A Study of Grounding Grid Characteristics with Conductive Concrete

Transcription

1 A Study of Grounding Grid Caracteristics wit Conductive Concrete Cun-Yao Lee and Siang-Ren Wang Abstract e purpose of tis paper is to improve electromagnetic caracteristics on grounding grid by applying te conductive concrete. e conductive concrete in tis study is under an extra ig voltage (EHV, 345kV) system located in a ig-tec industrial park or science park. Instead of surrounding soil of grounding grid, te application of conductive concrete can reduce equipment damage and body damage caused by switcing surges. e focus of te two cases on te EHV distribution system in a ig-tec industrial park is presented to analyze four soil styles. By comparing several soil styles, te study results ave sown tat te conductive concrete can effectively reduce te negative damages caused by electromagnetic transient. e adoption of te style of grounding grid located. (m) underground and conductive concrete located from te ground surface to.25 (m) underground can obviously improve te electromagnetic caracteristics so as to advance protective efficiency. Keywords Switcing surges, grounding gird, electromagnetic transient, conductive concrete. I. INRODUCION HE switcing surge in te EHV substation is always a topic for discussion, security in particular. e grounding system as to ensure personnel security and equipment security under ground fault, ligting and switcing surges []. is paper aims to study switcing surges of te two cases based on power delivery and distribution system in a ig-tec industrial park. Due to ig-frequency transient caracteristics of switcing surges, te injection of switcing surge current will result in some electromagnetic transient penomena on grounding system. For example, te value of Eart surface potential, Ground potential rise (GPR), Step voltage and ouc voltage may increase, and te magnitude of conductor wit current and electromagnetic field strengt will be canged. e application of a novel ig conductive concrete [2] for setting and arranging te soil s is focused. e conductive concrete surrounded te grounding grid is applied to reduce te damage of grounding equipment and accident of personnel and effectively improve te reliability and quality of power system in ig-tec industrial park. II. DESCRIPION OF SYSEM SRUCURE A. System Structure in EHV Substation is paper discusses te two cases related to te switcing surge in a typical EHV substation, as sown in Fig.. In tis diagram, a 345kV balanced tree-pase sources connects to a 345kV bus, and te 345kV bus connects four transmission lines, including te first circuit, te second circuit, te tird circuit and te backup circuit, and supplies te four main transformers. Every main transformer makes te voltage drop from 345kV to 6kV and supplies to te load troug te 6kV bus. e system structure also considers te 6kV equivalent sources wic supplies to te secondary side troug 6kV bus [3]. e six injecting points of te switcing surge on grounding grid assumed in tis paper is to observe electromagnetic transient penomena, as sown in able I. According to Fig., tis paper uses transients program of electromagnetic transients program to simulate te model in te EHV substation. e simulated model contains source, bus, cable, breaker and 345kV main transformers. We can use te program to calculate te capital parameter of transmission line and simulate te geometric position of conductor, te conductive resistivity and te number of conductors. is work was supported in part by te Ministry of Economic Affairs of te Republic of Cina, under Grant No. 98-EC-7-A-7-S2-29. C.-Y. Lee is wit te Department of Electrical Engineering, Cung Yuan Cristian University, aoyuan County, aiwan, (Pone: ; cyl@cycu.edu.tw). S.-R. Wang is wit te Department of Electrical Engineering, Cung Yuan Cristian University, aoyuan County, aiwan, 3223 ( g97782@cycu.edu.tw). Fig.. Single-line system in te EHV substation. 73

