Grounding grid downlead corrosion diagnosis method based on elastic waves

Abstract

A grounding grid downlead corrosion diagnosis method based on elastic waves is characterized by comprising the following steps: 1, constructing a grounding grid downlead corrosion diagnosis system; 2, the upper computer sends a collection instruction to the embedded control system; 3, the embedded control system generates a control signal, transmits a transmitting signal to the transient electromagnetic transmitter, and transmits an acquisition signal to an AD signal acquisition module; 4, after the transient electromagnetic transmitter receives the transmitting signal, generating an excitation signal to an elastic wave seismic source, exciting the elastic wave seismic source to work, and generating elastic waves in the down lead flat steel; 5, the elastic waves are spread in the down lead flat steel and pass through the acceleration sensor array back and forth, the acceleration sensor array collects elastic wave signals generated when the elastic waves pass through in real time, and the AD signal collection module collects the elastic wave signals and transmits the elastic wave signals to the upper computer; and 6, performing diagnosis processing on the elastic wave signal by the upper computer to obtain a corrosion diagnosis result. The effect is that the corrosion condition of the grounding grid downlead can be accurately detected.

Description

Technical Field

[0001] The invention relates to the technical field of grounding corrosion diagnosis, and in particular to a grounding grid down conductor corrosion diagnosis method based on elastic waves. Background Art

[0002] The grounding grid is an indispensable part of the power system and is the cornerstone of power consumption and transmission safety. Its function is to set a unified zero potential reference point for power equipment to ensure the normal operation of the power system and protect the safety of workers and equipment. When the power system is struck by lightning or a short circuit occurs, the grounding grid can quickly lead the fault current into the ground, reduce the increase in ground potential, and thus ensure that the power equipment is not damaged.

[0003] However, since the grounding grid is an underground concealed project, it is difficult to conduct an intuitive and comprehensive evaluation of the corrosion (including rust and fracture) of the grounding grid after it is put into operation. At present, there are many difficulties in the status detection of the grounding grid: one of which is that the corrosion and fracture of the grounding grid down conductor will cause the connection between the equipment and the grounding grid to be disconnected, but the existing means for the detection and evaluation of the corrosion of the grounding grid down conductor have many shortcomings such as low efficiency and difficulty in quantification. For example: it is difficult to ensure the safety of personnel and equipment by using direct excavation, and it is easy to cause permanent damage to the grounding grid structure and connection points; the grounding continuity test can only roughly determine whether there is a breakpoint in the grounding down conductor, and it is impossible to carry out precise positioning and corrosion status assessment of the down conductor; the electrochemical analysis method can only evaluate the overall corrosion status of the grounding grid, and cannot meet the refined requirements of corrosion positioning, imaging, etc.

[0004] The down conductor serves as a medium connecting the power equipment and the grounding grid. The lower end is connected to the grounding grid through the soil, and the upper end is exposed to the air and connected to the power equipment. Due to the special working environment of the down conductor, the down conductor buried in the soil is very prone to corrosion. The corrosion rate in the highly corrosive soil environment is 2-8mm / year.

[0005] The grounding grid is the main channel for power stations to discharge lightning current and equipment current. If it is severely corroded or even broken, the grounding resistance will increase and the ground potential will be unevenly distributed, thereby reducing the lightning current discharge capacity, causing damage to electrical equipment and even causing casualties and significant economic and property losses.

[0006] Grounding grid corrosion may cause equipment damage, power outages, and other situations, which in turn may lead to production interruptions and equipment repair or replacement costs, which will cause inestimable losses. In view of this, it is of great practical value to accurately judge the corrosion of the grounding grid. By testing the actual corrosion of the grounding grid, power accidents can be discovered and warned in advance to avoid power system paralysis, thereby reducing safety risks and economic losses.

[0007] From the installation posture, the grounding grid and the grounding down conductor are different. The down conductor is installed vertically to connect the power equipment and the grounding grid. It is shallowly buried and contains a large amount of oxygen, water, and chlorine, especially at the ground junction and welding point. It is more prone to corrosion than the grounding grid, so the probability of problems is greater.

[0008] Disadvantages of existing technologies: The current research on grounding grid corrosion diagnosis methods mainly focuses on the grid conductor, and research on grounding down conductor corrosion is extremely rare. In fact, due to the different positions and postures of grid conductors and down conductors, the inspection methods are also different. Summary of the invention

[0009] The present invention provides a grounding grid down conductor corrosion diagnosis method based on elastic waves, which can accurately detect the corrosion condition of the grounding grid down conductor.

