Powerware 9390 UPS 100160 kVA - Manual (Page 57)

UNDERSTANDING UPS OPERATION
EATON Powerware ® 9390 UPS (20–80 kVA) Installation and Operation Manual S 164201603 Rev 2 www.powerware.com 7-5
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)
static switch (SSW) and a backfeed protection contactor K5. The static switch is rated
as a continuous-duty device that is used anytime the inverter is unable to support the
applied load. The static switch is wired in series with the backfeed protection
contactor, 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 output contactor K3 opens to
isolates the inverter. The backfeed protection contactor is normally always closed,
ready to support the static switch unless the bypass input source becomes
unavailable.
During an outage, the UPS prohibits transfers to bypass and provides upstream
protection. The backfeed protection contactor is opened, preventing system output
voltage from bleeding across the static switch snubber components to the bypass
input source.
To ensure the load remains energized, the UPS automatically transfers the output to
the internal bypass when one of these abnormal conditions occur: the output of the
system exceeds acceptable voltage and frequency tolerances, the system is
overloaded, or the inverter fails. The transfer is initiated by turning on the static switch
and opening the output contactor K3, also called a Make-Before-Break transfer. The
transfer occurs in less than 4 mS (quarter cycle) to ensure that the output load is not
interrupted. The static switch remains on until either the inverter is able to support the
system output or the unit is placed into a maintenance position where repairs can be
made.
If the UPS initiates its own transfer to bypass for any reason other than operator
intervention, the UPS attempts to restart the inverter (if not running already) and
attempts a transfer back online to support the output of the system. Three attempts
are made within ten minutes to return the inverter online automatically before the
UPS locks out any further attempts. After three attempts, the UPS remains in bypass
and an alarm condition is enunciated. The UPS can also be transferred to bypass
using the front panel controls.
Bypass mode is a normal operating mode, not an alarm condition. However, if the
UPS is unable to return to Normal mode following an automatic transfer to Bypass
mode, an alarm condition is recorded.