11.1 Reasons for Substation Grounding System

Transcription

1 11 Subtation Grounding Richard P. Keil Commonwealth Aociate, Inc Reaon for Subtation Grounding Sytem Accidental Ground Circuit Condition Permiible Body Current imit Importance of High-Speed Fault Clearing Tolerable Voltage 11.3 Deign Criteria Actual Touch and Step Voltage Soil Reitivity Grid Reitance Grid Current Ue of the Deign Equation Selection of Conductor Selection of Connection Grounding of Subtation Fence Other Deign Conideration Reference Reaon for Subtation Grounding Sytem The ubtation grounding ytem i an eential part of the overall electrical ytem. The proper grounding of a ubtation i important for the following two reaon: 1. It provide a mean of diipating electric current into the earth without exceeding the operating limit of the equipment 2. It provide a afe environment to protect peronnel in the vicinity of grounded facilitie from the danger of electric hock under fault condition The grounding ytem include all of the interconnected grounding facilitie in the ubtation area, including the ground grid, overhead ground wire, neutral conductor, underground cable, foundation, deep well, etc. The ground grid conit of horizontal interconnected bare conductor (mat) and ground rod. The deign of the ground grid to control voltage level to afe value hould conider the total grounding ytem to provide a afe ytem at an economical cot. The following information i mainly concerned with peronnel afety. The information regarding the grounding ytem reitance, grid current, and ground potential rie can alo be ued to determine if the operating limit of the equipment will be exceeded. Safe grounding require the interaction of two grounding ytem: 1. The intentional ground, coniting of grounding ytem buried at ome depth below the earth urface 2. The accidental ground, temporarily etablihed by a peron expoed to a potential gradient in the vicinity of a grounded facility It i often aumed that any grounded object can be afely touched. A low ubtation ground reitance i not, in itelf, a guarantee of afety. There i no imple relation between the reitance of the grounding ytem a a whole and the maximum hock current to which a peron might be expoed. A ubtation with relatively low ground reitance might be dangerou, while another ubtation with very high ground reitance might be afe or could be made afe by careful deign /03/$0.00+$

2 11-2 Electric Power Subtation Engineering There are many parameter that have an effect on the voltage in and around the ubtation area. Since voltage are ite-dependent, it i impoible to deign one grounding ytem that i acceptable for all location. The grid current, fault duration, oil reitivity, urface material, and the ize and hape of the grid all have a ubtantial effect on the voltage in and around the ubtation area. If the geometry, location of ground electrode, local oil characteritic, and other factor contribute to an exceive potential gradient at the earth urface, the grounding ytem may be inadequate from a afety apect depite it capacity to carry the fault current in magnitude and duration permitted by protective relay. During typical ground fault condition, unle proper precaution are taken in deign, the maximum potential gradient along the earth urface may be of ufficient magnitude to endanger a peron in the area. Moreover, hazardou voltage may develop between grounded tructure or equipment frame and the nearby earth. The circumtance that make human electric hock accident poible are: Relatively high fault current to ground in relation to the area of the grounding ytem and it reitance to remote earth Soil reitivity and ditribution of ground current uch that high potential gradient may occur at point at the earth urface Preence of a peron at uch a point, time, and poition that the body i bridging two point of high potential difference Abence of ufficient contact reitance or other erie reitance to limit current through the body to a afe value under the above circumtance Duration of the fault and body contact and, hence, of the flow of current through a human body for a ufficient time to caue harm at the given current intenity The relative infrequency of accident i due largely to the low probability of coincidence of the above unfavorable condition. To provide a afe condition for peronnel within and around the ubtation area, the grounding ytem deign limit the potential difference a peron can come in contact with to afe level. IEEE Std. 80, IEEE Guide for Safety in AC Subtation Grounding [1], provide general information about ubtation grounding and the pecific deign equation neceary to deign a afe ubtation grounding ytem. The following dicuion i a brief decription of the information preented in IEEE Std. 80. The guide deign i baed on the permiible body current when a peron become part of an accidental ground circuit. Permiible body current will not caue ventricular fibrillation, i.e., toppage of the heart. The deign methodology limit the voltage that produce the permiible body current to a afe level Accidental Ground Circuit Condition There are two condition that a peron within or around the ubtation can experience that can caue them to become part of the ground circuit. One of thee condition, touch voltage, i illutrated in Figure 11.1 and Figure The other condition, tep voltage, i illutrated in Figure 11.3 and Figure Figure 11.1 how the fault current being dicharged to the earth by the ubtation grounding ytem and a peron touching a grounded metallic tructure, H. Figure 11.2 how the Thevenin equivalent for the peron feet in parallel, Z th, in erie with the body reitance, R B. V th i the voltage between terminal H and F when the peron i not preent. I b i the body current. When Z th i equal to the reitance of two feet in parallel, the touch voltage i ( ) E = I R + Z touch b B th (11.1)

