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Load Sharing Module Manual 02035
20 Woodward
Isochronous Load Sharing
Each comparator circuit compares the load signal voltage for its generator set to
two times the voltage on the load sharing lines and produces an error voltage
proportional to the difference. This error voltage is used to generate a pulse width
modulated signal which is output to the speed control. This output biases the
speed loop of the speed control until the load signal voltage is equal to that of
other generators on the load sharing lines.
Droop Operation
In droop operation, a portion of the load signal voltage is fed to the controller
circuit. This voltage is used by the comparator circuit to reduce the control output
by a percentage determined by the DROOP potentiometer. The output is
reduced, and the speed control reduces engine power output according to the
desired droop percentage.
When a generator set using the Load Sharing Module is paralleled in droop with
other generator sets, the common load signal on the paralleling lines is not used.
The frequency of the generator set will therefore vary with load, so it must be
determined by a different means. In an isolated system with two or more
generator sets paralleled, if isochronous speed control is required, one of the
generator sets must be running in the isochronous (constant speed) mode. This
generator set maintains the frequency of the system. If a generator set is in
droop and is paralleled with an infinite bus, the bus determines and maintains the
frequency. The DROOP percentage and the speed setting on the engine speed
control determine the amount of the load that is carried by the generator, when
running in droop.
Auxiliary Equipment
The Woodward SPM-A synchronizer functions by biasing the output of the Load
Sharing Module. All other Woodward auxiliary generating control equipment
functions by biasing the voltage on the load-sharing lines.
0.5–4.5 Vdc Output
The Load Sharing Module output to the engine control is a 0.5 to 4.5 Vdc signal.
The output nominally is at 2.5 Vdc when the difference between the generator
load and the signal on the load sharing lines is zero.
The speed control should be set up in Variable Speed Governor mode to
produce a ±10% variance in the engine speed with a ±2.0 Vdc input, as shown in
this example:
If 1800 rpm is equivalent to 60 Hz for a ±5.0% variance on the speed of the
engine, the control would be set up such that 0.5 Vdc output corresponds to 1710
rpm (VSG MIN SPD) and 4.5 Vdc output corresponds to 1890 rpm (VSG MAX
SPD).