Three Dimensional Grounding Grid Design

Transcription

1 Three Dimenional Grounding Grid Deign Fikri Bari Uzunlar 1, Özcan Kalenderli 2 1 Schneider Elecric Turkey, Ianbul, Turkey bari.uzunlar@r.chneider-elecric.com 2 Ianbul Technical Univeriy, Elecrical-Elecronic Faculy, Ianbul, Turkey ozcan@elk.iu.edu.r Abrac Thi udy preen an advanced mehodology and a compuer model for analyi of grounding yem conforming o andard IEEE Sd , IEEE Sd and IEEE Sd The mehodology and compuer program i validaed wih acual yem meauremen. The accuracy of compuer algorihm i dependen on how well he oil model and phyical layou reflec acual field condiion. The olerable volage limi and he maximum prediced volage value are calculaed uing empirical formula in given andard. The ep, ouch and meh volage and ho zone are calculaed according o he recommendaion in given andard and he difference beween hem are inveigaed and clarified. The imulaion are carried ou wih he aim o verify he poibiliy of inroducing ome pracical deign crieria helpful for engineering applicaion. Subaion grounding grid deign and analyi module i pecially deigned o help engineer opimize he deign of new grid and reinforce exiing grid, of any hape, by virue of eay o ue, buil-in danger poin evaluaion faciliie. 1. Inroducion Subaion grounding yem deign udie are increaing a available faul curren increae on oday elecrical power grid. The economic i an imporan facor a well a human afey. Engineer wan o deign yem ha proec human and equipmen while providing an opimized economic oluion wihou over deigning grounding yem. Tha i why he ue of more accurae compuer algorihm in deigning he grounding yem i neceary for ome of he cae aed a below: - Parameer exceed he limiaion of he equaion, - Due o ignifican variaion in oil reiiviy, mulilayer or wo-layer oil model i preferred, - By uing he approximae mehod, uneven grid conducor or ground rod pacing are unable o be analyzed, - To deermine local danger poin wih a flexible mehod, - For complex grid in which conducor and buried meallic rucure are no conneced o he grounding yem. 2. Purpoe The main objecive of hi udy are o: - Provide a uiable reference conaining he neceary guideline o ha a grounding yem deigner can focu quickly on he mo efficien deign, - Reain he ep and ouch volage wihin he afey olerance limi and o keep ground reiance mall, - Deign a ubaion grounding yem baed on IEEE Sd [1], IEEE Sd [2], IEEE Sd [3], andard in general, - Aure ha a peron in he viciniy of grounded faciliie i no expoed o he danger of criical elecric hock. - Deermine he required grid conducor pecificaion, - Compue he ubaion grounding reiance and udy he effec of reiance formula, ubaion area, number of ground rod, oil homogeneiy, grid conducor ize and grid burial deph on he calculaed value of ubaion reiance, - Compue he ground poenial rie, GPR, - Compue he olerable and maximum ouch and ep volage, - Diplay he urface poenial profile a a percenage of GPR along any redeermined direcion, - Inroduce a final repor conain all deign deail. 3. Grounding Grid Deign Sofware Compuer algorihm for modeling ground yem are baed on: - Individual componen modeling like grid conducor and ground rod, - Decripion of individual componen wih a e of equaion, - Ground-faul curren calculaion flowing ino he earh, - Surface poenial calculaion a any deired urface poin. Firly, definiion of a projec and udy hould be done in performing a grounding udy. Secondly, he oil model ha will be ued for he ubequen analye hould be deermined. In hi ep, he afey aemen calculaion have alo been performed including he maximum permiible ep and ouch volage for paricular urface and expoure condiion a defined in IEEE Sd [1]. The hird ep i he elecrode izing deerminaion (conducor and rod) which i aking ino accoun he wor cae faul parameer in he ubaion. The nex ep i enering he geomerical configuraion of he aion layou uch a coordinae, burial deph and phyical dimenion. Finally, i ha o be aured wheher he deign for he aion mee he neceary afey crieria or no. Poenial profile plo hould be generaed o acerain ha ouch and ep poenial are no exceeded. The grid deign may need o be modified and repeaed if any of he afey crieria i no me. The procedure ar from he hird ep unil accepable reul are obained. I-139

