An electromagnetic detection method for the corrosion degree of overhead ground wire

Abstract

The technical problem to be solved by the present invention is to provide an electromagnetic detection method that can determine the degree of corrosion of an overhead ground wire by reflecting the zinc coating thickness and the remaining cross-sectional area of ​​the overhead ground wire through the spatial signals around the overhead ground wire. This electromagnetic detection method for the degree of corrosion of an overhead ground wire is based on the skin effect, the magnetic effect of electric current, and the Fourier transform. By detecting the remaining thickness of the zinc coating on the surface of the overhead ground wire and the remaining cross-sectional area of ​​the steel substrate, the current degree of corrosion of the overhead ground wire is reflected. This overcomes the difficulty of non-destructive measurement of the zinc coating thickness and the remaining cross-sectional area of ​​the steel substrate during operation of the overhead ground wire. At the same time, this measurement method has good sensitivity and accuracy, which is of great significance for application in practical engineering.

Description

Technical Field

[0001] The present invention relates to the technical field of overhead ground wire detection, and in particular to an electromagnetic detection method for the corrosion degree of an overhead ground wire. Background Art

[0002] Overhead ground wires play an important role in ensuring the safe and reliable operation of power transmission and transformation equipment. In the construction of high-voltage and ultra-high-voltage substations, a large area is required to avoid lightning strikes, which requires the construction of special lightning protection lines to ensure safety. Overhead ground wires are usually used to prevent lightning damage to overhead transmission line conductors.

[0003] There are many types of overhead ground wires. Commonly, hot-dip galvanized steel strands are used. Zinc exhibits excellent corrosion resistance when exposed to atmospheric conditions, with a corrosion rate of only 1 / 15 that of steel. Furthermore, zinc's standard electrode potential is more negative than that of iron, and the galvanized layer acts as a sacrificial anode protecting the steel substrate in water and humid air. If the galvanized layer corrodes completely, the steel substrate of the overhead ground wire becomes completely exposed to the atmosphere and begins to corrode, significantly increasing the corrosion rate. The bearing capacity of an overhead ground wire is closely related to the remaining cross-sectional area of ​​the steel substrate. As the steel substrate corrodes, the remaining cross-sectional area gradually decreases, reducing the overall tensile strength of the overhead ground wire and significantly increasing the risk of strand and wire breakage, seriously threatening the safe and stable operation of overhead transmission lines. The thickness of the galvanized layer and the remaining cross-sectional area of ​​the overhead ground wire are closely related to the remaining life of the overhead ground wire. Therefore, the remaining thickness of the galvanized layer and the remaining cross-sectional area of ​​the overhead ground wire can be used to assess the current corrosion level of the overhead ground wire. The current non-destructive film thickness measurement method can only be applied to the measurement of film thickness of flat surfaces or specimens with a large curvature radius. Overhead ground wires are made of circular cross-section strands with a complex spatial structure and a small surface curvature radius. The current film thickness measurement technology cannot be applied to the measurement of surface coatings on overhead ground wires. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide an electromagnetic detection method which can determine the corrosion degree of the overhead ground wire by reflecting the zinc layer thickness of the overhead ground wire and the remaining cross-sectional area of ​​the overhead ground wire through the spatial signal around the overhead ground wire.

[0005] The technical solution adopted by the present invention to solve the above technical problems is: the electromagnetic detection method for the corrosion degree of the overhead ground wire includes the following steps:

[0006] S1. Prepare voltage excitation source;

[0007] S2. Determine the frequency and voltage of the excitation;

[0008] The appropriate excitation frequency range is determined according to the initial galvanized layer thickness of the overhead ground wire, and the appropriate voltage is selected according to the excitation frequency. The frequency calculation formula is shown in the following equations (1) and (2):

[0009] 2d0<δ<5d0 (1)

[0010]

[0011] Where δ is the skin depth, d0 is the initial thickness of the zinc coating, σ is the electrical conductivity, ω is the angular frequency, and μ is the magnetic permeability;

