UNDERSTANDING UPS OPERATION
Power Xpert
9395 UPS (450–550 kVA) Installation and Operation Manual
164201701 Rev 3
www.eaton.com/powerquality
6-5
6.2.4
Variable Module Management System
NOTE
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 in
order to force the remaining UPMs to carry a higher load. The efficiency rating for
each UPM is highest when loads are greater than 50% of the system rating.
Therefore, with multiple UPMs, a UPS can achieve higher efficiencies for lighter
loads.
When a UPM is placed into VMMS mode, the UPM will stop gating the inverter and
rectifier and keep the output contactor closed in order to maintain voltage match and
phase 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 (through
the rectifier diodes) and also to allow the UPM to immediately return to active mode
in 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. The
default setting is 80% of the UPM full power rating. If the limit is exceeded, additional
UPMs take on the increased load. The VMMS redundancy setting sets the number of
redundant UPMs that the customer requires. Both the VMMS UPM load limit and
UPM redundancy are customer configurable.
VMMS's UPM automatic scheduling rotates the idle UPMs every month with the
ABM cycle. This allows the UPMs to have equal load-share time and also conducts a
periodic UPM readiness test. The automatic ABM scheduler selects the next UPM in
turn after each charge-to-float transition. If ABM is disabled, the scheduler selects the
next UPM after conducting the automatic battery test. If ABM and automatic battery
tests are disabled, the scheduler selects the next UPM at the beginning of each
month.
In High Alert mode, the UPS defaults to double-conversion Online mode and all UPMs
go active for one hour (customer configurable). At the completion of the hour, the
UPS defaults back to VMMS mode. If the High Alert command is received during the
one hour, the one hour timer will be restarted.
