CN107462802B - State evaluation method for 500kV underground substation grounding system - Google Patents

Abstract

The invention relates to a 500kV underground substation grounding system state evaluation method, which comprises the following steps: 1) determining indexes of state evaluation of a grounding system of a 500kV underground substation, including contact and step voltage values, conductor thermal stability, grounding resistance of a ground grid, safety of the ground grid and thermal stability safety of an underground pile foundation; 2) simulating a 500kV underground substation grounding system, and respectively obtaining evaluation values of various indexes; 3) and comparing the evaluation values of the indexes with the safety threshold value to finally obtain the system state of the 500kV underground substation grounding system. Compared with the prior art, the invention has the advantages of suitability for 500kV underground transformer substations, wide consideration range and the like.

Description

State evaluation method for 500kV underground substation grounding system

Technical Field

The invention relates to a state evaluation method for a 500kV underground substation grounding system.

Background

With the rapid development of social economy, the demand of various industries on electric power is continuously increased, and the electric load of the center area of Shanghai cities is denser. In order to ensure safe and reliable electricity utilization in urban centers, properly solve the construction problems caused by short land utilization, difficult station site selection, expensive land and higher land acquisition and removal cost in the areas, and combine the overall requirements of area planning to improve the land utilization rate, improve urban landscapes and optimize urban environments, and the underground transformer substations are operated at the same time. Compare with traditional transformer substation, urban area underground substation compares with ordinary urban area transformer substation: the underground transformer substation occupies a large area, has more pile foundations and has large short-circuit current; the incoming and outgoing lines of the underground substation generally adopt cables, and when short-circuit faults occur, strong induction exists between a cable core and a sheath, so that the distribution of fault current is influenced; an underground transformer substation generally adopts a GIS (geographic information system), strong electromagnetic induction between a phase line and a metal shell causes the potential distribution of the metal shell of the GIS to be extremely uneven, and great potential difference exists among different positions of the GIS, so that the contact voltage safety of the GIS needs to be ensured; the urban underground transformer station coexists with commercial and civil buildings, the grounding system of the urban underground transformer station is directly or indirectly electrically connected with a pipeline or a building grounding system, and all coexisting facilities and buildings mutually influence each other.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a 500kV underground substation grounding system state evaluation method.

The purpose of the invention can be realized by the following technical scheme:

a500 kV underground substation grounding system state evaluation method comprises the following steps:

1) determining indexes of state evaluation of a grounding system of a 500kV underground substation, including contact and step voltage values, conductor thermal stability, grounding resistance of a ground grid, safety of the ground grid and thermal stability safety of an underground pile foundation;

2) simulating a 500kV underground substation grounding system, and respectively obtaining evaluation values of various indexes;

3) and comparing the evaluation values of the indexes with the safety threshold value to finally obtain the system state of the 500kV underground substation grounding system.

The step 2) specifically comprises the following steps:

apparent resistivity, electrically equivalent soil structure, grounding grid and power network structure parameters of the 500kV underground substation are respectively obtained, CDEGS software is adopted to simulate the grounding system of the underground substation, and evaluation values of all indexes are respectively obtained.

In the step 2), a plurality of soil models of air in the middle of the multi-layer floors of the 500kV underground substation are built, the soil resistivity of the 500kV underground substation is obtained by adopting a symmetrical four-pole electric sounding method and comprises large electrode spacing resistivity and short electrode spacing resistivity, the ground potential rise of a grounding system is obtained by adopting the large electrode spacing resistivity, and the contact and step voltage values are obtained by adopting the short electrode spacing resistivity.

And in the step 2), constructing a grounding grid three-dimensional model of the 500kV underground transformer substation, which comprises an outdoor grounding grid, an underground grounding trunk line, a main grounding grid and a GIS sub-model, adding an underground pile foundation on the grounding grid three-dimensional model to obtain a grounding resistance value of the grounding grid, simulating short-circuit faults of a circuit breaker, switches around the GIS and the main transformer at the fault occurrence positions, and obtaining potential rise, ground potential rise, contact and step voltage of the grounding grid corresponding to fault points.

In the step 2), a cable shunt equivalent network of the 500kV underground substation is constructed according to a network topology structure and line parameters of the 500kV underground substation, and fault current values when a three-phase short circuit fault and a single-phase grounding short circuit fault occur are obtained.

And in the step 2), acquiring current capacity according to the conductor type, the maximum fault duration, the conductor size and the frequency of the 500kV underground substation to evaluate the thermal stability of the conductor.

