A method for evaluating cable partial moisture absorption by combining ultra-low frequency dielectric loss and wide-band impedance spectrum test

Abstract

The application relates to a cable partial moisture evaluation method combining VLF dielectric loss and wide-frequency impedance spectrum testing, which comprises the following steps: S1, obtaining the dielectric loss change rate, dielectric loss stability over time and dielectric loss average value of the measured cable by using VLF; S2, obtaining a time-domain positioning spectrum by using BIS, and calculating the polarity deviation of each reflection peak; S3, calculating the moisture risk index of the measured cable; S4, establishing a partial moisture evaluation model; S5, inputting the data of S1-S3 into the partial moisture evaluation model of S4, calculating a moisture state evaluation value, and evaluating the partial moisture degree of the measured cable. The application comprehensively considers the indexes obtained by the VLF dielectric loss test (VLF) and the wide-frequency impedance spectrum method (BIS), combines the cable characteristics, establishes a new cable partial moisture evaluation model, and improves the accuracy of the cable partial moisture state evaluation.

Description

Technical Field

[0001] This invention belongs to the field of cable insulation defect detection technology, specifically relating to a method for assessing localized moisture absorption in cables by combining ultra-low frequency dielectric loss and broadband impedance spectrum testing. Background Technology

[0002] With the acceleration of urbanization, power cables, due to their advantages such as high reliability, high power transmission capacity, and space saving, are gradually replacing traditional overhead lines and are widely used in urban power distribution networks. According to relevant statistics, the average annual growth rate of power cable line length in my country has exceeded 10% in recent years, and the cable coverage rate in the central urban areas of cities such as Shanghai, Shenzhen, and Xiamen has reached over 90%. This demonstrates that power cables occupy a crucial position in the modernization of my country's power distribution network.

[0003] Cables are typically laid in cable trenches where water accumulates over long periods, or buried directly in damp soil. Operating in such a humid environment, cables with damaged outer sheaths or inadequate protection at joints are susceptible to moisture intrusion, leading to excessive dielectric loss or low insulation resistance, threatening safe operation. Furthermore, as the cable's service life increases, moisture damage worsens, potentially developing into permanent faults such as short circuits or open circuits. Cable faults can cause power distribution system outages or even loss of control, resulting in significant economic losses and social impact. Therefore, timely detection of cable moisture, locating the site of localized moisture, and assessing the degree of moisture absorption are crucial for improving the safety and stability of cable power transmission systems.

[0004] Currently, among commonly used methods for cable moisture detection and location, ultra-low frequency dielectric loss testing (VLF) can measure the overall insulation condition of the cable and assess its overall moisture status. However, its sensitivity in identifying localized moisture defects is low and cannot fully meet testing requirements. Broadband impedance spectroscopy (BIS), as a non-destructive testing method, is widely used in cable defect location and can pinpoint the location of localized moisture in a cable. However, the abnormal peak-to-peak values ​​reflecting the moisture location in its location spectrum are determined by multiple factors, including the location of the moisture, the degree of moisture, and cable characteristics, and cannot accurately reflect the degree of moisture at that location. Furthermore, existing methods for cable moisture detection and location lack a scheme for accurately assessing the degree of moisture.

[0005] Therefore, developing a method for assessing localized moisture absorption in cables by combining ultra-low frequency dielectric loss and broadband impedance spectrum testing is of great significance for evaluating the moisture condition of cables. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for assessing localized moisture absorption in cables by combining ultra-low frequency dielectric loss and broadband impedance spectrum testing.

