A method for diagnosing corrosion state of grounding round steel based on nonlinear conduction impedance

Abstract

The application discloses a grounding round steel corrosion state diagnosis method based on nonlinear conduction impedance and belongs to the technical field of power system grounding. The method constructs a one-dimensional non-uniform grid along the radius direction of the grounding round steel, and the grid is denser and denser from inside to outside. Taking the node electric field as the research object, the node electric field is expressed as a linear combination of the first and second types of zero-order Bessel special functions. The interlayer recursive relationship is established by using the continuous boundary conditions of the electric field and the continuous boundary conditions of the tangential magnetic field intensity, the full rank linear equation group is constructed by combining the convergence condition and the ampere loop theorem equation to solve the weighted coefficient, the node magnetic induction intensity and the relative magnetic permeability are iteratively updated until convergence, the conduction impedance under each scanning current is calculated, and the conduction impedance scanning result is compared with that of the intact conductor to evaluate the corrosion state. The application realizes the fast solution of the nonlinear conduction impedance of the grounding round steel and provides an efficient and reliable calculation method for the corrosion state diagnosis of the grounding system.

Description

Technical Field

[0001] This invention relates to the field of power system grounding technology, specifically to a method for diagnosing the corrosion status of grounding round steel based on nonlinear conduction impedance. Background Technology

[0002] Steel is widely used as the conductor material in my country's grounding systems. Steel exhibits nonlinear magnetic conductivity, resulting in varying conduction impedance and conductor voltage drop depending on the amplitude of the applied alternating current. During power system faults, the conductor voltage drop caused by the fault current acts on the substation secondary system, potentially causing overvoltage problems in secondary cables and relays, and affecting the normal operation of the secondary system. Therefore, the nonlinear conduction impedance of grounding steel is an important parameter for assessing the potential difference within the grounding system network.

[0003] Besides facing the high-amplitude current surges entering the grounding system during asymmetrical grounding faults in the power system, the grounding system also faces the problem of soil corrosion. Continuous corrosion leads to a reduction in the cross-sectional area of ​​the grounding conductor, reduces the conductivity of the grounding grid, triggers a series of grounding faults, and endangers the safe operation of the power system.

[0004] In the field of grounding grid corrosion diagnosis, some scholars have used the nodal voltage method based on unequal potentials to determine the corrosion status by calculating the change in grounding impedance before and after corrosion. Other studies have used ultra-low frequency excitation sources instead of DC or power frequency AC sources to excite the grounding grid, measuring the grid resistance based on electrical network theory to determine corrosion. Electrochemical impedance spectroscopy has also been used for in-situ monitoring of soil corrosion in grounded galvanized round steel. However, all of these methods treat the grounding conductor as a linear conductor or use DC or ultra-low frequency excitation to ignore the skin effect and nonlinear permeability, failing to fully consider the nonlinear magnetic properties of steel as a ferromagnetic material under power frequency and fault current conditions.

[0005] Regarding the consideration of the nonlinear characteristics of steel permeability, some scholars (such as Lu Zhiwei et al.) have proposed a method for calculating the grounding impedance of grounding grids that takes into account the saturation characteristics of steel permeability, based on the method of moments and the combination of field and circuit principles. They have also pointed out that the power frequency grounding impedance of large steel grounding grids decreases with the increase of the ground fault current. Other studies have used analytical methods based on classical electromagnetic field theory to solve for the nonlinear impedance effects of circular cross-section magnetic materials. However, the above methods still have shortcomings in terms of computational efficiency or engineering applicability, and none of them have applied nonlinear conduction impedance analysis to the diagnosis of corrosion status of grounding circular steel.

