CN116698975A - Power cable local defect detection method and system based on shielding layer injection signal - Google Patents
Power cable local defect detection method and system based on shielding layer injection signal Download PDFInfo
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Abstract
The invention relates to a method and a system for detecting local defects of a power cable based on shielding layer injection signals, wherein the method comprises the following steps: designing a detection signal, and generating an incident signal based on the detection signal; injecting an incident signal into the cable to obtain a reflected signal; measuring an incident signal and a reflected signal to obtain measurement data; acquiring a head-end reflection coefficient spectrum of the cable based on the measurement data; and detecting the local defect of the cable based on the first-end reflection coefficient spectrum of the cable, and obtaining a local defect detection result of the cable. The invention can realize the detection of the local defect of the power cable without changing the actual installation structure of the cable.
Description
Technical Field
The invention relates to the technical field of power cable local defect detection and positioning, in particular to a power cable local defect detection method and system based on shielding layer injection signals.
Background
The power cable plays an important role in the development process of national economy, however, the cable is easily influenced by external environment factors and human factors, so that defects appear in local positions of the cable, the local defects gradually evolve into cable faults along with the increase of the operational years, cable accidents are caused, typical accident explosion, fire and the like are caused, and the life and property safety of people is seriously threatened. At the same time when the cable is born, corresponding detection technology is developed, however, the current detection method is mainly concentrated in the field of off-line detection of the cable, the traditional off-line detection of the cable needs to cut off the power of the cable, and the cable is taken down from the connecting system, so that the practical application range of the off-line detection is limited. Therefore, there is a need to develop an on-line detection technique for a cable, which detects defects of the cable without affecting the operation of the cable.
Disclosure of Invention
The invention aims to provide a method and a system for detecting local defects of a power cable based on shielding layer injection signals, which do not need to change the actual installation structure of the cable, and realize the detection of the local defects of the power cable.
In order to achieve the above object, the present invention provides the following solutions:
the power cable local defect detection method based on the shielding layer injection signal comprises the following steps:
designing a detection signal, and generating an incident signal based on the detection signal;
injecting the incident signal into a cable to obtain a reflected signal;
measuring the incident signal and the reflected signal to obtain measurement data;
acquiring a head-end reflection coefficient spectrum of the cable based on the measurement data;
and detecting the local defect of the cable based on the head end reflection coefficient spectrum of the cable, and obtaining a local defect detection result of the cable.
Further, the detection signal is a step frequency signal, and the step frequency signal is:
wherein f i Is the frequency of the ith sinusoidal signal; TD (time division) i Waveform duration for the ith sinusoidal signal; d is the start time difference of the waveforms of the i-th and i+1-th sinusoidal signals.
Further, acquiring the reflected signal includes:
and injecting the incident signal into the head end of the cable through a shielding layer, wherein the incident signal is partially reflected back to the head end of the cable at the local defect of the cable due to impedance mismatch, so as to generate the reflected signal.
Furthermore, the incident signal is injected into the head end of the cable through the shielding layer and is transmitted through two transmission paths, wherein the first transmission path is a core wire, the shielding layer and a composite medium area surrounded by the boundaries of the core wire and the shielding layer, and the second transmission path is a shielding layer, a graphite layer and an insulating medium area surrounded by the boundaries of the shielding layer and the graphite layer.
Further, measuring the incident signal and the reflected signal, the obtaining measurement data comprising:
the oscilloscope is connected with the grounding wire of the cable to measure, and when the incident signal is injected into the head end of the cable through the shielding layer, the amplitude U of the incident signal is measured im The method comprises the steps of carrying out a first treatment on the surface of the Measuring the reflected signal amplitude U when the incident signal is partially reflected back to the head end of the cable fm 。
Further, obtaining a head-end reflectance spectrum of the cable based on the measurement data includes:
for the incident signal amplitude U im And the reflected signal amplitude U fm Performing operation, combining attenuation of signal amplitude to obtain the signalThe head end reflection coefficient spectrum of the cable is as follows:
wherein, I f U, which is the position of the local defect of the cable f To actually reflect the signal, U f ＝U fm X 3/2, beta is the phase coefficient, e is the natural constant.
Further, performing local defect detection on the cable based on the head end reflection coefficient spectrum of the cable, and obtaining a local defect detection result of the cable includes:
performing matched filtering processing on the first-end reflection coefficient spectrum of the cable, and constructing a cable local defect diagnosis model based on the reflection coefficient spectrum after the matched filtering processing; and acquiring a local defect detection result of the cable based on the cable local defect diagnosis model.
