Understanding UPS Operation5-4 Eaton 93PM UPS (20–100 kW, 480V – 100 kW Frame) Installation and Operation Manual P-164000227—Rev 6 www.eaton.com/powerqualityFigure 5-3. Path of Current Through the UPS in Normal Mode5.2.4 Bypass ModeCAUTIONThe critical load is not protected from voltage or frequency fluctuations or power outages whilethe UPS is in Bypass mode.The UPS automatically switches to Bypass mode if it detects an overload, load fault, or internal failure (ifconfigured for capacity). If configured for redundancy (N+1) the UPS would remain on line in Normal mode ifone UPM failed. The UPS can also be transferred from Normal mode to Bypass mode manually. The bypasssource supplies the commercial AC power to the load directly. Figure 5-4 shows the path of electrical powerthrough the UPS system when operating in Bypass mode.In Bypass mode, the output of the system is provided with three-phase AC power directly from the systeminput. While in this mode, the output of the system is not protected from voltage or frequency fluctuations orpower outages from the source. Some power line filtering and spike protection is provided to the load but noactive power conditioning or battery support is available to the output of the system in the Bypass mode ofoperation.The internal bypass is comprised of a solid-state, silicon-controlled rectifier (SCR) continuous static switch, andbypass switchgear. The static switch is used instantaneously anytime the inverter is unable to support theapplied load. The continuous static switch is wired in series with the bypass switchgear, and together they arewired in parallel with the rectifier and inverter.The static switch, being an electronically-controlled device, can be turned on immediately to pick up the loadfrom the inverter while the inverter switchgear opens to isolate the inverter. The bypass switchgear is normallyclosed, ready to support the static switch unless the bypass input source becomes unavailable.Rectifier InverterBattery ConverterBatterySwitchgearMain Power FlowOutputInverterSwitchgearBypass InputRectifierSwitchgearRectifier InputSwitchgearClosedOpenBatteryTrickle CurrentDe-EnergizedEnergizedStaticSwitchBypass Switchgear
