CN113419134A - 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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CN113419134A
CN113419134A CN202110542871.6A CN202110542871A CN113419134A CN 113419134 A CN113419134 A CN 113419134A CN 202110542871 A CN202110542871 A CN 202110542871A CN 113419134 A CN113419134 A CN 113419134A
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shielding layer
secondary cable
cable
core
safety
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CN113419134B (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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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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 grounding grid of the transformer substation. 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 invention discloses a method for equalizing the interference of a transformer substation grounding grid to a secondary cable by adopting a Thevenin circuit, namely a series connection of a voltage source and an 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 Z1、Z2、Zc、Zp、Yc、Yp、ZTAnd YT，Z1And Z2For equivalent impedance between the head and tail ends of the cable and the earth network, ZcAnd ZpBeing the self-impedance of the cable core and the shielding layer, YcFor admittance between cable core and shielding, YpFor admittance between shielding and earth grids, ZTFor transfer impedance between cable core and shielding layer, YTIs 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 is0For shielding the current flowing in the ground loop, V0For the voltage between the shield and ground, ZcIs the self-impedance of the cable core, ZTFor transfer impedance between cable core and shielding layer, YcFor admittance between cable core and shielding, YTIs the transmission admittance of the shielding layer to the cable core.
Preferably, there are shielding layer-grounding net loops
Figure BDA0003070356790000023
Figure BDA0003070356790000024
I0For shielding the current flowing in the ground loop, V0For the voltage between the shield and ground, ZTFor transfer impedance between cable core and shielding layer, ZpFor the self-impedance of the shield, I is the current flowing in the cable core-shield loop, YpIs 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
YT＝jωC12 (10)
Wherein I0And I1Zero and 1 st order Bessel functions, respectively, of the first kind, m ═ j ω μ σ1/2Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors0And ε1Are respectively airAnd the dielectric constant of the outer water-blocking layer of the shielding layer, d0And d1Respectively, the armor-to-ground and shield-to-armor distances, rcAnd q iscRespectively the outer diameter and the inner diameter r of the secondary cable insulation layereAnd riThe outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layere-ri，R0Resistance per unit length of the shielding layer, delta skin depth, C12The capacitance of the secondary cable core to the ground through the shielding layer. l is the cable length, K1Is a second class of 1 st order Bessel function, K0Is a zero order Bessel function of the second kind, mu0The dielectric constant is vacuum, 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 dielectric constant of the cable insulation layer.
Preferably, (2) the method for calculating the core sheath potential difference and the shielding layer current of the secondary cable comprises the following steps: with VcAnd VpRepresenting the voltage to ground of the cable core and the shield layer, with IcAnd IpRepresenting the current flowing in the cable core and the shielding layer, has Vc＝V+V0，Vp＝V0，Ic＝I，Ip＝-I+I0. 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 VDurableIf core-sheath potential difference Vc-VpGreater than VDurableThe secondary cable is subjected to insulation breakdown; screen of secondary cableThe shielding layer has overlarge current, the secondary cable can be burnt, and the maximum allowable current of the shielding layer of the secondary cable is IDurableIf the shield layer current IpIs greater than IDurableThis 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 Z1And Z2For equivalent impedance between the head and tail ends of the cable and the earth network, ZcAnd ZpBeing the self-impedance of the cable core and the shielding layer, YcFor admittance between cable core and shielding, YpFor admittance between shielding and earth grids, ZTFor transfer impedance between cable core and shielding layer, YTIs 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 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
YT＝jωC12 (10)
Wherein I0And I1Zero and 1 st order bessel functions, respectively, m ═ j ω μ σ1/2Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors0And ε1Dielectric constants of air and an outer insulating layer of the shield layer, d0And d1Respectively, the armor-to-ground and shield-to-armor distances, rcAnd q iscRespectively the outer diameter and the inner diameter r of the secondary cable insulation layereAnd riThe outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layere-ri，R0Resistance per unit length of the shielding layer, delta skin depth, C12The 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 VcAnd VpRepresenting the voltage to ground of the cable core and the shield layer, with IcAnd IpRepresenting the current flowing in the cable core and the shielding layer, has Vc＝V+V0，Vp＝V0，Ic＝I，Ip＝-I+I0. 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 VDurableIf core-sheath potential difference Vc-VpGreater than VDurableWill leadCausing insulation breakdown of the secondary cable; 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 IDurableIf the shield layer current IpIs greater than IDurableThis will result in secondary cable burnout. Safety ratings were four grades: very dangerous, medium dangerous, safer. 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 100m2The 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 C, and the size of the short-circuit current is 25 kA. The core-sheath potential difference V is obtained by calculationc-VpAt 2075V, the shield current was 525A. KVVP-22 cable insulating layer withstand voltage VDurable2000V, maximum allowable current of shielding layer is IDurableAt 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 Z1And Z2For equivalent impedance between the head and tail ends of the cable and the earth network, ZcAnd ZpBeing the self-impedance of the cable core and the shielding layer, YcFor admittance between cable core and shielding, YpFor admittance between shielding and earth grids, ZTFor transfer impedance between cable core and shielding layer, YTIs 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 network. 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
