UNDERSTANDING UPS OPERATIONPower Xpert 9395 UPS (450–550 kVA) Installation and Operation Manual 164201701 Rev 4 www.eaton.com/powerquality6-2The emergency bypass consists of a continuous-duty static switch and backfeedprotection contactor K5. The backfeed protection contactor is located in series withthe static switch. For manual transfers to bypass, the static switch is also used. Thestatic switch is armed and ready during both types of transfers.On a UPS configured as an Input Output Module (IOM), bypass circuitry is notinstalled. This configuration is primarily used in multiple UPS parallel systems that donot need a bypass for each UPS and use a separate System Bypass Module (SBM) toprovide system bypass capabilities.6.2 Single UPSA single UPS operates independently to support an applied load from the inverter,providing conditioned and uninterruptible AC power to the critical load from the outputof the module. During an outage, the inverter continues to operate, supporting powerto the load from the battery supply. If the unit requires service, applied loads aretransferred to the internal bypass, continuous-duty static switch either automaticallyor manually. With the exception of a battery cabinet, no other cabinets or equipmentare required for the single UPS to successfully support its applied loads.6.2.1 ModesThe Power Xpert 9395 UPS supports a critical load in three different modes ofoperation: In Online mode, the critical load is supplied by the inverter, which derives its powerfrom rectified utility AC power. In this mode, the battery charger also providescharging current for the battery, if needed. In Energy Saver mode, commercial AC power is supplied directly to the critical loadthrough the continuous static switch and transfers automatically to Online mode ifan abnormal condition is detected. The Energy Saver mode requires a UPS with acontinuous static switch. In Variable Module Management System mode, the UPS operates as a traditionaldouble-conversion UPS, but selectively shifts the load to fewer UPMs to increasethe efficiency of the UPS. In Bypass mode, the critical load is directly supported by utility power. In Battery mode, the battery provides DC power, which maintains inverteroperation. The battery supports the critical load.The following paragraphs describe the differences in the four UPS operating modes,using block diagrams to show the power flow during each mode of operation.6.2.2 Online ModeFigure 6‐2 shows the path of electrical power through the UPS system when the UPSis operating in Online mode.During normal UPS operation, power for the system is derived from a utility inputsource through the rectifier input contactor K1. The front panel displays “Normal,”indicating the incoming power is within voltage and frequency acceptance windows.Three-phase AC input power is converted to DC using IGBT devices to produce aregulated DC voltage to the inverter. When contactor K2 is closed the battery ischarged directly from the regulated rectifier output through a buck or boost DCconverter, depending on the system voltage and the size of the battery stringattached to the unit.
