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Chapter 12 Helpful information
Online or VFI UPS units
VFI stands for Voltage and Frequency Independent from mains supply. This type of UPS is
referred to as an online or double conversion UPS unit. UPS units of this design permanently
route the input voltage via a rectifier. The generated DC is fed to both the UPS batteries and
the inverter which in turn generates the AC voltage for the output. This design ensures that
there are no switching times in case of failure of supply by the power utility provider. This type
of UPS is the best choice for very sensitive devices that are to be protected.
On-line UPS units will only pass the input power to their output in case it is switched to
bypass mode for maintenance purposes.
For a Double Conversion On-Line UPS the primary power path is the inverter instead of the AC
mains. In the design of Double Conversion On-Line operation, failure of the input AC does not
cause activation of the transfer switch, because the input AC is not the primary source, but is
rather the backup source. Therefore, during an input AC power failure, on line operation results
in no transfer time. The on line mode of operation exhibits a transfer time when the power
from the primary battery charger/battery/inverter power path fails. This can occur when any of
the blocks in this power path fail. The inverter power can also drop out briefly, causing a
transfer, if the inverter is subjected to sudden changes in the load, or if the inverter experiences
an internal control "glitch".
Double Conversion On-Line UPS systems do exhibit a transfer time, however On-Line UPS
transfers are not related to AC input power failures as they are in a standby UPS. Both the
battery charger and the inverter convert the entire load power flow in this design, which
causes undesirable heat and results in reduced efficiency.
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6U Rack or Tower UPS RT8kVA and RT11kVA IUOMG
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