In the step 2), the maximum temperature rise of the underground pile foundation is obtained according to the maximum current of the underground pile foundation, the minimum diameter of the pile foundation reinforcing steel bar and the maximum fault duration flowing through the 500kV underground substation under the 220kV fault condition, so that the thermal stability safety of the underground pile foundation is evaluated.

Compared with the prior art, the invention has the following advantages:

the method is suitable for 500kV underground transformer substations: the existing substation evaluation method does not have a method for carrying out state evaluation on a grounding system of a 500kV underground substation, and the invention fills the gap.

Secondly, the consideration is wide: according to the method, the state of the grounding system of the 500kV underground substation is evaluated in multiple aspects of contact and step voltage values, conductor thermal stability, grounding resistance of the ground grid, ground grid safety, thermal stability safety of an underground pile foundation and the like, system parameters under a fault condition are obtained through simulation, and the state of the grounding system of the 500kV underground substation can be conveniently and accurately obtained.

Drawings

FIG. 1 is a flow chart of the method of the present invention.

Detailed Description

The invention is described in detail below with reference to the figures and specific embodiments.

Example (b):

the following is illustrated by taking the Hongyuan substation in the sea city as an example:

the Hongxuyang transformer station is located in the region of the land where the Yanpu area is east, the text garden district is west, the three-door road is south, the administrative road is north and the three-door road is close to. The Hongyu transformer substation adopts the design of three layers underground, and the design burial depth is 25 meters, and the transformer substation plans the usable floor area 24570 square meters.

The capacity of the main transformer of the Hongyao transformer substation is 2 multiplied by 1500MVA in the current period and 3 multiplied by 1500MVA in the long period. The transformer mainly comprises three voltage levels of 500kV/220kV/66kV, and adopts a single-phase, self-coupling and non-excitation voltage regulating transformer, and the neutral point of the transformer is directly grounded. The electrical main wiring scheme is as follows: 500kV adopts a wiring mode of a circuit transformer bank of a circuit breaker; the 220kV bus adopts a double-bus three-section wiring mode, and the current period is double-bus double-section; the 66kV bus adopts a single bus wiring which is changed into a unit from a main bus, and is provided with a main circuit breaker.

The rainbow poplar station access scheme is as follows:

500kV scheme

500kV rainbow poplar station adopts line transformer bank wiring mode to access 500kV row station through 2 times 500kV earth wire cable line in this period. The direction of the long-term loop 3 has certain uncertainty, which is determined according to the future power grid development condition.

220kV scheme

In this period, the 220kV outgoing line is 14 loops, 1 loop is respectively carried out in steel, Minhe and New Jiangwan, 2 loops are respectively carried out in algae, creeks, Yixian, Fangli and Jingan (220kV connection line), and 3 loops are respectively carried out in a pentagonal field. The outgoing line of 220kV in the long term is 21 times, and the final determination is not carried out at present due to certain changes of planning and initial setting conditions.

All business turn over lines in the station of hong Yang are cable run, and 500kV lays for the tunnel, and 220kV lays for calandria or tunnel. And (4) laying a tunnel, wherein the center burial depth is about 7.5m, and laying a calandria, and the center burial depth is about 1.5 m.

The Hongyan war equipment is miniaturized, and is convenient to transport and hoist; the occupied area is small, so that the occupied area of the transformer substation is reduced as much as possible, the recommended requirements of the transformer substation on peripheral facilities, particularly residences, are met, and therefore GIS equipment is adopted for 500kV, 220kV and 66kV power distribution devices.

The specific content of the invention comprises:

1. and analyzing and explaining the soil structure based on the measured data of the soil resistivity of the site area.

2. Based on data of an incoming and outgoing line cable and an overhead line of an underground 500kV Hongyun substation, a fault current distribution calculation model of a single-phase ground condition is established, and the maximum ground current entering the ground through a grounding network is determined.

3. The method comprises the steps of establishing a three-dimensional model of the grounding grid of the underground substation, and establishing a three-dimensional grounding system simulation calculation model based on the structure, the size and the material of the grounding system of the underground 500kV Hongyu substation.

4. And carrying out safety evaluation on the grounding system of the transformer substation, and determining parameters such as grounding resistance, contact voltage, step voltage, earth surface potential, GPR (general purpose voltage) of a grounding conductor and the like. The influence of the induction between the cable core and the sheath and different fault positions is considered in the analysis.

