CN113419134B - Method for evaluating safety of secondary cable during power frequency short circuit of grounding grid - Google Patents

Method for evaluating safety of secondary cable during power frequency short circuit of grounding grid Download PDF

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CN113419134B
CN113419134B CN202110542871.6A CN202110542871A CN113419134B CN 113419134 B CN113419134 B CN 113419134B CN 202110542871 A CN202110542871 A CN 202110542871A CN 113419134 B CN113419134 B CN 113419134B
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shielding layer
secondary cable
cable
core
cable core
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CN113419134A (en
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刘胜军
殷彦增
龚乐乐
沈辰
佟智勇
郭猛
刘钊
齐红斌
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State Grid Corp of China SGCC
State Grid Hebei Electric Power Co Ltd
Baoding Power Supply Co of State Grid Hebei Electric Power Co Ltd
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State Grid Corp of China SGCC
State Grid Hebei Electric Power Co Ltd
Baoding Power Supply Co of State Grid Hebei Electric Power Co Ltd
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
    • G01R31/083—Locating faults in cables, transmission lines, or networks according to type of conductors in cables, e.g. underground
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/088—Aspects of digital computing
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Abstract

The invention relates to a method for evaluating the safety of a secondary cable when a grounding grid is in power frequency short circuit, wherein the interference of a transformer substation grounding grid on the secondary cable is equivalent by adopting a Thevenin circuit, a calculation formula of each distribution parameter of the secondary cable is given, a secondary cable core skin potential difference and shielding layer current calculation method is given, and a secondary cable safety evaluation method is provided. The method can simply and conveniently calculate the potential difference of the core sheath of the secondary cable and the current of the shielding layer and evaluate the safety of the secondary cable when the grounding grid is short-circuited.

Description

Method for evaluating safety of secondary cable during power frequency short circuit of grounding grid
Technical Field
The invention relates to a method for calculating the core skin potential difference and the shielding layer current of a secondary cable in the case of power frequency short circuit of a grounding grid, in particular to a method for evaluating the safety of the secondary cable in the case of power frequency short circuit of the grounding grid.
Background
The safety evaluation of the secondary cable is an important task in the design of the substation grounding grid. At present, the power frequency electrical parameters of the transformer substation grounding grid are calculated more perfectly at home and abroad, but the core skin potential difference and the shielding layer current analysis and the safety evaluation of the secondary cable are not performed after the transformer substation grounding grid is connected with the secondary cable. The transformer substation grounding grid is equivalent to a voltage source, and the interference of the transformer substation grounding grid on a secondary cable is equivalent by adopting a Thevenin circuit, namely a series connection of the voltage source and internal resistance. The secondary cable adopts distribution parameters, provides a calculation formula of each distribution parameter of the secondary cable, obtains a calculation formula of the core skin potential difference and the shielding layer current of the secondary cable, and provides a secondary cable safety evaluation method based on the core skin potential difference and the shielding layer current.
Disclosure of Invention
The invention aims to provide a method for evaluating the safety of a secondary cable in the case of power frequency short circuit of a grounding grid, which can simply and conveniently calculate the potential difference of a core sheath of the secondary cable and the current of a shielding layer and evaluate the safety of the secondary cable in the case of short circuit of the grounding grid.
The technical scheme of the invention is as follows:
a method for evaluating the safety of a secondary cable during power frequency short circuit of a grounding grid is characterized in that interference of the grounding grid of a transformer substation on the secondary cable is equivalent by adopting a Thevenin circuit, a calculation formula of distribution parameters of the secondary cable is given, a secondary cable core skin potential difference and shielding layer current calculation method is given, and a secondary cable safety evaluation method is provided. Preferably, the method specifically comprises the following steps:
(1) establishing a secondary cable distribution parameter model;
(2) calculating the potential difference of the core skin of the secondary cable and the current of the shielding layer;
(3) the safety of the secondary cable is evaluated.
Preferably, (1) the quadratic cable distribution parameter model includes Z 1 、Z 2 、Z c 、Z p 、Y c 、Y p 、Z T And Y T ，Z 1 And Z 2 For equivalent impedance between the head and tail ends of the cable and the earth network, Z c And Z p Being the self-impedance of the cable core and the shielding layer, Y c For admittance between cable core and shielding, Y p Is a shielding layer andadmittance between earthed nets, Z T For transfer impedance between cable core and shielding layer, Y T Is the transmission admittance of the shielding layer to the cable core.
