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A
protective relay is a complex electromechanical
apparatus, often with more than one coil, designed
to calculate operating conditions on an electrical
circuit and trip circuit breakers when a fault was
found. Unlike switching type relays with fixed and
usually ill-defined operating voltage thresholds and
operating times, protective relays had
well-established, selectable, time/current (or other
operating parameter) curves. Such relays were very
elaborate, using arrays of induction disks,
shaded-pole magnets, operating and restraint coils,
solenoid-type operators, telephone-relay style
contacts, and phase-shifting networks to allow the
relay to respond to such conditions as over-current,
over-voltage, reverse power flow, over- and under-
frequency, and even distance relays that would trip
for faults up to a certain distance away from a
substation but not beyond that point. An important
transmission line or generator unit would have had
cubicles dedicated to protection, with a score of
individual electromechanical devices. The various
protective functions available on a given relay are
denoted by standard ANSI Device Numbers. For
example, a relay including function 51 would be a
timed overcurrent protective relay.
These protective relays provide various types of
electrical protection by detecting abnormal
conditions and isolating them from the rest of the
electrical system by circuit breaker operation. Such
relays may be located at the service entrance or at
major load centers.
Design and theory of these protective devices is an
important part of the education of an electrical
engineer who specializes in power systems. Today
these devices are nearly entirely replaced (in new
designs) with microprocessor-based instruments
(numerical relays) that emulate their
electromechanical ancestors with great precision and
convenience in application. By combining several
functions in one case, numerical relays also save
capital cost and maintenance cost over
electromechanical relays. However, due to their very
long life span, tens of thousands of these "silent
sentinels" are still protecting transmission lines
and electrical apparatus all over the world.
found in medium to large gas turbine engines, in
natural gas pumping stations, and within certain
chemical plants. |