[0007] The technical problem solved by this invention is achieved through the following technical solution:

[0008] A method for assessing localized moisture absorption in cables using a combination of ultra-low frequency dielectric loss and broadband impedance spectroscopy testing, comprising the following steps:

[0009] S1. The cable under test is measured using an ultra-low frequency dielectric loss measuring instrument to obtain the dielectric loss change rate Δtanδ and the dielectric loss stability over time σ. tanδ and the average value of dielectric loss tanδ a ;

[0010] S2. Measure the cable under test using the broadband impedance spectroscopy method to obtain a standardized time-domain localization spectrum. Record the positions of the reflection peaks except for the beginning and end, and label them sequentially according to their distance from the beginning. Calculate the polarity deviation δ of each reflection peak. p ;

[0011] S3. Check the ledger of the cable under test, and calculate the moisture risk index (DRI) of the cable under test based on the laying environment and laying method.

[0012] S4. Using principal component analysis to process the measurement data from S1 to S3, a localized dampness assessment model is established to obtain the weight ω of each indicator. j ;

[0013] S5. Substitute the measurement data from S1 to S3 into the local moisture assessment model to calculate the moisture status assessment value at that location, and assess the degree of local moisture in the cable under test accordingly.

[0014] Furthermore, in S2, the polarity deviation δ for the same position containing two reflection peaks p The calculation formula is:

[0015]

[0016] Where: δ p(k) It is the polarity deviation of the k-th reflection peak;

[0017] α is the bimodal correction factor, with a value of 0.4;

[0018] E r(k) It is the extreme value of the right peak at the position of the kth reflection peak;

[0019] E l(k) It is the extreme value of the left peak at the position of the kth reflection peak;

[0020] d(k) is the distance from the position of the kth reflection peak to the tail reflection peak;

[0021] A is the extreme value of the terminal reflection peak;

[0022] For the polarity deviation δ of a single reflection peak at the same location p The calculation formula is:

[0023]

[0024] Where E(k) is the extreme value of the kth reflection peak.

[0025] Furthermore, the formula for calculating the moisture risk index of the S3 cable is as follows:

[0026]

[0027] Where: I is the laying influence factor, which is determined according to the different laying methods of the cable under test;

[0028] T represents the cable laying time;

[0029] The average annual relative humidity of the cable laying location;

[0030] IP is the cable's waterproof rating. If IP = 0, then DRI = 1.

[0031] Furthermore, the weights ω of each indicator in S4 j The calculation formula is:

[0032]

[0033] Where: ω j It is the weight of the j-th indicator;

[0034] m is the number of principal components retained by principal component analysis;

[0035] p is the number of indicators;

[0036] λ i It is the variance contribution rate of the i-th principal component;

[0037] a ji It is the load of the j-th index in the i-th principal component.

[0038] Furthermore, the formula for calculating the moisture condition assessment value G of S5 is as follows:

[0039] G(k) = ω1Δtanδ + ω2tanδ a +ω3σ tanδ +ω4δ p(k) +ω5DRI (5)

[0040] Where: ω1, ω2, ω3, ω4 and ω5 are the rate of change of dielectric loss Δtanδ, the average dielectric loss tanδ, and so on, respectively. a Dielectric loss stability over time σtanδ The polarity deviation δ at this position p(k) The influence weight of the cable moisture risk index (DRI) is determined by comparing the value of G(k) with the moisture status assessment reference table to evaluate the degree of moisture at that location of the cable under test.

[0041] The positive effects that this invention can produce are:

[0042] 1. This invention comprehensively considers the indicators obtained from ultra-low frequency dielectric loss testing (VLF) and broadband impedance spectroscopy (BIS), and combines them with the characteristics of the cable itself to establish a new cable local moisture assessment model, thereby improving the accuracy of assessing the local moisture condition of the cable.

[0043] 2. This invention analyzes the tested cable at a wind farm testing site to determine the results of on-site disassembly and inspection.

