CN113656996A - Submarine cable rapid discharge strategy optimization method and grounding grid risk control method - Google Patents

Submarine cable rapid discharge strategy optimization method and grounding grid risk control method Download PDF

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CN113656996A
CN113656996A CN202110774303.9A CN202110774303A CN113656996A CN 113656996 A CN113656996 A CN 113656996A CN 202110774303 A CN202110774303 A CN 202110774303A CN 113656996 A CN113656996 A CN 113656996A
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submarine cable
grid
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CN113656996B (en
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曹京荥
陈杰
何康
杨景刚
张量
陶风波
何嘉弘
刘建军
李陈莹
谭笑
胡丽斌
张伟
董博文
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State Grid Corp of China SGCC
Southeast University
State Grid Jiangsu Electric Power Co Ltd
Electric Power Research Institute of State Grid Jiangsu Electric Power Co Ltd
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Southeast University
State Grid Jiangsu Electric Power Co Ltd
Electric Power Research Institute of State Grid Jiangsu Electric Power Co Ltd
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Abstract

The invention discloses a submarine cable rapid discharge strategy optimization method, which optimizes a discharge strategy in a cable terminal multi-resistor series discharge process by establishing a submarine cable discharge loop equivalent circuit model and according to current and power limiting conditions of a discharge resistor; the invention also discloses a grounding grid risk control method, which optimizes the design scheme of the grounding grid by establishing a calculation model of the step voltage and the electrostatic field of the submarine cable terminal site and according to the step voltage and electrostatic field distribution model; the invention gives consideration to two limiting conditions of the maximum allowable current and the maximum allowable power of the discharge resistor, ensures the optimized discharge strategy, can obtain the fastest discharge speed on the premise of ensuring the safe operation of the discharge resistor, properly reduces the grid spacing of the grounding grid around the submarine cable, and effectively reduces the material and installation cost of the grounding grid while ensuring that the field step voltage is lower than the safe voltage of a human body.

Description

Submarine cable rapid discharge strategy optimization method and grounding grid risk control method
Technical Field
The invention belongs to the field of cable terminal engineering, and particularly relates to a submarine cable rapid discharge strategy optimization method and a grounding grid risk control method.
Background
The direct current cable can store a large amount of charges after long-term operation or a direct current withstand voltage test, so that a cable terminal can keep a higher terminal voltage for a longer time after power failure, and the power failure overhaul or the subsequent treatment process of the test cannot be safely carried out. Therefore, it needs to be discharged quickly to shorten the time for power failure maintenance or test follow-up treatment. The cable terminal discharge resistor is formed by connecting a plurality of sections of resistors in series, and the plurality of sections of resistors are gradually short-circuited in the discharge process to accelerate the discharge speed. The discharge resistor is restricted by self-limiting conditions and safety conditions of surrounding sites in the discharge process, so that a submarine cable rapid discharge strategy and a submarine cable risk control method are urgently needed to monitor the safety and risk controllability of the submarine cable terminal discharge process.
Disclosure of Invention
The purpose of the invention is as follows: in order to make up for the lack of a technical means for monitoring the safety and risk controllability of the submarine cable terminal in the discharge process in the prior art, the invention provides a submarine cable rapid discharge strategy optimization method and a grounding grid risk control method, which can be used in the engineering fields of power failure overhaul or voltage withstand test and the like of a cable terminal and are suitable for the rapid discharge problem of long-distance high-voltage direct-current submarine cables.
The technical scheme is as follows: a submarine cable rapid discharge strategy optimization method comprises the following steps:
step 1: establishing an equivalent circuit model of a submarine cable discharge circuit:
step 2: judging whether m-1 is equal to 1, if so, stopping iteration, otherwise, executing the step 3;
and step 3: the resistance connected into the discharge loop is changed from m sections to (m-1) sections, and the discharge loop is discharged based on the submarine cableThe equivalent circuit model calculates the current and power at n time, and judges whether the current at n time exceeds the maximum allowable current ImaxAnd whether the power at the moment n exceeds the product of the maximum allowable power of a single node and the number of the resistance nodes connected into the discharge loop, if the current at the moment n does not exceed the maximum allowable current ImaxAnd the power at the moment n does not exceed the product of the maximum allowable power of a single section and the number of the resistance sections connected into the discharging loop, executing the step 4; if the current at the moment n exceeds the maximum allowable current ImaxOr the power at the moment n exceeds the product of the maximum allowable power of a single section and the number of the resistance sections connected into the discharging loop, executing the step 5;
and 4, step 4: recording the number m of discharge resistance nodes accessed to the submarine cable at n timen(m-1) and, at the same time, n ═ n +1 and m ═ m-1, step 2 is performed;
and 5: maintaining the number m of resistor nodes connected into the discharge loop unchanged, namely the number m of discharge resistor nodes connected into the submarine cable at the time nnAnd (5) executing step 2, wherein m is n + 1.
