Calculation sheet Station Grounding Grid Rev. 1 i CONTENTS 1.0 GENERAL 1 2.0 REFERENCES 1 3.0 CALCULATION 1 3.1 General 1 3.2 Conductor Size Calculation 1 3.3 Check of touch voltage and step voltage 2 3.4 Initial Design Parameters 3 3.5 Grid Resistance 3 3.6 Maximum Grid Current 4 3.7 Ground Potential Rise 4 3.8 Calculation of Mesh & Step Voltages 4 3.9 Verification on suitability of conductor length (L) 6 4.0 CONCLUSION 6 Calculation sheet Station Grounding Grid 1.0 GENERAL This calculation is prepared to design the station earthing with IEEE Standard 80 to achieve a low resistance earth path and to ensure safe touch and step voltages for the 240MW Coal Fired Power Plant. 2.0 REFERENCES 2.1 Resistivity Geophysical Analysis Puerto Chile Oct. 2006 2.2 ANSI/IEEE Std. 80-1986, IEEE Guide for Safety in AC Substation Grounding 2.3 IEEE 665-87 Guide for Generating Station Grounding 3.0 CALCULATION 3.1 General The worst fault current energy(MVA base) is occurred at highest voltage system. i.e. 220kV GIS. Hence, the soil resistivity data and fault currents are applied based on 220kV GIS system. 3.1.1 Maximum Ground Fault Current I = 20,000 A (please refer to Clause 3.6) 3.1.2 Soil Resistivity: 23.91 ohm-m Please refer to attachment 2 of site soil resistivity and analysis report. 3.1.3 Ground Grid Area : 35,000m 2 In this calculation, station earthing, around its perimeter is considered to be buried at 0.5m depth under the switchyard & power block area. Area A = 150×200 + 50×100 = 35,000 m 2 3.1.4 Depth of ground grid conductors in m: h = 0.5 m 3.1.5 Ground connection method : Thermal welding type 3.2 Conductor Size Calculation 200 m 100 m 150 m 250 m Calculation sheet Station Grounding Grid AI t TCAP TT KT mm crr ma a 2 10 1 4 0 = ⋅ + − + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ αρ ln (From IEEE 80, Eq. 33, page 67) where A mm 2 = conductor cross section in mm 2 I = rms max. Ground fault current in kA, : 20.000kA T m = maximum allowable temperature in °C (250°C). T a = ambient temperature in °C (31°C) T r = reference temperature for material constants in °C (20°C) α 0 = thermal coefficient of resistivity at 0 °C α r = thermal coefficient of resistivity at reference temperature T r (α 20 =0.00393) ρ r = the resistivity of the ground conductor at reference temperature T r in μΩ/cm 3 (ρ 20 =1.7241) K 0 = 1/α 0, or (1/α r )−T r , 234 for Standard Annealed Soft Copper Wire t c = time of current flow, in sec : 1sec. TCAP = thermal capacity factor, 3.422, in J/cm 3 /°C from IEEE Std. 80-1986, page 66, table 1 (Material Constants) ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + − + ××× ⋅= 31234 31250 1ln 422.3 107241.100393.01 20 4 2 mm A ≈ 114.7 mm 2 Thus, the ground conductor size shall be 120mm 2 .(IEC coded standard size) 3.3 Check of touch voltage and step voltage For a surface material with resistivity of 2094.8Ω-m, and for a soil resistivity of 23.91Ω-m, reflection factor K is 9774.0 8.209491.23 8.209491.23 −= + − = + − = s s K ρρ ρρ (From IEEE 80, Eq. 20, page 39) where ρ s = surface material resistivity in Ω-m, 2094.8 ρ = soil resistivity in Ω-m, 23.91 3.3.1 Touch Voltage Criterion Assuming the person’s weight to be at least 50 kg, the tolerable step and touch voltages shall be as follows: ()() [ ] VtKhCE sssstouch 7.365116.0,5.11000 50 =+= ρ (From IEEE 80, Eq. 26, page 46) Calculation sheet Station Grounding Grid C s reduction factor for derating the nominal value of surface layer resistivity determined as above, 0.685 ρ s the resistivity of the surface material in Ω-m, 2094.8 t s duration of shock current in sec., 1.0 3.3.2 Step Voltage Criterion ()() [ ] VtKhCE ssssstep 7.1114116.0,61000 50 =+= ρ (From IEEE 80, Eq. 24, page 46) C s reduction factor for derating the nominal value of surface layer resistivity determined as above, 0.685 ρ s the resistivity of the surface material in Ω-m, 2094.8 t s duration of shock current in s, 1.0 3.4 Initial Design Parameters 3.4.1 Spacing a) Horizontal Spacing: 20 m b) Vertical Spacing: 20 m 3.4.2 Total length of buried conductor (L) Main Conductors : ( 250(m) × 6(EA) ) + ( 200(m) × 3(EA) ) + ( 150(m) × 11(EA) ) + ( 100(m) × 3(EA) = 4,050 [m] The total buried length is 4,050 m. 3.4.3 Burial depth of ground grid conductors in m (h): 0.5 [m] 3.5 Grid Resistance ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ + + ⋅ += A h A L R g 20 1 1 1 20 11 ρ (From IEEE 80, Eq. 40, page 82) where R g = station ground resistance in Ω ρ = average earth resistivity in Ω-m A = the area occupied by the ground grid in m 2 L = total buried length of conductors in m h = depth of the grid in m, 0.5 Calculation sheet Station Grounding Grid ][063.0 35000 20 