3 Subtation Grounding 11-3 FIGURE 11.1 Expoure to touch voltage. FIGURE 11.2 Touch-voltage circuit. FIGURE 11.3 Expoure to tep voltage. Figure 11.3 and Figure 11.4 how the condition for tep voltage. Z th i the Thevenin equivalent impedance for the peron feet in erie and in erie with the body. Baed on the Thevenin equivalent impedance, the tep voltage i

4 11-4 Electric Power Subtation Engineering FIGURE 11.4 Step-voltage circuit. ( ) E = I R + Z tep b B th (11.2) The reitance of the foot in ohm i repreented by a metal circular plate of radiu b in meter on the urface of homogeneou earth of reitivity r (W-m) and i equal to: Auming b = 0.08 R f = r 4b R f = 3r (11.3) (11.4) The Thevenin equivalent impedance for 2 feet in parallel in the touch voltage, E touch, equation i Z Th Rf = = 15. r 2 The Thevenin equivalent impedance for 2 feet in erie in the tep voltage, E tep, equation i (11.5) Z Th = 2R = 6r (11.6) The above equation aume uniform oil reitivity. In a ubtation, a thin layer of high-reitivity material i often pread over the earth urface to introduce a high-reitance contact between the oil and the feet, reducing the body current. The urface-layer derating factor, C, increae the foot reitance and depend on the relative value of the reitivity of the oil, the urface material, and the thickne of the urface material. The following equation give the ground reitance of the foot on the urface material. f R f = È b C Î Í r ù ú 4 û (11.7) where C K r C b n = ÂK Rm( 2nh r ) n= 1 r K = - r r + r i the urface layer derating factor i the reflection factor between different material reitivitie i the urface material reitivity in W m (11.8) (11.9)

5 Subtation Grounding 11-5 FIGURE 11.5 C veru h. r i the reitivity of the earth beneath the urface material in W m h i the thickne of the urface material in m b i the radiu of the circular metallic dic repreenting the foot in m Rm(2nh ) i the mutual ground reitance between the two imilar, parallel, coaxial plate, eparated by a ditance (2nh ), in an infinite medium of reitivity r in W m A erie of C curve ha been developed baed on Equation 11.8 and b = 0.08 m, and i hown in Figure The following empirical equation by Sverak [2], and later modified, give the value of C. The value of C obtained uing Equation are within 5% of the value obtained with the analytical method [3]. C Ê r ˆ 009. Á1- Ë r = 1-2h (11.10) Permiible Body Current imit The duration, magnitude, and frequency of the current affect the human body a the current pae through it. The mot dangerou impact on the body i a heart condition known a ventricular fibrillation, a toppage of the heart reulting in immediate lo of blood circulation. Human are very uceptible to the effect of electric current at 50 and 60 Hz. The mot common phyiological effect a the current increae are perception, mucular contraction, unconcioune, fibrillation, repiratory nerve blockage, and burning [4]. The threhold of perception, the detection of a light tingling enation, i generally recognized a 1 ma. The let-go current, the ability to control the mucle and releae the ource of current, i recognized a between 1 and 6 ma. The lo of mucular control may be caued by 9 to 25 ma, making it impoible to releae the ource of current. At lightly higher current, breathing may become very difficult, caued by the mucular contraction of the chet mucle. Although very painful, thee level of current do not caue permanent damage to the body. In a range of 60 to 100 ma, ventricular fibrillation occur. Ventricular fibrillation can be a fatal electric hock. The only way to retore the normal heartbeat i through another controlled electric hock, called defibrillation. arger current will inflict nerve damage and burning, cauing other life-threatening condition. The ubtation grounding ytem deign hould limit the electric current flow through the body to a value below the fibrillation current. Dalziel [5] publihed a paper introducing an equation relating the

