A method, system and device for detecting cable insulation aging status based on frequency domain reflection method

Abstract

The present application discloses a method, system and equipment for detecting the aging state of cable insulation based on the frequency domain reflection method, and relates to the field of cable insulation state detection. The method comprises: measuring the head-end reflection coefficient spectrum and the head-end impedance spectrum of the cable under test; simulating a Gaussian pulse incident signal, and performing a time-frequency domain transformation in combination with the head-end reflection coefficient spectrum to obtain a positioning spectrum of the cable under test; determining the frequency period of the extreme value of the impedance amplitude spectrum according to the head-end impedance spectrum; determining the actual reflection peak position according to the positioning spectrum; determining the actual wave velocity of the cable under test according to the actual reflection peak position, the reference reflection peak position and the reference wave velocity; determining the relative change rate of the period according to the frequency period of the extreme value of the impedance amplitude spectrum and the reference frequency period; determining the relative change rate of the wave velocity according to the actual wave velocity and the reference wave velocity; and determining the aging degree of the cable under test according to the relative change rate. The present application can comprehensively diagnose the insulation state of the cable under test, and improve the performance and service life of the cable under test.

Description

Technical Field

[0001] The present application relates to the field of cable insulation status detection, and in particular to a cable insulation aging status detection method, system and equipment based on frequency domain reflection method. Background Art

[0002] In recent years, many scholars have conducted extensive research on the detection of local defects in power cable insulation, and proposed three types of non-destructive testing methods, including time domain reflectometry (TDR), time frequency domain reflectometry (TFDR) and frequency domain reflectometry (FDR). Among them, FDR injects a sweeping signal with a wide frequency range into the cable under test for testing. The average power of the high-frequency component of the signal is much greater than that of the pulse signal. Therefore, this method has higher sensitivity and spatial resolution, and has become the main research object in the field of cable local defect detection.

[0003] FDR can effectively detect local defects of cables such as electrical branches, water branches and mechanical damage, but it cannot effectively evaluate the overall aging state of cable insulation. During the operation of the cable, its main insulation will age under the influence of electrical, thermal and mechanical factors, resulting in reduced insulation performance and shortened cable service life. Therefore, developing a non-destructive testing method that can effectively evaluate the aging state of cable insulation is of great significance to the reliable and economical operation of the cable system. Summary of the invention

[0004] The purpose of this application is to provide a method, system and equipment for detecting the aging state of cable insulation based on the frequency domain reflection method, which can perform non-destructive detection of the insulation aging state of the cable on the basis of local defect detection of the cable using FDR.

[0005] To achieve the above objectives, this application provides the following solutions:

[0006] In a first aspect, the present application provides a method for detecting cable insulation aging status based on frequency domain reflection method, comprising:

[0007] Measure the reflection coefficient spectrum and impedance spectrum of the first end of the tested cable;

[0008] Simulating a Gaussian pulse incident signal, and performing a time-frequency domain transformation in combination with the head-end reflection coefficient spectrum to obtain a positioning spectrum of the tested cable;

[0009] Determine the frequency period of the extreme value of the impedance amplitude spectrum according to the first-end impedance spectrum;

[0010] Determine the actual reflection peak position according to the positioning spectrum, and determine the reference frequency period, reference reflection peak position and reference wave velocity;

[0011] Determining the actual wave velocity of the tested cable according to the actual reflection peak position, the reference reflection peak position and the reference wave velocity;

[0012] Determining a period relative change rate according to a frequency period of the extreme value of the impedance amplitude spectrum and the reference frequency period;

[0013] Determining the relative rate of change of wave velocity according to the actual wave velocity of the tested cable and the reference wave velocity;

[0014] The aging degree of the tested cable is determined according to the period relative change rate and the wave speed relative change rate.

[0015] In a second aspect, the present application provides a cable insulation aging state detection system based on frequency domain reflection method, comprising:

[0016] Cable measurement module, used to measure the head-end reflection coefficient spectrum and head-end impedance spectrum of the tested cable;

[0017] A positioning spectrum determination module, connected to the cable measurement module, is used to simulate a Gaussian pulse incident signal and perform a time-frequency domain transformation in combination with the head-end reflection coefficient spectrum to obtain a positioning spectrum of the measured cable;

[0018] A frequency period determination module, connected to the cable measurement module, for determining the frequency period of the extreme value of the impedance amplitude spectrum according to the head-end impedance spectrum;

[0019] A reference information determination module, connected to the positioning spectrum determination module, for determining the actual reflection peak position according to the positioning spectrum, and determining a reference frequency period, a reference reflection peak position and a reference wave velocity;

[0020] A wave speed determination module, connected to the reference information determination module, for determining an actual wave speed of the tested cable according to the actual reflection peak position, the reference reflection peak position and the reference wave speed;

[0021] A period change rate determination module, connected to the frequency period determination module and the reference information determination module respectively, for determining the period relative change rate according to the frequency period of the extreme value of the impedance amplitude spectrum and the reference frequency period;

[0022] A wave velocity change rate determination module, connected to the wave velocity determination module, for determining a relative wave velocity change rate according to an actual wave velocity of the tested cable and the reference wave velocity;

[0023] The aging degree determination module is connected to the period change rate determination module and the wave speed change rate determination module respectively, and is used to determine the aging degree of the tested cable according to the period relative change rate and the wave speed relative change rate.

