Case Study of Ground Potential Rise on Two Neighboring Substations

Transcription

1 Cae Study of Ground Potential Rie on Two Neighboring Subtation W. Pobporn, D. Rerkpreedapong, and A. Phayoho Abtract Thi paper preent the effect of contruction of a new peranent ubtation while the exiting ubtation ha not yet been reoved. the Iolation of ground grid of the two ubtation create ground potential rie () to be teep between the ground grid of two neighboring ubtation. odeling and iulation are perfored on the Current Ditribution lectroagnetic interference Grounding and Soil tructure (CDGS) progra. It i found that the percentage of ratio between the auxiliary grounding yte and the ain ground grid in unifor or hoogenou oil i contant while the percentage of ratio are different in the two layer oil. If the top layer oil reitivity i higher than the fixed botto layer oil reitivity, the percentage of ratio will decreae. However if the botto layer oil reitivity i higher than the fixed top layer oil reitivity, the percentage of ratio will increae. Thi iplie that only a riky cae can be conidered in ubtation deign, although the condition of oil i varied by eaon. oreover, the cae tudie are analyzed by varying the thickne of top layer and ditance between the ain ground grid yte and auxiliary grounding yte, which affect the percentage of ratio. The ore ditance between ain ground grid yte and auxiliary grounding yte i, the le the percentage of ratio i, a return of the auxiliary grounding yte i lower. Thi will ake the touch voltage higher due to the teepne of, which increae the rik of hazard. eyword Ground potential rie, Safety criteria, Step voltage, Touch voltage.. INTRODUCTION Thi paper preent a contruction procedure for a new peranent ubtation while the exiting ubtation ha not yet been reoved. While the ground grid of the two ditribution ubtation are iolated, the effect of the auxiliary grounding yte of the de-energized electrical power ite will exit. Thi create ground potential rie () to be teep between the ground grid of two neighboring ubtation. It i a concern for afety iue becaue a hort circuit can generate a large current that flow through the aboveground tructure and grounding yte and diipate in the oil, which the high potential ay caue a hazard to peronnel working nearby or in the area of ditribution ubtation. The ground grid deign for ditribution ubtation of the Provincial lectricity thority of Thailand (PA) i exained with the ain objective to ae grounding grid yte condition in ter of ground potential rie, axiu touch voltage and tep voltage. Thee three W. Pobporn (correponding author) i with the Departent of lectrical ngineering, Faculty of ngineering, aetart Univerity and with ngineering & aintenance Diviion, Provincial lectricity thority (PA), Area Central, Aia Rd., Hantra, Phranakorn Si Ayutthaya 3000, Thailand. Phone: ext. 343; Fax: ; -ail: watcharapob@hotail.co, watchara.pob@pea.co.th. D. Rerkpreedapong i with the Departent of lectrical ngineering, Faculty of ngineering, aetart Univerity, 50 Nga Wong Wan Rd., adyao, Chatuchak, angkok, 0900, Thailand. Phone: ext.543; -ail: dulpichet.r@ku.ac.th. A. Phayoho i with Power Syte Planning Departent, etropolitan lectricity thority (A), 9 Raa IV Rd., long Toey, angkok, 00, Thailand. Phone: ; Fax: ; -ail: att_powerea@hotail.co. value are analyzed to enure that they coply with the afety criteria defined in the I Std odeling and iulation are carried out on the Current Ditribution lectroagnetic interference Grounding and Soil tructure (CDGS) progra. The reult are found that ground grid iolation hould not be allowed during the tie of contruction becaue the auxiliary grounding yte of the de-energized ubtation can create teep ground potential rie and therefore the large voltage difference can har peronnel working nearby and caue a daage to equipent in the vicinity of fault, particularly when the ground grid of the two neighboring ubtation are not connected.. DIFFINITION OF TORA VOTAG In the proce of deigning the ground grid yte, afety criteria i firtly calculated to pecify a tolerable level, then the axiu touch and tep voltage are calculated to copare with the afety criteria to define whether it i afe to work on the area of ubtation. Thi part will how a calculation of afety criteria, touch and tep voltage. Touch Voltage Criteria The potential difference between the ground potential rie () and the urface potential at the point a peron i tanding while having a hand in contact with a grounded tructure. The tolerable touch voltage in volt i defined in (). touch ( R +.5 ) = I () 45

