Liebert 30-130kVA - Manual (Page 21)

Theory of Operation
13
Transfer and Retransfer Conditions
1. Automatic Transfers to Bypass:
Critical bus conditions that will initiate an automatic transfer of the critical load from the UPS
inverter output to the bypass source are:
a. Output Overload: overcurrent condition in excess of the current-versus-time overload capacity
curve.
b. Over/Under Voltage (OV/UV): critical bus voltage is outside the allowable tolerance.
c. Inverter Inoperative: inverter diagnostic circuitry senses an imminent inverter output OV/UV
condition:
d. Battery discharged to the shutdown voltage.
e. Inverter or rectifier fault condition (power, logic, or over-temperature) present or imminent.
f. Failure of system logic or logic power.
2. Manual Transfers:
Manual transfers may be initiated at any time provided no transfer inhibition conditions are
present.
3. Transfer Inhibited:
A manual transfer to the bypass source will be inhibited if any of the following conditions exist:
a. Bypass frequency deviates ±0.5 Hz from the nominal.
b. UPS system to bypass voltage difference (DV) exceeds a predetermined percentage (normally
10%).
c. OK to Transfer signal from the control logic is not present.
4. Automatic Retransfers to UPS:
Critical bus conditions that must be present to initiate an automatic retransfer (Auto-Rexfer) of
the critical load from the bypass source to the UPS system are:
a. The number of Auto-Rexfer Attempts selected must be greater than zero (0). If zero (0) is
selected, no automatic retransfer will occur.
b. Critical load was initially transferred to the bypass source due to a system overload only.
c. Overload has since been eliminated (the load has dropped below 100% of the rated load).
d. Both the Input contactor and Battery (MBD) circuit breakers have remained closed since the
overload transfer.
e. OK to Transfer signal received from the control logic for at least 10 seconds, within 5 minutes
of the overload transfer. (A manually initiated retransfer from bypass is required for
overloads lasting 5 minutes or more.)
f. Cyclic-type system overloads, which occur up to five (select range is 0 to 5) times in 60
minutes, are automatically returned to the UPS system for each event including the Nth
overload. A manually initiated retransfer from bypass is required for the N+1 overload.
5. Manual Retransfers:
Manual retransfers may be initiated at any time provided no retransfer inhibition conditions are
present.
6. Retransfer Inhibited:
A retransfer (automatic or manual) from the bypass source to the UPS system shall be inhibited if
any of the following conditions exist:
a. Retransfer Inhibitions:
1. Bypass frequency exceeds ±0.5 Hz of the nominal.
2. UPS system-to-bypass voltage difference (DV) exceeds a predetermined percentage
(normally 10%).
3. OK-to-Transfer signal from the control logic is not present.
4. Inverter or rectifier fault.
b. Automatic Retransfer Inhibitions (in addition to those above):
1. The load transfer to bypass was not caused by an output overload.
2. Excessive cyclical overloads within a one-hour period.
3. Retransfer conditions are not satisfied within 5 minutes of the initial transfer.