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TM 5-685/NAVFAC MO-912
counter depends on the frequency of the generator
stalled to protect generator stator windings from
output voltage. An electronic system also records
internal shorts and overheating are sensitive to
operating time on a digital measuring device. This
faults in the generator and do not respond to faults
system measures time by counting the number of
outside the generator. These relays act rapidly to
cycles produced by the frequency of the generator
prevent damage to the generator and isolate the
output voltage. Counter indications are propor-
generator from the system. Relay action includes
tional to frequency vs time.
de-energizing the generator field winding. Protec-
tive relays are provided in systems when reverse
5-8. Relays.
power flow occurs. Those relays operate on a succes-
Relays are used with the automatic controls for aux-
sion of power reversals and current impulses to
iliary power generating systems. A relay responds to
detect loss of synchronism. Protective relays include
electrical or other operating parameters and causes
the following types:
an abrupt change in the control circuits when the
(a) Overcurrent. Overcurrent relays function
measured values change. A relay consists of a sens-
when current flow exceeds the normal or desired
ing element and a control element with contacts.
value. Induction disk relays with time delay and
a. Types of relays. Relays used in switchgear in-
cup type relays (without time delay) are known as
clude general purpose and protective types.
electromechanical type relays. Solid state relays are
(1) General purpose. General purpose relays
normally used on more recently installed equip-
function as part of regulation and verification de-
vices throughout the system including the prime
(b) Overvoltage. Overvoltage relays function
when voltage exceeds the normal or desired value.
(a) Industrial. Portions of electrical systems
Induction disk relays with time delay and cup type
are energized or de-energized under normal or ab-
relays without time delay are used.
normal conditions by relays. Since the relays are
(c) Undervoltage. Undervoltage relays func-
usually used with subsystems or equipment circuit
tion when voltage is less than normal or desired
breakers, the overall operating plan must be electri-
value. Induction disk relays with time delay may be
cally coordinated. Coordination is usually accom-
used in a balanced position between minimum and
plished by designing the system circuitry to selec-
maximum voltages.
tively initiate the opening or closing of the relays.
(d) Reverse power. Reverse power relays
Relays constantly monitor the power system.
function whenever power flows in the reverse direc-
(b) Overload. 0 verload relays are used to
tion from normal or desired. These relays detect loss
provide overload protection for the auxiliary motors.
of synchronism.
When an overload condition occurs in any of the
(e) Underfrequency. Underfrequency relays
three phases in which heaters are inserted, it will
function whenever the desired frequency becomes
cause the relay to trip.
less than normal value. This condition is usually
(c) Time delay. Relays employed for time de-
the result of reduced prime mover speed and may be
lay purpose are usually solid-state type. Some pneu-
caused by the prime mover governor or excess elec-
matic relays may still be in use. Pneumatic relays
trical load.
utilize a bellows type arrangement to provide the
(f) Differential. Differential relays function
time delay. They can be adjusted for time periods of
due to the difference between two quantities of the
less than a second to several minutes.
same kind such as, two currents or two voltages.
(d) Solid-state. Solid-state relays derive their
Differential relays, usually used to detect stator
time delay from a combination of several electronic
winding electrical failure, respond to current per-
components. They are also adjustable between frac-
centage differences. Current or voltage transform-
tions of a second to several minutes.
ers used in differential network should be in
(e) Voltage sensitive. Voltage sensitive relays
matched sets. Percentage differential relays are
are used to sense an increase or decrease in a spe-
also used to prevent relay operation for faults due to
cific voltage. They provide an output signal when
current transformer ratio error outside the pro-
the voltages pass the preset level.
tected zone. In this application, the overcurrent re-
(2) Protective relays. Protective relays detect,
lay operates instantly when there is a bus short
isolate, and/or indicate abnormal electrical condi-
circuit but will not operate if a current transformer
tions. The operation of circuit breakers or other
secondary opens. The contacts of the two relays are
protective devices is initiated by relays as required.
connected in series.
Some of the electrical hazards protected against are
(g) Current balance. A current balance relay
short circuit, overcurrent, over or under voltage,
circuit monitors two or more current circuits and
and phase or frequency irregularities. Relays in-
provides an output if the difference between any

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