LOAD SHED switch at the switchboard. An operator
stop and opens the associated generator circuit breaker.
can start automatic load shedding by energizing either
An SSGTG will also automatically stop if certain
an overpower relay in a switchboard or an overcurrent
abnormal conditions, such as slow start, module fire,
relay on a shore power breaker.
and so forth, occur.
When one overpower relay is energized, its contacts
The output frequency of each SSGTG is controlled
operate tripping relays in each switchboard. The
by an electronic governor in the LOCCP. The electronic
tripping relays complete the power circuits to the trip
governor regulates the frequency by adjusting fuel flow
coils on selected circuit breakers. When a shore power
to the gas turbine. When the governor control is at the
overcurrent relay is energized, load shed stage 1 and
EPCC, frequency adjustments can only be made from
stage 2 relays in switchboard No. 2 are also energized.
the governor mode RAISE/LOWER switch at the
This completes the power circuits in the switchboards
EPCC. RAISE increases fuel flow to the gas turbine,
to trip the circuit breakers in the load shed system. All
while LOWER reduces the fuel flow to the engine.
circuit breakers opened during a load shed operation
There are two modes of governor operation:
must be closed locally. An exception is the
isochronous (ISO) and DROOP. The ISO mode is a
air-conditioning plant circuit breakers that can be closed
constant frequency mode where fuel flow is regulated
remotely from the EPCC. The tripped circuit breakers
DROOP mode is a constant fuel flow mode where
cannot be closed until an existing overpower or
increasing or decreasing the electrical load will cause
overcurrent condition is cleared.
the frequency to decrease or increase, respectively. At
the EPCC, the operator selects the mode by depressing
Air-Conditioning Plant Operation
either the ISPO or DROOP push button. When on shore
power, DROOP is automatically selected by the
The normal and alternate supply circuit breakers for
switchboard.
the air-conditioning (A/C) plant can be closed from the
EPCC. At the EPCC, all four A/C plants have a remote
Generator Control
restart capability to bring a plant that was operating
before a power interruption back online. Each A/C plant
Generator field excitation is controlled by one of
can be reconnected to the main bus and restarted from
two automatic voltage regulators in the EXCOP.
the EPCC after a stage 2 load shed or an unexpected
power loss.
Generator output voltage can be controlled either
manually or automatically. One manual mode and two
After a stage 2 load shed, all the primary and
automatic modes are available at both the switchboard
alternate sources of power to each A/C plant will have
and the EPCC. In the MANUAL mode, excitation
been tripped. Depressing the POWER RESTORE push
current from the voltage regulator is set by a
button at the EPCC closes the primary and alternate A/C
motor-driven manual voltage adjust circuit in the
plant circuit breakers. All eight breakers will close,
EXCOP. The two automatic modes are differential
regardless of the switchboard LOCAL CONTROL
(DIFF) and DROOP. In these modes, the voltage
alarm status, to energize each A/C plant controller.
regulator sets the general output voltage to a level
determined by the setting of a motor-operated
400-Hz Power System Operation
potentiometer in the EXCOP. The DIFF mode is used
during parallel operations to allow the on-line SSGTGs
The 400-Hz power systems consists of two
to share the reactive load equally at a selected output
60/400-Hz static converters that feed separate 400-Hz
voltage. The DROOP mode is used for operating
switchboards. On the DDG-51 class ship, the EPCC
generators on an isolated bus. The DROOP mode is
cannot control or monitor any part of the 400-Hz power
basically the DIFF mode without cross-current
system. Remote operation of the 400-Hz power system
compensation.
is accomplished in combat systems maintenance central.
The only 400-Hz monitoring done by the MCS is in the
Load Shedding System Operation
interior communication switchboards.
A load shedding operation can be started manually
Now that you have read about the monitoring and
or automatically. An operator can start manual load
control functions available at the PACC and EPCC, let's
shedding by depressing the appropriate INIT
look at some of these controls available at the
push-button indicator at the EPCC or the spring-loaded
EOOW/LU.
