UNDERSTANDING UPS OPERATIONEaton 9390 UPS (100–160 kVA) Installation and Operation Manual 164201604 Rev E www.eaton.com/powerquality 7-5In 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. Thestatic switch, being an electronically-controlled device, can be turned on immediatelyto pick up the load from the inverter while the inverter output contactor K3 opens toisolates the inverter. The backfeed protection contactor is normally always closed,ready to support the static switch unless the bypass input source becomesunavailable.During an outage, the UPS prohibits transfers to bypass and provides upstreamprotection. The backfeed protection contactor is opened, preventing system outputvoltage from bleeding across the static switch snubber components to the bypassinput source.To ensure the load remains energized, the UPS automatically transfers the output tothe internal bypass when one of these abnormal conditions occur: the output of thesystem exceeds acceptable voltage and frequency tolerances, the system isoverloaded, or the inverter fails. The transfer is initiated by turning on the static switchand opening the output contactor K3, also called a Make-Before-Break transfer. Thetransfer occurs in less than 4 mS (quarter cycle) to ensure that the output load is notinterrupted. The static switch remains on until either the inverter is able to support thesystem output or the unit is placed into a maintenance position where repairs can bemade.If the UPS initiates its own transfer to bypass for any reason other than operatorintervention, the UPS attempts to restart the inverter (if not running already) andattempts a transfer back online to support the output of the system. Three attemptsare made within ten minutes to return the inverter online automatically before theUPS locks out any further attempts. After three attempts, the UPS remains in bypassand an alarm condition is enunciated. The UPS can also be transferred to bypass usingthe front panel controls.Bypass mode is a normal operating mode, not an alarm condition. However, if theUPS is unable to return to Normal mode following an automatic transfer to Bypassmode, an alarm condition is recorded.