YT＝jωC12 (10)
Wherein I0And I1Zero and 1 st order bessel functions, respectively, m ═ j ω μ σ1/2Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors0And ε1Dielectric constants of air and an outer insulating layer of the shield layer, d0And d1Respectively, the armor-to-ground and shield-to-armor distances, rcAnd q iscRespectively the outer diameter and the inner diameter r of the secondary cable insulation layereAnd riAre respectively screensOuter and inner diameters of the shield layer, and a thickness Δ r of the shield layere-ri，R0Resistance per unit length of the shielding layer, delta skin depth, C12The 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 VcAnd VpRepresenting the voltage to ground of the cable core and the shield layer, with IcAnd IpRepresenting the current flowing in the cable core and the shielding layer, has Vc＝V+V0，Vp＝V0，Ic＝I，Ip＝-I+I0. 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 VDurableIf core-sheath potential difference Vc-VpGreater than VDurableThe 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 IDurableIf the shield layer current IpIs greater than IDurableThis 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 100100m2The 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 calculationc-Vp1660V, KVVP-22 cable insulation layer withstand voltage VDurableIs 2000V, in this case Vc-Vp/VDurable0.83. 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 IDurable200A, after laying drainage wire Ip/IDurable0.75. 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 Z1And Z2For equivalent impedance between the head and tail ends of the cable and the earth network, ZcAnd ZpBeing the self-impedance of the cable core and the shielding layer, YcFor admittance between cable core and shielding, YpFor admittance between shielding and earth grids, ZTFor transfer impedance between cable core and shielding layer, YTIs 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 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
YT＝jωC12 (10)
Wherein I0And I1Zero and 1 st order bessel functions, respectively, m ═ j ω μ σ1/2Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors0And ε1Dielectric constants of air and an outer insulating layer of the shield layer, d0And d1Respectively, the armor-to-ground and shield-to-armor distances, rcAnd q iscRespectively the outer diameter and the inner diameter r of the secondary cable insulation layereAnd riThe outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layere-ri，R0Resistance per unit length of the shielding layer, delta skin depth, C12The 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 VcAnd VpRepresenting the voltage to ground of the cable core and the shield layer, with IcAnd IpRepresenting the current flowing in the cable core and the shielding layer, has Vc＝V+V0，Vp＝V0，Ic＝I，Ip＝-I+I0. 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 VDurableIf core-sheath potential difference Vc-VpGreater than VDurableThe 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 IDurableIf the shield layer current IpIs greater than IDurableThis 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 100m2The 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 calculationc-Vp1188V, and the KVVP-22 cable insulation layer withstand voltage VDurableIs 2000V, in this case Vc-Vp/VDurable0.59. When no drainage line is laidThe current of the shielding layer is 264A, the current of the shielding layer after the drain wire is laid is 129A, and the maximum allowable current of the shielding layer is IDurable200A, after laying drainage wire Ip/IDurable0.65. 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 Z1And Z2For equivalent impedance between the head and tail ends of the cable and the earth network, ZcAnd ZpBeing the self-impedance of the cable core and the shielding layer, YcFor admittance between cable core and shielding, YpFor admittance between shielding and earth grids, ZTFor transfer impedance between cable core and shielding layer, YTIs 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 network. For the cable core-shielding layer
Figure BDA0003070356790000102
Figure BDA0003070356790000103
The secondary cable has the relevant parameters of
Figure BDA0003070356790000104
Figure BDA0003070356790000111
Figure BDA0003070356790000112
Figure BDA0003070356790000113
Figure BDA0003070356790000114
YT＝jωC12 (10)
Wherein I0And I1Zero and 1 st order bessel functions, respectively, m ═ j ω μ σ1/2Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors0And ε1Dielectric constants of air and an outer insulating layer of the shield layer, d0And d1Respectively, the armor-to-ground and shield-to-armor distances, rcAnd q iscRespectively the outer diameter and the inner diameter r of the secondary cable insulation layereAnd riThe outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layere-ri，R0Resistance per unit length of the shielding layer, delta skin depth, C12The 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 VcAnd VpRepresenting the voltage to ground of the cable core and the shield layer, with IcAnd IpRepresenting the current flowing in the cable core and the shielding layer, has Vc＝V+V0，Vp＝V0，Ic＝I，Ip＝-I+I0. 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 VDurableIf core-sheath potential difference Vc-VpGreater than VDurableThe 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 IDurableIf the shield layer current IpIs greater than IDurableThis will result in secondary cable burnout. Safety ratings were 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 100m2The 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 calculationc-VpThe voltage V is 212V, and the KVVP-22 cable insulation layer endures the voltage VDurableIs 2000V, in this case Vc-Vp/VDurable0.11. 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 IDurable200A, after laying drainage wire Ip/IDurable0.12. 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 (10)

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.
