Shielding performance calculation method for threading metal pipeline

Abstract

The invention provides a method for calculating the shielding performance of a threading metal pipeline. The method comprises the following steps: determining a calculation object and physical parameters; determining an electric field and a magnetic field in the shielding cavity according to coordinates and current elements of the radiation source in the physical parameters; obtaining radiation characteristics of a pipeline hole at the joint of the shielding cavity and the metal pipe, and determining an electric field and a magnetic field in the metal pipe according to the radiation characteristics; the method comprises the following steps: discretizing a transmission line into an equivalent circuit model formed by cascading a plurality of lumped parameter circuit units so as to calculate a transmission line coupling distributed current; determining a distributed voltage source in the equivalent circuit model according to the electric field in the shielding cavity and the electric field in the metal tube; and determining a leakage electric field and shielding effectiveness according to the distribution current coupled by the transmission line. According to the method, the vector Green function, the field line coupling theory and the electromagnetic compatibility theory are integrated, and the shielding performance of the shielding cavity threading metal pipeline is calculated.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic compatibility technology, and to a method for calculating the shielding performance of wired metal conduits. This method enables the calculation of the shielding performance of wired metal conduits within shielded cavities, serving the electromagnetic protection design of high-power equipment and high-voltage power distribution facilities. Background Technology

[0002] With the development of my country's economy and society, high-power and high-voltage equipment is becoming increasingly common, such as high-power motors for radio telescopes and high-voltage power distribution facilities. The high-performance electromagnetic protection design for these devices presents significant challenges. The difficulty lies in addressing the conducted radiation from transmission lines of high-power motors and high-voltage power distribution facilities. Traditional electromagnetic compatibility (EMC) design methods use signal filters or power filters to suppress conducted radiation from transmission lines. However, for high-power or high-voltage transmission lines, filter design is extremely difficult, costly, large in size, has low reliability, and is challenging to implement. To solve these problems, one approach is to lay the transmission line inside a metal conduit and extend the conduit. As electromagnetic waves propagate along the transmission line, they are continuously reflected and absorbed, reducing the energy of electromagnetic radiation and thus suppressing electromagnetic interference.

[0003] However, the problem with the above scheme is that we are unsure about the relationship between the length and cross-section of the pipe and the shielding performance, and there is currently no readily available calculation method for engineering scheme design. Summary of the Invention

[0004] The purpose of this invention is to provide a method for calculating the shielding performance of wired metal conduits, which integrates the dyadic Green's function, field-line coupling theory, and electromagnetic compatibility theory to calculate the shielding performance of wired metal conduits in shielded cavities.

[0005] To achieve the above objectives, the present invention provides a method for calculating the shielding performance of a wire-passing metal conduit, comprising:

[0006] Step S1: Determine the calculation object, which includes a shielding cavity, a radiation source located inside the shielding cavity, a transmission line that penetrates the shielding cavity and is connected to the radiation source, and a metal tube connected to the outer surface of the shielding cavity and sleeved on the transmission line.

[0007] Step S2: Determine the physical parameters of the object to be calculated;

[0008] Step S3: Determine the electric and magnetic fields inside the shielding cavity based on the coordinates of the radiation source and the current element in the physical parameters;

[0009] Step S4: Based on the results of the electric and magnetic fields inside the shielding cavity at the connection between the shielding cavity and the metal tube, obtain the radiation characteristics of the pipe hole at the connection between the shielding cavity and the metal tube, and determine the electric and magnetic fields inside the metal tube accordingly.

[0010] Step S5: The distributed current coupled in the transmission line is calculated by discretizing the transmission line into an equivalent circuit model composed of multiple cascaded lumped parameter circuit units; the distributed voltage source is determined in the equivalent circuit model based on the electric field inside the shielding cavity and the electric field inside the metal tube.

[0011] Step S6: Determine the leakage electric field and shielding effectiveness of point P in free space based on the distributed current coupled by the transmission line.

[0012] Step S2 specifically includes:

[0013] Step S21: Establish a rectangular coordinate system with one vertex of the shielding cavity as the origin;

[0014] Step S22: Determine the coordinates and current element of the radiation source;

[0015] Step S23: Determine the dimensions of the shielding cavity and the metal tube, and the coordinates of both ends of the shielding cavity and the metal tube;

[0016] Step S24: Determine the length and equivalent impedance of each segment of the transmission line;

[0017] In step S3, a current element is placed at the coordinates of the radiation source. The position vector is then set; subsequently, the electric and magnetic fields inside the shielded cavity are determined using the dyadic Green's function method.

[0018] Step S4 specifically includes:

[0019] Step S41: Obtain the radiation characteristics of the pipe hole at the connection between the shielding cavity and the metal pipe, wherein the radiation characteristics include the equivalent electric dipole moment P and the equivalent magnetic dipole moment M;

[0020] Step S42: Using equivalent electric dipole moment and equivalent magnetic dipole moment As the excitation source of the metal tube, the unit dipole moments of both are analyzed to obtain the electric and magnetic fields generated by the unit electric dipole moment along the y-axis and the electric and magnetic fields generated by the unit magnetic dipole moment along the x-axis.

[0021] Step S43: Determine the electric and magnetic fields inside the metal tube based on the electric and magnetic fields generated by the unit electric dipole moment along the y-axis and the unit magnetic dipole moment along the x-axis.

