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
You can preview the following Articles for more info:
Second: High And Medium
Voltage Electricity AC Substations

I began explaining Grounding Design Calculations for second type of buildings: AC Substations in Article " Grounding Design Calculations – Part Ten " where I explained the following:
 Design Procedures for grounding system design as per IEEE 80: Guide for safety in AC substation grounding,
 Step#1: Field Data Collection,
 Step#2: Earthing Grid Conductor Sizing.
Also in " Grounding Design Calculations – Part Eleven ", I explained Step#3: Calculation Of Tolerable Touch And Step Voltages.
And in Article " Grounding Design Calculations – Part Twelve ", I explained Step#4: Preliminary Design of Grounding System for AC Substations.
And In Article " Grounding Design Calculations – Part Thirteen ", I explained Step#5: Calculation of the Preliminary Grid Resistance, Rg, Of the Grounding System in Uniform Soil
Today, I will continue explaining other steps from the design procedures of grounding system for AC Substation.
Design
Procedures of Grounding System for AC Substations  Continued

Step#6: Determination Of Grid Current, IG

1 Terms Definitions for
Step#6
IF= If x Df
Where:
If
is the
symmetrical fault current,
Df is the Decrement Factor.
Notes:
I f = 3 I_{0}
Where:
If is the initial rms symmetrical ground fault
current,
I0 is the rms value of
zerosequence symmetrical current that develops immediately after the instant
of fault initiation, reflecting the subtransient reactances of rotating machines
contributing to the fault.
Note:
Not all of the Ground Fault Current will flow back through remote earth. A portion of
the Ground
Fault Current may have local
return paths (e.g. local generation) or there could be alternative return
paths other than remote earth (e.g. overhead earth return cables, buried
pipes and cables, etc). Therefore a current division factor Sf must be applied to account for the proportion of the Ground Fault Current flowing back through remote earth.
Ig = Sf x I f = Sf (3 I_{0})
Where:
Ig is the rms symmetrical grid current in A,
If is the rms symmetrical ground fault current
in A.
Sf is the fault current division
factor.
S f = Ig / I f = Ig / 3I_{0}
Where:
Sf is the fault current division factor,
Ig is the rms symmetrical grid current in A,
If is the rms symmetrical ground fault current
in A.
I0 is the zerosequence fault current in A.
Note:
The current division factor would change
during the fault duration. However, for the purposes of calculating the
design value of maximum grid current and symmetrical grid, the ratio is
assumed constant during the entire duration of a given fault.
IG= Df x Ig = Df x Sf x I f
Where:
IG is the maximum grid current in A,
Df is the decrement factor for the entire
duration of fault tf, given in sec,
Ig is the rms symmetrical grid current in A.

2 Calculation Steps for Maximum Grid Current IG
The following steps are involved in
determining the correct design value of maximum grid current IG for use in substation
grounding calculations:

Step#1: Assess the type and location
of the Worst ground fault producing the highest value of the maximum grid
current IG.
A Types of faults:
In threephase networks a distinction is
made between the following kinds of fault:
B Determining the Worst Fault Type:
The worst fault type for a given grounding
system is usually the one resulting in the highest value of the Maximum grid
current IG.
Since, Ig = Sf x I f = Sf (3 I_{0}) And since the Fault Current Division Factor
Sf is almost independent of the fault type,
So, the worst fault type can be defined as
the one resulting in the highest zero sequence or ground fault current flow
into the earth, 3I0.
As a general rule In a given location:
In the usual case where Z2 is assumed equal
to Z1, the above comparisons reduce to:
Where:
Z1: the positive sequence equivalent system
Impedance in Ω,
Z2: the negative sequence equivalent system
Impedance in Ω,
Z3: the zero sequence equivalent system
Impedance in Ω.
C Location of the Worst Fault Type:
The worst fault location may be either on
the high voltage side or on the low voltage side, and in either case may be
either inside the substation or outside on a line, at a certain distance from
the substation. There are no universal rules for the determination of the
worst fault location.