Further, the submarine cable discharge circuit equivalent circuit model is expressed as:
Figure BDA0003153933430000021
wherein U (t) represents the time-varying function of the earth voltage of the submarine cable terminal during the discharge process, U0The method comprises the steps that a submarine cable terminal at the initial discharge moment is grounded, t is time, m is the number of sections of resistors connected into a discharge loop, C is a capacitance value which enables the submarine cable to be equivalent to a capacitor, and R is the resistance value of each section of resistors connected into the discharge loop.
Further, the calculating of the current and the power at the n time based on the submarine cable discharge circuit equivalent circuit model specifically includes:
discretizing the time scale of the discharging process according to the equivalent circuit model of the submarine cable discharging loop, wherein the nth discrete moment is tnWhere Δ t is a time step and n-0 is an initial time, obtaining a time-dependent change in discharge resistance currentThe discretization expression is:
Figure BDA0003153933430000022
the discretization expression of the discharge resistance power along with the time change is as follows:
Figure BDA0003153933430000023
in the formula In、PnRespectively showing the current and power of the submarine cable terminal at the nth moment in the discharging process, mnThe node number of the discharge resistor connected into the submarine cable at the nth moment is shown;
the current and power at time n are calculated from equations (4) and (5).
The invention also discloses a submarine cable grounding grid risk control method, which adopts the submarine cable rapid discharge strategy optimization method to discharge the submarine cable, and optimizes the design scheme of the grounding grid according to the maximum voltage to earth appearing in the whole discharge process and the maximum voltage to earth appearing in the whole discharge strategy optimization process and the distribution rule of the field step voltage and the electrostatic field when accidental discharge to earth appears.
Further, the method comprises the following steps:
s100: the value of the potential to ground discharge point is set to the maximum voltage to ground that occurs during the entire discharge strategy optimization process.
S200: calculating the field intensity distribution of the submarine cable terminal when the submarine cable terminal is grounded without a grounding grid;
s300: comparing the field intensity distribution and the critical field intensity obtained by the calculation of S200 to obtain a safe area and the minimum transverse density d of the grounding grid required by each position in the grounding field of the submarine cable terminalx.maxAnd a minimum longitudinal density dy.max；
S400: on the basis of S300, the length of the edge around the grounded discharge point is lEWidth d of the grid in the squareEOf grids of other areasTransverse width dxLongitudinal width d of grid of other areayCombining, and screening out the size of the grounding grid meeting the step voltage limit;
s500: selecting the grounding grid size with the lowest manufacturing cost from the grounding grid sizes meeting the step voltage limit, and taking the grounding grid size as an optimal scheme;
s600: according to the optimal scheme in S500, a corresponding grounding grid is buried in the submarine cable terminal field, and a grounding discharge point is buried in the grounding grid with the width dEWithin a square grid of (a).
Further, in S100, the maximum voltage to ground appearing at the submarine cable terminal in the whole discharge strategy optimization process is calculated by:
the discretization expression of the variation of the submarine cable terminal to the ground voltage along with the time is as follows:
Figure BDA0003153933430000031
in the formula (I), the compound is shown in the specification,U nrepresenting the voltage to ground of the submarine cable terminal at the nth moment in the discharge process;
the voltage to ground U at each moment is calculated according to the formula (10)0、U1、U2…UnComparing the voltage to ground at each moment to obtain the maximum voltage to ground U appearing at the submarine cable terminal in the whole discharge strategy optimization processm：
Um＝max{U0,U1,U2,…Un} (11)。
Further, the S200 specifically includes the following steps:
using orthogonal grid subdivision on a submarine cable terminal grounding field to obtain a series of discrete points and calculating to obtain the potential of each discrete point;
calculating to obtain the step voltage between any two points of the submarine cable terminal site based on the electric potential of each discrete point;
and calculating to obtain the field intensity of the static field of the submarine cable terminal based on the electric potential of each discrete point.