[0024] In a third aspect, the present application provides a computer device, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the above-mentioned cable insulation aging status detection method based on the frequency domain reflection method.

[0025] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0026] The present application provides a method, system and device for detecting the aging state of cable insulation based on the frequency domain reflection method. By obtaining the head-end reflection coefficient spectrum, the head-end impedance spectrum and the positioning spectrum of the tested cable, the reference frequency period, the reference reflection peak position and the reference wave velocity are further obtained. The actual wave velocity of the tested cable and the frequency period of the extreme value of the impedance amplitude spectrum are calculated based on the reference data and the actual data of the tested cable. The relative change rate of the period and the relative change rate of the wave velocity are further determined to realize the detection of the aging degree of the tested cable. On the basis of using FDR to perform local defect detection on the cable, non-destructive detection of the insulation aging state of the tested cable can be realized, and then a more comprehensive diagnosis of the insulation state of the cable is performed, thereby improving the performance and service life of the tested cable. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 A schematic flow chart of a method for detecting cable insulation aging status based on frequency domain reflection method provided in one embodiment of the present application;

[0029] Figure 2 The diagram is a schematic diagram of the impedance amplitude spectrum of the first end of a 20-meter-long 10kV coaxial cable;

[0030] Figure 3 This is a schematic diagram of the positioning spectrum of a 20-meter-long 10kV defect-free cable;

[0031] Figure 4 Schematic diagram of impedance amplitude spectrum of defect-free cables at different insulation aging degrees;

[0032] Figure 5It is a schematic diagram of the positioning spectrum of defect-free cables under different insulation aging degrees;

[0033] Figure 6 Schematic diagram of impedance amplitude spectrum of defective cables at different insulation aging degrees;

[0034] Figure 7 It is a schematic diagram of the positioning spectrum of defective cables under different insulation aging degrees;

[0035] Figure 8 The impedance amplitude spectrum diagram of a 6kV three-phase cable is shown below;

[0036] Fig. 9 This is a partial enlarged view of the impedance amplitude spectrum of a 6kV three-phase cable;

[0037] Fig.10 This is a schematic diagram of the positioning spectrum of a 6kV three-phase cable. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0039] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0040] In an exemplary embodiment, Figure 1 As shown, a method for detecting the aging state of cable insulation based on the frequency domain reflection method is provided, including the following steps 101 to 108.

[0041] Step 101, measuring the head-end reflection coefficient spectrum and the head-end impedance spectrum of the cable under test.

[0042] Specifically, a vector network analyzer (VNA) is used to measure the reflection coefficient spectrum (RCS) and the broadband impedance spectrum (BIS) of the tested cable.

[0043] Since the test frequency of FDR is relatively high, the cable under test can be equivalent to a cascade of a series of distributed parameter micro-element circuits. The first-end impedance spectrum and the first-end reflection coefficient spectrum can be used to characterize the transmission characteristics of the signal in the cable.

[0044] First, make sure that the cable under test is not energized, and disconnect the electrical connection between the cable terminal and the switch cabinet, transformer and other equipment. At the test end, connect the cable terminal and the ground pigtail to the red and black alligator clips respectively, and connect the alligator clips to the VNA to measure the head-end reflection coefficient spectrum and head-end impedance spectrum of the cable under test. When measuring, set the test frequency of the VNA to 100kHz~100MHz, and the sampling interval to 10kHz. The relationship between the impedance spectrum and the reflection coefficient spectrum is as follows:

[0045]

[0046] Where, Γ(l) is the reflection coefficient spectrum, Z(l) is the impedance spectrum, l is the length of the cable under test, Γ(0) is the reflection coefficient spectrum at the head end, Z(0) is the impedance spectrum at the head end, and Z0 is the characteristic impedance of the cable under test, which can be calculated as follows:

[0047]

[0048] Where ε0 is the dielectric constant of vacuum, ε r is the relative dielectric constant of cross-linked polyethylene (XLPE) insulation, μ0 is the magnetic permeability in vacuum, r s is the outer diameter of the cable copper shield, r c Is the outer diameter of the cable core.

[0049] Step 102, simulating a Gaussian pulse incident signal, and performing a time-frequency domain transformation in combination with the head-end reflection coefficient spectrum to obtain a positioning spectrum of the tested cable.

[0050] The cable positioning spectrum contains information such as the entire cable length, the location of the intermediate joints, the location of local defects, and the insulation status of the cable. These location and status information will be presented in the positioning spectrum in the form of reflection peaks.

[0051] Specifically, step 102 includes:

[0052] (1) Generate a Gaussian pulse incident signal. Generate a Gaussian pulse incident signal by computer. The time-frequency domain expression of the Gaussian pulse incident signal is as follows:

[0053]

[0054] Among them, S i (t) is the time domain function of the Gaussian pulse incident signal, S i (w) is the frequency domain function of the Gaussian pulse incident signal, P is the amplitude factor of the Gaussian pulse incident signal, w is the frequency, n is the pulse width constant of the Gaussian pulse incident signal, m is the time delay constant of the Gaussian pulse incident signal, f s is the sampling frequency of the discrete Gaussian signal. The amplitude factor P and the time delay constant m have no effect on the Gaussian pulse incident signal, so P = 1, m = 1×10-6 The value of the pulse width constant n determines the accuracy and resolution of the positioning spectrum and should be calculated as follows:

[0055]

[0056] Among them, w u is the upper limit frequency of the Gaussian pulse incident signal, where w u is 100MHz, and then we can calculate n=4.833×10 -9 .

