Method and system for non-destructive testing of power cable multiple defects based on broadband impedance spectroscopy

Abstract

This invention provides a method and system for non-destructive testing of multiple defects in power cables based on broadband impedance spectroscopy. By measuring the impedance characteristics of the cable at different frequencies, the frequency domain data is converted into time domain data using inverse Fourier transform, enabling precise location and evaluation of internal defects in the cable. The method for non-destructive testing of multiple defects in power cables based on broadband impedance spectroscopy includes the following steps: Step 1, signal output and acquisition: A broadband sinusoidal voltage signal is output using a broadband impedance spectroscopy tester, and frequency domain measurement data is acquired, which is the impedance data of the cable at different frequencies; Step 2, time-frequency conversion: The frequency domain measurement data is converted into time domain data using inverse Fourier transform, generating a time-amplitude curve to analyze the electrical characteristics of the cable; Step 3, defect identification model: The electrical characteristics of the cable at different frequencies are analyzed using a transmission line model to evaluate the cable condition; Step 4, defect location and evaluation; Step 5, result display and analysis.

Description

Technical Field

[0001] This invention relates to the field of power equipment condition monitoring technology, specifically to a method and system for non-destructive testing of multiple defects in power cables based on broadband impedance spectrum. Background Technology

[0002] As a crucial component of the power system, the safe and stable operation of power cables is vital for ensuring power supply. However, during long-term operation, cables are prone to various defects due to environmental factors, external damage, and material aging, such as open / short circuit faults, joint / terminal defects, local / overall thermal aging, high resistance defects, water immersion in the cable body / joints, and outer sheath faults. If these defects are not detected and repaired in a timely manner, they will directly threaten the stability and security of the power system.

[0003] Traditional cable inspection methods, such as insulation resistance testing, power frequency or very low frequency dielectric loss testing, time domain reflection (TDR) testing, withstand voltage testing, and partial discharge testing, have many limitations. For example, insulation resistance testing and power frequency or very low frequency dielectric loss testing cannot locate defects; TDR testing has insufficient sensitivity for detecting high-resistance defects; withstand voltage testing requires using 2-3 times the rated voltage, which can cause cumulative damage to the cable. Therefore, developing an efficient, non-destructive testing method and system capable of accurately locating multiple defects in cables is particularly important. Summary of the Invention

[0004] This invention aims to address the technical deficiencies of existing technologies by providing a non-destructive testing method and system for multiple defects in power cables based on broadband impedance spectrum. By measuring the impedance characteristics of the cable at different frequencies, the frequency domain data is converted into time domain data using inverse Fourier transform, thereby achieving accurate location and evaluation of internal defects in the cable. This method has the advantages of being non-destructive, efficient, and convenient, and can detect multiple defects simultaneously, providing a strong guarantee for the safe operation of power cables.

[0005] This invention discloses a non-destructive testing method for multiple defects in power cables based on broadband impedance spectrum, comprising the following steps:

[0006] Step 1, Signal Output and Acquisition;

[0007] A wideband impedance spectrum analyzer is used to output a wideband sinusoidal voltage signal and collect frequency domain measurement data, which is the impedance data of the cable at different frequencies.

[0008] Step 2, time-frequency conversion;

[0009] The frequency domain measurement data is converted into time domain data by inverse Fourier transform, and a time-amplitude curve is generated to analyze the electrical characteristics of the cable.

[0010] Step 3, Defect Identification Model;

[0011] The electrical characteristics of cables at different frequencies are analyzed using transmission line models, including parameters such as resistance, inductance, capacitance, and parallel conductance, to evaluate the cable condition.

[0012] Step 4, Defect location and assessment;

[0013] Based on the characteristics of reflected signals in the time-amplitude curve, locate open / short circuit faults, joint / terminal defects, local / overall thermal aging, high resistance defects, water immersion of the cable body / joint, and outer sheath faults, and assess their severity.

[0014] Step 5: Results Display and Analysis;

[0015] The software system displays results such as pulse amplitude, step response, and impulse response, and uses any one of the analysis methods—time-based comparison, phase-to-phase comparison, and reflection peak analysis—to analyze the results and determine the cable condition.

[0016] Furthermore,

[0017] In step 3, a distributed parameter transmission line model is used to analyze how changes in the electrical characteristics of the cable cause changes in the inductance and capacitance parameters of corresponding regions, resulting in different reflected signals, and the relationship between the reflected voltage and the incident voltage. The expression is:

[0018] Where Z1 is the surge impedance of the cable under test, and Z0 is the surge impedance of the test cable. L is the inductance of the cable, and C is the capacitance of the cable.

