A method for judging the aging degree of a cable based on impedance spectrum and a related device

Abstract

The application belongs to the technical field of cable insulation material aging detection, and relates to a cable aging degree judgment method based on impedance spectrum and related devices. The method comprises the following steps: obtaining a reflection coefficient curve based on an impedance spectrum curve; obtaining a reflection coefficient real part curve based on the reflection coefficient curve, and extracting a resonance frequency corresponding to a resonance point of the reflection coefficient real part curve; calculating a relative dielectric constant real part of an insulation medium of a to-be-detected cable under different resonance frequencies; outputting an attenuation constant curve based on the reflection coefficient curve and a cable reflection coefficient amplitude calculation model; extracting multiple attenuation constant values of the attenuation constant curve at the resonance frequencies; inputting the attenuation constant values and the relative dielectric constant real part into a dielectric loss tangent calculation model to obtain a dielectric loss tangent value of the to-be-detected cable at the resonance frequencies; and judging the aging degree based on the dielectric loss tangent value. The method solves the problems that the existing methods are time-consuming or cannot obtain full-band dielectric information of cable insulation materials.

Description

Technical Field

[0001] This invention belongs to the field of cable insulation material aging detection technology, and particularly relates to a method and related device for judging the degree of cable aging based on impedance spectrum. Background Technology

[0002] In recent years, cross-linked polyethylene (XLPE) cables have been widely used in urban power distribution systems due to their excellent electrical performance and reliability. However, during long-term operation, cables are affected by the coupling of electro-thermal-mechanical stresses, leading to gradual aging of the insulation material. In severe cases, this can cause faults and threaten system safety. The dielectric loss tangent is an important indicator for assessing the state of insulation aging, moisture, impurities, or internal defects. It is also an effective means of detecting the health status of cable insulation and is used for condition monitoring and life prediction during cable operation, thereby helping to promptly identify potential problems, arrange maintenance plans, and prevent sudden failures. The measured dielectric loss tangent reflects the energy loss of the insulation material in an AC electric field due to polarization hysteresis and conductive current. The larger the dielectric loss tangent, the greater the loss and the more severe the aging.

[0003] Existing cable insulation testing technologies have made significant progress in identifying and locating local defects. Common methods include resonant cavity method, polarization and depolarization current method (PDC), and frequency domain dielectric response test (FDS).

[0004] Among them, the PDC and FDS methods have shown great potential in cable aging assessment due to their high sensitivity to minute changes in the dielectric parameters of insulating materials. However, these methods face significant challenges when applied to long-distance cables. On the one hand, both methods typically rely on sampling tests, resulting in insufficient representativeness. On the other hand, the FDS method is limited by the current output capability and frequency sweep range of the testing equipment, with its measurement frequencies mostly concentrated in the kilohertz (kHz) and below, making it difficult to cover dielectric behavior in higher frequency domains, and completing a full measurement takes a long time (usually more than 40 minutes). The PDC method is often used for detecting water treeing in cables, but it suffers from long testing times and sensitivity to noise, limiting its effective application in the field. In addition, although the resonant cavity method is faster than the aforementioned two methods, it also relies on sampling tests and has insufficient frequency resolution, which is not conducive to obtaining full-frequency dielectric information of the material. Summary of the Invention

[0005] The purpose of this invention is to provide a method and related device for judging the aging degree of cables based on impedance spectrum, which solves the technical problems of existing methods that rely on sampling and detection and therefore cannot obtain the full-frequency dielectric information of materials, as well as the long measurement time.

[0006] This invention is achieved through the following technical solution:

[0007] This invention discloses a method for determining the aging degree of cables based on impedance spectrum, comprising the following steps:

[0008] S1. Obtain the reflection coefficient curve based on the impedance spectrum curve of the cable under test;

[0009] S2. Based on the reflection coefficient curve, obtain the real part curve of the reflection coefficient and extract the resonant frequency corresponding to the resonant point of the real part curve of the reflection coefficient.

[0010] Based on the reflection coefficient curve and the cable reflection coefficient amplitude calculation model, the attenuation constant curve of the cable under test is output; multiple attenuation constant values ​​at the resonant frequency are extracted from the attenuation constant curve.

[0011] S3. Calculate the real part of the relative permittivity of the cable insulation medium at different resonant frequencies based on the resonant frequency and the position of the resonant point.

[0012] S4. Substitute the attenuation constant value and the real part of the relative permittivity into the dielectric loss tangent calculation model to obtain the dielectric loss tangent of the cable under test at the resonant frequency.

[0013] S5. Determine the degree of aging of the insulation material of the cable under test based on the dielectric loss tangent.

[0014] Furthermore, in S1, the process of obtaining the impedance spectrum curve of the cable under test is as follows:

[0015] A sweep frequency excitation signal is injected into the first end of the cable under test to obtain the impedance spectrum curve;

[0016] The test frequency range for the sweep excitation signal is 100MHz to 1GHz.