The underground transformer substation has large floor area and more pile foundations, is of an underground three-layer structure, and can correctly and accurately consider the characteristics of an underground large-scale transformer substation grounding system, and the adopted method and software can simulate an underground space layer and a soil model with any size at the periphery.

The calculations of the present invention are derived using the CDEGS package. CDEGS can calculate the current and electromagnetic fields of any network consisting of buried or above ground live conductors under transient conditions of normal, fault, lightning strike, etc. CDEGS can simulate simple conductors and composite conductors such as bare wire, coated pipe, or a system of tubular cables buried in complex soil structures. CDEGS can provide a solution that takes into account the complexity of sensing the complex state of buried or ground systems caused by lightning strikes, etc., from a simple grounded grid design. The CDEGS is the only software tool which can accurately simulate any soil model in the world at present, so that the grounding system of the complex transformer substation can be accurately calculated, and the safety performance of the grounding system can be evaluated.

The calculation of the ground current is also an important aspect in the analysis and design of the ground. Neglecting the calculation of the current to ground, it is a very conservative assumption to use the total short circuit current as the current to ground. This can result in significant waste in the design of the grounding grid. Sometimes the shunt factor is used to determine the incoming current. In this case, the selection of the correct division factor is very difficult because the factors affecting the division factor are many. The Right-of-Way module in the CDEGS package is used to calculate the ground current including any line very accurately.

The analysis of the grounding system mainly comprises the calculation of grounding resistance, potential rise, contact voltage and step voltage. A typical ground analysis is based on the assumption that the ground net is an equivalent potential. This assumption holds for small earth grids, high soil resistivity and small circulation. This is not the case in large earth grids (such as large underground substations), low resistivity or large currents. This problem is even more pronounced if the material of the earth grid is steel instead of copper. In addition, when a fault current flows in the line, an overhead earth wire, underground cable sheath or earth wire or the pipe casing of the GIL induces a current in the opposite direction. This current flow reduces the current flowing through the earth grid into the earth. Not considering this part of the current in the design may lead to erroneous conclusions. The present invention uses the MALZ in the CDEGS package to simulate these situations very accurately. Meanwhile, a complex soil structure can be simulated according to the actual situation of the site, the characteristics of the grounding system of the underground large-scale transformer substation are correctly and accurately considered, and a plurality of soil models of the underground space layer and the peripheral multi-layer soil models are simulated, so that the actual situation of the site can be approached to the maximum extent. Finally, due to the high voltage levels, a large part of fault currents of large underground substations are circulating currents, and the influence of the circulating currents on the safety performance of the ground grid must be considered in calculation. The flow of computing the design is shown in fig. 1.

Firstly, soil measurement data and model determination:

soil resistivity measurements form the basis of any grounding study, and the same grounding system exhibits completely different electrical characteristics in different soil models. Therefore, the first work of the project is to measure the resistivity of the soil and determine the electrically equivalent soil structure. Generally, Ground Potential Rise (GPR) of a grounded system is determined primarily by the deep soil (corresponding to large inter-electrode resistivity measurements); while the contact and step voltages as a percentage of ground potential rise depend on the surface soil characteristics (corresponding to short inter-electrode resistivity measurements).

The method is characterized in that the measurement of the soil resistivity of the iridescent poplar transformer substation is completed at 16 measuring point positions, a heavy WDDS-1 type digital resistivity measuring instrument is adopted, the measurement is carried out by combining a conventional symmetric quadrupole electrical sounding method, and a peripheral soil model of the iridescent poplar underground transformer substation is a four-layer soil model for the soil resistivity test of the iridescent poplar 500kV transformer substation.

Simulation and ground impedance calculation of ground system of Rainbow poplar station

The grounding network of the Hongyu transformer substation mainly comprises 6 parts: the system comprises an outdoor grounding grid of one layer, a grounding main line of the underground layer, a main grounding grid of the underground layer and a GIS with 2 voltage levels. All provided transformer substation grounding grids and GIS models are two-dimensional planes. Researchers successfully and accurately establish the whole transformer substation grounding grid, the GIS, the special grounding network of the GIS and all elements influencing the safety performance of the grounding grid according to the existing two-dimensional model and data, and the complete and huge three-dimensional model is used for calculating grounding impedance, potential rise of each part, contact and step voltage and the like of the transformer substation, so that the safety performance of the grounding grid is evaluated.