[0019] Furthermore,

[0020] In step 5, the time-based comparison uses the measurement results of the cable in a brand-new state as the reference curve. The current detection results are compared with the reference curve. The comparison time interval is set according to the cable operation status and detection requirements. When the change in the detection results exceeds the set threshold, it is determined that the cable state has changed.

[0021] Furthermore,

[0022] In step 5, the phase comparison uses the one with the highest insulation resistance among the three-phase cables as the reference curve, and the other two phases are compared with the reference curve. The point where the difference begins is the abnormal position.

[0023] Furthermore,

[0024] In step 5, the reflection peak value is determined by averaging the reflection values ​​over the entire length of the cable and combining this with the electromagnetic wave propagation attenuation coefficient to establish a baseline curve. Locations with larger reflection peak values ​​are identified as abnormal locations.

[0025] Furthermore,

[0026] In step 1, a broadband impedance spectrum test is performed by coupling a high-frequency signal.

[0027] Furthermore,

[0028] After generating the time-amplitude curve in step 2, the curve is smoothed using a moving average method. The size of the smoothing window is adaptively adjusted according to the data noise level, with an adjustment range of 3-10 data points.

[0029] Furthermore,

[0030] In step 4, when locating open / short circuit faults, the fault location is determined by analyzing the polarity and arrival time of the reflected signal in the time-amplitude curve. When the polarity of the reflected signal is opposite to that of the incident signal and the arrival time meets a specific condition, it is determined to be an open circuit fault. When the polarity of the reflected signal is the same as that of the incident signal and the arrival time meets a specific condition, it is determined to be a short circuit fault. The specific conditions are determined based on the cable length and the signal propagation speed.

[0031] Furthermore,

[0032] In step 4, when assessing the severity of the joint / terminal defect, a quantitative assessment is performed by comparing the amplitude of the reflected signal at the joint / terminal with the amplitude of the reflected signal at a normal joint / terminal. When the amplitude of the reflected signal exceeds 20% of the normal amplitude, it is judged as a minor defect; when it exceeds 50%, it is judged as a moderate defect; and when it exceeds 80%, it is judged as a severe defect.

[0033] This invention also discloses a multi-defect non-destructive testing system for power cables based on broadband impedance spectrum, comprising:

[0034] Wideband signal generator: used to output a wideband sinusoidal voltage signal and measure impedance data at different frequencies;

[0035] Data processing unit: connected to a broadband signal generator, used to convert the acquired frequency domain data into time domain data through inverse Fourier transform, and to perform defect location and evaluation;

[0036] Display and Recording Unit: Connected to the data processing unit, it is used to display test results and record test data.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] The purpose of this invention is to provide a method and system for non-destructive testing of multiple defects in power cables based on broadband impedance spectrum, so as to solve the problems of inaccurate defect location, low detection efficiency, and damage to cables in the existing technology.

[0039] Non-destructive testing: Employs low-voltage, non-invasive, and non-destructive testing methods that will not cause cumulative damage to the cable;

[0040] High-precision positioning: By increasing the number of measurement points and the frequency band range, high-precision defect positioning can be achieved, with a minimum resolution as low as 0.1m;

[0041] Multi-defect detection: It can simultaneously detect multiple defects in cables, such as open circuit / short circuit faults, joint / terminal defects, local / overall thermal aging, high resistance defects, water immersion of the cable body / joint, and outer sheath faults.

[0042] Early warning: By continuously monitoring the impedance spectrum of the cable, potential defects and faults can be detected in advance, thus achieving early warning of faults. Attached Figure Description

[0043] Figure 1 This invention relates to the time-frequency conversion principle of input and output signals.

[0044] Figure 2 This is a transmission line model for coaxial cables. Detailed Implementation

[0045] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0046] like Figures 1-2 As shown, it illustrates a specific embodiment of the present invention:

[0047] like Figures 1-2 As shown, this invention discloses a non-destructive testing method for multiple defects in power cables based on broadband impedance spectrum, comprising the following steps:

[0048] Step 1, Signal Output and Acquisition;

[0049] A wideband impedance spectrum analyzer is used to output a wideband sinusoidal voltage signal and collect frequency domain measurement data, which is the impedance data of the cable at different frequencies.

[0050] Step 2, time-frequency conversion;

[0051] The frequency domain measurement data is converted into time domain data by inverse Fourier transform, and a time-amplitude curve is generated to analyze the electrical characteristics of the cable.

[0052] Step 3, Defect Identification Model;

[0053] The electrical characteristics of cables at different frequencies are analyzed using transmission line models, including parameters such as resistance, inductance, capacitance, and parallel conductance, to evaluate the cable condition.

