CN112348359A - Method for determining safety design index of steel pipe tower grounding device of urban power transmission line - Google Patents
Method for determining safety design index of steel pipe tower grounding device of urban power transmission line Download PDFInfo
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Abstract
The invention provides a method for determining safety design indexes of an urban transmission line steel pipe tower grounding device, which comprises the following steps: the method comprises the steps that a typical grounding design of the existing transmission steel pipe tower is taken as a model, and safety indexes (step voltage and contact voltage) of a transmission tower grounding device are provided by combining relevant regulations and human short circuit resistance parameters; simulating and calculating safety index values under the lightning impulse short circuit and the power frequency short circuit by using CDEGS grounding calculation software; and obtaining key indexes influencing the grounding safety of the tower by comparing the target value with the actual value, and providing target parameters for safety design.
Description
Technical Field
The invention relates to the field of safety design of grounding devices of steel tube towers of urban power transmission lines.
Background
In the current-stage urban power transmission network, the problems of economic benefit and the like are considered, and the overhead line form is still adopted more. The steel tube tower is a typical tower type of an urban transmission line tower. The urban power transmission steel pipe towers are mostly distributed in flower beds at the roadside of a horse or in the center of a road, and belong to a people flow dense area. When a power frequency or lightning grounding short circuit fault occurs to the urban power transmission steel pipe tower, the ground potential around the tower is rapidly increased, and dangerous step voltage and contact voltage are generated around the tower, so that the personal safety is threatened. Therefore, in addition to the ground resistance, safety indexes such as contact voltage and step voltage should be considered as the ground index of the urban steel pipe tower. The safety design of the grounding device for the urban power transmission steel pipe tower has important significance for ensuring the safety of surrounding personnel.
Safety design and research of grounding devices at home and abroad mainly aim at grounding grids of power plants and transformer substations. By adjusting and controlling grids inside the rectangular ground net and paving asphalt, gravel and the like on the ground surface, the grounding safety parameters such as peripheral potential gradient, equipment contact voltage, step voltage and the like meet the requirement of a safety value. The research on the grounding of the power transmission line tower mainly focuses on the grounding resistance reduction aspect of high soil resistivity areas (particularly mountain rock areas), and novel grounding resistance reduction methods such as a needle-punching type grounding device, a flexible graphite grounding electrode, a long vertical grounding electrode and a hollow grounding device are provided.
However, the safety optimization design of the existing urban transmission tower lacks targeted index parameters, such as grounding safety indexes caused by lightning strike or power frequency grounding short circuit, namely, the optimization design object of the grounding safety indexes is unclear.
Disclosure of Invention
In order to solve the problems, the invention provides a method for determining a safety design index of an urban power transmission line steel pipe tower grounding device, which is used as a reference index of the urban power transmission line steel pipe tower grounding device in a safety design stage, so that the design of the grounding device is ensured to meet the safety specification, and a reasonable optimization target value is provided for the subsequent optimization process of the grounding device.
In order to solve the problems, the technical scheme provided by the invention is as follows:
a method for determining safety design indexes of an urban power transmission line steel tube tower grounding device comprises the following steps:
taking the typical grounding design of the existing transmission steel pipe tower as an object, and combining related regulations and human short circuit resistance parameters, providing a target value of safety indexes (step voltage and contact voltage) of a transmission tower grounding device; simulating and calculating the actual values of the safety indexes under the lightning impulse short circuit and the power frequency short circuit by using CDEGS grounding calculation software; and obtaining key indexes influencing the grounding safety of the tower by comparing the target value with the actual value, and providing target parameters for safety design.
In summary, the determination method proposed by this patent is divided into 3 steps:
step 1: calculating the safety index target value of the urban steel tube tower grounding device:
(1) safety index-power frequency step voltage and power frequency equipment contact voltage during power frequency grounding short circuit
According to the distribution parameter equivalent circuit of the human body in the electric shock accident, when the human walks on the ground, the contact resistance R of two feet of the human body and the groundFAnd a human body resistance RBConnected in series to obtain a step voltage U between two pinssComprises the following steps:
Us＝(RB+6ρ)×Ik (1)
in the formula: rBIs body resistance, rho is surface layer soil resistivity, IkIs the safe current of human body under different body weights k. Therefore, when no one stands on the ground, the potential difference between the two points corresponding to the two feet, namely the step voltage VsComprises the following steps:
when a person stands on the ground and contacts the metal conductor of the grounding body by hands, the contact resistance between the two feet of the person and the soil is in parallel connection, and the actual voltage between the hands and the feet of the person is the withstand contact voltage UTComprises the following steps:
UT＝(RB+1.5ρ)×Ik (3)
in the formula: rBIs body resistance, rho is surface layer soil resistivity, IkIs the safe current of human body under different body weights k. Therefore, when a person is not standing there, the potential difference between the two points, i.e. the contact voltage VTComprises the following steps:
according to the formulas (2) and (4), the target values of the power frequency step voltage and the contact voltage of the power frequency equipment can be calculated.