[0012] S3. Setting the voltage excitation source in step S1 according to the excitation frequency and the selected voltage calculated in step S2, and then applying voltage to both ends of the overhead ground wire to be detected through the voltage excitation source;

[0013] S4. Measure and calculate the magnetic field value in the space around the overhead ground wire;

[0014] Based on the Hall effect, the Hall sensor is used to measure the potential difference generated at both ends of the overhead ground wire and the magnetic field value in the space around the overhead ground wire is calculated according to the calculation formula; the calculation formula is shown in the following formula (3):

[0015]

[0016] Where U is the potential difference across the semiconductor, k is the Hall coefficient, B is the external magnetic field, I is the current perpendicular to the external magnetic field, and d is the thickness of the semiconductor sheet.

[0017] S5, feature signal extraction;

[0018] Input the analog signal output by the Hall sensor in step S4 into a computer, perform signal processing on the computer based on discrete Fourier transform, extract the magnetic field signal of the characteristic frequency, convert the magnetic field signal of the characteristic frequency into spectrum information, obtain the amplitude information of the characteristic frequency, and then obtain the original time domain amplitude of the characteristic frequency based on the relationship between the spectrum amplitude and the original time domain signal amplitude, and finally obtain the magnetic field value of the measurement point;

[0019] S6. Calibrate the magnetic field values ​​corresponding to different corrosion degrees;

[0020] Through steps S1-S5, the overhead ground wires in three conditions, namely, intact galvanized layer, no galvanized layer, and cross-section corrosion rate of 10%, are tested in sequence to obtain three corresponding magnetic field values ​​and set as follows: the magnetic field measurement value of the intact galvanized layer is B0, the magnetic field measurement value of the no galvanized layer is B1, and the magnetic field measurement value of the cross-section corrosion rate of 10% is B2;

[0021] S7. Use the actual detected magnetic field value to determine the current corrosion level

[0022] Measure the magnetic field value around the overhead ground wire in actual operation and set it as B 实测 , B 实测The current corrosion degree of the overhead ground wire is determined by comparing the magnetic field measurement values ​​B0, B1, and B2 calibrated in step S5. The specific comparison and determination process is as follows:

[0023] S71. When B 实测 =B0, it is determined that the overhead ground wire is not corroded;

[0024] S72. When B1≤B 实测 When <B0, the galvanized layer of the overhead ground wire is corroded, but the steel substrate is not corroded, and the overhead ground wire is judged to be slightly corroded;

[0025] S73. When B2<B 实测 When the corrosion rate is less than B1, the zinc coating of the overhead ground wire is completely corroded and falls off, the steel matrix begins to corrode, and the corrosion rate of the steel matrix cross section is within 10%. The overhead ground wire is judged to be moderately corroded.

[0026] S74. When B 实测 When ≤B2, the galvanized layer of the overhead ground wire is completely corroded and falls off, and the corrosion amount of the steel matrix cross section exceeds 10%, which determines that the overhead ground wire is severely corroded.

[0027] The beneficial effects of the present invention are as follows: the galvanized layer thickness and the remaining cross-sectional area of ​​the steel matrix of the overhead ground wire that has been put into use can be measured online and non-destructively, overcoming the problem of being unable to directly measure the remaining galvanized layer thickness and the remaining cross-sectional area of ​​the steel matrix of the overhead ground wire that has been put into use, and further setting standard values ​​for judging the mild, moderate and severe corrosion of the overhead ground wire, and using the measured value of the actual overhead ground wire to compare with the set calibration value to obtain the current corrosion degree of the overhead ground wire. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flow chart of the electromagnetic detection method for the corrosion degree of overhead ground wires according to the present invention;

[0029] Figure 2 This is a schematic structural diagram of the overhead ground wire GJ-50 1×7 of the present invention;

[0030] Figure 3 It is a schematic diagram of the magnetic field measurement trajectory in the present invention;

[0031] Figure 4 It is a curve diagram of the standard magnetic induction intensity signal distributed along the magnetic field measurement track when there is a galvanized layer, there is no galvanized layer, and the corrosion rate of the steel substrate cross section is 10%;

[0032] Figure 5 is a schematic cross-sectional view of the corrosion area of ​​the overhead ground wire in the present invention;

[0033] Figure 6 1 is a schematic diagram of a curve of magnetic induction intensity signals of overhead ground wires with different corrosion degrees according to the present invention; DETAILED DESCRIPTION

[0034] The following is a further description of the specific embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative work are within the scope of protection of this application.