[0010] To achieve the above object, the present invention provides a grounding grid down conductor corrosion diagnosis method based on elastic waves, the key of which is to include the following steps:

[0011] Step 1: construct a grounding grid down conductor corrosion diagnosis system, wherein the grounding grid down conductor corrosion diagnosis system is provided with a host computer and a down conductor flat steel, wherein the down conductor flat steel is a strip structure, which is divided into an upper end exposed above the ground and a lower half buried underground, wherein the host computer is connected to an embedded control system, and the embedded control system is connected to a transient electromagnetic transmitter and an AD signal acquisition module;

[0012] The transient electromagnetic transmitter is connected to an elastic wave source, which is fixed to the upper end of the down conductor flat steel. The upper end of the down conductor flat steel is also provided with an acceleration sensor array, which is connected to an AD signal acquisition module, and the AD signal acquisition module is connected to a host computer;

[0013] Step 2: The host computer sends a collection instruction to the embedded control system;

[0014] Step 3: The embedded control system generates a control signal, transmits the transmission signal in the control signal to the transient electromagnetic transmitter, and transmits the acquisition signal in the control signal to the AD signal acquisition module;

[0015] Step 4: After receiving the transmission signal, the transient electromagnetic transmitter generates a step excitation signal to the elastic wave source, which excites the elastic wave source to work, thereby generating elastic waves in the flat steel of the down conductor;

[0016] Step 5: the acceleration sensor array acquires the elastic wave signal in the flat steel of the down conductor, and the AD signal acquisition module acquires the elastic wave signal and transmits it to the host computer;

[0017] Step 6: The host computer performs diagnostic processing on the elastic wave signal to obtain a corrosion diagnosis result.

[0018] In the constructed grounding grid down conductor corrosion diagnosis system, under the command of the host computer, the embedded control system will generate a control signal as the enable signal of the transient electromagnetic transmitter. The control signal can be modified in real time on the host computer and output control signals of different frequencies. After receiving the control signal, the transient electromagnetic transmitter generates a step excitation signal to the elastic wave source, which excites the source to work, thereby generating elastic waves in the flat steel of the down conductor. An acceleration sensor array is placed on the flat steel of the down conductor. When the elastic wave propagates back and forth in the flat steel and passes through the sensor array, a voltage signal, i.e., an elastic wave signal, will be collected.

[0019] Through the above design, after using a precision seismic source to excite elastic waves in the flat steel of the down conductor, the sensor array is used to collect echo signals, which can intuitively reflect the buried depth and corrosion information of the flat steel of the down conductor, so as to judge the corrosion condition of the down conductor of the grounding grid at that location. By establishing a corresponding corrosion diagnosis platform, the collected signals are calculated and analyzed in real time, and the corrosion condition of the down conductor of the grounding grid can be quickly determined.

[0020] The upper computer analyzes the trend, amplitude and frequency information of the reflected wave from the corrosion point based on the elastic wave signal reflected in the grounding grid down conductor, and realizes the corrosion diagnosis of the grounding grid down conductor.

[0021] Using linear array sensors as elastic wave detection devices for down conductors overcomes the problems of small signal reception and non-directionality of single-channel sensors. The elastic wave source and acceleration sensor in the flat steel of the down conductor are fixed together at a fixed distance, which can realize rapid on-site detection of the down conductor.

[0022] Preferably, in step 5, the elastic wave signal includes a direct wave propagating in a forward direction and a reflected echo propagating in a reverse direction, and the reflected echo includes a corrosion point echo and an end echo;

[0023] The corrosion point echo and the end echo are related to the corrosion degree of the grounding grid down conductor;

[0024] The greater the degree of corrosion of the grounding grid down conductor, the stronger and greater the energy and amplitude of the echo at the corrosion point, and the weaker and smaller the energy and amplitude of the end echo; the smaller the degree of corrosion of the grounding grid down conductor, the weaker and smaller the energy and amplitude of the echo at the corrosion point, and the stronger and greater the energy and amplitude of the end echo.

[0025] After the source excites the elastic wave, it has two propagation paths. Path one is to first pass through the array sensor in the forward direction to the corrosion point, then reflect at the corrosion point and then pass through the array sensor in the reverse direction. Path two is to first pass through the array sensor to the end of the down conductor flat steel, then reflect at the end of the down conductor and then pass through the array sensor in the reverse direction. Based on the multi-channel signals received by the array sensor, the wave velocity of the elastic wave can be calculated and the reflected wave at the corrosion point can be extracted, so as to analyze the corrosion information of the down conductor flat steel.

[0026] The energy and amplitude of the reflected echo have certain regularities with the degree of corrosion of the grounding grid down conductor, but the linearity between the amplitude of the reflected echo and the degree of corrosion is higher, which can more accurately reflect the degree of corrosion of the grounding grid down conductor. Therefore, the degree of corrosion of the grounding grid down conductor can be evaluated by the amplitude of the corrosion point echo and the end echo.

[0027] Preferably, in step 6, the host computer performs the following steps to diagnose and process the elastic wave signal:

[0028] Step A1: The bandpass filter in the host computer acquires the elastic wave signal, performs bandpass filtering on it, and then uses FK transformation to separate the elastic wave propagating forward from the elastic wave propagating backward in the elastic wave signal to obtain a direct wave and a reflected echo, and transmit them to the corrosion diagnosis platform;

[0029] Step A2: The corrosion diagnosis platform performs cross-correlation calculation on the direct waves collected by any two adjacent acceleration sensors to obtain the time extension of the direct wave passing through the two adjacent acceleration sensors; then linear fitting is performed using the time of the direct wave passing through each acceleration sensor and the position information of the corresponding acceleration sensor to calculate the wave velocity of the elastic wave propagation in the down conductor. The wave velocity calculation formula is as follows:

[0030]

[0031] Where h is the slope of the fitting curve, d t is the sampling rate, l is the distance between two adjacent acceleration sensors;

[0032] Step A3: The corrosion diagnosis platform performs S transformation on the direct wave and the reflected echo collected by the same acceleration sensor to obtain the time when the direct wave and the reflected echo pass through the acceleration sensor, thereby obtaining the time delay when the direct wave and the reflected echo pass through the same acceleration sensor;

