Epilson STSTM
3.3Transfer Operations
3.3.1Automatic 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)
b)
c)
d)
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
h)SCR Gate Drive power supply failure on the active source (transfer to inactive source and inhibit
i)TVSS failure on the active source (if optional TVSS is installed).
3.3.2Manual 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
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.
3.3.3Automatic 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
3 | Operation |