[0044] The results are consistent with the evaluation of the present invention, while using VLF or BIS methods alone to determine the moisture condition of the tested cable cannot provide an accurate assessment of the cable's moisture condition. Attached Figure Description

[0045] Figure 1 This is a flowchart of the present invention;

[0046] Figure 2 This is a schematic diagram of a wind farm test site according to the present invention;

[0047] Figure 3 This is a location map obtained using the BIS method in this invention;

[0048] Figure 4 This is a schematic diagram of the on-site disassembly and inspection of the intermediate joint at 715m according to the present invention. Detailed Implementation

[0049] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0050] like Figure 1 As shown, a method for assessing localized moisture absorption in cables using a combination of ultra-low frequency dielectric loss and broadband impedance spectrum testing is innovative in that the method comprises the following steps:

[0051] S1. Measure the cable under test using an ultra-low frequency dielectric loss meter to obtain three indicators: dielectric loss change rate, dielectric loss stability over time, and average dielectric loss.

[0052] S2. After measuring the cable under test using the BIS method and obtaining the location spectrum, determine the location of the abnormal peak and calculate its polarity deviation.

[0053] S3. Check the cable ledger and calculate the cable moisture risk index based on the laying environment and laying method;

[0054] S4. Process the field test data containing the indicators in S1 to S3 using principal component analysis (PCA) to obtain a localized dampness assessment model.

[0055] S5. Substitute the indicators obtained from S1 to S3 into the local moisture assessment model to calculate the moisture status assessment value at that location, and assess the degree of local moisture in the cable accordingly.

[0056] The specific details of each process are as follows:

[0057] 1. Identify the cable to be tested, open the electrical connection between the cable terminal and the switchgear, transformer, and other equipment, check that the cable terminal is clean and in good condition, put an equalizing ring on the cable terminal lug, connect one end of the high-voltage connection cable to the cable terminal to be tested, and connect the other end to the test host, and ground the other phase cable terminals to the testing device.

[0058] Before testing, input the cable name, length, insulation type, and laying method on the testing host, and select the oil-paper cable or cross-linked cable test program. After clicking "Start Test," apply pressure to the cable under test, setting the frequency to 0.1Hz, and test the relevant dielectric loss data at three voltages: 0.5U0, 1.0U0, and 1.5U0, where U0 is the rated voltage of the cable under test. After obtaining the dielectric loss value and the test curve, calculate the dielectric loss change rate Δtanδ and the average dielectric loss tanδ. a Dielectric loss stability over time σ tanδ It can be calculated using the following formula:

[0059]

[0060] In the formula, n is the number of tests. It is the dielectric loss value of the i-th test at 0.5U0. and Similarly, tanδ can be calculated using the following formula:

[0061]

[0062] In the formula, δ is the dielectric loss angle, and I R It is the leakage current value of the resistor, I. C R is the capacitive current value, R is the DC leakage resistance, and C is the equivalent capacitance.

[0063] 2. Connect one end of the cable under test to Port 1 of the Vector Network Analyzer (VNA) using an alligator clip. Measure the impedance spectrum at the beginning of the cable under test using S11 mode. Keep the other end of the cable under test open. During the measurement, set the VNA's test frequency to 100kHz to 100MHz and the sampling interval to 10kHz.

[0064] The measured impedance at the front end is converted into the reflection coefficient at the front end, and the two have the following relationship:

[0065]

[0066] Wherein: Γ v (0) is the reflection coefficient at the front end, Z(0) is the measured impedance spectrum at the front end, Z a This is the characteristic impedance of the line.

[0067] By simulating a Gaussian pulse incident signal using a computer, the reflection coefficient and the spectrum of the Gaussian pulse are used to obtain the time-domain reflection signal of the Gaussian pulse. A time-space domain transformation is performed on the reflected signal to obtain the location map of the cable under test. Then, linear regression is used to compensate for the location map, resulting in a standardized time-domain location map.

[0068] The positions of the reflection peaks, excluding the first and last peaks, are determined using a peak-finding algorithm. After numbering them sequentially according to their distance from the first peak, the polarity deviation δ of each reflection peak is calculated. p .