6 11-6 Electric Power Subtation Engineering flow of current through the body for a pecific time that tatitically 99.5% of the population could urvive before the onet of fibrillation. Thi equation determine the allowable body current. I B = k t (11.11) where I B = rm magnitude of the current through the body, A t = duration of the current expoure, ec k = S B S B = empirical contant related to the electric hock energy tolerated by a certain percent of a given population Dalziel found the value of k = for peron weighing approximately 50 kg (110 lb) or k = for a body weight of 70 kg (154 lb) [6]. Baed on a 50-kg weight, the tolerable body current i I B = t (11.12) The equation i baed on tet limited to value of time in the range of 0.03 to 3.0 ec. It i not valid for other value of time. Other reearcher have uggeted other limit [7]. Their reult have been imilar to Dalziel for the range of 0.03 to 3.0 ec Importance of High-Speed Fault Clearing Conidering the ignificance of fault duration both in term of Equation and implicitly a an accident-expoure factor, high-peed clearing of ground fault i advantageou for two reaon: 1. The probability of expoure to electric hock i greatly reduced by fat fault clearing time, in contrat to ituation in which fault current could perit for everal minute or poibly hour. 2. Both tet and experience how that the chance of evere injury or death i greatly reduced if the duration of a current flow through the body i very brief. The allowed current value may therefore be baed on the clearing time of primary protective device, or that of the backup protection. A good cae could be made for uing the primary clearing time becaue of the low combined probability that relay malfunction will coincide with all other advere factor neceary for an accident. It i more conervative to chooe the backup relay clearing time in Equation 11.11, becaue it aure a greater afety margin. An additional incentive to ue witching time le than 0.5 ec reult from the reearch done by Biegelmeier and ee [7]. Their reearch provide evidence that a human heart become increaingly uceptible to ventricular fibrillation when the time of expoure to current i approaching the heartbeat period, but that the danger i much maller if the time of expoure to current i in the region of 0.06 to 0.3 ec. In reality, high ground gradient from fault are uually infrequent, and hock from thi caue are even more uncommon. Furthermore, both event are often of very hort duration. Thu, it would not be practical to deign againt hock that are merely painful and caue no eriou injury, i.e., for current below the fibrillation threhold Tolerable Voltage Figure 11.6 and Figure 11.7 how the five voltage a peron can be expoed to in a ubtation. The following definition decribe the voltage.

7 Subtation Grounding 11-7 FIGURE 11.6 Baic hock ituation. FIGURE 11.7 Typical ituation of external tranferred potential. Ground potential rie (GPR): The maximum electrical potential that a ubtation grounding grid may attain relative to a ditant grounding point aumed to be at the potential of remote earth. GPR i the product of the magnitude of the grid current, the portion of the fault current conducted to earth by the grounding ytem, and the ground grid reitance. Meh voltage: The maximum touch voltage within a meh of a ground grid.

