CN108075380B - Converter station grounding grid design method capable of replacing direct current grounding electrode and grounding grid - Google Patents

Abstract

The invention discloses a converter station grounding grid design method capable of replacing a direct current grounding electrode and a grounding grid. A, arranging a first feeder cable to be connected to a converter station grounding network from a valve hall grounding point; B. calculating the corrosion thickness of the grounding grid conductor under the conditions of monopolar earth running current and bipolar unbalanced current, and determining the section size of the grounding grid conductor; calculating to obtain the contact potential difference of the grounding grid, if the contact potential difference exceeds a preset contact potential difference limit value under the condition that the direct current transmission earth returns the running current, adding a grounding grid conductor at the position where the contact potential difference exceeds the limit value, and reducing the contact potential difference to be within the contact potential difference limit value; C. and establishing connection between the grounding equipment of the converter station grounding network and the converter station grounding network by using a second feeder cable. The converter station grounding grid established by the invention can simultaneously bear the functions of the traditional converter station grounding grid and the direct current grounding electrode, and has the advantages of safety, reliability, low cost and high benefit.

Description

Design method and grounding grid of converter station grounding grid that can replace DC grounding electrode

technical field

The invention relates to the technical field of direct current transmission, in particular to a design method for a grounding grid of a converter station that can replace a direct current grounding electrode, and the grounding grid of the converter station.

Background technique

The DC grounding electrode is an important part of the DC power transmission project. During unipolar operation, it directly transmits power to the system as part of the loop. During bipolar operation, it clamps the neutral point potential of the converter station (rectifier valve) to avoid the voltage difference between the two poles and the ground. balance and damage the device. With the rapid development of UHVDC transmission projects in my country, the location of DC grounding electrodes is becoming more and more difficult. In the actual project construction and construction process, due to external reasons such as land acquisition and site selection, the grounding electrode cannot be put into operation as scheduled, which affects the smooth operation of the bipolar HVDC transmission project and seriously hinders the economic and social benefits of large-capacity and long-distance power transmission. At present, few scholars, research institutions or equipment manufacturers at home and abroad have carried out research on other grounding devices that can replace the DC grounding electrode. The research on it is undoubtedly forward-looking and practical.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a design method for the grounding grid of the converter station and the grounding grid of the converter station that can replace the DC grounding electrode, which can make full use of the existing grounding grid of the converter station to realize the UHVDC power transmission project. It can reduce the construction investment of the grounding grid as much as possible and ensure the safe and reliable operation of the power grid.

In order to solve the above technical problems, the present invention provides a design method for the grounding grid of the converter station that can replace the DC grounding electrode, including:

A. Set the first feeder cable to connect from the grounding point of the valve hall to the grounding grid of the converter station;

B. Calculate the corrosion thickness of the grounding grid conductor under unipolar maximum operating current and bipolar unbalanced current, and determine the cross-sectional size of the grounding grid conductor according to the corrosion thickness;

According to the structure size of the grounding grid of the converter station, the cross-sectional size of the conductor, and the soil resistivity model of the surveyed site of the converter station, the contact potential difference of the grounding grid is calculated and obtained;

If the contact potential difference exceeds the preset contact potential difference limit when the DC transmission ground returns to the operating current, a grounding grid conductor is added at the position where the contact potential difference exceeds the limit value, so that the contact potential difference falls within the contact potential difference limit;

C. Use the second feeder cable to establish a connection between the grounding equipment of the grounding grid of the converter station and the grounding grid of the converter station.

Further, there are four first feeder cables, one end of the first feeder cable is respectively connected to the four corners of the grounding grid of the converter station, and the other end is connected to the grounding point of the valve hall.

Further, it also includes:

Calculate the maximum potential difference in the network,

If the superposition value of the maximum potential difference in the network exceeds the preset potential difference limit of the equipment safety network under the AC short-circuit fault into the ground current and the DC into the ground current, the conductive medium should be laid in the cable trench where the secondary cable is buried.