[0006] Currently, in engineering practice, the maximum magnetic permeability of the grounding steel is used as an approximation to analyze the upper limit of conduction impedance. This approach is relatively conservative, increasing engineering margins but also leading to a lack of refined design and parameters. In recent years, using conduction impedance to assess the condition of grounding grids has shown promise as a new technology for on-site testing of grounding systems. Solving the fundamental problem of nonlinear conduction impedance of grounding steel helps to analyze the conduction impedance of grounding grids under corrosion or fault conditions both theoretically and computationally, thus providing an effective calculation method for on-site testing. However, there is currently no fast and accurate engineering solution for the nonlinear conduction impedance problem of grounding round steel, and a technical solution for using nonlinear conduction impedance characteristics for quantitative diagnosis of corrosion conditions is also lacking. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for diagnosing the corrosion status of grounded round steel based on nonlinear conduction impedance. This invention establishes a one-dimensional non-uniform grid recursive model to achieve rapid solution of nonlinear conduction impedance, and quantitatively evaluates the corrosion status of the round steel by measuring the impedance characteristic changes under current amplitude scanning, thus providing an efficient and reliable computational method for corrosion diagnosis of grounding systems.

[0008] This invention provides a method for diagnosing the corrosion status of grounded round steel based on nonlinear conduction impedance, comprising the following steps: Step 1: Set the effective value sequence and angular frequency of the test current, the density coefficient of the grounding round steel mesh, the number of meshing nodes of the grounding round steel radius, the resistivity and BH curve of the grounding round steel material, and divide the grounding round steel into a one-dimensional mesh with equal mesh length according to the radial direction and the mesh density coefficient. Step 2: Determine whether the current scanning calculation is complete. If not, proceed to Step 3; otherwise, proceed to Step 9. Step 3: The conduction current is selected in a set order, and the initial magnetic field strength and initial relative permeability of the grid nodes are determined by the current magnitude and the grid division. Step 4: Construct the general solution for the electric field distribution along the radial direction of the grounded circular steel, and write out the boundary conditions, convergence conditions, and Ampere's circuital law equations. Set the number of iterations to 1. Step 5: Perform a recursive calculation of the boundary conditions for the boundary layer and solve for the weighting coefficients of the last layer; Step 6: Perform recursion for arbitrary boundary layer conditions, solve for the weighting coefficients of each layer, and update the magnetic field strength and relative permeability; Step 7: Perform convergence judgment and iterative calculation. If the convergence criterion is not met, go to step 5 to continue iterating. If the convergence criterion is met, go to step 8. Step 8: Calculate the conduction impedance per unit length of the conductor; Step 9: Summarize the on-resistance scan results; Step 10: Compare the on-resistance scan results with those of an intact conductor to assess the corrosion status.

[0009] In a further preferred embodiment, in step one, the axisymmetric conductor circular cross section is divided into a one-dimensional grounding circular steel mesh with equal proportions. The mesh density coefficient is used to subdivide the radial direction into a node distribution that becomes increasingly dense from the inside out, and the ratio of the lengths of adjacent meshes is equal to the mesh density coefficient.

[0010] In a further preferred embodiment, in step four, the nodal electric field distribution on the cross section is written as a linear combination of the first-type modified zero-order Bessel function and the second-type modified zero-order Bessel function, and a linear equation is formed with the weighting coefficients to be determined.

[0011] Further optimized, in step four, the electric field continuous boundary conditions, tangential magnetic field strength continuous boundary conditions, convergence conditions, and Ampere's circuital law equation at the nodes are used to solve the weighted coefficient equation of the nodal electric field. Among them, the electric field continuous boundary conditions and tangential magnetic field strength continuous boundary conditions form a recursive relationship of the nodal electric field from the inside to the outside, and the convergence conditions and Ampere's circuital law equation complete the additional equations of the first and last layers, so that the linear equation of the weighted coefficient reaches full rank.

[0012] In a further preferred embodiment, in step seven, the values ​​of the node magnetic induction intensity and relative permeability are updated according to the iterative principle, and the infinite norm of the relative permeability is used as the convergence criterion; if the iteration count reaches the preset upper limit and convergence is still not achieved, the failure current is output.

[0013] Further preferably, in step ten, spline fitting is performed on the conduction impedance scan results for the resistance and reactance components respectively, and the increase rates R% and X% of the peak values ​​of the resistance and reactance components relative to the intact conductor are calculated. The decrease rates I of the effective current values ​​corresponding to the peak values ​​of the resistance and reactance components relative to the intact conductor are also calculated. R % and I X Based on this, the corrosion state change rate P is calculated, and the corrosion level is determined according to the corrosion state change rate.

