematic diagram of the frequency components contained in different pulses in an embodiment of the present invention (frequency components contained in 10-100-10 pulses and 10-101-86 pulses).

[0051] Appendix Figure 2 This is a schematic diagram of the cable impedance spectrum calculated based on the 10-100-10 pulse in an embodiment of the present invention;

[0052] Appendix Figure 3 This is a schematic diagram of the cable impedance spectrum calculated based on the 10-101-86 pulse in an embodiment of the present invention;

[0053] Appendix Figure 4 This is a schematic diagram of the defect location results for different pulses in an embodiment of the present invention;

[0054] Appendix Figure 5 This is a schematic diagram of the measurement circuit in an embodiment of the present invention. Detailed Implementation

[0055] As shown in the figure, a cable defect location method based on pulse waveform modulation is described. The method uses a measurement circuit containing a waveform generator to locate cable defects. When the minimum pulse width of the measurement pulse in the measurement circuit is limited, the method uses a nonlinear programming method to adjust the pulse width, pulse rising edge, and pulse falling edge to obtain a complete cable impedance spectrum without missing frequency points, and maximizes the minimum amplitude of the frequency points to compensate for missing frequency components. This results in an impedance spectrum with wider frequency coverage and better integrity for cable defect location.

[0056] The cable defect location method employs a measurement circuit that includes a waveform generator, a voltage signal acquisition device, a T-connector and connecting cable, and a switching device.

[0057] The waveform generator is used to generate pulse signals, forming an excitation signal that is injected into the cable system under test.

[0058] The voltage signal acquisition device is responsible for acquiring the voltage signal at the port of the cable system under test.

[0059] T-shaped connectors are used to connect voltage signal acquisition devices, waveform generators, and the cable system under test.

[0060] The connecting cable is a signal cable with known parameters added at the measurement end. This is because there must be an electrical connection between the waveform generator, the signal acquisition device, and the cable system under test. Here, a coaxial signal cable is used for connection, and the impact of this connection on the test cannot be ignored.

[0061] The switching device is responsible for connecting or disconnecting the cable system under test.

[0062] Step 1: Connect the measurement circuit, check whether the connection is electrically reliable, and record the length of the connecting signal cable as l0;

[0063] Step 2: The waveform generator acts as a pulse generator, outputting a square wave signal with a pulse width of w and a repetition frequency of f, i.e., a pulse signal. Turn on the voltage signal acquisition device to prepare to acquire the port voltage signal within a single cycle in steady state.

[0064] Step 3: Connect switch K, collect the voltage signal at this time, record it as Uc, and the sampling rate is F;

[0065] Step 4: Disconnect switch K, collect the voltage signal at this time, record it as Uo, and the sampling rate is F;

[0066] Step 5: Using impedance spectrum analysis algorithms, based on the pulse waveform-controlled broadband impedance spectrum measurement method for cables, under the condition of limited minimum pulse width, the missing frequency components are compensated by adjusting the pulse width and rising and falling edges, thereby obtaining an impedance spectrum with wider frequency coverage and better integrity for defect location.

[0067] In steps two, three, and four, pulses are injected into the cable under test, and the pulse signal of the cable's pulse response is measured.

[0068] Step five includes the following steps;

[0069] Step 1: Determine the frequency range of the broadband impedance spectrum of the cable to be measured, as well as the adjustment range of the pulse width, rising edge, and falling edge of the pulse generator. Use nonlinear programming to change the pulse width, rising edge, and falling edge of the pulse so that the final generated pulse has no missing frequency points within the specified frequency range, and the amplitude of the smallest frequency component in the frequency band reaches the maximum.

[0070] Step 2: Inject the pulse calculated in Step 1 into the cable under test, and use the broadband impedance spectrum measurement method based on pulse voltage to calculate the impedance spectrum Z of the cable where there are no missing frequency points.

[0071] Step 3: Perform a fast Fourier transform on the impedance spectrum to obtain... The defect location results for the cable are given according to the following formula:

[0072]

[0073] Where x is the distance of the cable defect from the cable start-up point, and v is the speed at which the signal propagates in the cable, we have

[0074]

[0075] Where c is the speed of light, ε r is the relative permittivity of the cable insulation.

