| Seasonal
Influence on Safety of Substation Grounding
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Introduction
With
the development of modern power system to the direction of extra-high voltage,
large capacity, far distance transmission and application of advanced technologies
the demand on the safety, stability and economic operation of power system became
higher. A good grounding system is the fundamental insurance to keep the safe
operation of the power system. The good grounding system should ensure the following:
"
To provide safety to personnel during normal and fault conditions by limiting
step and touch potential.
" To assure correct operation of electrical
devices.
" To prevent damage to electrical apparatus.
" To dissipate
lightning strokes.
" To stabilize voltage during transient conditions
and therefore to minimize the probability of flashover during the transients
As it is stated in the ANSI/IEEE
Standard 80-1986 "IEEE Guide for Safety in AC substation grounding,"
a safe grounding design has two objectives:
"
To provide means to carry electric currents into the earth under normal and fault
condition without exceeding any operational and equipment limit or adversely affecting
continuity of service.
" To assure that a person in the vicinity of grounded
facilities is not exposed to the danger of critical electrical shock.
A
practical approach to safe grounding considers the interaction of two grounding
systems: The intentional ground, consisting of ground electrodes buried at some
depth below the earth surface, and the accidental ground, temporarily established
by a person exposed to a potential gradient at a grounded facility.
An
ideal ground should provide a near zero resistance to remote earth. In practice,
the ground potential rise at the facility site increases proportionally to the
fault current; the higher the current, the lower the value of total system resistance
which must be obtained. For most large substations the ground resistance should
be less than 1 Ohm. For smaller distribution substations the usually acceptable
range is 1-5 Ohms, depending on the local conditions.
When a grounding system
is designed, the fundamental method is to ensure the safety of human beings and
power apparatus is to control the step and touch voltages in their respective
safe region. step and touch voltage can be defined as follows. Step
Voltage
It is defined as the voltage between the feet of the person standing
in near an energized object. It is equal to the difference in voltage given by
the voltage distribution curve between two points at different distance from the
electrode.
Touch Voltage
It
is defined as the voltage between the energized object and the feet of the person
in contact with the object. It is equal to the difference in voltage between the
object and a point some distance away from it.
In different season, the resistivity
of the surface soil layer would be changed. This would affect the safety of grounding
systems. The value of step and touch voltage will move towards safe region or
to the hazard side is the main concerned question
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