8 11-8 Electric Power Subtation Engineering Metal-to-metal touch voltage: The difference in potential between metallic object or tructure within the ubtation ite that can be bridged by direct hand-to-hand or hand-to-feet contact. Note: The metal-to-metal touch voltage between metallic object or tructure bonded to the ground grid i aumed to be negligible in conventional ubtation. However, the metal-to-metal touch voltage between metallic object or tructure bonded to the ground grid and metallic object inide the ubtation ite but not bonded to the ground grid, uch a an iolated fence, may be ubtantial. In the cae of ga-inulated ubtation, the metal-to-metal touch voltage between metallic object or tructure bonded to the ground grid may be ubtantial becaue of internal fault or induced current in the encloure. Step voltage: The difference in urface potential experienced by a peron bridging a ditance of 1 m with the feet without contacting any other grounded object. Touch voltage: The potential difference between the ground potential rie (GPR) and the urface potential at the point where a peron i tanding while at the ame time having a hand in contact with a grounded tructure. Tranferred voltage: A pecial cae of the touch voltage where a voltage i tranferred into or out of the ubtation, from or to a remote point external to the ubtation ite. The maximum voltage of any accidental circuit mut not exceed the limit that would produce a current flow through the body that could caue fibrillation. Auming the more conervative body weight of 50 kg to determine the permiible body current and a body reitance of 1000 W, the tolerable touch voltage i and the tolerable tep voltage i E ( ) 50 = C r touch t (11.13) where E tep = tep voltage, V E touch = touch voltage, V C = determined from Figure 11.5 or Equation r = reitivity of the urface material, W-m = duration of hock current, ec t E ( ) 50 = C r tep t (11.14) Since the only reitance for the metal-to-metal touch voltage i the body reitance, the voltage limit i E mm- touch50 = 116 t (11.15) The hock duration i uually aumed to be equal to the fault duration. If recloing of a circuit i planned, the fault duration time hould be the um of the individual fault and ued a the hock duration time t Deign Criteria The deign criteria for a ubtation grounding ytem are to limit the actual tep and meh voltage to level below the tolerable tep and touch voltage a determined by Equation and The wortcae touch voltage, a hown in Figure 11.6, i the meh voltage.

9 Subtation Grounding Actual Touch and Step Voltage The following dicue the methodology to determine the actual touch and tep voltage Meh Voltage (E m ) The actual meh voltage, E m (maximum touch voltage), i the product of the oil reitivity, r; the geometrical factor baed on the configuration of the grid, K m ; a correction factor, K i, that account for ome of the error introduced by the aumption made in deriving K m ; and the average current per unit of effective buried length of the conductor that make up the grounding ytem (I G / M ). E m K K I = r m i G M (11.16) The geometrical factor K m [2] i a follow: K m ( ) È È 2 2 D D + h h ù K ii = Í + - ú hd Dd d Kh n Î Í û ú + È ùù 1 2 Í 8 ln lní úú 2 p Í ÎÍ p( 2-1) Î ûú ú û (11.17) For grid with ground rod along the perimeter, or for grid with ground rod in the grid corner, a well a both along the perimeter and throughout the grid area, K ii = 1. For grid with no ground rod or grid with only a few ground rod, none located in the corner or on the perimeter, K ii 1 = ( 2 n) 2 n (11.18) K h = 1 + h h o, h 0 = 1 m (grid reference depth) (11.19) Uing four grid-hape component [8], the effective number of parallel conductor in a given grid, n, can be made applicable to both rectangular and irregularly haped grid that repreent the number of parallel conductor of an equivalent rectangular grid: where n = na nb nc nd n a = 2 p C (11.20) (11.21) n b = 1 for quare grid n c = 1 for quare and rectangular grid n d = 1 for quare, rectangular, and -haped grid Otherwie, n b = p 4 A (11.22) È x y ù nc = Í ú ÎÍ A ûú 07. A x y (11.23)

10 11-10 Electric Power Subtation Engineering n d = D m x y (11.24) where c = total length of the conductor in the horizontal grid, m p = peripheral length of the grid, m A = area of the grid, m 2 x = maximum length of the grid in the x direction, m y = maximum length of the grid in the y direction, m D m = maximum ditance between any two point on the grid, m D = pacing between parallel conductor, m h = depth of the ground grid conductor, m d = diameter of the grid conductor, m = maximum grid current, A I G The irregularity factor, K i, ued in conjunction with the above-defined n, i K i = (11.25) For grid with no ground rod, or grid with only a few ground rod cattered throughout the grid, but none located in the corner or along the perimeter of the grid, the effective buried length, M, i n M = C + R (11.26) where R = total length of all ground rod, in meter. For grid with ground rod in the corner, a well a along the perimeter and throughout the grid, the effective buried length, M, i where r = length of each ground rod, m. M È Ê = C + Í Á Í Á ÎÍ Ë r x y ˆù ú ú ûú R (11.27) Step Voltage (E ) The maximum tep voltage i aumed to occur over a ditance of 1 m, beginning at and extending outide of the perimeter conductor at the angle biecting the mot extreme corner of the grid. The tep voltage value are obtained a a product of the oil reitivity (r), the geometrical factor K, the corrective factor K i, and the average current per unit of buried length of grounding ytem conductor (I G / S ): E K K I = r i G S (11.28) For the uual burial depth of 0.25 m < h < 2.5 m [2], K i defined a K ( ) 1 È 1 1 n = Í + Î h D + h + 1 D - - ù p 2 ú û (11.29) and K i a defined in Equation For grid with or without ground rod, the effective buried conductor length, S, i defined a = S C R (11.30)