[0069] This invention defines polarity deviation δ p The value δ is used to characterize the tendency of insulating materials to undergo dipole repolarization under the influence of an electric field. A value of 0 indicates a stable state, while a value greater than 0 indicates a tendency for repolarization to occur. When a cable is locally damp, the cable insulation material is prone to thermo-oxidative reactions and thermal degradation reactions, forming various small molecules that cause the polarity deviation at the current position to increase by δ. p Increase.

[0070] In localization patterns, reflection peaks typically exhibit a single-peak or double-peak shape. For double-peaked reflection peaks, the polarity deviation δ p Because the difference between the extreme values ​​of the two peaks is significant, the numerical value is generally larger than that of a single-peaked reflection peak, requiring numerical correction. Since the Gaussian pulse signal is input from the beginning, the left peak in the double-peaked signal undergoes a polarity change first. Furthermore, considering the differences in the frequency and power of the input signal, as well as the material and length of the cable, in different detection methods, the end reflection peak and the length from the reflection peak to the end are introduced. The calculation formula is as follows:

[0071]

[0072] Where: δ p(k) Let E be the polarity deviation of the k-th reflection peak, and α be the polarity correction index, taken as α = 0.4.r(k) E represents the extreme value of the right peak in the k-th reflection peak. l(k) Let l(k) be the extreme value of the left peak in the k-th reflection peak, l(k) be the length from the k-th reflection peak to the tail reflection peak, and A be the extreme value of the tail reflection peak. If it is a single reflection peak, the calculation formula is as follows:

[0073]

[0074] Where: E(k) is the extreme value of the kth reflection peak.

[0075] 3. After identifying the cable to be tested, the cable's inherent characteristics, laying method, time, and environment are important factors affecting cable moisture absorption. This invention defines the Cable Moisture Risk Index (DRI), which characterizes the ease with which a cable absorbs moisture and serves as a reference indicator for assessing the degree of cable moisture absorption. The DRI value ranges from 0 to 1; the higher the DRI value, the more easily the cable absorbs moisture.

[0076] The second characteristic in the cable protection rating indicates the cable's protection level in liquids, ranging from 0 to 8, representing protection from no protection to continuous immersion. A higher number indicates stronger moisture resistance. Combined with field testing statistics, it was found that different cable laying methods accounted for a significant proportion of cables judged to be damp, with direct burial accounting for the highest proportion and overhead laying the lowest. Meanwhile, cable laying time and the average humidity of the surrounding environment are also important factors affecting cable moisture absorption. Based on this, a formula for calculating the Cable Moisture Risk Index (DRI) is proposed:

[0077]

[0078] In the formula, I is the laying influence factor defined in this invention, which is determined according to the laying method of the cable under test and with reference to the moisture influence factor table of cable laying method described in Table 1; T is the cable laying time. The average annual relative humidity of the cable laying location is denoted by IP, which is the cable's waterproof rating. If IP = 0, then DRI = 1.

[0079] Table 1. Factors Affecting Moisture in Cable Laying Methods

[0080] Laying method direct burial cable trench Pipeline cable tray overhead Impact Factor 1.0 0.8 0.6 0.4 0.2

[0081] 4. To discover the inherent relationship between the indicators in the above steps and to determine their role in evaluating localized moisture in cables.

[0082] To assess the relative importance of the degree of dampness, this invention utilizes principal component analysis (PCA) to process field test data, establishes a local dampness assessment model, and obtains the dampness status assessment value G.

[0083] This invention proposes five indicators, using 104 sets of field test data. For ease of explanation, these are referred to as n samples and p indicators respectively. A sample matrix x of size n×p can be constructed:

[0084]

[0085] After standardizing the sample matrix, calculate its covariance matrix R.

[0086]

[0087] The eigenvalues ​​λ1≥λ2…λ can be calculated from the covariance matrix R. p ≥0 and eigenvectors a1, a2…a n ,in

[0088]

[0089] Principal components F i The proportion of a corresponding eigenvalue among all eigenvalues ​​is the contribution rate of that principal component. Principal components F are then classified according to their contribution rates. i Arrange from largest to smallest, and take the first and second values ​​corresponding to the feature values ​​with a cumulative contribution rate exceeding 80%.