[0017] Furthermore, in S2, the step of obtaining the real part curve of the reflection coefficient based on the reflection coefficient curve and extracting the resonant frequency corresponding to the resonant point of the real part curve of the reflection coefficient is specifically as follows:

[0018] Import the reflection coefficient curve data into MATLAB software, and use the real() and findpeaks() functions in sequence to obtain the real part curve of the reflection coefficient and the resonant frequency corresponding to the resonant point of the real part curve of the reflection coefficient.

[0019] Furthermore, in S3, the real part of the relative permittivity of the cable insulation medium of the cable under test at different resonant frequencies is calculated based on the resonant frequency and the resonant point position. The calculation expression is as follows:

[0020] ;

[0021] in, Let be the real part of the relative permittivity of the cable insulation medium at different resonant frequencies; c The speed of light in free space; L This refers to the cable length. The first part of the real part curve of the reflection coefficient n The resonant frequency of each resonant point; n The position of the resonant point. n= 1, 2...

[0022] Furthermore, in S2, the attenuation constant curve represents the relationship between the amplitude of the reflection coefficient and the attenuation constant.

[0023] The specific expression for the cable reflection coefficient amplitude calculation model is as follows:

[0024] ;

[0025] in, This represents the amplitude of the reflection coefficient; It is the attenuation constant. z' The distance from any point to the end of the transmission line; Represents an exponential function;

[0026] The attenuation constant curve is extracted to contain multiple attenuation constant values ​​at the resonant frequency, and the specific expression is as follows:

[0027] ;

[0028] in, It is a logarithmic function; L This refers to the cable length.

[0029] Furthermore, in S4, the calculation formula for the dielectric loss tangent value calculation model is as follows:

[0030] ;

[0031] in, For dielectric loss angle, This is the tangent of the dielectric loss angle;

[0032] c For the speed of light in free space, It is the attenuation constant; Pi The resonant frequency; Let be the real part of the relative permittivity of the cable insulation medium at different resonant frequencies.

[0033] This invention discloses a device for determining the aging degree of cables based on impedance spectroscopy, comprising:

[0034] The reflection coefficient curve acquisition unit is used to obtain the reflection coefficient curve based on the impedance spectrum curve of the cable under test.

[0035] The data extraction unit is used to obtain the real part curve of the reflection coefficient based on the reflection coefficient curve, and to extract the resonant frequency corresponding to the resonant point of the real part curve of the reflection coefficient.

[0036] The attenuation constant acquisition unit is used to output the attenuation constant curve of the cable under test based on the reflection coefficient curve and the cable reflection coefficient amplitude calculation model; and to extract multiple attenuation constant values ​​of the attenuation constant curve at the resonant frequency.

[0037] The relative permittivity real part calculation unit is used to calculate the relative permittivity real part of the cable insulation medium of the cable under test at different resonant frequencies based on the resonant frequency and the resonant point position.

[0038] The dielectric loss tangent calculation unit is used to substitute the attenuation constant value and the real part of the relative permittivity into the dielectric loss tangent calculation model to obtain the dielectric loss tangent value of the cable under test at the resonant frequency.

[0039] The judgment unit is used to determine the degree of aging of the insulation material of the cable under test based on the dielectric loss tangent.

[0040] Furthermore, the process of obtaining the impedance spectrum curve of the cable under test is as follows:

[0041] A sweep frequency excitation signal is injected into the first end of the cable under test to obtain the impedance spectrum curve;

[0042] The test frequency range for the sweep excitation signal is 100MHz to 1GHz.

[0043] Furthermore, the real part of the relative permittivity of the cable insulation medium at different resonant frequencies is calculated based on the resonant frequency and the resonant point position. The calculation expression is as follows:

[0044] ;

[0045] in, Let be the real part of the relative permittivity of the cable insulation medium at different resonant frequencies; c The speed of light in free space; L This refers to the cable length. The first part of the real part curve of the reflection coefficient n The resonant frequency of each resonant point;n The position of the resonant point. n= 1, 2...

[0046] Furthermore, the attenuation constant curve represents the relationship between the amplitude of the reflection coefficient and the attenuation constant.

[0047] The specific expression for the cable reflection coefficient amplitude calculation model is as follows:

[0048] ;

[0049] in, This represents the amplitude of the reflection coefficient; It is the attenuation constant. z' The distance from any point to the end of the transmission line; Represents an exponential function;

[0050] The attenuation constant curve is extracted to contain multiple attenuation constant values ​​at the resonant frequency, and the specific expression is as follows:

[0051] ;

[0052] in, It is a logarithmic function; L This refers to the cable length.