[0014] In the above technical solution, this invention transforms the axisymmetric conductor cross-section structure into a one-dimensional non-uniform mesh model. A mesh density coefficient is used to subdivide the radial direction into a node distribution that becomes increasingly dense from the inside out, thereby improving the simulation accuracy of the conductor skin effect during AC impedance evaluation. Based on this, the nodal electric field distribution on the cross-section is written as a linear combination of the first-kind modified zero-order Bessel function and the second-kind modified zero-order Bessel function. The undetermined weighting coefficients are solved using linear equations. A recursive relationship from the inside out is formed using continuous boundary conditions for the nodal electric field and continuous boundary conditions for the tangential magnetic field strength. Combined with convergence conditions and Ampere's circulation law equations, the additional equations for the first and last layers are completed, ensuring the linear equations for the weighting coefficients reach full rank. Based on this, the values ​​of the nodal magnetic induction and relative permeability are updated according to the iterative principle until the convergence criterion is met, ultimately completing the solution for the on-resistance corresponding to the specific frequency and effective value current.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention converts the axisymmetric conductor circular cross section into a one-dimensional non-uniform mesh and uses the Bessel function general solution combined with boundary conditions to solve the problem recursively, thereby realizing the rapid calculation of the nonlinear conduction impedance of the grounded circular steel. This avoids the large-scale meshing and long-term iteration required by numerical methods such as the finite element method when dealing with highly nonlinear problems, significantly improving the calculation efficiency and making it suitable for engineering applications.

[0016] (2) The present invention makes the grid gradually denser from the inside to the outside in the radial direction by using the grid density coefficient, which can accurately simulate the skin effect of alternating current; by iteratively updating the nodal magnetic induction intensity and relative permeability, it can truly reflect the distribution law of the permeability inside the steel conductor with the current amplitude and radial position, reveal the internal magnetic properties and nonlinear conduction impedance mechanism of the steel conductor, and overcome the shortcomings of the existing technology that assumes the permeability to be constant or calculates it approximately with the maximum permeability.

[0017] (3) The present invention obtains the complete scanning results of the conduction impedance changing with the current by scanning the current amplitude. By comparing it with the scanning results of the intact conductor, it can intuitively reflect the change in impedance characteristics caused by the reduction of cross-sectional area due to corrosion, and provides a quantitative basis for the diagnosis of corrosion status of grounded round steel.

[0018] (4) This invention provides clear convergence criteria and iterative strategies, which can effectively handle the highly nonlinear problem of magnetic permeability curves and provide stable and reliable solutions.

[0019] (5) The method of the present invention has flexible parameter adjustment. It can be applied to grounding round steel of different materials and sizes by simply changing the BH curve and resistivity parameters.

[0020] Instruction manual illustrations

[0021] Figure 1This is a flowchart illustrating the implementation of a method for diagnosing the corrosion status of grounded round steel based on nonlinear conduction impedance according to the present invention.

[0022] Figure 2 Example graph of BH curve for No. 45 round steel (horizontal axis is magnetic field strength H, vertical axis is magnetic induction intensity B).

[0023] Figure 3 This is a schematic diagram of a one-dimensional non-uniform grid distribution of round steel along the radial direction, with the node spacing decreasing proportionally outward along the radius.

[0024] Figure 4 According to Figure 2 The curve of relative permeability μ as a function of magnetic field strength H, obtained by converting the BH curve.

[0025] Figure 5 The curve shows the relative permeability distribution of a round steel bar with an effective current of 100A and a radius of 0.008m along the radial direction.

[0026] Figure 6 The curves show the variation of the real part (resistance component) and imaginary part (reactance component) of the conduction impedance of a round steel with a radius of 0.008m in the range of effective conduction current of 10A to 300A.

[0027] Figure 7 The curves show the real and imaginary parts of the conduction impedance of a round steel bar with a radius of 0.012m in the range of effective conduction current from 10A to 300A.

[0028] Figure 8 The curves show the real and imaginary parts of the conduction impedance of a round steel bar with a radius of 0.011m in the range of effective conduction current from 10A to 300A.

