CN103616582A

CN103616582A - Multidimensional evaluation method for large-scale grounding grid

Info

Publication number: CN103616582A
Application number: CN201310572050.2A
Authority: CN
Inventors: 李谦; 肖磊石; 饶章权; 张波
Original assignee: Tsinghua University; Electric Power Research Institute of Guangdong Power Grid Co Ltd
Current assignee: Tsinghua University; Electric Power Research Institute of Guangdong Power Grid Co Ltd
Priority date: 2013-11-13
Filing date: 2013-11-13
Publication date: 2014-03-05

Abstract

A multidimensional evaluation method for a large-scale grounding grid includes the following steps of on-site testing, numerical calculation based on the electromagnetic field principle and multidimensional evaluation on the grounding grid. The foothold of the multidimensional evaluation method for the grounding grid is based on basic functions of the grounding grid, the basic functions are stream breaking and potential holding safety performance under power frequency grounding faults and lightning stroke faults, and the evaluation problems of characteristic parameters reflecting the safety performance of the grounding grid under the power frequency grounding faults and in the state of lightning stroke are mainly solved, wherein the characteristic parameters relate to grounding grid potential rising, potential difference in the grid, a step voltage, a contact voltage and the like. The multidimensional evaluation on the grounding grid is achieved from the aspects of the characteristic parameters of the grounding grid, integrity of the grounding grid, corrosion evaluation, thermal stability check and the like, safety of the grounding grid is expressed more scientifically, the evaluation on the state of the grounding grid is more accurate, and the safety problem and the defects of the grounding grid are rectified more effectively in a more pertinent mode.

Description

The various dimensions evaluation method of large-scale grounding network

Technical field

The present invention relates to the various dimensions evaluation method of the medium-and-large-sized grounded screen of a kind of power industry, specifically refer to the method for evaluating safety of 200MW and above generating plant or 35kV and above transformer station (comprising current conversion station, high resistance station) large-scale grounding network (hereinafter to be referred as " grounded screen ").

Background technology

The basic role of grounded screen, except providing common reference ground for various electrical equipments in generating plant, transformer station, prior, when system generation earth fault or lightning ingress, play release fast fault current or lightning current, improve the effect that earth mat metallic conductor and earth's surface, place earth potential distribute, ensure malfunction next, secondary device and personal security.Grounded screen is to safeguard the important measures of power system security reliability service, support personnel and device security, and the basic role of grounded screen is embodied in the security performance aspect under power-frequency earthing fault and lightning fault.

For this reason, grounded screen characterisitic parameter as concentrated expression grounded screen security performance, especially reflected grounded screen while there is ground short circuit fault security performance comprise impedance ground, earth mat conductor potential rise and potential difference (PD), conductor thermal stability, ground wire shunting and diverting coefficient, place step voltage and touch voltage, parameter and the indexs such as place surface potential gradient and transfer current potential, and electrical integrity and grounded screen conductor corrosion condition, transformer station place equipment and personnel's security while having determined fault, it is the leading indicator of weighing substation ground network performance quality, above based on to the grounded screen security performance parameter index that ipsilateral is not evaluated, formed the various dimensions appraisement system of grounded screen.

Earth mat characterisitic parameter index depends on grounded screen discharge capacity on the one hand, and the latter is relevant with the factors such as shunting contribution of site soil resistivity height, earth mat impedance ground size and aerial earth wire; On the other hand, depend on the actual ground short circuit levels of current height that enters.Basic role from grounded screen, the impedance ground of reflection grounded screen discharge capacity macro-indicators, become the most important characterisitic parameter of grounded screen, but earth mat impedance ground is certain, when entering ground short circuit electric current when different, grounded screen correlation parameter all can change thereupon, so impedance ground is not again unique, absolute parameter index.

For a long time, people get used to only putting forward index of impedance ground to the evaluation of grounded screen, add integrality and equipment ground downlead conduction, and the content such as earth mat conductor corrosivity excavation inspection, evaluate the quality of a grounded screen.In real work, often pursue simply this index of impedance ground, cause the deviation to grounded screen performance evaluation, this idea is incorrect through practice test.Because Grounding impedance is no longer unique index of weighing operation substation ground network safety in operation, and after generating plant, grounding net of transformer substation put into operation, impedance ground cannot or be difficult to Measurement accuracy, to the evaluation of grounded screen, should consider indices, the impact of grounded screen potential rise on equipment and system operation during from integrality, corrosion condition, fault, and the many-sided comprehensive assessments such as impact on personal security, the parameters of grounded screen is carried out to various dimensions evaluation, according to indices, comprehensively judge the situation of grounded screen.

Along with electric system continues to stride forward to large capacity, robotization, digitizing, intelligentized direction, the a large amount of electromagnetic compatibility problem of bringing of using of weak electron equipment are day by day serious, the safe and stable operation of electric system, the security of grounded screen has been proposed to more and more higher requirement, adapt to the requirement of above-mentioned situation, requirement to the design of grounded screen, operation maintenance and preventive trial, can not stay in test and the evaluation of impedance ground, and the Mode change that should evaluate to grounded screen various dimensions, to comply with the requirement of status of electric power evaluation and repair based on condition of component situation.

Grounded screen various dimensions are evaluated as a brand-new theory, mainly from generating plant, on transformer station's short circuit current level and soil regime basis, site, occurring under earth fault short circuit and lightning ingress state, grounded screen potential rise and net inter potential, step voltage and touch voltage, grounded screen electrical integrity, the many aspects such as corrosion condition, evaluating in all directions under malfunction grounded screen and place surface potential distributes to once, secondary device operation, and personal security impact, in conjunction with grounded screen integrality (conduction) and corrosion diagnosis of grounding grid, the generating plant of complete, grounding net of transformer substation various dimensions evaluation method, on this basis, point out the problem that grounded screen security aspect exists, and measures to rectify and reform are proposed targetedly, design for transformer station and station grounding net, construction, check and accept, the work of the grounded screen such as operation maintenance and overhauling reform lifecycle management provides guidance.

The mainly basic role based on grounded screen of grounded screen various dimensions evaluation, release fast when equipment ground short trouble occurs for Ji Dang transformer station place fault current and improve transformer station's earth potential and distribute, for the characterisitic parameter analysis that characterizes grounded screen security performance, launch, because grounded screen has the feature of disguised engineering, safety evaluatio to whole grounded screen, the simple method of in-site measurement that adopts cannot realize the perfect measurement to above-mentioned characterisitic parameter, and workload is too large, the characterisitic parameter of all positions in station all can not be measured; On the other hand, the measurement of above characterisitic parameter and evaluation are the work of a very complex, are subject to the impact of the many factors such as size and shape, outlet grid structure, site surrounding soil homogeneity of grounding body itself, never break through.

Development along with computer technology, can the achieve a butt joint accurate simulation of earth mat characteristic of numerical analysis method based on electromagnetic field principle, and can consider the impact of on-the-spot complicated factor, make up inherent shortcoming and deficiency that in-site measurement exists, can solve: the problem that a) is difficult to Measurement accuracy with the earth mat impedance ground of overhead outlet ground wire and 10kV cable sheath; B) earth conductor potential rise distributes and place pressure reduction cannot field measurement, and then cannot judge the problem that noble potential draws and electrical secondary system is affected outward; C) in step voltage actual measurement, be difficult to find the problem of the test blindness that ground network edge and high-level step voltage region bring; D) in touch voltage actual measurement, be difficult to the problem of the test blindness that perfect measurement brings.Generating plant, the evaluation of grounding net of transformer substation various dimensions mainly adopt field measurement and the theoretical way combining of calculating, accurately provide the present state that grounded screen enters the characterisitic parameters such as earth fault electric current, impedance ground, earth mat Potential distribution, potential difference (PD), step voltage and touch voltage, comparison safety limit, the on this basis security of analysis grounding net.

Summary of the invention

Technical matters to be solved by this invention, just be to provide a kind of large-scale grounding network various dimensions evaluation method, it is scientific and reasonable, the deviation of having avoided traditional habit to understand grounded screen security, and evaluation means is simple, can reflect comparatively comprehensively, truly, exactly grounded screen present state.

Solve the problems of the technologies described above, the technical solution used in the present invention is:

A large-scale grounding network various dimensions evaluation method, is characterized in that: comprise the following steps:

S1 on-the-spot test

Apparent soil resistivity under the different spacing of S1-1 site is measured;

The impedance of S1-2 in-site measurement Grounding, overhead ground wire, comprise OPGW optical fiber ground wire, two-terminal-grounding cable jacket to minute flow vector of measuring current (comprise amplitude and with measuring current phase differential), and by mathematics manipulation, obtain picking out the earth mat impedance ground value of ground wire shunt influence;

S1-3 step voltage and touch voltage test;

S1-4 grounded screen integrality and equipment ground downlead conduction test;

The ground net corrosion comprehensive diagnos that S1-5 checks in conjunction with excavation;

The numerical evaluation of S2 based on electromagnetic field principle

S2-1 obtains site stratified soil structural model by site apparent soil resistivity test data inverting;

The grounded screen conductor topological structure simulation of S2-2 based on grounded screen completion design drawing;

S2-3 impedance ground field test data, completes the accuracy verification of grounded screen Simulation Calculation;

S2-4 is based on earth fault short-circuit current and outlet parameter, calculates the distribution of fault current, obtains that each place is actual enters earth fault electric current and grounding transformer neutral point circulation;

S2-5 is based on grounded screen topological structure, site soil model and enter earth fault electric current, calculates the distribution of earth mat conductor potential rise, net inter potential, step voltage and touch voltage;

When S2-6 calculates thunder direct attack transformer station, earth mat and earth's surface transient state current potential and step voltage distribute;

S3 grounded screen various dimensions are evaluated

S3-1 moves first and second equipment based on earth mat conductor potential rise and net inter potential under result of calculation check ground-fault condition, and the impact on stand outer personnel and equipment;

S3-2 checks the step voltage of whole grounded screen place and the touch voltage of station equipment place according to safety limit;

On the lightning rod that S3-3 investigation earth mat conductor transient state current potential is the highest and outlet structure, whether transient state Potential distribution and the potential difference (PD) of near zone, bring insulation to threaten and electromagnetic interference (EMI) near secondary device or electrical secondary system; Check near the transient state step voltage level of lightning strike spot, whether the interior personal security of standing is brought to threat;

S3-4 extends out the destroyed rear grounded screen security status predication of earth mat;

S3-5 considers the heat-stable calibration of the grounded screen conductor under earth conductor corrosion factor and transformer station's maximum short circuit current;

The grounded screen rectification suggestion of S3-6 based on grounded screen security, and verify by calculating.

