What does a good grounding of the Power Substation and Switching
Station really mean?
Earthing For Safety
Provision of adequate grounding in a substation and switching stations are very important
for the safety of operating personnel as well as electrical devices do not rise above tolerable
thresholds and that the earth connection is rugged to dissipate the fault to the earth.
What does a good grounding of the power substation and switching station really mean?
The importance of an effective, durable and a dependable earth for ensuring safety from
electrical hazards does not require to be elaborated upon more.
By earthing, connecting the electrical equipment to the general mass of the earth, this has a
very low resistance.
1. Requirements Of a Good Substation
Earthing
The object of an earthing system in a substation is to provide under and
around the substation a surface that shall be at a uniform potential and near
zero or absolute earth potential as possible.
The provision of such a
surface of uniform potential under and around
the substation
ensure that no human being in the substation subject in
shock of injury on the occurrence of a short circuit or development of other
abnormal conditions in the equipment installed in the yard.
The primary requirements of a good earthing system in a substation
are:
1.
It stabilizes circuit potentials with respect to ground and limits the
overall potential rise.
2.
It protects life and property from over voltage.
3.
It provides low impedance path to fault currents to ensure prompt and
consistent operation of protective devices during ground faults.
4.
It keeps the maximum voltage gradient along the surface inside and
around the substation within safe limits during ground fault.
2. Maximum Permissible Resistance Of
Earthing System
Large Power Station
0.5 ohm
Major Sub-Station
1.0 ohm
Small Sub-Station
2.0 ohms
In all other cases
5.0 ohms
3. Touch Voltage (E-TOUCH)
Definition – The potential difference between a ground metallic structure and
a point on the earth’s surface separated by a distance equal to the normal
maximum horizontal reach of a person, approximately one meter as shown in
figure 1.
Figure 1 – Touch voltage at a grounded structure
4. Step Voltage (E STEP)
Definition – The potential difference between two points on the earth surface
separated by distance of one pace that will be assumed to be one meter in
the direction of maximum potential gradient as shown in figure.
Fig
ure 2 – Step voltage at a grounded structure
5. Earthing System In a Substation
The earthing system comprises of earthing (or) grid, earthing electrodes,
earthing conductors and earth connections.
5.1 Earth Mat or Grid
The primary requirement of earthing is
to have a very low earth resistance
.