2 touch = tolerable touch voltage for huan (V) R = reitance of the huan body (Ω) = urface layer reitivity ( Ω ) The current through the body i deterined by () I = I = current through the body (A) k = 0.6 for 50 kg body weight 0.57 for 70 kg body weight t = duration of current expoe () The afety of a peron depend on preventing the critical aount of hock energy fro being aborbed before the fault i cleared and the yte i de-energized. To enure afety, the agnitude and duration of the current conducted through a huan body hould be le than the value that can caue ventricular fibrillation of the heart. Fibrillation current i aued to be a function of individual body weight. The tolerable body current liit for body weight of 50kg and 70kg can be found in [],[]. Step Voltage Criteria k t () The difference in urface potential experienced by a peron bridging a ditance of with the feet without contacting any other grounded object. The tolerable tep voltage in volt i defined in (3) [] tep ( R + 6 ) = I (3) tep = tolerable tep voltage for huan (V) 3. AXIU OF SH AND STP VOTAG The axiu touch voltage within a eh of a ground grid [] i calculated by (4) i I G a = (4) = eh voltage (V) a = apparent reitivity of oil (Ω-) = eh factor defined for n parallel conductor i = corrective factor for current irregularity I G = axiu r current flowing between ground grid and earth (A) = effective length of C + R for eh voltage () For grid with or without ground rod, the effective buried conductor length,, can be deterined by (5) = 0.75 C R (5) = effective length of C R C R voltage () + for tep = total length of grid conductor () = total length of ground rod () Then, the tep voltage i deterined fro (6) i I G a = (6) = tep voltage (V) = eh factor defined for n parallel conductor 4. NARY DISTRIUTION SUSTATION For, a new ditribution ubtation grounding grid cloe to the exiting ubtation whoe ground grid i depicted a a eh of rebar conductor, afety conideration require that the new and exiting ditribution ubtation grid are interconnected and thu the de-energized electrical power ite of ground grid act a an auxiliary grounding yte of the ubtation. However, if the effect of the exiting i taken into account for a grounding deign o a to reduce the perforance requireent of the ubtation grounding yte, the copper conductor ut be connected in a reliable anner to the ubtation grid [3]. 5. CAS STUDY In thi paper, cae tudie ue the cro ection of the ground grid conductor with ize of 95, and the ground rod i 3.0 long with in diaeter. All the grid conductor are buried 0.5 deep in the top layer oil. The figure of an intallation of ground rod will be pread out. The dienion of ground grid which preent the tatu of return will be categorized into 45 x 45. The ain one i of ediu ize 45 x45. Furtherore, the value of oil reitivity i choen to be, 50, 00 and,000 Ω for both top and botto layer of oil. In cae tudie, the top and botto layer ha difference reitivity due to a nuber of factor uch a oiture content of the oil, cheical copoition, concentration of alt diolved in the contained water, and grain ize [4]. Thu, the hort circuit current of 5 ka i pecified. Thi tudy i eparated into 3 cae a follow: Cae The ditance between ain ground grid and auxiliary grounding yte i 5 a hown in Fig.. The thickne of the top layer oil i. Cae Configuration i hown in Fig.. The thickne of the top layer oil i 4. Ditance between ground grid i the ae a cae. Cae 3 The ditance between ain ground grid and 46

3 auxiliary grounding yte i 5 a hown in Fig.. The thickne of the top layer oil i. contour area. etween ubtation, the touch voltage in Fig. 4 around the junction or the edge i very high. The tep voltage in Fig. 5 i alo high at the edge but lower than the touch voltage. ain 45x45 Fig.. Ground Grid Configuration for Cae and. ain 45x45 xiliary 45x45 Fig.. Ground Grid Configuration for Cae 3. The cae are of interet a follow: xiliary 45x45 Cae :, touch voltage and tep voltage fro the tudy of grounding yte intallation in variou oil reitivity, the reult are hown in Table. % ratio between auxiliary grounding yte and ain ground grid i deterined a percentage diplayed in Table. For detailed conideration, it can be divided into cae. 5.. Top layer reitivity ( ) i higher than the botto layer reitivity ( ) of ain ground grid, axiu touch voltage and tep voltage will be increaed when or increae. % ratio between auxiliary grounding yte and ain ground grid i found lower than the unifor oil cae. Therefore, the afety i alo wore than the unifor oil. 5.. Top layer reitivity (() i lower than the botto layer reitivity () of ain ground grid, axiu touch voltage and tep voltage will be increaed when or increae. % ratio between auxiliary grounding yte and ain ground grid i found higher than unifor oil cae. Therefore the afety i alo higher than unifor oil. For exaple, 3-dienion of ground grid deign in cae i hown in Fig. 3. Fig. 4 and 5 are the graph of touch and tep voltage, which illutrate the - dienion view to help deterine the afe and unafe Table., Touch Voltage and Step Voltage for Cae of voltage Touch Step Voltage level (V) 50 00, ,6.9 6,049.6, ,454 0,85 3, ,99,907 5,950, ,645 9, , ,8. 4,98.6, ,68.4 8,887 83, ,85.3 9,56.7 9,64, , ,64.6 9, , ,77.5 6,899.4, , ,555 34,830, ,80.4 4,80 85, , ,409.4,30.5 7, ,68.3,88.7,470, ,69.8 4,70.5 8,90 reitivity of top layer oil reitivity of botto layer oil ain ground grid yte auxiliary grounding yte Table. Ratio between xiliary and ain Ground Grid Configuration for Cae (%) 50 00, , Fig. 3. Ground Potential Rie on 000/50 Ω for Cae. 47