[0029] Figure 9 The curves show the real and imaginary parts of the conduction impedance of a round steel bar with a radius of 0.010m in the range of 10A to 300A effective conduction current.

[0030] Figure 10 The curves show the real and imaginary parts of the conduction impedance of a round steel bar with a radius of 0.009m in the range of effective conduction current from 10A to 300A.

[0031] Figure 11 The curve shows the relative permeability distribution of a round steel bar with an effective current of 150A and a radius of 0.012m along the radial direction. Detailed Implementation

[0032] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that these embodiments are only for helping to understand the present invention and do not constitute any limitation on the scope of protection of the present invention. Any equivalent transformations, modifications, or combinations made by those skilled in the art based on the disclosed technical concept of the present invention should fall within the scope of protection of the present invention. Experimental methods not specifying specific conditions in the embodiments are generally carried out under conventional conditions or according to the conditions recommended by the manufacturer. Unless otherwise specified, the materials, equipment, etc. used are all conventional products that can be obtained commercially.

[0033] like Figure 1 As shown, a method for diagnosing the corrosion status of grounded round steel based on nonlinear conduction impedance is as follows: The first step is to adjust the effective value sequence I of the test current and the angular frequency. The density coefficient of the grounding round steel mesh , radius of grounding round steel Number of partition nodes The resistivity of the grounding round steel conductor material The BH curve is used to divide the grounding round steel into a one-dimensional grid with equal grid length distribution according to the radial direction and grid density coefficient.

[0034] 1.1. Set the effective value sequence of the test current I = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 220, 250, 300} and the angular frequency. .

[0035] 1.2 Input the mesh density coefficient β (0 < β < 1) and the number of nodes N (N > 1) of the grounded round steel, and the resistivity of the steel conductor material. And the BH curve (see example of BH curve for No. 45 electrical steel) Figure 2 ).

[0036] 1.3 Divide the grounding round steel into a one-dimensional grounding round steel mesh with equal proportions according to the radial direction and the mesh density coefficient.

[0037] like Figure 3 As shown, a series of node positions R are set along the radial direction for the round steel. i (i = 1, …, N), the grid length is set to the distance between adjacent nodes. For a uniformly distributed one-dimensional grounded circular steel grid, the grid length decreases as the radius increases. The ratio of adjacent grid lengths is equal to the grid density coefficient. (1); In the formula, R iLet R be the position of node i. Therefore, node R... i The expression is: (2); in, The density coefficient is the i-th power of the mesh density coefficient. It is the Nth power of the mesh density coefficient.

[0038] The second step is to determine whether the current scanning calculation is complete. If yes, proceed to the third step; otherwise, proceed to the ninth step.

[0039] The third step involves taking the magnitude of the conduction current in a set order, and then calculating the initial magnetic field strength and initial relative permeability of the grid nodes based on the magnitude of the conduction current and the grid division of the grounding round steel.

[0040] 3.1 The conduction current C is set according to the order of the test current settings.

[0041] 3.2 Determining the initial magnetic field strength of a node based on the conduction current and node position: (3); In the formula, H i Let be the initial magnetic field strength at node i.

[0042] 3.3, From the initial magnetic field strength H i Obtaining the initial relative permeability μ i : (4); In the formula, f μ Let be the conversion function from magnetic field strength to relative permeability, regarding Figure 2 The relative permeability curve after data conversion is shown in the figure. Figure 4 .

[0043] The fourth step is to construct the nodal electric field equations for the grounded circular steel conductor distributed along the radial direction, and to write out the boundary conditions, convergence conditions, and Ampere's circuital law equations, setting the iteration number to 1.

[0044] 4.1 Regarding node position R i Write the equations for the electric field at the nodes along the radial direction of the grounded circular steel conductor: (5); In the formula, I0 and K0 are the first-order modified zeroth-order Bessel function and the second-order modified zeroth-order Bessel function, respectively, and A i and B i The weighting coefficients for I0 and K0 are respectively. Let be the initial relative permeability of node i. Let be the resistivity of the grounded round steel conductor, j be the imaginary unit, and r be the distance relative to the axis of the grounded round steel conductor. Let the node position R i The electric field strength at position r.