Step#2: Determine, by
computation, the fault current division factor Sf for the faults selected in Step#1
The split factor is used to take into
account the fact that not all the fault current uses the earth as a return
path. it is computed by the following equation:
S f = Ig / I f = Ig / 3I_{0}
Where:
Sf is the fault current division factor,
Ig is the rms symmetrical grid current in
A,
If is the rms symmetrical ground fault
current in A.
I_{0} is the zerosequence fault current in
A.
Note:
The current division factor would change
during the fault duration. However, for the purposes of calculating the
design value of maximum grid current and symmetrical grid, the ratio is
assumed constant during the entire duration of a given fault.
Sf is dependent on many parameters, some of
which are:
Methods For Calculation Of Sf
We have methods for determining the
percentage of the total fault current that flows into the earth which are:
1 Computer programs:
The most accurate method for determining
the percentage of the total fault current that flows into the earth is to use
a computer program such as EPRI’s SMECC, Substation Maximum Earth Current
Computation which requires an involved data collection effort.
2 Detailed methods such as:
3 Approximate methods such as:
In this Article, we will use the Garrett
and Patel Approximate method as follows.
Garrett and Patel Approximate method
It provides a quick and simple method to
estimate the current division factor that avoids the need for some of the
simplifying assumptions of the other approximate methods, though the results
are still only approximate. These curves, along with a few new curves and an impedance
table added for this guide, are included in Annex C in IEEE 80.
This method includes:
A Garrett and Patel’s table of split
factor equivalents (see part of Table C.1)
B Garret and Patel’s split factor Curves
(see Curve C.1)
It called " Garret and Patel’s split factor
curves ", the graphs are divided into the following four categories:
For method of using these graphs and
equivalent impedance table, limitations on this method and examples, please
Refer to Annex C in IEEE 80.

Step#3: Determine the
corresponding values of symmetrical ShortCircuit Current If from the power systems studies or from manual
calculation
Calculation of worst fault
type’s current value:
Initial symmetrical
shortcircuit currents are calculated with the equations in Table1:

Step#4: For each fault, based
on its duration time, tf, determine the value of
decrement factor Df to allow for the effects of asymmetry of the fault current wave
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 sec,
tf is the time duration of fault in s.
Ta = (X/R )(1/ω) = (X/R)(1/2∏f)
For
60 Hz, Ta = (X/R)(1/120π)
Where:
X/R is the X/R ratio at the fault
location,
f is the system frequency (Hz).
Typical values of the decrement factor for
various fault durations and X/R ratios are shown in Table2.
Note:
A decrement factor of 1.0 can be used for
fault durations of 30 cycles or more.

Step#5: Apply a
correction factor where appropriate to allow for future increase in fault
current due to expansion of the system.
Cp Corrective projection
factor : A factor to account for
increase in fault current due to system growth during life span of grid.
Note:
Typical values of Cp assumed
in design, lie in the range of 1.2 to 1.5 depending on the rate of growth of
the system.

Final
Equation Of The Maximum Grid Current IG
The maximum grounding grid fault
current is calculated using Eq.
IG = Sf. Df. Cp. If
Where:
IG = maximum grid current,
Sf = fault current division factor,
Df = decrement factor for the entire
duration of fault tf,
Cp = Corrective projection factor, if
applicable,
If = rms value of symmetrical ground
fault current.

Example:
A substation S/S132/11kV, 1X30/40MVA with the following
data:
Calculate the maximum
grounding grid fault current IG.
Solution:
Step#1: As a general rule in a given location and
in the usual case where Z2 is assumed equal to Z1 and since lZ0/Z1l=3, so Z0 > Z1. So, A
singlelinetoground fault will be the worst fault type.
Step#2: the fault current division factor Sf is given = 0.6
Step#3: Determine the corresponding values of
symmetrical shortcircuit current by using the formulas of Table1 in
above:
Firstly, calculate the Symmetrical three phase
shortcircuit current (r.m.s.) Ik3
Where:
Nominal system voltage Un=132 kV
Secondly, calculate the Single phase to earth
fault current Ik1 noting that Z2 is assumed equal to Z1
Step#4: For each fault, based on its duration
time, tf, determine the value of
decrement factor Df to allow for the effects of asymmetry of the fault current wave
Since, The X/R ratio at the fault is approximately
15, the maximum fault duration 150ms and the system nominal frequency is
50Hz.
Ta is then:
The decrement factor Df is
then:
Step#5: apply the Cp Corrective projection
factor =1.2.
So, the maximum grounding grid fault
current IG is:
IG = Sf. Df. Cp. If
So, IG = 0.6 x 1.1479 x 1.2 x
18.9 Ka = 15.62 KA

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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