Theoryof Operation 21
Transferand Retransfer Conditions
1. AutomaticTransfers to Bypass
Criticalbus conditions that will initiatean automatic transfer of the criticalload from the UPS system to the
bypasssource are:
a. System Overload:overcurrent condition in excess of the overload ratingof the System Control Cabinet
(SCC).
b. OutputOverload: overcurrent condition in excess of the current-versus-timeoverload capacity of UPS
modulesON LINE.
c. Over/UnderVoltage (OV/UV): critical bus voltageis outside the allowable tolerance.
d. Non-redundantUPS Module Inoperative: an essential UPS module is taken OFF LINE for one of the
reasonsbelow.
1. Batterydischarged to the shutdown voltage.
2. Inverteroverload capacity exceeded.
3. Inverteror rectifier fault condition (power, logic, or over-temperature)present or imminent.
4. Failureof UPS module logic or logic power.
e. SCC Logic:
1. EmergencyModule Off (EMO) circuit activated.
2. Failureof UPS system logic or logic power.
2. Manual Transfers
Manualtransfers may be initiated at any time provided no transfer inhibition conditionsare present.
3. TransferInhibited
Am anualtransfer to the bypass source shall be inhibited if any ofthe following conditions exist:
a. Bypass frequencydeviates ±0.5 Hz from the nominal.
b. UPSsystem to bypass voltage difference ( V) exceedsa predetermined percentage (normally 5%).
c. Staticswitch disconnects (manual switches) open.
d. OKto Transfer signal from the control logic is not present.
4. Automatic Retransfers toUPS
Criticalbus conditions that must be present to initiatean automatic retransfer (Auto-Rexfer) of the critical
loadfrom the bypass source to the UPS system are:
a. The number of Auto-RexferAttempts selected must be greater than zero (0). If zero (0) is selected, no
automaticretransfer will occur.
b. Criticalload was initially transferred to the bypass source due to as ystemoverload only.
c. Overload has sincedropped below 100% of the rated SCC current.
d. EnoughUPS modules are ON LINE to supply thecritical load.
e. OK to Transfersignal received from the control logic for at least 10 seconds,within 5 minutes of the
overloadtransfer. (A manually initiatedre transferfrom bypass is required for overloads lasting 5
minutes or more.)
f. Cyclic-type system overloads,which occurup to five (select range is 0 to 5) times in 60 minutes, are
automaticallyreturned to the UPS system for each eventincluding the Nth overload. A manually
initiatedretransfer from bypass is required for the N+1 overload.
NOTE
Aload transfer to the bypass line will be completed whenever an automatictransfer
tobypass is initiated If the Static Switch Unablealarm message is present for any
reason(including a ±20° phase lock synchronizationerror), the automatic transfer
willbe interrupted for 40 to 120 milliseconds. Because of the reliabilityof the UPS
components,an interrupted load transfer is a very unlikely occurrence.
DISCONTINUED
PROD U C T