Method and system for detecting partial defects in power cables based on shield layer injection signals

Abstract

The application relates to a power cable local defect detection method and system based on a shielding layer injection signal, and comprises the following steps: designing a detection signal, generating an incident signal based on the detection signal; injecting the incident signal into a cable, and acquiring a reflected signal; measuring the incident signal and the reflected signal, and acquiring measurement data; acquiring a first-end reflection coefficient spectrum of the cable based on the measurement data; and performing cable local defect detection based on the first-end reflection coefficient spectrum of the cable, and acquiring a cable local defect detection result. The application can realize power cable local defect detection without changing the actual installation structure of the cable.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of partial defect detection and positioning of power cables, and particularly relates to a partial defect detection method and system of a power cable based on a shield layer injection signal. BACKGROUND

[0002] Power cables play an important role in the development of the national economy. However, the cables are susceptible to external environmental factors and human factors, which can cause defects in local positions. With the increase of the operation period, the local defects will gradually evolve into cable faults, causing cable accidents, such as typical accidents of explosion and fire, which seriously threaten the safety of people's lives and property. Almost at the same time as the birth of the cable, the corresponding detection technology also develops. However, the current detection method is mainly concentrated in the offline detection field of the cable. The traditional cable offline detection needs to power off the cable and take the cable out of the connection system, so that the practical application range of the offline detection is limited. Therefore, it is necessary to develop an online detection technology of the cable to detect the defects of the cable without affecting the operation of the cable. SUMMARY

[0003] The purpose of the present application is to provide a partial defect detection method and system of a power cable based on a shield layer injection signal, which can realize the partial defect detection of the power cable without changing the actual installation structure of the cable.

[0004] To achieve the above purpose, the present application provides the following solutions.

[0005] The partial defect detection method of the power cable based on the shield layer injection signal comprises the following steps.

[0006] A detection signal is designed, and an incident signal is generated based on the detection signal.

[0007] The incident signal is injected into the cable, and a reflected signal is obtained.

[0008] The incident signal and the reflected signal are measured to obtain measurement data.

[0009] The first-end reflection coefficient spectrum of the cable is obtained based on the measurement data.

[0010] The partial defect detection of the cable is performed based on the first-end reflection coefficient spectrum of the cable to obtain a partial defect detection result of the cable.

[0011] Further, the detection signal is a step frequency signal, and the step frequency signal is as follows.

[0012]

[0013] In the formula, f i is the frequency of the i th sine signal; TDi is the waveform duration of the ith sinusoidal signal; D is the start time difference of the waveforms of the ith and (i+1)th sinusoidal signals.

[0014] Further, the acquiring the reflected signal comprises:

[0015] The incident signal is injected into the head end of the cable through the shielding layer, and at the local defect of the cable, the incident signal is partially reflected back to the head end of the cable due to impedance mismatch, generating the reflected signal.

[0016] Further, the incident signal is injected into the head end of the cable through the shielding layer and is transmitted through two transmission paths, the first transmission path being the composite medium region surrounded by the core wire, the shielding layer and their boundaries, and the second transmission path being the insulating medium region surrounded by the shielding layer, the graphite layer and their boundaries.

[0017] Further, the measuring the incident signal and the reflected signal to obtain the measurement data comprises:

[0018] The oscilloscope is connected to the ground wire of the cable for measurement, when the incident signal is injected into the head end of the cable through the shielding layer, the incident signal amplitude U im is measured; when the incident signal is partially reflected back to the head end of the cable, the reflected signal amplitude U fm is measured.

[0019] Further, the acquiring the cable head end reflection coefficient spectrum based on the measurement data comprises:

[0020] The incident signal amplitude U im and the reflected signal amplitude U fm are operated, and the signal amplitude attenuation is combined to obtain the cable head end reflection coefficient spectrum as follows:

[0021]

[0022] In the formula, l f is the position of the local defect of the cable, U f is the actual reflected signal, U f = U fm × 3 / 2, β is a phase coefficient, and e is a natural constant.

[0023] Further, the acquiring the cable local defect detection result based on the cable head end reflection coefficient spectrum comprises:

[0024] The reflection coefficient spectrum at the beginning of the cable is subjected to matched filtering, and a local defect diagnosis model of the cable is constructed based on the reflected coefficient spectrum after matched filtering; based on the local defect diagnosis model of the cable, the local defect detection results of the cable are obtained.

[0025] Furthermore, the cable local defect diagnosis model is as follows:

[0026]

[0027]

[0028] In the formula, RC cs (x j H is the defect diagnosis function for the first transmission path. cs (f i ,x j ) represents the matched filter set for the first transmission path, RC gs (x j H is the defect diagnosis function for the second transmission path. gs (f i ,x j ) represents the set of matched filters for the second transmission path, ρ(f i () represents the reflection coefficient spectrum.