Further, the cable local defect diagnosis model is as follows:
in the formula, RC cs (x j ) As a defect diagnosis function of the first transmission path, H cs (f i ,x j ) Matched filter set for first transmission path, RC gs (x j ) As a defect diagnosis function of the second transmission path, H gs (f i ,x j ) For the matched filter set of the second transmission path, ρ (f i ) Is the reflectance spectrum.
In order to further optimize the technical scheme, the invention also provides a power cable local defect detection system based on shielding layer injection signals, which is characterized by comprising the following steps: the system 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 used for designing a detection signal and sending the detection signal to the signal generation module;
the signal generation module is used for generating an incident signal based on the detection signal, injecting the incident signal into a cable, generating a reflected signal, and sending the incident signal and the reflected signal to the signal measurement module;
the signal measuring module is used for detecting the incident signal and the reflected signal, generating measurement data and sending the measurement data to the signal processing module;
the signal processing module is used for calculating the measurement data, acquiring a head-end reflection coefficient spectrum of the cable, and sending the head-end reflection coefficient spectrum of the cable to the defect detection module;
the defect detection module is used for detecting the local defects of the cable based on the reflection coefficient spectrum of the head end of the cable and obtaining the positioning of the local defects of the cable.
The beneficial effects of the invention are as follows:
the invention fully considers the actual laying condition of the power cable, realizes the detection of the local defects of the cable by injecting the detection signals into the shielding layer, can not change the original laying structure of the cable, and is the basis for realizing the online detection of the local defects of the power cable; compared with the traditional method of injecting signals between the cable shielding layer and the core wires, the method does not need to change the cable installation structure, and can detect the defects of the cable shielding layer along the radially inner and outer areas.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the drawings that are needed in the embodiments will be briefly described below, it being obvious that the drawings in the following description are only some embodiments of the present invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a flow chart of a method for detecting local defects of a power cable based on a shielding layer injection signal according to an embodiment of the invention;
FIG. 2 is a diagram of a step frequency signal according to an embodiment of the present invention;
fig. 3 is a basic structural diagram of a single-core coaxial power cable according to an embodiment of the present invention;
FIG. 4 is a diagram of a centralized pi-type circuit showing a coaxial transmission line in accordance with an embodiment of the present invention;
FIG. 5 is a schematic diagram of an equivalent circuit of two signal transmission paths of a power cable according to an embodiment of the present invention;
FIG. 6 is a schematic diagram illustrating a device connection of a local defect detection method according to an embodiment of the present invention;
fig. 7 is a power cable simulation model and a cross-sectional view thereof according to an embodiment of the present invention, wherein fig. 7 (a) is an overall view of the power cable simulation model, and fig. 7 (b) is a cross-sectional view of the power cable simulation model;
fig. 8 is a frequency spectrum diagram of a reflection coefficient of a cable head end according to an embodiment of the present invention, wherein fig. 8 (a) is a reflection amplitude-frequency characteristic spectrum diagram of the cable head end, and fig. 8 (b) is a reflection phase-frequency characteristic spectrum diagram of the cable head end;
fig. 9 is a graph of a detection result of a cable partial defect according to an embodiment of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
In order that the above-recited objects, features and advantages of the present invention will become more readily apparent, a more particular description of the invention will be rendered by reference to the appended drawings and appended detailed description.
The power cable local defect detection principle based on shielding layer injection signals is as follows:
the basic structure of the single core coaxial power cable is shown in fig. 3, and the core wire, the inner semiconductive layer, the insulating layer, the outer semiconductive layer, the shielding layer, the sheath and the graphite layer are respectively arranged from the inside to the outside. When an incident signal is injected into a cable through a cable shielding layer grounding wire for transmission, the cable shielding layer grounding wire comprises two transmission paths, wherein the first transmission path is a core wire, a shielding layer and a composite medium area surrounded by the boundaries of the core wire and the shielding layer, and the second transmission path is a shielding layer, a graphite layer and an insulating medium area surrounded by the boundaries of the shielding layer and the graphite layer.
