GE DMC490 Software Configuration Manual page 11

DMC490 Overview
Figure 5 Isochronous operating characteristic
An isochronous machine maintains a constant frequency for any value of power up to its maximum rating.
Having more than one generator operating in isochronous control creates a challenge. Measurement error
results in each machine having a slightly different idea of the actual system frequency, and each machine tries
to bring this value to the nominal frequency. To resolve this potential conflict, one machine is typically operated
in isochronous mode and the remaining generators are operated in droop control, resulting with the machine
receiving an additional power signal that it compares to a power reference.
Figure 6 Droop control used for two or more sources of power
At nominal frequency, the level of power output is determined by the power reference command because the
speed error signal is zero. When there is a drop in frequency, the generator increases output power according
to its droop setting. For example, a generator with a droop setting of 5% produces a 100% change in output
power for a 5% change in frequency.
Taken together the power and speed signals produce a sloped operating characteristic as shown in Figure 7.
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