5.01 1 1 3500020 1 4050 1 91.23 Ω= ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ ×+ + ⋅ += g R For most transmission and other large substations, the ground resistance should be about 1Ω or less. Hence, the value of the grid resistance obtained here (0.063[Ω]) meets usual requirements. (from IEEE 80, Section 12.1, page 81) 3.6 Maximum Ground Grid Current ICDI ISI Gpfg gff = = (from IEEE 80, Eq. 53&54, page 91) where I G = maximum ground grid current in A D f = decrement factor; 1.0 S f = current division factor relating the magnitude of fault current to that of its portion flowing between the grounding grid and surrounding earth, 0.5 I f = rms value of symmetrical fault current in A, 40kA C p = correction projection factor, 1.0 000,20000,405.00.10.1 = × × ×== fffpG ISDCI [A] 3.7 Ground Potential Rise To verify the suitability of the initial design parameters, it is necessary to compare the product of I G and R g , GRP, with the tolerable touch voltage, E touch50. Calculated according to the equation shown in IEEE 80, Eq. B4, page 177, [ ] VRIGRP gG 1260063.020000 =×=×= exceeds 365.7 [V], determined in Section 3.3.1 as the safe value of E touch50 . Therefore, further design evaluations are necessary. The calculation which accounts for geometrical factors, a corrective factor, the soil resistivity, and the average current density per unit of buried conductor can be found hereinafter in Section 3.8. 3.8 Calculation of Mesh & Step Voltages 3.8.1 Mesh Voltage (E m ) E m = ρK m K i I G /L (from IEEE 80, Eq. 66, page 113) a) K m , Spacing factor for mesh voltage, simplified method Calculation sheet Station Grounding Grid () K D hd Dh Dd h d K Kn m ii h =+ + − ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ + − ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ 1 216 2 84 8 21 2 2 ππ ln ln () (from IEEE 80, Eq. 68, page 113) where 5744.0 )2( 1 2 == n ii n K 225.11 0 =+= hhK h Corrective weighting factor that emphasizes the effects of grid depth, simplified method h 0 = 1m reference depth of grid 20=D Spacing between parallel conductors in m 22.11 149 = ×== BA nnn Number of parallel conductors in one direction 0142.0=d Diameter of grid conductors in m Thus, we get ( ) 1488.1 122.112 8 ln 2251 5744.0 0142.04 5.0 0142.0208 5.0220 014205.016 20 ln 2 1 2 2 = ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ −× + ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ × − ×× ×+ + ×× = )( ππ m K b) K i , Corrected factor for grid geometry, simplified method K i = 0.656 + 0.172×n =0.656 + 0.172× 11.22 = 2.5867 (from IEEE 80, Eq. 69, page 114) c) Mesh Voltage, E m E m = ρK m K i I G /L = 23.91×1.1488× 2.5867 × 20000 / 4050 = 350.9 < 365.7[V] (E touch50 ) 3.8.2 Step Voltage (E s ) E s = ρK s K i I G /L (from IEEE 80, Eq. 67, page 113) a) K s , Spacing factor for step voltage, simplified method () K hDhD s n =+ + +− ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ − 11 2 11 105 2 π . (from IEEE 80, Eq. 74, page 115) Calculation sheet Station Grounding Grid where 20=D Spacing between parallel conductors in m 14)14,9max(),max( = == BA nnn Number of parallel conductors in one direction h=0.5 Depth of ground grid conductors in m Thus, we get () 3498.05.01 20 1 5.020 1 5.02 11 214 = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ −+ + + × = − π s K b) K i , Corrected factor for grid geometry, simplified method K i = 0.656 + 0.172n =0.656 + 0.172×14 =3.064 (from IEEE 80, Eq. 69, page 114) c) Step Voltage, E s E s = ρK s K i I G /L=23.91 × 0.3498 × 3.064 × 20000 / 4050 = 126.5 < 1114.7[V] (E step50 ) 3.9 Verification on suitability of conductor length (L) The suitability of conductor length buried can be verified with the following equation. ()() ][886,3 8.2094685.0174.0116 12000091.235867.21488.1 ),(174.0116 m KhC tIKK L s sGim = ××+ ×××× = + > ρ ρ (from IEEE 80, Eq. 77, page 115) Since the design length (4,050 [m]) exceeds the minimum required length (3,886[m]), the proposed grounding design is acceptable. 4.0 CONCLUSION Area Grid Conductor Spacing Total Grid *E touch50 *E step50 Area Size Length resistance *E m *E s Chile 35000 120 20×20 4050 0.063 365.7[V] 1114.7[V] TPP [m 2 ] mm 2 [m×m] [m] [Ω] 350.9[V] 126.5[V] Note : 1. *E touch50 : Tolerable Touch potential Voltage 2. *E step50 : Tolerable Step potential Voltage 3. *E m : Actual Touch potential Voltage 4. *Es : Actual Step potential Voltage TPP: Thermal power plant . ANSI/IEEE Std. 8 0-1 986, IEEE Guide for Safety in AC Substation Grounding 2.3 IEEE 66 5-8 7 Guide for Generating Station Grounding 3.0 CALCULATION . Calculation sheet Station Grounding Grid Rev. 1 i CONTENTS 1.0 GENERAL 1 2.0 REFERENCES 1 3.0 CALCULATION 1 3.1 General