[0054] Step 4, Defect location and assessment;

[0055] Based on the characteristics of reflected signals in the time-amplitude curve, locate open / short circuit faults, joint / terminal defects, local / overall thermal aging, high resistance defects, water immersion of the cable body / joint, and outer sheath faults, and assess their severity.

[0056] Step 5: Results Display and Analysis;

[0057] The software system displays results such as pulse amplitude, step response, and impulse response, and uses any one of the analysis methods—time-based comparison, phase-to-phase comparison, and reflection peak analysis—to analyze the results and determine the cable condition.

[0058] This method is used to assess and locate open / short circuit faults, joint / terminal defects, local / overall thermal aging, high-resistivity defects, water immersion in the cable body / joints, and outer sheath faults in power cables. Compared to traditional time-domain reflectometry, broadband impedance spectroscopy provides more comprehensive and detailed information, thus significantly improving the accuracy and reliability of fault detection.

[0059] Its working principle is to convert frequency domain measurement data into time domain data through inverse Fourier transform. A wideband signal generator outputs a wideband sinusoidal voltage signal, then measures the cable impedance at different frequencies, and finally converts it into a time-amplitude curve using inverse Fourier transform. This time-amplitude curve is the same as the test result of TDR. Electromagnetic waves propagate in a uniform medium without reflection, but reflection occurs when they encounter areas where electrical characteristics have changed. The amplitude of the reflection is proportional to the degree of characteristic change. Newly laid cables only generate reflection signals at terminals and joints. Deterioration, water immersion, open circuits or short circuits, and mechanical damage that occur during cable operation will increase the areas of reflection. The severity of the defect determines the amplitude of the reflection. A completely lossless inverse Fourier transform requires impedance data across the entire frequency band, i.e., DC to +Hz. Due to hardware limitations, generally speaking, the wider the test frequency band, the higher the upper limit of the high frequency band, and the shorter the signal wavelength, the better the sensitivity to defect details. However, high-frequency signals are more prone to attenuation during propagation.

[0060] Preferred, such as Figure 2 As shown,

[0061] In step 3, a distributed parameter transmission line model is used to analyze how changes in the electrical characteristics of the cable cause changes in the inductance and capacitance parameters of corresponding regions, resulting in different reflected signals, and the relationship between the reflected voltage and the incident voltage. The expression is:

[0062] Where Z1 is the surge impedance of the cable under test, and Z0 is the surge impedance of the test cable. L is the inductance of the cable, and C is the capacitance of the cable.

[0063] Almost all voltage-level power cables use a coaxial structure, and the distributed parameters in their transmission line model are as follows:

[0064] Inductance L: Determined by the mechanical structure of the cable. The expression for a coaxial cable (outer diameter b, inner diameter a) is:

[0065]

[0066] Resistance R: determined by the resistivity of the cable conductor;

[0067] Capacitance C: Determined by the cable geometry and insulation material properties The expression for the coaxial cable (outer diameter b, inner diameter a) is determined as follows:

[0068]

[0069] Loss G: Equivalent to the dielectric loss characteristics of insulation;

[0070] The propagation speed of electromagnetic waves in a cable medium is mainly determined by the dielectric constant of the cable. The dielectric constant is generally proportional to the capacitance. The expression for the wave velocity is:

[0071]

[0072] For low-loss cables, the expression for wave impedance is:

[0073] L is the inductance of the cable, and C is the capacitance of the cable.

[0074] The common surge impedances for coaxial cables and testing instruments are 50Ω and 75Ω, while the surge impedance for buried cables is typically 25Ω. The surge impedance of test cables is... If the impedance of the cable under test is Z1, then what is the relationship between the reflected voltage and the incident voltage? This can be expressed as: .

[0075] Changes in the local electrical or mechanical characteristics of a cable can alter the inductance and capacitance parameters of the corresponding area, resulting in different reflected signals. For example, aging of the insulation and immersion in water can significantly increase capacitance, leading to a decrease in wave impedance and a reduction in reflection amplitude; mechanical damage can change inductance, and joints can cause changes in both capacitance and inductance.

[0076] Preferred,

[0077] In step 5, the time-based comparison uses the measurement results of the cable in a brand-new state as the reference curve. The current detection results are compared with the reference curve. The comparison time interval is set according to the cable operation status and detection requirements. When the change in the detection results exceeds the set threshold, it is determined that the cable state has changed.

[0078] Longitudinal comparisons based on time should take into account the impact of changes in the surrounding environment (temperature, humidity) on the results.

[0079] Temperature effect:

[0080] Resistance change: The resistance of a cable conductor increases with increasing temperature. Temperature coefficient of resistance formula.