[0035] It should be noted that, in the embodiments of the present application, all directional indication terms such as "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", and "outside" indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they cannot be understood as limiting the present invention. They are only used to explain the relative positional relationship, movement status, etc. between the components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0036] In this application, unless otherwise specified or limited, the terms "connection" and "fixation" should be understood in a broad sense. For example, "fixation" can mean fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; internal communication between two elements or interaction between two elements, unless otherwise specified. For those skilled in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0037] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present application, the descriptions of "first", "second", etc. are only for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of such features. In addition, the meaning of "and / or" appearing throughout the text includes three parallel schemes. Taking "A and / or B" as an example, it includes scheme A, or scheme B, or a scheme in which A and B are satisfied at the same time. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of ordinary technicians in this field to implement it. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by this application.

[0038] like Figure 1-6 As shown, the electromagnetic detection method for the corrosion degree of the overhead ground wire includes the following steps:

[0039] S1. Prepare voltage excitation source;

[0040] S2. Determine the frequency and voltage of the excitation;

[0041] The appropriate excitation frequency range is determined based on the thickness of the initial galvanized layer of the overhead ground wire, and the appropriate voltage is selected based on the excitation frequency. When alternating current passes through the conductor, a skin effect will occur. The skin depth is related to the conductor's conductivity, magnetic permeability, and current frequency. The frequency calculation formula is shown in the following equations (1) and (2):

[0042] 2d0<δ<5d0 (1)

[0043]

[0044] Where δ is the skin depth, d0 is the initial thickness of the zinc coating, σ is the electrical conductivity, ω is the angular frequency, and μ is the magnetic permeability;

[0045] S3. Set the voltage excitation source in step S1 according to the excitation frequency and selected voltage calculated in step S2. Then, apply voltage to both ends of the overhead ground wire to be tested through the voltage excitation source, and keep the voltage and frequency unchanged. Because a magnetic field is generated around a current-carrying conductor, and the magnitude of the magnetic field is closely related to the magnitude of the current flowing through the current-carrying conductor, when applying the excitation source, it is necessary to ensure that the current is mainly distributed on the surface of the overhead ground wire, so that the current flowing through the overhead ground wire is closely related to the thickness of the zinc coating on the surface of the overhead ground wire.

[0046] S4. Measure and calculate the magnetic field value in the space around the overhead ground wire;

[0047] Based on the Hall effect, the Hall sensor is used to measure the potential difference generated at both ends of the overhead ground wire and the magnetic field value in the space around the overhead ground wire is calculated according to the calculation formula; the calculation formula is shown in the following formula (3):

[0048]

[0049] Where U is the potential difference across the semiconductor, k is the Hall coefficient, B is the external magnetic field, I is the current perpendicular to the external magnetic field, and d is the thickness of the semiconductor sheet.

[0050] The Hall effect refers to the phenomenon that when current passes through a semiconductor perpendicular to an external magnetic field, the carriers are deflected, and an additional electric field is generated perpendicular to the direction of the current and the magnetic field, thereby generating a potential difference at both ends of the semiconductor; according to the Hall effect, the potential difference generated by a Hall element made of semiconductor material is very small, so the Hall element is usually integrated with an amplifier circuit, a temperature compensation circuit and a voltage-stabilized power supply circuit on a single chip, which is called a Hall sensor. The Hall sensor has the advantages of being able to measure dynamic and static magnetic fields, high precision, high sensitivity, good linearity, good dynamic performance, wide operating frequency band and small module size. The Hall sensor is divided into two types: linear Hall sensor and switch Hall sensor. The linear Hall sensor consists of a Hall element, a linear amplifier and an emitter follower, and outputs a voltage analog signal. The output voltage is linearly related to the strength of the external magnetic field. The magnetic field signal to be measured in the present invention is an alternating magnetic field signal, so a linear Hall sensor is selected;