[0029] To further optimize the technical solution, the present invention also provides a power cable local defect detection system based on shielding layer injection signal, characterized in that it includes: a signal design module, a signal generation module, a signal measurement module, a signal processing module, and a defect detection module;

[0030] The signal design module is used to design the detection signal and send the detection signal to the signal generation module;

[0031] The signal generation module is used to generate an incident signal based on the detection signal, inject the incident signal into the cable to generate a reflected signal, and send the incident signal and the reflected signal to the signal measurement module.

[0032] The signal measurement module is used to detect the incident signal and the reflected signal, generate measurement data, and send the measurement data to the signal processing module;

[0033] The signal processing module is used to perform calculations on the measurement data, obtain the reflection coefficient spectrum of the cable's head end, and send the reflection coefficient spectrum of the cable's head end to the defect detection module.

[0034] The defect detection module is used to detect local defects in the cable based on the reflection coefficient spectrum at the beginning of the cable, and to locate the local defects in the cable.

[0035] The present application has the following advantages:

[0036] The present application fully considers the actual laying condition of power cable, and realizes the detection of local defects of the cable by injecting a detection signal into the shielding layer. This method can realize the on-line detection of local defects of power cable without changing the original laying structure of the cable, and is the basis for realizing the on-line detection of local defects of power cable. Compared with the traditional method of injecting a signal between the shielding layer and the core of the cable, this method does not need to change the installation structure of the cable, and can detect defects in the radial inner and outer areas of the shielding layer of the cable. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments. Obviously, the drawings in the following description only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor based on these drawings.

[0038] Figure 1 The flow chart of the local defect detection method of the power cable based on the signal injection into the shielding layer according to the embodiment of the present application;

[0039] Figure 2 The stepped frequency signal diagram according to the embodiment of the present application;

[0040] Figure 3 The basic structure diagram of the single-core coaxial power cable according to the embodiment of the present application;

[0041] Figure 4 The lumped π-type circuit diagram representing the coaxial structure transmission line according to the embodiment of the present application;

[0042] Figure 5 The equivalent circuit model diagram of the two signal transmission paths of the power cable according to the embodiment of the present application;

[0043] Figure 6 The device connection schematic diagram of the local defect detection method according to the embodiment of the present application;

[0044] Figure 7 The power cable simulation model and its cross-sectional view according to the embodiment of the present application, wherein, Figure 7 (a) is the overall view of the power cable simulation model, Figure 7 (b) is the cross-sectional view of the power cable simulation model;

[0045] Figure 8 The cable head end reflection coefficient frequency spectrum diagram according to the embodiment of the present application, wherein, Figure 8 (a) is the reflection amplitude frequency characteristic spectrum diagram of the cable head end, Figure 8 (b) is the reflection phase frequency characteristic spectrum diagram of the cable head end;

[0046] Figure 9 The detection result graph of the partial defect of the cable of the embodiment of the present application. DETAILED DESCRIPTION

[0047] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0048] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0049] The principle of the partial defect detection of the power cable based on the signal injection of the shielding layer is as follows:

[0050] The basic structure of the single-core coaxial power cable is as shown in Figure 3 From inside to outside, it is core wire, inner semi-conductive layer, insulation layer, outer semi-conductive layer, shielding layer, sheath and graphite layer. When the incident signal is injected into the cable through the shielding layer grounding wire for transmission, there are two transmission paths, the first transmission path is the composite dielectric region surrounded by the core wire, the shielding layer and their boundaries, and the second transmission path is the insulation dielectric region surrounded by the shielding layer, the graphite layer and their boundaries.

[0051] The two transmission paths can be regarded as the coaxial structure transmission line processing. According to the introduction of electromagnetic compatibility, the coaxial structure transmission line can be represented by the lumped π-type circuit, as shown in Figure 4 , Figure 4 In the formula, R, L, C, G respectively represent the distributed resistance, distributed inductance, distributed capacitance and distributed conductance of the cable, and Δx is the length of the transmission line. The two signal transmission paths of the power cable can be equivalent to the lumped circuit model of the inner and outer two layers, as shown in Figure 5 , Figure 5 In the formula, R cs , L cs , C cs , G cs respectively represent the distributed parameters of the core wire, the shielding layer and the region surrounded by their boundaries, including the distributed resistance, the distributed inductance, the distributed capacitance and the distributed conductance, R gc , L gc , C gc , G gc represent the distributed parameters of the shielding layer, the sheath and the region surrounded by their boundaries, including the distributed resistance, the distributed inductance, the distributed capacitance and the distributed conductance, U s is the power frequency high voltage signal on the core wire, and u sThe high-frequency low-voltage signal applied on the shielding layer is a detection signal of the local defect of the cable, and the graphite layer of the cable is grounded through the grounded metal support.