2 The ofware ued in hi udy i compoed of hree main module Soil Analyi Module Thi module include below parameer neceary for grounding deign of ie which i being imulaed (Fig. 1). 1) Nework volage in vol, 2) Earh faul curren in ampere, 3) Lengh in meer and quaniy of grounding rod and conducor, 4) Ground poenial rie (GPR) in vol, 5) Calculaed ground reiance in ohm, 6) Equivalen impedance in ohm Three Dimenional Poenial Diribuion Module In hi module, hree dimenional poenial diribuion imulaed by finie elemen mehod can be oberved (Fig. 3). Fig. 1. Soil analyi module 1) Soil model (ingle or muli layered), 2) Upper layer hickne in meer, 3) Upper layer reiiviy in ohm-meer, 4) Lower layer hickne in meer, 5) Lower layer reiiviy in ohm-meer, 6) Faul duraion in econd, 7) Body weigh in kilogram Grounding Grid Analyi Module Elecrical characeriic of ie and properie of grid conducor ued exi in hi module (Fig. 2). Daa below are enered from hi module. Fig. 3. Three-dimenional poenial diribuion module 4. Grounding Grid Deign Mehodology Many graphical and analyical approache have been uggeed over he year in order o arrive a pracical oil model [4-8, 10]. Since i i more common o find raified oil which are compoed of layer having differen reiiviie, echnique o inerpre a e of oil reiiviy meauremen a a muli-layer oil model are currenly ued. For many year in ubaion grounding pracice, he wolayer model ha been followed a a pracical approach which ha an upper layer of a definie deph and a lower layer of an infinie deph wih a differen reiiviy. The ofware [9] uppor Wenner four-pin oil meauremen echnique in which he diance (a) beween each pair of probe i equal a given in Fig. 4 below: Fig. 4. Wenner four-pin mehod. Fig. 2. Grounding grid analyi module The volage (V) i meaured by he volmeer when a curren (I) i injeced and meaured by he ammeer. The meaured or apparen reiiviy (ρ) i given by: I-140

3 4 πav ( / I) ρ= 2a a a b a b or ρ = 2πa (V/I) if a >> b where b i he lengh of he probe. The maximum permiible ouch and ep volage under pecific urface and expoure condiion are eimaed. The afey aemen calculaion comply wih andard IEEE Guide for Safey in AC Subaion Grounding, 2000 ediion [1]. The purpoe of he calculaion i o arrive a a deraing facor ha will permi o ake advanage of he high reiiviy urface layer, hu permiing a higher ouch volage o be oleraed. The deraing facor C can eiher be calculaed a: 0.09 (1 ρ/ ρ) C = 1 2h (1) (2) Table 1. Daa ued in grid deign Properie Inpu Daa Body weigh 70 kg Cruhed rock urface layer reiiviy 2500.m Cruhed rock urface layer hickne m Clearing ime 0.50 ec Uniform oil reiiviy 400.m Max I G faul curren, X/R 6814 A, 16.2 I G faul curren 3180 A Spli facor S f 0.6 Conducor maerial Copper, hard-drawn Ambien emperaure 40 C Grid conducor diameer 0.01 m Burial deph 0.5 m For a 50 kg body weigh: E ouch50 = ( C ρ ) 0.116/ E ep50 = ( C ρ ) 0.116/ (3) (4) For a 70 kg body weigh: E ouch70 = ( C ρ ) 0.157/ E ep70 = ( C ρ ) 0.157/ (5) (6) where: i hock duraion in econd. ρ i he reiiviy of he urface maerial in ohm-meer. C i he deraing facor when high reiiviy urface maerial i preen. The reducion facor C i a funcion of he reflecion facor K and he hickne of he upper layer h. h i he hickne of he high reiiviy urface layer maerial in meer. ρ i he reiiviy of he urface maerial in ohm-meer. ρ i he reiiviy of he earh below he high reiiviy urface maerial in ohm-meer. 5. Simulaion The cae expoed in he Annex B and Annex E of he IEEE Sd are ued a a reference poin [1], for which here are following comparaive able and he correponding graph in order o validae he reul in hi documen. Deign daa i given in Table 1. Fig D grid layou for quare grid wih ground rod Table 2. Comparaive reul for quare grid wih ground rod IEEE Sd Sofware Touch Volage (V) Sep Volage (V) Reducion Facor, C Rg (Ω) GPR (V) The maximum limi for ouch volage i violaed on he corner of he grounding yem which can be eaily oberved a Fig Square Grid wih Ground Rod Square grid 70 m 70 m, 100 mehe wih ground rod placed along he perimeer i een below in Fig. 5. IEEE Sd mehod alo give reul for maximum allowable ouch and ep volage, a well a he maximum real volage in he yem, for comparion of he calculaion done by ofware. The obained reul uing boh echnique are hown in Table 2. I-141

4 Fig D grid layou for quare grid wih ground rod 5.2. L-Shaped Grid wih Ground Rod L-Shaped grid 70 m 105 m, 100 mehe wih ground rod placed along he perimeer i een below in Fig. 7. For comparion of he calculaion done by boh echnique, he obained reul are hown in Table 3. Fig D grid layou for L-haped grid wih ground rod 5.3. Unequally Spaced Grid wih Ground Rod Unequally paced grid m m, 64 mehe wih ground rod placed along he perimeer i een below in Fig. 9. For comparion of he calculaion done by boh echnique, he obained reul are hown in Table 4. Fig D grid layou for L-haped grid wih ground rod Table 3. Comparaive reul for L-haped grid wih ground rod IEEE Sd Sofware Touch Volage (V) Sep Volage (V) Reducion Facor, C Rg (Ω) GPR (V) The maximum limi for ouch volage i violaed on he ouide of he L-haped grounding yem which can be eaily oberved a Fig. 8. Fig D grid layou for unequally paced grid wih ground rod Table 4. Comparaive reul for unequally paced grid wih ground rod IEEE Sd Sofware Touch Volage (V) Sep Volage (V) Reducion Facor, C Rg (Ω) GPR (V) A eaily een alo from hi imulaion, if we keep ground reiance low, he dicrepancie beween reul are nearly negligible. The maximum limi for ouch volage of he unequally paced grounding yem can be oberved a Fig. 10. I-142