(2) Safety index of lightning flowing into ground
According to related research results, the harm of human body under the action of power frequency current of 50Hz-60Hz is the most serious, and the harm is greatly reduced when the current frequency is lower or higher than 50Hz-60 Hz. According to the amplitude-frequency characteristic of the lightning current, the energy of the lightning current is mainly concentrated in the middle-low frequency part below tens of thousands of Hz, so that 10kHz can be selected as a calculation reference point of the human body safety current.
When the frequency of the current is 10kHz, the threshold value of the human body feeling current can reach 13 times of the power frequency, so that the corresponding withstand voltage is 13 times of the power frequency. According to the calculation formula of the power frequency contact voltage and the step voltage, the safety step voltage allowed by the human body between two points when the lightning flows into the ground can be obtained by combining the frequency characteristics of the lightning current, and the calculation formula is as follows:
in the formula: i iskIs the safe current of human body under different body weights k.
The formula for calculating the safe contact voltage allowed by the human body between two points when the lightning flows into the ground is as follows:
in the formula: i iskIs the safe current of human body under different body weights k.
And (4) calculating the step voltage under the lightning stroke and the target value of the contact voltage of the equipment under the lightning stroke according to the formula (5) and the formula (6).
Step 2: calculating the actual safety index value of the grounding device in typical design
According to the actual design of the existing urban power transmission steel tube tower grounding device and parameters such as related soil and materials, the grounding device is built by utilizing an SECAD module in grounding calculation software CDEGS, and the step voltage and the contact voltage of the steel tube tower grounding device under the condition that power frequency grounding short circuit and lightning flow into the ground are 2 faults are calculated by the model device.
(1) A method for calculating the step voltage and the contact voltage of a grounding device during power frequency short circuit.
The MALZ module of CDEGS grounding calculation simulation software can be used for calculation, and the module can realize frequency domain grounding analysis of any soil structure.
1) The soil model (resistivity), the calculation frequency, the conductor type and the excitation type are defined in the definition options of the sescd tool. The method is characterized in that a soil model (resistivity) is based on a model (resistivity) of soil where an actual measurement grounding device is located, the frequency is 50Hz, the conductor type is based on the material and the size of an actual grounding body, a power frequency current source is selected according to the excitation type, and the power frequency current is the ground short circuit current I actually flowing into the tower grounding devicer. When a power frequency grounding short circuit fault occurs in the system, a part of fault current flows to the periphery through the overhead ground wire to be diffused; and a part of fault current is discharged into the ground through the tower grounding device after passing through the overhead ground wire. Research shows that the shunt coefficient of the tower grounding device is influenced by tower grounding resistance, the number of transmission line towers and substation grounding resistance, so that IrThe calculation method of (2) is as follows:
Ir＝(IS-Ig)×Kd (7)
in the formula: i isSIs power frequency fault current when the system is single-phase grounded, IgFor the current to diffuse all around through the overhead earth wire, KdThe value of the current shunt coefficient of the short circuit of the grounding device of the tower can be obtained by looking up relevant literature data.
2) Establishing a grounding device simulation model according to a grounding model shown in a typical design drawing of the grounding device of the existing urban power transmission steel pipe tower by using the conductor material obtained by the definition as a grounding body material through an object establishing option in an SESCAD tool; and a short-circuit current excitation point and a reasonable observation line are arranged, and the coverage area of the observation line is required to be larger than the size of the whole grounding device.
3) And changing the definition and the model updating into an Input Toolbox (Input Toolbox), returning to an Input panel of the MALZ, and clicking a running button to obtain a simulation result in the viewing output. By looking at the compute-scalar potential-step voltage/contact voltage-plot options in the output panel, profiles of step voltage and contact voltage under power frequency short circuits and their maximum values can be obtained.
