Vertiv™ | Liebert
®
STS2
™
100-1000A, 50/60Hz User Manual | Rev. 12 | 11/2017 57
9.0 THEORY OF OPERATION
9.1 General Description
The Liebert STS2 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 STS2 Static Transfer Switch
The Liebert STS2 is a 3-pole, double-throw, solid state automatic transfer switch that is fed from two sources.
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 parallel, 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 without an
interruption of power to the load longer than 1/4 cycle when utilizing the standard transfer algorithm. When
utilizing the Optimized transfer option the interruption does not exceed the ITIC/CBEMA standard.
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 allows manually initiated transfers between the two sources. On units without the Optimized
Transfer option the alternate source must be 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 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 until the two sources are
within the defined phase synchronization window.
The Liebert STS2 with Optimized Transfer option allows manually initiated transfer at any phase difference and
does not require synchronization, as long as the alternate source is within acceptable voltage limits with the
preferred source. Transfer is not delayed if the two frequencies are not in phase. When Optimized Transfer is
enabled, the out-of-sync alarm is NOT active.
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.
To Next Page
Loading...
