EARTHING SYSTEM DESIGN DESIGN CALCULATION FOR SUBSTATION SS 84-3. 1. INTRODUCTION The contract for EPC Works for the Halobutyl Project for A l-Jubail Petrochemical Company KEMYA has been awarded to Technip. Technip. Technip has awarded the Civil & Electrical works for the following buildings to Al Husam a. b. c. d. e.
Rubber Finishing Building Substation SS-84-2 Finishing Office & Substation SS-84-3 Remote Instruments Building Operators Shelters -4 Nos.
Earthing mesh in the Project/Plant area will be provided to protect the human being from the step and touch potentials and provide free path for earth fault current for equipment protection. Each mesh design, sizing of the conductor required for forming the earth mesh are done in accordance with IEEE Std.80. The results of this study will be used for forming the earthing mesh, depth of burial, driving depth of the electrode and total number of electrodes required.
This particular document covers the Calculations for Earthing System for the Substation SS 84-2.
2. REFERENCES a. IEEE Std.80-2000 : Guide for safety in AC Substation Grounding b. SABIC Engineering Standards c. Project Specific Drawings d. Geotechnical Reports inclkuding inclkuding Soil resitivity reports 3. EXPLANATION OF EARTHING SYSTEM The earthing system shall be composed of a earthing distribution grid system (meshed network) constructed by sub-grade earthing conductors and earthing electrodes. b. All connections are carried out by means of exothermic welding process. c. Adjacent to the transformer neutral grounding, earthing electrodes are t o be driven into the soil and connected to earthing mesh d. In order to achieve an overall earth resistance of 1 ohm, earth electrodes are to be driven at certain points into the soil and connected to earthing mesh a.
e. Earthing electrodes are 3 meter length with a diameter of 17.5 mm. f. Earthing resistance is required less than 1 ohm
4. SOIL RESISTIVITY Resistance (R ) of the soil was measured using Wenner’s method. Summary DATA are shown on Annex-B. Soil resistivity was computed by using the formula: ρ = 2π aR (“a” is electrode separation.) From the value measured in the Plant area, the average value of top layer resistivity is less than 656 Ω-m
5. EARTHING SYSTEM CALCULATIONS A. Earthing Grid conductor sizing calculation To determine the minimum cross sectional area of the main earthing conductor, followings are Considered: - Maximum fault current. - Material for the earth conductor is annealed copper stranded wire. - Following formula is used for to calculate the earthing conductor size, as per IEEE Std.80.Section 11 (Eq-37), Table 1.
This equation is can be arranged to give required conductor size as a function of conductor current.
Where,
rams current in KA
I
time of current flow in seconds conductor cross section 2 in mm
Tc A
400 A 1 sec
from Design Requirement from design requirement
max. Allowable temperature in °C
Tm
1083 °C
from IEEE 80-2000 Section
ambient temperature in °C
Ta
36 °C
from IEEE 80-2000 Section
= thermal coefficient of resistivity at 0 °C
αo
0.003 93(1/ °C )
from IEEE 80-2000 Section
thermal coefficient of resistivity at reference temperature T = the resistivity of the ground conductor at reference temperature T, in μΩ/cm3
αr 1.72 µΩm
from IEEE 80-2000 Section
234 °C
from IEEE 80-2000 Section
ρr
K o = 1/α0,or (1/αr)-Tr Thermal capacity factor 3 in j/cm /° C
TCAP
11, Table-1 11, Table-1 11, Table-1
11, Table-1
11, Table-1 from IEEE 80-2000 Section
3.42 3
11, Table-1
j/cm /° C
By substituting these values in the above formula, we g et A= 4.77 mm
2
2
Minimum conductor cross section selected as 95 mm , bare copper annealed soft drawn conductor for the main ground grid. Therefore the Earthing Ring/Main Grid Component for SS 84-2 will be formed using 2 95 mm bare copper annealed soft drawn conductors, laid at a depth of 0.76 Mtr.