[0051] S5, feature signal extraction;

[0052] Input the analog signal output by the Hall sensor in step S4 into a computer, perform signal processing on the computer based on discrete Fourier transform, extract the magnetic field signal of the characteristic frequency, convert the magnetic field signal of the characteristic frequency into spectrum information, obtain the amplitude information of the characteristic frequency, and then obtain the original time domain amplitude of the characteristic frequency based on the relationship between the spectrum amplitude and the original time domain signal amplitude, and finally obtain the magnetic field value of the measurement point;

[0053] In actual operation, overhead ground wires are exposed to interference signals generated by geomagnetism, industrial frequency alternating current, and other sources in the surrounding space. The ideal signal is an alternating magnetic field signal of a specific frequency generated by the alternating current in the overhead ground wire. Therefore, the actual measurement signal needs to be further processed. The analog signal output by the Hall sensor is input into a computer, and the signal is processed by the computer based on discrete Fourier transform to extract the magnetic field signal of the characteristic frequency. Fourier transform is a method for analyzing signals. Its typical use is to decompose the signal into a frequency spectrum and display the amplitude corresponding to the frequency. The signal input to the computer is a continuous signal, which is sampled using the δ(t) function. According to the sampling theorem, when the sampling frequency is greater than twice the highest frequency in the signal, the sampled digital signal can completely restore the original signal. In general, in practical applications, the sampling frequency is guaranteed to be 2.56 to 4 times the highest frequency of the signal. Assuming the number of sampling points is N, in order to facilitate DFT calculations, N is usually an integer power of 2.

[0054] The continuous signal x(t) is sampled at time T s Sampling is performed N timesδ(t-nT s ), and extend these N values ​​periodically to obtain a periodic discrete signal x[n], whose period T=N*T s, the frequency is f = 2π / T, within one period T, its expression is as follows (4):

[0055]

[0056] make The Fourier series of discrete periodic signals can be obtained as shown in the following formula (5):

[0057]

[0058] According to the screening property of the δ(t) function, we have:

[0059]

[0060] Let X[kω]·T s =X[k], we get:

[0061]

[0062] The above is the Fourier transform of discrete periodic signals, which realizes the conversion of continuous input signals into spectrum information, obtains the amplitude information of the characteristic frequency, and then obtains the amplitude of the characteristic frequency in the original time domain based on the relationship between the spectrum amplitude and the original time domain signal amplitude, and finally obtains the magnetic field value of the measurement point;

[0063] S6. Calibrate the magnetic field values ​​corresponding to different corrosion levels

[0064] Before the actual measurement, the magnetic field signals around the overhead ground wire are measured in sequence through steps S1-S5 for three conditions: intact galvanized layer, no galvanized layer, and cross-section corrosion rate of 10%. After the signal is processed by a computer, the magnetic field signal of the characteristic frequency is extracted and converted into spectrum information to obtain the amplitude information of the characteristic frequency. Then, based on the relationship between the spectrum amplitude and the original time domain signal amplitude, the original time domain amplitude of the characteristic frequency is obtained. Finally, the corresponding three magnetic field values ​​are obtained and set as follows: the magnetic field measurement value of the intact galvanized layer is B0, the magnetic field measurement value of the no galvanized layer is B1, and the magnetic field measurement value of the cross-section corrosion rate of 10% is B2. That is, the three reference values ​​for judgment are set to provide a benchmark for subsequent judgments.