Preferably, the secondary cable distribution parameter model is decomposed into two loops of a cable core-shielding layer and a shielding layer-grounding grid.
Preferably, for the cable core-shielding layer
Figure BDA0003070356790000021
Figure BDA0003070356790000022
Wherein V is the voltage between the cable core and the shielding layer, I is the current flowing through the cable core-shielding layer loop, I is 0 For shielding the current flowing in the ground loop, V 0 For the voltage between the shield and ground, Z c Is the self-impedance of the cable core, Z T For transfer impedance between cable core and shielding layer, Y c For admittance between cable core and shielding, Y T Is the transmission admittance of the shielding layer to the cable core.
Preferably, there are shielding layer-grounding net loops
Figure BDA0003070356790000023
Figure BDA0003070356790000024
I 0 For shielding the current flowing in the ground loop, V 0 For the voltage between the shield and ground, Z T For transfer impedance between cable core and shielding layer, Z p For the self-impedance of the shield, I is the current flowing in the cable core-shield loop, Y p Is the admittance between the shielding layer and the grounding grid;
the secondary cable has the relevant parameters of
Figure BDA0003070356790000025
Figure BDA0003070356790000026
Figure BDA0003070356790000027
Figure BDA0003070356790000028
Figure BDA0003070356790000029
Y T ＝jωC 12 (10)
In which I 0 And I 1 Zero and 1 st order Bessel functions, respectively, of the first kind, m ═ j ω μ σ 1/2 Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors 0 And ε 1 Dielectric constants of air and the outer water-blocking layer of the shielding layer, d 0 And d 1 Respectively, the armor-to-ground and shield-to-armor distances, r c And q is c Respectively the outer diameter and the inner diameter r of the secondary cable insulation layer e And r i The outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layer e -r i ，R 0 Resistance per unit length of the shielding layer, delta skin depth, C 12 The capacitance of the secondary cable core to the ground through the shielding layer. l is the cable length, K 1 Is a second class of 1 st order Bessel function, K 0 Is a zero order Bessel function of the second kind, mu 0 In terms of vacuum dielectric constant, R is the distance between the cable core wire unit and the shielding layer wire unit, j is an imaginary number unit, omega is the angular frequency of the power frequency short-circuit current, a is the radius of the cylindrical conductor, and epsilon is the electricityThe dielectric constant of the cable insulation.
Preferably, (2) the method for calculating the core sheath potential difference and the shielding layer current of the secondary cable comprises the following steps: at V c And V p Representing the voltage to ground of the cable core and the shield layer, with I c And I p Representing the current flowing in the cable core and the shielding layer, has V c ＝V+V 0 ，V p ＝V 0 ，I c ＝I，I p ＝-I+I 0 . Through simplification, have
Figure BDA0003070356790000031
Figure BDA0003070356790000032
The voltage on the secondary cable core and the shielding layer can be calculated by the equation (11), and the current flowing through the secondary cable core and the shielding layer can be calculated by the equation (12).
Preferably, the method for evaluating the safety of the secondary cable includes: the insulating layer of the secondary cable has a withstand voltage V Durable If core-sheath potential difference V c -V p Greater than V Durable The secondary cable is subjected to insulation breakdown; the secondary cable is burnt due to overlarge current of the shielding layer of the secondary cable, and the maximum allowable current of the shielding layer of the secondary cable is I Durable If the shield layer current I p Is greater than I Durable This will result in secondary cable burnout.
Preferably, the safety rating in the evaluation of safety is four grades: very dangerous, medium dangerous, safer.
Preferably, the security evaluation rule is as follows:
Figure BDA0003070356790000041
the invention has the beneficial effects that:
the interference of a transformer substation grounding grid on a secondary cable is equivalent by adopting a Thevenin circuit, and the core skin potential difference and the shielding layer current of the secondary cable are calculated by adopting a secondary cable distribution parameter model, so that the method has high calculation efficiency and high calculation precision; the method can conveniently evaluate the safety of the secondary cable when the grounding grid is short-circuited.
Drawings
Fig. 1 is a model diagram of quadratic cable distribution parameter calculation.