2 ABLE I GROUNDING POINS OF EQUIPMEN IN HE EHV SUBSAION Grounding point Grounding equipment grounding point neutral point of st main transformer grounding point 2 neutral point of 2nd, 3rd and 4t main transformer grounding point 3 345kV cable sielding conductors of te first circuit grounding point 4 345kV cable sielding conductors of te second circuit grounding point 5 345kV cable sielding conductors of te tird circuit grounding point 6 6kV cable sielding conductors B. Grounding Grid in EHV Substation In tis paper, we assume six injection points on grounding grid in te EHV substation, as sown in Fig. 2. Grounding grid area is 2m 85m and a mes area is 5m 5m. e conductive is composed of copper and sectional area of conductor is mm 2. e grounding grid contains 8 parallel conductors and 25 vertical conductors located under ground m in te bottom of te EHV substation [3]. Generally speaking, te soil composition is complex, and te soil resistivity is sligtly diverse in different environment, suc as moisture, umidity, temperature, etc. e parameter of soil and air caracteristic assumption is sown in able II [2, 4]. In tis paper, te of conductive concrete is composed of cement, aggregate, water and metal fiber. e advantages of te conductive concrete are good conductivity, durability and witout environmental pollution. is paper divides different soil into four styles to analyze te caracteristics of grounding grid. a. e soil style I : All soil is a general soil layer from ground surface to unlimited dept. b. e soil style II : e conductive concrete is applied from ground surface to.5m underground and a general soil layer from.5m underground to unlimited dept. c. e soil style III : A general soil layer from ground surface to.75m underground and from.25m underground to unlimited dept. e conductive concrete is applied from.75m underground to.25m underground. d. e soil style IV : All conductive concrete is applied from ground surface to.25m underground and a general soil layer from.25m underground to unlimited dept. ABLE II PARAMEER SEING OF GROUNDING GRID IN HE EHV SUBSAION Matter Resistivity (-m) Relative permeability Relative permittivity air 8 general soil 2 conductive concrete III. PROBLEM OF SWICHING SURGES AND ANALYSIS MEHOD OF ELECROMAGNEIC RANSIEN PHENOMENA A. Problem Analysis of Switcing Surges e injection of surge current on grounding system can be calculated, and te diagram of equivalent circuit is sown in Fig. 3. S a, S b and S c are tree-pase switces. E a, E b and Ec are te balanced tree-pase sources. Z s is zero-sequence impedance of te transmission line. C g, C 2g and C 3g is te equivalent capacitance corresponding to a, b and c pase respectively. I g is te equivalent current of injecting grounding system, namely, surge current. R g and Rg are equivalent grounding resistance of te source side and injection side of current respectively. e switc-close S a, S b and Sc occurs simultaneously called tree-poles operation and also termed tree-pase syncronous operation. And, te only one-pase switc-close is called single-pole operation. Bot tree-poles operation and single-pole operation can utilize te second-order equivalent circuit to analyze surge current I g. Fig. 3 can be simplified to R L C series second-order equivalent circuit as sown in Fig. 4 and furter inferred te current i []. g Fig. 3. ree-pase equivalent circuit of switcing surge current. Fig. 2. Position of injection points on grounding grid. Fig. 4. R L C series second-order equivalent circuit. 74

3 t ig() t ( VmVo) e sindt CVmsint () L d d = (2) 2 2 = (3) LC = 2 (4) R = (5) 2L : resonant frequency (rad/sec) d natural frequency (rad/sec) : angular frequency of voltage sources (rad/sec) : decay constant e values of R, L, C and amplitude of te voltage source V will be canged under different switcing mode, m and te initial value sould also be considered. B. Analysis of Electromagnetic ransient Penomena e operation of switc-open or switc-close will cause te switcing surge of te voltage and current wic leads to a ig magnetic field strengt, GPR, step voltage and touc voltage. Based on grounding bus on injecting grounding grid in te EHV substation, te following formulas are employed to estimate te values. (). Eart Surface Potential and Magnetic Field Strengt o analyze eart surface potential and electromagnetic field distribution, te analysis metods of electromagnetic teory is applied in tis paper [5]-[7]. Wen te range of grounding system is