A Simulation Method for Transient Electromagnetic Fields in the Lightning Environment of Ships

Abstract

The present invention discloses a method for simulating the transient electromagnetic field of the lightning environment of a ship, comprising the following steps: 1) Make the following assumptions for the lightning return stroke process, and consider the return stroke channel as an ideal vertical antenna; 2) Obtain the longitudinal electric field and angular magnetic field generated by the return stroke current at any time and any position when the return stroke current is known by the dipole method; 3) Model the return stroke current using the transmission line return stroke model; 4) Apply the Heidler function model to describe the characteristics of the lightning current, and combine with the transmission line return stroke model to obtain the analytical expression of the lightning current; 5) Substitute the analytical expression of the lightning current into the electric field and magnetic field formulas to obtain the electromagnetic field generated by the return stroke current at any time and any position. The simulation method of the present invention is applied to the overall design of the ship, and the electric field and magnetic field generated at the ship when lightning occurs in the ocean can be obtained, which is used to analyze and study the influence on microelectronic devices, and can further support the lightning protection design of the ship and the layout of shipboard equipment.

Description

Technical Field

[0001] The present invention relates to electromagnetic field simulation technology, and in particular to a transient electromagnetic field simulation method for the lightning environment of a ship. Background Art

[0002] Lightning generates a strong mechanical effect and thermal effect in its return stroke channel and its vicinity, and at the same time generates extremely harmful lightning induction. Lightning induction can be divided into electrostatic induction and electromagnetic induction. When a ship is in the electric field formed between a thundercloud and the ocean, a large amount of charges (referred to as bound charges) opposite in nature to the thundercloud will be induced on the metal parts of the ship. During the main discharge of the thundercloud, the electric field between the cloud and the sea suddenly disappears, and the charges on the metal parts do not have time to flow away immediately, thus generating a very high ground potential, which is called "electrostatic induction". At this time, the bound charges on the conductor become free charges and flow towards both ends of the conductor, forming an induced overvoltage wave.

[0003] Due to the extremely large amplitude and steepness of the lightning current, a transient magnetic field is generated, and the transient magnetic field will generate an induced electric field with a very high amplitude. Microelectronic devices have a low operating voltage and are very sensitive to electromagnetic pulses. Therefore, lightning electromagnetic pulses interfere with microelectronic devices within a certain range and are the greatest threat to microelectronic devices. The harm of lightning has expanded from the ship itself to the electrical and electronic equipment inside the cabin. Studying electromagnetic induction, simulating and emulating lightning electromagnetic fields, is of great significance for strengthening the lightning protection of ships. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a transient electromagnetic field simulation method for the lightning environment of a ship in view of the defects in the prior art.

[0005] The technical solution adopted by the present invention to solve its technical problems is: a transient electromagnetic field simulation method for the lightning environment of a ship, including the following steps:

[0006] 1) Make the following assumptions about the lightning return stroke process and consider the return stroke channel as an ideal vertical antenna;

[0007] Assumptions: The sea is an ideal planar conductor; there is no current reflection at the strike point; the instantaneous current at the strike point is known; the return wavefront has not reached the top of the channel;

[0008] 2) Obtain the longitudinal electric field and angular magnetic field generated by the return stroke current at any time and any position when the return stroke current is known by the dipole method;

[0009] When the return stroke current i(z', t) is known, the longitudinal electric field E and the angular magnetic field z generated by the return stroke current (divided into countless dipole current sources) at time t and position are respectively:

[0010]

[0011]

[0012] Among them, H is the height of the return stroke channel, z' is the height of the return stroke wavefront, R is the distance between the return stroke wavefront and the observation point, ε0 is the vacuum conductivity, μ0 is the magnetic permeability, and c represents the speed of light;

[0013] 3) Model the return stroke current using the transmission line return stroke model;

[0014] i(z',t) = u(t - z' / v f )P(z')i(0,t - z' / v)

[0015] 4) Apply the Heidler function model to describe the characteristics of the lightning current, and combine it with the transmission line return stroke model to obtain the analytical expression of the lightning current;

[0016]

[0017] Among them, I0 is the peak current, η is the current correction factor, n is the current steepness factor, τ1 is the front time constant, and τ2 is the tail time constant;

[0018] 5) Substitute the analytical expression of the lightning current into the electric field and magnetic field formulas to obtain the electromagnetic fields generated by the return stroke current at any time and any position.