Fig. 2 is a diagram of the arrangement of the secondary cable relative to the ground grid.
Detailed Description
The technical solutions in the embodiments of the present application will be described clearly and completely below, and it should be understood that the described embodiments are only a part of the embodiments of the present application, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the application, its application, or uses. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.
Example 1
The secondary cable safety evaluation process during the power frequency short circuit of the grounding grid is as follows:
(1) and establishing a secondary cable distribution parameter model. The quadratic cable distribution parametric model is shown in FIG. 1, where Z 1 And Z 2 For equivalent impedance between the head and tail ends of the cable and the earth network, Z c And Z p Being the self-impedance of the cable core and the shielding layer, Y c For admittance between cable core and shielding, Y p For admittance between shielding and earth grids, Z T For transfer impedance between cable core and shielding layer, Y T Is the transfer admittance of the shielding layer to the cable core.
According to the superposition theorem, the model shown in fig. 1 can be decomposed into two loops of cable core-shielding layer and shielding layer-grounding network. For the cable core-shielding layer
Figure BDA0003070356790000051
Figure BDA0003070356790000052
The secondary cable has the relevant parameters of
Figure BDA0003070356790000053
Figure BDA0003070356790000054
Figure BDA0003070356790000055
Figure BDA0003070356790000056
Figure BDA0003070356790000057
Y T ＝jωC 12 (10)
Wherein I 0 And I 1 Bessel function of zeroth and 1 st order, respectively, m ═ j ω μ σ 1/2 Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors 0 And ε 1 Dielectric constant of the outer insulating layer of the air and shield respectively, d 0 And d 1 Distances of armor-ground and shield-armor, r c And q is c Respectively the outer diameter and the inner diameter r of the secondary cable insulation layer e And r i The outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layer e -r i ，R 0 Resistance per unit length of the shielding layer, deltaTo the skin depth, C 12 The capacitance of the secondary cable core to the ground through the shielding layer.
(2) And calculating the potential difference of the core sheath of the secondary cable and the current of the shielding layer. With V c And V p Representing the voltage to ground of the cable core and the shield layer, with I c And I p Representing the current flowing in the cable core and the shielding layer, has V c ＝V+V 0 ，V p ＝V 0 ，I c ＝I，I p ＝-I+I 0 . Through simplification, have
Figure BDA0003070356790000058
Figure BDA0003070356790000059
The voltage on the secondary cable core and the shielding layer can be calculated by the equation (11), and the current flowing through the secondary cable core and the shielding layer can be calculated by the equation (12).
(3) The safety of the secondary cable is evaluated. The insulating layer of the secondary cable has a withstand voltage V Durable If core-sheath potential difference V c -V p Greater than V Durable The secondary cable is subjected to insulation breakdown; the secondary cable is burnt out due to overlarge current of the shielding layer of the secondary cable, and the maximum allowable current of the shielding layer of the secondary cable is I Durable If the shield layer current is I p Is greater than I Durable for This will result in secondary cable burnout. Safety ratings were four grades: very dangerous, medium dangerous, relatively safe. The security evaluation rules are as follows:
Figure BDA0003070356790000061
the secondary cable is arranged opposite to the earth grid as shown in fig. 2. The area of the grounding grid is 100 multiplied by 100m 2 The conductor spacing is 10m, the buried depth is 1m, the soil resistivity is 100 omega.m, the secondary cable is KVVP-22 cable, and the inner half of the shielding layerThe diameter is 3.88mm, the thickness is 0.06mm, the length is 100m, the secondary cable is positioned at the position A, power frequency short-circuit currents are respectively injected into the grounding grid from the C, and the size of the short-circuit current is 25 kA. The core-sheath potential difference V is obtained by calculation c -V p At 2075V, the shield current was 525A. KVVP-22 cable insulating layer withstand voltage V Durable 2000V, maximum allowable current of shielding layer is I Durable At 200A, the safety of the secondary cable under this arrangement and short circuit current was evaluated as "very dangerous" according to the criterion of equation (13).
Example 2
The secondary cable safety evaluation process during the power frequency short circuit of the grounding grid is as follows:
(1) and establishing a secondary cable distribution parameter model. The quadratic cable distribution parametric model is shown in FIG. 1, where Z 1 And Z 2 For equivalent impedance between the head and tail ends of the cable and the earth network, Z c And Z p Is the self-impedance of the cable core and the shielding layer, Y c For admittance between cable core and shielding, Y p For admittance between shielding and earth grids, Z T For transfer impedance between cable core and shielding layer, Y T Is the transfer admittance of the shielding layer to the cable core.