[0081] R = R0[1 + α(T−T0)], where R is the resistance at temperature T, R0 is the resistance at the reference temperature T0, and α is the temperature coefficient of resistance. Increased temperature leads to an increase in the measured resistance of the cable. This change in resistance affects the cable condition testing results based on resistance measurements, such as insulation resistance testing.

[0082] Changes in insulation performance: Increased temperature alters the dielectric constant and loss tangent of cable insulation materials. Generally, higher temperatures slightly increase the dielectric constant while significantly increasing the loss tangent. This leads to increased leakage current and decreased insulation resistance, thus affecting insulation performance testing results. For example, in high-temperature environments, partial discharge in the cable insulation layer may intensify, accelerating insulation aging. However, if the temperature effect is ignored, this might be misinterpreted as a sudden deterioration in the cable's condition.

[0083] Humidity effect:

[0084] Increased surface leakage current: When the ambient humidity around the cable is high, moisture will be absorbed onto the cable surface, forming a conductive layer, leading to an increase in surface leakage current. During insulation resistance testing, this surface leakage current will be superimposed on the measuring current, causing the measurement result to be lower than expected and failing to accurately reflect the true condition of the cable insulation. For example, during the humid rainy season, the measured insulation resistance of the cable may be significantly lower than in the dry season.

[0085] Insulation moisture: Prolonged exposure to high humidity can allow moisture to penetrate the cable insulation, degrading its performance. Moisture reduces breakdown strength, increases dielectric loss, and accelerates insulation aging. Ignoring the effects of humidity may lead to overlooked problems with moisture-damaged cable insulation, resulting in cable failure.

[0086] Environmental parameter recording and correction

[0087] Real-time monitoring of environmental parameters: When inspecting cables, the temperature and humidity parameters of the environment surrounding the cable should be recorded simultaneously. Temperature and humidity sensors can be used to monitor environmental data in real time, and this data should be recorded along with the inspection results.

[0088] Establish an environmental correction model: Based on the cable's material properties and testing methods, establish a correction model for environmental parameters on the test results. For example, for resistance measurement, the variation of resistance at different temperatures can be determined experimentally, and a temperature correction formula can be established; for insulation resistance measurement, the influence of humidity on surface leakage current can be considered, and a humidity correction model can be established. In actual comparisons, the current test results are corrected according to environmental parameters before being compared with the baseline curve.

[0089] In conclusion, in time-based longitudinal comparisons, it is crucial to fully consider the impact of changes in the cable's surrounding environment on the test results. Recording and correcting environmental parameters can improve the accuracy of cable condition detection and prevent misjudgments.

[0090] Preferred,

[0091] In step 5, the phase comparison uses the one with the highest insulation resistance among the three-phase cables as the reference curve, and the other two phases are compared with the reference curve. The point where the difference begins is the abnormal position.

[0092] The phase-to-phase comparison method is the most commonly used when historical data is lacking. This method uses the one with the highest insulation resistance in the three-phase cable as the reference curve, and compares the other two phases with the reference curve. The point where the difference begins is the location of the defect.

[0093] When a cable has localized defects (such as moisture, insulation aging, or mechanical damage), the insulation resistance of the defective phase will begin to change significantly at the location of the defect. The phase with the highest insulation resistance among the three phases is used as the baseline curve because this phase has the best insulation performance and best represents the cable's insulation characteristics under normal conditions. Comparing the other two phases with the baseline curve, the point where the difference begins indicates the location of the defect. This is because defects disrupt the uniformity of the cable insulation, causing the insulation resistance at that location to exhibit a different trend than the normal phase.

[0094] Determine a reference curve by comparing the insulation resistance curves of the three-phase cables and selecting the one with the highest insulation resistance value as the reference curve. For example, if the insulation resistance value of phase A cable is higher than that of phase B and phase C at all length positions, then the insulation resistance curve of phase A cable will be used as the reference.

[0095] Curve comparison involves comparing the insulation resistance curves of the other two phases (such as phase B and phase C) with the reference curve point by point. This can be done using numerical comparison or graphical overlay comparison methods.

[0096] When comparing values, the difference or ratio between the insulation resistance value at each position of the same length and the insulation resistance value at the corresponding position on the reference curve is calculated. The position where the difference or ratio exceeds a certain threshold is the starting point of the difference.

[0097] When comparing overlapping graphs, the insulation resistance curves of the three-phase cables are plotted on the same coordinate graph. The deviation of the other two phase curves from the reference curve can be observed visually, and the starting position of the difference is the defect location.