Further, the calculating to obtain the electric potential of each discrete point specifically includes:
and the potential distribution in the grounding field of the submarine cable terminal conforms to the Poisson equation, and the potential of each discrete point is calculated by applying a finite difference method.
Further, S300 specifically includes the following steps:
s301: taking the earth discharge point as the center of a circle, and selecting the radius standard as the minimum value of all radii which can satisfy the condition that the circle can surround all regions with the electric field intensity exceeding the critical field intensity; the area inside the circle is a dangerous area, and the area outside the circle is a safety range;
s302: the field grounding grid is a metal grid consisting of a plurality of longitudinal and transverse metal strips, and the number of the transverse metal strips and the longitudinal metal strips of the grounding grid is increased one by one to reduce the range of a dangerous area until the radius of the dangerous area meets the requirement; obtaining the minimum density d of the grounding grid according to the number of the metal strips of the grounding grid and the length and width of the fieldx.maxAnd dy.max。
Further, the S400 specifically includes the following steps:
s401: length of side around the grounding discharge point is lENumber n of inner bars of squareEThe number n of longitudinal steel bars in other areas of the fieldVONumber n of transverse steel bars in other areas of the fieldHORespectively as follows:
Figure BDA0003153933430000032
Figure BDA0003153933430000033
Figure BDA0003153933430000034
in selecting the grid width dE、dxAnd dyWhen n is to be ensuredE、nVOAnd nHOIs an integer, and dxShould not be greater than dx.max，dyShould not be greater than dy.max；
S402: length of side around the grounded discharge point is lEWidth d of the grid in the squareEThe lateral width d of the grid of the other regionxLongitudinal width d of grid of other areayAnd (4) combining, and screening out the size of the grounding grid meeting the limitation of the step voltage.
Has the advantages that: compared with the prior art, the invention has the following advantages:
(1) the discharge strategy of the invention gives consideration to two limiting conditions of the maximum allowable current and the maximum allowable power of the discharge resistor, ensures the optimized discharge strategy, and can obtain the fastest discharge speed on the premise of ensuring the safe operation of the discharge resistor;
(2) according to the ground net risk control method, the grid spacing of the ground net around the submarine cable is properly reduced, so that the ground net material and the installation cost are effectively reduced while the field step voltage is lower than the human body safety voltage.
Drawings
FIG. 1 is a schematic diagram of an equivalent circuit of a submarine cable discharge circuit;
FIG. 2 is a flow chart of submarine cable fast discharge strategy optimization;
FIG. 3 is a schematic diagram of the positions of the grounding grid and the ground discharging points;
FIG. 4 is a schematic diagram of a spatial orthogonal grid subdivision step length;
FIG. 5 is a schematic diagram of the number of grids of the grounding grid;
fig. 6 is a flow chart of ground net design optimization.
Detailed Description
The technical solution of the present invention will be further explained with reference to the accompanying drawings and embodiments.
The invention relates to a submarine cable rapid discharge strategy optimization method, which researches the change rule of discharge resistance current and power in the discharge process by establishing a submarine cable discharge loop equivalent circuit model, discretizes the expression of the change rule of the discharge resistance current and power in the discharge process, and optimizes the discharge strategy in the cable terminal multi-resistor series discharge process according to the current and power limiting conditions of the discharge resistance; the submarine cable is discharged by adopting a submarine cable rapid discharge strategy optimization method, according to the maximum voltage to ground appearing in the whole discharge process, the distribution rule of the field step voltage and the static field when accidental discharge to the ground appears is obtained by establishing a calculation model of the field step voltage and the static field of a submarine cable terminal, and the design scheme of a grounding grid is optimized.
The above process will now be described in detail.
