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The UPSs continually monitor themselves and the incoming utility power and
automatically switch between these modes as required, without operator
intervention, except when manually switching to Bypass mode. The sophisticated
detection and switching logic inside the UPSs ensures that operating mode changes
are automatic and transparent to the critical load, while internal monitoring systems
indicate the current mode of operation. The UPSs switch operating modes in
response to these system events:
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A command is an intervention that is externally initiated by an operator or by some
site action. A command causes the UPSs to switch operating modes; it usually
does not require any further action.
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A notice is a minor system event that may or may not require attention.
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An alarm is a system event that requires immediate intervention.
The following paragraphs describe the differences in the three parallel system
operating modes, using block diagrams to show the power flow during each mode of
operation.
6.4.2
Normal Mode – Distributed Bypass
In Normal mode, utility AC power is supplied to the UPSs. Each UPS then conditions
the incoming AC power and provides clean, regulated AC power to tie cabinet or
distribution panel for parallel systems up to four modules. The applied load is shared
equally among the available UPMs in the system.
Figure 6-10 shows the path of electrical power through the parallel system when
operating in Normal mode.
Bypass Input
Battery
Battery
Battery
Battery
Main Power Flow
Figure 6-10. Path of Current through the UPSs in Normal Mode – Distributed Bypass
EATON Powerware
®
9395 UPS (1000–1100 kVA) Installation and Operation Manual
UPS Input
UPS 1
UPS 2
UPS 3
UPS 4
Breakers
Closed
Open
UNDERSTANDING UPS OPERATION
Tie Cabinet
UPS 1 Output
UPS 2 Output
UPS 3 Output
UPS 4 Output
Output to
Critical Load
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164201764 Rev 1 www.powerware.com
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