[0022] The expressions for the equivalent electric dipole moment P and the equivalent magnetic dipole moment M are:

[0023] ,

[0024] In the formula, is the polarization coefficient of the pore. The vacuum permittivity, and These are the polarization coefficients in the x and z directions of the pipe orifice, respectively. The normal electric field at the center of the pipe hole; The magnetic field in the x-direction at the center of the pipe hole;

[0025] The electric field produced by the unit electric dipole moment along the y-axis The magnetic field produced by the unit electric dipole moment along the y-axis for:

[0026] ,

[0027] ,

[0028] The electric field produced by a unit magnetic dipole moment along the x-axis and magnetic field for:

[0029] ,

[0030] ,

[0031] electric field inside the metal tube and magnetic field for:

[0032] ,

[0033] in, These are the current spatial coordinates. , , These are the center coordinates of the connection between the shielding cavity and the metal tube. The imaginary unit, Angular frequency, Let n be the free space permeability, and n, m, and l be the wave mode indices in the x, y, and z directions, respectively. , and These are the Neumann coefficients in the x, y, and z directions; k is the free-space wavenumber. , For the operating wavelength, The square of the free space wavenumber; , , For wavenumber components, , , The dimensions of the metal tube. Let be the total wave number, which satisfies ; , , ω represents the unit vectors in the x, y, and z directions, respectively; j is the imaginary unit, and ω is the angular frequency. is the free space conductivity.

[0034] Discretizing the entire transmission line into After constructing the equivalent circuit model by cascading the total parameter circuit units, the distributed current of the transmission line coupling is calculated using the formula for establishing the equivalent circuit model based on the Agrawal model.

[0035] The first segment of the transmission line to the second segment The segment is the first type of lumped parameter circuit unit corresponding to the internal region of the shielded cavity, and the transmission line's first... Section to the first The segment corresponds to the second type of lumped parameter circuit unit in the region where the metal tube is located; the transmission line's first... to The segment is the third type of lumped parameter circuit unit corresponding to the external region of the shielded cavity; the first and second types of lumped parameter circuit units both include distributed voltage sources, distributed capacitance, and distributed inductance; the third type of lumped parameter circuit unit all include distributed capacitance and distributed inductance.

[0036] Step S5 includes:

[0037] Step S51: Based on the Agrawal model, establish the formulas for the corresponding equivalent circuit models for lumped parameter circuit units including distributed voltage sources and lumped parameter circuit units without distributed voltage sources.

[0038] Step S52: Determine the distributed inductance L per unit length, the distributed inductance capacitance C per unit length, the length dy of the lumped parameter circuit unit, the coordinates of the center point of each lumped parameter circuit unit, and the distributed voltage source;

[0039] Step S53: Apply Kirchhoff's laws to the formulas of the equivalent circuit model established in step S51 to establish a set of equations for the equivalent circuit model. Solve the set of equations for the equivalent circuit model to obtain the distributed current of the transmission line coupling.

[0040] In step S51, according to the Agrawal model, the formula for the equivalent circuit model of a lumped-parameter circuit unit including distributed voltage sources is:

[0041] ,

[0042] For a lumped-parameter circuit unit without distributed voltage sources, the formula for its equivalent circuit model is:

[0043] ,

[0044] In the formula, The imaginary unit, Angular frequency, This is the scattering voltage, the value of which is determined by the scattering electric field generated by the conductor; Let L be the incident electric field along the direction of the conductor; L and C are the inductance and capacitance per unit length, respectively. The current is on the y-axis. The voltage is on the y-axis;

[0045] In step S52, the expressions for the distributed inductance L per unit length and the distributed inductance capacitance C per unit length are:

[0046] ,

[0047] Among them, subscript The subscript is the ordinal number of the lumped parameter circuit unit type. ; The transmission line grounding distance for a first-class lumped-parameter circuit unit is the height of the transmission line from the transmission line inside the shielded cavity to the bottom wall of the shielded cavity. The grounding distance of the transmission line in a second-type lumped-parameter circuit unit is the height of the transmission line in the metal tube from the bottom wall of the metal tube. The transmission line grounding distance for a third type of lumped parameter circuit unit is the height of the transmission line from the ground in free space. and These are the vacuum permeability and vacuum permittivity, respectively. The radius of the cable's cross-section;

[0048] Distributed voltage source of lumped parameter circuit unit in segment g. for:

[0049] ,

[0050] in, Let G be the electric field at the center point of the g-th lumped parameter circuit unit. The length of the lumped parameter circuit unit. The total number of segments in the first and second type lumped parameter circuit units;

[0051] In step S53, the equivalent circuit model equations are:

[0052] ,

[0053] In the formula, , For equivalent impedance, For resistance, For resistance, , ; , For the i-th segment of the lumped parameter circuit unit, there are distributed voltage sources and distributed currents. This represents the total number of segments in a first-class lumped-parameter circuit unit. This represents the total number of segments in the first and second types of lumped parameter circuit units. This represents the total number of segments in a lumped parameter circuit unit.

[0054] The distributed current of transmission line coupling is obtained by solving the equations of the equivalent circuit model. Specifically, this includes: converting the equations of the equivalent circuit model into matrix form, establishing the matrix equation of the distributed current of transmission line coupling, and obtaining the distributed current of transmission line coupling based on the matrix equation of the distributed current of transmission line coupling.

[0055] In step S6, the transmission line outside the cavity is divided into multiple differentially segmented cables, each of which is equivalent to an electric dipole antenna. The distributed current coupled by the transmission line is used as the excitation current element of each electric dipole antenna. The leakage electric field of point P in free space is obtained by superimposing the electric fields of the excitation current elements of each electric dipole antenna and the excitation current elements of the image current.

