UNDERSTANDING UPS OPERATIONEATON Eaton 9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716 Rev 2 www.eaton.com/powerquality6-66.2.6 Battery ModeThe UPS automatically transfers to Battery mode 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 6‐4 shows the path of electrical power through the UPS system whenoperating in Battery mode.StaticSwitch K5Rectifier InverterK1 K3BatteryConverterBatteryBatteryBreakerMain Power FlowTrickle CurrentEnergizedDe-EnergizedClosedOpenBreakers ContactorsK2InputBreaker(CB1Bypass InputRectifierInput OutputFigure 6‐4. Path of Current Through the UPS in Battery ModeDuring a utility power failure, the rectifier no longer has an AC utility source fromwhich to supply the DC output current required to support the inverter. The inputcontactor K1 opens and the battery instantaneously supplies energy to the batteryconverter. The converter either bucks or boosts the voltage so that the inverter cansupport the customer's load without interruption. If bypass is common with therectifier input, the backfeed protection contactor K5 also opens. The opening ofcontactors K1 and K5 prevent system voltages from bleeding backwards through thestatic switch and rectifier snubber components and re-entering the input source.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 connected loads. When thisevent occurs, the UPS issues another set of audible and visual alarms indicatingSHUTDOWN IMMINENT. Unless the rectifier has a valid AC input soon, the outputcan be supported for only two minutes before the output of the system shuts down.If the bypass source is available, the UPS transfers to bypass instead of shuttingdown.If at any time during the battery discharge the input power becomes available again,contactors K1 and K5 close and the rectifier begins to supply DC current to theconverter and inverter. At this point, the unit returns to Online mode. Depending onthe total load and the duration of the battery discharge, battery current limit alarmsmay be seen for a short time due to the current required to recharge the battery.