[0098] Criteria for determining abnormal locations:

[0099] Sudden change in insulation resistance value

[0100] When the insulation resistance curves of the other two phase cables begin to deviate significantly from the reference curve at a certain point, and the degree of deviation exceeds the normal fluctuation range, that point is considered an abnormal position. For example, if the insulation resistance value of the reference curve is 1000MΩ at a certain length, and the insulation resistance value of the other phase curves suddenly drops below 800MΩ at that point, and the insulation resistance value at subsequent positions continues to be lower than the reference curve, then that drop point is considered an abnormal position.

[0101] Differences in trend changes

[0102] Besides abrupt changes in insulation resistance values, differences in trend changes are also an important basis for judging the location of anomalies. If the insulation resistance curves of other phase cables begin to deviate from the trend of the reference curve at a certain point, such as the reference curve showing a steady upward or downward trend, while the curves of other phases show a reverse change or a significantly accelerated rate of change after that point, then that point may also be an anomaly.

[0103] This method has the following advantages:

[0104] No historical data required: Even in the absence of historical insulation resistance test data for cables, this method can still effectively determine the location of cable defects, making it highly practical.

[0105] Intuitive and easy to understand: By comparing curves, operators can intuitively observe the differences in insulation resistance of three-phase cables, making it easy to quickly locate defects.

[0106] Low cost: This method only requires testing equipment, the testing process is relatively simple, and the cost is low.

[0107] To improve the accuracy of anomaly location identification, multiple tests can be conducted, and the results compared and analyzed. If the results from multiple tests are consistent, the reliability of the identification is high; if the results show significant differences, the cause should be further investigated.

[0108] In addition, it is combined with other detection methods to make a comprehensive judgment, so as to improve the accuracy and reliability of defect detection.

[0109] Preferred,

[0110] In step 5, the reflection peak value is determined by averaging the reflection values ​​over the entire length of the cable and combining this with the electromagnetic wave propagation attenuation coefficient to establish a baseline curve. Locations with larger reflection peak values ​​are identified as abnormal locations.

[0111] In cable fault detection, the time-domain reflectometry (TDR) method is commonly used. When an electromagnetic wave signal is sent to a cable, the electromagnetic wave propagates along the cable. If there is an abnormality in the cable (such as insulation damage, short circuit, open circuit, etc.), the electromagnetic wave will be reflected at the abnormal location. The strength of the reflected electromagnetic wave signal is related to the characteristics of the abnormal point (such as the degree of impedance mismatch).

[0112] By averaging the reflection values ​​along the entire length of the cable and combining this with the electromagnetic wave propagation attenuation coefficient, the influence of the cable's inherent characteristics (such as length and material) on the reflected signal can be eliminated, thus establishing a reference curve. The reference curve represents the cable's reflection characteristics under normal conditions. When the reflection peak at a certain location in the actual measured reflection curve is significantly higher than the reference curve, it indicates an anomaly at that location, as the anomaly results in a stronger reflected signal.

[0113] Electromagnetic waves attenuate when propagating in cables, and the attenuation coefficient is related to factors such as the cable material and frequency. Based on the cable material and the measurement frequency used, the electromagnetic wave propagation attenuation coefficient α is determined experimentally.

[0114] After considering the attenuation coefficient, the average reflection value is corrected to obtain the reference curve. Generally speaking, the farther away from the transmitter, the greater the attenuation of the reflected signal, and the reflection value of the reference curve will gradually decrease with increasing distance.

[0115] Compare the actual measured reflection curve with the reference curve. In the reflection curve, find the position where the reflection peak is significantly higher than that of the reference curve.

[0116] Peak reflection amplitude:

[0117] The peak reflection amplitude is a key indicator for determining the location of anomalies. When the peak reflection amplitude is significantly higher than the baseline curve, it indicates a larger reflection coefficient at that location, meaning there is a significant impedance mismatch. For example, in a normal cable, the peak reflection amplitude may only be about 1.2 times the baseline curve value. However, if the peak reflection amplitude at a certain location reaches 2 times or even higher than the baseline curve value, it can be generally determined that there is an anomaly at that location.

[0118] Reflection peak width:

[0119] Besides the amplitude of the reflected peak, the width of the reflected peak can also provide some information. If the reflected peak width is narrow, it may indicate that the anomaly is a point defect, such as a localized insulation failure; if the reflected peak width is wide, it may indicate that the anomaly is a regional defect, such as a section of cable being damp or aging.

[0120] It has the following advantages:

[0121] Accurate positioning: It can accurately determine the distance to abnormal locations in the cable, with positioning accuracy reaching the meter level or even higher, which is of great significance for cable maintenance and repair.

[0122] Wide range of applications: Applicable to various types of cables, including power cables, communication cables, etc., and is not limited by cable laying methods (such as direct burial, overhead, duct, etc.).

[0123] Real-time detection: It can detect potential cable faults in real time during cable operation, and prevent the faults from escalating and causing greater losses.