11 Subtation Grounding Evaluation of the Actual Touch- and Step-Voltage Equation It i eential to determine the oil reitivity and maximum grid current to deign a ubtation grounding ytem. The touch and tep voltage are directly proportional to thee value. Overly conervative value of oil reitivity and grid current will increae the cot dramatically. Underetimating them may caue the deign to be unafe Soil Reitivity Soil reitivity invetigation are neceary to determine the oil tructure. There are a number of table in the literature howing the range of reitivity baed on oil type (clay, loam, and, hale, etc.) [9 11]. Thee table give only very rough etimate. The oil reitivity can change dramatically with change in moiture, temperature, and chemical content. To determine the oil reitivity of a particular ite, oil reitivity meaurement need to be taken. Soil reitivity can vary both horizontally and vertically, making it neceary to take more than one et of meaurement. A number of meauring technique are decribed in detail in IEEE Std , Guide for Meauring Earth Reitivity, Ground Impedance, and Earth Surface Potential of a Ground Sytem [12]. The mot widely ued tet for determining oil reitivity data wa developed by Wenner and i called either the Wenner or four-pin method. Uing four pin or electrode driven into the earth along a traight line at equal ditance of a, to a depth of b, current i paed through the outer pin while a voltage reading i taken with the two inide pin. Baed on the reitance, R, a determined by the voltage and current, the apparent reitivity can be calculated uing the following equation, auming b i mall compared with a: r a = 2paR (11.31) where it i aumed the apparent reitivity, r a, at depth a i given by the equation. Interpretation of the apparent oil reitivity baed on field meaurement i difficult. Uniform and two-layer oil model are the mot commonly ued oil reitivity model. The objective of the oil model i to provide a good approximation of the actual oil condition. Interpretation can be done either manually or by the ue of computer analyi. There are commercially available computer program that take the oil data and mathematically calculate the oil reitivity and give a confidence level baed on the tet. Sunde developed a graphical method to interpret the tet reult. The equation in IEEE Std. 80 require a uniform oil reitivity. Engineering judgment i required to interpret the oil reitivity meaurement to determine the value of the oil reitivity, r, to ue in the equation. IEEE Std. 80 preent equation to calculate the apparent oil reitivity baed on field meaurement a well a example of Sunde graphical method. Although the equation and graphical method are etimate, they provide the engineer with guideline of the uniform oil reitivity to ue in the ground grid deign Grid Reitance The grid reitance, i.e., the reitance of the ground grid to remote earth without other metallic conductor connected, can be calculated baed on the following Sverak [2] equation: È Ê ˆù Rg = r Í Á1 + ú ÎÍ T 20A Ë 1 + h 20/ A ûú where R g = ubtation ground reitance, W r = oil reitivity, W-m A = area occupied by the ground grid, m 2 h = depth of the grid, m T = total buried length of conductor, m (11.32)

12 11-12 Electric Power Subtation Engineering FIGURE 11.8 Fault within local ubtation; local neutral grounded. FIGURE 11.9 Fault within local ubtation; neutral grounded at remote location Grid Current The maximum grid current mut be determined, ince it i thi current that will produce the greatet ground potential rie (GPR) and the larget local urface potential gradient in and around the ubtation area. It i the flow of the current from the ground grid ytem to remote earth that determine the GPR. There are many type of fault that can occur on an electrical ytem. Therefore, it i difficult to determine what condition will produce the maximum fault current. In practice, ingle-line-to-ground and line-to-line-to-ground fault will produce the maximum grid current. Figure 11.8 through Figure how the maximum grid current, I G, for variou fault location and ytem configuration. Overhead ground wire, neutral conductor, and directly buried pipe and cable conduct a portion of the ground fault current away from the ubtation ground grid and need to be conidered when determining the maximum grid current. The effect of thee other current path in parallel with the ground grid i difficult to determine becaue of the complexitie and uncertaintie in the current flow. Computer program are available to determine the plit between the variou current path. There are many paper available to determine the effective impedance of a tatic wire a een from the fault point.