[0052] The propagation coefficients of the two transmission paths are calculated respectively, and the calculation formula is:

[0053]

[0054] In the formula, γ cs represents the propagation coefficient of the first transmission path; γ gc represents the propagation coefficient of the second transmission path; R cs , L cs , C cs , and G cs respectively represent the distributed resistance, distributed inductance, distributed capacitance, and distributed conductance of the first transmission path; R gc , L gc , C gc , and G gc respectively represent the distributed resistance, distributed inductance, distributed capacitance, and distributed conductance of the second transmission path.

[0055] The real part of the two propagation coefficients is the attenuation coefficient, and the imaginary part is the phase coefficient. The attenuation coefficient represents the energy loss of the electromagnetic signal in the cable, and the phase coefficient represents the phase change of the electromagnetic signal along the cable. The energy attenuation of the electromagnetic signal in the propagation process can be compensated by the attenuation coefficient, and the local defect of the cable can be positioned by the phase coefficient.

[0056] Based on the above principle, the embodiment provides a power cable local defect detection method based on shielding layer injection signal, as shown in Figure 1 , which comprises the following steps:

[0057] S1. Designing a detection signal in the upper computer

[0058] An incident signal used as a detection signal is designed in the upper computer. In the embodiment, the incident signal is designed as a step frequency signal, as shown in Figure 2 , in which f0 is the starting frequency of the signal, Δf is the size of the step frequency, n is the frequency step number, D is the duration of a single frequency point signal, and T is the cycle time of a single frequency point. The step frequency signal contains both the time domain information of the signal, which is convenient for determining the time delay of the reflected signal, and the frequency domain information of the signal, so that the frequency spectrum of the head-end reflected signal of the cable can be obtained through one measurement.

[0059] For the step frequency signal, the following signal can be designed and input into an arbitrary function generator to generate a corresponding arbitrary waveform with an amplitude of 1V:

[0060]

[0061] where f i is the frequency of the i-th sinusoidal signal; TD i is the waveform duration of the i-th sinusoidal signal; D is the start time difference of the i-th and i+1-th sinusoidal waveform.

[0062] In order to ensure that the reflection signal of the i-th sinusoidal wave does not overlap with the i+1-th sinusoidal waveform at the input end, D and TD i need to be limited:

[0063]

[0064]

[0065] where L is the length of the 50Ω signal cable, v op is the signal transmission speed.

[0066] S2. The control signal generator generates an incident signal

[0067] The detection signal designed in the host computer is sent to the signal generator through the communication bus, and the incident signal is generated after being modulated by the signal generator. The incident signal is injected into the cable through the cable shielding layer grounding line. Due to the discontinuity of the characteristic impedance at both ends of the cable, the incident signal will be partially reflected back to the cable head at the local defect of the cable, generating a reflected signal. For the stepped frequency signal, both the incident signal and the reflected signal are a series of stepped frequency sinusoidal signals, and the frequency components between the incident signal and the reflected signal are the same, and the difference is mainly in the amplitude.

[0068] wherein the working process of signal reflection is:

[0069] As a coaxial structure transmission line, power cable needs to use distributed parameters R, L, C, G to describe the characteristics of the cable, and the distributed parameters are mainly determined by the structure and material parameters of the cable. The characteristic impedance calculation formula of the healthy cable is:

[0070]

[0071] When the cable has a local defect, the material or structure parameters at this position change, which causes the change of the distributed parameters of the cable, and further causes the change of the characteristic impedance, resulting in signal reflection. Let the characteristic impedance of the cable defect section be Z d , then the reflection coefficient of the signal at this position is:

[0072]

[0073] S3. The control oscilloscope measures the incident signal and the reflected signal

[0074] The oscilloscope connects the ground wire of the cable, and the host computer controls the oscilloscope to detect the incident signal amplitude U input into the cable im , and uploads it to the host computer storage. When the incident signal is partially reflected back to the cable head, the oscilloscope captures the reflected signal amplitude U fm , and uploads it to the host computer storage.