[0090] II. The m-th (m≤p) principal component F i .in

[0091] F i =a 1i X1+a 2i X2 + ... + a pi X p (14)

[0092] In the formula, X j The standardized x in equation (9) j Indicator, a ji It is indicator X j In principal component F i The load in.

[0093] For principal component F i In terms of each indicator X j Load a ji The larger the value, the greater the influence of that indicator on the principal component. Based on this, the X values ​​of each indicator can be calculated. j The weight of the moisture condition assessment value G is calculated using the following formula:

[0094]

[0095] A localized moisture assessment model was thus established, yielding the moisture condition assessment value G(k) for that point. This assessment value comprehensively considers the advantages of ultra-low frequency dielectric loss testing (VLF) for the overall cable assessment, the sensitive response of broadband impedance spectroscopy (BIS) to the localized areas of the cable, and the inherent moisture risk of the cable itself, providing a high level of confidence in assessing the localized moisture condition of the tested cable.

[0096] 5. After establishing a localized moisture assessment model, subsequent on-site test data can be directly input into the model to calculate the moisture content.

[0097] The tidal state assessment value G(k) is calculated using the following formula:

[0098] G(k) = ω1Δtanδ + ω2tanδ a +ω3σ tanδ +ω4δ p(k) +ω5DRI (5)

[0099] The accuracy of the evaluation model improves as sample data accumulates. By comparing the obtained moisture condition evaluation value G(k) with the moisture condition evaluation reference table in Table 2, the degree of moisture at that point of the cable under test can be evaluated.

[0100] Table 2 Reference Table for Assessing Moisture Condition

[0101] Cable status Intact cable Slightly damp Moderate dampness Severely damp Numerical range G(k)≤0.2 0.2<G(k)≤0.6 0.6<G(k)≤1.0 G(k)>1.0

[0102] In published invention patents, the use of VLF or BIS methods to determine the moisture condition of tested cables has been mentioned. However, using either method alone cannot provide an accurate assessment of cable moisture levels. The VLF method detects anomalies in the overall cable dielectric loss factor, rather than assessing local conditions, and therefore cannot pinpoint the location of localized moisture. Furthermore, this anomaly is influenced by two factors: the degree of localized cable aging and the proportion of localized aging. When the length of localized aging is relatively small compared to the total cable length, it will lead to a misjudgment of the degree of localized aging. The BIS method can obtain a location map of the tested cable, and the location of localized moisture can be determined by the abnormal reflection peaks. However, the size of the abnormal reflection peaks is affected by factors such as the input signal frequency, the total cable length, the location of localized moisture, and the degree of localized moisture, and therefore cannot be used to simply assess the degree of localized moisture.

[0103] Therefore, this invention combines the VLF and BIS methods, comprehensively considers the indicators obtained by the two methods, and combines them with the characteristics of the cable itself to establish a new cable local moisture assessment model, thereby improving the accuracy of assessing the local moisture status of the cable.

[0104] Figure 2 The image shows a test site at a wind farm. The cable under test is a 1866m long 35kV XLPE cable. The intermediate joint, located 715m from the beginning, is severely damp. Figure 3 The location map obtained by the BIS method has a frequency range of 100kHz to 50MHz and a frequency sampling interval of 2kHz. Table 3 shows the test data results.

[0105] Table 3 Test Data Table

[0106] Rate of change of dielectric loss Average dielectric loss Dielectric loss stability over time Polarity deviation Moisture Risk Index Moisture condition assessment value 37.96 0.01 0.01 1.11 0.36 1.39

[0107] Comparing the results of ultra-low frequency dielectric loss testing with the international standard IEEE400.2-2013, only the dielectric loss change rate is in a state of concern, while the other two indicators are in a normal state.