The present invention is based on grounded screen basic function, adopt the method combining of field measurement and theoretical analysis simulation calculation, in transformer station, occur under earth fault and thunderbolt state, to relating to the grounded screen potential rise of grounded screen security, net inter potential, step voltage and touch voltage, the several characterisitic parameters of conductor thermal stability are analyzed, in conjunction with integrality and corrosion condition, various dimensions are evaluated grounded screen characterisitic parameter level to once, the impact of secondary device and system operation, and the impact on personal security, provide safety limit, further improve science and the accuracy that large-scale grounding network is evaluated, after can damaging to operation grounded screen (grounded screen goes to pot as extended out or corrosion cracking etc.), grounded screen security state is predicted simultaneously, for the safety issue existing, propose targetedly rectification scheme and calculate demonstration.In addition, also can be by the application extension of large-scale grounding network various dimensions evaluation method to the stages of design and construction of grounded screen, for the grounded screen lifecycle management work such as ground grid design, construction, examination, operation maintenance, security rectification and overhauling reform provide the guidance of science.

Inventor utilizes large-scale grounding network various dimensions evaluation method, in dozens of operating 110kV to 500kV electric pressure generating plant and grounding net of transformer substation, completed safety evaluatio, comparison under the condition of not untiing lightning conducter, adopt and carry out retrial and phase difference measurement to leading to the shunting of lightning conducter, accurately obtain diverting coefficient and further obtain the validity of impedance ground, by the appraisal of grounded screen various dimensions, found that grounded screen potential rise exceeds standard, net inter potential is bigger than normal, impedance ground surpasses service requirement, the defects such as local step voltage and touch voltage exceed standard, rectification scheme is proposed targetedly, and obtain the theoretical checking of calculating, in addition after moving grounded screen and damaging (grounded screen goes to pot as extended out or corrosion cracking etc.), grounded screen security state is predicted, for grounded screen is in the future safeguarded, overhaul technological transformation provides foundation and guidance.

Inventor is the stages of design and construction to grounded screen by the application extension of large-scale grounding network various dimensions evaluation method also, complete a plurality of ground grid design evaluations, proved the transformer station not high for short circuit current level, outlet is more, suitably relax the feasibility of impedance ground design load and be put to design, and City Substation higher for short circuit current level, pure cable outlet, should strictly limit the requirement of impedance ground design load, be grounded the critical point reach of network technology supervision, for grounded screen operation maintenance is in the future laid a good foundation.

Beneficial effect: grounded screen various dimensions evaluation method of the present invention foothold is based on grounded screen basic function, focus on the security of grounded screen, solve and under transformer station's power-frequency earthing fault and thunderbolt state, reflect the characterisitic parameter evaluation problem of grounded screen security emphatically, compare with the conventional art of only focusing on impedance ground level, to the statement of grounded screen security science more, avoided the systematic bias of traditional evaluation means, that evaluates is with strong points, can achieve a butt joint the accurate evaluation of ground software safety, more effective to the rectification of grounded screen safety issue and defect, benefit is more remarkable.

Accompanying drawing explanation

Below in conjunction with the drawings and specific embodiments, the present invention is described in further details.

Fig. 1 is the basic comprising figure of generating plant, transformer station's large-scale grounding network various dimensions evaluation method: be divided into data and prepare (grounded screen drawing, outlet parameter, short-circuit current etc.), the actual measurement of site layering apparent soil resistivity and the analysis of site soil texture, grounded screen characterisitic parameter field measurement, the verification of grounded screen computation model, the evaluation of grounded screen security value, thermally-stabilised verification, various dimensions evaluation conclusion, and several basic steps such as demonstration are advised and calculated in rectification.

Fig. 2 is the basic flow sheet that the pith-grounded screen security value during grounded screen various dimensions are evaluated is evaluated: first at data (grounded screen drawing, outlet parameter, short-circuit current etc.) on collection and site layering apparent soil resistivity actual measurement basis, by site soil texture, analyze, comprise the grounded screen topological structure simulation of falling resistance measure, ground wire shunting and grounded screen enter earth fault calculation of short-circuit current, step voltage and touch voltage safety limit etc., complete grounded screen power frequency characterisitic parameter (impedance ground, earth mat conductor Potential distribution, step voltage and touch voltage etc.) calculate, and grounded screen transient state Potential distribution and step voltage distribution calculating under lightning ingress state, comparison safety limit, to grounded screen, security is evaluated, to expanding the substation safety of net, predict, simulation calculation demonstration to defect rectification scheme, complete on this basis grounded screen security accurate evaluation.

Fig. 3 is the basic flow sheet of computation model verification: in view of the disguised engineering properties of grounded screen and on-the-spot complicated influence factor, cannot be by the achieve a butt joint perfect measurement of earth mat safety features parameter of field measurement, the accurate simulation of the earth mat characteristic that need to the numerical analysis based on electromagnetic field principle achieves a butt joint, before the latter's application, need to adopt impedance ground, earth mat potential rise and ground wire shunting in-site measurement result, in conjunction with the adjustment of site soil texture, complete the verification of docking grounding network computation model accuracy;

Fig. 4 a is the vertical view of case one 500kV grounding net of transformer substation topological structure schematic diagram;

Fig. 4 b is 45 degree vertical views of case one 500kV grounding net of transformer substation topological structure schematic diagram;

Fig. 5 is case one 500kV substation location soil layering Structure Calculation illustraton of model;

Fig. 6 is 500kV grounding net of transformer substation typical case conductor branches conductor potential rise (GPR) the distribution result of calculation figure of case one;

Place surface potential distribution result of calculation figure when Fig. 7 is the 500kV of the 500kV transformer station equipment place singlephase earth fault of case one;

Place step voltage distribution result of calculation figure when Fig. 8 is the 500kV of the 500kV transformer station equipment place singlephase earth fault of case one;

Place touch voltage distribution result of calculation figure when Fig. 9 is the 500kV of the 500kV transformer station equipment place singlephase earth fault of case one;

Figure 10 a is 450 jiaos of schematic top plan view of case two 500kV grounding net of transformer substation topological structures;

Figure 10 b is case two 220kV grounding net of transformer substation topological structure schematic top plan view;

Figure 11 is case two 220kV substation location soil layering Structure Calculation illustratons of model;

Figure 12 is near the 500kV transformer station thunder of case two grounded screen conductor current potential transient state distribution result of calculation figure lightning rod while attacking lightning rod directly, and wherein Figure 12 a and Figure 12 b are respectively from the situation of A point and the B point lightning rod injection lightning current of Figure 10;

Figure 13 is near the 500kV transformer station thunder of case two step voltage transient state distribution result of calculation figure lightning rod while attacking the A point place lightning rod of Figure 10 directly;

Figure 14 is the 220kV grounding net of transformer substation topological structure schematic diagram of case three;

The 220kV substation location soil layering Structure Calculation illustraton of model of Figure 15 case three;

Figure 16 is the 500kV grounding net of transformer substation topological structure schematic diagram of case four;

Figure 17 is case four 500kV substation location soil layering Structure Calculation models.

Embodiment

Referring to Fig. 1, Fig. 2 and Fig. 3, large-scale grounding network various dimensions evaluation method of the present invention specifically comprises the steps:

(1) adopt four-electrode method or soil pattern analytic approach, measure the apparent soil resistivity under the Different electrodes spacing of site, grounded screen place, get maximum anode-cathode distance and equate with grounded screen size, adopt alternation direct supply to overcome the mutual inductance impact between slotted line.

(2) impedance ground of field measurement grounded screen (and potential difference (PD) of grounded screen and voltage pole), overhead ground wire (comprising OPGW optical fiber ground wire), the cable jacket of two-terminal-grounding and the transformer neutral point of ground connection to the divergent die value of measuring current and respectively shunt component and measuring current vector between phase angle difference, by measuring, each shunting is carried out to vector summation, obtain actual in the measuring current of grounded screen diffusing, the impact of picking out ground wire shunting, obtains earth mat impedance ground value comparatively accurately.