[0124] When analyzing reflection curves, factors such as the amplitude, width, and location of reflection peaks should be considered comprehensively, along with the actual condition of the cable and relevant standards. For some suspicious reflection peaks, multiple measurements or other testing methods can be used for verification.

[0125] Preferred,

[0126] In step 1, a broadband impedance spectrum test is performed by coupling a high-frequency signal.

[0127] Wideband impedance spectroscopy testing obtains the impedance characteristics of cables or circuit systems at different frequencies. The method of coupling high-frequency signals involves introducing a signal generated by a high-frequency signal source into the object under test (e.g., a cable), while minimizing the impact on the original state of the object and accurately measuring the signal response on the object. By changing the frequency of the signal source, the voltage and current across the object are measured at different frequencies, and the impedance value is calculated to plot the wideband impedance spectrum.

[0128] The purpose of coupling high-frequency signals is to allow the high-frequency signals to smoothly enter the object under test, and at the same time to extract the response signals that reflect the characteristics of the object under test for analysis of its internal characteristics.

[0129] Based on the measured voltage and current data, the impedance value of the cable at each frequency point is calculated. A curve showing the impedance changing with frequency is plotted, i.e., a broadband impedance spectrum. Analysis of the impedance spectrum can determine whether the cable has defects such as insulation aging or partial discharge. For example, if the impedance spectrum shows a significant decrease or fluctuation within a certain frequency range, it may indicate a defect in the cable at the corresponding length point of that frequency.

[0130] Preferred,

[0131] After generating the time-amplitude curve in step 2, the curve is smoothed using a moving average method. The size of the smoothing window is adaptively adjusted according to the data noise level, with an adjustment range of 3-10 data points.

[0132] In broadband impedance spectroscopy testing, generating time-amplitude curves is a crucial step in data processing. These curves reflect the amplitude changes of the measured signal (such as voltage or current) at different time points. However, due to various noise interferences during the measurement process, such as thermal noise from electronic components and external electromagnetic interference, the original time-amplitude curves often exhibit a rough and highly fluctuating shape, which can affect the accurate extraction and analysis of signal characteristics. Therefore, smoothing the curve is essential; it reduces the impact of noise, making the curve smoother and thus more clearly revealing the true trend and characteristics of the signal.

[0133] The moving average method is used because, for a time series of data, at each data point, the average of that point and a certain number of data points before and after it is taken as the new value for that point. In this way, the value of each data point is influenced by the surrounding data points, thus eliminating the interference of random noise to a certain extent and making the curve smoother.

[0134] The smoothing window size adaptively adjusts based on the data noise level, meaning the system automatically selects an appropriate window size according to the noise level of the measured data. Generally, the higher the noise level, the larger the required window size is needed to better smooth the noise; the lower the noise level, the smaller the window size can be to retain more signal details. The processed curve is significantly smoother, noise is effectively suppressed, and the main characteristics of the signal are still preserved. By analyzing the smoothed curve, the impedance characteristics of the cable can be extracted more accurately, and the presence of defects in the cable can be determined.

[0135] Preferred,

[0136] In step 4, when locating open / short circuit faults, the fault location is determined by analyzing the polarity and arrival time of the reflected signal in the time-amplitude curve. When the polarity of the reflected signal is opposite to that of the incident signal and the arrival time meets a specific condition, it is determined to be an open circuit fault. When the polarity of the reflected signal is the same as that of the incident signal and the arrival time meets a specific condition, it is determined to be a short circuit fault. The specific conditions are determined based on the cable length and the signal propagation speed.

[0137] Relationship between reflected signal polarity and fault type:

[0138] Open circuit fault

[0139] Principle: When an open circuit fault exists in a cable, the impedance at the fault point becomes extremely high, almost infinite. According to electromagnetic wave propagation theory, electromagnetic waves are reflected when they encounter a high impedance mismatch point, and the polarity of the reflected wave is opposite to that of the incident wave. Assuming the incident signal is a sine wave, its positive half-cycle represents the forward propagation of the signal. When the electromagnetic wave propagates to the open circuit fault point, the positive half-cycle of the reflected signal becomes the negative half-cycle, meaning the polarity of the reflected signal is opposite to that of the incident signal.

[0140] Short circuit fault

[0141] Principle: A short-circuit fault means that the impedance at the fault point is very small, close to zero. When an electromagnetic wave encounters a low impedance mismatch point, the polarity of the reflected wave is the same as that of the incident wave. Similarly, using a sine wave as the incident signal, when the electromagnetic wave propagates to the short-circuit fault point, the positive half-cycle of the reflected signal is still the positive half-cycle, that is, the polarity of the reflected signal is the same as that of the incident signal.