4 Fig. 4. Touch Voltage on 000/50 Fig. 5. Step Voltage on 000/50 Ω for Cae. Ω for Cae. Cae : The reult are hown in Table 3 and Table 4. For detailed conideration, it can be divided into cae. 5.. Top layer reitivity ( ) i higher than the botto layer reitivity ( ) The of the ain ground grid, axiu touch voltage and tep voltage have the ae trend a in cae, but thee 3 voltage in cae are higher than thoe in cae. Thi i becaue the ground rod of cae are till in the top layer oil with higher oil reitivity. Conideration of % ratio between auxiliary grounding yte and ain ground grid at the ae oil reitivity found that % ratio in cae i lower than in that in cae. Thi ean that afety of cae i wore than cae becaue the axiu touch voltage i higher. 5.. Top layer reitivity () i lower than the botto layer reitivity () The of the ain ground grid, axiu touch voltage and tep voltage have the ae trend a in cae but thee 3 voltage in cae are lower than in the cae. Thi i becaue the ground rod of cae are till in the top layer oil with lower oil reitivity. Conideration of % ratio between the auxiliary grounding yte and ain ground grid at the ae oil reitivity found that % ratio of cae i higher than that in cae. Thi ean that afety of cae i lower than in cae fro the reaon that the tolerable touch voltage i lower. For exaple, 3-dienion of ground grid deign in cae i hown in Fig. 6. Fig. 7 and 8 are the graph of the touch and tep voltage, which illutrate the - dienion view to help deterine the afe and unafe contour area. Table 3., Touch Voltage and Step Voltage for Cae Touch Step Voltage level (V) 50 00, ,., , ,53,454 7,84 7, ,837 5,3,907 98, ,333 57,9 68,33 9, ,756., , ,68.4 8, , , , , , , , ,9 4,77 5,59 9, ,77 6,683 8,555 7, ,055 40,50 43,830 85, , ,409,8 3,55 00,7,99,89 5, ,56 3,50 4,490 8,90 Table 4. Ratio between xiliary and ain Ground Grid Configuration for Cae of voltage (%) 50 00, , Fig. 6 Ground Potential Rie on 000/50 Fig. 7. Touch Voltage on 000/50 Ω for Cae. Ω for Cae. 48

5 Table 6. Ratio between xiliary and ain Ground Grid Configuration for Cae 3 Fig. 8. Step Voltage on 000/50 Ω for Cae. Cae 3: The reult are hown in Table 3 and 4. For detailed conideration, it can be divided into cae. (%) Top layer reitivity ( ) i higher than the botto layer reitivity ( ) The of ain ground grid, axiu touch voltage and tep voltage have the ae trend a in cae but thee 3 voltage in cae 3 are higher than thoe in cae. Thi i becaue the difference between of the ain ground grid and auxiliary grounding yte i ignificantly higher than that in cae. % ratio between the auxiliary grounding yte and ain ground grid for the ae oil reitivity in cae 3 i found lower than that in cae. Thi ean that afety of cae 3 i lower than cae becaue the axiu touch voltage i higher than that in cae. Fig.9. Ground Potential Rie on 000/50 Ω for Cae Top layer reitivity ( ) i lower than the upper ayer reitivity ( ) The of the ain ground grid, axiu touch voltage, tep voltage and % ratio between the auxiliary grounding yte and ain ground grid at different oil reitivity have the ae trend a in 3.. Table 5., Touch Voltage and Step Voltage for Cae 3 Touch Step Voltage level (V) 50 00, , , , ,87,30 34, ,99 3,744 65,860, ,885 30,8 37, , , , ,87,30 34, ,99 3,744 65,860, ,885 30,8 37, ,374.3,58, ,740.,453 50, ,747. 3,480 7,380, ,660 9,70 34, , ,9.6, , ,430.79, ,60.98, , , ,59 Fig. 0. Touch Voltage on 000/50 Fig.. Step Voltage on 000/50 Ω for Cae 3. Ω for Cae 3. For exaple, 3-dienion of the ground grid deign in cae 3 i hown in Fig. 9. Fig. 0 and are the graph of touch and tep voltage, which illutrate the - dienion view to help deterine the afe and unafe contour area. 49