[0033] Step A4: combining the time delay of the direct wave and the reflected echo through the same acceleration sensor with the corresponding wave velocity, calculating the distance between the acceleration sensor and the corrosion point, and then obtaining the position of the corrosion point based on the position information of the acceleration sensor;

[0034] The calculation expression of the position Y of the corrosion point is as follows:

[0035] Y=x+(v×Δt) / 2

[0036] Where x represents the position of the accelerometer, v represents the wave velocity, and Δt represents the time delay between the direct wave and the reflected echo through the same accelerometer;

[0037] Step A5: Using the offset imaging algorithm, combined with the end position of the down conductor, the position Y of the corrosion point is made into an offset imaging diagram, and the depth of color in the offset imaging diagram reflects the strength of the corrosion degree at that position. Combined with the echo amplitude of the corrosion point and the end echo amplitude, the corrosion diagnosis of the grounding grid down conductor is completed.

[0038] Through the above design, after filtering with a bandpass filter, clutter can be eliminated to obtain clear direct waves and reflected echoes. Then, by using the cross-correlation algorithm on the direct wave, the time delay of the direct wave passing through several acceleration sensors can be obtained. Since the position of the acceleration sensor is known, the wave speed can be calculated. For the accuracy of the data, a linear fit is made using the time of the direct wave passing through different acceleration sensors and the position of the acceleration sensor to obtain the wave speed.

[0039] By performing S transformation on the data of the same acceleration sensor, we can obtain the time it takes for the direct wave and the radiated echo to pass through the same acceleration sensor, and thus obtain the time delay Δt between the direct wave and the radiated echo passing through the same acceleration sensor.

[0040] After obtaining the time delay Δt of the direct wave and the radiated echo through the same acceleration sensor, combined with the wave velocity, the distance between the acceleration sensor and the corrosion point can be obtained. The position of the acceleration sensor is known, so the position of the corrosion point can be obtained, and then the corrosion degree of the grounding grid down conductor can be diagnosed.

[0041] Preferably, in step A2, the calculation expression of the cross-correlation algorithm is as follows:

[0042]

[0043] Among them, s1 and s2 represent the direct waves collected by two adjacent acceleration sensors, π represents the time extension between the two direct waves, and s2 * represents the conjugate signal of the direct wave s2, t represents the corresponding sampling time, and R represents the correlation.

[0044] The cross-correlation function can reflect the time delay of the direct wave passing through two adjacent acceleration sensors.

[0045] Preferably, in step A1, the apparent velocity of the direct wave is defined as positive, and the apparent velocity of the reflected echo is defined as negative, and the direct wave is extracted by using FK transformation, and the expression is as follows:

[0046] S(f,k)=∫∫S(x,t)E -j2π(Ft+kX) dxd

[0047] Wherein, x is the position of the acceleration sensor, t is the corresponding sampling time, F is the frequency of the elastic wave signal, K is the wave number; S represents the elastic wave signal collected by the acceleration sensor;

[0048] After the above transformation, the elastic wave signal is transformed from the distance-time (xt) domain to the frequency-wavenumber domain (FK). Since the wave number of the direct wave is greater than 0 and the wave number of the reflected echo is less than 0, the FK inverse transformation is used to extract the reflected echo. The expression is as follows:

[0049] S(x,t)=∫∫S(F,K)e j2π(ft+Kx) dFdk.

[0051] The principle of FK transformation is to extract useful, separate irrelevant signals and suppress clutter based on the difference in apparent velocity between useful signals and other irrelevant signals in the received signal. When using elastic waves to detect grounding grid down conductors, the signals received by the sensor include direct waves and reflected echoes. The two signals may overlap, making it difficult to diagnose corrosion information from the collected signals. In the grounding grid down conductor corrosion diagnosis system, after the earthquake source excites elastic waves in the down conductor, the elastic wave packet will pass through the sensor array. The order of the direct wave is from the first sensor to the last sensor, while the order of the reflected echo from the corrosion point and the echo from the end of the down conductor is from the last sensor to the first sensor. Therefore, the apparent velocity of the direct wave in the received signal can be considered positive, and the apparent velocity of the reflected echo can be considered negative. Since only the reflected echo contains the corrosion point, the FK filter can be used to process the received signal to extract the reflected echo containing the corrosion point information for subsequent analysis.

[0052] Preferably, in step A5, since the direct wave and the reflected echo collected by the acceleration sensor include secondary direct waves and reflected echoes, a gather superposition algorithm is used to process the extracted direct waves and reflected echoes, firstly, time shift correction is performed on the direct wave and the reflected echo respectively, and then the corrected signals are superimposed;

[0053] The peak amplitudes of the direct wave, end echo and corrosion point echo after the gather stacking are recorded, using the formula Quantitative processing is performed to obtain the corrosion degree assessment result, where A1 is the direct wave amplitude, A2 is the echo amplitude of the corrosion point, A3 is the end echo amplitude, and F is the corrosion degree.

[0054] The principle of the gather superposition algorithm is to use the common information in the multi-channel signal records to enhance the underground target signal through superposition processing, and to suppress or weaken the noise and other irrelevant signals. Through superposition processing, the reflection signal of the same underground structure will be enhanced in different signal records, and the noise signals will cancel each other out during the superposition process, thereby improving the quality of the detection data and more accurately reflecting the characteristics of the underground object.

