Cable defect online detection method based on frequency domain reflection

Abstract

The invention relates to a cable defect online detection method based on frequency domain reflection. The method comprises the following steps: S1, connecting a test device; s2, generating and injecting an anti-interference spread spectrum excitation signal; s3, acquiring a mixed response signal; s4, reflection response signal extraction based on cross-correlation; and S5, performing frequency domain transformation and positioning map calculation. According to the invention, power frequency interference can be effectively suppressed under the condition of live operation of the cable, broadband signal coupling and response acquisition are realized, the technical blank in the field is filled, and cable defect detection is completed.

Description

Technical Field

[0001] This invention belongs to the field of cable insulation defect detection technology, specifically relating to an online cable defect detection method based on frequency domain reflection. Background Technology

[0002] Power cables are a crucial component of modern power systems, and their operational status directly impacts power grid safety and public safety. With ongoing urbanization, the scale and service life of my country's existing power grid are continuously increasing, leading to increasingly prominent issues such as cable aging and insulation defects, which seriously threaten the safe operation of the power grid.

[0003] Currently, frequency domain reflection (FDR), as a non-destructive cable defect detection technology, has shown its application potential in the field of offline cable inspection. FDR can characterize changes in cable insulation over a wide frequency range and has high sensitivity to localized cable defects. However, existing FDR methods are mainly designed for cables under offline conditions and are difficult to implement during cable operation, failing to meet the requirements of continuous and reliable power supply in modern power grids. Furthermore, during online inspection, effectively injecting a wideband detection signal from strong power frequency signals and their harmonic interference and accurately extracting the weak reflection response remains a core technical obstacle to the engineering application of this technology. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings and deficiencies of existing technologies by providing an online cable defect detection method based on frequency domain reflection. This method can effectively suppress power frequency interference and achieve wideband signal coupling and response acquisition under the condition that the cable is energized, thereby filling the technological gap in this field and completing cable defect detection.