The two transmission paths can be regarded as a coaxial transmission line, and the coaxial transmission line can be represented by a centralized pi-type circuit according to an electromagnetic compatibility theory, as shown in fig. 4, R, L, C, G in fig. 4 represents a distributed resistance, a distributed inductance, a distributed capacitance and a 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 a centralized circuit model with inner and outer layers, as shown in FIG. 5, R in FIG. 5 cs 、L cs 、C cs 、G cs The distribution parameters of the core wire, the shielding layer and the boundary surrounding area respectively comprise distribution resistance, distribution inductance, distribution capacitance and distribution conductance, R gc 、L gc 、C gc 、G gc Representing the distribution parameters of the shielding layer, the sheath and the area surrounded by the boundary of the shielding layer and the sheath, including distributed resistance, distributed inductance, distributed capacitance and distributed conductance, U s Is the power frequency high-voltage signal on the core wire, u s The high-frequency low-voltage signal applied on the shielding layer is used as a detection signal of the local defect of the cable, and the graphite layer of the cable is grounded through the grounding metal support.
Respectively calculating the propagation coefficients of the two transmission paths, wherein the calculation formula is as follows:
wherein, gamma cs A propagation coefficient representing a first transmission path; gamma ray gc Representing a propagation coefficient of the second transmission path; r is R cs 、L cs 、C cs 、G cs Respectively represent the distributed resistance, the distributed inductance, the distributed capacitance and the distributed conductance of the first transmission path；R gc 、L gc 、C gc 、G gc The distributed resistance, the distributed inductance, the distributed capacitance and the distributed conductance of the second transmission path are respectively represented.
The real parts of the two propagation coefficients are attenuation coefficients, the imaginary parts are phase coefficients, the attenuation coefficients represent the energy loss condition of the electromagnetic signals in the cable, and the phase coefficients represent the phase change condition of the electromagnetic signals propagating 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.
Based on the above principle, the present embodiment provides a method for detecting a local defect of a power cable based on a shielding layer injection signal, as shown in fig. 1, including:
s1, designing a detection signal in an upper computer
The incident signal used as the detection signal is designed in the upper computer, and the incident signal is designed as a step frequency signal in the embodiment, as shown in fig. 2, f 0 For the starting frequency of the signal, Δf is the magnitude of the step frequency, n is the number of frequency steps, D is the signal duration of a single frequency point, and T is the cycle time of a single frequency point. The step frequency signal not only comprises time domain information of the signal, and is convenient for determining time delay of the reflected signal, but also comprises frequency domain information of the signal, and the frequency spectrum of the reflected signal at the head end of the cable can be obtained through one-time measurement.
For step frequency signals, the following signals can be designed and input into an arbitrary function generator to generate corresponding arbitrary waveforms with amplitude of 1V:
wherein f i Is the frequency of the ith sinusoidal signal; TD (time division) i Waveform duration for the ith sinusoidal signal; d is the start time difference of the i-th and i+1-th sine waveforms.
In order to ensure that the reflected signal of the ith sine wave does not overlap with the i+1 sine wave at the input end, D and TD are needed i Limiting:
wherein L is the length of a 50 omega signal cable, v op Is the signal transmission speed.
S2, controlling a signal generator to generate an incident signal
And sending the detection signal designed in the upper computer to a signal generator through a communication bus, modulating the detection signal by the signal generator to generate an incident signal, injecting the incident signal into a cable through a cable shielding layer grounding wire, and partially reflecting the incident signal back to the head end of the cable at the local defect of the cable due to discontinuous characteristic impedance at the two ends of the cable to generate a reflected signal. For a step frequency signal, the incident signal and the reflected signal are a series of step frequency sinusoidal signals, and the frequency components between the incident signal and the reflected signal are the same, and the difference is mainly in amplitude.
The working process of signal reflection is as follows:
as a coaxial structure transmission line, the power cable needs to be characterized by using a distribution parameter R, L, C, G, which is mainly determined by the structure and material parameters of the cable. The calculation formula of the characteristic impedance of the healthy cable is as follows:
local defects in the cable occur, and the material or structural parameters at the position change, so that the distribution parameters of the cable change, and the characteristic impedance changes, and the signal is reflected. The characteristic impedance of the cable defect section is recorded as Z d The reflection coefficient of the position signal is:
s3, controlling an oscilloscope to measure incident signals and reflected signals
The oscilloscope is connected with the grounding wire of the cable, and the upper computer controls the oscilloscope to detect the amplitude U of an incident signal input into the cable im And uploading the data to an upper computer for storage. When the incident signal is partially reflected back to the cable head end, the oscilloscope captures the amplitude U of the reflected signal fm And uploading the data to an upper computer for storage.