[0055] Since the propagation directions of the direct wave and the reflected echo are different, the time shift correction directions of the signals in the gather stacking processing are also different, and time shift correction needs to be performed separately.

[0056] Preferably, in step 1, the down conductor flat steel is arranged perpendicular to the ground, and the down conductor flat steel includes an above-ground part and an underground part, the upper end of the down conductor flat steel is connected to a primary power equipment, and the lower end is connected to a grounding grid.

[0057] As a preferred embodiment, the elastic waves conducted in the flat steel of the down conductor are Lamb waves and horizontal shear waves SH, the mode morphology of the Lamb waves includes A-type antisymmetric mode and S-type symmetric mode, and the Lamb waves and horizontal shear waves SH with a frequency below 100kHz are used for detection, that is, the two elastic waves A0 and SH0 are used for detection;

[0058] The modal equation of the S-type symmetric mode is:

[0059]

[0060] The modal equation of type A antisymmetric mode is:

[0061]

[0062] in,

[0063]

[0064] Where k represents the wave number transmitted in the horizontal direction, ρ is the density of the underground flat steel material, λ and μ are the Lame elastic constants; ω is the angular frequency, ω=2πF; c p is the group velocity, b is half of the plate thickness, c1 is the longitudinal wave velocity of the corresponding mode, c t is the shear wave velocity of the corresponding mode, q and p are intermediate numbers.

[0065] According to the above modal equation, it can be used to determine the propagation speed of the wave of the corresponding frequency.

[0066] Preferably, the elastic wave source comprises a giant magnetostrictive source and a magnetic focusing source, wherein the giant magnetostrictive source is used to excite A0 waves for detecting corrosion in the longitudinal direction; and the magnetic focusing source is used to excite SH0 waves for detecting corrosion in the transverse direction.

[0067] The giant magnetostrictive seismic source has the advantages of small size, wide vibration frequency band, fast response speed, large and adjustable output energy, etc. The working principle of the seismic source is: with the magnetostrictive material rod as the core, current is passed through the excitation coil to generate a magnetic field, so that the magnetostrictive material rod in the magnetic field is deformed and drives the output rod to hit the flat steel to generate elastic waves. The seismic source can effectively excite A0 waves.

[0068] The magnetic focusing source has the characteristics of wide frequency band, fast response speed, non-contact, stable output energy, superposition, etc. It can adapt to complex on-site environments and effectively stimulate SHO waves.

[0069] Preferably, the speed sensor array is connected to an AD signal acquisition module via a signal amplification module, and the AD signal acquisition module is connected to the host computer via a USB data cable.

[0070] The amplification module is used to amplify the collected elastic wave signal, and the USB data line is used to realize the communication between the AD signal acquisition module and the host computer to ensure the integrity and accuracy of the data.

[0071] The beneficial effects of the present invention are as follows: after using a precise elastic wave source to excite elastic waves in the flat steel of the down conductor, an acceleration sensor array is used to collect echo signals, which can intuitively reflect the buried depth and corrosion information of the flat steel of the down conductor, thereby judging the corrosion condition of the down conductor of the grounding grid at that location. By establishing a corresponding corrosion diagnosis platform and performing real-time calculation and analysis on the collected signals, the corrosion condition of the down conductor of the grounding grid can be quickly determined, and the accurate detection of the corrosion degree of the down conductor of the grounding grid can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 This is the structural diagram of the grounding grid down conductor corrosion diagnosis system;

[0073] Figure 2 This is a schematic diagram of the propagation path of elastic waves in the flat steel of the down conductor;

[0074] Figure 3 It is the dispersion curve of elastic wave phase velocity in the flat steel of down conductor;

[0075] Figure 4 It is the dispersion curve of elastic wave group velocity in the flat steel of down conductor;

[0076] Figure 5 The offset imaging diagrams are of the non-corrosion point and the corrosion point;

[0077] Figure 6 is a current waveform diagram in the excitation coil in the embodiment;

[0078] Figure 7 This is a diagram of the upper computer software supporting the grounding grid down conductor corrosion diagnosis system in the embodiment;

[0079] Figure 8 It is the signal diagram before and after FK transformation in the embodiment;

[0080] Fig. 9 This is a signal diagram obtained after bandpass filtering in the embodiment;

[0081] Fig.10 A linear fitting graph is obtained by performing a linear regression on the maximum correlation point obtained in the embodiment and performing a polynomial fitting on the data;

[0082] Fig.11 The signal diagram after filtering in the embodiment and the image after S transformation thereof;

[0083] Fig.12 The composite image of the direct wave and the corrosion point echo after time shift correction and the energy image after superposition in the embodiment;

[0084] Fig.13 It is a schematic diagram of the installation structure of the down conductor flat steel in the embodiment. DETAILED DESCRIPTION

[0085] The present invention is further described in detail below in conjunction with the accompanying drawings and specific examples. The following examples or drawings are used to illustrate the present invention, but are not used to limit the scope of the present invention.