Further, it also includes:

According to the preset limit of potential difference in the safety net of the equipment, set the limit of unipolar allowable running time in combination with the temperature rise;

Calculate the maximum current density J _m and grounding resistance R under the unipolar maximum operating current, and further calculate the unipolar allowable operating time t _m in combination with the maximum allowable temperature rise,

When t _m is less than the set single-pole allowable operating time limit, a conductor connected to the grounding grid of the converter station is arranged outside the grounding grid of the converter station, and coke is laid near the conductor to make t _m reach the required single-pole allowable operation. time.

Further, under the unipolar maximum operating current, the contact potential difference is limited to 50V.

Further, the limit value of the potential difference in the safety network of the equipment is: the limit value of the potential difference in the network that the secondary equipment withstands does not exceed 2.8kV.

Further, the unipolar allowable running time limit is 5h.

Further, the method for laying a conductive medium in a cable trench where a secondary cable is buried includes:

Add a preset number of conductive media in the horizontal and vertical middle of the grounding grid, and in the middle of the surrounding outer grids;

Calculate the maximum potential difference in the network again. If the superposition value of the maximum potential difference in the network still exceeds the preset value of the potential difference in the equipment safety network, continue to add a group of conductive media between the adjacent grids of the conductive medium added last time. The potential difference is less than the preset potential difference limit within the equipment safety net.

Further, the conductive medium laid in the cable trench is electrically connected to the grounding grid, and the cross-sectional dimension of the conductive medium is not less than 220 mm ² .

Further, the cross-sectional size of the coke laid around the conductor is in the range of 0.1m*0.1m～1m*1m, wherein the size of the coke is proportional to the current density of the conductor when the coke is laid.

The present invention also provides a grounding grid for a converter station, comprising: a grounding grid body, a first feeder cable, a second feeder cable, grounding equipment, and a conductive medium, wherein,

the first feeder cable is used to connect the valve hall grounding point and the grounding grid body;

the second feeder cable is used to connect the grounding equipment of the grounding grid of the converter station and the grounding grid body;

The first calculation module is used to calculate the corrosion thickness of the grounding grid conductor under the unipolar extreme operating current and the bipolar unbalanced current, and determine the cross-sectional size of the grounding grid conductor according to the corrosion thickness;

The second calculation module is used to calculate and obtain the contact potential difference of the grounding grid according to the structural size of the grounding grid of the converter station, the cross-sectional size of the conductor, and the soil resistivity model of the surveyed site of the converter station;

The grounding grid conductor is laid at the position where the contact potential difference exceeds the limit value and is connected to the grounding grid body.

Further, it also includes:

The third calculation module is used to calculate the maximum potential difference in the network;

Conductive medium shall be laid in the cable trench where the secondary cable is buried when the superposition value of the maximum potential difference in the network exceeds the preset value of the potential difference in the equipment safety network under the AC short-circuit fault into the ground current and the DC into the ground current.

Further, it also includes:

The fourth calculation module is used to calculate the maximum current density J _m and the grounding resistance R under the unipolar maximum operating current, and further calculate the unipolar allowable operating time t _m in combination with the maximum allowable temperature rise;

The conductor is used to be arranged at the periphery of the grounding grid of the converter station and connected to the grounding grid of the converter station when t _m is less than the set unipolar allowable operating time limit;

Coke, laid around the conductor.

In the present invention, the current of the valve hall is led to the ground through the grounding grid of the converter station through the first feeder cable, and the current of each equipment in the grounding grid of the original converter station is led to the ground through the second feeder cable. To increase the grounding grid conductor, the grounding grid of the converter station can be established, and the functions of the traditional converter station grounding grid and the DC grounding electrode can be assumed. The grounding grid makes full use of the existing grounding grid, saves the additional land acquisition and construction of the DC grounding pole, and ensures the safety of people and equipment in the converter station. It has obvious economic and social benefits and can be used in HVDC transmission projects. project.

Description of drawings

FIG. 1 is a flow chart of a method for designing a grounding grid of a converter station that can replace a DC grounding electrode according to the present invention.

FIG. 2 is a schematic diagram of the connection point between the first feeder cable and the grounding grid in the present invention.

FIG. 3 is a partial enlarged view of the connection point between the first feeder cable and the grounding grid shown in FIG. 2 .