For accurately reflecting the shunting situation of ground wire, inventor, invented the ground wire shunting Method for Phase Difference Measurement based on wireless transmission:

A) by the waveform of the wireless frequency selecting communication instrument real-time sampling class power frequency measuring current of measurement point, the waveform phase information of this measuring current is carried out to wireless transmit to the place of large-scale grounding network;

B) at each framework footing place, outlet structure ground of the on-site of large-scale grounding network, employing has the multi-functional frequency-selecting volometer of measuring current phase angle wireless receiving function, receive the reference current phase information of above-mentioned measuring current, automatically relatively receive each next partial current vector and the phase differential between class power frequency measuring current reference vector;

C) using class power frequency measuring current as reference vector, character based on vectorial, using the shunting of each footing and every cable jacket (also cluster cable) as a vector, first calculate the shunting vector sum of the partial current vector of all framework footings and cable jacket, obtain total minute flow vector (that is aerial earth wire and cable jacket diverting coefficient to measuring current);

D) then with class power frequency measuring current reference vector and total points flow vector, carrying out vector subtracts each other, obtain the actual vector of the part by earth mat diffusing, be the actual current value of large-scale grounding network diffusing, correctly to reject the impact of shunting on measurement result, obtain earth mat impedance ground value accurately.This disposal route physical significance is more clear and definite, and result is more accurate.

(3) step voltage of field measurement grounded screen (comprise and extend out grounded screen) place and the touch voltage of equipment place, and be converted to the value under actual short levels of current.

(4) complete the conduction test of grounded screen integrity test and equipment ground downlead and grounded screen.

(5) by grounded screen breakpoint and corrosion diagnosis and Evaluating Soil Corrosivity based on measurement of electric parameter, in conjunction with grounded screen excavation, check, complete based on electric and ground net corrosion comprehensive diagnos physicochemical property.

(6) based on the long distance measuring, middle distance and short distance array apparent soil resistivity data, inverting obtains site layering (level, vertical or complicated layering) soil texture model.

(7) according to completion design drawing and the construction experience of grounded screen, complete the simulation of grounded screen conductor topological structure.

(8) step (2) on-the-spot test process is carried out to simulation calculation, complete impedance ground (or potential difference (PD) of earth mat and voltage pole), and the comparison of the cable jacket of overhead ground wire (comprising OPGW optical fiber ground wire) and two-terminal-grounding shunting on-the-spot test result, by adjusting soil layering structural model, complete the accuracy verification of grounded screen computation model.

(9) based on generating plant, transformer station's singlephase earth fault short-circuit current, built on stilts and cable outlet structure and parameter, and the parameter such as offside substation grounding impedance, distribution to fault current is calculated, obtain the shunting to fault current of aerial earth wire and cable jacket, through phasor with after processing, obtain different electric pressures place the actual circulation that enters earth fault electric current and grounding transformer neutral point occurs.

(10) based on grounded screen topological structure, rationally the number of plies site soil model and enter earth fault electric current, equipment place for different electric pressures, and each side of main-transformer, choose representational several power-frequency earthing short-circuit current decanting point, the numerical evaluation of grounded screen characterisitic parameter while completing failed shorted, obtains the distribution of earth mat conductor potential rise, net inter potential, step voltage and touch voltage.

(11) while calculating thunder direct attack transformer station, earth mat and earth's surface transient state current potential and step voltage distribute.In generating plant, may there is Lightning Incoming Wave place in the lightning rod of transformer station and outlet structure place etc., carry out respectively grounded screen transient state Calculation of Potential Distribution under thunderbolt state.

(12) check earth mat conductor potential rise and net inter potential to primary equipment (being mainly 10kV and following electric pressure lightning arrester and cable jacket protector etc.) and secondary device (secondary cable, secondary module) operation, and the impact of the outer personnel in station and equipment.Main contents comprise:

(a) whether the relatively variation of impedance ground, there is deviation with check, or have the situation of bounce-back, checks impedance ground value and whether meet service requirement.

(b), while there is ground short circuit fault, grounded screen potential rise should be lower than 2000V, should not be higher than 5000V; In net, maximum potential difference should be lower than 1000V.

(c) check under ground short circuit malfunction, earth mat potential rise causes the zero-clearance zinc oxide lightning arrester two ends aperiodic component potential difference (PD) of 10kV and following electric pressure, whether surpass its superpotential tolerance in short-term, may cause and after lightning arrestor movement even moves, can not bear the energy (surpassing lightning arrester through-current capability) being endowed and blast.

(d) check the potential difference (PD) at power cable oversheath protector two ends, whether surpass its tolerance.

(13) check the step voltage of whole grounded screen (comprise and extend out grounded screen) place, and the touch voltage of station equipment place of the distance enclosure wall certain distance (3m～5m) of ground short circuit may occur, whether surpass safety limit (or recommendation).Pay close attention to the region on grounded screen edge, gate, enclosure wall limit, near the outgoing line side equipment of transformer station's enclosure wall, the equipment of often operation, the place, high-pressure side of main transformer etc., for higher region, consider the check under rainy or the moist situation in ground, whether meet personal security requirement, confirm that the step voltage of which subregion and (or touch voltage) level are higher.

(14) whether transient state Potential distribution and the potential difference (PD) of near zone on the lightning rod that investigation earth mat conductor transient state current potential is the highest and outlet structure, bring insulation to threaten and electromagnetic interference (EMI) near secondary device or electrical secondary system; Check near the transient state step voltage level of lightning strike spot, whether the interior personal security of standing is brought to threat.

(15) for adopting the resistance measure of falling of expanding net mode, extend out the destroyed rear grounded screen characterisitic parameter of earth mat and calculate, to grounded screen, security is predicted, evaluates and whether meets the requirement of grounded screen safety indexes.

(16) based on system maximum short circuit current level current or that generating plant, transformer station adjust in the future, consider the conductor corrosion factor in corrosion diagnosis of grounding grid, extent of corrosion to grounding body is proofreaied and correct, and completes down conductor and grounded screen cross-sectional area of conductor carries out heat-stable calibration.

(17) for on-the-spot test and security value, calculate the problem of the grounded screen security presence reflecting, for current or grid short circuit levels of current in the future, propose improvement suggestion, and scheme is carried out to security value calculating, instruct rectification or fall resistance practice.

In enforcement, should be according to generating plant, the importance of transformer station and the operation time limit, carry out large-scale grounding network various dimensions evaluation sampling observation, mainly for following several situations, select in principle: (1) operation time limit long (as over 10 years), (2) earth mat suffers local failure (especially extend out earth mat and suffer artificial destruction), (3) suspect earth mat seriously corroded or suffer artificial destruction, (4) suspect earth mat existing problems (especially transformer station of county telephone central office), (5) in servicely cross the equipment failure relevant to grounded screen, (6) performance under thunderbolt or power-frequency earthing short-circuit condition of suspection grounded screen does not meet the demands, (7) after ground network reconstruction.

The grounded screen various dimensions evaluation method of the implementation case also can be applied to the large-scale grounding networks such as generating plant, DC converter station, and implementation method is identical.

Concrete application implementation case one

1, the simulation of basic condition and grounded screen topological structure

Vertical view and 45 degree vertical views referring to the 500kV grounding net of transformer substation topological structure schematic diagram of Fig. 4 a and Fig. 4 b.

This 500kV transformer station has moved nearly 10 years, and having at present 4 capacity is respectively the 500kV main transformer of 1000MVA, has 3 times 500kV outlets, and outlet structure has 7 lightning conducters (wherein 3 OPGW optical fiber ground wires) and is connected with terminal shaft tower; Have 12 times 220kV outlets, outlet structure has 11 lightning conducters (wherein 7 OPGW optical fiber ground wires) and is connected with terminal shaft tower.Transformer station is positioned at knob, and transformer station is formed by a hillside leveling, this station right and left and three periderm mountains, rear around, the door of standing is that before a pool and ，Cong Xinwei village, an a slice orchard cement road slave station out, 20m passes through under slope, dead ahead.Substation location edaphic condition is poor, and it is main earth mat that the horizontal grounding objects consisting of φ 22 galvanizing round steel has laid in transformer station, and earth mat buried depth is 0.8m, and using steel length is 8200m; Make a call to altogether 10 vertical ground connection deep-wells that 30m is dark, and use 10 tons of friction reducers and ionic earthing array, the main grounding grid 230m * 196m of transformer station, the nearly 40000m2 of area.Get grounded screen equivalence catercorner length D ≈ 400m.Grounded screen topological structure as shown in Figure 1.Grounding impedance design value is 0.50 Ω.The 500kV of transformer station and the 220kV bus single-line to ground fault electric current adjusted are respectively 35.72kA and 22.05kA.

2, the foundation of site soil layering structural model

By on-the-spot apparent soil resistivity, measure and analog computation, inverting obtains the soil texture model of Fig. 5, can be divided into the hierarchy of four layers of levels, and deep soil resistivity is higher, and overall edaphic condition is poor.

3, grounded screen computation model accuracy verification

Under running status with 500kV, 220kV lightning conducter and OPGW optical fiber ground wire on outlet structure, what employing applied the little electric current of 6.0A class power frequency is 1.344V away from the measured substation ground network of the current-voltage method of direction and voltage pole potential difference (PD), impedance ground value is 0.224 Ω, the actual measurement diverting coefficient of 500kV and 220kV overhead ground wire and OPGW optical fiber ground wire is 72.52%, and the Grounding resistance value of rejecting ground wire shunt influence is 0.8151 Ω.

Based on grounded screen topological structure model, outlet circuit and shaft tower, arranging, survey the electric current utmost point and the voltage pole of process arranges, and soil layering structural model, while injecting 6.0A class power current, earth mat and voltage pole potential difference (PD) are 1.383V, the total diverting coefficient of ground wire is 68.27%, and grounded screen computation model meets grounded screen various dimensions and evaluates accuracy requirement.