S4, processing the incident signal and the reflected signal to obtain a reflection coefficient spectrum
The upper computer measures the amplitude U of the incident signal im And the reflected signal amplitude U fm And (5) performing operation to obtain a head-end reflection coefficient spectrum of the cable. As shown in FIG. 6, since the T-connector is present at the junction of the signal generator, oscilloscope and signal cable, the amplitude of the signal is attenuated by 1/3 while passing through the T-connector, so the actual reflected signal amplitude should be U fm X 3/2, the reflectance spectrum is therefore:
wherein, I f Is the location of a localized defect in the cable. A reflection coefficient can be calculated at each frequency point by using the step frequency signal as an incident signal, so that a complete reflection coefficient spectrum is obtained.
S5, processing the head-end reflection coefficient spectrum of the cable to realize the positioning of the local defects of the cable
And detecting the local defects of the cable by using the upper computer to detect the first section reflection coefficient spectrum of the cable, so as to realize the positioning of the local defects of the cable.
The reflectance spectrum processing process is as follows:
the reflection coefficient spectrum is subjected to matched filtering, and a matched filter set is firstly constructed:
and then compensating the attenuation of the signal in the cable, namely introducing an attenuation constant in the matched filter set, and correcting the expression as follows:
gamma is the propagation constant of the cable, and for the diagnosis of cable core wires, shielding layers and local defects of the cable in the area surrounded by the cable core wires and the shielding layers, the matched filter set is as follows:
for the diagnosis of local defects of the cable in the areas surrounded by the shielding layer and the graphite layer, the matched filter set is as follows:
after the reflection coefficient spectrum is matched and filtered, the reflection coefficient spectrum is accumulated along the frequency dimension and divided by the total number of frequency points to obtain a diagnosis function of the cable local defect:
the spatial spectrum of cable fault diagnosis is obtained through the processing, the peak value in the spatial spectrum curve represents the local defect of the cable, the peak value position is the position of the local defect, and the peak value size is the reflection coefficient of the local defect. The extent of the cable local defect can be assessed on the basis of the reflection coefficient.
To further optimize the technical solution, the present embodiment further provides a power cable local defect detection system based on a shielding layer injection signal, including: the system 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 embedded in the upper computer and is used for designing detection signals by using a window operating system.
The signal generating module adopts a signal generator for generating an incident signal based on the detection signal, the waveform function can be programmed, and the amplitude and the frequency of the incident signal can be changed at will within the performance range of the instrument; the signal generating module is also used for injecting the generated incident signal into the cable to generate a reflected signal.
The signal measuring module adopts an oscilloscope, and is mainly used for collecting waveform data of an incident signal and a reflected signal and uploading the data to the signal processing module.
The signal processing module is embedded in the upper computer and is used for calculating the incident signal and the reflected signal data and obtaining the head-end reflection coefficient spectrum of the cable.
The defect detection module is embedded in the upper computer and is used for detecting the local defects of the cable based on the reflection coefficient spectrum of the head end of the cable, judging whether the cable has defects, and if so, acquiring the positioning of the local defects of the cable.
The signal design module, the signal generation module, the signal measurement module, the signal processing module and the defect detection module are sequentially connected, the upper computer is connected with the signal generator and the oscilloscope through the communication bus, and the signal generator, the oscilloscope and the signal cable are connected through the T-shaped connector.
Verification of the validity of the invention:
the effectiveness of the invention is verified through CST simulation, the power cable simulation model and the cross-sectional view thereof are shown in the figure 7, the figure 7 (a) is the whole power cable simulation model, and the 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 respectively arranged in an insulating layer and a sheath area of the cable, wherein the defect positions of the insulating area are located at 4.5 meters and 9 meters, and the defect positions of the sheath area are located at 2 meters and 6 meters. The obtained frequency spectrum of the reflection coefficient of the cable head end is shown in fig. 8, wherein fig. 8 (a) is a reflection amplitude-frequency characteristic spectrum of the cable head end, and fig. 8 (b) is a reflection phase-frequency characteristic spectrum of the cable head end. The reflection coefficient spectrum is processed by using a cable local defect detection method based on the reflection coefficient spectrum, so that a detection result of the cable local defect is shown in fig. 9, four 5 obvious peaks can be seen from fig. 9, and the four peaks correspond to the local defect and the terminal position of the cable respectively. The detection results of the local defects of the insulating layer are 1.984 meters and 6.09 meters, the detection results of the local defects of the sheath area are 4.462 meters and 9.018 meters, and the cable terminal position is 10 meters. The detection result is basically consistent with the preset position, and the effectiveness of detecting the local defects of the cable through the injection signal of the cable shielding layer is verified.
The above embodiments are merely illustrative of the preferred embodiments of the present invention, and the scope of the present invention is not limited thereto, but various modifications and improvements made by those skilled in the art to which the present invention pertains are made without departing from the spirit of the present invention, and all modifications and improvements fall within the scope of the present invention as defined in the appended claims.