FIG. 4 is a schematic diagram of reconstruction of a grounding grid and its encrypted grounding grid according to an embodiment of the present invention.

FIG. 5 is a diagram of node numbers of a network for calculating the potential difference in a network according to an embodiment of the present invention.

6 is a schematic diagram of burying a copper tape in a secondary cable trench according to an embodiment of the present invention.

FIG. 7 is a schematic diagram of increasing the area of the grounding grid of the converter station according to an embodiment of the present invention.

FIG. 8 is a schematic diagram of laying a coke bed for the edge conductor of the grounding grid according to an embodiment of the present invention.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

During the process of the invention, the inventors noticed:

Although the current into the ground of the DC grounding pole is relatively large, the single pole running time of the current DC power transmission project is getting shorter and shorter. The grounding grid of the converter station is usually established in a place with low soil resistivity, and has the characteristics of large area, large number of conductors, and strong current capacity. It has the function of circulating DC current, thereby replacing the DC grounding electrode and eliminating the construction of additional DC grounding electrodes, which will have significant economic benefits. At present, the design of the grounding grid of the converter station is mainly used to ensure the safety of personnel and equipment under system fault conditions, to flow lightning strikes and fault short-circuit currents, and to provide grounding for primary and secondary equipment casings. Its design control indicators are mainly grounding resistance, contact potential difference and step potential difference level under the maximum short-circuit current. In the design of the grounding grid, the situation of providing a large current return path as DC is not considered. Therefore, if the grounding grid of the converter station is directly used instead of the bipolar operation of the grounding pole, it will bring many problems.

In addition, in the case of replacing the DC ground electrode, a strong DC current will be injected into the ground grid through the ground electrode during single-pole operation, and an unbalanced current will flow through the ground grid for a long time during two-pole operation. When the DC current is dissipated through the grounding grid, it not only causes the pole site potential to rise, but also causes problems such as step potential difference, contact potential difference, and soil heating under DC conditions. The neutral point grounding transformer within the range of the grounding pole current field will DC bias occurs, affecting the safe operation of the power grid. At the same time, the buried grounding grid conductor will also suffer a certain degree of corrosion due to DC intrusion.

Therefore, it is necessary to design the grounding grid scientifically to ensure that the above problems will not affect the safe operation of the system, and will not threaten the safety of humans and animals, as well as the safe operation of electrical facilities and underground metal pipeline facilities around the grounding grid. invest.

A design method of the grounding grid of the converter station that can replace the DC grounding electrode of the present invention is shown in FIG. 1 , and referring to FIGS. 2 and 3 at the same time, the present invention is based on the grounding grid of the converter station designed by the conventional method. , according to the personal and equipment safety problems caused when the DC grounding pole function is added to the grounding grid of the converter station, the limit values of parameters such as contact potential difference, potential difference in the network, and unipolar allowable running time are proposed. The parameters such as conductor corrosion thickness, contact potential difference, potential difference in the grid, and unipolar allowable running time controlled by the maximum allowable temperature rise are calculated and checked for the grounding grid of the converter station through the computer processing program with the current field calculation function and related formulas. Corresponding parameter limits adopt measures such as increasing the cross-sectional size of the grounding grid conductors, setting up encrypted conductors, connecting copper strips, adding conductors on the periphery to expand the grounding grid area, laying coke around the conductors, and setting feeder cable down conductors, so as to obtain alternative DC. The grounding grid of the converter station of the grounding electrode.

It takes the following steps:

(1) Set the first feeder cable 202 to be connected from the grounding point of the valve hall to the grounding grid of the converter station,

Preferably, four first feeder cables 202 are provided, one end of the first feeder cables 202 is respectively connected to the four corners 201 of the grounding grid of the converter station, and the other end is connected to the grounding point of the valve hall;

In the prior art, an overhead bare wire comes out of the valve hall, which is connected to the grounding electrode line, and then passes through the bus bar, and then is connected to the grounding electrode; and the present invention uses the first feeder cable to replace the overhead bare wires, The grounding electrode line, bus bar and grounding electrode can use the existing grounding grid to realize the grounding of the valve hall, thereby saving the construction cost.