Consider that on-the-spot test has numerous disturbing factors, and analog computation can better reflect the truth of earth mat, get simulation value 0.8891 Ω based on soil horizontal slice structure as the impedance ground value of grounding net of transformer substation.

4, the grounded screen of consideration transformer circulation enters ground short circuit Current calculation

According to the 500kV of transformer station and the 220kV bus single-line to ground fault levels of current adjusted, and substation location soil texture and outlet structure, calculate aerial earth wire diverting coefficient (mould) and be roughly 75% level.

During 500kV switch region singlephase earth fault, 73.255 ° of kA of trouble spot earth current 9.542 ∠, neutral point of main transformer earth current 1.312 ∠-51.173 ° kA; During 220kV switch region singlephase earth fault, 59.117 ° of kA of trouble spot earth current 5.617 ∠, 66.969 ° of kA of neutral point of main transformer earth current 10.931 ∠; 66.246 ° of kA of main transformer region 500kV side single-line to ground fault electric current 8.867 ∠; 64.306 ° of kA of main transformer region 220kV side single-line to ground fault electric current 16.513 ∠.

5, earth mat conductor Calculation of Potential Distribution during power-frequency earthing fault

Get this modal failure situations of transformer station's place single phase ground fault, calculate the distribution of grounded screen conductor potential rise (GPR), typical earth mat conductor branches Potential distribution as shown in Figure 6.Result of calculation shows, although transformer station's outlet lightning conducter is very large to the shunting contribution of single-phase grounding fault, but because transformer station's singlephase earth fault short circuit current level is higher and ground network ground resistance is higher, no matter be 500kV region, there is singlephase earth fault in 220kV region or main transformer region, the fault current that enters ground from earth mat all will cause very high earth mat conductor stable state power frequency potential rise, when 220kV place and main transformer medium voltage side generation singlephase earth fault in ground, place metallic conductor potential rise all more than 14.6kV, 8kV nearly when singlephase earth fault occurs for 500kV place and main transformer high-pressure side, personnel and equipment outside grounding net of transformer substation noble potential Wai Yindui transformer station cause sizable threat, must check 10kV metal zinc oxide lightning arrester in transformer station simultaneously and whether can tolerate the potential rise of grounded screen.

Substation Station has one group (3 phase) protection 10kV#0 station to use metal zinc oxide lightning arrester (model YH5WZ-16.5/45 that becomes use with turing indoor; rated voltage 16.5kV; continuous running voltage 13.6kV; direct current 1mA reference voltage 24kV; discharge capacity is 2ms, is not less than 200A 18 times); being arranged on station and refuting the place of connecing (#0 station is with in change 501 switch cubicles, lightning arrester not being installed) with what become chamber 10kV inlet wire cable end and autocratic line row, outside transformer station's enclosure wall, lightning arrester is not installed at 10kV inlet wire line terminal bar and the 10kV cable transferring head place of entering the station.

When getting earth short circuit current and continuing 0.5s, to tolerate in short-term coefficient be 1.1 to lightning arrester, and the superpotential in short-term that this group lightning arrester can bear is: 16.5 * √, 2 * 1.1=25.664(kV).

The severe situation of earth mat conductor current potential and lightning arrester phase voltage single spin-echo while considering transformer station's generation singlephase earth fault, when typical 10 points in 220kV place and 4 main transformer medium voltage side generation singlephase earth faults, even if the situation of the minimum 14.465kV(#4 main transformer region medium voltage side W point singlephase earth fault of earth mat conductor potential rise), arrester ground end institute grounded screen conductor current potential aperiodic component is superimposed upon the earth potential rising impact peak value causing in AC compounent: 14.465 * 2.55(coefficient of impact)=36.886(kV), the superpotential in short-term that now bear at lightning arrester phase and two ends, ground: 36.886+10/ √ 3=42.659(kV), this value will surpass the tolerance of superpotential in short-term of 10kV metal zinc oxide lightning arrester, may cause lightning arrester generation thermal runaway and explode.When typical 9 points in 500kV place and 4 main transformer high-pressure sides generation singlephase earth faults, consider the maximal value 7.949kV(500kV switch region G point of earth mat conductor potential rise, be the situation that singlephase earth fault occurs 500kV peace tabernaemontanus bulrush line CVT B mutually), the superpotential in short-term of bearing by the lightning arrester phase of above-mentioned accounting and two ends, ground is 26.044kV, the superpotential in short-term that this value can bear lower than lightning arrester, can not cause Explosion of on Arrester.As can be seen here, when 220kV place and 4 main transformer medium voltage side generation singlephase earth faults, the most serious in the situation that, there is the hidden danger that causes the blast of the 10kV of transformer station metal zinc oxide lightning arrester in earth mat conductor potential rise.

On the other hand, while there is ground short circuit fault, whole grounded screen place maximum differential pressure level is up to 237V, most of singlephase earth fault short dot that occurs is all below 200V, during single phase ground fault, earth mat conductor potential rise can not threaten to secondary device or secondary circuit insulation, and secondary device or secondary circuit insulation can be born the potential rise of grounded screen conductor.

6, step voltage distributes and calculates

When singlephase earth fault occurs in transformer station place, the surface potential of whole place distributes and step voltage distributes respectively as shown in Figure 7 and Figure 8.While there is singlephase earth fault in 500kV place and main transformer high-pressure side, the step voltage mxm. that comprises the whole earthed system place of enclosure wall earth mat fringe region is around 367.434V, lower than place step voltage safety limit US(530.66V), and when 220kV place and main transformer medium voltage side generation singlephase earth fault, the step voltage maximal value in network edge region, enclosure wall other places all (is up to 687.539V over place step voltage safety limit, generally more than 600V), and the step voltage maximal value of place is up to 520.173V in enclosure wall, generally more than 450V, though lower than safety limit, but also approach very much safety limit, need to draw attention.Mxm. is 69.019V, far below place step voltage safety limit US(375.23V).

7, touch voltage distributes and calculates

Consider operation actual conditions, investigate grid of distance ground network edge with interior touch voltage level, the touch voltage of whole equipment place distributes respectively as shown in Figure 9.High level touch voltage appears at the earth mat edge zone by enclosure wall, 220kV place and transformer medium voltage side region single phase ground fault failure situations touch voltage level are higher, and (mxm. is 908.128V, generally in 870V～900V scope), the touch voltage maximal value slightly low (in 450V～490V scope) causing when 500kV place and high voltage side of transformer region single phase ground fault fault, all over safety limit, the high-caliber touch voltage in place, transformer station edge brings threat to the safety of personnel in transformer station, and during in the future changes of operating modes or transformer station's dilatation, this situation is more serious, should draw attention.

8, electrical equipment grounding downlead and main earth mat connection

City Substation Electric Equipment down conductor and main earth mat connection are good, and grounded screen integrality is good.

9, ground net corrosion diagnostic evaluation

Transformer station's horizontal earth mat general corrosion situation of having moved nearly 10 years is general, and horizontal earth mat is all right, and earth mat horizontal metal conductor average corrosion rate is 6.22%, and average weightlessness is 9.82g/50mm, and annual weightlessness is 1.24%.

In cable duct, lay 50 * 5 zinc-coated flat steel sheets, zinc coat is substantially destroyed, and corrosion, and except indivedual corrosion conditions are partially serious, overall corrosion condition is general.

10, grounded screen various dimensions comprehensive evaluation

The deep soil resistivity of substation location is higher, and overall edaphic condition is poor.Substation ground network grounding resistance surpasses Substation Design and requires (0.50 Ω).

While there is ground short circuit fault, the potential rise level of whole grounded screen is more even, and earth mat conductor potential difference (PD) does not threaten to secondary device or secondary circuit insulation; But personnel and equipment outside the outer Yin Yejiangdui of earth mat conductor noble potential transformer station cause sizable threat; And the most serious in the situation that, exist earth mat conductor potential rise to cause the hidden danger of 10kV Explosion of on Arrester.

In standing, electrical equipment grounding downlead and main earth mat connection are good, and grounded screen integrality is good.

Transformer station's horizontal earth mat general corrosion situation is general, and horizontal earth mat is all right.In cable duct, lay 50 * 5 zinc-coated flat steel sheets, zinc coat is substantially destroyed, and corrosion, and except indivedual corrosion conditions are partially serious, overall corrosion condition is general.

While there is singlephase earth fault in place, enclosure wall earth mat fringe region step voltage around will approach or over safety limit, earth mat edge zone touch voltage by enclosure wall will be over safety limit, bring threat to the safety of personnel in transformer station, and during in the future changes of operating modes or transformer station's dilatation, this situation is more serious, should draw attention.

Although under current system service condition, the requirement of transformer station's safe operation of station equipment and personnel when flatly net state can meet station district single phase ground fault fault substantially, but because main grounding grid impedance ground surpasses design load, to bring following risk: when singlephase earth fault occurs in place, the too high lightning arrester that may cause of earth mat conductor potential rise is blasted, outside noble potential Wai Yindui transformer station, personnel and equipment cause sizable threat, step voltage and touch voltage near enclosure wall region will generally surpass safety limit, and during in the future changes of operating modes or transformer station's dilatation, this situation is more serious.

Suggestion:

(1) in transformer station's red line, take the rational measure of Technological Economy to realize and effectively fall resistance, earth mat conductor potential rise when effectively reducing singlephase earth fault, to eliminate interior primary equipment and secondary device (or secondary circuit) insulation of standing, and personnel's security threat.