[0056] The method of this invention integrates the dyadic Green's function, field-line coupling theory, and electromagnetic compatibility theory to calculate the shielding performance of metal conduits passing through shielded cavities. It can quickly calculate the suppression effect of metal conduits of different physical sizes on interference signals. Compared with existing simulation software, it significantly improves the calculation speed and has higher accuracy, and can serve the design of electromagnetic protection schemes for transmission lines of high-voltage and high-power facilities.

[0057] The method of this invention, through comparative analysis of calculated and simulated data, takes into account the influence of low-frequency cutoff waveguide effects, and is therefore more suitable for calculating the shielding performance of the 500MHz-6GHz frequency band. It is also suitable for obtaining the transmission line protection equipment performance of high-power and low-power devices, such as transmission line protection equipment for high-power motors and high-voltage power distribution facilities. Attached Figure Description

[0058] Figure 1 This is a schematic diagram of the structure of the metal conduit to which the shielding performance calculation method for the metal conduit of the present invention is applicable.

[0059] Figure 2 This is a structural dimension diagram of the metal conduit to which the shielding performance calculation method for the metal conduit used in this invention is applicable.

[0060] Figure 3 This is a schematic diagram of the equivalent dipole moment model of a hole.

[0061] Figure 4 It is a topology diagram of the equivalent circuit model of a transmission line running through a metal conduit.

[0062] Figure 5 This is a diagram of a differential segmented cable electric dipole model.

[0063] Figure 6 This is a schematic diagram of the parameters of the numerical example and the CST simulation model.

[0064] Figure 7 This is a comparison chart of the results from the computational model and the simulation model. Detailed Implementation

[0065] The invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0066] This invention provides a method for calculating the shielding performance of wired metal conduits. It takes into account the influence of low-frequency cutoff waveguide effects, and is therefore more suitable for calculating the shielding performance of the 500MHz-6GHz frequency band. It is also suitable for obtaining the transmission line protection equipment performance of both high-power and low-power devices, such as transmission line protection equipment for high-power motors and high-voltage power distribution facilities.

[0067] According to an embodiment of the present invention, a method for calculating the shielding performance of a metal conduit for wiring includes the following steps:

[0068] Step S1: Determine the object of calculation; where, for example... Figure 1 As shown, the calculation object includes a shielded cavity 10, a radiation source 20 located inside the shielded cavity 10, a transmission line 30 that passes through the shielded cavity 10 and is connected to the radiation source 20, and a metal tube 40 that is connected to the outer surface of the shielded cavity 10 and sleeved on the transmission line 30.

[0069] The metal tube 40 can be a rectangular tube or a circular tube. In this embodiment, the metal tube 40 is installed to the shielding cavity 10 by welding (full welding) or by flange connection (a conductive rubber gasket needs to be installed in the flange groove, which is a mature method in the shielding field).

[0070] The length of the metal pipe 40 is greater than 10 times the cross-sectional dimension (when the cross-section of the metal pipe is circular, the cross-sectional dimension is the diameter of the cross-section; when the cross-section of the metal pipe is rectangular, the cross-sectional dimension is the length of the longer side of the cross-section; when the cross-section of the metal pipe is square, the cross-sectional dimension is the length of the side of the cross-section).

[0071] Therefore, the above-mentioned calculation objects can be used to calculate the shielding performance from the radiation source to the pipeline outlet, providing technical support for the design of electromagnetic protection schemes for transmission lines of high-power and low-power equipment.

[0072] Step S2: Determine the physical parameters of the object to be calculated;

[0073] Step S2 specifically includes:

[0074] Step S21: Establish a rectangular coordinate system with one vertex of the shielding cavity 10 as the origin;

[0075] The established rectangular coordinate system is as follows Figure 2 As shown.

[0076] Step S22: Determine the coordinates and current element of radiation source 20;

[0077] The coordinates of the center of radiation source 20 are ( , , Radiation source 20 is represented as a current element. ,in For current intensity, The length is the dipole length.

[0078] In this embodiment, its direction is the z-axis direction. In other embodiments, it can also be set to the x and y directions. In this case, the corresponding formula can be replaced by changing the coordinate system.

[0079] Step S23: Determine the dimensions of the shielding cavity 10 and the metal tube 40, and the coordinates of both ends of the shielding cavity 10 and the metal tube 40;

[0080] The dimensions of the shielding cavity 10 are × × When the metal pipe 40 is a rectangular pipe 1, the dimensions are as follows: × × When the pipe is circular, the diameter is D and the length is (y direction).

[0081] The center coordinates of the connection between the shielding cavity and the metal tube 40 are ( , , The coordinates of the other end of the metal tube 40 are ( , , ).

[0082] Step S24: Determine the length and equivalent impedance of each segment of transmission line 30.

[0083] Wherein, the radius of transmission line 30 is R, and the length of transmission line 30 within the shielded cavity is... The length of the transmission line 30 inside the metal tube is The length of transmission line 30 in free space is And the equivalent impedance of the cable is , . , The equivalent impedance is taken as follows: since our calculation frequency is 500MHz-6GHz, which is a high-frequency circuit field, the equivalent impedance is taken as 50Ω for coaxial and power lines, 100Ω for high-speed signal transmission lines (such as UPS, HDMI, Ethernet, PCIe, SATA, etc.), 120Ω for differential transmission lines of industrial fieldbuses such as RS485 and CAN bus, and 50Ω for other transmission lines.

[0084] Step S3: Determine the electric and magnetic fields inside the shielding cavity 10 based on the coordinates of the radiation source 20 and the current element in the physical parameters.

[0085] In step S3, a current element is placed at the coordinates of the radiation source 20. and set the position vector Subsequently, the electric and magnetic fields inside the shielded cavity 10 were determined using the dyadic Green's function method.