[0065] S7. Use the actual detected magnetic field value to determine the current corrosion level

[0066] Measure the magnetic field value in the space around the overhead ground wire during actual operation. Since the magnetic field signal in the space around the energized overhead wire will gradually decrease as the degree of corrosion deepens, the magnetic field value in the space around the overhead ground wire during actual operation needs to be amplified and filtered to obtain the best signal measurement value and set the best signal measurement value as B. 实测 , B 实测The current corrosion degree of the overhead ground wire is determined by comparing it with the magnetic field measurement values ​​B0, B1, and B2 set in step S6. The specific comparison and determination process is as follows:

[0067] S71. When B 实测 =B0, it is determined that the overhead ground wire is not corroded;

[0068] S72. When B1≤B 实测 When <B0, the galvanized layer of the overhead ground wire is corroded, but the steel substrate is not corroded, and the overhead ground wire is judged to be slightly corroded;

[0069] S73. When B2<B 实测 When the corrosion rate is less than B1, the zinc coating of the overhead ground wire is completely corroded and falls off, the steel matrix begins to corrode, and the corrosion rate of the steel matrix cross section is within 10%. The overhead ground wire is judged to be moderately corroded.

[0070] S74. When B 实测 When ≤B2, the galvanized layer of the overhead ground wire is completely corroded and falls off, and the corrosion amount of the steel matrix cross section exceeds 10%, which determines that the overhead ground wire is severely corroded.

[0071] In order to further clearly illustrate the technical solution of the present invention, the applicant conducted a specific simulation using the finite element simulation software Comsol to elaborate on the present invention and verify the feasibility of the technical solution.

[0072] The specific embodiment steps are as follows:

[0073] (1) Determine the frequency and voltage of voltage excitation

[0074] like Figure 2 As shown, taking the typical overhead ground wire GJ-50 1×7 as an example, the diameter of each strand is 1.5 mm, the initial thickness of the galvanized layer is 40 μm, the steel base material is Q235 steel, and its electrical conductivity is 1×10 7 S / m, relative magnetic permeability is 400;

[0075] According to formulas (1) and (2), it can be calculated that the appropriate frequency range of the excitation source is 1.5kHz to 10kHz. In this embodiment, the frequency is 5kHz, the voltage is 1V, and the length L of the overhead ground wire to be measured is set to 1m.

[0076] (2) Calibration of intact galvanized layer B0, no galvanized layer B1 and 10% cross-section corrosion rate of steel substrate B2

[0077] Based on the parameters of step (1), the electromagnetic field analysis module in Comsol is used to construct a finite element simulation model for calculation and analysis. It should be noted that the magnetic field measurement signal needs to be processed by a computer in the actual measurement. In this embodiment, the magnetic field strength value of the space around the overhead ground wire is directly calculated by the finite element simulation software, and then a suitable magnetic field measurement position is selected. The magnetic field measurement position is selected at L = 0.5m, and the magnetic field value around the surface of the overhead ground wire is measured. The magnetic field measurement trajectory is as follows: Figure 3 As shown in the figure, the magnetic induction intensity on the scanning track when the galvanized layer is intact and when there is no galvanized layer is as follows Figure 4 As shown, from Figure 4 It can be seen that each trajectory corresponds to 6 maximum values, and the arithmetic average of the 6 maximum values ​​is taken as the magnetic field calibration values ​​B0, B1 and B2 of the 10% cross-section corrosion rate of the galvanized layer, the non-galvanized layer and the steel substrate, respectively. After calculation, B0 = 19G, B1 = 11.5G, and B2 = 10.2G in this embodiment.

[0078] (3) Measuring the corrosion degree of overhead ground wire

[0079] For the actual overhead ground wire, the magnetic field signal is scanned along the measurement trajectory. The magnetic field measurement value is amplified and filtered to obtain the optimal measurement signal. The processed magnetic field signal curve has 6 maximum values. The arithmetic average of the 6 maximum values ​​is B. 实测 , and then compared with B0, B1 and B2, the current corrosion degree of the overhead ground wire can be obtained;

[0080] Measurements were conducted for the cases where the galvanized layer thickness remained at 30 μm, the case where the galvanized layer thickness remained at 10 μm, and the case where the galvanized layer was completely corroded and the corrosion area of ​​the steel substrate reached 5% and 15% of the total cross-sectional area. The schematic diagram of the steel substrate corrosion area is shown in the figure. Figure 5 As shown, the measured magnetic field signal is Figure 6 As shown;

