P-164000476 9395P 500kVA / 600 kVA 75Revision 002 User’s and Installation GuideThe UPSs continually monitor themselves and the incoming utility power and automatically switchbetween these modes as required, without operator intervention, except when manually switching toBypass mode. The sophisticated detection and switching logic inside the UPSs ensures that operatingmode changes are automatic and transparent to the critical load, while internal monitoring systemsindicate the current mode of operation. The UPSs switch operating modes in response to thesesystem events:• A command is an intervention that is externally initiated by an operator or by some site action. Acommand causes the UPSs to switch operating modes; it usually does not require any furtheraction.• A notice is a minor system event that may or may not require attention.• An alarm is a system event that requires immediate intervention.The following sections describe the differences in the five parallel system operating modes, usingblock diagrams to show the power flow during each mode of operation.6.4.2 Normal mode – distributed bypassIn Normal mode, utility AC power is supplied to the UPSs. Each UPS then conditions the incoming ACpower and provides clean, regulated AC power to tie cabinet or distribution panel for parallel systemsof up to four modules. The applied load is shared equally among the available UPMs in the system.Figure 6-11 shows the path of electrical power through the parallel system when operating in Normalmode.Figure 6-11. Path of current through the UPSs in normal mode – distributed bypassIf the utility AC power is interrupted or is out of specification, the UPSs automatically switch to Batterymode to support the critical load without interruption. When utility power returns, the UPSs return toNormal mode.If the UPSs become overloaded or unavailable, the distributed bypass system switches to Bypassmode. The distributed bypass system automatically returns to Normal mode when the overloadcondition is cleared and system operation is restored within specified limits.Main Power Flow Output toCritical LoadUPS Input Tie CabinetUPS 1 Output(closed)(closed)(closed)(closed)Bypass InputUPS 2 OutputUPS 3 OutputUPS 4 OutputDe-energizedBatteryBatteryBatteryBatteryUPS 1UPS 2UPS 3UPS 4