[0086] Among them, electric field It can be represented as:

[0087] (1)

[0088] Where j is the imaginary unit, and ω is the angular frequency. Permeability in free space Indicates the location vector The unit point source at the location The generated electric dyadic Green's function.

[0089] Substituting the coordinates and current element of the radiation source 20 from step S1, the electric field inside the shielding cavity 10 is obtained. The specific expression;

[0090] electric field inside shielded cavity 10 for:

[0091] (2)

[0092] Where j is the imaginary unit, and ω is the angular frequency. Let I be the free space permeability, and I be the current intensity. Let be the dipole length, n, m, and l be the wave mode indices in the x, y, and z directions, respectively; and k be the free-space wavenumber. , For the operating wavelength, The square of the free space wavenumber; , , For wavenumber components, , , , , , The dimensions of the shielding cavity, Let be the total wave number, which satisfies ; , and These are the Neumann coefficients in the x, y, and z directions. The value can be: when hour, ; hour, x, y, z represent the position coordinates of the electric field, , , () represents the location coordinates of the radiation source. , , These are unit vectors in the x, y, and z directions, respectively.

[0093] Similarly, the magnetic field inside the shielding cavity 10 for:

[0094] (3)

[0095] Where I is the current intensity, Let be the dipole length, n, m, and l be the wave mode indices in the x, y, and z directions, respectively; and k be the free-space wavenumber. , For wavelength, The square of the free space wavenumber; , , For wavenumber components, , , , , , The dimensions of the shielding cavity, Let be the total wave number, which satisfies ; , and These are the Neumann coefficients in the x, y, and z directions. The value can be: when hour, ; hour, x, y, z represent the position coordinates of the electric field, , , () represents the location coordinates of the radiation source. , , These are unit vectors in the x, y, and z directions, respectively.

[0096] As can be seen from formula (3), the magnetic field inside the shielding cavity 10 z-component It is 0.

[0097] Step S4: Based on the results of the electric and magnetic fields in the shielding cavity 10 at the connection between the shielding cavity 10 and the metal tube 40, obtain the radiation characteristics of the pipe hole at the connection between the shielding cavity 10 and the metal tube 40, and determine the electric and magnetic fields in the metal tube 40 accordingly.

[0098] Step S4 specifically includes:

[0099] Step S41: Obtain the radiation characteristics of the pipe hole at the connection between the shielded cavity 10 and the metal tube 40, wherein the radiation characteristics include the equivalent electric dipole moment P and the equivalent magnetic dipole moment M.

[0100] In this embodiment, as Figure 3 As shown, based on the hole equivalent dipole moment model, the equivalent electric dipole moment P is perpendicular to the hole plane (along the y-axis direction), and the equivalent magnetic dipole moment M is parallel to the hole plane (along the x-axis and z-axis directions).

[0101] The expressions for the equivalent electric dipole moment P and the equivalent magnetic dipole moment M are:

[0102] (4)

[0103] In the formula, is the polarization coefficient of the pore. The vacuum permittivity, and These are the polarization coefficients in the x and z directions of the pipe orifice, respectively. The polarization coefficients of the pipe orifice are given in Table 1. The normal electric field at the center of the pipe hole is calculated by formula (2); The magnetic field in the x-direction at the center of the pipe hole is calculated by formula (3).

[0104] Since the magnetic field value at any point in the z-direction is... The value is 0, therefore the magnetic field in the z-direction at the center of the pipe hole is 0. The value is zero.

[0105] Table 1: Polarization coefficients of pipe orifices of different shapes

[0106]

[0107] Step S42: Using equivalent electric dipole moment and equivalent magnetic dipole moment As the excitation source of the metal tube 40 (taking a rectangle as an example), the unit dipole moments of both are analyzed to obtain the electric and magnetic fields generated by the unit electric dipole moment along the y-axis and the electric and magnetic fields generated by the unit magnetic dipole moment along the x-axis.

[0108] Wherein, the electric field produced by the unit electric dipole moment along the y-axis direction The magnetic field produced by the unit electric dipole moment along the y-axis for:

[0109] (5)

[0110] (6)

[0111] in, These are the current spatial coordinates. , , These are the center coordinates of the connection between the shielding cavity and the metal tube. The imaginary unit, Angular frequency, Let n be the free space permeability, and n, m, and l be the wave mode indices in the x, y, and z directions, respectively. , and These are the Neumann coefficients in the x, y, and z directions. The value can be: when hour, ; hour, k is the free space wavenumber. , For the operating wavelength, The square of the free space wavenumber; , , For wavenumber components, , , , , , The dimensions of the metal tube. Let be the total wave number, which satisfies ; , , These are unit vectors in the x, y, and z directions, respectively.

[0112] According to equation (6), the magnetic field generated by the unit electric dipole moment along the y-axis direction is... y-component .

[0113] Due to the magnetic field inside the shielding cavity 10 z-component Therefore, the magnetic dipole moment exists only in the x-direction.

[0114] The electric field produced by a unit magnetic dipole moment along the x-axis and magnetic field for:

[0115] (7)

[0116] (8)

[0117] According to formula (8), the electric field generated by the unit magnetic dipole moment along the x-axis direction is... x-component .

[0118] Step S43: Determine the electric and magnetic fields inside the metal tube 40 based on the electric and magnetic fields generated by the unit electric dipole moment along the y-axis and the unit magnetic dipole moment along the x-axis.

[0119] electric field inside metal tube 40 and magnetic field for:

[0120] (9)

[0121] Similarly, for circular pipes, only the size parameters need to be changed, while everything else remains the same.