[0057] (2) According to the head-end reflection coefficient spectrum and the spectrum of the Gaussian pulse incident signal, the time domain signal of the Gaussian pulse reflection signal is determined by using an inverse fast Fourier transform:

[0058] S r (t) = abs[IFFT(S i (w)Γ(l) * )] (6)

[0059] Among them, S r (t) is the time domain signal of the Gaussian pulse reflection signal, S i (w) is the frequency domain function of the Gaussian pulse incident signal, Γ(l) * It represents the signal after the negative frequency domain conjugate extension of the head-end reflection coefficient spectrum, abs represents the absolute value, and IFFT represents the inverse fast Fourier transform.

[0060] (3) Convert the time domain signal of the Gaussian pulse reflection signal into the space domain to obtain the positioning spectrum of the measured cable.

[0061] Since the S r (t) is a time domain signal, which needs to be converted from time to space domain to obtain the positioning spectrum of the tested cable. The conversion process is as follows:

[0062]

[0063] Among them, v c is the transmission speed of electromagnetic waves in a vacuum, i.e. 3×10 8 m / s, v is the propagation speed of electromagnetic waves in the tested cable, that is, the wave velocity of the tested cable, which is approximately equal to 1.75×10 8 m / s, t is the time it takes for the electromagnetic wave to propagate in the cable. Combining formula (6) and formula (7), the positioning spectrum of the tested cable can be obtained.

[0064] Step 103: determining the frequency period of the extreme values ​​of the impedance amplitude spectrum according to the first-end impedance spectrum.

[0065] According to the transmission line theory, when the end of the measured cable is open, the input impedance amplitude of the measured cable head end can be expressed as:

[0066]

[0067] Among them, A and δ are intermediate variables, A=Γ L e -2αl ,δ=-2βl,Γ L is the reflection coefficient at the end of the cable under test. When the end of the cable under test is open circuit, Γ L =1, α is the attenuation constant of the cable under test, and β is the phase shift constant of the cable under test.

[0068] From formula (2), we can know that the characteristic impedance Z0 of the measured cable has nothing to do with the frequency and is basically a constant. L e -2αl , Γ L = 1, it can be deduced that A is always greater than 0. Therefore, formula (8) will reach a maximum value when cosδ = 1. At this time, δ = 2kπ, k is an integer, that is, -βl = kπ.

[0069] The phase shift constant of the tested cable is β = 2π / λ, where λ = v / f is the wavelength of the tested cable and f is the frequency of the signal. Therefore, the frequency and period of the extreme value of the input impedance amplitude at the head end of the tested cable are:

[0070]

[0071] It can be seen that the extreme values ​​of the input impedance amplitude spectrum at the head end will appear periodically, and its frequency period is related to the wave velocity v of the measured cable and the length l of the measured cable. The impedance amplitude spectrum of the head end of a 20-meter-long 10kV coaxial cable is as follows: Figure 2 As shown, it can be seen that the extreme values ​​of the impedance amplitude spectrum appear periodically. The peak-finding method is used to find the extreme points in the spectrum and their corresponding frequencies, the frequencies of adjacent extreme points are subtracted, and then the frequency differences of all extreme points are averaged to obtain the frequency period of the extreme values ​​of the impedance amplitude spectrum.

[0072] Specifically, import the first-end impedance amplitude spectrum in Matlab or Python, traverse the first-end impedance amplitude spectrum, find the frequency corresponding to the amplitude extreme point and calculate the frequency period. First, the peak finding method is used to determine the extreme point and the frequency of the extreme point in the first-end impedance spectrum. Then, according to the frequency of each extreme point in the first-end impedance spectrum, the frequency period of the impedance amplitude spectrum extreme value is determined by the following formula:

[0073]

[0074] Among them, T |Z(l)|max is the frequency period of the extreme value of the impedance amplitude spectrum, num is the number of extreme value points in the impedance spectrum of the first end, and f sub(k) is the frequency difference between the k-th extreme point and the (k + 1)-th extreme point, f |Z(l)|max (k + 1) is the frequency of the (k + 1)-th extreme point, f |Z(l)|max (k) is the frequency of the k-th extreme point.

[0075] Step 104: Determine the actual reflection peak position according to the positioning spectrum, and determine the reference frequency period T Z(l)max(base) 、the reference reflection peak position and the reference wave velocity.

[0076] In this application, the peak search method is adopted to determine the actual reflection peak position according to the positioning spectrum. The actual reflection peak position is the actual end reflection peak position or the actual fault reflection peak position. The reference reflection peak position is the reference end reflection peak position or the reference fault reflection peak position.

