UNDERSTANDING UPS OPERATIONEATON Eaton 9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716 Rev 2 www.eaton.com/powerquality6-14 In Bypass mode, the critical load is directly supported by utility power through thebypass circuit in each UPS. In Battery mode, the battery provides DC power, which maintains UPM operation.The UPM batteries support the critical load.The UPSs 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 UPSs ensures that operating mode changesare automatic and transparent to the critical load, while internal monitoring systemsindicate the current mode of operation. The UPSs 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 UPSs 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.The following paragraphs describe the differences in the parallel system operatingmodes, using block diagrams to show the power flow during each mode of operation.6.4.2 Online Mode – Distributed BypassIn Online mode, utility AC power is supplied to the UPSs. Each UPS then conditionsthe incoming AC power and provides clean, regulated AC power to the tie cabinet ordistribution panel for parallel systems up to four modules. The applied load is sharedequally among the available UPMs in the system.Figure 6‐10 shows the path of electrical power through the parallel system whenoperating in Online mode.If the utility AC power is interrupted or is out of specification, the UPSs automaticallyswitch to Battery mode to support the critical load without interruption. When utilitypower returns, the UPSs return to Online mode.If the UPSs become overloaded or unavailable, the distributed bypass systemswitches to Bypass mode. The distributed bypass system automatically returns toOnline mode when the overload condition is cleared and system operation is restoredwithin specified limits.