[0053] Compared with the prior art, the present invention has the following beneficial technical effects:

[0054] This invention provides a method for judging the aging degree of cables based on impedance spectroscopy. First, the reflection coefficient curve of the cable is obtained based on the impedance spectrum curve. Obtaining the impedance spectrum curve only requires injecting a sweeping signal at the cable end, eliminating the need for sample extraction, thus achieving non-invasive and non-destructive testing of in-service cables. Then, the real part curve of the reflection coefficient is obtained based on the cable's reflection coefficient curve. By analyzing the resonance characteristics of the real part curve and establishing a calculation model for the dielectric loss tangent, the real part of the relative permittivity, the attenuation constant, and the dielectric loss tangent of the cable insulation medium at multiple resonance points can be accurately obtained. This enables multi-dimensional quantitative analysis of the cable's dielectric properties and aging state, overcoming the limitations of traditional methods in terms of measurement frequency band. It achieves a comprehensive judgment of the aging state of the cable insulation material, significantly improving the reliability of the evaluation results. This invention achieves accurate calculation of the cable's dielectric loss characteristics by establishing a dielectric loss tangent calculation model. Through multi-resonance point parameter analysis, this method can sensitively capture dielectric parameter drift caused by changes in the microstructure of the insulation medium, thereby enabling highly sensitive identification of early aging signs of cables and providing reliable early warnings for potential defects.

[0055] Furthermore, compared to traditional polarization / depolarization current methods and frequency domain dielectric spectroscopy methods, this invention overcomes the bottlenecks of limited test frequency bands and long test cycles. Using a wideband excitation signal from 100MHz to 1GHz, impedance spectrum data covering the entire frequency domain can be acquired in a short time, thereby obtaining the real part of the relative permittivity, attenuation constant, and dielectric loss tangent of the cable insulation medium at multiple resonant points, achieving multi-dimensional quantitative analysis of the cable's dielectric properties. This feature effectively overcomes the problem of missing frequency domain information caused by the narrow test bandwidth in traditional methods, significantly improving detection efficiency and frequency domain resolution. Attached Figure Description

[0056] Figure 1 This is a flowchart of a cable aging degree determination method based on impedance spectrum according to the present invention;

[0057] Figure 2 This is a schematic diagram of transmission line termination conditions;

[0058] Figure 3 This is a structural diagram of a 10kV single-core cross-linked polyethylene power distribution cable;

[0059] In the diagram, 1. Wire core; 2. Inner semiconductive layer; 3. Insulation layer; 4. Outer semiconductive layer; 5. Copper tape shielding layer; 6. Sheath;

[0060] Figure 4 The effect of dispersion on the measurement of the real part of the relative permittivity;

[0061] Figure 5 Simulation modeling results for loss tangent values ​​of insulation media with different aging;

[0062] Figure 6 A broadband dielectric parameter measurement platform for cables;

[0063] Figure 7 Measured curves of dielectric loss tangent values ​​for insulation materials of three aged cables;

[0064] Figure 8 Measured curves of relative permittivity for three types of cable insulation materials under aging conditions;

[0065] Figure 9 Impedance spectrum curve;

[0066] Figure 10 This is the reflection coefficient curve;

[0067] Figure 11 This is the curve showing the real part of the reflection coefficient;

[0068] Figure 12 This is a schematic diagram of a device for determining the aging degree of cables based on impedance spectrum according to the present invention. Detailed Implementation

[0069] To make the objectives, technical solutions, and advantages of the present invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0070] The detailed description of the embodiments of the present invention provided in the following figures is not intended to limit the scope of the claimed invention, but merely to illustrate one selected embodiment of the invention. All other embodiments obtained by those skilled in the art based on the figures and embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0071] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0072] like Figure 1 As shown, this invention discloses a method for determining the aging degree of cables based on impedance spectroscopy, comprising the following steps:

[0073] S1. Obtain the reflection coefficient curve based on the impedance spectrum curve of the cable under test;

[0074] S2. Based on the reflection coefficient curve, obtain the real part curve of the reflection coefficient and extract the resonant frequency corresponding to the resonant point of the real part curve of the reflection coefficient.

[0075] Based on the reflection coefficient curve and the cable reflection coefficient amplitude calculation model, the attenuation constant curve of the cable under test is output; multiple attenuation constant values ​​at the resonant frequency are extracted from the attenuation constant curve.

[0076] S3. Calculate the real part of the relative permittivity of the cable insulation medium at different resonant frequencies based on the resonant frequency and the position of the resonant point.

[0077] S4. Substitute the attenuation constant value and the real part of the relative permittivity into the dielectric loss tangent calculation model to obtain the dielectric loss tangent of the cable under test at the resonant frequency.

[0078] S5. Determine the degree of aging of the insulation material of the cable under test based on the dielectric loss tangent.

