A special type of relay is one which monitors the current,
voltage, frequency, or any other type of electric power measurement either
from a generating source or to a load for the purpose of triggering a
circuit breaker to open in the event of an abnormal condition. These relays
are referred to in the electrical power industry as protective relays.
The circuit breakers which are used to switch large quantities of
electric power on and off are actually electromechanical relays, themselves.
Unlike the circuit breakers found in residential and commercial use which
determine when to trip (open) by means of a bimetallic strip inside that
bends when it gets too hot from overcurrent, large industrial circuit
breakers must be "told" by an external device when to open. Such breakers
have two electromagnetic coils inside: one to close the breaker contacts and
one to open them. The "trip" coil can be energized by one or more protective
relays, as well as by hand switches, connected to switch 125 Volt DC power.
DC power is used because it allows for a battery bank to supply close/trip
power to the breaker control circuits in the event of a complete (AC) power
failure.
Protective relays can monitor large AC currents by means of current
transformers (CT's), which encircle the current-carrying conductors exiting
a large circuit breaker, transformer, generator, or other device. Current
transformers step down the monitored current to a secondary (output) range
of 0 to 5 amps AC to power the protective relay. The current relay uses this
0-5 amp signal to power its internal mechanism, closing a contact to switch
125 Volt DC power to the breaker's trip coil if the monitored current
becomes excessive.
Likewise, (protective) voltage relays can monitor high AC voltages by
means of voltage, or potential, transformers (PT's) which step down the
monitored voltage to a secondary range of 0 to 120 Volts AC, typically. Like
(protective) current relays, this voltage signal powers the internal
mechanism of the relay, closing a contact to switch 125 Volt DC power to the
breaker's trip coil is the monitored voltage becomes excessive.
There are many types of protective relays, some with highly specialized
functions. Not all monitor voltage or current, either. They all, however,
share the common feature of outputting a contact closure signal which can be
used to switch power to a breaker trip coil, close coil, or operator alarm
panel. Most protective relay functions have been categorized into an ANSI
standard number code. Here are a few examples from that code list:
ANSI protective relay designation numbers
12 = |
Overspeed |
24 = |
Overexcitation |
25 = |
Syncrocheck |
27 = |
Bus/Line undervoltage |
32 = |
Reverse power (anti-motoring) |
38 = |
Stator overtemp (RTD) |
39 = |
Bearing vibration |
40 = |
Loss of excitation |
46 = |
Negative sequence undercurrent (phase current
imbalance) |
47 = |
Negative sequence undervoltage (phase voltage
imbalance) |
49 = |
Bearing overtemp (RTD) |
50 = |
Instantaneous overcurrent |
51 = |
Time overcurrent |
51V = |
Time overcurrent -- voltage restrained |
55 = |
Power factor |
59 = |
Bus overvoltage |
60FL = |
Voltage transformer fuse failure |
67 = |
Phase/Ground directional current |
79 = |
Autoreclose |
81 = |
Bus over/underfrequency |
REVIEW: Large electric circuit breakers do not contain within themselves the
necessary mechanisms to automatically trip (open) in the event of
overcurrent conditions. They must be "told" to trip by external devices.
Protective relays are devices built to automatically trigger
the actuation coils of large electric circuit breakers under certain
conditions.
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