[0086] A grounding grid down conductor corrosion diagnosis method based on elastic waves comprises the following steps:

[0087] Step 1: Construct a grounding grid down conductor corrosion diagnosis system, such as Figure 1 , Figure 2 As shown, the grounding grid down conductor corrosion diagnosis system is provided with a host computer and a down conductor flat steel, the down conductor flat steel is a strip structure, which is divided into an upper end exposed above the ground and a lower half buried underground, the host computer is connected to an embedded control system, and the embedded control system is connected to a transient electromagnetic transmitter and an AD signal acquisition module;

[0088] The transient electromagnetic transmitter is connected to an elastic wave source, which is fixed to the upper end of the down conductor flat steel. The upper end of the down conductor flat steel is also provided with an acceleration sensor array, which is connected to an AD signal acquisition module via a signal amplification module, and the AD signal acquisition module is connected to the host computer via a USB data cable;

[0089] Step 2: The host computer sends a collection instruction to the embedded control system;

[0090] Step 3: The embedded control system generates a control signal, transmits the transmission signal in the control signal to the transient electromagnetic transmitter, and transmits the acquisition signal in the control signal to the AD signal acquisition module;

[0091] Step 4: After receiving the transmission signal, the transient electromagnetic transmitter generates a step excitation signal to the elastic wave source, which excites the elastic wave source to work, thereby generating elastic waves in the flat steel of the down conductor;

[0092] Step 5: the acceleration sensor array acquires the elastic wave signal in the flat steel of the down conductor, and the AD signal acquisition module acquires the elastic wave signal and transmits it to the host computer;

[0093] Step 6: The host computer performs diagnostic processing on the elastic wave signal to obtain a corrosion diagnosis result.

[0094] In step 5, the elastic wave signal includes a direct wave propagating in a forward direction and a reflected echo propagating in a reverse direction, and the reflected echo includes a corrosion point echo and an end echo;

[0095] The corrosion point echo and the end echo are related to the corrosion degree of the grounding grid down conductor;

[0096] The greater the degree of corrosion of the grounding grid down conductor, the stronger and greater the energy and amplitude of the echo at the corrosion point, and the weaker and smaller the energy and amplitude of the end echo; the smaller the degree of corrosion of the grounding grid down conductor, the weaker and smaller the energy and amplitude of the echo at the corrosion point, and the stronger and greater the energy and amplitude of the end echo.

[0097] like Figure 2 As shown in the figure, after the earthquake source excites the elastic wave, there are two propagation paths. Path one is to first pass through the array sensor in the forward direction to reach the corrosion point, then pass through the array sensor in the reverse direction after being reflected at the corrosion point; Path two is to first pass through the array sensor to reach the end of the down conductor flat steel, then pass through the array sensor in the reverse direction after being reflected at the end of the down conductor.

[0098] In step 6, the host computer performs the following steps to diagnose and process the elastic wave signal:

[0099] Step A1: The bandpass filter in the host computer obtains the elastic wave signal, performs bandpass filtering on it, and then uses FK transformation to separate the elastic wave propagating forward from the elastic wave propagating backward in the elastic wave signal to obtain the direct wave and the reflected echo, and transmit them to the corrosion diagnosis platform; the signal obtained after bandpass filtering is as shown in FIG. Fig. 9 As shown, the signal diagram before and after FK transformation is as follows Figure 8 shown.

[0100] Step A2: The corrosion diagnosis platform performs cross-correlation calculation on the direct waves collected by any two adjacent acceleration sensors to obtain the time extension of the direct wave passing through the two adjacent acceleration sensors; then linear fitting is performed using the time of the direct wave passing through each acceleration sensor and the position information of the corresponding acceleration sensor to calculate the wave velocity of the elastic wave propagation in the down conductor. The wave velocity calculation formula is as follows:

[0101]

[0102] Where h is the slope of the fitting curve, d t is the sampling rate, l is the distance between two adjacent acceleration sensors;

[0103] The direct waves collected by two adjacent acceleration sensors are cross-correlated to obtain the maximum correlation point of the direct waves collected by the two adjacent acceleration sensors. The obtained maximum correlation point is linearly regressed and a polynomial fit is performed on the data, such as Fig.10 shown.

[0104] Step A3: The corrosion diagnosis platform performs S transformation on the direct wave and the reflected echo collected by the same acceleration sensor to obtain the time when the direct wave and the reflected echo pass through the acceleration sensor, thereby obtaining the time delay when the direct wave and the reflected echo pass through the same acceleration sensor; Fig.11 shown.

[0105] Step A4: combining the time delay of the direct wave and the reflected echo through the same acceleration sensor with the corresponding wave velocity, calculating the distance between the acceleration sensor and the corrosion point, and then obtaining the position of the corrosion point based on the position information of the acceleration sensor;

[0106] The calculation expression of the position Y of the corrosion point is as follows:

[0107] Y=x+(v×Δt) / 2

[0108] Where x represents the position of the accelerometer, v represents the wave velocity, and Δt represents the time delay between the direct wave and the reflected echo through the same accelerometer;

[0109] Step A5: Using the offset imaging algorithm, combined with the end position of the down conductor, the position Y of the corrosion point is made into an offset imaging diagram, and the depth of color in the offset imaging diagram reflects the strength of the corrosion degree at that position. Combined with the echo amplitude of the corrosion point and the end echo amplitude, the corrosion diagnosis of the grounding grid down conductor is completed. Figure 5 (a) is the image of the non-corroded point offset. Figure 5 (b) is an offset imaging image showing a corrosion point at 1.3 m and 30 mm in the horizontal direction.