[0045] 4.2 Solve the boundary condition equations for the continuity of the electric field and the tangential magnetic field intensity at the boundary layer interface.

[0046] List the boundary layer boundary nodes R i The boundary condition equations for the continuous electric field on the surface are: (6); For node R i The electric field strength at that location; List the boundary layer boundary nodes R i The boundary condition equation for the continuous tangential magnetic field strength on the surface is as follows: (7); 4.3. Since the magnetic field is zero at the center of the conductor and does not diverge, write down the magnitude of the electric field: (8); in, The electric field strength at the center of the round steel bar. The I0 weighting coefficient for node 1. The K0 weighting coefficient for node 1. To output the zero value of the first-order corrected zeroth-order Bessel function, For the zero-value output of the second-type modified zero-order Bessel function, since K0(0) diverges in equation (8), we have: (9); 4.4. Regarding the outer surface R of the conductor N =a, write the equation of Ampere's circuital law: (10); In the formula, I1 and K1 are the first-order modified first-order Bessel function and the second-order modified first-order Bessel function, respectively, and μ N The relative permeability of the outer surface of the conductor. and These are the weighting coefficients I0 and K0 for node N, respectively.

[0047] 4.5, Number of iterations m = 1: Fifth, perform a recursive calculation of the boundary layer conditions to solve for A. N and B N .

[0048] 5.1. Write the recursive relationship of the boundary layer boundary conditions as a recursive equation: (11); In the formula, the recursive matrix M i The specific format of the element is: (12); (13); (14); (15); in, For the first kind of modified zeroth order Bessel function, For the first kind of modified first-order Bessel function, For the first kind of modified first-order Bessel function, This is a modified zeroth-order Bessel function of the first kind.

[0049] 5.2 Complete the recursion from the first floor to the last floor: (16); In the formula, M is the recursive boundary condition matrix from the first layer to the last layer. From equation (16), we have (17); 5.3 Solving for the coefficient A of the last layer N and B N .

[0050] Combining equations (10) and (17), we have (18); Solving equation (18) yields the coefficient A of the last layer. N and B N .

[0051] Step 6: Perform a recursive calculation for the arbitrary boundary layer conditions and solve for A. i and B i , (i = 1, …, N-2).

[0052] 6.1. Complete the recursion from the i-th (i = 1, …, N-1) layer to the last layer according to equation (16): (19); In the formula, M i+ For node R i The recursive matrix to the conductor surface.

[0053] 6.2 Solving for the iterative magnetic field strength : (20); Let be the iterative magnetic field strength at node i; 6.3 Update the magnetic field strength H i With relative permeability μ i+ : (twenty one); (twenty two); in, The magnetic field strength of node i after the update; Let i be the permeability of node i after the update. This is the conversion function from magnetic field strength to relative permeability.

[0054] Step 7: Convergence assessment and iterative calculation.

[0055] 7.1 List the relative permeability μ of the nodes i+ Infinite norm criterion: (twenty three); 7.2 If the infinite norm criterion holds, proceed to step 8.

[0056] 7.3 If the infinite norm criterion does not hold, (twenty four); (25); 7.4 If the number of iterations m < 100, proceed to step 5. Otherwise, output the failure current and proceed to step 2.

[0057] Step 8: Calculate the conduction impedance Z per unit length of the grounded round steel conductor.

[0058] (26); (27); in, Let N be the electric field strength at node N.

[0059] Step 9: Summarize the on-resistance scan results.

[0060] The on-resistance scan results are as follows: (28); In the formula, I represents the set of on-resistance scan results, where I is the series of test currents.

[0061] Step 10: Compare the on-resistance scan results with those of an intact conductor to assess the corrosion status.

[0062] 10.1 Perform spline fitting on the resistance and reactance components of the conduction impedance scan results.

[0063] 10.2 Calculate the peak values ​​of the resistive and reactive components, and give the increase rate R% and the increase rate X of the peak values ​​of the resistive and reactive components relative to the intact conductor.