[0019] The beneficial effects produced by the present invention are:

[0020] The simulation method of the present invention is applied to the overall design of ships, and the electric and magnetic fields generated at the ship during lightning strikes in the ocean can be obtained, which are used to analyze the impact on microelectronic devices, and can further support the lightning protection design of ships, and has reference significance for the layout of shipboard equipment. Description of the Drawings

[0021] The following will further illustrate the present invention in conjunction with the drawings and embodiments. In the drawings:

[0022] Figure 1 is the flowchart of the method of the embodiment of the present invention. Detailed Embodiments

[0023] In order to make the objectives, technical solutions and advantages of the present invention clearer, the following further details the present invention in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] Such as Figure 1As shown in the figure, a transient electromagnetic field simulation method for the lightning environment of a ship includes the following steps:

[0025] Step 1: Make assumptions about the lightning return stroke process

[0026] Lightning is a random event with non-repeatability. Each lightning strike is not exactly the same, and the return stroke channel and the surrounding environment where the lightning occurs also have strong randomness. For the convenience of theoretical and experimental research, the following assumptions are made about the lightning return stroke process before establishing the model:

[0027] (1) The vertical lightning channel is an ideal transmission line;

[0028] (2) The sea is an ideal planar conductor;

[0029] (3) There is no current reflection at the strike point;

[0030] (4) The instantaneous current at the strike point is known;

[0031] (5) The return stroke wavefront has not reached the top of the channel.

[0032] Step 2: Obtain the longitudinal electric field and azimuthal magnetic field generated by the return stroke current

[0033] The return stroke channel is considered to be an ideal vertical antenna. The electromagnetic field generated by the transient current on the ideal vertical conductor (lightning channel) on the ideal conductive plane (sea) can be divided into an electrostatic field component (E Q ), an induced field component (E i , H i ) and a far-field component (i.e., radiation field, E di , H di ). Among them, E Q , E i , E di are the corresponding electric field strengths respectively, and H i , H di are the corresponding magnetic field strengths respectively. The lightning electromagnetic field is the sum of the fields generated by the transient current i(z,t) on an infinite number of infinitesimal vertical dipoles dz in the return stroke channel.

[0034] Assume that the height of the return stroke channel is H, the height of the return stroke wavefront is z’, and the distance between the return stroke wavefront and the observation point is R. The vacuum permittivity, permeability, and speed of light are represented by ε0, μ0, and c respectively. In the cylindrical coordinate system, when the return stroke current i(z',t) is known, the longitudinal electric field E and the azimuthal magnetic field z generated by the return stroke current (divided into an infinite number of dipole current sources) at time t and position are respectively:

[0035]

[0036]

[0037] The first term in Equation (2.1) is the electrostatic field, the second term is the induced field, and the third term is the radiation field; the first term in Equation (2.2) is the induced field, and the second term is the magnetic radiation field.

[0038] Step 3: Model the return stroke current using the transmission line return stroke model

[0039] Different return stroke models can be obtained based on different assumptions about the current parameters in the lightning return stroke channel. Since the transmission line return stroke model is the most widely used return stroke model, there are mainly the following types: TL model, MTLL model, MTLE model, and MTLD model.

[0040] (1) TL model: The return stroke current flows on a lossless transmission line, and the return stroke speed is equal to the propagation speed of the return stroke current and is affected by the height.

[0041] (2) MTLL model: Based on the TL model, consider that the amplitude of the return stroke current linearly decays with the increase in height.

[0042] (3) MTLE model: Based on the TL model, consider that the amplitude of the return stroke current exponentially decays with the increase in height.

[0043] (4) MTLD model: Based on the TL model, consider that the amplitude and rise time of the return stroke current linearly decay with the increase in height.

[0044] These models can all be described by a simplified expression, that is:

[0045] i(z',t) = u(t - z' / v f )P(z')i(0,t - z' / v)

[0046] Different return stroke models can be obtained from different parameters P(z'). The parameters of the four transmission line type return stroke models are shown in the following table.

[0047] Table 1 Parameters of Four Transmission Line Type Return Stroke Models

[0048]

[0049] MTLL: H is the total height of the return stroke channel.

[0050] MTLE: λ is the current decay constant. (λ = 2000m.)

[0051] MTLD: τ(z' / λ p)Control the rise time of the channel current (the rise time increases linearly with the increase of height z'), and (1 - z' / H) controls the decay of the channel current. τ = 0.3s, λ p = 1000m.

[0052] Step 4: Propose an analytical expression for the channel current of the return stroke model

[0053] Lightning current, as the main characteristic of lightning, provides an important reference basis for lightning protection research. The present invention uses the widely used Heidler function model to describe the characteristics of lightning current.