[0079] In S1, a vector network analyzer can be used to measure the cable under test and obtain the impedance spectrum curve. The core of obtaining the reflection coefficient curve based on the impedance spectrum curve is to use the inherent mathematical relationship between impedance and reflection coefficient in transmission line theory, combined with the calibration parameters of the measurement system (such as reference impedance) to complete the calculation.

[0080] As a typical two-conductor transmission line structure, the electromagnetic wave propagation of coaxial lines is dominated by the transverse electromagnetic mode (TEM). Under this propagation mode, due to the spatial distribution characteristics of the electromagnetic field, traditional lumped parameter circuit theory can no longer accurately characterize its transmission behavior, and a precise description of the transmission line model must be based on distributed parameter theory.

[0081] Transmission line termination condition model as follows Figure 2 As shown, Z g and E g These are the internal resistance and voltage of the excitation source, respectively; Z L For transmission line load; Z 0 represents the characteristic impedance of the transmission line; I ( z )and U ( z ( ) represent the current and voltage values ​​at any point on the transmission line, respectively; z Let be the distance from any point to the beginning of the transmission line. z' The distance from any point to the end of the transmission line.

[0082] According to the long-line equation, on the transmission line z' Input impedance at Z in ( z' The voltage at that point can be used. With current The ratio represents:

[0083] (1)

[0084] In the formula, Let be the propagation constant. , j The imaginary unit, It is the attenuation constant. The phase shift constant is f The frequency of the sweep excitation signal is denoted as .

[0085] For lossless transmission lines Substituting α=0 into formula (1), we get:

[0086] (2)

[0087] The reflection coefficient is the ratio of the reflected voltage to the input voltage, and can be expressed using the characteristic impedance of the transmission line. Z 0 and the transmission coefficient are expressed as:

[0088] (3)

[0089] in, Represents the reflection coefficient. For the transmission coefficient, Г L The load reflection coefficient.

[0090] For a lossless transmission line, equation (3) can also be expressed as:

[0091] (4)

[0092] Let be the hysteresis angle of the reflection coefficient, and its expression is:

[0093] (5)

[0094] In the formula, λ p The operating wavelength of electromagnetic waves on the transmission line. Pi is the mathematical constant of a circle.

[0095] According to equations (2) and (5), the input impedance of a lossless transmission line is... Z in ( z’ and reflection coefficient It exhibits periodic changes. This characteristic stems from the inherent property of the sine function contained in equation (2), namely, it possesses... π / 2 repeatability and π / 4 inverse invariance. Regarding the reflection coefficient, if... z′ and( z′ + λ p / 2) Substituting into equation (5) respectively, we can find that the results differ by 2. π The phase of the reflection coefficient is equivalent to the phase being the same, thus confirming that the reflection coefficient also has the same periodicity. Therefore, at a fixed frequency, the phase distance on the transmission line is... λ p The two points at point / 2 have the same input impedance and reflection coefficient, and are separated by a distance of 1 / 2. λ p The input impedance at point / 4 shows an inversion.

[0096] When the transmission line length is fixed, a sweep frequency signal is used as the excitation. Since the wavelength of the electromagnetic wave changes with the frequency, the transmission line... λ p The position of the 2 / 2 spacing point exhibits periodic frequency-varying characteristics. This phenomenon also causes the reflection coefficient measured at the beginning of the transmission line to exhibit periodicity in the frequency domain.

[0097] For lossless transmission lines, the periodicity of the reflection coefficient is characterized by a constant amplitude and a periodic change in phase; for lossy transmission lines, the periodicity of phase still exists, but the amplitude decreases as the frequency increases.

[0098] Based on the reflection coefficient curve obtained by formula (3), the following describes how to obtain the resonant frequency corresponding to the resonant point of the real part curve of the reflection coefficient.

[0099] Import the reflection coefficient curve data into MATLAB (Matrix Laboratory) software, and use the real() and findpeaks() functions in sequence to obtain the real part curve of the reflection coefficient and the frequency corresponding to its resonant point.

[0100]

[0101]

[0102] In the formula, This is the curve showing the real part of the reflection coefficient. The first part of the real part curve of the reflection coefficient n The resonant frequency of each resonant point.

[0103] The following describes the method for calculating the real part of the relative permittivity of a cable.

[0104] Based on the repeatability of transmission line input impedance and reflection coefficient, for a coaxial cable transmission line with an open-circuit termination, when the transmission line length is the electromagnetic wave... λ p When the value is an integer multiple of 2, the real part of the reflection coefficient measured at its head resonates, and the resonant frequency can be expressed as:

[0105] (6)

[0106] In the formula, n The position of the resonant point. n= 1, 2...; The first part of the real part curve of the reflection coefficient n The resonant frequency of a resonant point is defined as the maximum value of the real part of the reflection coefficient within the period. v p The wave velocity of electromagnetic waves in the cable; L The cable length is the same as the transmission line length. Let λ be the wavelength of the electromagnetic wave at the nth resonant frequency.