(2) The lightning inflow ground fault parameters are calculated by utilizing FFTSES and HIFREQ modules, and the method comprises the following steps:
1) a time domain lightning signal, namely a lightning current function, is added in an FFTSES module, the function adopts a double-exponential function waveform, and the mathematical expression of the function is as follows:
i(t)＝kIm(e-αt-e-βt) (8)
in the formula: i (t) is the instantaneous value of lightning current at time t, ImThe amplitude of lightning current flowing into the tower grounding device; k is the regulating coefficient of the lightning current peak value, and alpha and beta are parameters set according to the wave front time and the wave tail time, so that the response of the electromagnetic field is obtained by calculating frequencies recommended by the module.
According to the industry standard, the empirical formula of the probability P that the lightning current amplitude exceeds I in the general area of China is as follows:
therefore, the value of I can be taken according to the probability P of lightning current (by determining the lightning protection probability P of the power facility)1The corresponding P should be (1-P)1) Within). Because the tower shunting action under the lightning fault is influenced by the distribution parameters of the line and the tower, the input time domain lightning signal amplitude is as follows:
Im＝KI (10)
in the formula: and K is the shunt coefficient of the tower grounding device under the lightning fault.
2) Constructing a model of the grounding device of the urban power transmission steel pipe tower in the HIFREQ module, and obtaining the electric field frequency domain response of the grounding device by using the calculation frequency recommended by the forward transformation of the FFTSES module;
3) and (3) obtaining the electric field time domain response under the lightning current function by utilizing the Fourier inversion of the FFTSES module, obtaining the contact voltage and the step voltage value required in the electric field response, and checking the mode in the third step in the step (1).
And step 3: obtaining key indexes influencing the grounding safety of the urban steel tube tower:
comparing the target value of the safety index calculated by the formulas (1) to (6) with the actual value of the safety index calculated according to a typical design, determining whether the index values under the conditions of power frequency faults and lightning faults meet the standard, and carrying out discussion and analysis on the result so as to determine the key index influencing the grounding safety of the urban power transmission steel pipe tower.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:
FIG. 1 is a basic type of a grounding body of a linear steel tube tower according to a preferred embodiment of the present invention;
FIG. 2 is a schematic diagram of a simulation model of a steel tube tower grounding body according to a preferred embodiment of the invention;
FIG. 3 is a power frequency step voltage distribution two-dimensional color block diagram of the preferred embodiment of the present invention;
fig. 4 is a two-dimensional color block diagram of power frequency contact voltage distribution in accordance with a preferred embodiment of the present invention.
The reference numerals in the figures denote:
1. the earth surface; 2. an earth current point; 3. a horizontal ground body; 4. vertical ground body spacing; 5. a concrete foundation; 6. a vertical ground body; 7. and (5) burying deeply.
Detailed Description
The following detailed description of embodiments of the invention, but the invention can be practiced in many different ways, as defined and covered by the claims.
In the method provided by the embodiment, a person with the weight of 50kg is used as a standard, and the safety specification index value under the power frequency short circuit and the lightning impulse is calculated according to a formula; based on the existing 110kV urban power transmission steel pipe tower grounding device type, a simplified model is built by using CDEGES software, and actual index values under power frequency and lightning impulse are obtained through simulation; by comparing the calculated value with the actual value, the safety design index of the steel tube tower grounding device of the urban power transmission line is determined, and the key index influencing the safety of the grounding device is obtained. In this embodiment, taking the actual grounding device type of the urban power transmission steel pipe tower as an example, the specific steps in execution are as follows:
step 1: calculating the safety index value of the urban steel tube tower grounding device:
(1) safety index during power frequency grounding short circuit
According to the distribution parameter equivalent circuit of the human body in the electric shock accident, when the human walks on the ground, the contact resistance R of two feet of the human body and the groundFAnd a human body resistance RBConnected in series to obtain a step voltage U between two pinssComprises the following steps:
Us＝(RB+6ρ)×Ik (1)
in the formula: rBIs body resistance, rho is surface layer soil resistivity, IkIs the safe current of human body under different body weights k. When no one stands on the ground, the potential difference between the two points corresponding to the two feet, namely the step voltage VsComprises the following steps:
according to the statistical result of the test, the current value I of the heart fibrillation can be obtained under the power frequency condition that the human body can bear large current without appearingkThe following formula is satisfied:
in the formula: t is the power frequency fault duration; the coefficient k is an energy coefficient related to the body weight of the human body. Since the example selects a person with a weight of 50kg as the standard, the corresponding energy coefficient K50When the formula (3) is substituted with 0.0135, the compound can be obtained
Substituting the formula (4) into the formula (2) can obtain the allowable safe step voltage V of the human bodysComprises the following steps:
the following can be obtained in the same way: when a person stands on the ground and contacts the metal conductor of the grounding body by hands, the contact resistance between the two feet of the person and the soil is in parallel connection, and the actual voltage between the hands and the feet of the person is the withstand contact voltage UTComprises the following steps:
UT＝(RB+1.5ρ)×Ik (6)
selecting a person with the weight of 50kg as a standard, and allowing the person to safely contact with the voltage VTComprises the following steps:
looking up related data: resistance of human body RBAnd taking 1000 omega, taking the power frequency fault duration t as 0.25s, and taking the soil resistivity rho as 500 omega m. Substituting the parameters into a formula (5) and a formula (7), and calculating to obtain a step voltage safety value of 3712V and a contact voltage safety value of 710.5V which are allowed by the human body under the power frequency.