[0110] In terms of corrosion point positioning and imaging, the signal bandwidth and imaging method are key factors affecting positioning accuracy. The present invention studies the ultrasonic elastic wave multi-wave multi-component reverse time migration imaging method, derives the near-field dynamics equation of the flat steel structure to realize ultrasonic elastic wave simulation, sets the boundary absorption conditions of the specific structure for the "D" type, "L" type and "X" type flat steel structures, realizes the theoretical formula derivation of forward extension and reverse time extension, makes full use of longitudinal waves, SH waves and Lamb waves to realize fine imaging of corrosion points, improves the positioning accuracy of corrosion points, and accurately identifies multiple corrosion points.

[0111] The frequency, phase, and amplitude of the elastic waves in the grounding grid down conductors with different corrosion levels will be significantly different. After multiple processing steps, the elastic wave signals of the down conductors will eventually be reflected in the positioning image. Due to different corrosion levels, the energy of the reflected waves is also different. Through a large amount of simulation and experimental data, the connection between the corrosion degree of different materials and the image (energy of the reflected wave) is established, and a corrosion degree assessment model is generated, and a corrosion grade model is divided.

[0112] In step A2, the calculation expression of the cross-correlation algorithm is as follows:

[0113]

[0114] Among them, s1 and s2 represent the direct waves collected by two adjacent acceleration sensors, τ represents the time extension between the two direct waves, and s2 * represents the conjugate signal of the direct wave s2, t represents the corresponding sampling time, and R represents the correlation.

[0115] In step A1, the apparent velocity of the direct wave is defined as positive, and the apparent velocity of the reflected echo is defined as negative. The direct wave is extracted by using FK transformation, and the expression is as follows:

[0116] S(F,K)=∫∫S(x,t)e -j2π(Ft+Kx) dxd

[0117] Wherein, x is the position of the acceleration sensor, t is the corresponding sampling time, F is the frequency of the elastic wave signal, K is the wave number; S represents the elastic wave signal collected by the acceleration sensor;

[0118] After the above transformation, the elastic wave signal is transformed from the distance-time (xt) domain to the frequency-wavenumber domain (FK). Since the wave number of the direct wave is greater than 0 and the wave number of the reflected echo is less than 0, the FK inverse transformation is used to extract the reflected echo. The expression is as follows:

[0119] S(x,t)=∫∫S(F,K)e j2π(ft+Kx) dFdk.

[0121] In step A5, since the direct wave and reflected echo collected by the acceleration sensor include secondary direct waves and reflected echoes, a gather superposition algorithm is used to process the extracted direct waves and reflected echoes. First, time shift correction is performed on the direct wave and reflected echo respectively, and then the corrected signals are superimposed.

[0122] The peak amplitudes of the direct wave, end echo and corrosion point echo after the gather stacking are recorded, using the formula Quantitative processing is performed to obtain the corrosion degree assessment result, where A1 is the direct wave amplitude, A2 is the echo amplitude of the corrosion point, A3 is the end echo amplitude, and F is the corrosion degree.

[0123] Fig.12 It is the composite image after time shift correction of the direct wave and the corrosion point echo, as well as the energy image after superposition.

[0124] like Fig.13 As shown: the upper end of the down conductor flat steel is connected to the primary power equipment, and the lower half is connected to the grounding grid.

[0125] The elastic waves conducted in the flat steel of the down conductor are Lamb waves and horizontal shear waves. The modes of Lamb waves include A-type antisymmetric mode and S-type symmetric mode. Lamb waves and horizontal shear waves with a frequency below 100kHz are used for detection, that is, two elastic waves, A0 waves and SH0 waves, are used for detection.

[0126] The modal equation of the S-type symmetric mode is:

[0127]

[0128] The modal equation of type A antisymmetric mode is:

[0129]

[0130] in,

[0131]

[0132] Where k represents the number of waves transmitted in the horizontal direction, ρ is the density of the underground flat steel material, λ and μ are the Lame elastic constants; ω is the angular frequency, ω=2πf; c p is the group velocity, b is half of the plate thickness, c1 is the longitudinal wave velocity of the corresponding mode, c t is the shear wave velocity of the corresponding mode, q and p are intermediate numbers.

[0133] Figure 3 and Figure 4 They are the dispersion curves of phase velocity and group velocity of the down conductor flat steel respectively.

[0134] Depend on Figure 3 , Figure 4 It can be obtained that (1) the wave velocity of a Lamb wave of a mode is not fixed, but changes with the change of frequency; (2) at the same frequency, there are two or more Lamb waves of two modes at the same time. The specific manifestation of the Lamb wave dispersion phenomenon is that as the frequency of the wave changes, waves of different modes will propagate at different speeds, resulting in the dispersion of wave packets. This dispersion effect makes Lamb waves exhibit complex propagation characteristics in thin plate structures, providing rich information for wave detection and analysis. Therefore, studying the dispersion phenomenon of Lamb waves is of great significance for understanding the elastic properties of materials and structural health monitoring. In summary, for down conductors, when using Lamb waves for down conductor detection, in order to use frequency bands with fewer modes as much as possible, Lamb waves below 100kHz are used for detection, that is, elastic waves of two modes, A0 wave and S0 wave, are used for detection.

