58
UPS 225 - 275 kVA/300kVAPF0.8
1027212
User’s and Installation Guide
Revision E
Figure 6-5. 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 current required to support the inverter. The input contactor K1 opens and the battery
instantaneously supplies energy to the battery converter. The converter either bucks or 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 contactor K5 also opens. The opening of contactors K1
and K5 prevent system voltages from bleeding backwards through the static switch and rectifier
snubber components and re-entering the input 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, contactors K1 and
K5 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,
status
Charging at full current
may be seen for a short time due to the current required to
recharge the battery.
