It is, of course, desirable to remove a
fault from the power system as quickly as possible. However, the
relay must make its decision based upon voltage and current
waveforms, which are severely distorted due to transient phenomena
that follow the occurrence of a fault.
The relay must separate the meaningful
and significant information contained in these waveforms upon which a
secure relaying decision must be based.
These considerations demand that the
relay take a certain amount of time to arrive at a decision with the
necessary degree of certainty. The relationship between the relay
response time and its degree of certainty is an inverse one and is
one of the most basic properties of all protection systems.
Although the operating time of relays
often varies between wide limits, relays are generally classified by
their speed of operation as follows:
1. Instantaneous—These relays operate
as soon as a secure decision is made. No intentional time delay is
introduced to slow down the relay response.
2. Time-delay—An intentional time
delay is inserted between the relay decision time and the initiation
of the trip action.
3. High-speed—A relay that operates
in less than a specified time. The specified time in present practice
is 50 milliseconds (3 cycles on a 60 Hz system).
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