[0142] Specific steps for fault location:

[0143] Data Acquisition and Curve Generation

[0144] An electromagnetic wave signal was sent to the cable using a broadband impedance spectroscopy tester, and the time-amplitude curve of the reflected signal was recorded. This curve reflects the changes of the incident and reflected signals over time.

[0145] Polarity Analysis

[0146] Analyze the time-amplitude curve to determine the polarity of the reflected signal. The polarity can be determined by observing whether the positive and negative half-cycles of the reflected signal are consistent with those of the incident signal. If the positive half-cycle of the reflected signal is opposite to that of the incident signal, it indicates an open-circuit fault; if they are the same, it indicates a short-circuit fault.

[0147] Arrival Time Measurement

[0148] Measure the arrival time of the reflected signal. A threshold can be set, and when the amplitude of the reflected signal exceeds the threshold, the corresponding time point is recorded as the arrival time of the reflected signal.

[0149] Fault location calculation

[0150] Based on the known cable length L, signal propagation speed v, and the measured arrival time t of the reflected signal, the fault location is calculated using the formula x=2vt.

[0151] Result Validation

[0152] To ensure accurate fault location, multiple measurements can be performed, and the results averaged. Furthermore, it can be combined with other fault detection methods for verification.

[0153] Preferred,

[0154] In step 4, when assessing the severity of the joint / terminal defect, a quantitative assessment is performed by comparing the amplitude of the reflected signal at the joint / terminal with the amplitude of the reflected signal at a normal joint / terminal. When the amplitude of the reflected signal exceeds 20% of the normal amplitude, it is judged as a minor defect; when it exceeds 50%, it is judged as a moderate defect; and when it exceeds 80%, it is judged as a severe defect.

[0155] In broadband impedance spectroscopy testing, when electromagnetic waves propagate through a cable and encounter a joint or termination, reflection occurs due to impedance discontinuities. Under normal circumstances, the impedance matching of a joint / termination is within a reasonable range, and the amplitude of the reflected signal is relatively stable. However, when defects exist in the joint / termination, such as poor contact, oxidation, or insulation damage, the impedance mismatch is exacerbated, leading to a significant change in the amplitude of the reflected signal. By comparing the amplitude of the reflected signal at the joint / termination with that of a normal joint / termination, the severity of the defect can be quantitatively assessed.

[0156] When the reflected signal amplitude exceeds 20% of the normal amplitude, it indicates that the impedance mismatch of the connector / terminal has increased to some extent, but the impact on the overall performance of the cable system is relatively small. For example, during signal transmission, a small amount of signal reflection and energy loss may occur, but it generally will not lead to serious signal distortion or transmission interruption. This level of defect may be caused by slight loosening of the connector / terminal, a small amount of oxidation on the contact surface, etc.

[0157] A reflected signal amplitude exceeding 50% of the normal amplitude indicates a severe impedance mismatch at the connector / terminal. In this case, signal reflection and energy loss will increase significantly, potentially leading to a decrease in signal quality, such as reduced signal amplitude and waveform distortion. Moderate defects may affect the normal operation of the cable system, for example, potentially causing an increased bit error rate in data transmission. This defect may be caused by poor contact at the connector / terminal, localized damage to the insulation, or other reasons.

[0158] When the reflected signal amplitude exceeds 80% of the normal amplitude, the impedance mismatch of the connector / terminal is already very serious, resulting in extremely high signal reflection and energy loss, which may lead to signal transmission interruption or system failure. Severe defects can pose a serious threat to the safety and reliability of cable systems; for example, in power cables, they may cause problems such as localized overheating and insulation breakdown. Such defects are usually caused by severe damage to the connector / terminal, such as connector breakage or large-area damage to the insulation layer.

[0159] This invention also discloses a multi-defect non-destructive testing system for power cables based on broadband impedance spectrum, comprising:

[0160] Wideband signal generator: used to output a wideband sinusoidal voltage signal and measure impedance data at different frequencies;

[0161] Data processing unit: connected to a broadband signal generator, used to convert the acquired frequency domain data into time domain data through inverse Fourier transform, and to perform defect location and evaluation;

[0162] Display and Recording Unit: Connected to the data processing unit, it is used to display test results and record test data.

[0163] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. These changes involve related technologies well known to those skilled in the art, and all of them fall within the protection scope of the present invention.

[0164] Many other changes and modifications can be made without departing from the concept and scope of this invention. It should be understood that this invention is not limited to the specific embodiments, and the scope of this invention is defined by the appended claims.