13 Subtation Grounding FIGURE Fault in ubtation; ytem grounded at local tation and alo at other point. FIGURE Typical current diviion for a fault on high ide of ditribution ubtation. The fault current diviion factor, or plit factor, repreent the invere of a ratio of the ymmetrical fault current to that portion of the current that flow between the grounding grid and the urrounding earth. S f I g = 3 I o (11.33) where S f = fault current diviion factor I g = rm ymmetrical grid current, A I 0 = zero-equence fault current, A The proce of computing the plit factor, S f, conit of deriving an equivalent repreentation of the overhead ground wire, neutral, etc., connected to the grid and then olving the equivalent to determine what fraction of the total fault current flow between the grid and earth, and what fraction flow through the ground wire or neutral. S f i dependent on many parameter, ome of which are:

14 11-14 Electric Power Subtation Engineering 1. ocation of the fault 2. Magnitude of ubtation ground grid reitance 3. Buried pipe and cable in the vicinity of or directly connected to the ubtation ground ytem 4. Overhead ground wire, neutral, or other ground return path Becaue of S f, the ymmetrical grid current I g and maximum grid current I G are cloely related to the location of the fault. If the additional ground path of item 3 and 4 above are neglected, the current diviion ratio (baed on remote v. local current contribution) can be computed uing traditional ymmetrical component. However, the current I g computed uing uch a method may be overly peimitic, even if the future ytem expanion i taken into conideration. IEEE Std. 80 preent a erie of curve baed on computer imulation for variou value of ground grid reitance and ytem condition to determine the grid current. Thee plit-current curve can be ued to determine the maximum grid current. Uing the maximum grid current intead of the maximum fault current will reduce the overall cot of the ground grid ytem Ue of the Deign Equation The deign equation above are limited to a uniform oil reitivity, equal grid pacing, pecific buried depth, and relatively imple geometric layout of the grid ytem. It may be neceary to ue more ophiticated computer technique to deign a ubtation ground grid ytem for nonuniform oil or complex geometric layout. Commercially available computer program can be ued to optimize the layout and provide for unequal grid pacing and maximum grid current baed on the actual ytem configuration, including overhead wire, neutral conductor, underground facilitie, etc. Computer program can alo handle pecial problem aociated with fence, interconnected ubtation grounding ytem at power plant, cutomer ubtation, and other unique ituation Selection of Conductor Material Each element of the grounding ytem, including grid conductor, connection, connecting lead, and all primary electrode, hould be deigned o that for the expected deign life of the intallation, the element will: Have ufficient conductivity, o that it will not contribute ubtantially to local voltage difference Reit fuing and mechanical deterioration under the mot advere combination of a fault current magnitude and duration Be mechanically reliable and rugged to a high degree Be able to maintain it function even when expoed to corroion or phyical abue Copper i a common material ued for grounding. Copper conductor, in addition to their high conductivity, have the advantage of being reitant to mot underground corroion becaue copper i cathodic with repect to mot other metal that are likely to be buried in the vicinity. Copper-clad teel i uually ued for underground rod and occaionally for grid conductor, epecially where theft i a problem. Ue of copper, or to a leer degree copper-clad teel, therefore aure that the integrity of an underground network will be maintained for year, o long a the conductor are of an adequate ize and not damaged and the oil condition are not corroive to the material ued. Aluminum i ued for ground grid le frequently. Though at firt glance the ue of aluminum would be a natural choice for GIS equipment with encloure made of aluminum or aluminum alloy, there are everal diadvantage to conider: Aluminum can corrode in certain oil. The layer of corroded aluminum material i nonconductive for all practical grounding purpoe. Gradual corroion caued by alternating current can alo be a problem under certain condition.

