MGE UPS Systems STS 400A Installation And User Manual Download

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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:

Under-voltage on any phase of the

active

source.

Over-voltage on any phase of the

active

source.

Under-frequency of the

active

source.

Over-frequency of the

active

source.

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).

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).

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).

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).

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:

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.

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:

If retransfer is not prohibited, retransfer will occur after an owner selected time delay after the preferred source
becomes available and within acceptable limits.

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

Operation

3 —4