[0005] The technical problem solved by this invention is achieved through the following technical solution: An online cable defect detection method based on frequency domain reflection, the method comprising the following steps: S1. Test device connection: The frequency domain reflectance tester is connected to the adapter via test leads, and the adapter is connected to the voltage testing interface of the live indicator of the operating cable line. The live indicator is installed in the switch cabinet or ring main unit of the cable line to indicate the operating voltage status. The live indicator is connected to the cable under test via a bushing sensor. The bushing sensor contains a capacitive voltage divider, which is composed of a high-voltage arm coupling capacitor and a low-voltage arm concentrated capacitor connected in series. The voltage testing interface of the live indicator is used to connect a voltage testing device or a signal acquisition device. The voltage detection interface is the injection and acquisition port for high-frequency detection signals. The excitation signal generated by the frequency domain reflectance tester is transmitted to the live indicator through the adapter and voltage detection interface, and then transmitted to the cable under test after being coupled by the high and low voltage arms of the capacitive voltage divider inside the bushing sensor. S2. Generation and injection of anti-interference spread spectrum excitation signal: The signal generation module of the frequency domain reflectance tester generates a periodic pseudo-random binary sequence as the excitation signal, employing a Gold sequence with sharp autocorrelation characteristics. The Gold sequence is generated by a preferred pair of m-sequences of equal length through modulo-2 addition. Its energy is dispersed in a wide frequency band. The pseudo-random characteristics of the Gold sequence make its spectrum approximate white noise. After digital-to-analog conversion and power amplification, the sequence is injected into the live indicator through the adapter. Then, the wideband detection signal is injected into the cable under test through the capacitive coupling path inside the sleeve sensor. The spectrum of the injected excitation signal covers the preset diagnostic frequency band. S3. Acquisition of mixed response signals: The synchronous acquisition module of the frequency domain reflectance tester receives the mixed response signal returned from the original path simultaneously with the injection of the excitation signal. The mixed response signal is a mixture of the injected excitation signal and the reflected signal formed by the reflection at impedance discontinuities in the cable under test, superimposed with significant power frequency interference and background noise. The synchronous acquisition module and the signal generation module are synchronized under the same master clock control. The synchronous acquisition module performs analog-to-digital conversion on the mixed response signal at a sampling rate more than twice that of the highest frequency component of the excitation signal, obtaining a digitized mixed response signal sequence. The hybrid response signal sequence Represented as: ; (1) in: The effective signal components include the incident signal. and the reflected signals reflected through M impedance discontinuities in the cable under test. ; It is a periodic narrowband interference, consisting of the fundamental power frequency and its harmonics; This is background noise; S4. Extraction of reflection response signal based on cross-correlation: The acquired mixed response signal sequence is processed in the signal processing module of the frequency domain reflectance tester. With the original stimulus Gold sequence stored locally Perform cross-correlation calculations to extract the effective response signal: ; (2) in: The result of cross-correlation calculation; for Length; For time delay variables; Substituting equation (1) into equation (2), and utilizing the autocorrelation properties of the Gold sequence, equation (2) can be approximately simplified to: ; (3) in: This represents the actual reflection response of the cable under test, with its peak value corresponding to the time delay at the defect point. ; The remaining interference and noise after significant suppression; Accurately extracting weak reflection response signals against a strong noise background ; S5. Frequency Domain Transformation and Location Map Calculation: The reflection response signal For discrete-time signals, the signal processing module uses a Fast Fourier Transform to convert them into frequency-domain data. ; (4) in: The input reflection coefficient of the cable under test characterizes the reflection characteristics of the cable under test for signals of different frequencies. ( ) represents the Fast Fourier Transform; Obtain the input reflection coefficient of the cable under test. Then, with the selected virtual excitation signal Perform frequency domain multiplication and inverse Fourier transform to obtain the equivalent time-domain reflection waveform: ; (5) in: The time-domain waveform of the reflected signal; ( ) represents the inverse Fourier transform operation; ( ) represents the modulo operation; Will The speed of electromagnetic wave propagation in the cable under test Multiplying them yields the spatial domain waveform of the reflection response signal. The discretization calculation formula is shown below: ; (6) in: The time shift factor in the selected virtual excitation signal represents the peak time of the signal; Equation (6) can be used to convert the time domain to the spatial domain, obtain the location map of the cable under test, and complete the online detection of defects in the cable under test.

[0006] Furthermore, the adapter has a built-in core circuit, including: The power frequency suppression circuit is used to block power frequency voltage from entering the frequency domain reflection tester, ensuring front-end safety. Impedance matching circuit is used to achieve efficient matching between the characteristic impedance of the test wire and the impedance of the voltage detection interface; The frequency domain reflectance tester integrates signal generation, power amplification, synchronous acquisition, and signal processing.

[0007] The advantages and beneficial effects of this invention are as follows: 1. This invention enables safe online detection, supporting the continuous and reliable operation of the power grid: It innovatively utilizes the voltage detection interface of the existing live indicator as a signal path, and couples the detection signal to the cable under test through the internal capacitor structure of the bushing sensor. No power outage or electrical modification is required for the cable under test. This fundamentally changes the traditional detection operation mode that must rely on power outages, realizing the transformation from "planned power outage maintenance" to "active online monitoring", directly supporting the core requirements of modern power grids for high reliability and high continuity of power supply.

[0008] 2. This invention has strong anti-interference capabilities and a high signal-to-noise ratio: It adopts a signal processing mechanism that combines Gold sequence spread spectrum excitation with synchronous cross-correlation processing. Even if the amplitude of the injected detection signal is much lower than the power frequency and harmonic interference in the cable under test, it can still lock and extract the weak defect reflection signal from the strong power frequency interference and noise background. This makes it possible to achieve reliable detection in the extreme electromagnetic environment of the cable under test being energized, thus solving the fundamental problem of online detection. Attached Figure Description

[0009] Figure 1 This is a flowchart of the present invention; Figure 2 This is a schematic diagram of the connection of the testing device of the present invention; Figure 3 This is a schematic diagram of the Gold sequence generation method of the present invention; Figure 4 This is a schematic diagram of m-sequence generation according to the present invention; Figure 5 This is a schematic diagram of the physical connection of the testing device of the present invention; Figure 6 This is a time-domain recovery spectrum curve for testing the present invention. Detailed Implementation