The size of the main grounding grid of the iris poplar substation is 148.2m multiplied by 68.4m, the grid is 8m multiplied by 8m, and the main grounding grid is positioned at 25.6m and four under the groundThe corners are arcs, and grounding rods with the length of 10.5 meters are arranged at a plurality of positions. Horizontal conductor 150mm²The equivalent radius of the copper stranded wire is 6.9mm, the relative resistivity (for copper) is 1, and the relative permeability (for air) is 1. The grounding rod is made of copper-clad steel, has the relative resistivity (to copper) of 12 and the relative magnetic permeability (to air) of 250, and has the radius of 7.1 mm.

The underground substation area is great, and the pile foundation is more, and this section is to how the influence that influences underground substation security performance of pile foundation makes analytical calculation research to do preliminary discussion to the possibility that the pile foundation replaces main earth mat.

The pile foundation simulation is that a total of 600 reinforcing steel bars are divided into five types, namely P1, P2, P3, P4 and P5. The pile is used as a vertical column pile in the reverse construction stage of P1, P2, P3 and P4 piles, is used as an uplift pile under the anti-floating working condition, has the pile diameter phi of 1000, and is designed to have the length and the tail of 56.1 meters, and a steel pipe concrete column is arranged in the pile top. The uplift pile under the P5 pile position anti-floating working condition has the pile diameter phi of 800 and the designed pile length of 45 meters. P1, P2, P3, P4 and P5 are connected with a horizontal ground net, a first layer of underground, a second layer of underground and a third layer of cement steel bars and a main ground net at different positions.

When the power station grounding resistance is considered in the pile foundation, the original 0.083ohm is reduced to 0.0714ohm, and the reduction rate is reduced by 14 percent (the defined reduction rate C is (R) ═_{Pile-foundation-free}-R_{With pile foundation})/R_{Pile-foundation-free}100%). It can be seen that: the huge pile foundation has a relatively obvious influence on the grounding performance of the underground station.

Third, calculation and analysis of maximum ground short-circuit current of power station

The calculation of the ground current is also an important aspect in the analysis and design of the ground. Neglecting the calculation of the ground current, it is a very conservative assumption that the total short circuit current is used as the ground current, which causes a great waste in the design of the grounding grid. Some designers can determine the value of the earth current based on typical shunt coefficients, and it can be imagined that such typical data are difficult to fit each power station under study, and the obtained results are very marginal. Furthermore, it is difficult to actually measure the shunt factor. It is known that for a power grid system comprising cables, it is more difficult to determine the correct shunt factor, because the factors influencing the shunt factor are many, and the strong induction between the cable sheath and the cable core causes a large amount of current to return from the sheath to the remote power station. By means of the tool software CDEGS, each power network component including a ground network, a ground wire, a tower grounding system, a cable, an in-station transformer and the like can be simulated to be accurate to each element, and then fault current distribution is calculated, and the return current of the ground current, the ground wire and a sheath is determined.

The inlet and outlet cables of the Hongxuyang transformer substation are all in a direct grounding mode of two ends of the cable metal sheath which are in cross interconnection, and the grounding resistance of each far-end transformer substation grounding system is considered according to 0.5 ohm.

The calculation of the shunting coefficient of the Hongyao transformer substation depends on the operation mode of a topological network and is based on data provided by Shanghai Power economic research institute. When the single phase-to-ground fault occurs to the 500kV or 220kV bus of the iridescent substation, only the poplar station provides short-circuit current, the circulating current of the neutral point of the transformer is negligible, and under the condition that the circulating current is not considered, namely the current enters the ground from a fault position, the safety indexes (contact voltage and step voltage) of the grounding system are in direct proportion to the magnitude of the grounding current, so that the current entering the ground through the grounding system only needs to be calculated under the condition that the single phase-to-ground fault occurs to the 220kV bus of the iridescent substation (the fault current is 45.5 kA).

When a fault current flows in the line, there will be a current in the opposite direction in the overhead earth wire or cable sheath. This current flow reduces the current flowing through the earth grid into the earth. Design considerations of this part of the current and the return current of the main transformer neutral point can lead to erroneous conclusions. Similarly, neglecting the calculation of the ground current, it is a very conservative assumption that the total short-circuit current is used as the ground current, which causes a great waste in the design of the grounding grid. Some designers can determine the value of the earth current based on typical shunt coefficients, and it can be imagined that such typical data are difficult to fit each power station under study, and the obtained results are very marginal. Furthermore, it is difficult to actually measure the shunt factor.

