Understanding UPS OperationEaton 93PM UPS (20–100 kW, 480V – 100 kW Frame) Installation and Operation Manual P-164000227—Rev 6 www.eaton.com/powerquality 5-5If 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.Figure 5-4. Path of Current Through the UPS in Bypass Mode5.2.5 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 5-5 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 outputvoltage required to support the inverter. The rectifier switchgear opens and the battery instantaneouslysupplies energy 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 bypass switchgear alsoopens. The opening of the rectifier and bypass switchgear prevent system voltages from bleeding backwardsthrough the static switch and rectifier snubber components to the utility 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 output can be supportedfor only two minutes before the output of the system shuts down. If the bypass source is available, the UPStransfers to bypass instead of shutting down.If at any time during the battery discharge the input power becomes available again, the rectifier and the bypassswitchgear close and the rectifier begins to supply DC current to the converter and inverter. At this point, theunit returns to Normal mode. Depending on the total load and the duration of the battery discharge, batterycurrent limit alarms may be seen for a short time due to the current required to recharge the battery.Bypass Input StaticSwitchBypass SwitchgearRectifier InverterInverterSwitchgearRectifierSwitchgearRectifier Input OutputBattery ConverterBatterySwitchgearMain Power FlowSwitchgearClosedOpenBatteryTrickle CurrentDe-EnergizedEnergized
