Vertiv™ | Liebert® STS2/PDU
™
User Manual | Rev. 6 | 10/2017 71
9.0 THEORY OF OPERATION
9.1 General Description
The Liebert STS2/PDU contains all the equipment necessary to prevent an interruption in power flow to the AC
load, should your primary source fail.
9.1.1 Liebert Static Transfer Switch 2/Power Distribution Unit
The Liebert STS2/PDU is a combination automatic static transfer switch and power distribution unit. The Liebert
STS2/PDU shall include two isolation transformers connected to a solid-state, three-pole, dual-position static
transfer switch designed to automatically and manually switch between two synchronized three-phase AC power
sources without an interruption of power to the load longer than 4 milliseconds (1/4 cycle). Input, output, bypass
circuit breakers and a distribution section shall be included to form an integrated system in a single freestanding
enclosure.
The preferred source is the input source that normally carries the load when both sources are available. The
preferred source is selectable by the user from the LCD panel or an optional Remote Source Selection. The
unselected source is referred to as the alternate source and carries the load if the preferred source should fail or
if a manual transfer is initiated.
The unit uses Silicon Controlled Rectifiers (SCRs) connected in paralleled, opposing pairs to function as an AC
switch configuration. Three pairs of SCRs connect the AC load to the selected preferred input source as long as
the preferred source is available. A second set of SCRs stands ready to transfer the AC load to the alternate input
source should the preferred source fail. The switching action is a very fast, break-before-make with less than a
1/4 cycle break in the AC waveform.
During static switch transfers and retransfers, the conduction state of the SCRs is carefully monitored and
controlled to prevent a current path from one source to the other. This technique prevents a faulted source from
feeding into the other good source.
9.1.2 Source Transfer
The Liebert STS2/PDU allows manually initiated transfers between the two sources, providing the alternate
source is within acceptable voltage limits and phase tolerances with the preferred source. You begin a manual
transfer by selecting the inactive source as the preferred source. On a manual transfer, the Liebert STS2/PDU
transfers between the two sources with less than one (1) millisecond interruption of power to the load. The
transfer can only be completed when both sources are available and synchronized within the user-adjustable
phase synchronization window. For sources where the two frequencies are not exactly in phase, manually-
initiated transfers are delayed by the Liebert STS2/PDU until the two sources are within the defined phase
synchronization window.
Source selection can also be done remotely using the Remote Source Selection option, if installed.
9.1.3 Automatic Transfer/Retransfer
If the preferred source is out of voltage and/or frequency limits, the load is automatically transferred to the
alternate source. The default transfer voltage limits comply with the limits of the IEEE Std. 446-1995 computer
voltage tolerance envelope.
If the unit is so configured, the load is retransferred automatically to the preferred source after the preferred
source returns to within the acceptable voltage and frequency limits for at least the defined retransfer time delay
and is in phase with the alternate source. You can enable and disable automatic retransfer to the preferred
source through a user configuration setting.
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