For a power grid system with cables, it is more difficult to determine the correct shunt coefficient, because the factors influencing the shunt coefficient are many, and the strong induction between the cable sheath and the cable core causes a large amount of current to return to a remote power station from the sheath. By means of the tool software CDEGS, each power network component including a ground network, a ground wire, a tower grounding system, a cable, an in-station transformer and the like can be simulated to be accurate to each element, and then fault current distribution is calculated, and the return current of the ground current, the ground wire and a sheath is determined.

When the condition of single-phase short circuit fault in a station of a 220kV Hongyun substation is calculated, a network topology model of the distribution of fault current in a grounding system and a cable sheath of the Hongyun substation is calculated and analyzed by using a ROW (TRALIN/SPLITS) module of CDEGS software.

The ROW (TRALIN/SPLITS) module is carried out in a circuit mode, namely, a TRALIN module is adopted to model and obtain line parameters: self, transimpedance and shunt impedances; the SPLITS module establishes a circuit model of the whole topological network, and obtains current distribution of current in a grounding system, an overhead ground wire and a cable sheath based on specified excitation.

Four, ground net safety performance evaluation

1. The contact and step voltage safety values are calculated according to the standard GB/T50065-2011, and the results are shown in Table 1.

TABLE 1 maximum contact and step voltage safety values

2. Thermal stability of conductor

The conductor of the rainbow poplar ground net is 150mm²For a maximum fault duration of 0.33s, the current heat capacity was 68.5ka (rms) using the sesampability module for calculation analysis. The maximum total fault current is 45.5 kA. Thus, even if it is assumed that the entire total fault current flows through one conductor, such as the conductor of a fault point connection device or structure, 150mm²The copper strands of (a) are sufficient to meet the thermal capacity requirement.

3. Analysis of ground resistance of ground network

The grounding system of the Hongyu transformer substation is placed in a four-layer multi-block soil model considering air between layers of the transformer substation, an MALZ module is adopted for simulation calculation, the calculated value of the grounding impedance of the grounding system of the underground power station is 0.083 ∠ 2.94 & lt 2.94 & gt, according to the GB/T50065-2001 standard of China' grounding design Specification of alternating current electrical equipment, the grounding impedance of the grounding system is preferably smaller than 2000/I & lt omega & gt (I is an earth current flowing through the grounding system), the 220kV single-phase fault of the Hongyu underground station is realized, under a very conservative calculation condition, the maximum earth current is 5.446kA, 2/5.446 is 0.367 & lt omega, and the grounding resistance of the Hongyu underground transformer substation is far smaller than a required value.

4. Safety evaluation of ground net

Under different short-circuit faults, the potential of the power grid of the underground substation, the ground potential distribution of key positions, the contact voltage and the step voltage are calculated. The mallz grounding software module in CDEGS is used. The location of the fault is selected from the group consisting of a circuit breaker (F1), a GIS proximity switch (F2), a main transformer (F3), and where typical single-phase ground faults typically occur. In each case taking into account the circulating currents between the fault point and the cable sheath and the connection point of the earth mat. When short-circuit fault occurs, the ground current greatly contributes to the potential rise of the whole earth screen. In order to take into account the effects of circulating currents, the total fault current of the current is injected at the fault location F1 or F2 or F3, and the current diverted by the sheath is injected at each sheath-to-earth connection point. Note that if the circulating currents are ignored (as is typical with conventional software, the grounded system is considered an equivalent), an underestimated error conclusion!

For different fault points and different voltage grades, corresponding ground network potential rise, ground potential rise, contact and step voltage are calculated.

Considering an air layer of an underground substation, adopting a plurality of soil models, considering sheath circulation for 3 typical fault points of a breaker (F1), a switch (F2) near a GIS, a main transformer (F3) and 3 fault points, calculating 220kV bus short circuit, and obtaining corresponding ground potential rise, key part potential, surface potential, contact and step voltage distribution of a total station grounding system under the condition of maximum conservative fault current. As shown in tables 2, 3 and 4, it can be seen from the calculation results that the potential of the ground conductor of the total-iris-poplar underground station is increased to 642.64V, and the highest ground potential is 580.29V. The maximum step voltage of any place where all underground station personnel can go is 23.27V and is smaller than the conservative step voltage safety value 339.9V. The maximum contact voltage of the place which can be contacted by the personnel in the station is 203.21V, which is less than the conservative contact voltage safety value of 311.9V.