(2) Based on existing regulations, operating experience and actual conditions, set the contact potential difference limit according to personal safety requirements;

Calculate the corrosion thickness of the grounding grid conductor under unipolar maximum operating current and bipolar unbalanced current, and determine the cross-sectional size of the grounding grid conductor of the converter station according to the corrosion thickness; usually, the cross-sectional size is larger than the original grounding grid conductor. Increasing the cross-sectional size of the grounding grid conductor can solve the problem of electrolytic corrosion of DC current;

According to the parameters such as the structure size of the grounding grid of the converter station, the cross-sectional size of the conductor, the soil resistivity model of the pole site of the converter station and other parameters, the computer processing program is used to establish the model of the grounding grid, and the contact potential difference of the grounding grid is obtained by running the calculation; When the contact potential difference exceeds the preset contact potential difference limit when the DC transmission ground returns to the operating current, add grounding grid conductors where the contact potential difference is large, and then calculate the contact potential difference again until the contact potential difference drops to the preset contact potential difference. The contact potential difference is within the limit value; the personal safety problem of the personnel in the station can be solved by encrypting the grounding grid conductor;

(3) Use the second feeder cable to establish a connection between the grounding equipment of the grounding grid of the converter station and the grounding grid of the converter station.

Among them, the model formed by the operation of the computer processing program can also be used to calculate the maximum potential difference in the network. If the superposition value of the maximum potential difference in the network exceeds the preset potential difference in the equipment safety network under the AC short-circuit fault into the ground current and the DC into the ground current The limit value, as shown in FIG. 6 , is to lay at least one conductive medium 602 with a cross-sectional dimension of not less than 220 mm ² in the cable trench 601 where the secondary cable is buried.

Wherein, the method for laying the conductive medium 602 in the cable trench where the secondary cable is buried includes:

Add a preset number of conductive media in the horizontal and vertical middle of the grounding grid, and in the middle of the surrounding outer grids;

Calculate the maximum potential difference in the network again. If the superposition value of the maximum potential difference in the network still exceeds the preset value of the potential difference in the safety network of the equipment, continue to add a group of conductive media between the adjacent grids where the conductive medium was added last time, until the potential difference is contacted. Less than the preset potential difference limit within the equipment safety net.

Among them, the conductive medium is preferably copper tape or copper wire, and the copper tape or copper wire should be electrically connected to the grounding grid at intervals; The potential difference in the network of equipment insulation breakdown.

In addition, it is also possible to set the limit of the allowable running time of the unipolar according to the preset limit of the potential difference in the safety net of the equipment, combined with the temperature rise, and use the model formed by the computer processing program to calculate the maximum current under the maximum operating current of the unipolar. Density J _m and grounding resistance R, and further combined with the maximum allowable temperature rise to calculate the unipolar allowable operating time t _m , when t _m is less than the set unipolar maximum operating time limit, according to the actual converter station grounding grid pole location conditions A conductor connected to the grounding grid of the original converter station is arranged on the periphery of the grounding grid of the converter station to expand the area of the grounding grid, and 0.1m*0.1m~1m*1m of coke is laid near the conductor, and then run and calculate _tm until _tm reaches the required value. The temperature rise of the grounding grid is controlled by measures such as increasing the area of the grounding grid of the converter station and laying a coke bed, and the allowable operating time of the single-pole operating mode of the single-pole operating mode is extended. The wire can solve the temperature rise problem of the grounding down-conductor.

Among them, according to the requirements of personal safety, the contact potential difference can be set to be no more than 50V, and according to the requirements of equipment safety, it can be set to the limit value of the potential difference in the network that the secondary equipment can withstand is no more than 2.8kV. In order to meet the fault and maintenance requirements of the DC transmission system, the grounding The allowable running time limit of the single pole of the net is 5h.