(2) Safety index of lightning flowing into ground
According to related research results, the harm of human body under the action of power frequency current of 50Hz-60Hz is the most serious, and the harm is greatly reduced when the current frequency is lower or higher than 50Hz-60 Hz. According to the amplitude-frequency characteristic of the lightning current, the energy of the lightning current is mainly concentrated in the middle-low frequency part below tens of thousands of Hz, so that 10kHz can be selected as a calculation reference point of the human body safety current.
When the frequency of the current is 10kHz, the threshold value of the human body feeling current can reach 13 times of the power frequency, so that the corresponding withstand voltage is 13 times of the power frequency. According to the calculation formula of the power frequency contact voltage and the step voltage, the safety step voltage allowed by the human body between two points when the lightning flows into the ground can be obtained by combining the frequency characteristics of the lightning current, and the calculation formula is as follows:
in the formula: i iskIs the safe current of human body under different body weights k.
The formula for calculating the safe contact voltage allowed by the human body between two points when the lightning flows into the ground is as follows:
in the formula: i iskFor the safe current of human body under different body weights k
The standard for selecting a person with a weight of 50kg is as follows:
substituting equation (10) into equation (8) to equation (9) yields:
as above, the resistance R of the human bodyBAnd taking 1000 omega, taking the power frequency fault duration t as 0.25s, and taking the soil resistivity rho as 500 omega m. Substituting the parameters into a formula (11), and calculating to obtain the allowable step voltage safety value of 1.97 multiplied by 10 of the human body under the lightning impulse7V, contact voltage safety value of 3.78 x 105V。
Step 2: the CDEGS software simulates and calculates the actual safety index value of the grounding device:
the basic type of the existing 110kV urban power transmission steel tube tower grounding body is shown in the attached drawing 1, an SESCAD module in grounding calculation software CDEGS is utilized to simplify and build a grounding body model shown in the attached drawing 2, and actual values of two indexes, namely step voltage and contact voltage, of the steel tube tower grounding device are obtained through simulation calculation under the conditions of power frequency grounding short circuit and 2 faults of lightning inflow to the ground.
(1) And calculating power frequency fault parameters by using an MALZ module:
firstly, defining a soil model, a calculation frequency, a conductor type and an excitation type by an SESCAD module: the soil model is a uniform soil model, and the urban resistivity rho is 500 omega · m; the grounding conductor is made of round steel with the relative resistivity of 1 and the relative magnetic permeability of 636; according to the consulted data, when the 110kv system has a short-circuit fault, when the tower grounding resistance is 10 omega and the number of the towers is more than 15, the shunt coefficient of the tower grounding device is 0.4, so that the power frequency excitation source current with the amplitude of 5280A is adopted by calculation.
And modeling the model by means of creating an object according to the typical design of the existing urban power transmission steel pipe tower grounding device, and building a simulation model as shown in figure 2. Wherein the total length of the horizontal grounding electrode is 88m, and the buried depth is 0.8 m; the total number of the vertical grounding electrodes is 23, and each vertical grounding electrode is 2m in length; and setting short circuit current excitation points and reasonable observation lines.
Finally, fault calculation under power frequency short circuit is carried out on the model by using an MALZ module, the maximum power frequency step voltage value of the grounding device model is 5319.1V, and the voltage distribution two-dimensional color block diagram is shown in the attached figure 3; the contact voltage of the grounding body is calculated by selecting the potential difference between 2 points of the tower body which is 1.8 meters away from the ground and 0.8m away from the tower from the ground, the maximum value of the power frequency contact voltage is 13898V, and the voltage distribution two-dimensional color block diagram is shown in figure 4.