15 Subtation Grounding Thu, aluminum hould be ued only after full invetigation of all circumtance, depite the fact that, like teel, it would alleviate the problem of contributing to the corroion of other buried object. However, it i anodic to many other metal, including teel and, if interconnected to one of thee metal in the preence of an electrolyte, the aluminum will acrifice itelf to protect the other metal. If aluminum i ued, the high-purity electric-conductor grade are recommended a being more uitable than mot alloy. Steel can be ued for ground grid conductor and rod. Of coure, uch a deign require that attention be paid to the corroion of the teel. Ue of a galvanized or corroion-reitant teel, in combination with cathodic protection, i typical for teel grounding ytem. A grid of copper or copper-clad teel form a galvanic cell with buried teel tructure, pipe, and any of the lead-baed alloy that might be preent in cable heath. Thi galvanic cell can haten corroion of the latter. Tinning the copper ha been tried by ome utilitie becaue tinning reduce the cell potential with repect to teel and zinc by about 50% and practically eliminate thi potential with repect to lead (tin being lightly acrificial to lead). The diadvantage of uing tinned copper conductor i that it accelerate and concentrate the natural corroion, caued by the chemical in the oil, of the copper in any mall bare area. Other often-ued method are: Inulation of the acrificial metal urface with a coating uch a platic tape, aphalt compound, or both. Routing of buried metal element o that any copper-baed conductor will cro water pipe line or imilar object made of other uncoated metal a nearly a poible at right angle, and then applying an inulated coating to one metal or the other where they are in proximity. The inulated coating i uually applied to the pipe. Cathodic protection uing acrificial anode or impreed current ytem. Ue of nonmetallic pipe and conduit Conductor Sizing Factor Conductor izing factor include the ymmetrical current, aymmetrical current, limitation of temperature to value that will not caue harm to other equipment, mechanical reliability, expoure to corroive environment, and future growth cauing higher grounding-ytem current. The following provide information concerning ymmetrical and aymmetrical current Symmetrical Current The hort-time temperature rie in a ground conductor, or the required conductor ize a a function of conductor current, can be obtained from Equation and 11.35, which are taken from the derivation by Sverak [13]. Thee equation evaluate the ampacity of any conductor for which the material contant are known. Equation and are derived for ymmetrical current (with no dc offet). Ê - TCAP 10 ˆ Ê K o + T ˆ m I = A 2 mm Á t ln Ë carr Á r K o + T Ë a where I = rm current, ka A 2 mm = conductor cro ection, mm 2 T m = maximum allowable temperature, C T a = ambient temperature, C T r = reference temperature for material contant, C a 0 = thermal coefficient of reitivity at 0 C, 1/ C a r = thermal coefficient of reitivity at reference temperature T r, 1/ C r r = reitivity of the ground conductor at reference temperature T r, mw-cm K 0 = 1/a 0 or (1/a r ) T r, C t c = duration of current, ec TCAP = thermal capacity per unit volume, J/(cm 3 C) (11.34)