(2) as interim measure, completing before grounded screen effectively falls resistance, change 10kV#0 station and use 10kV lightning arrester with becoming, significantly improve its discharge capacity (being 2ms, 18 times, 200A at present); Also can consider to take existing parameter lightning arrester to organize in parallel to improve total discharge capacity more, thereby reduce the scheme of single lightning arrester superpotential energy absorption burden, to improve the safety in operation of lightning arrester.For the synthetic overcoat Zinc-Oxide Arrester technical parameter of the gapless of changing type selecting reference, be: rated voltage 16.5～17kV, continuous running voltage 13.2～13.6kV, the residual voltage 42～45kV under 5kA nominal direct-current discharge electric current, 1mA reference voltage is not less than 24kV; Discharge capacity 2ms, be not less than 800A(for 18 times and can select parallel capacitor group lightning arrester), producer has ready conditions and can select 1000A even higher.

(3) according to relevant rules and counter arranging, carry out the quarantine measures of drawing outer metallic water pipe, communication line and power supply etc., the danger that while avoiding because of place single phase ground fault fault, the outer leader tape of noble potential comes.

(4) for the purpose of safety is safe, in the device ground near edge, enclosure wall place, lay high resistant gravel or resistive formation, also can consider to adopt in measures such as equipment frame or device housings spraying resistive formations (to 2m height), eliminate the threat of touch voltage to the interior personnel that stand.

(5) time limit in strict accordance with the standard Q/CSG1007-2004 < < of CHINA SOUTHERN POWER incorporated business electrical equipment preventive test rules > > regulation carries out close follow-up investigations to full station grounded screen conductor corrosion condition, the zinc-coated flat steel sheet laying in cable duct is generally investigated simultaneously, corrosion condition very severe patient arrangement is carried out large repairs.

(6) according to the variation of system short-circuit electric current, regularly check the thermal capacity of grounding body, and potential rise level, step voltage and the touch voltage level of earth mat metallic conductor during single-phase grounding fault, whether meet personnel and equipment safety operation requirement.

Case study on implementation two

1, the simulation of basic condition and grounded screen topological structure

Certain 500kV transformer station has moved 12 years, and through second phase enlarging, having at present 3 capacity is respectively the 500kV main transformer of 750MVA, has 5 times 500kV outlets, and outlet structure has 12 lightning conducters (wherein 5 OPGW optical fiber ground wires); Have 13 times 220kV outlets, outlet structure has 15 lightning conducters (wherein 4 OPGW optical fiber ground wires); There is no 10kV cable outlet.Substation location is positioned at hilly country, and main grounding grid is 351.5m * 289.5m at present, does not expand net; Horizontal grounding band adopts 60mm * 60mm * 6mm zinc-coated flat steel sheet of overall length 19000m, depth of burying 0.8m, vertical grounding electrode adopts the coating steel pipe (overall length 450m) of Φ 50mm, long 2.5m, at ground network edge, buy altogether 4 mouthfuls of 30m deep-wells pressure injection friction reducer, grounded screen topological structure is as shown in Figure 10 a and Figure 10 b.Grounded screen power-frequency earthing impedance design value is 0.60 Ω.Transformer station place has been installed 17 lightning rods altogether, is all effectively connected with main grounding grid.The 500kV of transformer station and 220kV bus single-line to ground fault electric current are respectively 9.94kA and 12.63kA.

2, the foundation of site soil layering structural model

By on-the-spot apparent soil resistivity, measure and analog computation, inverting obtains the soil texture model of Figure 11, can be divided into the soil layering structure of four layers of levels, site stratified soil unevenness is larger, shallow-layer and BC soil condition are better, and deep soil condition is general, and overall edaphic condition is better.

3, grounded screen computation model accuracy verification

Under running status with 500kV, 220kV lightning conducter and OPGW optical fiber ground wire on outlet structure, employing applies the measured substation ground network of the little current method of 6.0A class power frequency and voltage pole potential difference (PD) is 1.40V, impedance ground value is 0.234 Ω, the actual measurement diverting coefficient of 500kV and 220kV overhead ground wire and OPGW optical fiber ground wire is 43.25%, and the Grounding resistance value of rejecting ground wire shunt influence is 0.412 Ω.

Based on grounded screen topological structure model, outlet circuit and shaft tower, arranging, survey the electric current utmost point and the voltage pole of process arranges, and soil layering structural model, while injecting 6.0A class power current, earth mat and voltage pole potential difference (PD) are 1.48V, the total diverting coefficient of ground wire is 46.65%, and grounded screen computation model meets grounded screen security value and evaluates accuracy requirement.

Consider that on-the-spot test has numerous disturbing factors, and analog computation can better reflect the truth of earth mat, get simulation value 0.420 Ω based on soil horizontal slice structure as the impedance ground value of grounding net of transformer substation.

According to the 500kV of transformer station and the 220kV bus single-line to ground fault levels of current adjusted, and substation location soil texture and outlet structure, calculate aerial earth wire diverting coefficient (mould) and be roughly 58% level.

During 500kV outgoing line side singlephase earth fault, 105.8509 ° of kA of trouble spot earth current 8.4331 ∠, neutral point of main transformer earth current 4.2286 ∠-77.2090 ° kA; During 220kV outgoing line side singlephase earth fault, 111.0084 ° of kA of trouble spot earth current 10.6228 ∠, neutral point of main transformer earth current 5.3990 ∠-56.8222 ° kA; 116.8448 ° of kA of main transformer region 500kV side single-line to ground fault electric current 4.2683 ∠; 98.9881 ° of kA of main transformer region 220kV side single-line to ground fault electric current 5.4649 ∠.

Get this modal failure situations of transformer station's place single phase ground fault, when calculating 220kV place and main transformer medium voltage side and breaking down, earth mat conductor potential rise reaches 2200V above (typical earth mat conductor Potential distribution as shown in figure 12), and place maximum differential pressure can reach 260.86V; 500kV place and main transformer high-pressure side are no more than 2000V while breaking down, and place maximum differential pressure is 307.75V, between age at failure to once, the operation of secondary device can not impact.

6, step voltage distributes and calculates

When break down in 500kV place and main transformer high-pressure side, the step voltage mxm. of whole grounded screen place is 65.01V, far below place step voltage safety limit U _s500(530.66V); When 220kV place and main transformer medium voltage side fault, step voltage maximal value is 101.34V, far below safety limit U _s220(375.23V).

7, touch voltage distributes and calculates

When break down in 500kV place and main transformer high-pressure side, the equipment place touch voltage mxm. in the enclosure wall of being concerned about is 184.13V, lower than touch voltage safety limit U _t500(351.52V); When 220kV place and main transformer medium voltage side fault, touch voltage mxm. is 245.74V, surpasses or approaches safety limit U _t220(248.56V), consider that current transformer station short circuit current level is still lower, unfavorable factor, the especially ground such as power system operating mode variation in the future or transformer station's dilatation are moist, and when soil resistivity is lower, this problem is more serious.

8, transient state Calculation of Potential Distribution under Lightning Incoming Wave condition

The likelihood ratio of direct lightning strike For Lightning-Rod Protection Disign In Substations and transformer station's near region thunderbolt is higher, and while investigating lightning rod and outlet structure injection lightning current, the transient state Potential distribution of grounded screen and step voltage distribute.Near lightning strike spot, surface potential rises very fast, and decay rapidly, and transient state noble potential mainly concentrates on lightning rod near zone, and the transient state potential rise of all the other most equipment places is relatively low.

Wave mode is that surface potential and the earth mat conductor current potential that the lightning current of 2.6/50 μ s, amplitude 50kA causes reaches respectively 250kV and level more than 275kV; The highest apart from the potential difference (PD) within the scope of decanting point conductor segment 5m, can reach level more than 120kV; Near step voltage level lightning strike spot is the highest, and mxm. reaches the level of 40kV, and the step voltage level of all the other most equipment places is very low.

The earth mat conductor potential rise that thunder direct attack transformer station causes can not threaten to the safe operation of primary equipment, and may bring threat to the safe operation of near secondary device thunderbolt point and secondary cable, especially for thunder, attack near the situation of lightning rod place, master control building directly.

Typical case's earth mat conductor current potential transient state distributes and place step voltage transient state distributes respectively as shown in Figure 12 and Figure 13.

9, grounded screen various dimensions comprehensive evaluation

The overall edaphic condition of substation location is relatively better, and ground network ground resistance value meets design requirement.Substation equipment place will cause certain earth mat conductor stable state power frequency potential rise while there is earth fault, personnel outside earth mat noble potential Wai Yindui transformer station and equipment cause certain threat ，Dan transformer station place earth mat conductor potential difference (PD) not threaten to secondary device or secondary circuit insulation; Whole grounded screen place step voltage level is far below safety limit; Whole equipment place touch voltage level when 500kV equipment and region, main transformer high-pressure side are short-circuited fault is lower than safety limit, but when 220kV place and main transformer medium voltage side generation singlephase earth fault, equipment place touch voltage level in enclosure wall is higher, surpasses or approaches safety limit.

