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Powerware 9335 (80 kVA and120 kVA) Installation and Operation 5-7
164201396 REV. A 071103
contactor and together are wired to enable the inverter and rectifier to be bypassed when
this path is active. The static switch, as an electronically controlled device, can be turned on
immediately to pick up the load from 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, transfers to bypass are prohibited and for the safety of those who may be
working on the power lines upstream from the UPS, the back feed protection contactor is
opened, preventing system output voltage from bleeding across the static switch snubber
components to the bypass input source. In the bypass mode, which can be used for higher
efficiency operation, the static switch is turned on to supply the load current directly from the
bypass supply, with the inverter ready to take over the load if the bypass supply should vary
outside preset limits.
If the inverter is unable to support the load on the output of the system, the UPS will transfer
the critical load to the internal bypass. The critical load will be transferred automatically to
the internal bypass if any of the following abnormal conditions occur on the output of the
system: If the output of the system exceeds acceptable voltage tolerances; Critical load
current exceeds the overload specifications of the inverter; Inverter failure. The transfer is
initiated by turning on the static switch and blocking the IGBT gate signal. This kind of
transfer is normally referred to as a Make-Before-Break transfer. The transfer takes place in
less than 4 ms (one-quarter cycle) to ensure loads on the systems output are 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 will attempt to restart the inverter (if not running already) and attempt
a retransfer back on line to support the output of the system. Three attempts will be made
within ten minutes to bring the inverter back on-line automatically before the UPS will lock
out any further attempts. After three attempts have been made, the UPS will remain in
bypass and an alarm condition will be annunciated. The UPS can also be transferred to
bypass using the front panel controls.
Bypass mode is a normal operating mode, not requiring 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.