large, grounding conductor sould be divided to several small parts by using tis metod. Firstly, te current distribution of every part will be analyzed, and ten scalar potential and vector potential A are computed. Finally, bot te electric field strengt E and te magnetic filed strengt H can be estimated as follows: A E = (6) t H = A (7) = xˆ+ yˆ + zˆ (8) x y z : permeability of te media (H/m) e electric field strengt and magnetic field strengt are sum of te entire conductor s effect. Formula (6) and (7) are applied to analyze any waveform. Once obtaining, E and H, we can furter analyze eart surface potential, electric field strengt and magnetic field strengt, tat is, we compute potential distribution of conductor and electromagnetic field distribution corresponding to every point of ground one by one. (2). Conductor wit te Maximum Current and GPR Coose te serious one to analyze te switc mode caused by surge current, and compute switcing surge current I g. e fault decrement factor D f of duration calculation is considered to estimate te current I G. e grounding resistance R g and conductor wit te maximum current I G can be estimated as te following formulas according to IEEE Std. 8-2 [8]: GPR = R I (9) g G R = + + g L () 2A + 2 A I = D I () G f g : soil resistivity (-m) L : total effective lengt of grounding system conductor (m) A : total area enclosed by of grounding grid (m 2 ) : dept of grounding grid conductors (m) GPR : ground potential rise (V) (3). Maximum Step Voltage and Maximum ouc Voltage According to IEEE Std. 8-2 [8], maximum step voltage E can be estimated as te following formulas for grounding s grid of te rectangular uniform mes: E Ki I K K G i s = (2) s L = n (3) n-2 K s = + + ( -.5 ) 2 D+ D (4) 2L n = c L (5) p K : correction factor for grounding grid geometry i K : spacing factor for step voltage s n : geometric factor D : spacing between parallel conductors (m) L : total lengt of grounding system conductor (m) c L : te periperal lengt of grounding grid (m) p According to IEEE Std. 8-2 [8], maximum touc voltage E can be estimated as te following formulas for touc grounding grid of te rectangular uniform mes: E K I K K G i m = (6) touc m L 2 D ( D+ 2 + ) K 8 ii = ln ln 2 6d 8D 4d K ( 2n-) (7) K = + (8) 75

4 K ii = (9) ( ) 2 2n n K : spacing factor for mes voltage m K : corrective weigting factor tat empasizes te effects of grounding grid dept K : corrective weigting factor tat adjusts for te effects of ii inner conductors on te corner mes d : diameter of grounding grid conductor (m) : reference dept of grounding grid ( = m ) IV. CASE SUDY AND RESULS A. Analysis of Switcing Surge Current According to (), te switcing surge current could be estimated; owever, too many factors would lead a quite large error of current value. is paper mainly uses te program model to simulate te magnitude of switcing surge current in tese two cases. Case : e power supplies to load in te EHV substation. In tis situation, te tird circuit of EHV supplies power to te 345kV bus and te switc-close is not syncronization. e application of te program model simulates te switcing surge current caracteristics on grounding system as sown in able III. Case 2: e power supplies to load in te EHV substation. In tis situation, te 345kV bus supplies power to te No. 4 main transformer and te switc-close is not syncronization. e application of te program model simulates te switcing surge current caracteristics on grounding system as sown in able IV. B. Analysis of Electromagnetic ransient caracteristic e program model is applied to simulate te value of surge current in every grounding bus and te maximum surge current in different conditions of switc mode is obtained. Moreover, te switcing surge current of injecting grounding system is analyzed and various electromagnetic caracteristics on grounding grid including eart surface potential, touc voltage, step voltage, conductor wit te current, magnetic field strengt and GPR are observed. All te analyses consider ABLE III SURGE CURREN CHARACERISICS ON GROUNDING SYSEM FOR CASE Injection point injection point 3 (29-j4.7) injection point 4 (29-j4.7) injection point 5 (22-j5.5) injection point 6 (-5-j2.) Maximum value of current (A) Degrees ( ) Frequency (Hz) ABLE IV SURGE CURREN CHARACERISICS ON GROUNDING SYSEM FOR CASE 2 Injection point Injection point (.8-j6.87) Injection point 2 (36.4-j95.7) Injection point 3 (-7.3+j72) Injection point 4 (-7.3+j72) Injection point 5 (-7.3+j72) Injection point 6 (.9-j7.8) Maximum value of current (A) Degrees ( ) Frequency (Hz) different styles of soil layers. e EHV substation in te vicinity of eart surface potential and magnetic field distributions can be estimated as sown in (6) and (7). e magnitude of switcing surge current on injecting grounding system in te EHV substation is simulated according to te program model. By employing (), te fault decrement factor D f of duration calculation sould be considered to estimate conductor wit te maximum current I G. Based on te grounding grid structure and te parameters (area, dept, total lengt of conductors and soil resistivity) in te EHV substation, te equivalent grounding resistance of grounding grid R g is computed according to (), and ten te magnitude of GPR is computed according to (9). Similar to te description mentioned above, based on grounding grid structure and te parameter, (3) to (5) and (7) to (9) are employed to compute every factor value on grounding grid structure, and te reference of its grounding grid dept is m. From (2), (6) and conductor wit te maximum current of estimation I G, te magnitude of step voltage and touc voltage are estimated. e analyzed result of electromagnetic transient penomena in different styles of te soil for case is presented in able V and for case 2 is presented in able VI. ABLE V ELECROMAGNEIC RANSIEN PHENOMENA ON GROUNDING SYSEM FOR CASE Electromagnetic transient penomena style I style II style III style IV ground surface potential (V) touc voltage (V) step voltage (V) conductor wit te current (A) magnetic field strengt (A/m) GPR (V)

5 ABLE VI ELECROMAGNEIC RANSIEN PHENOMENA ON GROUNDING SYSEM FOR CASE 2 Electromagnetic transient penomena style I style II style III style IV ground surface potential (V) touc voltage (V) step voltage (V) conductor wit te current (A) magnetic field strengt (A/m) GPR (V) V. DAA ANALYSIS AND DISCUSSION e two cases demonstrate tat te grounding grid is located.(m) underground. e switcing surge current of ig frequency transient on injecting grounding grid affect te values of eart surface potential, touc voltage, step voltage, magnetic field strengt, GPR and conductor wit te current. e soil style I can be seen as a general soil layer, tat is, te soil style I is a reference compared to every soil style in tis paper. able V and able VI indicate tat wen te soil is style I, te magnitude of various electromagnetic transient caracteristics is te largest value. Wen te conductive concrete is applied around te grounding grid, it can effectively improve te magnitude of eart surface potential, touc voltage and step voltage. able V and able VI also reveal tat te soil style II decreases te degree of value more tan te soil style III in eart surface potential, GPR and conductor wit te current. Since te conductive concrete is firstly applied under ground surface in te soil style II and its soil resistivity is low and conductivity is great, te switcing surge current of injecting grounding grid spreads to eart quickly. e magnitude of electromagnetic transient caracteristics is lower tan te soil style III. Due to te conductive concrete is located in te grounding grid.25m above and.25m below tat makes current spreading rate lower tan conductive concrete located from ground surface to.5m underground, te fact tat te style III of voltage difference is closer style II, and te magnitude of touc voltage and step voltage is smaller tan te soil style II. e soil style III improves effectively tan te soil style II in touc voltage and step voltage parts. e soil style IV improves obviously for grounding system. e magnitude of eart surface potential reduces 2.5%, te touc voltage reduces 54.62% and step voltage reduces 36.3%. It is noted tat te soil style II and style III make magnitude of magnetic filed strengt a little iger tan te reference value (e soil style I). Particularly, te percentage of soil style II in case 2 is iger about 6.62% tan reference