Although under the service condition of current system underload and low short circuit current level, the requirement of station equipment and personnel's safe operation when substation ground network security can meet station district single phase ground fault fault substantially, but exist noble potential to draw and higher the brought risk of part place touch voltage outward, consider that current transformer station singlephase earth fault short circuit current level is still lower, power system operating mode changes in the future, the unfavorable factors such as system loading and capacity of short circuit rising and transformer station's dilatation, especially ground is moist, when soil resistivity is lower, this problem should draw attention, suggestion:

(1) according to relevant rules and counter arranging, carry out the quarantine measures of drawing outer metallic water pipe, communication line and power supply etc., the danger that while avoiding because of place single phase ground fault fault, the outer leader tape of noble potential comes, can consider: (a) external running water pipe enters in station and plugs into the long pvc pipe of a hop count rice, prevents that noble potential from drawing; (b) external communication line installs isolating transformer additional and isolates in station; (c) entrance guard and master control building supply water and lay with pvc pipe.

(2) the higher problem of equipment outgoing line side equipment place touch voltage level of network edge closely while breaking down for current 220kV equipment place and main transformer medium voltage side, near the region, 220kV equipment place of enclosure wall, taking the measure of the horizontal earth mat grid of encryption targetedly, effectively to reduce touch voltage level, also can consider to lay high resistant gravel or resistive formation on the equipment region ground near enclosure wall, or in measures such as equipment frame or device housings spraying resistive formations (to 2m height), eliminate the threat of touch voltage to the interior personnel that stand, improve the security level of personnel under ground short circuit fault.

(3) when ground short circuit fault occurs in transformer station, mainly to have benefited from the lower short circuit current level that causes of the current capacity of transformer station very low for relatively low characterisitic parameter (conductor potential rise, place pressure reduction, step voltage and touch voltage) the numerical value level of earth mat, along with the quick rising of system loading and capacity of short circuit in the future, should regularly check according to the variation of system short-circuit electric current the thermal capacity of grounding body, and potential rise level and the touch voltage level of earth mat metallic conductor during earth fault, whether meet personnel and equipment safety operation requirement.

(4) the earth mat potential rise that thunder is attacked directly or lightning surge intrusion transformer station causes will bring electrical secondary system and personnel's security risk, and the layout of suggestion secondary device, secondary circuit or secondary cable etc. is left lightning rod or outlet structure one segment distance as far as possible; While having ready conditions, near the lightning rod of close together place, master control building is moved to a segment distance far away, improve electrical secondary system in master control building and at thunder, attack the security moving under transformer station's state directly.Should, in strict accordance with relevant secondary device lightning protection rules (standard), improve the links such as SPD type selecting of ground connection, shielding, power supply and the signal system of transformer station secondary system and secondary cable.

(5) thunder and lighting process reduces the activity in Outdoor Trusses For Transformer Substation place as far as possible, especially avoids movable in the place of contiguous lightning rod and outlet structure, to lower step voltage that Lightning Incoming Wave causes to personal threat.

Case study on implementation three

1, the simulation of basic condition and grounded screen topological structure

Certain 220kV transformer station has moved 19 years, and having at present 2 capacity is respectively the 220kV main transformer of 150MVA, has 5 times 220kV outlets, and outlet structure has 6 lightning conducters (wherein 3 OPGW optical fiber ground wires); Have 5 times 110kV outlets, outlet structure has 8 lightning conducters (wherein 4 OPGW optical fiber ground wires); Have 18 times 10kV outlets, as the arrive at a station transition of the not far 10kV of the turning overhead outlet of outer enclosure wall of the 10kV of transformer station hyperbaric chamber, substantially, in extending out within the scope of grounded screen, can be used as a part for substation ground network.Transformer station is positioned at coastal knob, and around physical features is comparatively mild in transformer station, and edaphic condition is general, and the road both sides of entering the station are field, and edaphic condition is good, extends out earth mat and mainly concentrates on this region.Main grounding grid 284m * 252m in transformer station's enclosure wall, the about 70000m2 of area, horizontal earth mat adopts φ 16 galvanizing round steel to amount to 8400m, buried depth 0.8m, in cable duct, the equal pressure zone of ground connection adopts φ 14 galvanized steel bars to lay; Vertical grounding electrode adopts ∠ 50 * 5, and the galvanizing angle steel of long 2.5m amounts to 178.Because substation location edaphic condition is general, adopt station to extend out the resistance mode of falling of net and deep-well pressure injection friction reducer, extend out earth mat mainly in the transformer station good farmland of edaphic condition, road both sides of entering the station, extend out the earth mat total area and main grounding grid suitable, adopt in addition the prolongation earthing pole of 5 200m～500m to transformer station both sides and extension behind.Extend out the employing of horizontal grounding band and amount to 6462.6m by φ 16 galvanizing round steel, buried depth is 1.0m; At transformer station's peripheral edge, make a call to 7 mouthfuls of vertical ground connection deep-wells of φ 150 that 30m～40m is dark.Grounded screen topological structure as shown in figure 14.Grounding impedance design value is 0.50 Ω.The 220kV of transformer station and 110kV bus single-line to ground fault electric current are respectively 16.69kA and 15.95kA.

2, the foundation of site soil layering structural model

By on-the-spot apparent soil resistivity, measure and analog computation, inverting obtains the soil texture model of Figure 15, can be divided into the soil layering structure of four layers of levels, and top layer and thin solum resistivity are general, middle level and deep soil resistivity are higher, and overall edaphic condition is general.

3, grounded screen computation model accuracy verification

Under running status with 220kV, 110kV lightning conducter and OPGW optical fiber ground wire on outlet structure, measured substation ground network and the voltage pole potential difference (PD) of the electric current-voltage method away from direction that employing applies the little electric current of 6.0A class power frequency is 1.57V, impedance ground value is 0.262 Ω, the actual measurement diverting coefficient of 220kV and 110kV overhead ground wire and OPGW optical fiber ground wire is 41.40%, and the Grounding resistance value of rejecting ground wire shunt influence is 0.447 Ω.

Based on grounded screen topological structure model, outlet circuit and shaft tower, arranging, survey the electric current utmost point and the voltage pole of process arranges, and soil layering structural model, while injecting 6.0A class power current, earth mat and voltage pole potential difference (PD) are 1.52V, the total diverting coefficient of ground wire is 44.72%, and grounded screen computation model meets grounded screen various dimensions and evaluates accuracy requirement.

Consider that on-the-spot test has numerous disturbing factors, and analog computation can better reflect the truth of earth mat, get simulation value 0.4322 Ω based on soil horizontal slice structure as the impedance ground value of grounding net of transformer substation.

According to the 220kV of transformer station and the 110kV bus single-line to ground fault levels of current adjusted, and substation location soil texture and outlet structure, calculate aerial earth wire diverting coefficient (mould) and be roughly 77% level.

While there is singlephase earth fault in 220kV switch region, 51.9246 ° of kA of trouble spot earth current 4.2361 ∠, neutral point of main transformer earth current 0.7217 ∠-137.1237 ° kA; During 110kV switch region singlephase earth fault, 47.8495 ° of kA of trouble spot earth current 9.3408 ∠, neutral point of main transformer earth current 7.3007 ∠-137.4426 ° kA; 53.7696 ° of kA of main transformer region 220kV side single-line to ground fault electric current 3.5252 ∠; 65.8592 ° of kA of main transformer region 110kV side single-line to ground fault electric current 2.1779 ∠.

Get this modal failure situations of transformer station's place single phase ground fault, while calculating most regions generation ground short circuit fault, the potential rise of grounded screen is all not high, when singlephase earth fault occurs for 220kV place and main transformer high-pressure side, earth mat conductor current potential is up to 1657.264V(generally between 1500～1700V); When 110kV place and main transformer medium voltage side generation singlephase earth fault, earth mat conductor current potential is the highest generally between 1100～1200V, and personnel and equipment outside earth mat noble potential Wai Yindui transformer station cause certain threat.During ground short circuit fault, the potential rise level of whole grounded screen is more even, place maximum differential pressure level is up to 764.576V, when wherein singlephase earth fault occurs in 110kV place, whole grounded screen place maximum differential pressure level is higher, generally between 500～700V; When 220kV place and 2 main transformer generation singlephase earth faults, whole grounded screen place maximum differential pressure level is lower, and generally, between 100～300V, secondary device insulation can be born the potential rise of grounded screen conductor.6, step voltage distributes and calculates

When singlephase earth fault occurs in transformer station place, the step voltage mxm. of whole place is 69.019V, far below place step voltage safety limit US(375.23V).

7, touch voltage distributes and calculates

Equipment place touch voltage mxm. in place single phase ground fault fault Shi， transformer station of transformer station enclosure wall is 144.562V, all far below safety limit U _t(248.56V).

8, ground net corrosion diagnostic evaluation

8.1 corrosion diagnosis of grounding grids based on electrical specification

In view of transformer station's floor area is larger, during on-the-spot test, be divided into two regions of 220kV and 110kV, in every region, divide 4 measurement subregions simultaneously and carry out piecemeal measurement.Then in each measures subregion, select suitable down conductor as isolated node, measure successively the resistance between different nodes and isolated node, wherein different nodes refer to other nodes in the independent partitions at corresponding isolated node place.According to resulting measurement result, to in substation ground network branch road and node data input diagnostic routine, carry out simulation calculation, obtain corresponding diagnostic result, find 110kV region the 259th, it is 10 times that No. 273 branch road resistance increase multiple, these two branch roads lay respectively under No. 2 main transformer lightning arresters of 110kV side and No. 1 main transformer by, be judged to be compared with heavy corrosion, excavation checking has been carried out to these two branch roads in scene, confirm that No. 2 main transformer lightning arrester downleads effectively do not weld by designing requirement with earth mat conductor, cause No. 2 main transformer lightning arresters there is no effective grounding.Comprehensive diagnos result the ground net corrosion appraisement system based on electric measurement, grounding net of transformer substation general corrosion is assessed, grounded screen has 18 branch road generation mild corrosions, article 43, branch road generation moderate corrosion, article 15, branch road generation heavy corrosion, substation ground network corrosion branch road ratio KN is 0.38, belongs to the corrosion of III level.

