UNDERSTANDING UPS OPERATIONPower Xpert 9395 UPS (450–550 kVA) Installation and Operation Manual 164201701 Rev 3 www.eaton.com/powerquality 6-25In a distributed bypass system, each UPS operates similar to a single UPS, but inparallel with each other. The bypass source for the load is derived from the bypassinput of one, two, three, or four UPSs, depending on the system configuration,through the internal static switches. If a module is taken offline, the other modulesremain online to support the load. If more modules than can support the load must betaken offline, the load must be transferred to maintenance bypass or shut down.The distributed bypass system can be transferred from Normal mode to Bypass modemanually. However, the distributed bypass system automatically switches to Bypassmode whenever the UPSs can no longer supply the critical load. If the distributedbypass system transfers to Bypass mode from Normal mode due to an output voltagedeviation, the distributed bypass system automatically attempts to return to Normalmode (up to three times within a 10‐minute period). After three transfer attempts oran overload, the system locks the critical load to the bypass source and requiresoperator intervention to transfer.Bypass mode is a normal operating mode, not an alarm condition. However, if thedistributed bypass system is unable to return to Normal mode following an automatictransfer to Bypass mode, an alarm condition is recorded.Bypass may also be used when the UPSs or UPMs in the system must be shut downto perform routine maintenance or repairs.In the Parallel Redundant (N+1) arrangement, the bypass circuitry in each UPSoperates to support the applied loads on bypass. If the UPSs are in ONLINE and oneUPS trips offline, the remaining UPSs do not go to bypass as long as they have thecapacity to support the load.In the Parallel Capacity (N+0) arrangement, if one UPS trips offline and goes tobypass, the remaining UPSs also go to bypass.6.4.4 Battery Mode – Distributed BypassThe UPSs transfer to Battery mode automatically if a utility power outage occurs, or ifthe utility power does not conform to specified parameters. In Battery mode, thebattery provides emergency DC power that the inverter converts to AC power.Figure 6‐21 shows the path of electrical power through the distributed bypass systemwhen operating in Battery mode.While in Battery mode, the UPSs sound an audible horn, illuminate a visual indicatorlamp on the front panel (Online and On Battery), and create an entry into the alarmevent history. As the battery discharges, the boost converter and inverter constantlymake minute adjustments maintaining a steady output. The UPSs remain in thisoperating mode until the input power to the rectifier is again within the specifiedvoltage or frequency acceptance windows.