[0075] S4. Process incident signal and reflected signal to obtain reflection coefficient spectrum

[0076] The host computer calculates the incident signal amplitude U im and the reflected signal amplitude U fm , and obtains the head reflection coefficient spectrum of the cable. As shown in Figure 6 , due to the existence of T-type connectors at the connection of the signal generator, the oscilloscope and the signal cable, when passing through the T-type connector, the amplitude of the signal is attenuated by 1 / 3, therefore the actual reflected signal amplitude should be U fm ×3 / 2, so the reflection coefficient spectrum is:

[0077]

[0078] In the formula, l f is the position of the local defect of the cable. Using a step frequency signal as the incident signal, a reflection coefficient can be calculated at each frequency point, so a complete reflection coefficient spectrum is obtained.

[0079] S5. Process the head reflection coefficient spectrum of the cable to realize the positioning of the local defect of the cable

[0080] The host computer is used to process the head reflection coefficient spectrum of the cable to realize the positioning of the local defect of the cable.

[0081] The reflection coefficient spectrum processing process is as follows:

[0082] Matched filtering is performed on the reflection coefficient spectrum. First, a matched filter set is constructed:

[0083]

[0084] Then, the attenuation of the signal in the cable is compensated, that is, an attenuation constant is introduced into the matched filter set, and its expression is corrected as:

[0085]

[0086] γ is the propagation constant of the cable. For the diagnosis of the cable core, the shielding layer and the local defect of the region surrounded by them, the matched filter set is:

[0087]

[0088] For the diagnosis of the shielding layer, the graphite layer and the cable partial defect of the region surrounded by them, the matched filter set is:

[0089]

[0090] After the reflection coefficient spectrum is matched filtered, the diagnosis function of the cable partial defect is obtained by accumulating along the frequency dimension and dividing by the total number of frequency points:

[0091]

[0092]

[0093] The spatial spectrum of the cable fault diagnosis is obtained through the above processing, the peak value in the spatial spectrum curve represents that the cable has a partial defect, the peak position is the position of the partial defect, and the peak size is the reflection coefficient of the partial defect. According to the reflection coefficient, the degree of the cable partial defect can be evaluated.

[0094] To further optimize the technical scheme, the embodiment also provides a power cable partial defect detection system based on a shielding layer injection signal, which comprises a signal design module, a signal generation module, a signal measurement module, a signal processing module and a defect detection module.

[0095] The signal design module is embedded in an upper computer, uses a window operating system, and is used for designing a detection signal.

[0096] The signal generation module adopts a signal generator and is used for generating an incident signal based on the detection signal. The waveform function can be programmed, and the amplitude and frequency of the incident signal can be changed arbitrarily within the performance range of the instrument. The signal generation module is also used for injecting the generated incident signal into the cable to generate a reflected signal.

[0097] The signal measurement module adopts an oscilloscope and is mainly used for collecting waveform data of the incident signal and the reflected signal and uploading the data to the signal processing module.

[0098] The signal processing module is embedded in the upper computer and is used for operating the incident signal and the reflected signal data to obtain the head-end reflection coefficient spectrum of the cable.

[0099] The defect detection module is embedded in the upper computer and is used for detecting the cable partial defect based on the head-end reflection coefficient spectrum of the cable, determining whether the cable has a defect, and obtaining the positioning of the cable partial defect if there is a defect.

[0100] The signal design module, the signal generation module, the signal measurement module, the signal processing module and the defect detection module are connected in sequence, the upper computer is connected with the signal generator and the oscilloscope through a communication bus, and the signal generator, the oscilloscope and the signal cable are connected through a T-shaped connector.

[0101] Verification of the effectiveness of the present application:

[0102] The effectiveness of the present application is verified by CST simulation, and the power cable simulation model and its cross-sectional view are shown in Figure 7, Figure 7 (a) is the overall view of the power cable simulation model, Figure 7 (b) is the cross-sectional view of the power cable simulation model. The total length of the cable is 10 meters, and local defects are set in the insulation layer and the sheath area of the cable, respectively. The defect positions in the insulation area are located at 4.5 meters and 9 meters, and the defect positions in the sheath area are located at 2 meters and 6 meters. The obtained cable head end reflection coefficient frequency spectrum is shown in Figure 8 , wherein, Figure 8 (a) is the cable head end reflection amplitude frequency characteristic spectrum, Figure 8 (b) is the cable head end reflection phase frequency characteristic spectrum. The reflection coefficient spectrum is processed using the cable local defect detection method based on the reflection coefficient spectrum, and the detection result of the cable local defect is shown in Figure 9 From Figure 9 , it can be seen that there are four obvious peaks, respectively corresponding to the local defects and the terminal position of the cable. The detection results of the insulation layer local defects are 1.984 meters and 6.09 meters, the detection results of the local defects in the sheath area are 4.462 meters and 9.018 meters, and the cable terminal position is 10 meters. The detection results are basically consistent with the preset positions, verifying the effectiveness of detecting the cable local defects by injecting signals through the cable shielding layer.