According to the superposition theorem, the model shown in fig. 1 can be decomposed into two loops of cable core-shielding layer and shielding layer-grounding grid. For the cable core-shielding layer
Figure BDA0003070356790000062
Figure BDA0003070356790000063
The secondary cable has the relevant parameters of
Figure BDA0003070356790000071
Figure BDA0003070356790000072
Figure BDA0003070356790000073
Figure BDA0003070356790000074
Figure BDA0003070356790000075
Y T ＝jωC 12 (10)
Wherein I 0 And I 1 Zero and 1 st order bessel functions, respectively, m ═ j ω μ σ 1/2 Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors 0 And ε 1 Dielectric constants of air and an outer insulating layer of the shield layer, d 0 And d 1 Distances of armor-ground and shield-armor, r c And q is c Respectively the outer diameter and the inner diameter r of the secondary cable insulation layer e And r i The outer diameter and the inner diameter of the shielding layer, respectively, and the thickness of the shielding layer Δ r e -r i ，R 0 Resistance per unit length of the shielding layer, delta skin depth, C 12 The capacitance of the secondary cable core to the ground through the shielding layer.
(2) And calculating the potential difference of the core sheath of the secondary cable and the current of the shielding layer. With V c And V p Representing the voltage to earth of the cable core and the shield layer, with I c And I p Representing the current flowing in the cable core and the shielding layer, then has V c ＝V+V 0 ，V p ＝V 0 ，I c ＝I，I p ＝-I+I 0 . Through simplification, have
Figure BDA0003070356790000076
Figure BDA0003070356790000077
The voltage on the secondary cable core and the shielding layer can be calculated by the equation (11), and the current flowing through the secondary cable core and the shielding layer can be calculated by the equation (12).
(3) The safety of the secondary cable is evaluated. The insulating layer of the secondary cable has a withstand voltage V Durable Core sheath potential difference V c -V p Greater than V Durable for The secondary cable is subjected to insulation breakdown; the secondary cable is burnt due to overlarge current of the shielding layer of the secondary cable, and the maximum allowable current of the shielding layer of the secondary cable is I Durable for If the shield layer current is I p Is greater than I Durable This will result in secondary cable burnout. Safety ratings were four grades: very dangerous, medium dangerous, safer. The security evaluation rules are as follows:
Figure BDA0003070356790000081
the secondary cable is arranged opposite to the earth grid as shown in fig. 2. The area of the grounding grid is 100 multiplied by 100m 2 The conductor spacing is 10m, the buried depth is 1m, the soil resistivity is 100 omega.m, the secondary cable is a KVVP-22 cable, the radius in the shielding layer is 3.88mm, the thickness is 0.06mm, the length is 100m, the secondary cable is positioned at the position A, power frequency short-circuit currents are respectively injected into the grounding grid from F, and the magnitude of the short-circuit current is 40 kA. The core-sheath potential difference V is obtained by calculation c -V p 1660V, KVVP-22 cable insulation layer withstand voltage V Durable for Is 2000V, in this case V c -V p /V Durable 0.83. When no drain wire is laid, the current of the shielding layer is 445A, the current of the shielding layer after the drain wire is laid is 150A, and the maximum allowable current of the shielding layer is I Durable 200A, after laying drainage wire I p /I Durable 0.75. According to the criterion of the formula (13), the safety of the secondary cable under the condition of laying the drainage line and the short-circuit current is evaluated as medium danger.
Example 3
The secondary cable safety evaluation process during the power frequency short circuit of the grounding grid is as follows:
(1) and establishing a secondary cable distribution parameter model. The quadratic cable distribution parametric model is shown in FIG. 1, where Z 1 And Z 2 For equivalent impedance between the head and tail ends of the cable and the earth network, Z c And Z p Being the self-impedance of the cable core and the shielding layer, Y c For admittance between cable core and shielding, Y p For admittance between shielding and earth grids, Z T For transfer impedance between cable core and shielding layer, Y T Is the transfer admittance of the shielding layer to the cable core.