[0076] In step 1, for a pulse signal U1(t) with amplitude E, pulse width τ, and period T1, ignoring rising and falling edges, it can be expressed by the formula:

[0077]

[0078] Where a0 is the DC component of the pulse, ak is the amplitude of the kth harmonic of the pulse, and ω1 is the fundamental frequency of the pulse, satisfying:

[0079]

[0080]

[0081]

[0082] When satisfied ,Right now When ak=0, it means that the frequency point is missing and the amplitude is 0.

[0083] Without considering rise and fall edges, it is generally believed that the smaller the pulse width, the higher the upper limit of the measurable frequency. When the cable length is short, to maintain the accuracy of defect detection, it is often necessary to increase the upper limit of the impedance spectrum measurement frequency. However, an excessively high upper limit frequency may result in some frequency points not being effectively acquired, leading to incomplete spectral data. If the frequency information is incomplete, the impedance spectrum will be unable to fully represent the defect characteristics, thus affecting the accuracy of defect location.

[0084] In step 1, for a pulse signal U2(t) with amplitude E, pulse width τ, rising edge t1, falling edge t2, and period T1, it can be expressed by the formula:

[0085]

[0086] in

[0087]

[0088]

[0089] .

[0091] In step 1, when the spectrum range to be measured is large and the minimum pulse width is limited, the amplitude of each frequency component is not zero by changing the pulse width, rising edge and falling edge.

[0092] In step 1, by adjusting the pulse width and rising and falling edge parameters of the pulse, the missing frequency components in the broadband impedance spectrum of the cable are effectively supplemented, thereby constructing more complete impedance spectrum data to improve the accuracy of cable defect location.

[0093] In step 1, the pulse width, rising edge, and falling edge of the pulse are optimized by nonlinear programming to maximize the minimum amplitude of the frequency point, so as to effectively suppress noise interference at the cable defect location operation site.

[0094] In the measurement circuit, the internal resistance of the waveform generator needs to be matched with the impedance between the connecting signal cable, and the input impedance of the voltage signal acquisition device is set at the MΩ level, i.e., approximately open circuit.

[0095] Example:

[0096] This example demonstrates a test application of the positioning method of this invention to a 140m long RG58 coaxial cable. A 510 Ohm resistor was connected in parallel between the core wire and the shielding layer at 60m of the cable to simulate a cable defect.

[0097] The lower limit of the pulse width is set to 100ns, and the lower limits of the rise and fall edges are 10ns. The frequency range to be measured is 1-51MHz. The optimal pulse parameters are calculated using nonlinear programming, which is a pulse with a rise edge of 10ns, a pulse width of 101ns, and a fall edge of 86ns (referred to as the 10-101-86 pulse, and the same applies below).

[0098] Figure 1 The frequency components contained in the 10-100-10 pulse and the 10-101-86 pulse are shown. It can be seen that the 10-100-10 pulse has multiple missing frequency points, while the amplitude of the 10-101-86 pulse in the frequency range of 1-51MHz is greater than 0.

[0099] Figures 2-3 The cable impedance spectrum calculated based on experimental pulse voltages is presented. A comparison shows that the broadband impedance spectrum calculated using 10-100-10 pulses exhibits significant jumps at multiple frequency points. These jumps are related to… Figure 1 The missing frequency points in the pulse waveform shown correspond perfectly; however, the impedance spectrum calculated using the 10-101-86 pulse does not show such anomalies.

[0100] Figure 4The results further demonstrate the defect location obtained from impedance spectrum inversion. In the results corresponding to the 10-100-10 pulse, besides the main peak at 60m, stray peaks that could lead to misjudgment also appeared at other locations, which is directly related to the absence of multiple frequency points. In contrast, the location results of the 10-101-86 pulse only show a clear defect peak at 60m, with smooth responses and no obvious interference in the remaining segments. These experimental results demonstrate that eliminating frequency point omissions in the impedance spectrum is crucial for accurate location and also verify the effectiveness of this patent.

Claims

1. A method for cable defect location based on pulse waveform regulation, characterized in that: The method uses a measurement circuit containing a waveform generator to locate cable defects. When the minimum pulse width of the measurement pulse in the measurement circuit is limited, the method uses a nonlinear programming method to adjust the pulse width, pulse rising edge, and pulse falling edge to obtain a complete cable impedance spectrum and maximize the minimum amplitude of the frequency point to compensate for the missing frequency components.