16 11-16 Electric Power Subtation Engineering Note that a r and r r are both to be found at the ame reference temperature of T r degree Celiu. If the conductor ize i given in kcmil (mm = kcmil), Equation become Ê TCAP ˆ Ê - K I = Akcmil Á Ë t ln a r Á Ë K c r r o + T ˆ m + T o a (11.35) Aymmetrical Current: Decrement Factor In cae where accounting for a poible dc offet component in the fault current i deired, an equivalent value of the ymmetrical current, I F, repreenting the rm value of an aymmetrical current integrated over the entire fault duration, t c, can be determined a a function of X/R by uing the decrement factor D f, Equation 11.35, prior to the application of Equation and Equation I F = I f Df (11.36) D f T Ê a = 1+ 1-e t Á f Ë -2t f Ta ˆ (11.37) The reulting value of I F i alway larger than I f becaue the decrement factor i baed on a very conervative aumption that the ac component doe not decay with time but remain contant at it initial ubtranient value. The decrement factor i dependent on both the ytem X/R ratio at the fault location for a given fault type and the duration of the fault. The decrement factor i larger for higher X/R ratio and horter fault duration. The effect of the dc offet are negligible if the X/R ratio i le than five and the duration of the fault i greater than 1 ec Selection of Connection All connection made in a grounding network above and below ground hould be evaluated to meet the ame general requirement of the conductor ued, namely electrical conductivity, corroion reitance, current-carrying capacity, and mechanical trength. Thee connection hould be maive enough to maintain a temperature rie below that of the conductor and to withtand the effect of heating, be trong enough to withtand the mechanical force caued by the electromagnetic force of maximum expected fault current, and be able to reit corroion for the intended life of the intallation. IEEE Std. 837, Qualifying Permanent Connection Ued in Subtation Grounding [14], provide detailed information on the application and teting of permanent connection for ue in ubtation grounding. Grounding connection that pa IEEE Std. 837 for a particular conductor ize range and material hould atify all the criteria outlined above for that ame conductor ize, range, and material Grounding of Subtation Fence Fence grounding i of major importance, ince the fence i uually acceible to the general public, children and adult. The ubtation grounding ytem deign hould be uch that the touch potential on the fence i within the calculated tolerable limit of touch potential. Step potential i uually not a concern at the fence perimeter, but thi hould be checked to verify that a problem doe not exit. There are variou way to ground the ubtation fence. The fence can be within and attached to the ground grid, outide and attached to the ground grid, outide and not attached to the ground grid, or eparately grounded uch a through the fence pot. IEEE Std. 80 provide a very detailed analyi of the different grounding ituation. There are many afety conideration aociated with the different fence-grounding option.

17 Subtation Grounding Other Deign Conideration There are other element of ubtation grounding ytem deign that have not been dicued here. Thee element include the refinement of the deign, effect of directly buried pipe and cable, pecial area of concern including control- and power-cable grounding, urge arreter grounding, tranferred potential, and intallation conideration. Reference 1. Intitute of Electrical and Electronic Engineer, IEEE Guide for Safety in AC Subtation Grounding, IEEE Std , IEEE, Picataway, NJ, Sverak, J.G., Simplified analyi of electrical gradient above a ground grid: part I how good i the preent IEEE method? IEEE Tran. Power Appar. Sytem, 103, 7 25, Thapar, B., Gerez, V., and Kejriwal, H., Reduction factor for the ground reitance of the foot in ubtation yard, IEEE Tran. Power Delivery, 9, , Dalziel, C.F. and ee, W.R., ethal electric current, IEEE Spectrum, 44 50, Feb Dalziel, C.F., Threhold 60-cycle fibrillating current, AIEE Tran. Power Appar. Syt., 79, , Dalziel, C.F. and ee, R.W., Reevaluation of lethal electric current, IEEE Tran. Ind. Gen. Applic., 4, , Biegelmeier, U.G. and ee, W.R., New conideration on the threhold of ventricular fibrillation for AC hock at Hz, Proc. IEEE, 127, , Thapar, B., Gerez, V., Balakrihnan, A., and Blank, D., Simplified equation for meh and tep voltage in an AC ubtation, IEEE Tran. Power Delivery, 6, , Rüdenberg, R., Baic conideration concerning ytem, Electrotechniche Zeitchrift, 11 and 12, Sunde, E.D., Earth Conduction Effect in Tranmiion Sytem, Macmillan, New York, Wenner, F., A method of meauring earth reitance, Rep. 258, Bull. Bur. Stand., 12, , Intitute of Electrical and Electronic Engineer, IEEE Guide for Meauring Earth Reitivity, Ground Impedance, and Earth Surface Potential of a Ground Sytem, IEEE Std , IEEE, Picataway, NJ, Sverak, J.G., Sizing of ground conductor againt fuing, IEEE Tran. Power Appar. Syt., 100, 51 59, Intitute of Electrical and Electronic Engineer, IEEE Standard for Qualifying Permanent Connection Ued in Subtation Grounding, IEEE Std (reaffirmed 1996), IEEE, Picataway, NJ, 1996.