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TM 5-685/NAVFAC MO-912
6-5. Paralleled with the electric utility system.
tive load (vars) can be controlled by adjusting the
voltage control. To remove load from the generator,
a. Stations that can be paralleled with electric
reduce the load by decreasing the governor control
utility systems have the same basic characteristics
while observing the kW meter for the generator.
as those discussed in paragraph 6-2. They have
When the kW meter indicates zero, open the gen-
additional features including synchronizing cir-
erator circuit breaker. The load will now be trans-
cuitry and, in the case of the unattended station, an
ferred to the electric utility bus. The prime mover
automatic mode.
can then be shut down by following normal proce-
b. The prime movers in these stations can be
operated in the manual or automatic mode. Opera-
tion in the manual mode is discussed in paragraph
6-6. Paralleled with other generating units.
6 - 2 a . In the automatic mode, relays in the
switchgear will sense the loss of commercial power
a. Stations that have two or more generators that
and provide a starting signal to the prime mover. It
can be operated in parallel have the same basic
will then accelerate under control of the governor to
characteristics as previously discussed in para-
the operating speed. The remainder of the prime
graphs 6-2 and 6-3. In addition, they may have
mover operation is as previously discussed.
automatic synchronizing circuitry and also droop
c. This discussion is the same as paragraph 6-4d
circuits for the voltage regulators. The automatic
with the exception that if the station is in auto-
synchronizing circuitry includes speed-matching re-
matic, excitation will be applied by the automatic
lays, voltage-matching relays, and automatic syn-
circuitry. Also, in automatic the main circuit
chronizing relays. These relays function when the
breaker will close automatically, provided the in-
station is in the automatic mode and when two or
coming line circuit breaker is open.
more AC sources are in agreement within specified
d. The comments regarding proper switchgear
limits of phase angle and frequency. The voltage
operation as noted in paragraph 6-4h pertain to
regulator droop circuits are energized when two or
paralleling with the utility system. In addition, this
more generators are operated in parallel. Their pur-
type of system requires paralleling circuitry which
pose is to prevent the undesirable condition of cir-
is part of the switchgear. It includes one or more
culating currents between generators.
synchronizing switches, a synchroscope, synchroniz-
b. Parallel operation of generators with regula-
ing lights, incoming voltmeter, incoming frequency
tors is accomplished by appropriate cross-current
meter, running voltmeter, and a running frequency
compensation. The method employs an equalizing
meter. The synchronizing circuitry is energized by
reactor or compensator which adds a small voltage,
turning the synchronizing switch on.
proportional to the reactive current delivered by the
e. The synchroscope indicates the condition of the
generator, to the voltage delivered by the potential
incoming machine with respect to the bus. If the
frequency of the incoming machine is higher than
transformers. This gives a slight droop to the volt-
that of the bus, the synchroscope pointer will re-
age held by the regulator on reactive loads and
volve in a clockwise or "fast" direction. The operator
divides reactive currents in proportion to load cur-
should adjust the governor control of the incoming
rents. Differential compensation is used when line-
prime mover until the synchroscope pointer is re-
droop compensators are installed to automatically
volving slowly in the "fast" direction. The machines
increase the voltage as the load increases. With this
should be paralleled by closing the breaker of the
connection, all the equalizing reactors or compensa-
incoming generator when the pointer reaches 12
tors are connected in series. There is no current flow
o'clock. Because there is a slight lag in the breaker
in the equalizing reactors under balanced load con-
or switching mechanism, it is good practice to start
ditions. If the load is unbalanced, the currents flow
the breaker closing operation at about the 11:30
through the regulators to decrease the excitation of
position or slightly before the pointer reaches 12
the generator carrying excessive reactive currents.
This increases the excitation of the generator carry-
f.  Synchronizing lamps provide a means of check-
ing low reactive current.
ing the synchroscope for proper operation. As the
6-7. Operational testing.
pointer revolves, the lamps go alternately bright
and dark in unison. Both lamps must be dark as the
a. Emergency generator power units must be op-
pointer passes 12 o'clock or the synchroscope is de-
erated under load conditions periodically to insure
their reliability. The period for this exercising will
g. Now that the generator is paralleled with the
vary from station to station. It is important that
electric utility system, the load (kW) can be con-
accurate logs be kept of the conditions encountered
trolled by adjusting the governor control. The reac-
during the exercising.

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