In Article " Grounding Design Calculations – Part One ", I indicated the following:
Grounding System Design
Calculations according to type of the building
The procedures for performing the Grounding System Design
Calculations can differ slightly according to the type of the building
as follows:

First: Domestic,
commercial and industrial premises
We mean by domestic, commercial and
industrial premises, all installations up to 1,000 V ac and 1,500 V dc 
between phases, with some minor exceptions.

And I explained Methods of Grounding Design Calculations of Domestic, commercial and industrial premises in the following Articles:
 Grounding Design Calculations – Part One and Grounding Design Calculations – Part Two: Equations Method and solved examples.
 Grounding Design Calculations – Part Three: Nomographs Method
 Grounding Design Calculations – Part Four: Excel Spreadsheets Method
 Grounding Design Calculations – Part Five and Grounding Design Calculations – Part Six: using Tables Method
 Grounding Design Calculations – Part Seven and Grounding Design Calculations – Part Eight: Using Online Earthing Calculators
 Grounding Design Calculations – Part Nine: Software Programs Method
Today, I will explain Grounding Design Calculations of High and Medium Voltage Electricity AC Substations.
You can preview the following Articles for more info:
Grounding System Design
Calculations Of High And Medium Voltage AC Substations

1 Standards
Standard used for ground calculations
of High and medium voltage electricity AC substations is IEEE 80: Guide for
safety in AC substation grounding.
Note:
This standard is primarily concerned with
outdoor ac substations, either conventional or gasinsulated. Distribution,
transmission, and generating plant substations are included. With proper
caution, the methods described herein are also applicable to indoor portions
of such substations, or to substations that are wholly indoors. It doesn’t
cover the grounding problems peculiar to dc substations.

Step#1: Field Data Collection
The following information is required / desirable before
starting the calculation:

Step#2: Earthing Grid
Conductor Sizing
In this step, we will know how to size the
earthing grid conductor in the following two cases:

Terms Definitions for
Step#2
Note:

First: Earthing
Grid Conductor Sizing For Symmetrical Fault Currents

Earthing Grid
Conductor Sizing for Symmetrical Fault Currents
Where:
I is the rms current in kA
Amm2 is the conductor cross section in mm2
Tm is the maximum allowable temperature in °C
Ta is the ambient temperature in °C
Tr is the reference temperature for material
constants in °C
αo is the thermal coefficient of resistivity at 0 °C in 1/°C
αr is the thermal coefficient of resistivity at
reference temperature Tr in 1/°C
ρr is the resistivity of the
ground conductor at reference temperature Tr in μΩcm
Ko = [(1/αo– Tr )]or [(1/αr) – Tr] in °C
tc is the duration of current in s
TCAP is the thermal capacity per unit volume
from Table1, in J/(cm3·°C)
Notes:
1 It should be noted that αr and ρr are both to be found at the
same reference temperature of Tr °C. Table1 provides data for αr and ρr at 20 °C.
2 If the conductor size is given in
kcmils:
Since, kcmils = mm2 × 1.974 , so the above Equation becomes as follows:
3 calculation of TACP for materials not
listed in Table1:
TCAP can be calculated for materials not listed
in Table1 from the specific heat and specific weight. Specific heat, SH, in cal/(grams × °C) and
specific weight, SW, in gram/cm3 are related to
the thermal capacity per unit volume in J/(cm3 × °C) as follows:
Since 1 calorie =
4.184 J
Therefore, TCAP is defined by;
TCAP [cal/(cm3 · _°C)] = SH [cal/(gram · °C)] · SW (gram/cm3)
or
TCAP [J/(cm3 ·°C)] = 4.184 (J/cal) · SH [(cal/(gram · °C)] · SW (gram/cm3)
4 The Equation in English units can be
simplified to the following:
Akcmil = I x Kf x √ tc
Where:
Akcmil is the area of conductor in kcmil,
I is the rms fault current in kA,
tc is the current duration in s,
Kf is the constant from Table2 for the
material at various values of
Tm (fusing temperature or limited conductor
temperature) and using ambient temperature (Ta) of 40°C.

Example#1:
Calculate the Earthing Grid Conductor Size for 30% copperclad steel
conductor for 1 KA fault current and duration = 1 sec.
Solution:
For instance, to calculate the 1 s size of
a 30% copperclad steel conductor, one gets
tc = 1.0, α20 = 0.003 78, ρ20 =
5.86, TCAP = 3.85, Tm = 1084, Ta = 40, K0 = 245
Thus, for I = 1 kA
Akcmil = 197.4/ √267.61 = 12.06 k cmil or 12.06 kcmil/kA
Example#2:
A 20 kA, 3 s fault current, calculate the Earthing Grid Conductor Size using, the English unit
equation, for each of the following conductor’s material:
Solution:
1 For soft drawn copper
Akcmil = 20 x7.00√3
= 242.5 kcmil
use 250 kcmil
2 For 40% conductivity copperclad steel
conductor
Akcmil = 20x 10.45 = 362.0 kcmil
use 19/#7 conductor
3 For steel conductor
Akcmil = 20 x 15.95 = 552.5 kcmil
use 7/8 inch diameter conductor

Second:
Earthing Grid Conductor Sizing For Asymmetrical Fault Currents

Earthing Grid
Conductor Sizing For Asymmetrical Fault Currents
The minimum conductor size of the earthing grid conductors in
case of Asymmetrical
Fault Currents can be calculated by one of the following two methods:
Method#1: Using Decrement Factor
In cases where accounting for a possible dc
offset component in the fault current is desired, an equivalent value of the
symmetrical current, IF, will be used in above equations for Earthing Grid Conductor Sizing For Symmetrical Fault Currents
as follows:
IF= If x Df
Where:
If
is the
symmetrical fault current,
Df is the Decrement Factor.
Notes:
Calculation of Decrement Factor Df:
The following Equation can be used to
compute the decrement factor for specific X/R ratios and fault durations:
Where:
Ta is the dc offset time constant in s [Ta = X/(ωR), for 60 Hz, Ta = X/(120πR)],
tf is the time duration of fault in s.
However, Typical values of the decrement factor for
various fault durations and X/R ratios are shown in Table3.
Method#2: Using Asymmetrical Current
Tables
Fusing characteristics for various sizes of
copper conductor with various degree of dc offset are presented in Table4
through Table7 as follows:
Notes:

Additional Conductor
Sizing Factors

In the next Article, I will explain Other Steps from the Design Procedures of Grounding System Design for AC Substation. Please, keep following.
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