[0107] Wave velocity of electromagnetic waves in cables v p It can have the following relationship with the real part of the relative permittivity:

[0108] (7)

[0109] In the formula, c The speed of light in free space; Let be the real part of the relative permittivity of the cable insulation medium at different resonant frequencies.

[0110] Substituting equation (7) into equation (6), we can obtain the real part of the relative permittivity of the cable insulation medium at different resonant frequencies:

[0111] (8)

[0112] The following describes the process of obtaining the attenuation constant curve of the cable under test in S2.

[0113] According to equation (3), the reflection coefficient of the cable depends on the load reflection coefficient Γ. L and transmission coefficient e -2γ(f)z' When the cable end is open-circuited, Г L It is always equal to 1. Therefore, equation (3) can be simplified to equation (9).

[0114] (9)

[0115] The amplitude of the reflection coefficient is:

[0116] (10)

[0117] Therefore, the attenuation constant at each frequency point can be derived from the amplitude of the reflection coefficient:

[0118] (11)

[0119] in, It is the attenuation constant. It is a logarithmic function; This represents the amplitude of the reflection coefficient; This represents an exponential function.

[0120] The resonant point is a specific set of frequencies, and the relationship between the two is one of inclusion and being included; the resonant point is included in each frequency.

[0121] If only the medium loss of the transmission line is considered, the propagation constant can be expressed as:

[0122] (12)

[0123] In the formula, ε ( f () is the dielectric constant; μ The dielectric permeability; It is the angular frequency of the electromagnetic wave.

[0124] When the cable insulation medium is mostly non-magnetic, the above equation (12) can be rewritten as:

[0125] (13)

[0126] In the formula, μ 0 represents the permeability of free space; ε' ( f )and ε'' ( f ) are the real part and the imaginary part of the dielectric constant, respectively.

[0127] Furthermore, the tangent of the dielectric loss angle tan δ ( f )= ε'' ( f ) / ε' ( f Substituting into equation (13), we get:

[0128] (14)

[0129] From formula (14) and the above-mentioned It can be seen that, for Extracting the real part of the , we get:

[0130] (15)

[0131] In the formula, is the attenuation constant.

[0132] In summary, the formula for calculating the tangent of the dielectric loss angle can be obtained from formula (15):

[0133] (16)

[0134] in, For dielectric loss angle, This is the tangent of the dielectric loss angle.

[0135] To address the aforementioned method for judging cable aging based on impedance spectrum, this invention proposes corresponding simulation verification.

[0136] To verify the correctness of the broadband dielectric parameter measurement method for cables based on resonant frequency and attenuation constant, a simulation model of a 10kV single-core cross-linked polyethylene distribution cable was constructed on the MATLAB (Matrix Laboratory) platform. The cable structure is as follows: Figure 3 As shown, it mainly includes a core 1, an inner semiconductive layer 2, an insulating layer 3, an outer semiconductive layer 4, a copper strip shielding layer 5, and a sheath 6. The insulating layer 3 is made of cross-linked polyethylene. Simulation parameters are shown in Table 1.

[0137] Table 1

[0138]

[0139] At high frequencies, the relative permittivity of cross-linked polyethylene insulation material satisfies the Cole-Cole equation:

[0140] (17)

[0141] in, The dielectric constant of the cable; N , M , P These are the fitting parameters, N =2.34、 M =2.65×10 -9 and P =0.715; is the real part of the dielectric constant; This represents the imaginary part of the dielectric constant. It is the vacuum permittivity; i The imaginary unit; ω is the angular frequency.

[0142] The Cole-Cole equation, proposed by American scientists Kenneth S. Cole and Robert H. Cole, describes the relationship between the dielectric constant of a dielectric and frequency under an alternating electric field, and can more accurately reflect the relaxation characteristics of actual dielectrics.

[0143] Simulation tests were conducted on a simulation model of a 10kV single-core cross-linked polyethylene distribution cable. First, the following results were obtained: Figure 9 The impedance spectrum curve shown is used to obtain the following results: Figure 10 The reflection coefficient curve shown is a 3D curve, with its three axes representing the real part of the reflection coefficient, the imaginary part of the reflection coefficient, and the frequency. Taking the real part of the reflection coefficient curve means only observing the relationship between the real part axis and the frequency axis, i.e., a top view of the 3D image.

[0144] Based on the reflection coefficient curve, as shown below Figure 11 The curve showing the real part of the reflection coefficient is used to extract the resonant frequency corresponding to the resonant point of the real part in MATLAB software. The real part curve of the reflection coefficient is a periodically changing curve (with peaks and troughs). The resonant point is the point where the peak of the curve is located, and the resonant frequency is the frequency corresponding to the resonant point. The frequency of the sweep excitation signal is... Figure 11 The horizontal axis represents the resonant frequency, which is a partial point on the horizontal axis.