According to the superposition theorem, the model shown in fig. 1 can be decomposed into two loops of cable core-shielding layer and shielding layer-grounding network. For the cable core-shielding layer
Figure BDA0003070356790000082
Figure BDA0003070356790000083
The secondary cable has the relevant parameters of
Figure BDA0003070356790000091
Figure BDA0003070356790000092
Figure BDA0003070356790000093
Figure BDA0003070356790000094
Figure BDA0003070356790000095
Y T ＝jωC 12 (10)
Wherein I 0 And I 1 Zero and 1 st order bessel functions, respectively, m ═ j ω μ σ 1/2 Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductor 0 And ε 1 Dielectric constant of the outer insulating layer of the air and shield respectively, d 0 And d 1 Respectively, the armor-to-ground and shield-to-armor distances, r c And q is c Respectively the outer diameter and the inner diameter r of the secondary cable insulation layer e And r i The outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layer e -r i ，R 0 Resistance per unit length of the shielding layer, delta skin depth, C 12 The capacitance of the secondary cable core to the ground through the shielding layer.
(2) And calculating the potential difference of the core sheath of the secondary cable and the current of the shielding layer. With V c And V p Representing the voltage to ground of the cable core and the shield layer, with I c And I p Representing the current flowing in the cable core and the shielding layer, then has V c ＝V+V 0 ，V p ＝V 0 ，I c ＝I，I p ＝-I+I 0 . Through simplification, have
Figure BDA0003070356790000096
Figure BDA0003070356790000097
The voltage on the secondary cable core and the shielding layer can be calculated by the equation (11), and the current flowing through the secondary cable core and the shielding layer can be calculated by the equation (12).
(3) The safety of the secondary cable is evaluated. The insulating layer of the secondary cable has a withstand voltage V Durable If core-sheath potential difference V c -V p Greater than V Durable The secondary cable is subjected to insulation breakdown; the secondary cable is burnt due to overlarge current of the shielding layer of the secondary cable, and the maximum allowable current of the shielding layer of the secondary cable is I Durable If the shield layer current I p Is greater than I Durable This will result in secondary cable burnout. Safety ratings were four grades: very dangerous, medium dangerous, safer. The security evaluation rules are as follows:
Figure BDA0003070356790000101
the secondary cable is arranged opposite to the earth grid as shown in fig. 2. The area of the grounding grid is 100 multiplied by 100m 2 The conductor spacing is 10m, the buried depth is 1m, the soil resistivity is 100 omega.m, the secondary cable is a KVVP-22 cable, the radius in the shielding layer is 3.88mm, the thickness is 0.06mm, the length is 100m, the secondary cable is positioned at the position A, power frequency short-circuit currents are respectively injected into the grounding grid from the position D, and the short-circuit current is 30 kA. The core-sheath potential difference V is obtained by calculation c -V p 1188V, and the KVVP-22 cable insulation layer withstand voltage V Durable Is 2000V, in this case V c -V p /V Durable 0.59. The current of the shielding layer is 264A when the drain wire is not laid, the current of the shielding layer is 129A after the drain wire is laid, and the maximum allowable current of the shielding layer is I Durable 200A, after laying drainage wire I p /I Durable 0.65. According to the criterion of the formula (13), the safety of the secondary cable under the condition of laying the drainage line and the short-circuit current is evaluated as 'dangerous'.
Example 4
The secondary cable safety evaluation process during the power frequency short circuit of the grounding grid is as follows:
(1) and establishing a secondary cable distribution parameter model. The quadratic cable distribution parametric model is shown in FIG. 1, where Z 1 And Z 2 For equivalent impedance between the head and tail ends of the cable and the earth network, Z c And Z p Is the self-impedance of the cable core and the shielding layer, Y c For admittance between cable core and shielding, Y p For admittance between shielding and earth grids, Z T For cable cores and shielding layersTransfer impedance between, Y T Is the transmission admittance of the shielding layer to the cable core.