value. e decreased percentage of te four soil styles for analyzing te results of te two cases is sown in able VII. erefore, we ABLE VII HE DECREASED PERCENAGE FOR ELECROMAGNEIC RANSIEN PHENOMENA IN DIFFEREN SOIL MAERIAL SYLE Electromagnetic transient penomena ground surface potential ype of cases style II ave to evaluate te magnitude of magnetic field strengt and to confirm if it is consistent wit security standards before setting. VI. CONCLUSION Grounding grid in te EHV substation is an important facility for bot te security of personnel and operating reliability in power system. e magnitude of grounding resistance is te main tecnical parameter for grounding grid. erefore, tis paper indicated te two cases of te EHV substation in a ig-tec industry presented to analyze switcing surges make some electromagnetic transient penomena on grounding system. e results ave sown tat te conductive concrete applied around grounding grid not only makes te value of grounding resistance decrease in te wole grounding system but also improves te magnitude of electromagnetic transient penomenon effectively. It is difficult for grounding resistance to meet te security standards wen we set a iger soil resistivity in te EHV substation. Bot te decrease values of touc resistance between grounding grid and te soil, and resistance of te current spreading to eart could acieve tis purpose. us, te conductive concrete applied around grounding grid is workable. e advantages of tis metod are small and stable value of grounding resistance, long lifespan and decreased maintenance cost, wic can be promoted to te practical application of grounding engineering in te area of ig soil resistivity. For a better adoption, te cooperation wit te arranging and programming of grounding grid will meet te standards of grounding resistance and decrease cost effectively. ACKNOWLEDGMEN style III style IV Case (%) Case touc voltage (%) Case Case step voltage (%) Case Case magnetic field strengt Case (%) Case GPR (%) Case Case conductor wit te Case current (%) Case e researc was supported by te Ministry of Economic Affairs of te Republic of Cina, under Grant No. 98-EC-7-A-7-S2-29. And te autors are greatly indebted to National aipei University of ecnology for supporting te application software. 77

6 REFERENCES [] W. Xiong, F. P. Dawalibi, ransient Performance of Substation Grounding Systems Subjected to Ligtning and Similar Surge Currents, IEEE rans. on Power Delivery, Vol. 9, No. 3, July 994. [2] Serif Yeia, Cristoper Y. uan, David Ferdon, and Bing Cen, Conductive Concrete Overlay for Bridge Deck Deicing: Mixture Proportioning, Optimization, and Properties ACI Materials Journal, No.97-M23, PP.72-8, Mar. 2. [3] Y.. Can, te literature of transmission system planning in aiwan, 26. [4] F. Dawalibi, D. Mukedkar, Parametric Analysis of Grounding Systems, IEEE rans. on Power Apparatus and Systems, Vol. PAS-98, No. 5, pp , Sept./Oct [5] F. Dawalibi, Electromagnetic Fields Generated by Overead and Buried Conductors. Part - Single Conductor, IEEE rans. on Power Delivery, Vol PWRD-, No.4, PP.5-, Oct [6] F. Dawalibi, Electromagnetic Fields Generated by Overead and Buried Conductors. Part 2 - Ground Networks, IEEE rans. on Power Delivery, Vol PWRD-, No.4, PP.2-9, Oct [7] M. Heimbac, L. D. Grcev, Grounding System Analysis in ransients Programs Applying Electromagnetic Field Approac, IEEE rans. on Power Delivery, Vol. 2, No., pp , Janu [8] ANSI/IEEE Std.8-2, IEEE Guide for Safety in AC Substation Grounding, by IEEE Society, New York 2. Cun-Yao Lee (S 5 M 8) received is P. D. in electrical engineering from aiwan University of Science and ecnology in 27. During 2-27, e was a distribution system designer in te engineering division, aipei Government. In August 27, e joined Cung Yuan Cristian University as a faculty member. He is presently an Assistant Professor. His major areas of researc include power distribution and power filter design. Siang-Ren Wang was born in aiwan in 985. He received is B.S. degree in electrical engineering from Cung Yuan Cristian University in 28. He is presently a graduate student toward is M.S. program in electrical engineering department of Cung Yuan Cristian University in aiwan. 78