UNDERSTANDING UPS OPERATIONEaton 9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716 Rev 2 www.eaton.com/powerquality 6-176.4.4 Battery Mode – Distributed BypassThe UPSs transfer to Battery mode automatically if a utility power outage occurs, or ifthe utility power does not conform to specified parameters. In Battery mode, thebattery provides emergency DC power that the inverter converts to AC power.Figure 6‐12 shows the path of electrical power through the distributed bypass systemwhen operating in Battery mode.While in Battery mode, the UPSs sound an audible horn, illuminate visual indicatorlamps on the front panel (Online and On Battery), and create an entry into the alarmevent history. As the battery discharges, the boost converter and inverter constantlymake minute adjustments maintaining a steady output. The UPSs remain in thisoperating mode until the input power to the rectifier is again within the specifiedvoltage or frequency acceptance windows.If the input power fails to return or is not within the acceptance windows required fornormal operation, the battery continues discharging until a DC voltage level is reachedwhere the inverter output can no longer support the shared loads. When this eventoccurs, each UPS issues another set of audible and visual alarms that indicate atwo-minute SHUTDOWN IMMINENT warning. Unless the system has a valid ACinput soon, redundant UPMs begin shutting down until there are no longer enoughUPMs online to support the connected load. When this event occurs, the systemshuts down. If the bypass source is available, the system transfers to bypass insteadof shutting down.Main Power FlowUPS 1UPS 2UPS 3UPS 4Output toCritical LoadBatteryUPS Input Tie CabinetUPS 1 OutputBypass InputUPS 2 OutputUPS 3 OutputUPS 4 OutputBatteryBatteryBatteryClosedOpenBreakersFigure 6‐12. Path of Current through the UPSs in Battery Mode – Distributed Bypass
