UNDERSTANDING UPS OPERATIONEaton 9395 550/275 UPS (225–550 kVA) Installation and Operation Manual 164201716 Rev 2 www.eaton.com/powerquality 6-5StaticSwitch K5Rectifier InverterK1 K3BatteryConverterBatteryBatteryBreakerMain Power FlowTrickle CurrentEnergizedDe-EnergizedClosedOpenBreakers ContactorsK2Bypass InputRectifierInput OutputInputBreaker(CB1Figure 6‐3. Path of Current Through the UPS in Bypass ModeIn Bypass mode, the output of the system is provided with three-phase AC powerdirectly from the system input. While in this mode, the output of the system is notprotected from voltage or frequency fluctuations or power outages from the source.Some power line filtering and spike protection is provided to the load but no activepower conditioning or battery support is available to the output of the system in theBypass mode of operation.The internal bypass is comprised of a solid-state, silicon-controlled rectifier (SCR)static switch (SSW) and a backfeed protection contactor K5. The static switch is ratedas a continuous-duty device that is used anytime the inverter is unable to support theapplied load. The static switch is wired in series with the backfeed protectioncontactor, and together they are wired in parallel with the rectifier and inverter.The static switch, being an electronically-controlled device, can be turned onimmediately to pick up the load from the inverter while the inverter outputcontactor K3 opens to isolate the inverter. The backfeed protection contactor isnormally always closed, ready to support the static switch unless the bypass inputsource becomes unavailable.If the UPS transfers to Bypass mode from Online mode due to any reason other thanoperator intervention, the UPS automatically attempts to transfer back to Online mode(up to three times within a ten minute period). The fourth transfer locks the criticalload to the bypass source and requires operator intervention to transfer.