[0064] 10.3 Calculate the effective current values ​​corresponding to the peak values ​​of the resistive and reactive components, and give the reduction rate I of the effective current values ​​corresponding to the peak values ​​of the resistive and reactive components relative to the effective current values ​​corresponding to the peak values ​​of the resistive and reactive components in a healthy conductor. R % and the effective value of current I corresponding to the peak value of the reactance component X %.

[0065] 10.4 Calculate the rate of change of corrosion state P.

[0066] (29); In the formula, the reduction rate I of the effective value of the current corresponding to the peak value of the resistive component needs to be calculated. R Taking the absolute value of %, it is also necessary to consider the reduction rate I of the effective current value corresponding to the peak value of the reactance component. X The percentage is taken as an absolute value to prevent the peak current from increasing in the opposite direction due to nonlinearity of the magnetic permeability.

[0067] 10.5. The corrosion status of round steel is assessed by the corrosion status change rate P.

[0068] P<5%, the round steel is in good corrosion condition.

[0069] 5%≤P<10% indicates slight corrosion in the round steel.

[0070] If 10% ≤ P < 20%, the round steel exhibits moderate corrosion; enhanced monitoring is recommended.

[0071] If 20% ≤ P < 50%, the round steel has a relatively severe degree of corrosion, and it is recommended to conduct an excavation inspection.

[0072] P≥50% indicates severe corrosion of the round steel; excavation inspection is recommended.

[0073] Supplementary embodiment.

[0074] Test current RMS value C = 100A and angular frequency The density coefficient β of the grounding round steel mesh is 0.99, the radius of the grounding round steel is a = 0.008m, the number of meshing nodes is N = 100, and the resistivity is... .

[0075] The calculated nonlinear conduction impedance of the grounded round steel is (3.3090 + j2.9356) mΩ.

[0076] The relative permeability curves distributed along the radius are as follows: Figure 5 As shown. Figure 5 Data shows that the relative permeability curve distributed along the radius initially increases with the increase of the radius. However, near the conductor surface, due to the skin effect, current concentration leads to magnetic saturation, resulting in a peak and a decrease in the relative permeability curve.

[0077] Figure 6 The curves of the real and imaginary parts of the conduction impedance calculated for conduction currents of 10A to 300A. Figure 6 The results show that the effective current value corresponding to the peak value of the resistive component is about 80A, while the effective current value corresponding to the peak value of the inductive component is about 50A, indicating that the two are out of sync.

[0078] Comparing the four cases where the radius of the grounding round steel is a = 0.012m, 0.011m, 0.010m, and 0.009m, the curves of the real and imaginary parts of the conduction impedance are shown below. Figures 7-10 .

[0079] The peak values ​​of the resistive and reactive components of a 0.012m grounding round steel bar are approximately 2.2Ω. Grounding round steel bars with a radius of 0.012m and 0.008m (with peak values ​​of resistive and reactive components of approximately 3.2Ω) exhibit different peak characteristics in their resistive and reactive components. Therefore, by comparing the conduction impedance, it can be analyzed whether corrosion has occurred in the grounding grid, leading to a significant reduction in the conductor cross-section. Similar results apply to other radii.

[0080] The effective current corresponding to the peak resistive component of a 0.012m grounding round steel bar is approximately 130A, while the effective current corresponding to the peak inductive component is approximately 80A. Grounding round steel bars with a radius of 0.012m and 0.008m exhibit different peak current characteristics. Therefore, comparing the peak currents can help analyze whether corrosion in the grounding grid has led to a significant reduction in the conductor cross-section. Similar patterns exist for other radii.

[0081] The relative permeability curve along the radius of a grounded round steel with an effective current of 150A and a radius of 0.012m is shown below. Figure 11 As shown, the specific relative permeability curves are... Figure 5 Consistent. Figure 11 Data shows that when the effective current is 150A, the relative permeability of the grounded round steel with a radius of 0.012m initially increases with increasing radius, peaks near the conductor surface due to magnetic saturation caused by the skin effect, and then decreases. Its trend is similar to... Figure 5 The results were consistent, further verifying the correct simulation of the permeability distribution under different radii and current conditions using this method.

[0082] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations, substitutions, or improvements that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims. Furthermore, the method steps of the present invention are not necessarily limited to the order described; the order of some steps can be adjusted according to actual needs without violating the principles of the invention.