8.2 ground net corrosion based on soil physico-chemical characteristic assessments

Transformer station's soil property be take silt as main, the collection point of pedotheque lays respectively at: #1 main transformer step-down bus bridge place (sampling spot #1), #1 main transformer uprise that 22011 disconnecting link A get along (sampling spot #2), #2 main transformer uprises that 22021 disconnecting link A get along (sampling spot #3), #2 main transformer become in 1023 disconnecting link A get along (sampling spot #4), fragrant A-wire A phase post insulator place (sampling spot #5), 110kV osmanthus, 22-05 cat ladder place, 220kV place (sampling spot #6), soil parameters on-site measurement and lab analysis the results are shown in Table one.

The two 220kV transformer stations soil parameters on-site measurement of table one case and lab analysis result

According to soil three indexs and eight index evaluation standards, transformer station's soil corrosion grade is III level.

With maximum short circuit current value (26.69kA), carry out thermally-stabilised verification calculating, trying to achieve the average corrosion index Kavg of grounding net of transformer substation is 3.1, current earth mat diameter of section is 15.2mm, according to the smallest cross-sectional of the thermally-stabilised requirement of grounded screen conductor require and residual Life Calculation in Table two.

Table two case two 220kV grounding net of transformer substation residual Life Calculation tables

8.2 based on ground net corrosion transformation strategy electric and soil physico-chemical characteristic

Transformer station has moved 19 years, and according to the ground net corrosion appraisement system based on electric, grounded screen belongs to III level heavy corrosion, suggestion full station transformation.

According to earth mat Soil Characteristic Analysis, the former employing of main grounding grid Φ 16 round steel, according to current short-circuit current value (26.69kA), carry out heat-stable calibration, when equivalent short circuit duration is got 0.35s and 0.7s, the smallest cross-sectional diameter of thermally-stabilised requirement is respectively 14.68mm and 17.45mm; According to long-term short-circuit current value, carry out thermally-stabilised verification, smallest cross-sectional diameter requires respectively 16.73mm and 19.90mm.Transformer station's soil corrosion grade is III level, the average INF prediction drawing according to soil corrosion grade, and grounded screen residual life is respectively-1.4 years and-38.4 years, has not met thermal stability requirement, need to transform.

According to the average INF (0.05mm/a) of grounding net of transformer substation conductor, according to 30 years serviceable life of ground grid design and short-circuit current distant view, increase 1.3 times of considerations, when if equivalent short circuit duration is got 0.7s, grounded screen smallest cross-sectional requires as 20.65mm, therefore Φ 20 round steel are used in suggestion while transforming.

9, consider to extend out complete substation ground network safety prediction when destroyed of earth mat

The consideration calculating extend out earth mat complete when destroyed in enclosure wall main grounding grid impedance ground value be 0.8368 Ω, this value surpasses designing requirement (0.50 Ω).

Extend out earth mat completely destroyed after, while there is earth fault, the level that aerial earth wire diverting coefficient (mould) is 86%.During 220kV switch region singlephase earth fault, 55.8074 ° of kA of trouble spot earth current 2.9363 ∠, neutral point of main transformer earth current 0.7217 ∠-137.1237 ° kA; During 110kV switch region singlephase earth fault, 48.8026 ° of kA of trouble spot earth current 8.5808 ∠, neutral point of main transformer earth current 7.3007 ∠-137.4426 ° kA; 59.9443 ° of kA of main transformer region 220kV side single-line to ground fault electric current 2.2387 ∠; 79.7708 ° of kA of main transformer region 110kV side single-line to ground fault electric current 1.5434 ∠.

There is all more current grounded screen nearly 1 times of left and right that raises of the caused earth mat conductor of singlephase earth fault stable state power frequency potential rise in substation equipment place, place maximum differential pressure level approaches with current grounded screen, and during ground short circuit, earth mat conductor potential rise is little to the threat of secondary device insulation.

The generation caused grounded screen of singlephase earth fault place, substation equipment place step voltage maximal value all more current grounded screen has raise largely (between 70%～120%), the step voltage mxm. that comprises the whole earthed system place that extends out earth mat fringe region is that 130.110V(is generally between 70V～130V), all far below place step voltage safety limit.The interior equipment of transformer station's enclosure wall place touch voltage maximal value all more current grounded screen has rising (between 70%～150%) largely, and the touch voltage mxm. of the equipment place of close enclosure wall is 184.079V, all lower than place touch voltage safety limit.

10, grounded screen various dimensions comprehensive evaluation

The overall edaphic condition of substation location is general, during due to capital construction, adopts large area to extend out earth mat, the measure in conjunction with deep-well with friction reducer, and ground network ground resistance value can meet design requirement.

Due to the current system loading of transformer station and short circuit current level very low, when ground short circuit fault occurs in transformer station, the conductor potential difference (PD) of whole grounded screen does not threaten to secondary device or secondary circuit insulation, and the touch voltage of the outer step voltage level of on-site and the interior equipment of enclosure wall place is far below safety limit.

During single phase ground fault fault from earth mat enter ground fault current still cause certain earth mat conductor stable state power frequency potential rise, between age at failure earth mat noble potential draw outward to station outside personnel and equipment cause certain threat.

Whole grounded screen integrality is good, but exists indivedual visual plant down conductors to be connected the bad substantial equipment defect even disconnecting with main earth mat.

Transformer station's horizontal earth mat general corrosion situation is serious, suggestion full station transformation.Cable duct internal corrosion situation is general.The substation ground network inadequate standard of constructing, steel welding weld bond overlap joint multiple is not enough, it is general that indivedual equipment downlead weld bonds have even snapped phenomenon, has equipment ground downlead series connection phenomenon (especially connecing main earth mat after three-phase lightning protector series connection), and horizontal earth mat reliability is had a direct impact.

Although under current system service condition, the requirement of transformer station's safe operation of station equipment and personnel when flatly software safety can meet station district single phase ground fault fault substantially, but exist noble potential to draw the risk of bringing with high-level touch voltage outward, suggestion:

(1) because horizontal earth mat corrosion condition is serious, the transformation that should arrange in time to carry out large repairs, simultaneously in conjunction with technological transformation overhaul engineering is not enough to steel welding weld bond overlap joint multiple, equipment downlead weld bond snaps, meet the earth mats such as the main earth mat place lack of standardization that constructs after equipment ground downlead series connection (especially three-phase lightning protector series connection) and rectify and improve and reinforcement.The former employing of main grounding grid Φ 16 round steel, carry out thermally-stabilised verification according to long-term short-circuit current value, and when equivalent short circuit duration is got 0.35s and 0.7s, the smallest cross-sectional diameter of thermally-stabilised requirement is respectively 16.73mm and 19.90mm.According to the average INF (0.05mm/a) of grounding net of transformer substation conductor, according to 30 years serviceable life of ground grid design and short-circuit current distant view, increase 1.3 times of considerations, when if equivalent short circuit duration is got 0.7s, grounded screen smallest cross-sectional requires as 20.65mm, therefore Φ 20 round steel are used in suggestion while transforming.

(2) according to relevant rules and counter arranging, carry out the quarantine measures of drawing outer metallic water pipe, communication line and power supply etc., the danger that while avoiding because of place single phase ground fault fault, the outer leader tape of noble potential comes.

(3) along with the quick rising of system loading and capacity of short circuit in the future, should regularly check according to the variation of system short-circuit electric current the thermal capacity of grounding body, and potential rise level and the touch voltage level of earth mat metallic conductor during earth fault, whether meet personnel and equipment safety operation requirement.

(4) in the situation that around transformer station, wrecking completely future because the reasons such as construction and artificial destruction cause extending out earth mat, in transformer station's enclosure wall, main grounding grid impedance ground surpasses design load, personnel and equipment when singlephase earth fault occurs in place outside earth mat conductor noble potential Wai Yindui transformer station cause sizable threat, consider the unfavorable factor such as changes of operating modes or transformer station's dilatation in the future, after suggestion confirms that earth mat all destroyed, can consider to take the rational measure of Technological Economy to realize in transformer station's red line and effectively fall resistance, earth mat conductor stable state power frequency potential rise and place pressure reduction when ground short circuit fault occurs in overall reduction transformer station, eliminate earth mat conductor potential rise and the place pressure reduction potential threat to electrical secondary system and secondary device operation, improve its reliability of operation under ground short circuit malfunction.On the other hand, can consider in the measure of taking to encrypt earth mat grid near the equipment region of enclosure wall, effectively to reduce the level of touch voltage; Also can consider to lay high resistant gravel or resistive formation on the equipment region ground near enclosure wall, or in measures such as equipment frame or device housings spraying resistive formations (to 2m height), eliminate the threat of touch voltage to the interior personnel that stand.

According to transformation Proposals, owner was the second half year in 2011, and transform at the full station that adopts Φ 20 galvanized steel bars to complete grounding net of transformer substation, has improved the general level of the health of earth mat.