[0077] Specifically, import the positioning spectrum in Matlab or Python, traverse the data within the entire cable length range, and use the peak search function to find the actual end reflection peak position l end or the actual fault reflection peak position l defect .

[0078] For a cable without defects, the positioning spectrum will show reflection peaks at the head and end of the cable. Figure 3 shows the positioning spectra of defect-free cables with different wave velocities (v1, v2, v3), where the wave velocity v1 < v2 < v3, and the total length of the cable is 20 meters. Theoretically, the end reflection peaks of the positioning spectra calculated by substituting the wave velocities v1, v2, and v3 into formula (7) should all be at the 20-meter position. However, during actual testing, the actual wave velocity of each cable under test is unknown, and only the set reference wave velocity (1.75×10 8 m / s) can be used for calculation. Therefore, the horizontal axis of the positioning spectrum is shown in the spatial domain coordinates calculated based on the reference wave velocity, while the measured reflection coefficient spectrum is obtained according to the actual wave velocity of the cable. So the S r (t) calculated by formula (6) is also the result under the actual wave velocity, which will cause the positioning peaks at different actual wave velocities to shift on the positioning spectrum. The greater the difference between the actual wave velocity and the reference wave velocity, the more serious the offset degree of the positioning peak.

[0079] Therefore, this application selects reference data and calculates the reference frequency period and the reference reflection peak position. Generally, the reference can be the previous test data of the cable under test (such as the BIS and RCS measured when the cable was just put into operation) or the test data of different phases in the same circuit. The cable lengths of different phases in the same circuit are similar and the operation times are the same. If the test data of one of the phases is selected as the reference, the relative aging degrees of the three phases can be evaluated. Therefore, this application uses the previous test data of the cable under test as the reference, which can better reflect the aging degree of the cable under test at different operation stages.

[0080] Step 105: determining the actual wave velocity of the measured cable according to the actual reflection peak position, the reference reflection peak position and the reference wave velocity.

[0081] Figure 3 In the positioning spectrum shown, the wave speed v1 is the reference wave speed of the 10kV cross-linked polyethylene insulated cable, so its end reflection peak is located at the position of 20 meters of the entire cable length. The wave speeds v2 and v3 are approximately 0.9 and 0.8 times of v1 respectively. The end reflection peak positions corresponding to the three wave speeds are l v1 , l v2 and l v3 It can be seen that the greater the wave velocity difference, the farther the end peak position is offset. The actual wave velocity of the measured cable can be determined by the position difference of the reflection peak. The calculation formula is as follows:

[0082]

[0083] Among them, V test is the actual wave velocity of the cable under test, V base is the reference wave speed, l base It is the actual physical length of the cable, which is also the distance between the end reflection peak and the beginning of the cable under the reference wave speed. Δl is the difference between the actual reflection peak position and the reference reflection peak position.

[0084] for Figure 3 For example, its reference wave velocity V base =v1, actual physical length of the cable l base is 20 meters, the reflection peak difference Δl1=l v2 -l v1 , Δl2=l v3 -l v1 , then the calculation formulas for wave speed v2 and wave speed v3 are as follows:

[0085]

[0086] Step 106, determining the period relative change rate according to the frequency period of the extreme value of the impedance amplitude spectrum and the reference frequency period:

[0087] RCR(T |Z(l)|max )=(T |Z(l)|max -T |Z(l)|max(base) ) / T |Z(l)|max(base) ×100% (13)

[0088] Among them, RCR(T |Z(l)|max ) is the relative rate of change of the period, T |Z(l)|max is the frequency period of the extreme value of the impedance amplitude spectrum, T |Z(l)|max(base) is the base frequency period.

[0089] Step 107, determining the relative rate of change of wave velocity according to the actual wave velocity of the tested cable and the reference wave velocity:

[0090] RCR(V test )=(V test -V base ) / V base ×100% (14)

[0091] Among them, RCR (V test ) is the relative rate of change of wave velocity, V test is the actual wave velocity of the cable under test, V base is the base wave speed.

[0092] Step 108: Determine the aging degree of the tested cable according to the period relative change rate and the wave speed relative change rate.

[0093] Specifically, if the period relative change rate is greater than or equal to the wave speed relative change rate, the period relative change rate is used as an evaluation index; otherwise, the wave speed relative change rate is used as an evaluation index.

[0094] If the evaluation index is greater than or equal to a preset first index value, the aging degree of the tested cable is slight aging.

[0095] If the evaluation index is less than or equal to a preset second index value, the aging degree of the tested cable is severe aging.

[0096] If the evaluation index is smaller than the first index value and larger than the second index value, the aging degree of the tested cable is moderate aging; the first index value is larger than the second index value.

[0097] The service life of the cable is up to 30 years, and its main insulation will be aged under the continuous action of electricity, heat, environment and other factors. This application is analyzed by taking common cable insulation heat aging and insulation dampness as examples. When the cable is subjected to high temperature, the XLPE molecular chain will be thermally cracked and react with oxygen to generate peroxide free radicals. When the peroxide free radicals collide with each other, a diradical coupling reaction will occur to generate carbonyl compounds. When thermal aging is continuously carried out, the concentration of peroxide free radicals will increase, and the carbonyl compounds as aging products will also increase. Carbonyl, as a polar group, increases the dielectric constant of XLPE after aging. When the cable insulation is damp, the polar groups inside the insulation increase under the action of water molecules, resulting in an increase in the relaxation polarization process, which will also increase the dielectric constant of XLPE insulation.