[0054] Based on the Heidler model and the transmission line return stroke model, an analytical expression for lightning current for analysis is proposed:

[0055]

[0056] In the formula, I0 is the peak current, η is the current correction factor, n is the current steepness factor, τ1 is the wavefront time constant, τ2 is the wave tail time constant, and each parameter in the formula allows independent selection.

[0057] The channel current of the lightning return stroke model is a function of time t and height h.

[0058] Step 5: Electromagnetic field calculation

[0059] The present invention selects the Heidler function to describe the current at the bottom of the return stroke channel, and calculates the electromagnetic fields generated by four typical transmission line return stroke models respectively.

[0060] (1) Taking the channel current as the input quantity, the return stroke speed is taken as 1.3×10 8 m / s, and substitute it into the general expression of the electromagnetic field obtained by the dipole method. Calculate the electromagnetic fields at distances of 50m, 5km, and 100km respectively, and simulate the scenarios when lightning occurs on the ship, near the ship, and at a relatively long distance respectively, to obtain the calculated electromagnetic fields.

[0061] (2) When the return stroke speeds are taken as 0.5×10 8 m / s, 1×10 8 m / s, 2×10 8 m / s respectively, substitute them into the general expression of the electromagnetic field obtained by the dipole method, and calculate the electromagnetic fields at 50m, 5km, and 100km respectively.

[0062] Analyze the calculated electric field and magnetic field waveform diagrams, and it can be seen that:

[0063] The electromagnetic field at 50 m has the following characteristics: the electric field is mainly composed of the electrostatic field and the induced field. As the return stroke speed increases, the amplitude of the near-field electric field decreases, and the variation trend after the initial peak changes from a slow decline to being basically unchanged and finally to a slow rise. The magnetic field is mainly composed of the induced field, and the waveform of the magnetic field is basically the same as that of the current at the bottom of the channel because the magnetic field here is mainly the induced field determined by the current at the bottom of the channel. The peak time of the magnetic field also decreases as the return stroke speed increases.

[0064] The electromagnetic field at 5 km has the following characteristics: the electric field is mainly composed of the electrostatic field, and both the induced field and the radiation field are influencing factors. The radiation field determines the amplitude of the electric field, and its amplitude is proportional to the speed. The magnetic field is composed of the superposition of the induced field and the radiation field. The radiation field determines the amplitude of the magnetic field, and its amplitude is proportional to the speed.

[0065] The waveforms of the electromagnetic fields at 100 km are basically the same because the electromagnetic fields here are mainly composed of the radiation field.

[0066] It should be understood that for those of ordinary skill in the art, improvements or transformations can be made according to the above description, and all such improvements and transformations should fall within the protection scope of the appended claims of the present invention.

Claims

1. A transient electromagnetic field simulation method for the lightning environment of a ship, characterized in that, Including the following steps: 1) Make the following assumptions for the lightning return stroke process and consider the return stroke channel as an ideal vertical antenna; Assumptions: The sea is an ideal planar conductor; there is no current reflection at the strike point; the instantaneous current at the strike point is known; The return wavefront has not reached the top of the channel; 2) Obtain the longitudinal electric field and azimuthal magnetic field generated by the return current at any time and any position when the return current is known by the dipole method; When the return stroke current \(i(z',t)\) is known, the longitudinal electric field \(E\) generated by the return stroke current at time \(t\) and position z and the azimuthal magnetic field are respectively: where, H is the height of the return stroke channel, z’ is the height of the return wavefront, R is the distance between the return wavefront and the observation point, ε0 is the vacuum permittivity, μ0 is the permeability, and c represents the speed of light; 3) Use the transmission line return stroke model to model the return current; 4) Apply the Heidler function model to describe the characteristics of the lightning current, and combine with the transmission line return stroke model to obtain the analytical expression of the lightning current; Wherein, I0 is the peak current, η is the current correction factor, n is the current steepness factor, τ1 is the front time constant, and τ2 is the tail time constant; 5) Substitute the analytical expression of the lightning current into the electric field and magnetic field formulas to obtain the electromagnetic field generated by the return current at any time and any position.

2. The transient electromagnetic field simulation method for the lightning environment of a ship according to claim 1, wherein In step 3), the following several transmission line return stroke models are adopted: TL model, MTLL model, MTLE model, and MTLD model.

3. The transient electromagnetic field simulation method for the lightning environment of a ship according to claim 1, characterized in that In step 3), using the transmission line return stroke model to model the return current is uniformly simplified to the following model; i(z',t) = u(t - z' / v f )P(z')i(0,t - z' / v) where P(z') is a parameter of the transmission line return stroke model; v f is the electromagnetic wave propagation velocity; i represents the return stroke current.

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