CN104568719B - A kind of appraisal procedure of grounding net of transformer substation - Google Patents

Abstract

The invention relates to a method for evaluating a substation grounding grid, comprising steps: (1) applying Faraday's law to deduce the thickness thinning rate of the grounding grid metal; (2) determining the minimum cross-sectional area S _g of the grounding wire when corrosion is not considered; (3) Determine the minimum cross-sectional area S _g2 of the grounding wire when considering the influence of corrosion factors; (4) Determine the evaluation period of the grounding grid. The calculation process of this method is clear and the accuracy is high, which provides a strong theoretical support for the safe operation of the power system grounding grid.

Description

An Evaluation Method for Substation Grounding Grid

technical field

The invention belongs to the technical field of substation grounding grids, in particular to an evaluation method for substation grounding grids.

Background technique

Grounding technology involves numerical analysis and optimal design of electrical parameters of the grounding system, analysis of soil structure parameters, resistance reduction technology in areas with high soil resistivity, scientific measurement of grounding resistance, corrosion diagnosis technology of grounding grids, and impact characteristics of grounding devices. A large number of literatures show that the main research results of the grounding grid are concentrated in three aspects. One is the testing, analysis and optimal design of grounding system parameters. Not only grounding impedance, but also its step voltage, contact voltage and potential distribution on the grounding grid should be considered. Secondly, the electrical integrity, heat capacity, and resistance of the grounding grid should also be considered. For parameters such as corrosion performance, it is generally necessary to measure soil resistivity to establish a layered model of soil, and obtain more accurate measurement and analysis results and guide optimal design through numerical analysis. The second is the status of the grounding grid and its evaluation. According to the grounding grid test and analysis results, the safety of the grounding grid is judged, mainly including the tolerance of grounding resistance, contact voltage, step voltage, grounding downconductor and heat capacity of the grounding grid. Value, with reference to domestic and foreign standards, evaluate the safety of the grounding system. The third is grounding grid fault monitoring and diagnosis, measuring the connectivity of the grounding system and diagnosing the corrosion of the grounding grid through the grounding down conductor, and proposing corresponding rectification and maintenance measures to provide guarantee for the safe operation of the grounding grid.

Research on effective grounding grid corrosion detection and fault diagnosis methods and grounding grid measurability issues, for correctly grasping the operation status of the grounding grid, evaluating the feasibility and applicability of various grounding grid fault diagnosis algorithms, and timely discovering hidden dangers of the grounding grid And taking corresponding measures has important guiding significance, and is more conducive to protecting people's lives, property and safety.

Many of the electrical accidents in the power system are related to the defects of the grounding device. The reason is that the grounding grid is a hidden facility buried deep underground in the power system. The main technical problems exposed on site are corrosion of the grounding grid, small cross-section of the grounding down-conductor, and poor connection between grounding bodies. These problems are caused by power failures. hidden dangers. Grounding grid corrosion is a gradual process. When the corrosion reaches a certain level, it will enter a fault state, reduce reliability or even fail performance, and cannot meet the requirements of safe operation of the power system. Due to resource and economic reasons in my country, the material used for the grounding grid is mainly ordinary carbon steel. The corrosion of the grounding grid usually presents a localized corrosion form. After corrosion, the carbon steel material of the grounding grid becomes brittle, peeled off, loose, thinned, and even fractured. , so the serious corrosion of the power grounding grid is currently the most prominent problem in my country's power grounding grid.

According to the order of corrosion degree, the most serious is the corrosion of the grounding belt in the cable trench, followed by the corrosion of the grounding down-conductor, and finally the corrosion of the main grounding grid. The corrosion time is generally 5-20 years after being put into operation. Serious Corrosion occurred within 3 years after commissioning. Depending on the soil resistivity, soil electrochemical properties, distribution of microorganisms, stray current, climatic conditions, materials used, construction techniques, and anti-corrosion measures, the maximum annual corrosion rate also varies.

Contents of the invention

The purpose of the present invention is to provide a method for evaluating the substation grounding grid in order to overcome the deficiencies of the prior art.