[0098] From formula (7), we can know that the propagation speed v of electromagnetic waves in the measured cable is equal to the transmission speed v of electromagnetic waves in vacuum. cIt is related to the relative dielectric constant; as the dielectric constant increases, the propagation speed v of the electromagnetic wave in the measured cable will decrease. From formula (9), it can be seen that the frequency period of the extreme value of the impedance amplitude spectrum is proportional to the cable wave velocity. Based on the above conclusions, the aging of cable insulation will lead to an increase in the dielectric constant of XLPE insulation, thereby causing a decrease in the cable wave velocity, and then shortening the frequency period of the extreme value of the impedance amplitude spectrum.

[0099] Since the frequency period of the extreme value of the impedance amplitude spectrum is proportional to the wave velocity of the cable, the relative change rate of the two should be close. Cable insulation aging will lead to an increase in the dielectric constant of XLPE insulation, which will cause a decrease in the wave velocity of the cable, and then shorten the frequency period of the extreme value of the impedance amplitude spectrum. Therefore, the frequency period of the extreme value of the impedance amplitude spectrum and the relative change rate of the cable wave velocity together reflect the aging degree of the cable insulation. When evaluating, take RCR (T |Z(l)|max ) and RCR(V test ) is used as the evaluation index. The first index value is -7 and the second index value is -19. If the benchmark data is the previous test data of the same cable, the absolute aging degree of the cable insulation is evaluated (the degree of aging increases with the increase in operating time); if the test data of one of the three phases is selected as the benchmark, the relative aging degree between the three-phase cables is evaluated.

[0100] This application verifies the feasibility of the cable insulation aging status detection method based on the frequency domain reflection method through simulation, and the simulation process is as follows.

[0101] (1) Calculate the relative dielectric constant of cable insulation at different aging degrees.

[0102] According to existing research, within the test frequency range of 100kHz to 100MHz, its complex dielectric constant can be fitted by the following formula:

[0103]

[0104] Among them, ε(ω) is the complex dielectric constant, A1, B1, P1 are fitting coefficients, j is the imaginary unit, ω is the test angular frequency, which ranges from 100kHz to 100MHz, and the frequency interval is 10kHz. The fitting coefficients of the dielectric constant of cable insulation with different aging degrees are shown in Table 1.

[0105] Table 1 Fitting coefficients of cable insulation at different aging degrees

[0106]

[0107] Substituting the fitting coefficient into formula (15) and taking the real part, the relative dielectric constant of XLPE insulation at various aging degrees can be obtained, as shown in Table 2.

[0108] Table 2 Relative dielectric constant of cable insulation at different aging degrees

[0109] Aging degree ε Not aged 2.779 Mild aging 3.203 Severe aging 4.085

[0110] (2) Calculate the extreme frequency period and actual wave velocity of defect-free cables at different aging degrees.

[0111] The simulation cable is 20 meters long YJLV-1×35mm 2 -10kV cable, in order to simulate the different aging states of the cable insulation, the relative dielectric constant ε at different aging degrees in Table 2 r The impedance amplitude spectrum and positioning spectrum calculated by respectively bringing them into the distributed parameter model are as follows Figure 4 and Figure 5 As shown. In order to simulate the real measurement scenario, a white noise signal with a mean of zero is added to RCS and BIS. With the help of MATLAB software, the peak-finding method is used to calculate the frequency period and actual wave velocity of the extreme value of the impedance amplitude spectrum according to formula (10) and formula (11), as shown in Table 3 (based on the extreme frequency period and wave velocity when the cable insulation is not aged).

[0112] Table 3 Extreme frequency period and actual wave speed under different aging degrees of cable insulation

[0113] Aging degree <![CDATA[T |Z(l)|max (MHz)]]> <![CDATA[V test (×10 8 m / s)]]> Not aged 4.49 1.80 Mild aging 4.18 1.67 Severe aging 3.60 1.42

[0114] In order to verify whether the actual wave velocity of cable insulation under different aging conditions calculated by formula (11) is accurate, the present application combines Table 2 and formula (7) to calculate the theoretical wave velocity V under different aging conditions. def , and calculated V test With V def The relative error RE between the two conditions (because the non-aging state is taken as the benchmark, only the mildly aged and severely aged results are evaluated) is shown in Table 4.

[0115] Table 4 Relative error between actual wave velocity and theoretical wave velocity

[0116] Aging degree <![CDATA[V def (×10 8 m / s)]]> <![CDATA[V test (×10 8 m / s)]]> RE(%) Mild aging 1.68 1.67 0.59 Severe aging 1.44 1.42 1.4

[0117] The results show that the actual wave velocity calculation method proposed in this application can accurately calculate the cable wave velocity after cable insulation aging.

[0118] (3) Assessment of the aging condition of defect-free cable insulation.

[0119] From formula (9), it can be seen that the extreme frequency period is proportional to the cable wave speed, so the relative change rate of the extreme frequency period and the actual cable wave speed should be close, as shown in Table 5.