[0081] After calculation, when the thickness of the galvanized layer is 30μm, B 实测 =17.6G, which is less than B0 and greater than B1, so the corrosion degree of the overhead ground wire is judged to be mild corrosion;

[0082] When the thickness of the galvanized layer is 10 μm, B 实测 =14.2G, which is less than B0 and greater than B1, so the corrosion degree of the overhead ground wire is judged to be mild corrosion;

[0083] When the galvanized layer is completely corroded and the corrosion area of ​​the steel substrate reaches 5% of the total cross-sectional area, B 实测 =10.7G, which is less than B1 and greater than B2, so the corrosion degree of the overhead ground wire is determined to be moderate corrosion;

[0084] When the galvanized layer is completely corroded and the corrosion area of ​​the steel substrate reaches 15% of the total cross-sectional area, B 实测 =9.6G, which is less than B2, so the corrosion degree of the overhead ground wire is determined to be severe corrosion.

[0085] The above are only preferred embodiments of the present application and do not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. An electromagnetic detection method for the corrosion degree of an overhead ground wire, characterized by: The steps include: S1. Prepare voltage excitation source; S2. Determine the frequency and voltage of the excitation; The appropriate excitation frequency range is determined according to the initial galvanized layer thickness of the overhead ground wire, and the appropriate voltage is selected according to the excitation frequency. The frequency calculation formula is shown in the following equations (1) and (2): （1） （2） Where, is the skin depth, Initial thickness of galvanized layer, is the conductivity, is the magnetic permeability; S3. Setting the voltage excitation source in step S1 according to the excitation frequency and the selected voltage calculated in step S2, and then applying voltage to both ends of the overhead ground wire to be detected through the voltage excitation source; S4. Measure and calculate the magnetic field value in the space around the overhead ground wire; Based on the Hall effect, the Hall sensor is used to measure the potential difference generated at both ends of the overhead ground wire and the magnetic field value in the space around the overhead ground wire is calculated according to the calculation formula; the calculation formula is shown in the following formula (3): （3） Where U is the potential difference across the semiconductor, k is the Hall coefficient, B is the external magnetic field, I is the current perpendicular to the external magnetic field, and d is the thickness of the semiconductor sheet. S5, feature signal extraction; Input the analog signal output by the Hall sensor in step S4 into a computer, perform signal processing on the computer based on discrete Fourier transform, extract the magnetic field signal of the characteristic frequency, convert the magnetic field signal of the characteristic frequency into spectrum information, obtain the amplitude information of the characteristic frequency, and then obtain the original time domain amplitude of the characteristic frequency based on the relationship between the spectrum amplitude and the original time domain signal amplitude, and finally obtain the magnetic field value of the measurement point; S6. Calibrate the magnetic field values ​​corresponding to different corrosion degrees; Through steps S1-S5, the overhead ground wires with intact galvanized layer, no galvanized layer, and cross-section corrosion rate of 10% are tested in turn to obtain the corresponding three magnetic field values ​​and set as follows: The magnetic field measurement value of the intact galvanized layer is , the magnetic field measurement value without zinc coating is The magnetic field measurement value when the cross-section corrosion rate is 10% is ; S7, using the actually detected magnetic field value to determine the current corrosion degree; Measure the magnetic field value around the overhead ground wire in actual operation and set it as ,Will The magnetic field measurement value calibrated in step S6 、 and Compare and determine the current corrosion degree of the overhead ground wire. The specific comparison and determination process is as follows: S71. When = When the overhead ground wire is not corroded; S72. When When the galvanized layer of the overhead ground wire is corroded, but the steel substrate is not corroded, the overhead ground wire is judged to be slightly corroded; S73. When When the zinc coating of the overhead ground wire is completely corroded and falls off, the steel matrix begins to corrode, and the corrosion amount of the steel matrix cross section is within 10%, the overhead ground wire is judged to be moderately corroded; S74. When When the galvanized layer of the overhead ground wire is completely corroded and falls off, and the corrosion amount of the steel matrix cross section exceeds 10%, the overhead ground wire is judged to be severely corroded.

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