2. The method of claim 1, wherein the safety of the secondary cable is evaluated when the grounding grid is in power frequency short circuit,
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.
3. The method for evaluating the safety of the secondary cable in the case of the power frequency short circuit of the grounding grid according to claim 2, wherein (1) the secondary cable distribution parameter model comprises Z1、Z2、Zc、Zp、Yc、Yp、ZTAnd YT，Z1And Z2For equivalent impedance between the head and tail ends of the cable and the earth network, ZcAnd ZpBeing the self-impedance of the cable core and the shielding layer, YcFor admittance between cable core and shielding, YpFor admittance between shielding and earth grids, ZTFor transfer impedance between cable core and shielding layer, YTIs the transmission admittance of the shielding layer to the cable core.
4. The method for evaluating the safety of the secondary cable in the power frequency short circuit of the grounding grid according to claim 3, wherein the secondary cable distribution parameter model is decomposed into two loops of a cable core-shielding layer and a shielding layer-grounding grid.
5. The method of claim 4, wherein the cable core-shielding layer is provided with
Figure FDA0003070356780000011
Figure FDA0003070356780000012
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 is0For shielding the current flowing in the ground loop, V0For the voltage between the shield and ground, ZcIs the self-impedance of the cable core, ZTFor transfer impedance between cable core and shielding layer, YcFor admittance between cable core and shielding, YTIs the transmission admittance of the shielding layer to the cable core.
6. The method of claim 5, wherein the shield-ground network circuit comprises
Figure FDA0003070356780000021
Figure FDA0003070356780000022
I0For shielding the current flowing in the ground loop, V0For the voltage between the shield and ground, ZTFor transfer impedance between cable core and shielding layer, ZpFor the self-impedance of the shield, I is the current flowing in the cable core-shield loop, YpIs the admittance between the shielding layer and the grounding grid;
the secondary cable has the relevant parameters of
Figure FDA0003070356780000023
Figure FDA0003070356780000024
Figure FDA0003070356780000025
Figure FDA0003070356780000026
Figure FDA0003070356780000027
YT＝jωC12 (10)
Wherein I0And I1Zero and 1 st order Bessel functions, respectively, of the first kind, m ═ j ω μ σ1/2Mu and sigma are respectively the permeability and conductivity, epsilon, of the corresponding conductors0And ε1Dielectric constant of air and the outer water-blocking layer of the shielding layer, d0And d1Respectively, the armor-to-ground and shield-to-armor distances, rcAnd q iscRespectively the outer diameter and the inner diameter r of the secondary cable insulation layereAnd riThe outer diameter and the inner diameter of the shielding layer respectively, and the thickness delta r of the shielding layere-ri，R0Resistance per unit length of the shielding layer, delta skin depth, C12The capacitance of the secondary cable core to the ground through the shielding layer. l is the cable length, K1Is a second class of 1 st order Bessel function, K0Is a zero order Bessel function of the second kind, mu0The dielectric constant is vacuum, 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 dielectric constant of the cable insulation layer.
7. The method for evaluating the safety of the secondary cable in the power frequency short circuit of the grounding grid according to claim 6, wherein the method for calculating the core skin potential difference and the shielding layer current of the secondary cable comprises the following steps: with VcAnd VpRepresenting the voltage to ground of the cable core and the shield layer, with IcAnd IpRepresenting the current flowing in the cable core and the shielding layer, has Vc＝V+V0，Vp＝V0，Ic＝I，Ip＝-I+I0. Through simplification, have
Figure FDA0003070356780000031
Figure FDA0003070356780000032
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).
8. The method for evaluating the safety of the secondary cable in the power frequency short circuit of the grounding grid according to claim 7, 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 VDurableIf core-sheath potential difference Vc-VpGreater than VDurableThe 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 IDurableIf the shield layer current IpIs greater than IDurableThis will result in secondary cable burnout.
9. The method for evaluating the safety of the secondary cable during the power frequency short circuit of the grounding grid according to claim 8, wherein the safety level in the evaluation is four levels: very dangerous, medium dangerous, safer.
10. The method for evaluating the safety of the secondary cable in the power frequency short circuit of the grounding grid according to claim 9, wherein the safety evaluation rule is as follows:
Figure FDA0003070356780000033
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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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