[0145] Using formula (8), the real part of the relative permittivity of the cable insulation medium at different resonant frequencies is calculated based on the resonant frequency and the position of the resonant point. Based on the Cole-Cole model, a simulation test of the real part of the relative permittivity is conducted on the cable model considering material dispersion effects. The simulation results are as follows: Figure 4 As shown, the measured real part of the relative permittivity-frequency curve was compared with the actual real part of the relative permittivity-frequency curve in the simulation settings. The results show that the two have good consistency in their trends, but the measured curve is slightly higher than the actual value in terms of amplitude, showing a regular deviation, that is, the actual curve is shifted upward along the vertical axis. The measured value of the real part of the relative permittivity is basically consistent with the actual value, indicating that the measurement results of the calculation method of the present invention and the actual simulation results have high consistency, and the calculation method of the present invention is feasible.

[0146] This deviation primarily stems from the inherent characteristics of the resonance method measurement. The relative permittivity obtained by the resonance method is the equivalent relative permittivity of all dielectric materials between the electrodes. Specifically, in the context of cable structure, the measured value reflects the equivalent relative permittivity under the combined action of the main insulation material and the semiconductive layer between core 1 and the copper tape shielding layer 5. Because the permittivity of the semiconductive material is higher than that of cross-linked polyethylene, the measured result is higher than the actual relative permittivity of the single cross-linked polyethylene material.

[0147] Nevertheless, because the inner semiconductive layer 2 and the outer semiconductive layer 4 are relatively thin (only about 1 / 10 the thickness of the insulation layer 3), their influence on the equivalent dielectric constant is limited, thus the experimental error can be controlled within 0.2%. This measurement method still possesses high accuracy and can reliably reflect the dielectric characteristics of the cable's main insulation.

[0148] In the simulation modeling, it is approximated that the dielectric loss angle does not change with frequency, and three typical operating conditions are set: normal cable, slightly aged cable, and severely aged cable. Through comparative analysis, the following can be obtained: Figure 5 The dielectric loss tangent values ​​for the three types of cables are shown. Specifically, the dielectric loss tangent value for a normal cable is 0.0025 at high frequencies, the dielectric loss tangent value for a slightly aged cable is 0.005 at high frequencies, and the dielectric loss tangent value for a severely aged cable is 0.01 at high frequencies.

[0149] Simulation results show that under three typical insulation conditions, this method can accurately measure the dielectric loss tangent of the cable insulation material with an error of no more than 4%.

[0150] To further verify the effectiveness of the method described in this invention in practical experiments, a system was constructed as follows: Figure 6The diagram shows a broadband dielectric parameter measurement platform for cables. The host computer is connected to a 10kV distribution cable via a vector network analyzer. kV represents kilovolt, a unit of voltage measurement.

[0151] A 10kV power cable with a characteristic impedance of 50Ω and a length of 30m was selected as the test sample for the experiment. To simulate different insulation aging states, the cable was placed in a constant temperature oven and subjected to accelerated thermal aging treatment at 90℃ for 24h and 48h, respectively.

[0152] The test was conducted in a constant temperature environment of 25℃. A network analyzer was used to inject a swept-frequency excitation signal into the open-circuit cable end, and its reflection coefficient curve was measured. The network analyzer model was KEYSIGHT E5063A, the test frequency range of the swept-frequency excitation signal was 100MHz to 1GHz, and the number of sampling points was set to 10000 to ensure the frequency resolution of the measurement data. Here, MHz represents megahertz, and GHz represents gigahertz.

[0153] As the resonant frequency increases, the measured curves of the dielectric loss tangent of the insulation materials of the three types of aged cables are as follows: Figure 7 As shown, at the same resonant frequency, the dielectric loss tangent of a normal cable is the smallest, and the longer the aging time, the larger the dielectric loss tangent.

[0154] As the testing frequency increases, the measured curves of the real part of the relative permittivity of the three types of cable insulation materials are as follows: Figure 8 As shown, at the same frequency, the relative permittivity of a normal cable is the smallest, and the longer the aging time, the larger the relative permittivity.

[0155] The experimental results show that with the extension of thermal aging time, the relative permittivity and dielectric loss tangent of the cable insulation material both show a gradual increasing trend. This trend is consistent with the physical mechanisms of gradual deterioration of the internal microstructure, increase of polar groups, and enhanced interfacial polarization during the thermal aging process of the insulation material, and conforms to the general law of insulation performance degradation.

[0156] On the other hand, frequency domain analysis shows that as the test frequency increases, both the relative permittivity and the dielectric loss tangent exhibit a decreasing trend. This is mainly due to the different response times in the polarization mechanisms: in the low-frequency region, slow polarization mechanisms such as dipole polarization and interface polarization can fully respond to the applied electric field, thus contributing significantly to the dielectric response; while in the high-frequency region, these polarization mechanisms struggle to follow changes in the electric field, resulting in decreased polarization capability and a weakened overall dielectric response, manifested as a reduction in the permittivity and loss factor.