Table 2 when a single-phase short-circuit fault occurs, the safety evaluation calculation results of each part of the iris underground substation: the fault occurred at typical circuit breaker position F1

Table 3, when a single-phase short-circuit fault occurs, the safety evaluation calculation results of each part of the iris underground substation: the fault occurs in a typical GIS switch F2

Table 4, when a single-phase short-circuit fault occurs, the safety evaluation calculation results of each part of the iris underground substation: the fault occurs at a typical main transformer position F3

5. Underground pile foundation thermal stability safety calculation evaluation

The maximum surface temperature of the component is regulated according to the current national standard GB 50010-2002. When a building is struck by lightning, the lightning current flowing through the components requiring fatigue checking is shunted small enough to ensure that the conductor temperature rise is within safe limits. Through the coordination and communication with the experts of the national grid and sea power design company Limited, the temperature of the pile foundation reinforcing steel bar can be controlled within 100 ℃ according to a thermal stability formula under the influence of power frequency current temperature. The initial temperature of the reinforcing steel bar is 40 degrees, so the temperature rise of the pile foundation reinforcing steel bar caused by fault current is controlled within 60 degrees.

Under the 220kV fault of the rainbow poplar station, the maximum current flowing through the pile foundation steel bar is 1193A, the diameter (minimum) of the pile foundation steel bar is 80mm, the maximum fault duration time is 0.33s, the SESAmparity module is used for calculation and analysis, the temperature rise on the surface of the pile foundation steel bar is only 0.004, the initial temperature of the steel bar is 40 degrees, the highest temperature of the steel bar is 40.004 degrees, the requirement value is far less than 100 degrees, and the thermal stability meets the safety requirement.

The safety performance evaluation result of the ground grid of the iris poplar underground station shows that the ground grid has good performance. It is to be emphasized that: in the research, data such as short-circuit current, a GIS structure, a grounding point, a cable and the like are obtained based on typical data set conservative estimation. It is recommended that after obtaining these data, a corresponding earth-network security calculation, evaluation and accounting is performed to ensure the safe operation of the iridescent underground station.

Claims (1)

1. A500 kV underground substation grounding system state evaluation method is characterized by comprising the following steps:

1) determining indexes of state evaluation of a grounding system of a 500kV underground substation, including contact and step voltage values, conductor thermal stability, grounding resistance of a ground grid, safety of the ground grid and thermal stability safety of an underground pile foundation;

2) the method comprises the following steps of simulating a 500kV underground substation grounding system to respectively obtain evaluation values of various indexes:

respectively acquiring apparent resistivity, electrically equivalent soil structure, grounding grid and power network structure parameters of a 500kV underground substation, and simulating a grounding system of the underground substation by using CDEGS software to respectively obtain evaluation values of various indexes;

constructing a plurality of soil models of air in the middle of multiple floors of a 500kV underground substation, acquiring soil resistivity of the 500kV underground substation by adopting a symmetrical four-pole electric sounding method, wherein the soil resistivity comprises large electrode spacing resistivity and short electrode spacing resistivity, acquiring ground potential rise of a grounding system by adopting the large electrode spacing resistivity, and acquiring contact and step voltage values by adopting the short electrode spacing resistivity;

constructing a grounding grid three-dimensional model of a 500kV underground substation, wherein the grounding grid three-dimensional model comprises an outdoor grounding grid, an underground grounding trunk line, a main grounding grid and a GIS sub-model, adding an underground pile foundation on the grounding grid three-dimensional model to obtain a grounding resistance value of the grounding grid, simulating short-circuit faults of a circuit breaker, switches around the GIS and a main transformer at the fault occurrence position, and obtaining potential rise of the grounding grid, ground potential rise, contact and step voltage of the corresponding fault point;

constructing a cable shunt equivalent network of the 500kV underground substation according to a network topology structure and line parameters of the 500kV underground substation, and acquiring fault current values when a three-phase short circuit fault and a single-phase ground short circuit fault occur;

acquiring current capacity according to the conductor type, the maximum fault duration, the conductor size and the frequency of the 500kV underground substation to evaluate the thermal stability of the conductor;

acquiring the maximum temperature rise of the underground pile foundation according to the maximum current flowing through the underground pile foundation, the minimum diameter of the pile foundation steel bar and the maximum fault duration time of a 500kV underground substation under the 220kV fault condition, so as to evaluate the thermal stability safety of the underground pile foundation;

3) and comparing the evaluation values of the indexes with the safety threshold value to finally obtain the system state of the 500kV underground substation grounding system.

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