The length of the discharged submarine cable is l, and the unit length capacitance is C0The submarine cable is equivalent to a capacitor, the capacitance value of the capacitor is C, the resistance connected into the discharge loop is m sections, the resistance value of each section is R, and the total resistance value is mR, as shown in fig. 1, the equivalent circuit model of the submarine cable discharge loop is a first-order RC zero-input response loop model, the discharge time constant is τ mRC, the cable terminal follows a first-order RC zero-input response to the ground voltage, and the specific form is as follows:
Figure BDA0003153933430000051
wherein U (t) represents the time-varying function of the earth voltage of the submarine cable terminal during the discharge process, U0The cable terminal at the initial moment of discharge is grounded, and t is time.
According to the equivalent first-order RC zero input response loop, the change of the discharge resistance current along with the time in the discharge process is derived as follows:
Figure BDA0003153933430000052
according to the equivalent first-order RC zero input response loop, the time variation of the discharge resistance power in the discharge process is derived as follows:
Figure BDA0003153933430000053
because a plurality of resistors need to be short-circuited in the discharging process, the discharging speed is accelerated, and the number m of the resistors connected into a discharging loop can be changed, so that the formula (2) and the formula (3) can not be directly used, and the time scale of the discharging process needs to be discretized.
The time step adopted by discretization is delta t, and the nth discrete moment is tnN Δ t, and an initial time n is defined as 0.
The discretization expression of the discharge resistance current along with the time change is as follows:
Figure BDA0003153933430000054
the discretization expression of the discharge resistance power along with the time change is as follows:
Figure BDA0003153933430000055
in the formula, Δ t represents the step of time taken for discretization, the index n represents the nth time, In、PnRespectively showing the current and power of the submarine cable terminal at the nth moment in the discharging process, mnIndicating the number of discharge resistance nodes connected into the submarine cable at the nth time.
Wherein, the current limiting condition of the discharge resistor is as follows: the current I (t) of the discharge resistor during discharge must not exceed the maximum allowable current Imax。
The power limiting conditions for the discharge resistor are: the power P (t) of the discharge resistor in the discharge process must not exceed the product mP of the maximum allowable power of a single resistor and the number of resistor nodes connected into the discharge loopmax。
As shown in fig. 2, the discharge strategy in the process of optimizing the multi-resistor series discharge of the cable terminal is as follows: changing the resistance connected into the discharge loop from m sections to (m-1) sections at each iteration moment, calculating the current and power at the moment by using the formulas (4) and (5), comparing the current and power with a limiting condition, and taking the moment as the moment for short-circuiting one section of resistance if the current and power are both below the limiting condition; if the current or power is above the limit condition, the number of the resistance nodes connected into the discharge loop at the moment keeps m nodes unchanged. The method is iterated until only one section of the resistor section connected into the discharge loop is left.
According to the maximum voltage to ground appearing in the whole discharge strategy optimization process, the embodiment provides a submarine cable grounding grid risk control method, and the design scheme of the grounding grid is optimized according to the distribution rule of the field step voltage and the electrostatic field when accidental discharge to ground appears.
The process of optimizing the design scheme of the grounding grid is as follows: firstly, calculating the field intensity distribution of the high-voltage electrode when the high-voltage electrode is grounded in the absence of a net: comparing and calculating the field intensity and the critical field intensity, designing the minimum density of the grounding grid required by each position of the safety area and the field, and designing the minimum density of the grounding grid according to the actual three grid widths dE、dx、dyThen select different dE、dx、dyAnd (4) combining, and verifying the influence of the grounding grid under the grid width on the field step voltage. Maximum allowable value U of given step voltagemaxAnd screening out the grounding grid type meeting the step voltage limitation. This control method will now be described in detail.
As shown in fig. 3, a grounding grid is buried in the submarine cable terminal in the field, and the discharge point to ground is located in a certain grid of the grounding grid. Based on a finite difference method, the distribution rule of the field step voltage and the static field under the condition of accidental ground discharge in the discharge process needs to be calculated, and the design scheme of the grounding grid is optimized based on the distribution rule of the field step voltage and the static field.
Firstly, establishing a finite difference method calculation model of the stepping voltage and the electrostatic field of the submarine cable terminal site:
the potential distribution in the field conforms to the poisson equation and is expressed as:
Figure BDA0003153933430000061
in the formula (I), the compound is shown in the specification,
Figure BDA0003153933430000062
representing an electrical potential.
