Basic of protection systemIntroduction
The purpose of an electrical power generation system is to
distribute energy to amultiplicity of points for diverse
applications.
The system should be designed and managed to deliver this energy
to the utilizationpoints with both reliability and economy.
As these two requirements are largely opposed, it is instructive
to look at therelationship between the reliability of a system and
its cost and value to the consumer,which is shown in Figure 1.
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Figure 1 Relationship between reliability of supply,its cost and
value to the consumer.
It is important to realize that the system is viable only
between thecross-over points A and B. The diagram illustrates the
significance of reliability insystem design, and the necessity of
achieving sufficient reliability.
On the other hand, high reliability should not be pursued as an
end in itself,regardless of cost, but should rather be balanced
against economy, taking all factors
Basic of protection system
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into account.
Security of supply can be bettered by improving plant design,
increasing thespare capacity margin and arranging alternative
circuits to supply loads. Sub-divisionof the system into zones,
each controlled by switchgear in association with protectivegear,
provides flexibility during normal operation and ensures a minimum
ofdislocation following a breakdown.
The greatest threat to a secure supply is the shunt fault or
short circuit, whichimposes a sudden and sometimes violent change
on system operation.
The large current which then flows, accompanied by the localized
release of aconsiderable quantity of energy, can cause fire at the
fault location, and mechanicaldamage throughout the system,
particularly to machine and transformer windings.Rapid isolation of
the fault by the nearest switch-gear will minimize the damage
anddisruption caused to the system.
A power system represents a very large capital investment . To
maximize thereturn on this outlay, the system must be loaded as
much as possible. For this reasonit is necessary not only to
provide a supply of energy which is attractive to prospectiveusers
by operating the system within the range AB (Figure 1.1), but also
to keep thesystem in full operation as far as possible
continuously, so that it may give the bestservice to the consumer,
and earn the most.
Revenue for the supply authority. Absolute freedom from failure
of the plant andsystem network cannot be guaranteed.
The risk of a fault occurring, however slight for each item, is
multiplied by thenumber of such items which are closely associated
in an extensive system, as anyfault produces repercussions
throughout the net-work. When the system is large, thechance of a
fault occurring and the disturbance that a fault would bring are
both sogreat that without equipment to remove faults the system
will become, in practicalterms, inoperable.
The object of the system will be defeated if adequate provision
for fault clearanceis not made. Nor is the installation of
switchgear alone sufficient; discriminativeprotective gear,
designed according to the characteristics and requirements of
thepower system, must be provided to control the switchgear.
A system is not properly designed and managed if it is not
adequately protected.This is the measure of the importance of
protective systems in modern practice and ofthe responsibility
vested in the protection engineer.
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