UNDERSTANDING UPS OPERATIONEaton 9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716 Rev 2 www.eaton.com/powerquality 6-136.4 Multiple UPS Distributed Bypass SystemDistributed bypass parallel operation extends the normal operation of Eaton9395UPSs by offering increased capacity and/or redundant capability. The parallel systemcontinues to maintain power to the critical loads during commercial electrical powerbrownout, blackout, overvoltage, undervoltage, and out‐of‐tolerance frequencyconditions.The output of the system is normally supplied by Uninterruptible Power Modules(UPMs) contained in each UPS. Multiple UPMs are connected with their outputs inparallel (tied together) to provide a load level greater than the rating of one UPMand/or for redundancy. The paralleled UPMs supply the output load with protectedpower as long as the load does not exceed the combined rating of the paralleledUPMs.The power system is redundant as long as one of the UPMs can be disconnectedfrom the output bus and the remaining UPMs can continue to supply power to theload without exceeding their ratings.When the load is being supplied by the UPMs, the system output bus is continuouslymonitored for an overvoltage or undervoltage condition. If an out of limits condition isdetected, the paralleled UPMs transfer the load to bypass using the UPS staticswitches.Communication is required between the UPSs for system metering and mode control.System level communication and control are accomplished using a Controller AreaNetwork (CAN). A single building alarm in each UPS, connected to the other UPSs inparallel and tied to the bypass contactor auxiliary contacts in each UPS, are used for asecondary communication path. This arrangement ensures bypass control even if theCAN bus is lost.The system is paralleled for redundancy (1+1) in a system where two UPMs areparalleled together and the load is less than the supporting capability of one of theUPMs. The system is paralleled for capacity (2+0) if both UPMs in a system arerequired to support the load.A parallel capacity system can also be redundant (2+1), as long as there is always oneor more UPMs online than required to support the load.6.4.1 Multiple UPS Parallel System ModesSimilar to the single UPS system, the Eaton9395 UPS parallel system supports acritical load in five different modes of operation. The standard operation modes are:NOTE The Variable Module Management System and Energy Saver System modes are mutually exclusive. In Online mode, the paralleled UPMs supply the critical load with clean, filteredpower. Each UPM battery charger also provides charging current for the battery ifneeded. In ESS mode, commercial AC power is supplied directly to the critical load throughthe continuous static switch and transfers automatically to Online mode if anabnormal condition is detected. ESS mode in a parallel system operates the sameas it does in a single UPS system (see paragraph 6.2.3). In VMMS mode, the UPS operates as a traditional double-conversion UPS, butselectively shifts the load to fewer UPMs to increase the efficiency of the UPS. Ina parallel system each UPS will require at least one UPM to be indouble-conversion mode at all times; otherwise, VMMS mode operates the sameas it does in a single UPS system (see paragraph 6.2.4).
