UNDERSTANDING UPS OPERATION
Power Xpert
9395 UPS (450–550 kVA) Installation and Operation Manual
164201701 Rev 3
www.eaton.com/powerquality
6-22
6.4
Multiple UPS Distributed Bypass System
Distributed bypass parallel operation extends the normal operation of Power Xpert
9395 UPSs by offering increased capacity and/or redundant capability. The parallel
system continues to maintain power to the critical loads during commercial electrical
power brownout, blackout, overvoltage, undervoltage, and out‐of‐tolerance frequency
conditions.
The output of the system is normally supplied by Uninterruptible Power Modules
(UPMs) contained in each UPS. Multiple
UPMs are connected with their outputs in
parallel (tied together) to provide a load level greater than the rating of one UPM, for
redundancy, or both. The paralleled UPMs supply the output load with protected
power as long as the load does not exceed the combined rating of the paralleled
UPMs.
The power system is redundant as long as one of the UPMs can be disconnected
from the output bus and the remaining UPMs can continue to supply power to the
load without exceeding their ratings.
When the load is being supplied by the UPMs, the system output bus is continuously
monitored for an overvoltage or undervoltage condition. If an out of limits condition is
detected, the paralleled UPMs transfer the load to bypass using the UPS static
switches.
Communication is required between the UPSs for system metering and mode
control. System level communication and control are accomplished using a Controller
Area Network (CAN). A single building alarm in each UPS, connected to the other
UPSs in parallel and tied to the bypass contactor auxiliary contacts in each UPS, are
used for a secondary communication path. This arrangement ensures bypass control
even if the CAN bus is lost.
The system is paralleled for redundancy (1+1) in a system where two UPMs are
paralleled together and the load is less than the supporting capability of one of the
UPMs. The system is paralleled for capacity (2+0) if both UPMs in a system are
required to support the load.
A parallel capacity system can also be redundant (2+1), as long as there is always one
or more UPMs online than required to support the load.
6.4.1
Multiple UPS Parallel System Modes
Similar to the single UPS system, the Power Xpert 9395 UPS parallel system supports
a critical load in three different modes of operation. The standard operation
modes are:
In Normal mode, the paralleled UPMs supply the critical load with clean, filtered
power. Each UPM battery charger also provides charging current for the battery if
needed.
In Bypass mode, the critical load is directly supported by utility power through the
bypass circuits in each UPS.
In Battery mode, the battery provides DC power, which maintains UPM operation.
The UPM batteries support the critical load.
