The manual SSGTG start controls at the EPCC are
on-line SSGTGs to share the reactive load equally at a
the ON, HIGH PRESS, and BLEED pushbuttons. When
selected output voltage. The DROOP mode is used for
ON is depressed, it is combined with the HIGH PRESS
operating generators on an isolated bus. It is basically
or BLEED indication signal in the EPCC input/output
the DIFF mode without cross-current compensation.
multiplexer hardware. The start air push buttons provide
only a hardware logic signal within the EPCC and do
Load Shedding System Operation
n o t cause the start air piping valves to realign.
Depending upon which start air indicator is illuminated,
A load shedding operation can be started manually
the EPCC will send a high-pressure start or bleed air
or automatically. Manual load shedding is started by
start signal to the LOCOP.
operating the appropriate INIT push-button indicator at
the EPCC or the spring-loaded LOAD SHED switch at
A n SSGTG stop is always available from its
the switchboard. Automatic load shedding is started
LOCOP, its associated switchboard, and the EPCC. The
when an overpower relay in a switchboard or an
EPCC OFF push-button switch sends a signal to the
overcurrent relay on a shore power breaker is energized.
LOCOP, which initiates a normal SSGTG stop and
opens the associated generator circuit breaker. An
When one overpower relay is energized, its contacts
SSGTG will also automatically stop if certain abnormal
o p e r a t e tripping relays in each switchboard. The
conditions occur (slow start, module fire, and so forth).
tripping relays complete power circuits to the trip coils
on selected circuit breakers. When a shore power
The output frequency of each SSGTG is controlled
overcurrent relay is energized, load shed stage 1 and
by an electronic governor in the LOCOP. The electronic
stage 2 relays in switchboard No. 2 are also energized.
governor regulates frequency by adjusting fuel flow to
These relays complete power circuits in the
the gas turbine. When the governor control is at the
switchboards to trip circuit breakers in the load shed
EPCC, frequency adjustments can only be made from
system. All circuit breakers opened during a load shed
t h e governor mode RAISE/LOWER switch at the
operation must be closed locally. An exception is the
EPCC. RAISE increases fuel flow to the gas turbine,
air-conditioning plant circuit breakers that can be closed
while LOWER reduces the fuel flow to the engine.
remotely from the EPCC. The tripped circuit breakers
There are two modes of governor operation:
cannot be closed until any overpower or overcurrent
isochronous (ISO) and DROOP. The ISO mode is a
condition clears.
constant frequency mode where fuel flow is regulated
to maintain a constant generator output frequency. The
Air-Conditioning Plant Operation
DROOP mode is a constant fuel flow mode where
increasing or decreasing the electrical load will cause
The A/C plant normal and alternate supply circuit
frequency to decrease or increase, respectively. The
breakers can be closed from the EPCC. All four A/C
mode is selected by depressing either the ISO or
plants have a remote restart capability to bring a plant
DROOP push button at the EPCC. When on shore
that was operating before a power interruption back on
p o w e r , DROOP is automatically selected by the
line. Each A/C plant can be reconnected to the main bus
switchboard.
and restarted from the EPCC after a stage 2 load shed
or an unexpected power loss.
Generator Control
After a stage 2 load shed, all the primary and
alternate sources of power to each A/C plant will have
Generator field excitation is controlled by one of
been tripped. Depressing the POWER RESTORE push
t w o automatic voltage regulators in the EXCOP.
button at the EPCC closes the primary and alternate A/C
Generator output voltage can be controlled manually or
plant circuit breakers. All eight breakers will close,
automatically. One manual mode and two automatic
regardless of switchboard LOCAL CONTROL alarm
modes are available at the switchboard and at the EPCC.
status, to energize each A/C plant controller.
In the MANUAL mode, excitation current from the
voltage regulator is set by a motor-driven manual
voltage adjust circuit in the EXCOP. The two automatic
400-Hz Power System Operation
modes are differential (DIFF) and DROOP. In these
modes, the voltage regulator sets the generator output
The 400-Hz power system consists of two
voltage to a level determined by the setting of a
60/400-Hz static converters that feed separate 400-Hz
motor-operated potentiometer in the EXCOP. The DIFF
switchboards. The EPCC on the DDG-51 class ships
mode is used during parallel operations to allow the
neither controls nor monitors any part of the 400-Hz
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