The present invention solves its technical problem and realizes by taking the following technical solutions:

A method for evaluating a substation grounding grid, the method comprising steps as follows:

(1) Apply Faraday's law to deduce the thickness reduction rate of the grounding grid metal;

The corrosion process of the grounding grid material in the soil is applicable to Faraday's law: △m=MQ/zF, where △m is the mass of the dissolved metal; M is its atomic weight; Q is the transferred charge; z is the valence of the metal ion; F is Faraday's constant, and the following formula can be derived by means of the density ρ _s :

J _A ＝Q/St＝△m/St×zF/M-△s/t×zFρ _s /M

In the formula, J _A is the current density of the anode sub-reaction of the metal ion channel: S is the surface area of the electrode: t is the time; △s is the thickness to be stripped;

Wf _a J _A ＝△s/t

f _a =M/zFρ _s =3.2684/z(M/g·mol ^-1 )(g·cm ^-3 /ρ _s )mm·a ^-1 /mA·cm ^-2

v＝f _b J _A ＝△m/St

f _b = M/z F = 0.32311/z (M/g·mol ^-1 ) g·cm ^-2 ·h ^-1 /mA·cm ^-2

In the formula, W is the thickness thinning rate, and v is the mass loss rate per unit surface area;

W=f _c v (I)

f _c =f _a /f _b =1/ρ _s =8.76 ((g·cm ^-3 /ρ _s )mm·a ^-1 /g·cm ^-2 ·h ^-1 )

Among them, f _a , f _b , f _c are conversion factors of grounding grid materials;

The thickness reduction rate of the grounding grid metal can be obtained by the above formula (I) and the conversion factors of f _a , f _b and f _c ;

(2) When it is determined that corrosion is not considered, the minimum cross-sectional area S _g1 of the grounding wire;

According to thermal stability conditions, when corrosion is not considered, the minimum cross-sectional area S _g1 of the grounding wire is in mm ² ,

S _g1 =Ig/c·t _e ^-2 (II)

Among them, in the formula, Ig is the stable value of the short-circuit current flowing through the grounding wire, c is the thermal stability coefficient of the grounding wire material, and t _e is the equivalent duration of the short circuit; the specific provisions of Ig, c, and t _e are:

① Ig is the stable value of short-circuit current flowing through the grounding wire, unit: A. For effective grounding systems and low-resistance grounding systems, consider single or two grounding short-circuit currents; for non-grounding, arc suppression coil grounding and high-resistance grounding systems, then Considering two grounding short-circuit currents at different points, Ig is determined according to the 5-10-year development plan of the system and the maximum operating mode of the system, and Ig is calculated based on 75% of the maximum short-circuit current of the busbar of the substation;

②c is the thermal stability coefficient of the grounding wire material, which is determined according to the type, performance, maximum allowable temperature of the material and the initial temperature of the grounding wire before short circuit. Steel takes 70, copper takes 210, and aluminum takes 120;

③t _e is the equivalent duration of short circuit, unit: s, 0.35s for 500kV system, 0.6-0.7s for 200kV system, fault duration of 110kV and below voltage level system is selected according to the actual situation, considering that the relay protection configuration is close to , take the same level value of 220kV system as 0.6-0.7s;

(3) When determining the influence of corrosion factors, the minimum cross-sectional area S _g2 of the grounding wire;

S _g2 = (A-2×w×n)(B-2×w×n) (III)

Among them, when the grounding grid is a round steel type, in the formula, A indicates the width of the grounding grid, B indicates the thickness of the grounding grid, n indicates the service life, and w indicates the thinning rate of the grounding grid material, unit: mm/a;

(4) Determine the evaluation period of the grounding grid, the specific method is:

① For the grounding grid that has been used, compare the implementation specification of the actual grounding grid with the value of S _g2 , take the n value when the implementation specification value is closest to the calculated S _g value, and obtain the estimated service life of the actual grounding grid;

②For the calculated minimum cross-sectional area S _g of the grounding wire when corrosion is not considered, the in-service life of the design value of S _g1 is obtained by comparing the values of S _g1 and S _g2 calculated according to different years.

Moreover, for the conversion factors of f _a , f _b , and f _c in the step (1), the common conversion factors of grounding grid materials are as follows:

Advantage and positive effect of the present invention are:

The present invention measures the average corrosion rate (g/dm ² ·a) of different materials and the anode reaction current density J _A to know the thickness of the metal stripping material, and to know the transferred charge by measuring the mass loss Δm of different materials . Then know the thickness of the metal peeling material, calculate the grounding grid material thinning rate w (mm/a), consider the influence of grounding grid corrosion factors, obtain the scientific material specifications of the grounding grid, and deduce the service life of the grounding grid. The calculation process of this method is clear and the accuracy is high, which provides a strong theoretical support for the safe operation of the power system grounding grid.

detailed description

The implementation of the present invention will be described in further detail below. The following examples are only descriptive, not restrictive, and cannot limit the protection scope of the present invention.