5 Fig D grid layou for unequally paced grid wih ground rod 6. Concluion Alhough analyi echnique baed on compuer imulaion are expenive ye, i i clear ha he uage of hem caue cheap and reliable earhing grid regulaion. I i neceary o regenerae parameric analyi o ha include muli layer oil condiion. Earhing grid regulaing andard need harmonizaion beween hem. If he reul of hi udy are ummarized a below, i how ha: 1. The afey level for equipmen and people can be enhanced uing unequally paced grounding grid by decreaing earh urface poenial gradien and making conducor leakage curren diribuion more uniform, 2. A non-uniform conducor pacing, having more conducor a he edge of he grid, provide he mo efficien deign, 3. An unequally paced grounding grid i an economical and reaonable echnique a hown in imulaion, 4. The deign of recangular grid buried in uniform oil wihou ground rod could be adjued and applied o any pracical ubaion grounding grid deign. Tracking he baic rule uggeed by IEEE Sd , IEEE Sd and many compuaional algorihm o obain afe grounding yem, an exper yem mehodology for analyi and deign of he ubaion grounding yem i given. Sep by ep deign echnique wih neceary explanaion i ued o repreen he required informaion. The developed exper yem can be very ueful ool in analyi and deign of he ubaion grounding yem. When deigning earhing grid yem for large elecrical ubaion, calculaion performed in hi udy how ha IEEE Sd i he mo conervaive ince i predic he lowe level of olerable volage and alo he body curren formula provide adequae margin of afey for modern ranmiion and diribuion yem. The afey crierion i compared and heir difference are hown o prove ha IEEE Sd provide he mo ueful procedure for grounding yem afey aemen. The mo criical ouch volage are found in he corner of grounding grid cae a hown in imulaion. To achieve more accurae reul, compuer aided deign program are needed for complex algorihm of calculaion requiring he ue of compuaional ool. Since i offer he poibiliy of making a cloe o he realiy deailed analyi, he finie elemen mehod i he be way o repreen he grounding yem udy nowaday. More accurae compuer reul are compared wih everal equaion for E and E m from cae wih variou meh ize, grid hape, lengh and number of ground rod and hey are found o be beer han he equaion. The cae include quare, recangular, L-haped, equally and unequally paced grid. They are run wihou and wih ground rod. The oal ground rod lengh i varied wih differen number of ground rod lengh and locaion. Mo pracical example of grid deign are conidered. The comparion how ha i i poible o rack he compuer reul wih accepable accuracy. 7. Reference [1] IEEE Sd , IEEE Guide for Safey in AC Subaion Grounding, New York, USA, [2] IEEE Sd , IEEE Guide for Meauring Earh Reiiviy, Ground Impedance and Earh Surface Poenial of a Ground Syem, New York, USA, [3] IEEE Sd. 837, IEEE Sandard for Qualifying Permanen Connecion Ued in Subaion Grounding, New York, USA, [4] C. H. Lee, A. P. S. Meliopoulo, "Comparion of ouch and ep volage beween IEEE Sd. 80 and IEC 479-1", IEE Proc. on Generaion, Tranmiion and Diribuion, vol. 146, no. 6, pp , [5] H. Zhao, H. Griffih, A. Haddad, A. Ainley, "Safey-limi curve for earhing yem deign: appraial of andard recommendaion", IEE Proc. on Generaion, Tranmiion and Diribuion, vol. 152, no. 6, pp , [6] M. H. Hocaoglu, A. T. Hocaoglu, "Comparion of earhing andard for high-volage yem", 8h Naional Congre on Elecrical, Elecronic and Compuer Engineering, Gazianep, Turkey, pp , [7] L. M. Coa, "Comparaive udy beween IEEE Sd and finie elemen mehod, applicaion for grounding yem analyi", Tranmiion & Diribuion Conference and Expoiion, Lain America, pp. 1-5, [8] J. Ma, F. P. Dawalibi, R. D. Souhey, "Effec of he change in IEEE Sd. 80 on he deign and analyi of power yem grounding", PowerCon In. Conf., vol. 2, pp , [9] CYMGRD, Uer Guide and Reference Manual, Canada, [10] S. Meliopoulo, Power Syem Grounding and Tranien, Marcel Dekker, New York, I-143