Claims

1. A method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance, characterized in that, Includes the following steps: Step 1: Set the effective value sequence and angular frequency of the test current, the density coefficient of the grounding round steel mesh, the number of meshing nodes of the grounding round steel radius, the resistivity and BH curve of the grounding round steel material, and divide the grounding round steel into a one-dimensional mesh with equal mesh length according to the radial direction and the mesh density coefficient. Step 2: Determine whether the current scanning calculation is complete. If not, proceed to Step 3; otherwise, proceed to Step 9. Step 3: The conduction current is selected in a set order, and the initial magnetic field strength and initial relative permeability of the grid nodes are determined by the current magnitude and the grid division. Step 4: Construct the general solution for the electric field distribution along the radial direction of the grounded circular steel, and write out the boundary conditions, convergence conditions, and Ampere's circuital law equations. Set the number of iterations to 1. Step 5: Perform a recursive calculation of the boundary conditions for the boundary layer and solve for the weighting coefficients of the last layer; Step 6: Perform recursion for arbitrary boundary layer conditions, solve for the weighting coefficients of each layer, and update the magnetic field strength and relative permeability; Step 7: Perform convergence judgment and iterative calculation. If the convergence criterion is not met, go to step 5 to continue iterating. If the convergence criterion is met, go to step 8. Step 8: Calculate the conduction impedance per unit length of the conductor; Step 9: Summarize the on-resistance scan results; Step 10: Compare the on-resistance scan results with those of an intact conductor to assess the corrosion status.

2. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 1, characterized in that, In step one, the axisymmetric conductor circular cross section is divided into a one-dimensional grounding circular steel mesh with equal proportions. The mesh density coefficient is used to subdivide the radial direction into a node distribution that becomes increasingly dense from the inside to the outside. The ratio of the lengths of adjacent meshes is equal to the mesh density coefficient.

3. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 1, characterized in that, In step four, the nodal electric field distribution on the cross section is written as a linear combination of the first-type modified zero-order Bessel function and the second-type modified zero-order Bessel function, and a linear equation is formed with the weighting coefficients to be determined.

4. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 1, characterized in that, In step four, the electric field weighting coefficient equation at the node is solved using the continuous boundary conditions of the electric field and the tangential magnetic field strength, the convergence condition, and the Ampere's circuital law equation. The continuous boundary conditions of the electric field and the tangential magnetic field strength form a recursive relationship of the node electric field from the inside to the outside. The convergence condition and the Ampere's circuital law equation complete the additional equations of the first and last layers, so that the linear equation of the weighting coefficient reaches full rank.

5. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 1, characterized in that, In step seven, the values ​​of the node magnetic induction intensity and relative permeability are updated according to the iterative principle, and the infinite norm of the relative permeability is used as the convergence criterion; if the iteration count reaches the preset upper limit and convergence is still not achieved, the failure current is output.

6. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 1, characterized in that, In step ten, spline fitting is performed on the conduction impedance scan results for the resistance and reactance components, respectively. The increase rates R% and X% of the peak values ​​of the resistance and reactance components relative to the intact conductor are calculated, and the decrease rates I of the effective current values ​​corresponding to the peak values ​​of the resistance and reactance components relative to the intact conductor are calculated. R % and I X Based on this, the corrosion state change rate P is calculated, and the corrosion level is determined according to the corrosion state change rate.

7. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 6, characterized in that, The formula for calculating the corrosion state change rate P is: 。 8. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 6, characterized in that, The criteria for determining the corrosion level are as follows: P < 5% indicates good corrosion; 5% ≤ P < 10% indicates slight corrosion; 10% ≤ P < 20% indicates moderate corrosion; 20% ≤ P < 50% indicates severe corrosion; and P ≥ 50% indicates severe corrosion.

9. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 1, characterized in that, The test frequency is selected within the range of 25 to 900 Hz.

10. The method for diagnosing the corrosion state of grounded round steel based on nonlinear conduction impedance according to claim 1, characterized in that, The effective value sequence of the test current is selected from multiple scanning points within the range of 10A to 300A.

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