2. The method of claim 1, wherein: The cable defect location method employs a measurement circuit that includes a waveform generator, a voltage signal acquisition device, a connector and connecting cable, and a switching device. The waveform generator is used to generate pulse signals, forming an excitation signal that is injected into the cable system under test. The voltage signal acquisition device is responsible for acquiring the voltage signal at the port of the cable system under test. The connector is used to connect voltage signal acquisition devices, waveform generators, and the cable system under test. The connecting cable is a signal cable with known parameters added at the measurement end. This is because there must be an electrical connection between the waveform generator, the signal acquisition device, and the cable system under test. Here, a coaxial signal cable is used for connection, and the impact of this connection on the test cannot be ignored. The switching device is responsible for connecting or disconnecting the cable system under test.

3. The method of claim 1, wherein: Step 1: Connect the measurement circuit, check whether the connection is electrically reliable, and record the length of the connecting signal cable as l0; Step 2: The waveform generator acts as a pulse generator, outputting a square wave signal with a pulse width of w and a repetition frequency of f, i.e., a pulse signal. Turn on the voltage signal acquisition device to prepare to acquire the port voltage signal within a single cycle in steady state. Step 3: Connect switch K, collect the voltage signal at this time, record it as Uc, and the sampling rate is F; Step 4: Disconnect switch K, collect the voltage signal at this time, record it as Uo, and the sampling rate is F; Step 5: Using impedance spectrum analysis algorithms, based on the pulse waveform-controlled broadband impedance spectrum measurement method for cables, under the condition of limited minimum pulse width, the missing frequency components are compensated by adjusting the pulse width and rising and falling edges, thereby obtaining an impedance spectrum with wider frequency coverage and better integrity for defect location.

4. The cable defect location method based on pulse waveform modulation according to claim 3, characterized in that: In steps two, three, and four, pulses are injected into the cable under test, and the pulse signal of the cable's pulse response is measured.

5. The cable defect location method based on pulse waveform modulation according to claim 4, characterized in that: Step five includes the following steps; Step 1: Determine the frequency range of the broadband impedance spectrum of the cable to be measured, as well as the adjustment range of the pulse width, rising edge, and falling edge of the pulse generator. Use nonlinear programming to change the pulse width, rising edge, and falling edge of the pulse so that the final generated pulse has no missing frequency points within the specified frequency range, and the amplitude of the smallest frequency component in the frequency band reaches the maximum. Step 2: Inject the pulse calculated in Step 1 into the cable under test, and use the broadband impedance spectrum measurement method based on pulse voltage to calculate the impedance spectrum Z of the cable where there are no missing frequency points. Step 3: Perform a fast Fourier transform on the impedance spectrum to obtain... The defect location results for the cable are given according to the following formula: Where x is the distance of the cable defect from the cable start-up point, and v is the speed at which the signal propagates in the cable, we have where c is the speed of light, ε r is the relative permittivity of the cable insulation.

6. The cable defect location method based on pulse waveform modulation according to claim 5, characterized in that: In step 1, for a pulse signal U1(t) with amplitude E, pulse width τ, and period T1, ignoring rising and falling edges, it can be expressed by the formula: Where a0 is the DC component of the pulse, ak is the amplitude of the kth harmonic of the pulse, and ω1 is the fundamental frequency of the pulse, satisfying: When satisfied ,Right now When ak=0, it means that the frequency point is missing and the amplitude is 0.

7. The cable defect location method based on pulse waveform modulation according to claim 5, characterized in that: In step 1, for a pulse signal U2(t) with amplitude E, pulse width τ, rising edge t1, falling edge t2, and period T1, it can be expressed by the formula: in 。 8. The cable defect location method based on pulse waveform modulation according to claim 5, characterized in that: In step 1, when the spectrum range to be measured is large and the minimum pulse width is limited, the amplitude of each frequency component is not zero by changing the pulse width, rising edge and falling edge.

9. The cable defect location method based on pulse waveform modulation according to claim 5, characterized in that: In step 1, by adjusting the pulse width and rising and falling edge parameters of the pulse, the missing frequency components in the broadband impedance spectrum of the cable are supplemented to construct more complete impedance spectrum data, thereby improving the accuracy of cable defect location. In step 1, the pulse width, rising edge, and falling edge of the pulse are optimized by nonlinear programming to maximize the minimum amplitude of the frequency point, so as to suppress noise interference at the cable defect location operation site.

10. A cable defect location method based on pulse waveform modulation according to claim 2, characterized in that: In the measurement circuit, the internal resistance of the waveform generator needs to be matched with the impedance between the connecting signal cable, and the input impedance of the voltage signal acquisition device is set at the MΩ level, i.e., approximately open circuit.

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