[0120] Table 5 Relative change rate of extreme frequency period and actual wave speed under different aging conditions

[0121]

[0122] From the results in Table 5, it can be seen that the aging of cable insulation will lead to a decrease in the frequency period of the extreme value of the impedance amplitude spectrum and the cable wave velocity, and the relative change rate of the two is close, which jointly reflects the aging degree of the cable insulation. Therefore, the insulation aging status evaluation index can be obtained as shown in Table 6 (where the evaluation index is RCR (T |Z(l)|max ) and RCR(V test )).

[0123] Table 6 Cable insulation aging evaluation indicators

[0124] Evaluation index (%) Aging degree ≥-7 Mild aging -8～-18 Moderate aging ≤-19 Severe aging

[0125] (4) Assessment of insulation aging status of defective cables.

[0126] In order to study whether local defects in cable insulation have an impact on the assessment of insulation aging status, this application sets a local defect at a distance of 6.95m-7.05m from the cable head end to increase the distributed capacitance at this location to simulate a water tree defect; at the same time, in order to simulate the actual measurement scenario, a white noise signal with a mean of zero is added to the calculated RCS and BIS. The impedance amplitude spectrum and positioning spectrum obtained are as follows: Figure 6 and Figure 7 As shown in Figure 2. By observing the localization spectrum, it can be found that an obvious double-peak reflection peak appears at the local defect.

[0127] l in formula (11) base It can also be replaced by the length l of the defect reflection peak from the cable head end at the reference wave speed defect(base) , then formula (11) becomes:

[0128]

[0129] From formula (16), we can see that the closer the reflection peak is to the cable head end, the greater the difference Δl between the actual fault reflection peak position and the reference fault reflection peak position. defect The smaller the value, the smaller the actual wave velocity of the tested cable will be. Therefore, the present application will simultaneously use the local defect reflection peak and the cable end reflection peak to calculate the actual wave velocity of the tested cable.

[0130] The peak-finding method is used to traverse the data. The frequency period and actual wave velocity of the extreme value of the impedance amplitude spectrum calculated according to formula (10), formula (11) and formula (16) are shown in Table 7.

[0131] Table 7 Extreme frequency period and actual wave speed of defective cable insulation at different aging degrees

[0132]

[0133] Comparing the data in Table 4 and Table 7, it can be seen that the influence of a single local defect on the frequency period of the extreme value of the cable impedance amplitude spectrum is small. When the actual wave velocity of the cable is calculated by the difference of the reflection peak at the end of the cable, the influence of a single local defect on the actual wave velocity is less than 10 -3 The order of magnitude has little impact. The actual wave velocity of the cable calculated using the difference of the local defect fault reflection peak is different from the actual wave velocity of the cable calculated using the difference of the end reflection peak. The main reason for the difference is that compared with the end reflection peak, the local defect reflection peak is closer to the head end of the cable, resulting in a smaller difference Δl of the defect reflection peaks with different insulation aging degrees, which is more affected by white noise, so the calculated actual wave velocity will also be disturbed.

[0134] Since the frequency period and actual wave velocity of the impedance amplitude spectrum extreme value are similar to those when the cable insulation is defect-free, the insulation state evaluation results are also similar. It can be inferred that the local defects of the cable have little effect on the insulation aging state evaluation method proposed in this application.

[0135] This application further evaluates the relative aging degree of a 6kV three-phase cable. The impedance amplitude spectrum and positioning spectrum measured on site are as follows: Figures 8 to 10 As shown. Figure 8 It can be seen that the effective frequency range of the impedance amplitude spectrum is about 100kHz~2MHz. In this frequency range, the surface wave can be ignored, and the electromagnetic field structure in the cable can be regarded as a transverse electromagnetic mode. When the frequency is higher than this range, the surface wave cannot be ignored. At this time, the electromagnetic field structure of the cable is a combination of the transverse electric wave mode and the transverse magnetic wave mode. The transmission properties of the signal will change and it is no longer applicable to the model of this application. Therefore, the impedance amplitude spectrum in the frequency range of 100kHz~2MHz is taken to calculate the frequency period, such as Fig. 9 shown.

[0136] Depend on Fig.10 The positioning spectrum shown shows that, in addition to the reflection peaks at the beginning and end of the cable, there is an obvious abnormal reflection peak at 54 meters in the three-phase positioning spectrum. Field verification shows that the abnormal reflection peak is caused by excessive bending of the cable. The reflection peak at the end of the spectrum can be used to determine that the total length of the cable is about 365 meters, and there is an obvious offset at the end reflection peak of the three-phase cable, indicating that the insulation aging degree of the three-phase cable is inconsistent. Therefore, this application can detect the aging degree of the three-phase cable insulation.

[0137] Since the excessive bending point of the cable is close to the head end of the cable, according to formula (16), the offset of the fault reflection peak position of cables with different insulation aging degrees will be smaller. In addition, the noise level of the field test is much higher than that of the laboratory test, so the actual wave velocity calculated by the difference of the fault reflection peak position of the cable will produce a large error. For the above reasons, this application uses the position difference of the reflection peak at the end of the cable to calculate the actual wave velocity of the cable, as shown in Table 8. Since there is no previous test data for the tested cable, the relative aging degree of the three-phase cable can only be evaluated based on the data of phase A with the smallest offset.

