Powerware 9395 UPS and Plus 1 UPS 650825 kVA - Manual (Page 117)

UNDERSTANDING UPS OPERATION
EATON Powerware ® 9395 UPS (650–825 kVA) Installation and Operation Manual S 164201725 Rev 2 www.powerware.com 6−5
The internal bypass is comprised of a solid−state, silicon−controlled rectifier (SCR)
static switch (SSW), a motorized wraparound bypass breaker CB4, and a backfeed
protection contactor K5. The static switch is rated as a momentary−duty device that is
used instantaneously anytime the inverter is unable to support the applied load. The
static switch is wired in series with the backfeed protection contactor, and CB4 is
wired in parallel with this combination. 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 bypass breaker CB4 closes
and inverter output contactor K3 opens to isolate the inverter. When breaker CB4
closes, the static switch stops conducting. The backfeed protection contactor is
normally always closed, ready to support the static switch unless the bypass input
source becomes unavailable.
If the UPS transfers to Bypass mode from Normal mode due to any reason other than
operator intervention, the UPS automatically attempts to transfer back to Normal
mode (up to three times within a ten minute period). The fourth transfer locks the
critical load to the bypass source and requires operator intervention to transfer.
6.2.4 Battery Mode
The UPS automatically transfers to Battery mode if a utility power outage occurs, or if
the utility power does not conform to specified parameters. In Battery mode, the
battery provides emergency DC power that the inverter converts to AC power.
Figure 6-4 shows the path of electrical power through the UPS system when
operating 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, battery current limit alarms
may be seen for a short time due to the current required to recharge the battery.