And (3) performing orthogonal grid division on the field to obtain a series of discrete points, wherein the division step length is shown in figure 4. For the (i, j, k) -th discrete point, the finite difference method form of equation (6) is:
Figure BDA0003153933430000063
the finite difference method is used for calculating the electric potential of each discrete point, and certain boundary conditions are needed.
(1) Infinite boundary conditions:
Figure BDA0003153933430000064
(2) known conditions for the ground discharge point potential:
Figure BDA0003153933430000065
in the formula (I), the compound is shown in the specification,
Figure BDA0003153933430000066
representing the potential to the ground discharge point.
Figure BDA0003153933430000067
Numerically taking the maximum voltage-to-ground U occurring at the submarine cable terminal in the whole discharge strategy optimization processmThe calculation method comprises the following steps:
the discretization expression of the variation of the submarine cable terminal to the ground voltage along with the time is as follows:
Figure BDA0003153933430000068
in the formula (I), the compound is shown in the specification,U nrepresenting the voltage to ground of the submarine cable termination at the nth moment in the discharge process. The voltage to ground U at each moment is calculated according to the formula (10)0、U1、U2…UnComparing the voltage to ground at each moment to obtain the maximum voltage to ground U appearing at the submarine cable terminal in the whole discharge strategy optimization processm：
Um＝max{U0，U1，U2，…Un} (11)
(3) Continuous conditions of current densities of different media:
Figure BDA0003153933430000071
in the formula of gamma1、γ2Respectively represents the electrical conductivity of the two media,
Figure BDA0003153933430000072
representing the component of the gradient of the potential at the interface of the two media along the normal direction of the interface.
(4) Suspended potential conductor boundary conditions:
Figure BDA0003153933430000073
Figure BDA0003153933430000074
this type of boundary condition requires the common expression of equations (13) and (14).
The expression (13) indicates a conductor occupying n discrete points, the potential of which is equal everywhere, and is
Figure BDA0003153933430000075
Is a pending amount); the expression (14) is the Gaussian theorem, which means the normal component of the potential gradient (electric field strength) at each point on the boundary of the conductorThe sum is 0.
After the electric potential of each discrete point in the field is solved, the step voltage between any two points in the field is calculated:
Figure BDA0003153933430000076
the field intensity of the electrostatic field in the field is as follows:
Figure BDA0003153933430000077
in the formula, "grad" indicates gradient calculation.
One approximate gradient method is:
Figure BDA0003153933430000078
wherein the content of the first and second substances,
Figure BDA0003153933430000079
representing the unit component along the direction of axis X, Y, Z.
The submarine cable terminal is buried with a grounding net in the ground, the grounding net is a metal grid formed by conductors, as shown in figure 5, the transverse length of the field is lxLongitudinal length of lyThe grounding grid is a metal grid consisting of a plurality of steel bars, has a limiting effect on the diffusion of potential distribution around an accidental discharge point, and enables the step voltage of most areas on the site to be maintained at a lower level.
Taking the earth discharge point as the center of a circle, and selecting the radius standard as the minimum value of all radii which can satisfy the condition that the circle can surround all regions with the electric field intensity exceeding the critical field intensity; the area inside the circle is a dangerous area, and the area outside the circle is a safety range; the number of the transverse metal strips and the longitudinal metal strips of the grounding grid is increased one by one to reduce the range of the dangerous area until the radius of the dangerous area meets the requirement; obtaining the minimum density of the grounding grid according to the number of the metal strips of the grounding grid and the length and width of the fielddx.maxAnd dy.max。
Length of side around discharge point to ground is lEThe grid density in the square is different from the grid density in other areas of the field. Length of side around discharge point to ground is lEHas a grid width dEThe transverse width of the grids in other areas of the field is dxA longitudinal width of dy. So that the length of the edge around the point of discharge to ground is lENumber n of inner bars of squareEThe number n of longitudinal steel bars in other areas of the fieldVONumber n of transverse steel bars in other areas of the fieldHORespectively as follows:
Figure BDA0003153933430000081
Figure BDA0003153933430000082
Figure BDA0003153933430000083
when selecting the grid width, n should be guaranteedE、nVOAnd nHOIs an integer, and dxShould not be greater than dx.max，dyShould not be greater than dy.max
Number n of longitudinal steel bars of grounding gridVNumber n of transverse barsHRespectively as follows:
Figure BDA0003153933430000084
Figure BDA0003153933430000085
the grounding grid with the lowest manufacturing cost is selected from the types of grounding grids meeting the limitation of the step voltage as an optimal scheme, so that the field step voltage is lower than the human body safety voltage, and meanwhile, the grounding grid material and the installation cost are effectively reduced.