[0138] Table 8 Extreme frequency period and actual wave speed of a 6kV three-phase cable

[0139] Separation <![CDATA[T |Z(l)|max (MHz)]]> <![CDATA[V test (×10 8 m / s)]]> A 0.249 <![CDATA[1.75(V base )]]> B 0.237 1.67 C 0.248 1.74

[0140] Table 9 Relative change rate of extreme frequency period and actual wave speed of a 6kV three-phase cable

[0141] Separation <![CDATA[RCR(T |Z(l)|max )%]]> <![CDATA[RCR(V test )%]]> B -4.82 -4.57 C -0.60 -0.65

[0142] Combining the results of Table 8 and Table 9 with the evaluation indicators of Table 6, it can be seen that the B-phase cable is slightly aged compared to the A-phase cable, and the insulation state of the C-phase cable is basically the same as that of the A-phase. Since there is no previous test data for this cable, the actual aging state of the three-phase cable insulation cannot be accurately evaluated. Therefore, it is recommended to perform periodic BIS testing on the commissioned cables to better evaluate the insulation aging state.

[0143] This application enriches the functions of the FDR detection method. On the basis of using FDR to detect local defects in the cable, it can perform non-destructive testing on the insulation aging status of the cable, and then conduct a more comprehensive diagnosis of the cable insulation status to improve the performance and service life of the cable.

[0144] In an exemplary embodiment, a cable insulation aging status detection system based on frequency domain reflection method is provided, including a cable measurement module, a positioning spectrum determination module, a frequency period determination module, a reference information determination module, a wave speed determination module, a period change rate determination module, a wave speed change rate determination module and an aging degree determination module.

[0145] The cable measurement module is used to measure the head-end reflection coefficient spectrum and head-end impedance spectrum of the tested cable.

[0146] The positioning spectrum determination module is connected to the cable measurement module, and is used to simulate a Gaussian pulse incident signal and perform time-frequency domain transformation in combination with the head-end reflection coefficient spectrum to obtain the positioning spectrum of the measured cable.

[0147] The frequency period determination module is connected to the cable measurement module, and is used to determine the frequency period of the extreme value of the impedance amplitude spectrum according to the head-end impedance spectrum.

[0148] The reference information determination module is connected to the positioning spectrum determination module, and the reference information determination module is used to determine the actual reflection peak position according to the positioning spectrum, and determine the reference frequency period, reference reflection peak position and reference wave speed.

[0149] The wave speed determination module is connected to the reference information determination module, and the wave speed determination module is used to determine the actual wave speed of the measured cable according to the actual reflection peak position, the reference reflection peak position and the reference wave speed.

[0150] The period change rate determination module is connected to the frequency period determination module and the reference information determination module respectively, and is used to determine the period relative change rate according to the frequency period of the extreme value of the impedance amplitude spectrum and the reference frequency period.

[0151] The wave speed change rate determination module is connected to the wave speed determination module, and the wave speed change rate determination module is used to determine the relative wave speed change rate according to the actual wave speed of the tested cable and the reference wave speed.

[0152] The aging degree determination module is connected to the period change rate determination module and the wave speed change rate determination module respectively, and the aging degree determination module is used to determine the aging degree of the tested cable according to the period relative change rate and the wave speed relative change rate.

[0153] In an exemplary embodiment, a computer device is provided, including: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement a method for detecting cable insulation aging status based on a frequency domain reflection method.

[0154] It should be noted that the data involved in this application (including but not limited to data used for analysis, stored data, displayed data, etc.) are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of relevant data must comply with relevant regulations.

[0155] Those of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to the memory, database or other medium used in the embodiments provided in the present application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM may be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM).

[0156] The database involved in each embodiment provided in this application may include at least one of a relational database and a non-relational database. The non-relational database may include a distributed database based on blockchain, etc., but is not limited thereto. The processor involved in each embodiment provided in this application may be a general-purpose processor, a central processing unit, a graphics processor, a digital signal processor, a programmable logic device, a data processing logic device based on quantum computing, etc., but is not limited thereto.

[0157] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0158] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A method for detecting cable insulation aging status based on frequency domain reflection method, characterized in that: The cable insulation aging state detection method based on frequency domain reflection method includes: Measure the reflection coefficient spectrum and impedance spectrum of the first end of the tested cable; Simulating a Gaussian pulse incident signal, and performing a time-frequency domain transformation in combination with the head-end reflection coefficient spectrum to obtain a positioning spectrum of the tested cable; Determine the frequency period of the extreme value of the impedance amplitude spectrum according to the first-end impedance spectrum; Determine the actual reflection peak position according to the positioning spectrum, and determine the reference frequency period, reference reflection peak position and reference wave velocity; Determining the actual wave velocity of the tested cable according to the actual reflection peak position, the reference reflection peak position and the reference wave velocity; Determining a relative rate of change of a period according to a frequency period of an extreme value of the impedance amplitude spectrum and a reference frequency period; According to the actual wave velocity of the tested cable and the reference wave velocity, the relative rate of change of wave velocity is determined: RCR (V test )=(V test -V base ) / V base ×100%; where RCR(V test ) is the relative rate of change of wave velocity, V test is the actual wave velocity of the cable under test, V base is the reference wave speed; The aging degree of the tested cable is determined according to the period relative change rate and the wave speed relative change rate.