[0103] The above-described embodiments are only descriptions of the preferred modes of the present application and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method for detecting local defects in a power cable based on a shield layer injecting a signal, characterized in that, The method comprises: designing a detection signal, generating an incident signal based on the detection signal; injecting the incident signal into the cable, and obtaining a reflected signal; measuring the incident signal and the reflected signal, and obtaining measurement data; obtaining a first-end reflection coefficient spectrum of the cable based on the measurement data; performing cable partial defect detection based on the first-end reflection coefficient spectrum of the cable, and obtaining a partial defect detection result of the cable; the incident signal is injected into the first end of the cable through the shielding layer and is transmitted through two transmission paths, the first transmission path is a composite medium region surrounded by the core wire, the shielding layer and their boundaries, and the second transmission path is an insulating medium region surrounded by the shielding layer, the graphite layer and their boundaries.

2. The shielded layer injection signal-based power cable partial defect detection method according to claim 1, characterized by, The detection signal is a stepped frequency signal, and the stepped frequency signal is: ； wherein is the frequency of the ith sinusoidal signal; is the waveform duration of the ith sinusoidal signal; D is the difference in the starting time of the waveforms of the i-th and i+1-th sinusoidal signals.

3. The shielded layer injection signal-based power cable partial defect detection method according to claim 1, characterized by, Obtaining the reflected signal comprises: injecting the incident signal into the first end of the cable through the shielding layer, and due to impedance mismatch at the partial defect of the cable, the incident signal is partially reflected back to the first end of the cable to generate the reflected signal.

4. The shielded layer injection signal-based power cable partial defect detection method according to claim 3, characterized by, Measuring the incident signal and the reflected signal, and obtaining measurement data comprises: connecting the oscilloscope to the ground line of the cable, measuring the incident signal amplitude U when the incident signal is injected into the first end of the cable through the shield im ; measuring the reflected signal amplitude U when the incident signal is partially reflected back into the first end of the cable fm .

5. The shielded layer injection signal based power cable partial defect detection method according to claim 4, characterized in that, obtaining the first-end reflection coefficient spectrum of the cable based on the measurement data comprises: The incident signal amplitude U im and the reflected signal amplitude U fm are operated on, and the attenuation of the signal amplitude is combined to obtain the head-end reflection coefficient spectrum of the cable: ； wherein is the position of the local defect of the cable, U f is the actual reflected signal, U f = β is a phase coefficient and e is the natural constant.

6. The shielded layer injection signal-based power cable partial defect detection method according to claim 5, characterized by, performing cable partial defect detection based on the first-end reflection coefficient spectrum of the cable, and obtaining a partial defect detection result of the cable comprises: performing matched filtering processing on the first-end reflection coefficient spectrum of the cable, constructing a cable partial defect diagnosis model based on the matched filtering processed reflection coefficient spectrum, and obtaining the partial defect detection result of the cable based on the cable partial defect diagnosis model.

7. The shielded layer injection signal based power cable partial defect detection method according to claim 6, characterized in that, The cable partial defect diagnosis model is: ； ； where RC cs (x j ) is a defect diagnostic function for the first transmission path, H cs (f i ,x j ) is a matched filter set for the first transmission path, RC gc (x j ) is a defect diagnostic function for the second transmission path, H gc (f i ,x j ) is a matched filter set for the second transmission path, and p(f i ) is a reflection coefficient spectrum.

8. A power cable partial defect detection system based on shield layer injection signal, characterized by, The method comprises: a signal design module, a signal generation module, a signal measurement module, a signal processing module and a defect detection module; the signal design module is configured to design a detection signal and send the detection signal to the signal generation module; the signal generation module is configured to generate an incident signal based on the detection signal, inject the incident signal into a cable, generate a reflected signal, and send the incident signal and the reflected signal to the signal measurement module; the incident signal is injected into the first end of the cable through the shielding layer and is transmitted through two transmission paths, the first transmission path is a composite medium region surrounded by the core wire, the shielding layer and their boundaries, and the second transmission path is an insulating medium region surrounded by the shielding layer, the graphite layer and their boundaries; the signal measurement module is configured to detect the incident signal and the reflected signal, generate measurement data, and send the measurement data to the signal processing module; the signal processing module is configured to operate on the measurement data, obtain a first-end reflection coefficient spectrum of the cable, and send the first-end reflection coefficient spectrum of the cable to the defect detection module; the defect detection module is configured to perform cable partial defect detection based on the first-end reflection coefficient spectrum of the cable, and obtain the positioning of the cable partial defect.

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