[0122] Step S5: Discretize the transmission line 30 into an equivalent circuit model composed of multiple cascaded lumped parameter circuit units to calculate the distributed current coupled in the transmission line; in the equivalent circuit model, determine the distributed voltage source based on the electric field inside the shielding cavity and the electric field inside the metal tube.

[0123] Radiation unit inside the shielded cavity ( This will generate an electromagnetic field. This electromagnetic field will induce a common-mode current on the transmission line 30 that passes through the cavity, thus turning the transmission line 30 into a conduction channel. The induced distributed current flows along the transmission line 30 to the external free space, making the transmission line 30 act as a secondary antenna, ultimately leading to electromagnetic energy radiation leakage. Therefore, the ultimate goal of step S5 is to calculate the distributed current coupled to the transmission line.

[0124] Based on transmission line theory, the entire transmission line 30 is discretized into... After constructing an equivalent circuit model by cascading the lumped-parameter circuit units, the formula for establishing the equivalent circuit model using the Agrawal model is used to calculate the distributed current of the transmission line coupling.

[0125] Specifically, the first segment to the second segment of transmission line 30 The segment is the first type of lumped parameter circuit unit corresponding to the internal region of the shielded cavity 10, and the transmission line 30 is the first Section to the first The segment corresponds to the second type of lumped parameter circuit unit in the area where the metal tube is located. Both the first type of lumped parameter circuit unit and the second type of lumped parameter circuit unit include a distributed voltage source. The distributed voltage source is generated by an electric field, and the value of the distributed voltage source is calculated and determined according to the formula of the distributed voltage source (formula (15)) and the electric field in the shielded cavity 10 (formula (2)) and the electric field in the metal tube 40 (formula (9)). to The segment corresponds to the third type of lumped parameter circuit unit in the external region of the shielded cavity 10. It has no external field excitation and therefore no distributed voltage source. All lumped parameter circuit units are simplified to a single-wire-to-ground structure. When the transmission line 30 is inside the shielded cavity, ground refers to the inner wall of the shielded cavity; when the transmission line 30 is inside a metal conduit, ground is the inner wall of the metal conduit; when the transmission line 30 is in free space, ground is the actual ground surface.

[0126] The topology of the equivalent circuit model of transmission line 30 is as follows: Figure 4 As shown. The lengths of the first and second type lumped parameter circuit units of transmission line 30 are both the length of the lumped parameter circuit unit dy, and each includes its own distributed voltage source V1, V2...V m The distributed capacitance Cdy and distributed inductance Ldy represent the distributed capacitance and distributed inductance over the length dy, respectively, with C and L representing the distributed capacitance and distributed inductance per unit length, respectively. The length of each type of lumped parameter circuit unit in transmission line 30 is the lumped parameter circuit unit length dy, and each includes both the distributed capacitance Cdy and the distributed inductance Ldy. It is important to note that the length of each segment of the lumped parameter circuit unit should be much smaller than the operating wavelength. This satisfies the validity condition of the lumped parameter approximation.

[0127] Step S5 includes:

[0128] Step S51: Based on the Agrawal model, establish the formulas for the corresponding equivalent circuit models for lumped parameter circuit units including distributed voltage sources and lumped parameter circuit units without distributed voltage sources.

[0129] According to the Agrawal model, for a lumped-parameter circuit unit including distributed voltage sources, the formula for its equivalent circuit model is:

[0130] (10)

[0131] For a lumped-parameter circuit unit without distributed voltage sources, the formula for its equivalent circuit model is:

[0132] (11)

[0133] In the formula, The imaginary unit, Angular frequency, This is the scattering voltage, the value of which is determined by the scattering electric field generated by the conductor; The incident electric field along the direction of the conductor corresponds to the magnitude of the distributed voltage source per unit length in a single-wire-to-ground structure, which can be obtained from formula (16) below; L and C are the distributed inductance and distributed capacitance per unit length, respectively; The current is on the y-axis. Let be the voltage on the y-axis.

[0134] Step S52: Determine the distributed inductance L per unit length, the distributed inductance capacitance C per unit length, the length dy of the lumped parameter circuit unit, the coordinates of the center point of each lumped parameter circuit unit, and the distributed voltage source;

[0135] In a single-wire-to-ground configuration, the distributed inductance L and distributed capacitance C per unit length vary with position. The expressions for the distributed inductance L and distributed capacitance C per unit length are:

[0136] (12)

[0137] Among them, subscript The subscript is the ordinal number of the lumped parameter circuit unit type. ; The transmission line grounding distance for a first-class lumped-parameter circuit unit is the height of the transmission line from the transmission line inside the shielded cavity to the bottom wall of the shielded cavity. The grounding distance of the transmission line in a second-type lumped-parameter circuit unit is the height of the transmission line in the metal tube from the bottom wall of the metal tube. The transmission line grounding distance for a third type of lumped parameter circuit unit is the height of the transmission line from the ground in free space. and These are the vacuum permeability and vacuum permittivity, respectively. The radius of the cable cross-section.

[0138] Because the height of the transmission line from the bottom wall of the shielded cavity to the bottom wall of the metal tube is different from that of the transmission line in the metal tube, the calculated distributed inductance L and distributed capacitance C per unit length are different. Therefore, the capacitance and inductance of the transmission line at the metal tube and the cavity are different.

[0139] The entire transmission line is evenly divided into 30 sections. In the case of segments, the length of lumped parameter circuit units for:

[0140] (13)

[0141] in, The total number of segments in a lumped parameter circuit unit. , and These refer to the lengths of the transmission line 30 inside the shielded cavity, inside the metal tube, and in free space, respectively.