A method for evaluating a substation grounding grid, the steps of the method are as follows:

(1) Apply Faraday's law to deduce the thickness reduction rate of the grounding grid metal;

The corrosion process of the grounding grid material in the soil is applicable to Faraday's law: △m=MQ/zF, where △m is the mass of the dissolved metal; M is its atomic weight; Q is the transferred charge; z is the valence of the metal ion; F is Faraday's constant, and the following formula can be derived by means of the density ρ _s :

J _A ＝Q/St＝△m/St×zF/M-△s/t×zFρ _s /M

In the formula, J _A is the current density of the anode sub-reaction of the metal ion channel: S is the surface area of the electrode: t is the time; △s is the thickness to be stripped;

Wf _a J _A ＝△s/t

f _a =M/zFρ _s =3.2684/z(M/g·mol ^-1 )(g·cm ^-3 /ρ _s )mm·a ^-1 /mA·cm ^-2

v＝f _b J _A ＝△m/St

f _b = M/z F = 0.32311/z (M/g·mol ^-1 ) g·cm ^-2 ·h ^-1 /mA·cm ^-2

In the formula, W is the thickness thinning rate, and v is the mass loss rate per unit surface area;

W=f _c v (I)

f _c =f _a /f _b =1/ρ _s =8.76 ((g·cm ^-3 /ρ _s )mm·a ^-1 /g·cm ^-2 ·h ^-1 )

Common grounding grid material conversion factors and standard potentials are shown in Table 1

Table 1 Conversion coefficients of metal-metal ion electrochemical reactions

The thickness reduction rate of grounding grid metal can be obtained by the conversion factor in above-mentioned formula (1) and table 1;

(2) When it is determined that corrosion is not considered, the minimum cross-sectional area S _g1 of the grounding wire;

According to DL/T 621-1996 regulations on thermal stability checking of substation grounding devices, according to thermal stability conditions, when corrosion is not considered, the minimum cross-sectional area of the grounding wire S _g1 unit: mm ² :

S _g1 =Ig/c·t _e ^-2 (II)

Among them, the parameters Ig, c, and _te ^-2 in the formula are specified as follows:

① Ig is the stable value of short-circuit current flowing through the grounding wire (unit: A). For effective grounding systems and low-resistance grounding systems, consider single or two grounding short-circuit currents; for non-grounding, arc suppression coil grounding and high-resistance grounding systems, Considering the two grounding short-circuit currents at different points, Ig should be determined according to the 5-10-year development plan of the system and the maximum operating mode of the system, and Ig should be calculated according to 75% of the maximum short-circuit current of the busbar of the substation;

②c is the thermal stability coefficient of the grounding wire material, which is determined according to the type, performance, maximum allowable temperature of the material and the initial temperature of the grounding wire before short circuit. Generally, 70 is taken for steel, 210 for copper, and 120 for aluminum;

③t _e is the equivalent duration of short circuit (unit: s). Generally, 0.35s is used for 500kV system, 0.6-0.7s is used for 200kV system, and the fault duration of 110kV and below voltage level system is selected according to the actual situation. The electrical protection configuration is close, and the same level as the 220kV system (0.6-0.7s) is desirable;

(3) When determining the influence of corrosion factors, the minimum cross-sectional area S _g2 of the grounding wire;

S _g2 = (A-2×w×n)(B-2×w×n) (III)

When the grounding grid is a round steel type, in the formula, A indicates the width of the grounding grid, B indicates the thickness of the grounding grid, n indicates the service life (unit: year), and w indicates the thinning rate of the grounding grid material (mm/a);

(4) For the grounding grid that has been used, compare the implementation specification of the actual grounding grid with the value of S _g2 , take the value of the implementation specification, and the n value when it is closest to the calculated S _g value, and obtain the evaluation and use of the actual grounding grid Years; for the calculated minimum cross-sectional area S _g of the grounding wire when corrosion is not considered, by comparing the values of S _g1 and S _g2 calculated according to different years, the in-service life of the design value of S _g1 can be obtained.

example

Choose Q235 steel in coastal areas,

W = f _c v,

By calculating w=0.877(mm/a);

S _g1 ＝Ig/c·t _e ^-2 , the stable value of short-circuit current Ig flowing through the grounding wire is calculated according to 75% of the maximum short-circuit current of the bus bar in the 220kV substation site, that is, Ig is 46.319×0.75＝34.74kA, and c is 70 . t _e =0.2s.

S _g1 =Ig/c·t _e ^-2 =34.74×10 ³ /70·0.2 ^-2 =221.94 mm ²

n select 1 year.