(2) The lightning inflow ground fault parameters are calculated by utilizing FFTSES and HIFREQ modules, and the method comprises the following steps:
firstly, adding a time domain lightning signal, namely a lightning current function, in an FFTSES module, wherein the function adopts a double-exponential function waveform, and the mathematical expression of the function is as follows:
i(t)＝kIm(e-αt-e-βt) (12)
in the formula: i (t) is the instantaneous value of lightning current at time t, k is the regulating coefficient of the lightning current peak, and alpha and beta are parameters set according to the wave front time and the wave tail time. Lightning protection recommended by national regulationsThe waveform parameters were calculated as 2.6/50 μ s standard lightning current waveform, where k is 1.0474, α is 14790.18, and β is 1877833. When the lightning protection probability of the power facility is 98.5%, the shunt coefficient of the corresponding tower grounding device is taken to be 0.08, and the amplitude I of the lightning current input into the tower grounding device and entering the ground is calculatedm20 kA. The forward transformation obtains 16 equal difference frequencies under the condition that the recommended frequency under the lightning current function is 0-3413333.5 Hz.
And then constructing a simplified model of the grounding device of the urban power transmission steel pipe tower shown in the figure 2 in the HIFREQ module, introducing the 16 arithmetic frequency, and keeping the rest settings consistent with the grounding model and parameter settings under the power frequency, thereby obtaining the electric field frequency domain response of the grounding device.
And finally, obtaining the electric field time domain response under the lightning current function by utilizing the Fourier inverse transformation of the FFTSES module. Calculating a simulation result to obtain a maximum value of step voltage of the grounding device under the lightning stroke fault of 43437V; and the contact voltage of the grounding body is calculated by selecting the potential difference between 2 points of the tower body and the ground, wherein the distance between the tower body and the ground is 1.8 m, and the distance between the ground and the tower is 0.8m, so that the maximum value of the contact voltage under the lightning stroke fault is 180144V.
And step 3: determining safety design indexes of the steel tube tower grounding device of the urban power transmission line:
according to the above example steps, the safety index value under formula calculation can be compared with the actual index value under simulation calculation, and the following important conclusions can be drawn:
(1) under the power frequency short circuit fault, the actual value of the step voltage of the grounding body obtained through simulation calculation is 5310.1V, and exceeds the safe step voltage value of 3712V obtained through formula calculation; the actual value of the contact voltage calculated by simulation is 13898V, which exceeds the safe contact voltage value of 710.5V, namely, the two indexes of the contact voltage and the step voltage under the power frequency fault do not meet the safety requirement.
(2) Under the lightning strike impact current, the actual values of the contact voltage and the step voltage of the grounding body are both smaller than the safe voltage value calculated by a formula, namely the contact voltage and the step voltage under the lightning strike current both meet the safety requirement.
From the above comparison it can be derived: the key problem influencing the grounding safety of the urban steel pipe tower is to reduce the power frequency stepping voltage and the contact voltage of power frequency equipment. For the power frequency contact voltage, if the surface soil resistivity is increased, the contact voltage safety value is also increased, and the contact electric shock is possible only when people contact the tower body. According to the formula (7), if insulating paint with high resistivity is coated in the range that the distance between the steel tube tower body and the ground is 2 meters, the resistivity rho of the soil is increased, and the contact voltage safety value of the tower can be effectively increased; for the power frequency step voltage, the power frequency step voltage needs to be reduced to be lower than a safety requirement value through subsequent related optimization design, and therefore personal safety of surrounding personnel is guaranteed.
In conclusion, a person with the weight of 50kg is taken as a standard, and the specific parameters are substituted into a formula for calculation, so that a safety index value capable of ensuring personal safety is obtained; simplifying and modeling the grounding device of the steel tube tower of the 110kV urban transmission line by using CDEGS grounding software, and simulating to obtain an actual index value of the grounding device; whether the safety design index of the steel tube tower grounding device of the urban power transmission line meets the requirement or not can be effectively determined by comparing the results of the two types of the grounding device, namely whether the contact voltage and the step voltage value under the two fault conditions of power frequency and lightning grounding meet the safety requirement or not can be effectively determined, and therefore reasonable reference standards and design ideas are provided for the subsequent safety design of the grounding device.
The above description is only a preferred example of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (5)
1. A method for determining safety design indexes of an urban transmission line steel pipe tower grounding device is characterized in that the safety indexes (step voltage and contact voltage) of the transmission tower grounding device are provided by taking the typical grounding design of the existing transmission steel pipe tower as a model and combining related regulations and human short circuit resistance parameters; simulating and calculating safety index values under the lightning impulse short circuit and the power frequency short circuit by using CDEGS grounding calculation software; and obtaining key indexes influencing the grounding safety of the tower by comparing the target value with the actual value, and providing target parameters for safety design.