[0142] The coordinates of the center point of the g-th lumped parameter circuit unit ( , , The following formula is satisfied:

[0143] (14)

[0144] In this configuration, transmission line 30 coincides with radiation source 20 along the x-axis and is arranged along the y-axis. The y-coordinate of the starting end of transmission line 30 is... ; The transmission line grounding distance for a first-class lumped-parameter circuit unit is the height of the transmission line from the transmission line inside the shielded cavity to the bottom wall of the shielded cavity. The grounding distance of the transmission line in a second-type lumped-parameter circuit unit is the height of the transmission line in the metal tube from the bottom wall of the metal tube. The transmission line grounding distance of the third type of lumped parameter circuit unit is the height of the transmission line from the ground in free space.

[0145] Since the cables are distributed along the y-axis, the distributed voltage source of the lumped parameter circuit unit in segment g is... for:

[0146] (15)

[0147] in, Let G be the electric field at the center point of the g-th lumped parameter circuit unit. The length of the lumped parameter circuit unit. This represents the total number of segments in the first and second type lumped parameter circuit units.

[0148] Step S53: Apply Kirchhoff's laws to the formulas of the equivalent circuit model established in step S51 to establish a set of equations for the equivalent circuit model. Solve the set of equations for the equivalent circuit model to obtain the distributed current of the transmission line coupling.

[0149] The equivalent circuit model equations are as follows:

[0150] (16)

[0151] In the formula, , For equivalent impedance, For resistance, For resistance, , The values ​​of L and C can be determined by equation (12); , The distributed voltage source and distributed current of the lumped parameter circuit unit in segment i are given by formula (15). The distributed voltage source of the lumped parameter circuit unit in segment g is determined by formula (15). To obtain the electric field components within it. The value of needs to be distinguished between the two cases where the cable is located in the cavity or in the pipeline, and is determined by formula (2) and formula (9); This represents the total number of segments in a first-class lumped-parameter circuit unit. This represents the total number of segments in the first and second types of lumped parameter circuit units. This represents the total number of segments in a lumped parameter circuit unit.

[0152] The distributed current of transmission line coupling is obtained by solving the equations of the equivalent circuit model. Specifically, this includes: converting the equations of the equivalent circuit model into matrix form, establishing the matrix equation of the distributed current of transmission line coupling, and obtaining the distributed current of transmission line coupling based on the matrix equation of the distributed current of transmission line coupling.

[0153] The equivalent circuit model equations in matrix form are as follows:

[0154] (17)

[0155] The matrix equation for the distributed current in transmission line coupling is as follows:

[0156] (18)

[0157] in, For the impedance matrix, It is the inverse of the impedance matrix. The matrix of distributed current, It is a matrix of distributed voltage sources.

[0158] Therefore, the ultimate goal of step S5 is to calculate the distributed current of the transmission line coupling, which is represented as a matrix of distributed currents. In step S6, the distributed current from step S5 is used to calculate the leakage field strength at point P in free space.

[0159] Step S6: Determine the leakage electric field and shielding effectiveness of point P in free space based on the distributed current coupled by the transmission line.

[0160] The coordinates of point P in free space are ( , , Its electric field strength is mainly generated by the radiation of the distributed current coupled through the transmission line. Therefore, as... Figure 5 As shown, in step S6, the transmission line outside the cavity is divided into multiple differential segment cables, each of which is equivalent to an electric dipole antenna. The distributed current coupled to the transmission line obtained by formula (18) is used as the excitation current element of each electric dipole antenna. The leakage electric field at point P in free space is obtained by superimposing the electric fields of the excitation current elements of each electric dipole antenna and the excitation current element of the image current. That is, each segment of the electric dipole antenna is represented as an excitation current element. The magnitude I of the current element is the distributed current coupled by the transmission line.

[0161] When a segmented cable is equivalent to an electric dipole antenna, the electric field expression of the electric dipole antenna is:

[0162] (19)

[0163] in, The imaginary unit, Angular frequency, The vacuum permittivity, Represents the excitation current element. The amplitude of the current element. The length of the current element. For wave number, Let P be the distance from the current element to the field point P. Let be the angle between the line connecting the current element and the field point and the y-axis. The angle between the line connecting the current element and the field point and the x-axis.

[0164] Therefore, given the coordinates of point P, the electric field components in any direction at that point can be calculated.

[0165] Considering the influence of the ground plane, according to the mirror principle, the total radiation field of point P in free space is the vector sum of the electric fields of the excitation current elements of each electric dipole antenna and their mirror current excitation current elements. Let the coordinates of the g-th current element be... The coordinates of the mirror current element g' of the g-th current element are determined by equation (14). Then the leakage electric field at point P in free space can be expressed as:

[0166] (20)

[0167] In the formula, and Let g' be the electric field generated at point P in free space by the mirror current element g'. The excitation current element of the mirror current is opposite in direction to the excitation current element of the original electric dipole antenna, has the same magnitude, and their positions are symmetrical with respect to the ground plane.

[0168] Determining the shielding effectiveness of point P in free space specifically includes: obtaining the electric field inside the shielding cavity 10 according to formula (2). And the leakage electric field of point P in free space obtained from formula (20) According to the definition of shielding effectiveness, taking the logarithm and subtracting the two values, we can obtain the shielding effectiveness as follows:

[0169] (twenty one)

[0170] It should be noted that the leakage electric field and shielding effectiveness of point P in free space calculated in step S6 are related to the position coordinates of point P. Therefore, selecting different points P will lead to different shielding effectiveness calculation results.