As shown in Figures 2-4, combined with Figures 6 and 7, the grounding grid of the converter station of the present invention includes a grounding grid body, a first feeder cable 202, a second feeder cable, grounding equipment, and a conductive medium, wherein , :

The first feeder cable 202 is used to connect the valve hall grounding point and the grounding grid body;

The second feeder cable is used to connect the grounding equipment of the grounding grid of the converter station and the grounding grid body;

The first calculation module is used to calculate the corrosion thickness of the grounding grid conductor under the unipolar extreme operating current and the bipolar unbalanced current, and determine the cross-sectional size of the grounding grid conductor according to the corrosion thickness;

The second calculation module is used to calculate and obtain the contact potential difference of the grounding grid according to the structural size of the grounding grid of the converter station, the cross-sectional size of the conductor, and the soil resistivity model of the surveyed site of the converter station;

The grounding grid conductor is laid at the position where the contact potential difference exceeds the limit value and is connected to the grounding grid body.

In addition, the converter station grounding grid of the present invention also includes:

The third calculation module is used to calculate the maximum potential difference in the network;

Conductive medium shall be laid in the cable trench where the secondary cable is buried when the superposition value of the maximum potential difference in the network exceeds the preset value of the potential difference in the equipment safety network under the AC short-circuit fault into the ground current and the DC into the ground current.

In addition, the converter station grounding grid of the present invention also includes:

The fourth calculation module is used to calculate the maximum current density J _m and the grounding resistance R under the unipolar maximum operating current, and further calculate the unipolar allowable operating time t _m in combination with the maximum allowable temperature rise;

The conductor 701 is used to be arranged on the periphery of the grounding grid of the converter station and connected to the grounding grid of the converter station when t _m is less than the set unipolar allowable running time limit;

Coke is laid around the conductor 701 .

The following is an example of the grounding grid of the original converter station with a grounding grid size of 200m*200m and a mesh size of 10m*10m. Figures 4 to 8 show the specific application of the present invention in the grounding grid of the original converter station.

1. Calculation of corrosion thickness of grounding grid

From the operating conditions of DC transmission, the corrosion of the grounding grid can be divided into electrolytic corrosion caused by unbalanced operating current and electrolytic corrosion caused by unipolar operating fault current. The amount of corrosion G per ampere current flowing through the iron anode is:

The unbalanced current of the DC transmission project within one year is taken as the upper limit value of the regulations, that is, 1% of the rated current. Calculated according to the current main DC transmission projects with a rated current of 3000A, the maximum local corrosion G of the grounding grid can be obtained as

In the formula, y is the design operating life, k is the current non-uniformity coefficient, L1 and L2 are the width and height of the grounding grid conductor flat steel, δ is the corrosion thickness, and L is the total conductor length. Substituting the relevant data of the grounding grid conductors, it can be obtained that the corrosion thickness of the grounding grid conductors is about 3 to 6 mm under the design life of the grounding grid of the converter station of 30 to 50 years.

2. Calculation and verification of contact potential difference

Since the contact potential difference is usually more serious than the step potential difference, the criterion of personal safety can be checked by the contact potential difference. As shown in Figure 4, on the basis of the grounding grid of the original converter station (Figure 4a), the cross-sectional size of the grounding grid conductor is increased by 3 mm to offset the effect of electrolytic corrosion. According to the parameters such as the size and shape of the grounding grid, the grounding grid model of the converter station is established by the MALZ module of the grounding analysis software CDEGS. The contact potential difference of the grounding grid, the maximum contact potential difference is 107.5V, and the larger value of the contact potential difference mainly occurs around and corners of the grounding grid. Arrange 6 32*3.5mm ² copper tape 401 encrypted grounding grids in the middle and center of the quadrilateral grid with high contact potential difference (as shown in Figure 4b). The encrypted grounding grid is modeled again, and the contact potential difference is calculated and checked. The maximum contact potential difference is reduced to 46.3V, which is within the personal safety limit (that is, the contact potential difference limit).

3. Calculation and verification of the potential difference in the network

The MALZ module of the grounding analysis software CDEGS is used to model the grounding grid formed by the above steps, and the potential difference in the grid under the AC short-circuit fault current is calculated. 50V, no longer calculated in the model.

As shown in Figure 5, the calculation conditions are: 50kA power frequency fault current flows from point 40, –20kA flows from point 22, representing 500kV line shunting, –15kA flowing from No. 400 represents 220kV line shunting, calculate the maximum network The potential difference is 3153V.

