MGE UPS Systems STS 600A - Manual (Page 42)

3.3 Transfer Operations
3.3.1 Automatic Transfers
Automatic transfer of the load from one source to the other is accomplished by turning off the active source SCR's
and turning on the inactive source SCR's in a sequence and timing that allows a combined sense and on transfer
time of less than one quarter cycle (4.17ms). Automatic transfer is initiated under any of the following conditions,
provided transfer is authorized and the “inactive” source is not more degraded than the “active” source:
a) Under-voltage on any phase of the active source.
b) Over-voltage on any phase of the active source.
c) Under-frequency of the active source.
d) Over-frequency of the active source.
e) Open SCR on the active source (transfer to inactive source, then shunt trip the active C.B. and inhibit
retransfer until repair is made and the system is reset).
f) Shorted SCR on the inactive source (transfer to shorted inactive source, then shunt trip the previously active
C.B. to prevent retransfer. Inhibit transfers and retransfers until repair is made and the system is reset).
g) SCR Over-temperature on the active source (if there are no current overload faults, transfer to inactive source
and inhibit re-transfer until repair is made and the system is reset. If inactive source subsequently fails, transfer
back to the active side if it is within acceptable limits, including over-temperature limits).
h) SCR Gate Drive power supply failure on the active source (transfer to inactive source and inhibit re-transfer
until repair is made and the system is reset).
i) TVSS failure on the active source (if optional TVSS is installed).
3.3.2 Manual Transfers
The STS can be operated as a manual switch when the Manual Control keyed switch on the front panel is in the
ON position, enabling the manual control pushbuttons and some setpoint changes, for the following manual opera-
tions:
a) Manual Change Preferred Source Pushbutton – Selects other source as Preferred, causing a transfer (without
break) to the selected source if it is available and within acceptable limits and is not the active source. If the
two sources are outside the preset in sync phase window, a transfer (with break) can be made by pressing the
manual Override Limits pushbutton simultaneously.
b) Manual Return to Preferred Source Pushbutton - If auto-retransfer is disabled, pressing this pushbutton will
initiate a retransfer back to the preferred source, overriding owner selected auto retransfer time delay.
To prevent normal automatic transfers, the operator can use the Change Preferred Source pushbutton to transfer
to the desired source, and then shut down the feeder to the other inactive source.
3.3.3 Automatic Retransfers
Automatic retransfer of the load from the alternate source back to the preferred is accomplished by turning off the
active alternate source SCR's and turning on the inactive preferred source SCR's in a sequence and timing that
allows a combined sense and transfer time of less than one quarter cycle (4.17ms). Providing the inactive preferred
source is not more degraded than the active alternate source, retransfer will occur under the following conditions:
a) If retransfer is not prohibited, retransfer will occur after an owner selected time delay after the preferred source
becomes available and within acceptable limits.
b) If retransfer is prohibited, retransfer can be manually initiated by pressing the manual Return to Preferred
Source Pushbutton (Pre-conditions: Manual Control switch in ON position; inactive preferred source is
available and within acceptable limits). Transfer is temporarily authorized for a preset time.
Epilson STS TM
Operation 3 —4 86-504004-00 B03