If the unit cost of the steel bars of the grounding grid is c, the cost of the whole grounding grid is:
C＝c(nVly+nHlx) (23)。

Claims (10)

1. a submarine cable rapid discharge strategy optimization method is characterized by comprising the following steps: the method comprises the following steps:
step 1: establishing an equivalent circuit model of a submarine cable discharge circuit:
step 2: judging whether m-1 is equal to 1, if so, stopping iteration, otherwise, executing the step 3;
and step 3: changing the resistance connected into the discharge loop from m sections to (m-1) sections, calculating the current and power at n moments based on the submarine cable discharge loop equivalent circuit model, and judging whether the current at n moments exceeds the maximum allowable current ImaxAnd whether the power at the moment n exceeds the product of the maximum allowable power of a single node and the number of the resistance nodes connected into the discharge loop, if the current at the moment n does not exceed the maximum allowable current ImaxAnd the power at the moment n does not exceed the product of the maximum allowable power of a single section and the number of the resistance sections connected into the discharging loop, executing the step 4; if the current at the moment n exceeds the maximum allowable current ImaxOr the power at the moment n exceeds the product of the maximum allowable power of a single section and the number of the resistance sections connected into the discharging loop, executing the step 5;
and 4, step 4: recording the number m of discharge resistance nodes accessed to the submarine cable at n timen(m-1) and, at the same time, n ═ n +1 and m ═ m-1, step 2 is performed;
and 5: maintaining the number m of resistor nodes connected into the discharge loop unchanged, namely the number m of discharge resistor nodes connected into the submarine cable at the time nnAnd (5) executing step 2, wherein m is n + 1.
2. The submarine cable rapid discharge strategy optimization method according to claim 1, wherein: the submarine cable discharge circuit equivalent circuit model is expressed as:
Figure FDA0003153933420000011
wherein U (t) represents the time-varying function of the earth voltage of the submarine cable terminal during the discharge process, U0The method comprises the steps that a submarine cable terminal at the initial discharge moment is grounded, t is time, m is the number of sections of resistors connected into a discharge loop, C is a capacitance value which enables the submarine cable to be equivalent to a capacitor, and R is the resistance value of each section of resistors connected into the discharge loop.
3. The submarine cable rapid discharge strategy optimization method according to claim 1, wherein: the method for calculating the current and the power at n moments based on the submarine cable discharge circuit equivalent circuit model specifically comprises the following steps:
discretizing the time scale of the discharging process according to the equivalent circuit model of the submarine cable discharging loop, wherein the nth discrete moment is tnWhere Δ t is a time step, and n is 0 as an initial time, the discretization of the discharge resistance current with time is expressed as:
Figure FDA0003153933420000012
the discretization expression of the discharge resistance power along with the time change is as follows:
Figure FDA0003153933420000013
in the formula In、PnRespectively showing the current and power of the submarine cable terminal at the nth moment in the discharging process, mnThe node number of the discharge resistor connected into the submarine cable at the nth moment is shown;
the current and power at time n are calculated from equations (4) and (5).
4. A submarine cable grounding grid risk control method is characterized by comprising the following steps: the submarine cable rapid discharge strategy optimization method of any one of claims 1 to 3 is adopted to discharge submarine cables, and the design scheme of the grounding grid is optimized according to the maximum voltage to ground appearing in the whole discharge process and the distribution rule of the field step voltage and the electrostatic field when accidental discharge to ground appears.