[0135] According to the signal loading theory of Lamb wave, when the surface of the down conductor is excited by a single side, there will be A0 mode and S0 mode in the down conductor; when the surface of the down conductor is excited by two sides in phase, there will be only A0 mode elastic wave in the down conductor; when the surface of the down conductor is excited by two sides in opposite phase, there will be S0 mode in the down conductor. In the frequency band below 100KHz, since the S0 wave velocity is too fast and the displacement is mainly in the Z direction, and the SH wave in the Z direction is not conducive to detection, a single excitation method is selected, and the A0 wave is used as the detection wave. By detecting the displacement in the X direction, the corrosion diagnosis of the longitudinal direction (X direction) of the down conductor is realized. This provides a theoretical basis for the corrosion diagnosis of the grounding grid down conductor using the elastic wave method.

[0136] The elastic wave source includes a giant magnetostrictive source and a magnetic focusing source. The giant magnetostrictive source is used to stimulate A0 waves for detecting corrosion in the longitudinal direction; the magnetic focusing source is used to stimulate SHO waves for detecting corrosion in the transverse direction.

[0137] Different corrosion types are identified by using the different sensitivity of waves of different modes to corrosion in different directions. In this embodiment, A0 wave and SH0 wave are used for joint corrosion judgment.

[0138] The working principle of the giant magnetostrictive source is: with the magnetostrictive material rod as the core, current is passed through the excitation coil to generate a magnetic field, so that the magnetostrictive material rod in the magnetic field is deformed and drives the output rod to hit the flat steel to generate elastic waves. The source can effectively excite A0 waves.

[0139] In order for the giant magnetostrictive source to work properly, the excitation signal of the source is crucial. When the excitation signal of the source is a sharp and narrow pulse, the elastic wave packet generated by the source will also be narrower, and the signal collected by the sensor will have less overlap and higher resolution. In order to further shorten the discharge time of the excitation coil, the constant voltage clamp fast shutdown technology is used when the switch tube is turned off to reduce the current in the excitation coil more quickly, so that its discharge time can basically approach 0. The current waveform in the excitation coil in this embodiment is as follows Figure 6 As shown, the data was acquired using a current clamp with a current clamp setting of 100mV / A.

[0140] In order to intuitively display the elastic wave morphology and detection information, a host computer software supporting the grounding grid down conductor corrosion diagnosis system is designed. Figure 7 As shown in the figure, it is written in Microsoft's C# language. The software can control the detection system to emit current, generate elastic waves in the down conductor through the source, and control the AD to collect and save data, and process the data after the collection is completed. The algorithm part calls the DLL file generated by MATLAB. After the data collection is completed, the algorithm can be called in real time to process the data. The system can set the sampling rate, sampling depth and number of stacking times, and different parameters can be set according to different scenarios. After the collection is completed, the DLL generated by MATLAB can be called to analyze and evaluate the data. Figure 7 As shown, on-site drawing can be realized to improve detection efficiency.

[0141] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A grounding grid down conductor corrosion diagnosis method based on elastic waves, characterized in that: The following steps are involved: Step 1: construct a grounding grid down conductor corrosion diagnosis system, wherein the grounding grid down conductor corrosion diagnosis system is provided with a host computer and a down conductor flat steel, wherein the down conductor flat steel is a strip structure, which is divided into an upper end exposed above the ground and a lower half buried underground, wherein the host computer is connected to an embedded control system, and the embedded control system is connected to a transient electromagnetic transmitter and an AD signal acquisition module; The transient electromagnetic transmitter is connected to an elastic wave source, which is fixed to the upper end of the down conductor flat steel. The upper end of the down conductor flat steel is also provided with an acceleration sensor array, which is connected to an AD signal acquisition module, and the AD signal acquisition module is connected to a host computer; Step 2: The host computer sends a collection instruction to the embedded control system; Step 3: The embedded control system generates a control signal, transmits the transmission signal in the control signal to the transient electromagnetic transmitter, and transmits the acquisition signal in the control signal to the AD signal acquisition module; Step 4: After receiving the transmission signal, the transient electromagnetic transmitter generates a step excitation signal to the elastic wave source, which excites the elastic wave source to work, thereby generating elastic waves in the flat steel of the down conductor; Step 5: the acceleration sensor array acquires the elastic wave signal in the flat steel of the down conductor, and the AD signal acquisition module acquires the elastic wave signal and transmits it to the host computer; Step 6: The host computer performs diagnostic processing on the elastic wave signal to obtain a corrosion diagnosis result.

2. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 1 is characterized by: In step 5, the elastic wave signal includes a direct wave propagating in a forward direction and a reflected echo propagating in a reverse direction, and the reflected echo includes a corrosion point echo and an end echo; The corrosion point echo and the end echo are related to the corrosion degree of the grounding grid down conductor; The greater the degree of corrosion of the grounding grid down conductor, the stronger and greater the energy and amplitude of the echo at the corrosion point, and the weaker and smaller the energy and amplitude of the end echo; the smaller the degree of corrosion of the grounding grid down conductor, the weaker and smaller the energy and amplitude of the echo at the corrosion point, and the stronger and greater the energy and amplitude of the end echo.

3. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 2 is characterized by: In step 6, the host computer performs the following steps to diagnose and process the elastic wave signal: Step A1: The bandpass filter in the host computer acquires the elastic wave signal, performs bandpass filtering on it, and then uses FK transformation to separate the elastic wave propagating forward from the elastic wave propagating backward in the elastic wave signal to obtain a direct wave and a reflected echo, and transmit them to the corrosion diagnosis platform; Step A2: The corrosion diagnosis platform performs cross-correlation calculation on the direct waves collected by any two adjacent acceleration sensors to obtain the time extension of the direct wave passing through the two adjacent acceleration sensors; then linear fitting is performed using the time of the direct wave passing through each acceleration sensor and the position information of the corresponding acceleration sensor to calculate the wave velocity of the elastic wave propagation in the down conductor. The wave velocity calculation formula is as follows: Where h is the slope of the fitting curve, d t is the sampling rate, l is the distance between two adjacent acceleration sensors; Step A3: The corrosion diagnosis platform performs S transformation on the direct wave and the reflected echo collected by the same acceleration sensor to obtain the time when the direct wave and the reflected echo pass through the acceleration sensor, thereby obtaining the time delay when the direct wave and the reflected echo pass through the same acceleration sensor; Step A4: combining the time delay of the direct wave and the reflected echo through the same acceleration sensor with the corresponding wave velocity, calculating the distance between the acceleration sensor and the corrosion point, and then obtaining the position of the corrosion point based on the position information of the acceleration sensor; The calculation expression of the position Y of the corrosion point is as follows: Y=x+(v×Δt) / 2 Where x represents the position of the accelerometer, v represents the wave velocity, and Δt represents the time delay between the direct wave and the reflected echo through the same accelerometer; Step A5: Using the offset imaging algorithm, combined with the end position of the down conductor, the position Y of the corrosion point is made into an offset imaging diagram, and the depth of color in the offset imaging diagram reflects the strength of the corrosion degree at that position. Combined with the echo amplitude of the corrosion point and the end echo amplitude, the corrosion diagnosis of the grounding grid down conductor is completed.

4. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 3 is characterized by: In step A2, the calculation expression of the cross-correlation algorithm is as follows: Among them, s1 and s2 represent the direct waves collected by two adjacent acceleration sensors, τ represents the time extension between the two direct waves, and s2 * represents the conjugate signal of the direct wave s2, t represents the corresponding sampling time, and R represents the correlation.

5. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 3 is characterized by: In step A1, the apparent velocity of the direct wave is defined as positive, and the apparent velocity of the reflected echo is defined as negative. The direct wave is extracted by using FK transformation, and the expression is as follows: s(F,K)=∫∫S(x,t)e -j2π(Ft+Kx) dxdt Wherein, x is the position of the acceleration sensor, t is the corresponding sampling time, F is the frequency of the elastic wave signal, K is the wave number; S represents the elastic wave signal collected by the acceleration sensor; After the above transformation, the elastic wave signal is transformed from the distance-time (xt) domain to the frequency-wavenumber domain (FK). Since the wave number of the direct wave is greater than 0 and the wave number of the reflected echo is less than 0, the FK inverse transformation is used to extract the reflected echo. The expression is as follows: S(x,t)=∫∫S(F,K)ej 2π(ft+Kx) dFdk。 6. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 3 is characterized by: In step A5, since the direct wave and reflected echo collected by the acceleration sensor include secondary direct waves and reflected echoes, a gather superposition algorithm is used to process the extracted direct waves and reflected echoes. First, time shift correction is performed on the direct wave and reflected echo respectively, and then the corrected signals are superimposed. The peak amplitudes of the direct wave, end echo and corrosion point echo after the gather stacking are recorded using the formula Quantitative processing is performed to obtain the corrosion degree assessment result, where A1 is the direct wave amplitude, A2 is the echo amplitude of the corrosion point, A3 is the end echo amplitude, and F is the corrosion degree.

7. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 1 is characterized by: In step 1, the upper end of the down conductor flat steel is connected to a primary power device, and the lower half is connected to a grounding grid.

8. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 2 is characterized by: The elastic waves conducted in the flat steel of the down conductor are Lamb waves and horizontal shear waves. The modes of Lamb waves include A-type antisymmetric mode and S-type symmetric mode. Lamb waves and horizontal shear waves with a frequency below 100kHz are used for detection, that is, two elastic waves, A0 waves and SH0 waves, are used for detection. The modal equation of the S-type symmetric mode is: The modal equation of type A antisymmetric mode is: in, Where k represents the number of waves transmitted in the horizontal direction, ρ is the density of the underground flat steel material, λ and μ are the Lame elastic constants; ω is the angular frequency, ω=2πf; c p is the group velocity, b is half of the plate thickness, c1 is the longitudinal wave velocity of the corresponding mode, c t is the shear wave velocity of the corresponding mode, q and p are intermediate numbers.

9. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 8, characterized in that: The elastic wave source includes a giant magnetostrictive source and a magnetic focusing source. The giant magnetostrictive source is used to stimulate A0 waves for detecting corrosion in the longitudinal direction; the magnetic focusing source is used to stimulate SHO waves for detecting corrosion in the transverse direction.

10. The grounding grid down conductor corrosion diagnosis method based on elastic waves according to claim 1, characterized in that: The speed sensor array is connected to the AD signal acquisition module via a signal amplification module, and the AD signal acquisition module is connected to the host computer via a USB data cable.

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