PROTECTION
OBJECTIVE : To quickly isolate a faulty section from both ends so that the rest of the
System can function satisfactorily.
THE FUNCTIONAL REQUIREMENTS OF THE RELAY:
i) Reliability : The most important requisite of protective relay is reliability
since they supervise the circuit for a long time before a fault
occurs; if a fault then occurs, the relays must respond
instantly and correctly.
ii) Selectivity : The relay must be able to discriminate(select) between those
conditions for which prompt operation is required and those
for which no operation, or time delayed operation is required.
iii) Sensitivity : The relaying equipment must be sufficiently sensitive so that
it operates reliably when required under the actual conditions that
produces least operating tendency.
iv) Speed : The relay must operate at the required speed. It should neither
be too slow which may result in damage to the equipment nor
should it be too fast which may result in undesired operation.
IMPORTANT ELEMENTS :
Switch gear:
Circuit breaker Bulk oil, Minimum oil, SF6, Airblast, Vacuum etc.
depending on medium used for quenching the arc. Different operating
mechanisms such as solenoid, spring, pneumatic, hydraulic etc. are
employed.
Protective gear:
Relays (current, voltage, impedance, power, frequency, etc. based on
operating parameter, definite time, inverse time, stepped etc. as per
operating characteristic, logic wise such as differential, over fluxing
etc.
Station Battery:
A Station battery containing a number of cells accumulate energy
during the period of availability of A.C supply and discharge at the
time when relays operate so that relevant circuit breaker is tripped.