Since the superposition value of the potential difference in the AC and DC network exceeds 2.8kV, as shown in Figure 6, a 38mm*6mm copper tape (or copper stranded wire of equal cross-section) is laid in the cable trench where the secondary cable is buried. And weld the copper tape to the grounding grid every 20m. After laying the copper tape, calculate the potential difference in the network again under the ground current of the AC short-circuit fault, which is 1739V, which is within the safety limit of the equipment.

The potential difference at both ends of the secondary cable is smaller than the maximum potential difference in the network. When the maximum potential difference in the network meets the safety requirements of the equipment, the potential difference at both ends of the secondary cable must also be within the safety limit.

4. The maximum unipolar allowable running time is calculated and checked

In practical engineering, the running time of the grounded-pole monopolar ground loop is generally no more than 5 hours per year, so the allowable running time of the monopolar grounding circuit is set to 5 hours.

The MALZ module of the grounding analysis software CDEGS is used to model the grounding grid formed in the above steps, and the maximum current density J _m = 15.9A/m ² and grounding resistance R = 0.197Ω under the unipolar operating current into the ground are calculated and obtained, The allowable running time t _m is calculated in combination with the maximum allowable temperature rise. The obtained t _m is about 0.79h.

Among them, the allowable running time t _m is calculated according to formula (2) to formula (4):

In the formula, T _r is the time constant; τ _m is the maximum allowable temperature rise, taking 60°C; τ _ω is the steady-state temperature rise of the grounding grid; I is the injection current of the grounding grid, in A; R is the grounding resistance of the grounding grid, in the unit Ω; λ is the thermal conductivity, the unit is W/m·℃; γ is the specific heat coefficient, the unit is J/kg·℃; ρ is the soil resistivity, the unit is Ω·m; L is the total length of the conductor, the unit is m ; S is the conductor cross-sectional area, in m ² ; J _m is the maximum current density, in A/m ² .

As shown in Figures 7 and 8, the grounding grid is reconstructed, the 200m*200m grounding grid is expanded to 240m*240m, and 0.1m*0.1m of coke 801 is laid around the outermost two circles of grounding grid conductors 701. The t _m after the transformation is calculated and checked again, and the t _m is obtained to be 22.8h, which meets the requirements of the operating time limit.

5. Grounding down lead setting

Set the second feeder cable as the grounding downconductor of each device in the grounding grid of the original converter station. The size of the second feeder cable is determined according to DL/T 5224-2014.

The above-mentioned embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Equivalent substitutions or transformations made by those skilled in the art on the basis of the present invention are all within the protection scope of the present invention. The protection scope of the present invention is subject to the claims.

Claims (11)

