DSLC Digital Synchronizer and Load Control
Manual 02007
68
Woodward
Droop is expressed as the percentage reduction in speed that occurs when the
generator is fully loaded. With a given droop setting, a generator set will always
produce the same power output at a particular speed or frequency. Droop
sometimes is called the percent speed regulation.
If all generator sets in a droop system have the same droop setting, they will
each share load proportionally. The amount of load will depend on their speed
settings. If the system load changes, the system frequency will also change. A
change in speed setting will then be required to offset the change in feedback
and return the system to its original speed or frequency. In order for each
generator set in the system to maintain the same proportion of the shared load,
each generator will require the same change in speed setting.
Figure 5-1. Droop Mode
Isochronous
Isochronous means repeating at a single rate or having a fixed frequency or
period. A generating set operating in the isochronous mode will operate at the
same set frequency regardless of the load it is supplying up to the full load
capability of the generator set, as illustrated in Figure 5-2. This mode can be
used on one generator set running by itself in an isolated system.
The isochronous mode can also be used on a generator set connected in parallel
with other generator sets. Unless the governors are load sharing and speed
controls, however, no more than one of the generator sets operating in parallel
can be in the isochronous mode. If two generator sets operating in the
isochronous mode without load sharing controls are tied together to the same
load, one of the units will try to carry the entire load and the other will shed all of
its load. In order to share load with other units, some additional means must be
used to keep each generator set from either trying to take all the load or from
motorizing.