[0171] Experimental results:

[0172] The model dimensions of the wire-threaded metal conduit of the present invention are as follows: Figure 6 As shown (unit: mm). The excitation current element is located at coordinates (250, 300, 150), with an amplitude of 1 A·m and a direction along the z-axis. The cavity dimensions are 500mm × 600mm × 300mm. The rectangular cross-section pipe dimensions are 80mm × 800mm × 50mm; the diameter of the circular cross-section pipe is 70mm. The cable radius is 3mm, and the total length is 800mm. The interface between the cavity and the pipe is located at (250, 600, 75). The coordinates of monitoring point P are (250, 1800, 75). The height parameters are h1=75mm, h2=25mm, h3=75mm; the cable segment lengths are l1=200mm, l2=800mm, l3=200mm, and the total number of segments is n0=120. The terminal impedance is set to Z1=Z2=50Ω. The simulation frequency range is 150–3000MHz, and the sampling interval is 50MHz.

[0173] The algorithm of this invention is compared with the electric field results of the three-dimensional electromagnetic field simulation software CST on pipes of different shapes and diameters. Figure 7As shown, the two match well, verifying the correctness of the leakage mechanism analysis and radiation electric field calculation method when cables pass through pipes of different shapes.

[0174] Under fixed simulation frequency band, computing platform (CPU: Intel(R) Xeon(R) Gold 6240C, 36 threads) and mesh size (1.5×10⁻⁶), the following conditions were met. 7 Under the condition of (unit), the comparison results shown in Table 2 show that the method of this patent has a smaller amount of simulation calculation compared with CST, which can significantly shorten the calculation time and significantly improve the calculation efficiency.

[0175] Table 2: Comparison of computation time between the method of the present invention and existing CST simulation methods

[0176]

[0177] The method of this invention integrates the dyadic Green's function, field-line coupling theory, and electromagnetic compatibility theory to calculate the shielding performance of metal conduits passing through shielded cavities. It can quickly calculate the suppression effect of metal conduits of different physical sizes on interference signals. Compared with existing simulation software, it significantly improves the calculation speed and has higher accuracy, and can serve the design of electromagnetic protection schemes for transmission lines of high-voltage and high-power facilities.

[0178] The method of this invention, through comparative analysis of calculated and simulated data, takes into account the influence of low-frequency cutoff waveguide effects, and is therefore more suitable for calculating the shielding performance of the 500MHz-6GHz frequency band. It is also suitable for obtaining the transmission line protection equipment performance of high-power and low-power devices, such as transmission line protection equipment for high-power motors and high-voltage power distribution facilities.

[0179] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Various variations can be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made based on the claims and description of this invention fall within the protection scope of the claims of this patent. All aspects not described in detail in this invention are conventional technical content.

Claims

1. A method for calculating the shielding performance of a wire-carrying metal conduit, characterized in that, include: Step S1: Determine the calculation object, which includes a shielding cavity, a radiation source located inside the shielding cavity, a transmission line that penetrates the shielding cavity and is connected to the radiation source, and a metal tube connected to the outer surface of the shielding cavity and sleeved on the transmission line. Step S2: Determine the physical parameters of the object to be calculated; Step S3: Determine the electric and magnetic fields inside the shielding cavity based on the coordinates of the radiation source and the current element in the physical parameters; Step S4: Based on the results of the electric and magnetic fields inside the shielding cavity at the connection between the shielding cavity and the metal tube, obtain the radiation characteristics of the pipe hole at the connection between the shielding cavity and the metal tube, and determine the electric and magnetic fields inside the metal tube accordingly. Step S5: The distributed current coupled in the transmission line is calculated by discretizing the transmission line into an equivalent circuit model composed of multiple cascaded lumped parameter circuit units; the distributed voltage source is determined in the equivalent circuit model based on the electric field inside the shielding cavity and the electric field inside the metal tube. Step S6: Determine the leakage electric field and shielding effectiveness of point P in free space based on the distributed current coupled by the transmission line.

2. The method for calculating the shielding performance of a wire-threaded metal conduit according to claim 1, characterized in that, Step S2 specifically includes: Step S21: Establish a rectangular coordinate system with one vertex of the shielding cavity as the origin; Step S22: Determine the coordinates and current element of the radiation source; Step S23: Determine the dimensions of the shielding cavity and the metal tube, and the coordinates of both ends of the shielding cavity and the metal tube; Step S24: Determine the length and equivalent impedance of each segment of the transmission line.

3. The method for calculating the shielding performance of a wire-threaded metal conduit according to claim 1, characterized in that, In step S3, a current element is placed at the coordinates of the radiation source. The position vector is then set; subsequently, the electric and magnetic fields inside the shielded cavity are determined using the dyadic Green's function method.

4. The method for calculating the shielding performance of a wire-threaded metal conduit according to claim 1, characterized in that, Step S4 specifically includes: Step S41: Obtain the radiation characteristics of the pipe hole at the connection between the shielding cavity and the metal pipe, wherein the radiation characteristics include the equivalent electric dipole moment P and the equivalent magnetic dipole moment M; Step S42: Using equivalent electric dipole moment and equivalent magnetic dipole moment As the excitation source of the metal tube, the unit dipole moments of both are analyzed to obtain the electric and magnetic fields generated by the unit electric dipole moment along the y-axis and the electric and magnetic fields generated by the unit magnetic dipole moment along the x-axis. Step S43: Determine the electric and magnetic fields inside the metal tube based on the electric and magnetic fields generated by the unit electric dipole moment along the y-axis and the unit magnetic dipole moment along the x-axis.

