Thomas & Betts 9395 - Manual (Page 44)

UPS 225 - 275 kVA
User’s and Installation Guide
1027212
Revision B
44
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, battery current limit alarms may be seen for a short time due to the current
required to recharge the battery.
4.3 UPS system one line configurations
One line drawing UPS model
Voltage
System type
Input Output
Figure 4-5 9395-275/225 400 400
Single module – Reverse transfer
dual-feed
Figure 4-6 9395-275/275 400 400
Single module – reverse transfer
dual-feed with typical maintenance
bypass panel