Claims (9)
1. The power cable local defect detection method based on the shielding layer injection signal is characterized by comprising the following steps of:
designing a detection signal, and generating an incident signal based on the detection signal;
injecting the incident signal into a cable to obtain a reflected signal;
measuring the incident signal and the reflected signal to obtain measurement data;
acquiring a head-end reflection coefficient spectrum of the cable based on the measurement data;
and detecting the local defect of the cable based on the head end reflection coefficient spectrum of the cable, and obtaining a local defect detection result of the cable.
2. The method for detecting a local defect of a power cable based on a shielding layer injection signal according to claim 1, wherein the detection signal is a step frequency signal, and the step frequency signal is:
wherein f i Is the frequency of the ith sinusoidal signal; TD (time division) i Waveform duration for the ith sinusoidal signal; d is the start time difference of the waveforms of the i-th and i+1-th sinusoidal signals.
3. The method for detecting a local defect in a power cable based on a shield injection signal according to claim 1, wherein acquiring the reflected signal comprises:
and injecting the incident signal into the head end of the cable through a shielding layer, wherein the incident signal is partially reflected back to the head end of the cable at the local defect of the cable due to impedance mismatch, so as to generate the reflected signal.
4. A method for detecting a local defect of a power cable based on a shielding layer injection signal according to claim 3, wherein 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 is a core wire, the shielding layer and a composite medium area surrounded by the boundaries of the core wire and the shielding layer, and the second transmission path is a shielding layer, a graphite layer and an insulating medium area surrounded by the boundaries of the shielding layer and the graphite layer.
5. A method of detecting a local defect in a power cable based on a shield injection signal as in claim 3, wherein measuring the incident signal and the reflected signal, obtaining measurement data comprises:
the oscilloscope is connected with the grounding wire of the cable to measure, and when the incident signal is injected into the head end of the cable through the shielding layer, the amplitude U of the incident signal is measured im The method comprises the steps of carrying out a first treatment on the surface of the Measuring the reflected signal amplitude U when the incident signal is partially reflected back to the head end of the cable fm 。
6. The method for detecting a local defect in a power cable based on a shield injection signal according to claim 5, wherein acquiring a head-end reflectance spectrum of the cable based on the measurement data comprises:
for the incident signal amplitude U im And the reflected signal amplitude U fm Performing operation, and combining attenuation of signal amplitude to obtain a head-end reflection coefficient spectrum of the cable, wherein the head-end reflection coefficient spectrum is as follows:
wherein, I f U, which is the position of the local defect of the cable f To actually reflect the signal, U f ＝U fm X 32, β is the phase coefficient, e is the natural constant.
7. The method for detecting a local defect of a power cable based on a shielding layer injection signal according to claim 6, wherein performing a local defect detection of the cable based on a head end reflection coefficient spectrum of the cable, obtaining a local defect detection result of the cable comprises:
performing matched filtering processing on the first-end reflection coefficient spectrum of the cable, and constructing a cable local defect diagnosis model based on the reflection coefficient spectrum after the matched filtering processing; and acquiring a local defect detection result of the cable based on the cable local defect diagnosis model.
8. The method for detecting a local defect in a power cable based on a shield injection signal according to claim 7, wherein the cable local defect diagnosis model is:
in the formula, RC cs (x j ) As a defect diagnosis function of the first transmission path, H cs (f i ,x j ) Matched filter set for first transmission path, RC gs (x j ) As a defect diagnosis function of the second transmission path, H gs (f i ,x j ) For the matched filter set of the second transmission path, ρ (f i ) Is the reflectance spectrum.
9. Power cable local defect detecting system based on shielding layer injection signal, characterized by comprising: the system 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 used for designing a detection signal and sending the detection signal to the signal generation module;
the signal generation module is used for generating an incident signal based on the detection signal, injecting the incident signal into a cable, generating a reflected signal, and sending the incident signal and the reflected signal to the signal measurement module;
the signal measuring module is used for detecting the incident signal and the reflected signal, generating measurement data and sending the measurement data to the signal processing module;
the signal processing module is used for calculating the measurement data, acquiring a head-end reflection coefficient spectrum of the cable, and sending the head-end reflection coefficient spectrum of the cable to the defect detection module;
the defect detection module is used for detecting the local defects of the cable based on the reflection coefficient spectrum of the head end of the cable and obtaining the positioning of the local defects of the cable.
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