UNDERSTANDING UPS OPERATIONEATON Eaton 9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716 Rev 2 www.eaton.com/powerquality6-4If commercial power voltage or frequency disturbances are detected, the systemforward transfers to Battery mode for minor disturbances and then todouble-conversion mode for more severe disturbances. During a complete poweroutage, the transfer takes place within 2 ms to ensure loads being fed by the systemoutput are not interrupted. All forward transfers transfer to Battery mode beforereturning to double-conversion mode.In High Alert mode the unit transfers from ESS mode to double-conversion mode or ifin double-conversion mode remains in double-conversion mode for a default timeperiod of one hour (customer configurable). At the completion of the time period, theunit defaults back to ESS mode. If the High Alert command is received during thetime period, the timer will be restarted.ESS mode is a normal operating mode, and not an alarm condition. While the UPS isin this mode, the Online light on the front display will illuminate.6.2.4 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.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.6.2.5 Bypass ModeThe UPS automatically switches to Bypass mode if it detects an overload, load fault,or internal failure. The UPS can also be transfered from Online mode to Bypass modemanually. The bypass source supplies the commercial AC power to the load directly.Figure 6‐3 shows the path of electrical power through the UPS system whenoperating in Bypass mode.C A U T I O NThe critical load is not protected from voltage or frequency fluctuations or power outages while the UPS is inBypass mode.
