are connected so an unbalance because of uneven
mode, the motor automatically drives to and
loads causes an input to each governor amplifier.
remains at 60 Hz. This position is established by
This forces proportional fuel adjustments until
the motor's limit switches. External adjustments
the loads are balanced between the two units. This
to the governor system are done by additional
also balances the bridge circuits. The amplifier
inputs to the amplifier. These inputs come from
either the master frequency trimmer (2) or the
input is again returned to algebraic zero volts dc.
APD (3). The master frequency trimmer in the
Sudden shifts in load demand cause pulses to be
developed in the load sensor. This upsets the
EPCC provides frequency control to any two or
algebraic zero voltage of the governor amplifier.
all three generators when operating in parallel on
This results in quicker response to load changes.
either the model 104 or the model 139 GTGS. You
may use the EPCC panel SYSTEM FREQ
Polarity of the pulse is also sensed to determine
the direction of load changes.
RAISE/LOWER switch to demand a change of
During droop mode some of the load sensor
frequency for the paralleled units. This control
output opposes the action of the amplifier speed
inputs 115 volts ac into a reversible motorized
reference. The input to the amplifier will be
potentiometer assembly. The potentiometer
decreased by an amount proportional to load,
output is a dc signal. Its amplitude is proportional
to the correction demanded in the generator
resulting in droop.
If the generator is not paralleled with another
output frequency. The polarity dictates the
source, this droop will result in a decrease in
direction of change.
frequency. The decrease is proportional to the
The summing amplifier (4) provides the
current to the fuel control actuator (5). This
increase in load. If the generator is paralleled with
an infinite bus (such as shore power), droop
current is varied in response to the inputs to the
provides load control. When paralleled with an
amplifier. This includes the reference, frequency
infinite bus, the speed of the engine is held
feedback, and load sensing. Input changes
constant by the bus. The governor system, in this
because of load, speed, or reference cause the
amplifier current to reposition the actuator output
situation, cannot control speed. Any attempt to
increase or decrease speed will only result in an
shaft. This increases or decreases fuel flow.
increase or decrease in load. Without the droop
Amplifier current then stabilizes at a new setting
characteristics, the governor system would
that satisfies all inputs. The amplifier is reverse
attempt to adjust the frequency to satisfy the
acting. That is, the larger the input (error signal),
reference exactly causing the load to increase
the smaller the output current to the actuator. The
beyond generator capacity or decrease until the
actuator output shaft is designed to work so a
flow of power reverses. The droop input,
decrease in current causes it to drive the LFV
however, will modify the speed reference. The
toward the maximum fuel position. If the
governor will reach a stable operating point even
amplifier fails and the current goes to zero, the
though the frequency does not match the re-
actuator will be positioned in the maximum fuel
ference. This operating point is set by the speed
position. (The centrifugal governor assumes
reference and droop input (since frequency is
control if engine speed increases to the preset
constant). It determines the load on the generator.
limit.)
Under this condition, the load on the generator
The PMA input to the control unit provides
will remain constant for any reference setting.
voltage for the two power supplies (6) and a
frequency feedback signal to the frequency sensor
Electrohydraulic Governor Actuator
(7). One power supply feeds the amplifier; the
second provides power for the motor-operated
The EG actuator is engine driven and is
potentiometer. The frequency sensor converts the
mounted on the left side of the AGB. The
PMA output (8) (about 120 volts ac at 420 Hz)
to a proportional dc voltage. This is used for the
actuators output shaft is mechanically linked to
the LFV. The actuator receives signals from the
frequency feedback input to the amplifier.
GCU and positions the LFV, which, in turn,
The load sensor module (9) controls load
meters fuel flow to the engine. The actuator
sharing in parallel isochronous operation. It is
incorporates normal (electronic) control by the
used to generate the droop characteristics during
GCU and backup (mechanical) control by a
droop operation. Power generated by the
centrifugal governor. Both systems are indepen-
generator is measured by transformers. They
dently capable of positioning the actuator output
supply voltage to a bridge circuit. For load
shaft.
sharing, the bridges of each paralleled generator
3-34