[0108] Based on the moisture assessment values ​​proposed in this invention, referring to Table 2, it was determined that the intermediate joint at this location was severely damp. Based on the test results, the staff disassembled and inspected the intermediate joint at 715m, and the results were as follows... Figure 4 The disassembly and inspection results showed that the intermediate joint at 715m had a severe moisture fault, seriously affecting the safety and reliability of cable operation. The on-site disassembly and inspection results were consistent with the assessment of this invention.

[0109] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for assessing localized moisture absorption in cables using a combination of ultra-low frequency dielectric loss and broadband impedance spectroscopy testing, characterized in that: The steps of the method are as follows: S1. The cable under test is measured using an ultra-low frequency dielectric loss measuring instrument to obtain the dielectric loss change rate Δtanδ and the dielectric loss stability over time σ. tanδ and the average value of dielectric loss tanδ a ; S2. Measure the cable under test using the broadband impedance spectroscopy method to obtain a standardized time-domain localization spectrum. Record the positions of the reflection peaks except for the beginning and end, and label them sequentially according to their distance from the beginning. Calculate the polarity deviation δ of each reflection peak. p ; S3. Check the ledger of the cable under test, and calculate the moisture risk index (DRI) of the cable under test based on the laying environment and laying method. S4. Process the data from S1 to S3 using principal component analysis to establish a localized dampness assessment model and obtain the weight ω of each indicator. j ; S5. Substitute the measurement data from S1 to S3 into the local moisture assessment model to calculate the moisture status assessment value at that location, and assess the degree of local moisture in the cable under test accordingly. In S2, the polarity deviation δ for two reflection peaks at the same location p The calculation formula is: Where: δ p(k) It is the polarity deviation of the k-th reflection peak; α is the bimodal correction factor, with a value of 0.4; E r(k) It is the extreme value of the right peak at the position of the kth reflection peak; E l(k) It is the extreme value of the left peak at the position of the kth reflection peak; d(k) is the distance from the position of the kth reflection peak to the tail reflection peak; A is the extreme value of the terminal reflection peak; For the polarity deviation δ of a single reflection peak at the same location p The calculation formula is: Where E(k) is the extreme value of the kth reflection peak.

2. The method for assessing localized moisture absorption in cables based on combined ultra-low frequency dielectric loss and broadband impedance spectrum testing according to claim 1, characterized in that: The formula for calculating the moisture risk index of the S3 cable is as follows: Where: I is the laying influence factor, which is determined according to the different laying methods of the cable under test; T represents the cable laying time; The average annual relative humidity of the cable laying location; IP is the cable's waterproof rating. If IP = 0, then DRI = 1.

3. The method for assessing localized moisture absorption in cables based on combined ultra-low frequency dielectric loss and broadband impedance spectrum testing according to claim 1, characterized in that: The weights ω of each indicator in S4 j The calculation formula is: Where: ω j It is the weight of the j-th indicator; m is the number of principal components retained by principal component analysis; p is the number of indicators; λ i It is the variance contribution rate of the i-th principal component; a ji It is the load of the j-th index in the i-th principal component.

4. The method for assessing localized moisture absorption in cables based on combined ultra-low frequency dielectric loss and broadband impedance spectrum testing according to claim 1, characterized in that: The formula for calculating the moisture condition assessment value G of S5 is as follows: G(k)=ω1Δtanδ+ω2tanδ a +ω3σ tanδ +ω4d p(k) +ω5DRI (5) Where: ω1, ω2, ω3, ω4 and ω5 are the rate of change of dielectric loss Δtanδ, the average dielectric loss tanδ, and so on, respectively. a Dielectric loss stability over time σ tanδ The polarity deviation δ at this position p(k) The influence weight of the cable moisture risk index (DRI) is determined by comparing the value of G(k) with the moisture status assessment reference table to evaluate the degree of moisture at that location of the cable under test.

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