appendix

APPENDIX A
Figure A–1. GenSize–New Project Parameters Dialog Box
Rev. Jan 2011
Application Manual – Liquid Cooled Generator Sets
Minimum Genset Load/Capacity
Running a generator set under light load can lead to engine damage and reduced
generator set reliability. Cummins Power Generation does not recommend running
generator sets at less than 30 percent of rated load –– this is the default setting in
GenSize. Load banks should be used to supplement the regular loads when loading falls
below the recommended value. A generator set should not run at less than 10 percent of
rated load for any extended period.
Maximum Voltage Dip (Starting and Peak)
As you reduce the maximum allowable voltage dip during initial startup, or when loads
cycle under automatic controls or have high peak surges, the size of the recommended
generator set increases. Choosing a lower allowable voltage dip results in a larger
recommended generator set. However, setting allowable voltage dips of more than 40
percent can lead to relay and contactor malfunctions. The default Maximum Voltage Dip
in GenSize is 35 percent.
Maximum Frequency Dip
As you reduce the maximum allowable frequency dip, you increase the size of the
recommended generator set. Since a generator set is a limited power source (compared
to a utility), voltage and frequency excursions will occur during transient loading events.
The generator set must be sized to limit these excursions to an appropriate level for
proper load performance. The default Maximum Frequency Dip in GenSize is 10 percent.
This number may have to be set lower when supplying frequency–sensitive loads, such
as UPS systems. Check with the UPS manufacturer for information on the UPS system’s
sensitivity to frequency excursions when operating on a standby generator set.
A–5