[0010] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0011] like Figure 1 As shown, an online cable defect detection method based on frequency domain reflection is innovative in that the method comprises the following steps: S1. Test device connection: The frequency domain reflectance tester is connected to an adapter via test leads, and the adapter is then connected to the voltage testing interface of a live indicator on the operating cable line. The live indicator is installed in the cable line switchgear or ring main unit to indicate the operating voltage status. The live indicator is connected to the cable under test via a bushing sensor. The bushing sensor contains a capacitive voltage divider, which consists of a high-voltage arm coupling capacitor and a low-voltage arm concentrated capacitor connected in series. The voltage testing interface of the live indicator is used to connect a voltage testing device or signal acquisition equipment. Figure 2 , 5 As shown.

[0012] The voltage detection interface is the injection and acquisition port for high-frequency detection signals. The excitation signal generated by the frequency domain reflectance tester is transmitted to the live indicator through the adapter and voltage detection interface, and then transmitted to the cable under test after being coupled by the high and low voltage arms of the capacitive voltage divider inside the bushing sensor. The adapter has a built-in core circuit, including: The power frequency suppression circuit is used to block power frequency voltage from entering the frequency domain reflection tester, ensuring front-end safety. Impedance matching circuit is used to achieve efficient matching between the characteristic impedance of the test wire and the impedance of the voltage detection interface; The frequency domain reflectance tester integrates signal generation, power amplification, synchronous acquisition, and signal processing.

[0013] S2. Generation and injection of anti-interference spread spectrum excitation signal: The signal generation module of the frequency domain reflectance tester generates a periodic pseudo-random binary sequence as the excitation signal, employing a Gold sequence with sharp autocorrelation characteristics. It is generated by a preferred pair of m-sequences of equal length through a modulo-2 addition operation, such as Figure 3 As shown; the m-sequence can be passed as follows Figure 4 The m-sequence generator shown controls the feedback coefficient. The value of the Gold sequence is generated. The energy of the Gold sequence is dispersed over a wide bandwidth, and its pseudo-random characteristics make its spectrum approximate white noise. After digital-to-analog conversion and power amplification, the sequence is injected into the live indicator through an adapter, and then injected into the cable under test through the capacitive coupling path inside the sleeve sensor. The spectrum of the injected excitation signal covers the preset diagnostic frequency band.

[0014] S3. Acquisition of mixed response signals: The synchronous acquisition module of the frequency domain reflectance tester receives the mixed response signal returned from the original path simultaneously with the injection of the excitation signal. The mixed response signal is a mixture of the injected excitation signal and the reflected signal formed by the reflection at impedance discontinuities in the cable under test, superimposed with significant power frequency interference and background noise. The synchronous acquisition module and the signal generation module are synchronized under the same master clock control. The synchronous acquisition module performs analog-to-digital conversion on the mixed response signal at a sampling rate more than twice that of the highest frequency component of the excitation signal, obtaining a digitized mixed response signal sequence. The hybrid response signal sequence Represented as: ; (1) in: The effective signal components include the incident signal. and the reflected signals reflected through M impedance discontinuities in the cable under test. ; It is a periodic narrowband interference, consisting of the fundamental power frequency and its harmonics; This is background noise.

[0015] S4. Extraction of reflection response signal based on cross-correlation: The acquired mixed response signal sequence is processed in the signal processing module of the frequency domain reflectance tester. With the original stimulus Gold sequence stored locally Perform cross-correlation calculations to extract the effective response signal: ; (2) in: The result of cross-correlation calculation; for Length; For time delay variables; Substituting equation (1) into equation (2), and utilizing the autocorrelation properties of the Gold sequence, equation (2) can be approximately simplified to: ; (3) in: This represents the actual reflection response of the cable under test, with its peak value corresponding to the time delay at the defect point. ; The remaining interference and noise after significant suppression; Accurately extracting weak reflection response signals against a strong noise background .

