Understanding UPS Operation
Eaton 9E UPS (40–60 kVA, 208/220V) Installation and Operation Manual P-164000058—Rev 4 www.eaton.com/powerquality 5-5
5.2.5 Battery Mode
The UPS automatically transfers to Battery mode if a utility power outage occurs, or if the utility power does
not conform to specified parameters. In Battery mode, the battery provides emergency DC power that the
inverter converts to AC power.
Figure 5-4 shows the path of electrical power through the UPS system when operating in Battery mode.
Figure 5-4. Path of Current Through the UPS in Battery Mode
During a utility power failure, the rectifier no longer has an AC utility source from which to supply the DC output
voltage required to support the inverter. The input relay opens and the battery instantaneously supplies energy
to the battery converter. The converter boosts the voltage so that the inverter can support the customer's load
without interruption. If bypass is common with the rectifier input, the backfeed protection relay also opens. The
opening of the input and the backfeed protection relays prevent system voltages from bleeding backwards
through the static switch and rectifier snubber components to the utility source.
If the input power fails to return or is not within the acceptance windows required for normal operation, the
battery continues discharging until a DC voltage level is reached where the inverter output can no longer
support the connected loads. When this event occurs, the UPS issues another set of audible and visual alarms
indicating SHUTDOWN IMMINENT. Unless the rectifier has a valid AC input soon, the output can be supported
for only two minutes before the output of the system shuts down. If the bypass source is available, the UPS
transfers to bypass instead of shutting down.
If at any time during the battery discharge the input power becomes available again, the input and the backfeed
protection relays close and the rectifier begins to supply DC current to the converter and inverter. At this point,
the unit returns to Normal mode. Depending on the total load and the duration of the battery discharge, battery
current limit alarms may be seen for a short time due to the current required to recharge the battery.
Bypass Input
Static
Switch
Backfeed
Protection Relay
Rectifier
Inverter
Output
Relay
Input
Relay
Input
Breaker
Rectifier
Input
Output
Battery
Converter
Battery
Relay
Main Power Flow
Relays
Breakers
Closed
Open
Battery
Trickle Current
De-Energized
Energized
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