1. a converter station grounding grid design method that can replace the direct current grounding electrode, is characterized in that, comprises: A. Set the first feeder cable to connect from the grounding point of the valve hall to the grounding grid of the converter station; B. Calculate the corrosion thickness of the grounding grid conductor under unipolar maximum operating current and bipolar unbalanced current, and determine the cross-sectional size of the grounding grid conductor according to the corrosion thickness; According to the structure size of the grounding grid of the converter station, the cross-sectional size of the conductor, and the soil resistivity model of the surveyed site of the converter station, the contact potential difference of the grounding grid is calculated and obtained; If the contact potential difference exceeds the preset contact potential difference limit when the DC transmission ground returns to the operating current, a grounding grid conductor is added at the position where the contact potential difference exceeds the limit value, so that the contact potential difference falls within the contact potential difference limit; C. Use the second feeder cable to establish a connection between the grounding equipment of the grounding grid of the converter station and the grounding grid body of the converter station, and set the second feeder cable as the grounding downconductor of each equipment in the grounding grid of the original converter station; Also includes: According to the preset limit of potential difference in the safety net of the equipment, set the limit of unipolar allowable running time in combination with the temperature rise; Calculate the maximum current density J _m and grounding resistance R under the unipolar maximum operating current, and further calculate the unipolar allowable operating time t _m in combination with the maximum allowable temperature rise, When t _m is less than the set single-pole allowable operating time limit, a conductor connected to the grounding grid of the converter station is arranged outside the grounding grid of the converter station, and coke is laid near the conductor to make t _m reach the required single-pole allowable operation. time. 2 . The method for designing a grounding grid of a converter station that can replace a DC ground electrode according to claim 1 , wherein there are four first feeder cables, and one end of the first feeder cables is respectively connected to the converter station. 3 . The four corners of the station grounding grid, and the other end is connected to the valve hall grounding point. 3. The design method for the grounding grid of the converter station that can replace the DC grounding electrode as claimed in claim 1, characterized in that, further comprising: Calculate the maximum potential difference in the network, If the superposition value of the maximum potential difference in the network exceeds the preset potential difference limit of the equipment safety network under the AC short-circuit fault into the ground current and the DC into the ground current, the conductive medium should be laid in the cable trench where the secondary cable is buried. 4 . The method for designing a grounding grid of a converter station that can replace a DC grounding electrode according to claim 1 , wherein, under a single pole operating current, the limit value of the contact potential difference is 50V. 5 . 5. The design method for the grounding grid of the converter station that can replace the DC grounding electrode as claimed in claim 3, wherein the limit value of the potential difference in the equipment safety network is: the limit value of the potential difference in the secondary equipment withstand network does not exceed 2.8 kV. 6 . The method for designing the grounding grid of a converter station that can replace the DC grounding pole according to claim 1 , wherein the allowable operating time limit of the single pole is 5h. 7 . 7. The design method for the grounding grid of the converter station that can replace the DC grounding electrode according to claim 3, wherein the method for laying a conductive medium in the cable trench where the secondary cable is buried comprises: Add a preset number of conductive media in the horizontal and vertical middle of the grounding grid, and in the middle of the surrounding outer grids; Calculate the maximum potential difference in the network again. If the superposition value of the maximum potential difference in the network still exceeds the preset value of the potential difference in the equipment safety network, continue to add a group of conductive media between the adjacent grids of the conductive medium added last time. The potential difference is less than the preset potential difference limit within the equipment safety net. 8. The method for designing a grounding grid of a converter station that can replace a DC grounding electrode according to claim 3, wherein the conductive medium laid in the cable trench is electrically connected to the grounding grid, and the cross-section of the conductive medium is electrically connected to the grounding grid. The size is not less than 220mm ² . 9. The design method for the grounding grid of a converter station that can replace the DC grounding electrode according to claim 1, wherein the cross-sectional size of the coke laid around the conductor is in the range of 0.1m*0.1m~1m*1m, wherein, The size of the coke as it is laid is proportional to the current density of the conductor. 10. A grounding grid for a converter station, comprising: a grounding grid body, a first feeder cable, a second feeder cable, grounding equipment, and a conductive medium, wherein, the first feeder cable is used to connect the valve hall grounding point and the grounding grid body; The second feeder cable is used to connect the grounding equipment of the grounding grid of the converter station and the grounding grid body, and is used as the grounding down conductor of each equipment in the grounding grid of the original converter station; The first calculation module is used to calculate the corrosion thickness of the grounding grid conductor under the unipolar extreme operating current and the bipolar unbalanced current, and determine the cross-sectional size of the grounding grid conductor according to the corrosion thickness; The second calculation module is used to calculate and obtain the contact potential difference of the grounding grid according to the structural size of the grounding grid of the converter station, the cross-sectional size of the conductor, and the soil resistivity model of the surveyed site of the converter station; The grounding grid conductor is laid at the position where the contact potential difference exceeds the limit value and is connected to the grounding grid body; Also includes: The fourth calculation module is used to calculate the maximum current density J _m and the grounding resistance R under the unipolar maximum operating current, and further calculate the unipolar allowable operating time t _m in combination with the maximum allowable temperature rise; The conductor is used to be arranged at the periphery of the grounding grid of the converter station and connected to the grounding grid of the converter station when t _m is less than the set unipolar allowable operating time limit; Coke, laid around the conductor. 11. The converter station grounding grid of claim 10, further comprising: The third calculation module is used to calculate the maximum potential difference in the network; Conductive medium shall be laid in the cable trench where the secondary cable is buried when the superposition value of the maximum potential difference in the network exceeds the preset value of the potential difference in the equipment safety network under the AC short-circuit fault into the ground current and the DC into the ground current.

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