According to the superposition theorem, the model shown in fig. 1 can be decomposed into two loops of cable core-shielding layer and shielding layer-grounding grid. For the cable core-shielding layer
Figure BDA0003070356790000102
Figure BDA0003070356790000103
The secondary cable has a related parameter of
Figure BDA0003070356790000104
Figure BDA0003070356790000111
Figure BDA0003070356790000112
Figure BDA0003070356790000113
Figure BDA0003070356790000114
Y T ＝jωC 12 (10)
Wherein I 0 And I 1 Zero and 1 st order bessel functions, respectively, m ═ j ω μ σ 1/2 Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors 0 And ε 1 Dielectric constant of the outer insulating layer of the air and shield respectively, d 0 And d 1 Are respectively provided withIs the distance between armor-ground and shield-armor, r c And q is c Respectively the outer diameter and the inner diameter r of the secondary cable insulation layer e And r i The outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layer e -r i ，R 0 Resistance per unit length of the shielding layer, δ is skin depth, C 12 The capacitance of the secondary cable core to the ground through the shielding layer.
(2) And calculating the potential difference of the core sheath of the secondary cable and the current of the shielding layer. With V c And V p Representing the voltage to earth of the cable core and the shield layer, with I c And I p Representing the current flowing in the cable core and the shielding layer, has V c ＝V+V 0 ，V p ＝V 0 ，I c ＝I，I p ＝-I+I 0 . Through simplification, have
Figure BDA0003070356790000115
Figure BDA0003070356790000116
The voltage on the secondary cable core and the shielding layer can be calculated by the equation (11), and the current flowing through the secondary cable core and the shielding layer can be calculated by the equation (12).
(3) The safety of the secondary cable is evaluated. The insulating layer of the secondary cable has a withstand voltage V Durable If core-sheath potential difference V c -V p Greater than V Durable The secondary cable is subjected to insulation breakdown; the secondary cable is burnt due to overlarge current of the shielding layer of the secondary cable, and the maximum allowable current of the shielding layer of the secondary cable is I Durable If the shield layer current is I p Is greater than I Durable This will result in secondary cable burnout. Safety ratings were rated in four grades: very dangerous, medium dangerous, safer. The security evaluation rules are as follows:
Figure BDA0003070356790000121
the secondary cable is arranged opposite to the earth grid as shown in fig. 2. The area of the grounding grid is 100 multiplied by 100m 2 The conductor spacing is 10m, the buried depth is 1m, the soil resistivity is 100 omega.m, the secondary cable is a KVVP-22 cable, the radius in the shielding layer is 3.88mm, the thickness is 0.06mm, the length is 100m, the secondary cable is positioned at a position B, power frequency short-circuit currents are respectively injected into the grounding grid from E, and the magnitude of the short-circuit current is 20 kA. The core-sheath potential difference V is obtained by calculation c -V p The voltage V is 212V, and the KVVP-22 cable insulation layer endures the voltage V Durable Is 2000V, in this case V c -V p /V Durable for 0.11. The current of the shielding layer is 50A when no drain wire is laid, the current of the shielding layer is 23.2A after the drain wire is laid, and the maximum allowable current of the shielding layer is I Durable 200A, after laying drainage wire I p /I Durable for 0.12. According to the criterion of the formula (13), the safety of the secondary cable under the condition of laying the drainage line and the short-circuit current is evaluated as 'very safe'.

Claims (5)

1. A method for evaluating the safety of a secondary cable during power frequency short circuit of a grounding grid is characterized in that interference of the grounding grid of a transformer substation on the secondary cable is equivalent by adopting a Thevenin circuit, a calculation formula of distribution parameters of the secondary cable is given, a secondary cable core potential difference and shielding layer current calculation method is given, and a secondary cable safety evaluation method is provided;
the method specifically comprises the following steps:
(1) establishing a secondary cable distribution parameter model;
(2) calculating the potential difference of the core skin of the secondary cable and the current of the shielding layer;
(3) evaluating the safety of the secondary cable;
the secondary cable distribution parameter model in the step (1) comprises Z 1 、Z 2 、Z c 、Z p 、Y c 、Y p 、Z T And Y T ，Z 1 And Z 2 For equivalent impedance between the head and tail ends of the cable and the earth network, Z c And Z p Is the self-impedance of the cable core and the shielding layer，Y c For admittance between cable core and shielding, Y p For admittance between shielding and earth grids, Z T For transfer impedance between cable core and shielding layer, Y T A transmission admittance of the shielding layer to the cable core;