Case study on implementation four

1, the simulation of basic condition and grounded screen topological structure

Certain 500kV transformer station has moved 15 years, and having at present 3 capacity is respectively the 500kV main transformer of 750MVA, has 4 times 500kV outlets, and outlet structure has 8 lightning conducters (wherein 4 OPGW optical fiber ground wires); Have 11 times 220kV outlets, outlet structure has 12 lightning conducters (wherein 6 OPGW optical fiber ground wires); There is no 10kV cable outlet.Substation location is leaning against hills, and facing to open coastal river plain, standing is above fish pond and paddy field, and deep soil condition is better.Consider that it is 802134m2 that station extends out the rear whole 500kV of net Shantou grounding net of transformer substation total area; Horizontal grounding band adopts the Φ 22mm galvanizing round steel (the wherein outer 11950m in station) of overall length 24500m, buried degree of depth 0.8m, the galvanizing angle steel of 188 50 * 50 * 5 of vertical grounding electrode employings, long 2.5m, outside station, beaten the deep-well (total depth 1240m) that 31 degree of depth are about 40m, diameter of phi 150, use 60 tons of friction reducers, grounded screen topological structure as shown in figure 16.Grounding impedance design value is 0.50 Ω.The 500kV of transformer station and 220kV bus single-line to ground fault electric current are respectively 23.25kA and 49.42kA.

2, the foundation of site soil layering structural model

By on-the-spot apparent soil resistivity, measure and analog computation, inverting obtains the soil texture model of Figure 17, can be divided into the soil layering structure of four layers of levels, site stratified soil unevenness is larger, middle level is rock, and soil property is poor, and soil resistivity is higher; Transformer station is facing to open coastal river plain, and standing is above fish pond and paddy field, and soil property compares better, and deep soil resistivity decreases, and overall edaphic condition is general.

3, grounded screen computation model accuracy verification

Under running status with 500kV, 220kV lightning conducter and OPGW optical fiber ground wire on outlet structure, measured substation ground network and the voltage pole potential difference (PD) of 30 ° of angle electric current-voltage methods that employing applies the little electric current of 8.0A class power frequency is 1.67V, impedance ground value is 0.2091 Ω, the actual measurement diverting coefficient of 500kV and 220kV overhead ground wire and OPGW optical fiber ground wire is 68.55%, and the Grounding resistance value of rejecting ground wire shunt influence is 0.305 Ω.

Based on grounded screen topological structure model, outlet circuit and shaft tower, arranging, survey the electric current utmost point and the voltage pole of process arranges, and soil layering structural model, while injecting 8.0A class power current, earth mat and voltage pole potential difference (PD) are 1.58V, the total diverting coefficient of ground wire is 64.65%, and grounded screen computation model meets grounded screen various dimensions and evaluates accuracy requirement.

Consider that on-the-spot test has numerous disturbing factors, and analog computation can better reflect the truth of earth mat, get simulation value 0.3213 Ω based on soil horizontal slice structure as the impedance ground value of grounding net of transformer substation.

During 500kV outgoing line side earth fault, 58.84 ° of kA of trouble spot earth current 1.243 ∠, 65.46 ° of kA of neutral point of main transformer earth current 6.697 ∠; During 220kV switch region fault, 66.19 ° of kA of trouble spot earth current 23.376 ∠, neutral point of main transformer earth current 14.613 ∠-61.29 ° kA; 64.42 ° of kA of main transformer region 500kV side single-line to ground fault electric current 7.932 ∠; 27.51 ° of kA of main transformer region 220kV side single-line to ground fault electric current 18.553 ∠.

Get this modal failure situations of transformer station's place single phase ground fault, the potential rise of grounded screen all higher (5800V left and right) when calculating most regions ground short circuit fault occurring, personnel and equipment outside earth mat noble potential Wai Yindui transformer station cause certain threat.During ground short circuit fault, the potential rise level of whole grounded screen is more even, place maximum differential pressure level not high (550V～700V scope), lower when high voltage side of transformer is short-circuited (200V～300V scope), secondary device insulation can be born the potential rise of grounded screen conductor.

6, step voltage distributes and calculates

When singlephase earth fault occurs in overwhelming majority place, enclosure wall earth mat fringe region step voltage level (160V～170V scope) is around far below place step voltage safety limit US(530.66V).

7, touch voltage distributes and calculates

Fault causes network edge grid in distance ground to distribute all in the scope of 150V～340V, lower than safety limit with interior equipment place touch voltage.

8, consider to extend out complete substation ground network safety prediction when destroyed of earth mat

The consideration calculating extend out earth mat complete when destroyed in enclosure wall main grounding grid impedance ground value be 0.5646 Ω, this value surpasses designing requirement (0.50 Ω).

Extend out earth mat completely destroyed after, while there is earth fault, the level that aerial earth wire diverting coefficient (mould) is 83.2%.During 500kV switch region fault, trouble spot earth current 2.068 ∠-30.79 ° kA, 65.46 ° of kA of neutral point of main transformer earth current 6.697 ∠; During 220kV switch region fault, 66.70 ° of kA of trouble spot earth current 20.546 ∠, neutral point of main transformer earth current 14.613 ∠-61.29 ° kA; 47.84 ° of kA of main transformer region 500kV side single-line to ground fault electric current 6.791 ∠; 21.79 ° of kA of main transformer region 220kV side single-line to ground fault electric current 16.312 ∠.

Overwhelming majority region all causes very high earth mat conductor stable state power frequency potential rise (can reach the level of 10kV) while there is earth fault, during fault because personnel and equipment outside grounded screen noble potential Wai Yindui transformer station cause sizable threat, and earth mat conductor potential rise, by surpassing the tolerance of superpotential in short-term of 10kV metal zinc oxide lightning arrester, may cause lightning arrester to be blasted.Whole grounded screen place maximum differential pressure level is up to 1kV left and right, and during single-line to ground fault, earth mat conductor potential rise is little to the threat of secondary device insulation.

During overwhelming majority places fault, enclosure wall earth mat fringe region step voltage is around in higher level (330V～370V scope), but still surpasses place step voltage safety limit US(530.66V).When ground short circuit fault occurs in overwhelming majority substation equipments place, place touch voltage all surpasses or approaches safety limit, and during changes of operating modes or transformer station's dilatation in the future, this situation more seriously, should draw attention.

9, grounded screen various dimensions comprehensive evaluation

While there is earth fault, the potential rise level of whole grounded screen is more even, and earth mat conductor potential difference (PD) does not threaten to secondary device or secondary circuit insulation.The outer step voltage level of on-site is far below its safety limit.

The fault current that enters ground from earth mat while there is earth fault still causes certain earth mat conductor stable state power frequency potential rise, and personnel and equipment between age at failure outside earth mat noble potential Wai Yindui transformer station cause certain threat; Although transformer station does not surpass safety limit near the touch voltage of enclosure wall equipment region, but still in higher level, when changes of operating modes or transformer station's dilatation in the future, this situation is even more serious, should draw attention.

(1) according to relevant rules and counter arranging, carry out the quarantine measures of drawing outer metallic water pipe, communication line and power supply etc., the danger that while avoiding because of place single phase ground fault fault, the outer leader tape of noble potential comes.

(2) suggestion is laid high resistant gravel or resistive formation on the equipment region ground near enclosure wall, also can consider to adopt in measures such as equipment frame or device housings spraying resistive formations (to 2m height), eliminates the threat of touch voltage to the interior personnel that stand.

(3) according to the variation of system short-circuit electric current, regularly check the thermal capacity of grounding body, and potential rise level and the touch voltage level of earth mat metallic conductor during single-phase grounding fault, whether meet personnel and equipment safety operation requirement.

(4) after confirming that earth mat all destroyed, in transformer station's red line, take the rational measure of Technological Economy to realize and effectively fall resistance, when earth mat impedance ground is reduced to place singlephase earth fault occurs earth mat conductor potential rise meet 10kV lightning arrester in short-term superpotential tolerance check and meet near enclosure wall place touch voltage the level of safety limit requirement, the potential rise of earth mat conductor while effectively reducing single-phase grounding fault, to eliminate the security threat to the interior primary equipment of standing, secondary device (or secondary circuit) and personnel.

Claims

1. a large-scale grounding network various dimensions evaluation method, is characterized in that comprising the following steps:

S1 on-the-spot test, it comprises following sub-step:

Apparent soil resistivity under the different spacing of S1-1 site is measured;

The impedance of S1-2 in-site measurement Grounding, overhead ground wire, minute flow vector of cable jacket to measuring current that comprises OPGW optical fiber ground wire, two-terminal-grounding, and by mathematics manipulation, obtain picking out the earth mat impedance ground value of ground wire shunt influence;

S1-3 step voltage and touch voltage test;

S1-4 grounded screen integrality and equipment ground downlead conduction test;

The numerical evaluation of S2 based on electromagnetic field principle;

S3 grounded screen various dimensions are evaluated.

2. large-scale grounding network various dimensions evaluation method according to claim 1, is characterized in that: described step S2 comprises following sub-step:

When S2-6 calculates thunder direct attack transformer station, earth mat and earth's surface transient state current potential and step voltage distribute.

3. large-scale grounding network various dimensions evaluation method according to claim 1, is characterized in that: described step S3 comprises following sub-step:

S3-3 investigates transient state Potential distribution and the potential difference (PD) of lightning rod and outlet structure near zone, the impact near secondary device or electrical secondary system; Near and the impact of transient state step voltage lightning strike spot on the interior personal security of standing;

S3-5 considers the grounded screen conductor heat-stable calibration of earth conductor corrosion factor;