**JOURNAL OF RESEARCH IN NATIONAL DEVELOPMENT VOLUME 8 NO 2, DECEMBER, 2010**

**
COMPUTER-BASED
MODEL FOR DESIGN OF
A
132/33kV
SUBSTATION EARTHING SYSTEM**

**
A.O.
Ibe, E.
Sofimieari, and Uma Uzubi**

**
Department of Electrical/Electronics**** Engineering,
University of Port Harcourt, Port Harcourt**

**
E-mail:gbogbonna@yahoo.co.uk**

**
**

**
Abstract**

*
**This
p**aper
presents the design of earthing system for 132 /
33kV
substation and its simulation for required
parameters** using MATLAB GUI.
This paper reviews earthing practices with special
reference to safety and also provides guidance and
information pertinent to safe earthing practices
in AC substation design. *
*
Standard
Equations are used in determination of design
parameters, such as step and t**ouch**
potential,
mesh and
grid
potential, diameter of required conductor,
substation resistance, the number of conductor in
x and y axis of the mesh and total length of
conductor required.*

**
Keywords:**Earthing
design,
Earth
mat and
MATLAB program.

**
**

**
Introduction**

Earthing system, sometimes simply called
“earthing” is the total set of measures used to
connect an electrically conductive part of earth
(Gutta,
2007).
This is realized by connecting the non current
carrying metallic parts in every electrical
installation to the underground electrical
conductor or electrode placed in intimate contact
with the soil some distance below the ground level
(IEEE, 1985). The intent of this paper is to
provide information pertinent to safe earthing
practices in AC substation
design. The objective of an earthing system for an
electrical installation can be classified into
four groups

Ø
To ensure that, the non current carrying metal
work of the electrical equipment does not attain a
dangerous potential with respect to the general
mass of earth when fault occurs.

Ø
To allow the easily flow of large fault current
into the ground mat. So that the protective
equipment has time to operate and thus isolate the
faulty circuit in the event of fault.

Ø
To limit the potential of any part of an
installation to a pre – determine value with
respect to the general mass of the earth.

Ø
Provide earthing for lightning impulses and the
surges occurring from the switching of substation
equipment, which reduces damage of equipment.

Grounding resistance should be
low enough to permit the flow of fault current.
The resistance of the mat should not be of such
a
magnitude as to permit the flow of fatal
current in the live body (IEEE, 1976).

This paper briefly shows the procedures for
measuring the resistance of the installed earthing
system, diameter of conductor required for the
design and total length of conductor for the
design. The ultimate of goal of this paper is to
design earthing system to safety.
To
evaluate and simulate grid conductor size,
vertical and horizontal electrode size,
permissible potential difference and required
facts for design procedure by using MATLAB
Programme.

**
**

**
Basic design requirements**

Ø
No earth plate should be less than
2ft
2ft in area
and
inch
for copper, so the plates are buried at one foot
below the permanent level.

Ø
If lower resistance is required for an
installation, more than two plates are connected
in parallel keeping each plate at least 10 feet
away from each other.

Ø
If electrical apparatus is present then at least
two plates are buried at maximum distance which is
never less than 10 feet.

**
****E****arthing****
d****esign e****quations**

**
**Measurement of soil resistivity

The resistivity of the soil is measured by the
formula

= resistivity
of soil in

s = horizontal distance between two
successive spikes in (m)

b = depth of electrode in meters

I = known current passed between electrode

R=resistance in ohms

V = potential drop across electrodes.

For
equation
(1) becomes

Calculation of step voltage limit

This is the potential difference between the feet
of a person standing on the floor of substation,
with 0.5m spacing between his feet, during the
flow of earthing fault current to the grounding
system. It can be calculated by the equation.

= resistivity
of the surface material in

t = duration of shock circuit in seconds

Calculation of touch voltage limit

This is potential difference between the fingers
of raised hard touching the faulted structure and
the feet of the person standing on substation
floor. The person should not get shock even if the
ground structure is carrying fault current. The
expression below can be used for its calculation.

Minimum conductor size

It can be calculated using the equation below;

= conductor
sectional size in mm^{2}

= maximum
allowable temperature in ^{O}C

T_{a
}= ambient temperature for material constants
in ^{O}C

= thermal
coefficient of resistivity at O^{0}C

= thermal
coefficient of resistivity at reference
temperature