the secondary cable distribution parameter model is decomposed into two loops of a cable core, a shielding layer and a grounding grid;
for the cable core-shielding layer
Figure FDA0003601407210000011
Figure FDA0003601407210000012
Wherein V is the voltage between the cable core and the shielding layer, I is the current flowing through the cable core-shielding layer loop, I is 0 For shielding the current flowing in the ground loop, V 0 For the voltage between the shield and ground, Z c Is the self-impedance of the cable core, Z T For transfer impedance between cable core and shielding layer, Y c For admittance between cable core and shielding, Y T The transmission admittance of the shielding layer to the cable core; to the shielding layer-grounding net loop have
Figure FDA0003601407210000021
Figure FDA0003601407210000022
I 0 For shielding the current flowing in the ground loop, V 0 For the voltage between the shield and ground, Z T For transfer impedance between cable core and shielding layer, Z p For the self-impedance of the shield, I is the current flowing in the cable core-shield loop, Y p For shielding layer and grounding gridAdmittance in between;
the secondary cable has the relevant parameters of
Figure FDA0003601407210000023
Figure FDA0003601407210000024
Figure FDA0003601407210000025
Figure FDA0003601407210000026
Figure FDA0003601407210000027
Y T ＝jωC 12 (10)
Wherein J 0 And J 1 First order zero and 1 st order Bessel functions, respectively, m ═ j ω μ σ 1/2 Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors 0 And ε 1 Dielectric constant of air and the outer water-blocking layer of the shielding layer, d 0 And d 1 Respectively, the armor-to-ground and shield-to-armor distances, r c And q is c Respectively the outer diameter and the inner diameter r of the secondary cable insulation layer e And r i The outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layer e -r i ，R 0 Resistance per unit length of the shielding layer, delta skin depth, C 12 The capacitance of the secondary cable core to the ground through the shielding layer; l is the cable length, K 1 As a second class of 1 st order Bessel function, K 0 Is a zero order Bessel function of the second kind, mu 0 Is a dielectric constant in vacuumR is the distance between the cable core wire unit and the shielding layer wire unit, j is an imaginary number unit, omega is the angular frequency of the power frequency short-circuit current, a is the radius of the cylindrical conductor, and epsilon is the dielectric constant of the cable insulation layer.
2. The method for evaluating the safety of the secondary cable in the power frequency short circuit of the grounding grid according to claim 1, wherein the method for calculating the core skin potential difference and the shielding layer current of the secondary cable comprises the following steps: with V c And V p Representing the voltage to earth of the cable core and the shield layer, with I c And I p Representing the current flowing in the cable core and the shielding layer, has V c ＝V+V 0 ，V p ＝V 0 ，I c ＝I，I p ＝-I+I 0 (ii) a Through simplification, have
Figure FDA0003601407210000031
Figure FDA0003601407210000032
The voltage on the secondary cable core and the shielding layer can be calculated by the equation (11), and the current flowing through the secondary cable core and the shielding layer can be calculated by the equation (12).
3. The method for evaluating the safety of the secondary cable in the power frequency short circuit of the grounding grid according to claim 2, wherein the method for evaluating the safety of the secondary cable comprises the following steps: the insulating layer of the secondary cable has a withstand voltage V Durable for If core-sheath potential difference V c -V p Greater than V Durable The secondary cable is subjected to insulation breakdown; the secondary cable is burnt due to overlarge current of the shielding layer of the secondary cable, and the maximum allowable current of the shielding layer of the secondary cable is I Durable If the shield layer current I p Is greater than I Durable This will result in secondary cable burnout.
4. The method for evaluating the safety of the secondary cable during the power frequency short circuit of the grounding grid according to claim 3, wherein the safety level in the safety evaluation is four levels: very dangerous, medium dangerous, relatively safe.
5. The method for evaluating the safety of the secondary cable in the power frequency short circuit of the grounding grid according to claim 4, wherein the safety evaluation rule is as follows:
Figure FDA0003601407210000041
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CN108387770A (en) * 2018-03-08 2018-08-10 清华大学 In a kind of impact transient process in substation secondary cable overvoltage computational methods
CN111830364A (en) * 2020-06-04 2020-10-27 国网冀北电力有限公司经济技术研究院 Calculation method for calculating transient interference signals at two ends of secondary cable of transformer substation
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JPH06292325A (en) * 1993-04-02 1994-10-18 Chubu Electric Power Co Inc Power cable insulation deterioration detecting circuit
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