S _g2 = (A-2×w×n)(B-2×w×n)=(6-2×0.877×1)(60-2×0.877×1)

=247.31mm ²

Through the comparative analysis of S _g1 and S _g2 , it is concluded that the service life of the grounding grid in this area is 1 year.

Claims (2)

1. An evaluation method of a substation grounding grid, characterized in that the method comprises steps as follows: (1) Apply Faraday's law to deduce the thickness reduction rate of the grounding grid metal; The corrosion process of the grounding grid material in the soil is applicable to Faraday's law: △m=MQ/zF, where △m is the mass of the dissolved metal; M is its atomic weight; Q is the transferred charge; z is the valence of the metal ion; F is Faraday's constant, and the following formula can be derived by means of the density ρ _s : J _A ＝Q/St＝△m/St×zF/M-△s/t×zFρ _s /M In the formula, J _A is the current density of the anode sub-reaction of the metal ion channel: S is the surface area of the electrode: t is the time; △s is the thickness to be stripped; Wf _a J _A ＝△s/t f _a =M/zFρ _s =3.2684/z(M/g·mol ^-1 )(g·cm ^-3 /ρ _s )mm·a ^-1 /mA·cm ^-2 v＝f _b J _A ＝△m/St f _b = M/z F = 0.32311/z (M/g·mol ^-1 ) g·cm ^-2 ·h ^-1 /mA·cm ^-2 In the formula, W is the thickness thinning rate, and v is the mass loss rate per unit surface area; W=f _c v (I) f _c =f _a /f _b =1/ρ _s =8.76 ((g·cm ^-3 /ρ _s )mm·a ^-1 /g·cm ^-2 ·h ^-1 ) Among them, f _a , f _b , f _c are conversion factors of grounding grid materials; The thickness reduction rate of the grounding grid metal can be obtained by the above formula (I) and the conversion factors of f _a , f _b and f _c ; (2) When it is determined that corrosion is not considered, the minimum cross-sectional area S _g1 of the grounding wire; According to thermal stability conditions, when corrosion is not considered, the minimum cross-sectional area S _g1 of the grounding wire is in mm ² , S _g1 =Ig/c·t _e ^-2 (II) Among them, in the formula, Ig is the stable value of the short-circuit current flowing through the grounding wire, c is the thermal stability coefficient of the grounding wire material, and t _e is the equivalent duration of the short circuit; the specific provisions of Ig, c, and t _e are: ① Ig is the stable value of short-circuit current flowing through the grounding wire, unit: A. For effective grounding systems and low-resistance grounding systems, consider single or two grounding short-circuit currents; for non-grounding, arc suppression coil grounding and high-resistance grounding systems, then Considering two grounding short-circuit currents at different points, Ig is determined according to the 5-10-year development plan of the system and the maximum operating mode of the system, and Ig is calculated based on 75% of the maximum short-circuit current of the busbar of the substation; ②c is the thermal stability coefficient of the grounding wire material, which is determined according to the type, performance, maximum allowable temperature of the material and the initial temperature of the grounding wire before short circuit. Steel takes 70, copper takes 210, and aluminum takes 120; ③t _e is the equivalent duration of short circuit, unit: s, 0.35s for 500kV system, 0.6-0.7s for 200kV system, fault duration of 110kV and below voltage level system is selected according to the actual situation, considering that the relay protection configuration is close to , take the same level value of 220kV system as 0.6-0.7s; (3) When determining the influence of corrosion factors, the minimum cross-sectional area S _g2 of the grounding wire; S _g2 = (A-2×w×n)(B-2×w×n) (III) Among them, when the grounding grid is a round steel type, in the formula, A indicates the width of the grounding grid, B indicates the thickness of the grounding grid, n indicates the service life, and w indicates the thinning rate of the grounding grid material, unit: mm/a; (4) Determine the evaluation period of the grounding grid, the specific method is: ① For the grounding grid that has been used, compare the implementation specification of the actual grounding grid with the value of S _g2 , take the n value when the implementation specification value is closest to the calculated S _g value, and obtain the estimated service life of the actual grounding grid; ②For the calculated minimum cross-sectional area S _g of the grounding wire when corrosion is not considered, the in-service life of the design value of S _g1 is obtained by comparing the values of S _g1 and S _g2 calculated according to different years. 2. the evaluation method of substation grounding grid according to claim 1, is characterized in that: in described step (1), for f _a , f _b , f _c conversion factor, common grounding grid material conversion factor is as follows: