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Understanding UPS Operation
Bypass Input
Rectifier Input
Rectifier
Switchgear
Figure 5-3. Path of Current Through the UPS in Normal Mode
5.2.4
Bypass Mode
The critical load is not protected from voltage or frequency fluctuations or power outages while
the UPS is in Bypass mode.
The UPS automatically switches to Bypass mode if it detects an overload, load fault, or internal failure (if
configured for capacity). If configured for redundancy (N+1) the UPS would remain on line in Normal mode if
one UPM failed. The UPS can also be transferred from Normal mode to Bypass mode manually. The bypass
source supplies the commercial AC power to the load directly. Figure 5-4 shows the path of electrical power
through the UPS system when operating in Bypass mode.
In Bypass mode, the output of the system is provided with three-phase AC power directly from the system
input. While in this mode, the output of the system is not protected from voltage or frequency fluctuations or
power outages from the source. Some power line filtering and spike protection is provided to the load but no
active power conditioning or battery support is available to the output of the system in the Bypass mode of
operation.
The internal bypass is comprised of a solid-state, silicon-controlled rectifier (SCR) continuous static switch, and
bypass switchgear. The static switch is used instantaneously anytime the inverter is unable to support the
applied load. The continuous static switch is wired in series with the bypass switchgear, and together they are
wired in parallel with the rectifier and inverter.
The static switch, being an electronically-controlled device, can be turned on immediately to pick up the load
from the inverter while the inverter switchgear opens to isolate the inverter. The bypass switchgear is normally
closed, ready to support the static switch unless the bypass input source becomes unavailable.
5-4
Eaton 93PM UPS (20–50 kW, 480V Four Wire – 50 kW Frame) Installation and Operation Manual P-164000540—Rev 4
Bypass Switchgear
Rectifier
Switchgear
Closed
Open
Static
Switch
Inverter
Battery Converter
Battery
Switchgear
Battery
CAUTION
Output
Inverter
Switchgear
Main Power Flow
Trickle Current
Energized
De-Energized
www.eaton.com/powerquality
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