04-06-2013, 01:15 PM
Introduction to Power Systems Protection & Relays
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Why we need the Protection ?
To detect abnormalities (faults).
To eliminate such abnormality by
isolating smallest portion of the system
in a shortest period of time.
To prevent injury to personnel.
To prevent damage to Equipment.
Enable Continuous service in
undamaged part of the network.
Protective System –What are the Requirements?
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. Damage can be Minimised.
Sensitivity
The relaying equipment must be sufficiently sensitive so that it operates reliably
when required under the actual conditions that produces least operating tendency
and to detect all possible Shunt and series Faults.
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. Isolate only Faulty Section without disturbing Healthy
Section.
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. The Operation Capability of the Protection
System is summerised in “ reliability”, which includes both the security in fault
clearance and the security against undesired clearances. The security in fault
clearance is called dependability and the security against undesired clearances
is called Security.
Relay operation when a fault occurs
Each Relay should Protect a specific
Zone in the System.
If fault is inside its Zone, Relay should
operate and isolate the faulted Zone.
If fault is outside zone, Relay should
not operate,
Some other Relay should operate and
isolate