2. The method for determining the safety design index of the steel tube tower grounding device of the urban transmission line according to claim 1, wherein the potential difference between two points corresponding to two feet, namely the step voltage V, is obtained according to the distribution parameter equivalent circuit of the human body during the electric shock accident when no one stands on the groundsComprises the following steps:
when a person is not standing on the position, the potential difference between the two points is the contact voltage VTComprises the following steps:
according to the statistical result of the test, the current value I of the heart fibrillation can be obtained under the power frequency condition that the human body can bear large current without appearingkThe following formula is satisfied:
in the formula: t is the power frequency fault duration; the coefficient k is an energy coefficient related to the body weight of the human body.
3. The method for determining the safety design index of the urban transmission line steel tube tower grounding device according to claim 1, wherein the safety step voltage allowed by a human body between two points when lightning flows into the ground is calculated according to the formula:
in the formula: i iskIs the safe current of human body under different body weights k.
The formula for calculating the safe contact voltage allowed by the human body between two points when the lightning flows into the ground is as follows:
4. the method for determining the safety design index of the urban transmission line steel tube tower grounding device according to claim 1, is characterized in that the method for calculating the step voltage and the contact voltage of the grounding device during power frequency short circuit comprises the following steps:
1) and the CDEGS is grounded and calculated by an MALZ module of simulation software. The soil model (resistivity), the calculation frequency, the conductor type and the excitation type are defined by the sescd module. The soil model (resistivity) takes the model (resistivity) of the soil where the grounding device is actually measured as a value basis; the conductor type is based on the material and size of the actual grounding body, and the excitation type is selected from a power frequency current source, wherein the power frequency current is the ground short-circuit current I actually flowing into the grounding devicer。
IrThe calculation method of (2) is as follows:
Ir＝(IS-Ig)×Kd (7)
in the formula: i isSIs power frequency fault current when the system is single-phase grounded, IgFor the current to diffuse all around through the overhead earth wire, KdThe value of the current shunt coefficient of the short circuit of the grounding device of the tower can be obtained by looking up relevant literature data.
2) Establishing a grounding device simulation model according to a grounding model shown in a typical design drawing of the grounding device of the existing urban power transmission steel pipe tower by using the conductor material obtained by the definition as a grounding body material through an object establishing option in an SESCAD tool; setting short circuit current excitation point and reasonable observation line, and the coverage area of observation line is larger than the size of whole grounding device
3) And changing the definition and the model updating into an input box (InputToolbox) and returning to an input panel of the MALZ, and clicking a running button to obtain a simulation result in the viewing output.
5. The method for determining the safety design index of the urban transmission line steel tube tower grounding device according to claim 1, wherein the lightning inflow ground fault parameter is calculated by using FFTSES and HIFREQ modules, and the method comprises the following steps:
1) a time domain lightning signal, namely a lightning current function, is added in an FFTSES module, the function adopts a double-exponential function waveform, and the mathematical expression of the function is as follows:
i(t)＝kIm(e-αt-e-βt) (8)
in the formula: i (t) is the instantaneous value of lightning current at time t, ImIs the lightning current amplitude; k is the regulating coefficient of the lightning current peak value, and alpha and beta are parameters set according to the wave front time and the wave tail time, so that the response of the electromagnetic field is obtained by calculating frequencies recommended by the module.
According to the industry standard, the empirical formula of the probability P that the lightning current amplitude exceeds I in the general area of China is as follows:
therefore, the value of I can be taken according to the probability P of lightning current (by determining the lightning protection probability P of the power facility)1The corresponding P should be (1-P)1) Within). Because the tower shunting action under the lightning fault is influenced by the distribution parameters of the line and the tower, the input time domain lightning signal amplitude is as follows:
Im＝KI (10)
in the formula: and K is the shunt coefficient of the tower grounding device under the lightning fault.
2) Building a model of the grounding device of the urban power transmission steel pipe tower in the HIFREQ module, and obtaining the electric field frequency domain response of the grounding device by using the calculation frequency recommended by the forward transformation of the FFTSES module;
3) and obtaining the electric field time domain response under the lightning current function by utilizing the Fourier inverse transformation of the FFTSES module, and obtaining the contact voltage and the step voltage value required in the electric field response.
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