UNDERSTANDING UPS OPERATIONEaton 9390 UPS (100–160 kVA) Installation and Operation Manual 164201604 Rev E www.eaton.com/powerquality 7-97.2.1 Multiple Module Parallel System ModesSimilar to the single module system, the Eaton 9390 UPS parallel system supports acritical load in three different modes of operation. The standard operation modes are: In Normal mode, the paralleled UPMs supply the critical load with clean, filteredpower. Each UPM battery charger also provides charging current for the battery ifneeded. In Battery mode, the battery provides DC power, which maintains UPM operation.The UPM batteries support the critical load. In Bypass mode, the critical load is directly supported by utility power through thecontinuous-duty static bypass switch in each UPM.The UPMs continually monitor themselves and the incoming utility power, andautomatically switch between these modes as required, without operatorintervention, except when manually switching to Bypass mode. The sophisticateddetection and switching logic inside the UPMs ensures that operating mode changesare automatic and transparent to the critical load, while internal monitoring systemsindicate the current mode of operation. The UPMs switch operating modes inresponse to these system events: A command is an intervention that is externally initiated by an operator or by somesite action. A command causes the UPMs to switch operating modes; it usuallydoes not require any further action. A notice is a minor system event that may or may not require attention. An alarm is a system event that requires immediate intervention.System events, alarm horns, and indicator lights are described in Chapter 11,“Responding to System Events.”To assist in developing an understanding of the parallel system modes of operation,review and understand thoroughly the single module modes of operation.The following paragraphs describe the differences in the three parallel systemoperating modes, using block diagrams to show the power flow during each mode ofoperation.