6 Table 7. Safety Criteria for 50 kg ody Weight, 000/50 Ω Surface ayer Reitivity ( Ω ) Fault Clearing Tie 0. ec Touch Voltage Step Voltage (V) (V) Foot Reitance: Foot (Ω) None 74,096 3,5, , ,475 3,000, ,65.8 8,43.9 4,000,863 6,588 0,89 8,000 3,335,47,066,000 4,80 8,34 3,5,6000 6,68 4,08 4,356 0,000 7,734 30,07 5,495 4,000 9,00 35,935 6,633 Table 7 i the afety criteria of 000/50 Ω oil tructure by aterial urface covering with 0 c thick. For the bae cae, it i found that at the ae oil reitivity, the axiu touch voltage i equal to 7,80.4 V and tep voltage i,69.8 V. To coply with the afety criteria, it ut be covered by 0,000 Ω reitivity aterial. The touch voltage alo eet the afety criteria. The tep voltage doe not violate the afety criteria and it can be eaily olved.. Generally, PA will pread the ground with cruhed rock No. (Reitivity of cruhed rock No. i about 3,000 Ω ). The tep voltage can be olved. Fro Table 7, preading with 3,000 Ω aterial, the tep voltage criteria i 5,65.8 Ω, which can be et. The tudy found that the danger ay occur at the edge of ground grid, o the tudy concentrate at ground grid connection between the ubtation. It i found that, axiu touch voltage and tep voltage are equal to 0, V, 3, V and,408 V repectively. The decreae i obtained by the reduction of reitance of electrode yte. 6. CONCUSION The ground grid deign for the ditribution ubtation i exained with the ain objective to ae it grounding yte condition in ter of ground potential rie, touch voltage, tep voltage and % ratio between the auxiliary grounding yte and ain ground grid. Thee value are analyzed to enure that they coply with the afety criteria defined in the I Std with three cae claified by 5 ka Power Ditribution in PA. It i found that when the ground grid i eparated or two neighboring ubtation are diconnected, the afety iue ut be taken into account. In cae of ground grid of two neighboring ditribution ubtation, connecting ground grid between two ditribution ubtation can reduce the voltage to eet the afety criteria. In the procedure of renovation of the exiting ditribution ubtation that require a all ditribution ubtation in order to upply teporary electricity, a large ground potential difference between two eparate ground grid of the ditribution ubtation can occur when the ground grid of two neighboring ditribution ubtation are not connected together. Thi high can daage intelligent electronic device (ID), which will be ued in ditribution ubtation in the future or electronic controller which are currently ued. Thi incident can occur after a fault or lightning in a ditribution yte. oreover, thi high i alo dangerou to peronnel operating in the ditribution ubtation or nearby. The connection between ground grid of two neighboring ditribution ubtation i a iple and econoical ethod with effectivene to reduce the daage of device and danger to peronnel that can lead to power upply outage in indutrial zone or denely populated area. Therefore, thi ethod ha ore advantage copared with other ethod e.g. intalling ore protection device which need ore invetent cot but it cannot copletely olve the proble. A far a intallation cot and other neceary expene in grounding yte planning are concerned, the length of ground rod, the ize of conductor, the hort circuit current hould financially reflect increental cot and worth for variou alternative while repecting the etablihed afety criteria [5]. ACNOWDGNT The firt author would like to expre hi deepet gratitude to late Aoc. Prof. Dr. Janarn Hokierti, aertart Univerity, Thailand. The author would like to expre hi incere thank to Reearch Diviion, Provincial lectricity thority (PA) for CDGS progra and the technical data ued in thi reearch work. RFRNCS [] I STD Guide for Safety in AC Subtation Grounding. [] Jayawarna, N. and et al., 006. Safety Analyi of a icrogrid. International Journal of Ditributed nergy Reource, vol., no. 4, pp [3] Safe ngineering Service & Technologie td., Grounding & lectroagnetic Field, Technical Seinar. Chapter Fundaental Grounding Concept, 996, pp _ - _ 5. [4] S td 7430:998. Code of Practice for arthing. [5] Phayoho, A. and et al., 0. fficient Solution for A Subtation Grounding Syte during Subtation Upgrading. In Proc. HAPUA - JPIC Sypoiu, alayia. 8-0 October. 50