[0157] like Figure 12 As shown, this invention discloses a device for judging the aging degree of cables based on impedance spectrum, comprising:

[0158] The reflection coefficient curve acquisition unit is used to obtain the reflection coefficient curve based on the impedance spectrum curve of the cable under test.

[0159] The data extraction unit is used to obtain the real part curve of the reflection coefficient based on the reflection coefficient curve, and to extract the resonant frequency corresponding to the resonant point of the real part curve of the reflection coefficient.

[0160] The attenuation constant acquisition unit is used to output the attenuation constant curve of the cable under test based on the reflection coefficient curve and the cable reflection coefficient amplitude calculation model; and to extract multiple attenuation constant values ​​of the attenuation constant curve at the resonant frequency.

[0161] The relative permittivity real part calculation unit is used to calculate the relative permittivity real part of the cable insulation medium of the cable under test at different resonant frequencies based on the resonant frequency and the resonant point position.

[0162] The dielectric loss tangent calculation unit is used to substitute the attenuation constant value and the real part of the relative permittivity into the dielectric loss tangent calculation model to obtain the dielectric loss tangent value of the cable under test at the resonant frequency.

[0163] The judgment unit is used to determine the degree of aging of the insulation material of the cable under test based on the dielectric loss tangent.

[0164] This invention discloses a method for judging the aging degree of cables based on impedance spectroscopy. This method utilizes frequency domain impedance spectroscopy measurement technology to collect impedance characteristic parameters of the cable at different frequencies, achieving non-destructive and non-invasive cable condition detection and assessing the aging degree while ensuring the cable's normal operation. The aging process of cables causes significant changes in their internal microstructure and electrical parameters, which are directly reflected in the abnormal characteristics of the impedance spectrum and reflection coefficient curve. This method, by analyzing these characteristic curves, can effectively identify subtle signs of degradation in the cable insulation layer. Compared to traditional detection methods such as DC withstand voltage testing, this method has higher detection sensitivity, enabling early warning of potential faults and providing more accurate aging degree assessment results.

[0165] Based on impedance spectral analysis, this method can accurately locate aging sections and concentrated defects in non-uniformly aged cables by analyzing characteristic parameters such as reflection coefficient curves (including but not limited to orthogonal transformation analysis of input impedance phase spectrum), thereby achieving a quantitative assessment of the local aging state of the cable.

[0166] This method uses multi-dimensional parameter calculations (including key indicators such as the real part of the relative permittivity, attenuation constant, and dielectric loss tangent) to comprehensively reflect the aging state of cables from different physical characteristics, providing a more systematic and accurate assessment basis for determining the degree of aging, and helping maintenance personnel to gain a deeper understanding of the actual aging status of cables.

[0167] By accurately assessing the aging degree and spatial distribution characteristics of cable insulation materials, this method helps maintenance personnel to promptly identify vulnerable areas and implement targeted maintenance strategies, effectively preventing faults. Furthermore, based on the quantitative assessment results of aging degree, the remaining service life of the cable can be scientifically predicted.

[0168] This method only requires analyzing the characteristics of the reflected signal at the cable's head end, without damaging the cable structure. It possesses excellent non-invasive and non-destructive testing characteristics, making it suitable for on-site testing of long-distance cables in actual operating environments. By extracting the amplitude-frequency characteristic parameters of the reflection coefficient, the insulation dielectric parameters are inverted and calculated, fundamentally avoiding measurement noise problems caused by electromagnetic interference, ground potential fluctuations, and capacitive coupling of the test leads in traditional methods, thus significantly improving the reliability of the test results.

[0169] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. An impedance spectrum-based method of judging a degree of aging of a cable, characterized by, The method comprises the following steps: S1, obtaining a reflection coefficient curve based on an impedance spectrum curve of the cable to be measured; S2, obtaining a real part curve of the reflection coefficient based on the reflection coefficient curve, and extracting a resonance frequency corresponding to a resonance point of the real part curve of the reflection coefficient; Based on the reflection coefficient curve and a cable reflection coefficient amplitude calculation model, an attenuation constant curve of the cable to be measured is outputted; and a plurality of attenuation constant values on the resonance frequency of the attenuation constant curve are extracted; S3, calculating a relative dielectric constant real part of the cable insulation medium of the cable to be measured at different resonance frequencies according to the resonance frequency and a resonance point order; S4, substituting the attenuation constant value and the relative dielectric constant real part into a dielectric loss tangent calculation model to obtain a dielectric loss tangent of the cable to be measured at the resonance frequency; S5, judging the aging degree of the insulation material of the cable to be measured based on the dielectric loss tangent; In S2, the attenuation constant curve is a relationship between the amplitude of the reflection coefficient and the attenuation constant. The specific expression of the cable reflection coefficient amplitude calculation model is: ； wherein, is the magnitude of the reflection coefficient; is the attenuation constant, z' is the distance of an arbitrary point from the end of the transmission line; represents the exponential function; The specific expression for extracting the plurality of attenuation constant values on the resonance frequency of the attenuation constant curve is: ； wherein is a logarithmic function; L is the cable length.