5. The submarine cable grounding grid risk control method according to claim 4, wherein: the method comprises the following steps:
s100: the value of the potential to the ground discharge point is set to the maximum voltage to ground U occurring during the entire discharge processm；
S200: based on the maximum voltage to earth UmCalculating the field intensity distribution of the submarine cable terminal when the submarine cable terminal is grounded without a grounding grid;
s300: comparing the field intensity distribution and the critical field intensity obtained by the calculation of S200 to obtain a safe area and the minimum transverse density d of the grounding grid required by each position in the grounding field of the submarine cable terminalx.maxAnd a minimum longitudinal density dy.max；
S400: on the basis of S300, the length of the edge around the grounded discharge point is lEWidth d of the grid in the squareEThe lateral width d of the grid of the other regionxLongitudinal width d of grid of other areayCombining, and screening out the size of the grounding grid meeting the step voltage limit;
s500: selecting the grounding grid size with the lowest manufacturing cost from the grounding grid sizes meeting the step voltage limit, and taking the grounding grid size as an optimal scheme;
s600: according to the optimal scheme in S500, a corresponding grounding grid is buried in the submarine cable terminal field, and a grounding discharge point is buried in the grounding grid with the width dEWithin a square grid of (a).
6. The submarine cable grounding grid risk control method according to claim 5, wherein: in S100, the maximum voltage to ground occurring in the whole discharging process is calculated by:
the discretization expression of the variation of the submarine cable terminal to the ground voltage along with the time is as follows:
Figure FDA0003153933420000021
in the formula (I), the compound is shown in the specification,U nrepresenting the voltage to ground of the submarine cable terminal at the nth moment in the discharge process;
the voltage to ground U at each moment is calculated according to the formula (10)0、U1、U2…UnComparing the voltage to ground at each moment to obtain the maximum voltage to ground U appearing in the whole discharge processm：
Um＝max{U0,U1,U2,…Un} (11)。
7. The submarine cable grounding grid risk control method according to claim 5, wherein: the S200 specifically includes the following steps:
using orthogonal grid subdivision on a submarine cable terminal grounding field to obtain a series of discrete points and calculating to obtain the potential of each discrete point;
calculating to obtain the step voltage between any two points of the submarine cable terminal site based on the electric potential of each discrete point;
and calculating to obtain the field intensity of the static field of the submarine cable terminal based on the electric potential of each discrete point.
8. The submarine cable grounding grid risk control method according to claim 6, wherein: the calculation to obtain the electric potential of each discrete point specifically comprises:
and the potential distribution in the grounding field of the submarine cable terminal conforms to the Poisson equation, and the potential of each discrete point is calculated by applying a finite difference method.
9. The submarine cable grounding grid risk control method according to claim 5, wherein: the S300 specifically comprises the following steps:
s301: taking the earth discharge point as the center of a circle, and selecting the radius standard as the minimum value of all radii which can satisfy the condition that the circle can surround all regions with the electric field intensity exceeding the critical field intensity; the area inside the circle is a dangerous area, and the area outside the circle is a safety range;
s302: the field grounding grid is a metal grid consisting of a plurality of longitudinal and transverse metal strips, and the number of the transverse metal strips and the longitudinal metal strips of the grounding grid is increased one by one to reduce the range of a dangerous area until the radius of the dangerous area meets the requirement; obtaining the minimum density d of the grounding grid according to the number of the metal strips of the grounding grid and the length and width of the fieldx.maxAnd dy.max。
10. The submarine cable grounding grid risk control method according to claim 5, wherein: the S400 specifically includes the following steps:
s401: length of side around the grounding discharge point is lENumber n of inner bars of squareEThe number n of longitudinal steel bars in other areas of the fieldVONumber n of transverse steel bars in other areas of the fieldHORespectively as follows:
Figure FDA0003153933420000031
Figure FDA0003153933420000032
Figure FDA0003153933420000033
in selecting the grid width dE、dxAnd dyWhen n is to be ensuredE、nVOAnd nHOIs an integer, and dxShould not be greater than dx.max，dyShould not be greater than dy.max；
S402: length of side around the grounded discharge point is lEWidth d of the grid in the squareEThe lateral width d of the grid of the other regionxLongitudinal width d of grid of other areayAnd (4) combining, and screening out the size of the grounding grid meeting the limitation of the step voltage.
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