Claims

1. A non-destructive testing method for multiple defects in power cables based on broadband impedance spectrum, characterized in that, Includes the following steps: Step 1, Signal Output and Acquisition; A wideband impedance spectrum analyzer is used to output a wideband sinusoidal voltage signal and collect frequency domain measurement data, which is the impedance data of the cable at different frequencies. Step 2, time-frequency conversion; The frequency domain measurement data is converted into time domain data by inverse Fourier transform, and a time-amplitude curve is generated to analyze the electrical characteristics of the cable. Step 3, Defect Identification Model; The electrical characteristics of cables at different frequencies are analyzed using transmission line models, including parameters such as resistance, inductance, capacitance, and parallel conductance, to evaluate the cable condition. Step 4, Defect Location and Assessment; Based on the characteristics of reflected signals in the time-amplitude curve, locate open / short circuit faults, joint / terminal defects, local / overall thermal aging, high resistance defects, water immersion of the cable body / joint, and outer sheath faults, and assess their severity. Step 5: Results Display and Analysis; The software system displays results such as pulse amplitude, step response, and impulse response, and uses any one of the analysis methods—time-based comparison, phase-to-phase comparison, and reflection peak analysis—to analyze the results and determine the cable condition. The time-based comparison uses the measurement results of the cable in a brand-new state as the benchmark curve, compares the current detection results with the benchmark curve, sets the comparison time interval according to the cable operating conditions and detection requirements, and determines that the cable state has changed when the change in the detection results exceeds the set threshold. In the time-based comparison, the detection results are corrected according to the established environmental correction model before being compared with the benchmark curve. The phase-to-phase comparison uses the curve with the highest insulation resistance among the three-phase cables as the reference curve, and compares the other two phases with the reference curve. The point where the difference begins is the abnormal position.

2. The method for non-destructive testing of multiple defects in power cables based on broadband impedance spectrum according to claim 1, characterized in that, In step 3, a distributed parameter transmission line model is used to analyze how changes in the electrical characteristics of the cable cause changes in the inductance and capacitance parameters of corresponding regions, resulting in different reflected signals, and the relationship between the reflected voltage and the incident voltage. The expression is: Where Z1 is the surge impedance of the cable under test, and Z0 is the surge impedance of the test cable. L is the inductance of the cable, and C is the capacitance of the cable.

3. The method for non-destructive testing of multiple defects in power cables based on broadband impedance spectrum according to claim 2, characterized in that, In step 5, the reflection peak value is determined by averaging the reflection values ​​over the entire length of the cable and combining this with the electromagnetic wave propagation attenuation coefficient to establish a baseline curve. Locations with larger reflection peak values ​​are identified as abnormal locations.

4. The method for non-destructive testing of multiple defects in power cables based on broadband impedance spectrum according to claim 1, characterized in that, In step 1, a broadband impedance spectrum test is performed by coupling a high-frequency signal.

5. The method for non-destructive testing of multiple defects in power cables based on broadband impedance spectrum according to claim 1, characterized in that, After generating the time-amplitude curve in step 2, the curve is smoothed using a moving average method. The size of the smoothing window is adaptively adjusted according to the data noise level, with an adjustment range of 3-10 data points.

6. The method for non-destructive testing of multiple defects in power cables based on broadband impedance spectrum according to claim 1, characterized in that, In step 4, when locating open / short circuit faults, the fault location is determined by analyzing the polarity and arrival time of the reflected signal in the time-amplitude curve. When the polarity of the reflected signal is opposite to that of the incident signal and the arrival time meets a specific condition, it is determined to be an open circuit fault. When the polarity of the reflected signal is the same as that of the incident signal and the arrival time meets a specific condition, it is determined to be a short circuit fault. The specific conditions are determined based on the cable length and the signal propagation speed.

7. The method for non-destructive testing of multiple defects in power cables based on broadband impedance spectrum according to claim 1, characterized in that, In step 4, when assessing the severity of the joint / terminal defect, a quantitative assessment is performed by comparing the amplitude of the reflected signal at the joint / terminal with the amplitude of the reflected signal at a normal joint / terminal. When the amplitude of the reflected signal exceeds 20% of the normal amplitude, it is judged as a minor defect; when it exceeds 50%, it is judged as a moderate defect; and when it exceeds 80%, it is judged as a severe defect.

8. A multi-defect non-destructive testing system for power cables based on broadband impedance spectrum, characterized in that, The detection method described in any one of claims 1-7 includes: Wideband signal generator: used to output a wideband sinusoidal voltage signal and measure impedance data at different frequencies; Data processing unit: connected to a broadband signal generator, used to convert the acquired frequency domain data into time domain data through inverse Fourier transform, and to perform defect location and evaluation; Display and Recording Unit: Connected to the data processing unit, it is used to display test results and record test data.

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