B. Calculation of Step & Touch Potentials The maximum tolerable voltages for step and touch scenarios can be calculated empirically from IEEE Std Section 8.3 for body weights of 50kg and 70kg: Touch voltage limit - the maximum potential difference between the surface potential and the potential of an earthed conducting structure during a fault (due to ground potential rise):
Step voltage limit - is the maximum difference in surface potential experience by a person bridging a distance of 1m with the feet wit hout contact to any earthed object
Where,
Touch voltage limit (V) Step voltage limit (V) Surface Layer Derating Factor
0.8
C s
for crushed rocks
2500 Ω.m
the resistivity of the surface layer material (Ω.m) Maximum Fault Clearing Time (s)
15 sec
The choice of body weight (50kg or 70kg) depends on the expected weight of the personnel at the site. We shall assume 70 kg as the average weight of a human of the region. Assuming the values above, The maximum allowable touch potential is = 162.15 Volts The maximum allowable Step Potential is = 526.98 Volts
C. Calculation of earthing resistance As per IEEE80-2000 section14 (eq52)
Where,
R g Ground Resistance ρ Soil Resistivity A Area occupied by the ground grid LT Total buried length of conductors h Depth of the grid
: --
(ohm)
: 656 (ohm-m) : 3772.98 (m2) : 447.32 (m) : 0.76 (m)
using the above values Rg = 6.1174 (Ohm)
D. Maximum grid current IG The X/R ratio at the fault is approximately 15, the maximum fault duration 150ms and the system nominal frequency is 50Hz. The DC time offset is therefore:
= 0.047745
Considering T f = 0.15 msec
D f
= 1.148436
Then, finally the maximum grid current is
Ig = Ike * Sf In the most conservative case, a current Sf =1, division factor of can be applied, meaning that 100% of earth fault current flows back through remote earth.
= 400*1 = 400 A
Therefore,
IG
= Ig * Df = 400 * 1.148436
=459.3742 A
E. Ground potential Rise GPR
=459.3742 * 6.1174 =2810.1757 Vol ts The maximum allowable touch potential calculated earlier is = 1621.49 Volts The maximum allowable Step Potential calculated earlier is = 5269.84 Volts The Ground Potential Rise is greater than the than the Maximum allowable touch potential but lower than the maximum allowable step potentials.
6.
Earthing Grid Design Verification A. Mesh Voltage Calculation The geometric factor n is calculated from IEEE Std 80 Equation 85
Where is Lc the total length of horizontal grid conductors (m)
LP is the length of grid conductors on the perimeter (m) A is the total area of the grid (m2)
L x and Ly are the maximum length of the grids in the x and y directions (m) Dm is the maximum distance between any two points on the grid (m) na nb
=
5.953857
=
0.721860306
nc = nc
= 1
Therefore,
n
= 5.953857 * 0.721860306 * 1 * 1 = 4.297853473
The average spacing D between parallel grid conductors is:
Where, W g and Lg are the width and length of the grid respectively
W g = 36.2 Mtr , Lg = 64.575 Mtr nr and nc is the number of parallel rows and columns respectively nr = 3 and nc = 2 D = 73.625 The geometric spacing factor K m is:
Where
is the spacing between parallel grid conductors (m) is the depth of buried grid conductors (m) is the cross-sectional diameter of a grid conductor (m) is a weighting factor for depth of burial = is a weighting factor for earth electrodes /rods on the corner mesh for grids with earth electrodes along the gr id perimeter or corners
for grids with no earth electrodes on the corners or on the
perimeter
is a geometric factor (see below)
= 0.118255
The irregularity factor K i is
=1.28008
The effective buried length LM is:
Where Lc is the total length of horizontal grid conductors (m)
LR is the total length of earthing electrodes / rods (m) Lr is the length of each earthing electrode / rod (m) L x and Ly are the maximum length of the grids in the x and y directions (m) LM
= 2569.25
Where is the soil resistivity (Ω.m)
I G is the maximum grid current found earlier in Step 4 (A) K m is the geometric spacing factor (see below) K i is the irregularity factor (see below) LM is the effective buried length o f the grid (see below)
EM
= 91.969 Volts
The maximum allowable touch potential is 1,621.49 V, which exceeds the mesh voltage calculated above and the earthing system passes the touch potential criteria.