[0016] S5. Frequency Domain Transformation and Location Map Calculation: The reflection response signal For discrete-time signals, the signal processing module uses a Fast Fourier Transform to convert them into frequency-domain data. ; (4) in: The input reflection coefficient of the cable under test characterizes the reflection characteristics of the cable under test for signals of different frequencies. ( ) represents the Fast Fourier Transform; Obtain the input reflection coefficient of the cable under test. Then, with the selected virtual excitation signal Perform frequency domain multiplication and inverse Fourier transform to obtain the equivalent time-domain reflection waveform: ; (5) in: The time-domain waveform of the reflected signal; ( ) represents the inverse Fourier transform operation; ( ) represents the modulo operation; Will The speed of electromagnetic wave propagation in the cable under test Multiplying them yields the spatial domain waveform of the reflection response signal. The discretization calculation formula is shown below: ; (6) in: The time shift factor in the selected virtual excitation signal represents the peak time of the signal; Equation (6) can be used to convert the time domain to the spatial domain and obtain the location map of the cable under test, such as... Figure 6 As shown. Through analysis of the location map, the defect points can be found. Figure 6 The defect detection of the cable under test is completed by circling the white frame.

[0017] The innovative aspects of this invention include: (1) Non-contact online signal coupling (2) This invention creatively utilizes existing live indicators in power systems as signal transmission media to achieve safe, non-contact coupling with the cable under test. Specific protection measures include: Coupling path: The test device is connected to the preset voltage detection interface of the live indicator via a dedicated adapter. This interface achieves electrical isolation from the cable under test via a capacitive voltage divider inside the bushing sensor.

[0018] Signal transmission mechanism: The detection signal is injected through this path, entering from the voltage detection interface and being transmitted non-contactly to the conductor of the live cable under test through the electric field coupling effect of the capacitive voltage divider. The reflected signal is collected in the reverse direction along the same path.

[0019] Key features and advantages: This method does not require changes to the primary equipment wiring or interruption of cable operation. It directly reuses existing standardized interfaces and isolation structures, achieving inherent safety and engineering convenience for online detection, and forming the physical basis of the entire online detection solution.

[0020] (2) Anti-interference signal extraction based on spread spectrum sequence and correlation processing This invention employs specific signal encoding and processing algorithms to solve the critical challenges of extremely low signal-to-noise ratio and suppression of external interference in online detection. Specific protection measures include: Excitation signal design: A Gold sequence is used as the broadband excitation signal. The Gold sequence is generated by modulo-2 addition of a pair of m sequences of equal length that are preferred to each other.

[0021] Anti-interference and noise suppression mechanisms: This method achieves signal extraction under strong noise background through the following synergistic mechanism: Processing Gain: At the receiving end, a cross-correlation operation is performed between the acquired mixed signal and the locally stored original Gold sequence. Utilizing the excellent autocorrelation properties of the Gold sequence, this operation can refocus the signal energy dispersed across a wide frequency band into a sharp correlation peak in the time domain, generating a processing gain proportional to the sequence length. This allows the weak reflected signal amplitude to be amplified to a level far exceeding the background noise.

[0022] Interference suppression: Power frequency interference, harmonics, and random noise uncorrelated with the Gold sequence are greatly suppressed in cross-correlation calculations. The discrete spectral energy of periodic interference is whitened and diffused, while the average correlation output of random noise tends to zero.

