Understanding UPS Operation6-6 Eaton ® Power Xpert® 9395P-300 (300kVA, 300kW) UPS Installation and Operation Manual P-164000563—Rev 04 www.eaton.com/powerqualityFigure 6-3. Path of Current Through the UPS in Bypass ModeIf the UPS transfers to Bypass mode from Normal mode due to any reason other than operator intervention,the UPS automatically attempts to transfer back to Normal mode (up to three times within a ten minute period).The fourth transfer locks the critical load to the bypass source and requires operator intervention to transfer.6.2.6 Battery ModeThe UPS automatically transfers to Battery mode if a utility power outage occurs, or if the utility power doesnot conform to specified parameters. In Battery mode, the battery provides emergency DC power that theinverter converts to AC power.Figure 6-4 shows the path of electrical power through the UPS system when operating in Battery mode.During a utility power failure, the rectifier no longer has an AC utility source from which to supply the DC outputcurrent required to support the inverter. The input contactor K1 opens and the battery instantaneously suppliesenergy to the battery converter. The converter boosts the voltage so that the inverter can support thecustomer's load without interruption. If bypass is common with the rectifier input, the backfeed protectioncontactor K5 also opens. The opening of contactors K1 and K5 prevent system voltages from bleedingbackwards through the static 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 for normal operation, thebattery continues discharging until a DC voltage level is reached where the inverter output can no longersupport the connected loads. When this event occurs, the UPS issues another set of audible and visual alarmsindicating SHUTDOWN IMMINENT. Unless the rectifier has a valid AC input soon, the battery continuesdischarging until a DC voltage level is reached where the inverter output can no longer support the sharedloads. If the bypass source is available, the UPS transfers to bypass instead of shutting down.StaticSwitch K5retrevnIreifitceRK1 K3BatteryConverterBatteryBatteryBreakerMain Power FlowTrickle CurrentEnergizedDe-EnergizedClosedOpenBreakers ContactorsK2Bypass InputRectifierInput OutputInputBreaker(CB1NOTE On a UPS configured as an IOM, the bypass input, static switch, bypass breaker (CB4), and K5 are not present.NOTE On a UPS configured with a Continuous Static Switch (CSS), bypass breaker (CB4) is not present.
