S&C Instruction Sheet 515-600 9
Overview
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The
Unbalance Detection feature should not be programmed in applications using three-phase voltage
sensing provided by two line-to-line connected voltage transformers per source .
Depending on the control settings, return to the normal circuit configuration—
preferred-source interrupter switch closed, alternate-source switch open—may be
performed automatically on restoration of normal voltage to the preferred source after a
delay sufficient to establish that the return is not temporary (
Automatic Return
mode
or
Window Return
mode) or manually at a convenient time (
Hold Return
mode).
In the
Automatic Return
mode, such return-of-source transfer may be accomplished
with an open transition or a closed transition. With an open transition, retransfer—used
when the power sources are not to be paralleled—the alternate-source interrupter
switch opens before the pre ferred-source interrupter switch closes . . . with a momen tary
interruption of service to the load. With a closed transition, retransfer—selected when
it is permissible to parallel the sources so that there will be no interruption of service to
the load—the alternate-source interrupter switch will open after the preferred-source
interrupter switch closes.
The
Window Return
mode is functionally identical to the
Automatic Return
mode,
except that return-of-source transfer is permitted to occur only if the time of day is within
a user-specified time “window,” typically at a time when the implications are least severe
for critical loads.
In
Hold Return
mode, if the preferred source voltage meets the criteria for being
considered normal and if the alternate-source is lost (or reduced below a predetermined
level) for a period of time sufficient to confirm that the loss is not transient, an automatic
open transition loss-of-source transfer will take place so that the load is served from the
preferred-source following the loss-of-source transfer.
The voltage-sensing input circuitry of the Micro-AT Source-Transfer Control In
Weatherproof Enclosure accommodates either of the following single-phase or three-
phase voltage-sensing schemes, using user-furnished voltage transformers having
240/120-Volt, 60-Hertz secondaries:
• For single-phase sensing, one line-to-ground or line-to-line connected voltage trans-
former per source
• For three-phase sensing, three line-to-ground con nected voltage transformers per
source or—on delta-connected systems only—two line-to-line connected voltage
transformers per source
In instances where three-phase source voltage sensing is used, an output-voltage
magnitude unbalance and/or phase-angle unbalance will likely exist between the volt age
transformers on each source. The Micro-AT control must be normalized to compensate
for such differences on the left source and the right source, respectively. A source can
be normalized only if each phase has measur able voltage and its sequence of rotation is
the same as on the other source.
The base voltages on phase 2 of the left and right sources can also be calibrated to
known values.
An
Unbalance Detection
feature may be field-programmed in the Micro-AT control in
instances where three-phase voltage sensing is provided by three line-to-ground
con nected voltage transformers per source. This feature pro tects the loads from
any source-side open-phase condition at the same system voltage level as the S&C
Alduti-Rupter Switches—whether caused by utility-line burndown, broken conductors,
single-phase switching, equipment malfunctions, or single-phasing resulting from blown
source-side fuses. The unbalance detection feature continuously develops and monitors
the negative-sequence and zero-sequence voltages to detect any unbal ance present as
the result of an open-phase condition
●
.
If the voltage unbalance exceeds a preset reference level for a period of time
sufficient to confirm that the loss is not transient, the Micro-AT control will initiate an
automatic loss-of-source transfer to the other source. By monitor ing negative-sequence
and zero-sequence voltages, the unbalance detection feature detects virtually all source-
side open-phase conditions, even those where backfeed defeats simple voltage-magnitude
sensing schemes.
Voltage Sensing
Unbalance Detection