[0023] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for online detection of cable defects based on frequency domain reflection, characterized in that: The steps of the method are as follows: S1. Test device connection: The frequency domain reflectance tester is connected to the adapter via test leads, and the adapter is connected to the voltage testing interface of the live indicator of the operating cable line. The live indicator is installed in the switch cabinet or ring main unit of the cable line to indicate the operating voltage status. The live indicator is connected to the cable under test via a bushing sensor. The bushing sensor contains a capacitive voltage divider, which is composed of a high-voltage arm coupling capacitor and a low-voltage arm concentrated capacitor connected in series. The voltage testing interface of the live indicator is used to connect a voltage testing device or signal acquisition equipment. The voltage detection interface is the injection and acquisition port for high-frequency detection signals. The excitation signal generated by the frequency domain reflectance tester is transmitted to the live indicator through the adapter and voltage detection interface, and then transmitted to the cable under test after being coupled by the high and low voltage arms of the capacitive voltage divider inside the bushing sensor. S2. Generation and injection of anti-interference spread spectrum excitation signal: The signal generation module of the frequency domain reflectance tester generates a periodic pseudo-random binary sequence as the excitation signal, employing a Gold sequence with sharp autocorrelation characteristics. The Gold sequence is generated by a preferred pair of m-sequences of equal length through modulo-2 addition. Its energy is dispersed in a wide frequency band. The pseudo-random characteristics of the Gold sequence make its spectrum approximate white noise. After digital-to-analog conversion and power amplification, the sequence is injected into the live indicator through the adapter. Then, the wideband detection signal is injected into the cable under test through the capacitive coupling path inside the sleeve sensor. The spectrum of the injected excitation signal covers the preset diagnostic frequency band. S3. Acquisition of mixed response signals: The synchronous acquisition module of the frequency domain reflectance tester receives the mixed response signal returned from the original path simultaneously with the injection of the excitation signal. The mixed response signal is a mixture of the injected excitation signal and the reflected signal formed by the reflection at impedance discontinuities in the cable under test, superimposed with significant power frequency interference and background noise. The synchronous acquisition module and the signal generation module are synchronized under the same master clock control. The synchronous acquisition module performs analog-to-digital conversion on the mixed response signal at a sampling rate more than twice that of the highest frequency component of the excitation signal, obtaining a digitized mixed response signal sequence. The hybrid response signal sequence Represented as: ； （1） in: The effective signal components include the incident signal. and the reflected signals reflected through M impedance discontinuities in the cable under test. ; It is a periodic narrowband interference, consisting of the fundamental power frequency and its harmonics; This is background noise; S4. Extraction of reflection response signal based on cross-correlation: The acquired mixed response signal sequence is processed in the signal processing module of the frequency domain reflectance tester. With the original stimulus Gold sequence stored locally Perform cross-correlation calculations to extract the effective response signal: ； （2） in: The result of cross-correlation calculation; for Length; For time delay variables; Substituting equation (1) into equation (2), and utilizing the autocorrelation properties of the Gold sequence, equation (2) can be approximately simplified to: ； （3） in: This represents the actual reflection response of the cable under test, with its peak value corresponding to the time delay at the defect point. ; The remaining interference and noise after significant suppression; Accurately extracting weak reflection response signals against a strong noise background ; S5. Frequency Domain Transformation and Location Map Calculation: The reflection response signal For discrete-time signals, the signal processing module uses a Fast Fourier Transform to convert them into frequency-domain data. ； （4） in: The input reflection coefficient of the cable under test characterizes the reflection characteristics of the cable under test for signals of different frequencies. ( ) represents the Fast Fourier Transform; Obtain the input reflection coefficient of the cable under test. Then, with the selected virtual excitation signal Perform frequency domain multiplication and inverse Fourier transform to obtain the equivalent time-domain reflection waveform: ； （5） in: The time-domain waveform of the reflected signal; ( ) represents the inverse Fourier transform operation; ( ) represents the modulo operation; Will The speed of electromagnetic wave propagation in the cable under test Multiplying them yields the spatial domain waveform of the reflection response signal. The discretization calculation formula is shown below: ； （6） in: The time shift factor in the selected virtual excitation signal represents the peak time of the signal; Equation (6) can be used to convert the time domain to the spatial domain, obtain the location map of the cable under test, and complete the online detection of defects in the cable under test.

2. The online cable defect detection method based on frequency domain reflection according to claim 1, characterized in that: The adapter has a built-in core circuit, including: The power frequency suppression circuit is used to block power frequency voltage from entering the frequency domain reflection tester, ensuring front-end safety. Impedance matching circuit is used to achieve efficient matching between the characteristic impedance of the test wire and the impedance of the voltage detection interface; The frequency domain reflectance tester integrates signal generation, power amplification, synchronous acquisition, and signal processing.

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