5. The method for calculating the shielding performance of a wire-threaded metal conduit according to claim 4, characterized in that, The expressions for the equivalent electric dipole moment P and the equivalent magnetic dipole moment M are: ， In the formula, is the polarization coefficient of the pore. The vacuum permittivity, and These are the polarization coefficients in the x and z directions of the pipe orifice, respectively. The normal electric field at the center of the pipe hole; The magnetic field in the x-direction at the center of the pipe hole; The electric field produced by the unit electric dipole moment along the y-axis The magnetic field produced by the unit electric dipole moment along the y-axis for: ， ， The electric field produced by a unit magnetic dipole moment along the x-axis and magnetic field for: ， ， electric field inside the metal tube and magnetic field for: ， in, These are the current spatial coordinates. , , These are the center coordinates of the connection between the shielding cavity and the metal tube. The imaginary unit, Angular frequency, Let n be the free space permeability, and n, m, and l be the wave mode indices in the x, y, and z directions, respectively. , and These are the Neumann coefficients in the x, y, and z directions; k is the free-space wavenumber. , For the operating wavelength, The square of the free space wavenumber; , , For wavenumber components, , , The dimensions of the metal tube. Let be the total wave number, which satisfies ; , , ω represents the unit vectors in the x, y, and z directions, respectively; j is the imaginary unit, and ω is the angular frequency. is the free space conductivity.

6. The method for calculating the shielding performance of a wire-threaded metal conduit according to claim 1, characterized in that, Discretizing the entire transmission line into After constructing the equivalent circuit model by cascading the total parameter circuit units, the distributed current of the transmission line coupling is calculated using the formula for establishing the equivalent circuit model based on the Agrawal model. The first segment of the transmission line to the second segment The segment is the first type of lumped parameter circuit unit corresponding to the internal region of the shielded cavity, and the transmission line's first... Section to the first The segment corresponds to the second type of lumped parameter circuit unit in the region where the metal tube is located; the transmission line's first... to The segment is the third type of lumped parameter circuit unit corresponding to the external region of the shielded cavity; the first and second types of lumped parameter circuit units both include distributed voltage sources, distributed capacitance, and distributed inductance; the third type of lumped parameter circuit unit all include distributed capacitance and distributed inductance.

7. The method for calculating the shielding performance of a wire-threaded metal conduit according to claim 6, characterized in that, Step S5 includes: Step S51: Based on the Agrawal model, establish the formulas for the corresponding equivalent circuit models for lumped parameter circuit units including distributed voltage sources and lumped parameter circuit units without distributed voltage sources. Step S52: Determine the distributed inductance L per unit length, the distributed inductance capacitance C per unit length, the length dy of the lumped parameter circuit unit, the coordinates of the center point of each lumped parameter circuit unit, and the distributed voltage source; Step S53: Apply Kirchhoff's laws to the formulas of the equivalent circuit model established in step S51 to establish a set of equations for the equivalent circuit model. Solve the set of equations for the equivalent circuit model to obtain the distributed current of the transmission line coupling.

8. The method for calculating the shielding performance of a conduit for wiring according to claim 7, characterized in that, In step S51, according to the Agrawal model, the formula for the equivalent circuit model of a lumped-parameter circuit unit including distributed voltage sources is: ， For a lumped-parameter circuit unit without distributed voltage sources, the formula for its equivalent circuit model is: ， In the formula, The imaginary unit, Angular frequency, This is the scattering voltage, the value of which is determined by the scattering electric field generated by the conductor; Let L be the incident electric field along the direction of the conductor; L and C are the inductance and capacitance per unit length, respectively. The current is on the y-axis. The voltage is on the y-axis; In step S52, the expressions for the distributed inductance L per unit length and the distributed inductance capacitance C per unit length are: ， Among them, subscript The subscript is the ordinal number of the lumped parameter circuit unit type. ; The transmission line grounding distance for a first-class lumped-parameter circuit unit is the height of the transmission line from the transmission line inside the shielded cavity to the bottom wall of the shielded cavity. The grounding distance of the transmission line in a second-type lumped-parameter circuit unit is the height of the transmission line in the metal tube from the bottom wall of the metal tube. The transmission line grounding distance for a third type of lumped parameter circuit unit is the height of the transmission line from the ground in free space. and These are the vacuum permeability and vacuum permittivity, respectively. The radius of the cable's cross-section; Distributed voltage source of lumped parameter circuit unit in segment g. for: ， in, Let G be the electric field at the center point of the g-th lumped parameter circuit unit. The length of the lumped parameter circuit unit. The total number of segments in the first and second type lumped parameter circuit units; In step S53, the equivalent circuit model equations are: ， In the formula, , For equivalent impedance, For resistance, For resistance, , ; , For the i-th segment of the lumped parameter circuit unit, there are distributed voltage sources and distributed currents. This represents the total number of segments in a first-class lumped-parameter circuit unit. This represents the total number of segments in the first and second types of lumped parameter circuit units. This represents the total number of segments in a lumped parameter circuit unit.

9. The method for calculating the shielding performance of a conduit for wiring according to claim 7, characterized in that, The distributed current of transmission line coupling is obtained by solving the equations of the equivalent circuit model. Specifically, this includes: converting the equations of the equivalent circuit model into matrix form, establishing the matrix equation of the distributed current of transmission line coupling, and obtaining the distributed current of transmission line coupling based on the matrix equation of the distributed current of transmission line coupling.

10. The method for calculating the shielding performance of a wire-threaded metal conduit according to claim 1, characterized in that, In step S6, the transmission line outside the cavity is divided into multiple differentially segmented cables, each of which is equivalent to an electric dipole antenna. The distributed current coupled by the transmission line is used as the excitation current element of each electric dipole antenna. The leakage electric field of point P in free space is obtained by superimposing the electric fields of the excitation current elements of each electric dipole antenna and the excitation current elements of the image current.

Images