2. The method of claim 1, wherein the method is characterized by, In S1, the obtaining process of the impedance spectrum curve of the cable to be measured is specifically as follows: A sweep excitation signal is injected into the first end of the cable to be measured to obtain the impedance spectrum curve; The test frequency range of the sweep excitation signal is 100 MHz to 1 GHz.

3. The method of claim 1, wherein the method is characterized by: In S2, the real part curve of the reflection coefficient is obtained based on the reflection coefficient curve, and the resonance frequency corresponding to the resonance point of the real part curve of the reflection coefficient is extracted, which is specifically as follows: The data of the reflection coefficient curve is imported into MATLAB software, and the real() function and the findpeaks() function are used in sequence to obtain the real part curve of the reflection coefficient and the resonance frequency corresponding to the resonance point of the real part curve of the reflection coefficient.

4. The method of claim 1, wherein the method is characterized by: In S3, the relative dielectric constant real part of the cable insulation medium of the cable to be measured at different resonance frequencies is calculated according to the resonance frequency and the resonance point order, and the calculation expression is: ； wherein, is the real part of the relative permittivity of the cable insulation medium at different resonance frequencies; c is the speed of light in free space; L is the length of the cable; is the resonance frequency of the n th resonance point in the real part curve of the reflection coefficient; n is the order of the resonance point, n= 1, 2,...

5. The method of claim 1, wherein the method is characterized by: In S4, the calculation formula of the dielectric loss tangent calculation model is: ； wherein is the dielectric loss angle, is the dielectric loss angle tangent; c is the speed of light in free space, is the attenuation constant; is the circle constant, is the resonance point frequency; is the relative dielectric constant real part of the cable insulation medium at different resonance frequencies.

6. An apparatus for determining the degree of aging of a cable based on impedance spectroscopy, comprising: It comprises: A reflection coefficient curve acquisition unit is configured to obtain a reflection coefficient curve based on an impedance spectrum curve of the cable to be measured; A data extraction unit is configured to obtain a real part curve of the reflection coefficient based on the reflection coefficient curve, and extract a resonance frequency corresponding to a resonance point of the real part curve of the reflection coefficient; An attenuation constant acquisition unit is configured to output an attenuation constant curve of the cable to be measured based on the reflection coefficient curve and a cable reflection coefficient amplitude calculation model; and extract a plurality of attenuation constant values on the resonance frequency of the attenuation constant curve; A relative dielectric constant real part calculation unit is configured to calculate a relative dielectric constant real part of a cable insulation medium of the cable to be measured at different resonance frequencies according to the resonance frequency and a resonance point order; A dielectric loss tangent calculation unit is configured to substitute an attenuation constant value and a relative dielectric constant real part into a dielectric loss tangent calculation model to obtain a dielectric loss tangent of the cable to be measured at the resonance frequency; A judgment unit is configured to judge the aging degree of the insulation material of the cable to be measured based on the dielectric loss tangent. The attenuation constant curve is a relationship between the amplitude of the reflection coefficient and the attenuation constant. The specific expression of the amplitude calculation model of the cable reflection coefficient is: ； wherein, is the magnitude of the reflection coefficient; is the attenuation constant, z' is the distance of an arbitrary point from the end of the transmission line; represents the exponential function; The specific expression of the multiple attenuation constant values of the extraction attenuation constant curve at the resonance frequency is: ； wherein is a logarithmic function; L is the cable length.

7. The device for determining the aging degree of a cable based on impedance spectrum according to claim 6, characterized in that, The obtaining process of the impedance spectrum curve of the to-be-tested cable is specifically: The sweep excitation signal is injected into the first end of the to-be-tested cable, and the impedance spectrum curve is obtained. The test frequency range of the sweep excitation signal is 100 MHz to 1 GHz.

8. The device for determining the aging degree of a cable based on impedance spectrum according to claim 6, characterized in that, The specific expression of the relative dielectric constant real part of the cable insulation medium of the to-be-tested cable at different resonance frequencies is calculated according to the resonance frequency and the resonance point order, and the calculation expression is: ； wherein, is the real part of the relative permittivity of the cable insulation medium at different resonance frequencies; c is the speed of light in free space; L is the length of the cable; is the resonance frequency of the n th resonance point in the real part curve of the reflection coefficient; n is the order of the resonance point, n= 1, 2,...

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