UNDERSTANDING UPS OPERATIONPower Xpert 9395 UPS (450–550 kVA) Installation and Operation Manual 164201701 Rev 4 www.eaton.com/powerquality 6-56.2.4 Variable Module Management SystemNOTE VMMS and ESS modes are mutually exclusive. If VMMS mode is enabled, ESS mode is disabled.In VMMS mode, the UPS operates as a traditional double-conversion UPS. However,the UPS will selectively shift the load to fewer UPMs based on the required load inorder to force the remaining UPMs to carry a higher load. The efficiency rating foreach UPM is highest when loads are greater than 50% of the system rating.Therefore, with multiple UPMs, a UPS can achieve higher efficiencies for lighterloads.When a UPM is placed into VMMS mode, the UPM will stop gating the inverter andrectifier and keep the output contactor closed in order to maintain voltage match andphase lock to the critical bus. In this mode the UPS actively monitors the critical bus,keeping the UPM input contactors closed in order to keep the DC link primed (throughthe rectifier diodes) and also to allow the UPM to immediately return to active modein the event of a disturbance or load step on the critical bus.VMMS limits the maximum power available from a UPM when in VMMS mode. Thedefault setting is 80% of the UPM full power rating. If the limit is exceeded, additionalUPMs take on the increased load. The VMMS redundancy setting sets the number ofredundant UPMs that the customer requires. Both the VMMS UPM load limit andUPM redundancy are customer configurable.VMMS's UPM automatic scheduling rotates the idle UPMs every month with theABM cycle. This allows the UPMs to have equal load-share time and also conducts aperiodic UPM readiness test. The automatic ABM scheduler selects the next UPM inturn after each charge-to-float transition. If ABM is disabled, the scheduler selects thenext UPM after conducting the automatic battery test. If ABM and automatic batterytests are disabled, the scheduler selects the next UPM at the beginning of eachmonth.In High Alert mode, the UPS defaults to double-conversion Online mode and all UPMsgo active for one hour (customer configurable). At the completion of the hour, theUPS defaults back to VMMS mode. If the High Alert command is received during theone hour, the one hour timer will be restarted.