2. The cable insulation aging state detection method based on frequency domain reflection method according to claim 1 is characterized in that: A vector network analyzer is used to measure the head-end reflection coefficient spectrum and head-end impedance spectrum of the tested cable.

3. The cable insulation aging state detection method based on frequency domain reflection method according to claim 1 is characterized in that: Determining the frequency period of the extreme value of the impedance amplitude spectrum according to the first-end impedance spectrum specifically includes: Using a peak-finding method to determine the extreme point and the frequency of the extreme point in the impedance spectrum of the first end; According to the frequency of each extreme point in the first-end impedance spectrum, the frequency period of the extreme value of the impedance amplitude spectrum is determined by the following formula: f sub (k)=f |Z(l)|max (k+1)-f |Z(l)|max (k); Among them, T |Z(l)|max is the frequency period of the extreme value of the impedance amplitude spectrum, num is the number of extreme value points in the impedance spectrum of the first end, and f sub (k) is the frequency difference between the kth extreme point and the k+1th extreme point, f |Z(l)|max (k+1) is the frequency of the k+1th extreme point, f |Z(l)|max (k) is the frequency of the kth extreme point.

4. The cable insulation aging state detection method based on frequency domain reflection method according to claim 1 is characterized in that: The peak-finding method is used to determine the actual reflection peak position according to the positioning spectrum; the actual reflection peak position is the actual end reflection peak position or the actual fault reflection peak position; the reference reflection peak position is the reference end reflection peak position or the reference fault reflection peak position.

5. The cable insulation aging state detection method based on frequency domain reflection method according to claim 1 is characterized in that: The actual wave velocity of the cable under test is determined using the following formula: Among them, V test is the actual wave velocity of the cable under test, V base is the reference wave speed, l base is the actual physical length of the cable, and Δl is the difference between the actual reflection peak position and the reference reflection peak position.

6. The cable insulation aging state detection method based on frequency domain reflection method according to claim 1 is characterized in that: The relative rate of change of the cycle is determined using the following formula: RCR(T |Z(l)|max )=(T |Z(l)|max -T |Z(l)|max(base) ) / T |Z(l)|max(base) ×100%; Among them, RCR(T |Z(l)|max ) is the relative rate of change of the period, T |Z(l)|max is the frequency period of the extreme value of the impedance amplitude spectrum, T |Z(l)|max(base) is the base frequency period.

7. The cable insulation aging state detection method based on frequency domain reflection method according to claim 1 is characterized in that: The aging degree of the tested cable includes mild aging, moderate aging and severe aging; Determining the aging degree of the tested cable according to the period relative change rate and the wave speed relative change rate specifically includes: If the period relative change rate is greater than or equal to the wave speed relative change rate, the period relative change rate is used as an evaluation index, otherwise the wave speed relative change rate is used as an evaluation index; If the evaluation index is greater than or equal to a preset first index value, the aging degree of the tested cable is slight aging; If the evaluation index is less than or equal to a preset second index value, the aging degree of the tested cable is severe aging; If the evaluation index is smaller than the first index value and larger than the second index value, the aging degree of the tested cable is moderate aging; the first index value is larger than the second index value.

8. A cable insulation aging state detection system based on frequency domain reflection method, applied to the cable insulation aging state detection method based on frequency domain reflection method according to any one of claims 1 to 7, characterized in that: The cable insulation aging state detection system based on frequency domain reflection method includes: Cable measurement module, used to measure the head-end reflection coefficient spectrum and head-end impedance spectrum of the tested cable; A positioning spectrum determination module, connected to the cable measurement module, is used to simulate a Gaussian pulse incident signal and perform a time-frequency domain transformation in combination with the head-end reflection coefficient spectrum to obtain a positioning spectrum of the measured cable; A frequency period determination module, connected to the cable measurement module, for determining the frequency period of the extreme value of the impedance amplitude spectrum according to the head-end impedance spectrum; A reference information determination module, connected to the positioning spectrum determination module, for determining the actual reflection peak position according to the positioning spectrum, and determining a reference frequency period, a reference reflection peak position and a reference wave velocity; A wave speed determination module, connected to the reference information determination module, for determining an actual wave speed of the tested cable according to the actual reflection peak position, the reference reflection peak position and the reference wave speed; A period change rate determination module, connected to the frequency period determination module and the reference information determination module respectively, for determining the period relative change rate according to the frequency period of the extreme value of the impedance amplitude spectrum and the reference frequency period; A wave velocity change rate determination module, connected to the wave velocity determination module, for determining a relative wave velocity change rate according to an actual wave velocity of the tested cable and the reference wave velocity; The aging degree determination module is connected to the period change rate determination module and the wave speed change rate determination module respectively, and is used to determine the aging degree of the tested cable according to the period relative change rate and the wave speed relative change rate.

9. A computer device comprising: A memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the cable insulation aging state detection method based on the frequency domain reflection method as described in any one of claims 1 to 7.

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