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Theory of Operation
60
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 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 moni-
tored 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, providing the alter-
nate 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 trans-
fer, the Liebert STS2 transfers between the two sources with less than one (1) millisecond interrup-
tion 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.
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 auto-
matic retransfer to the preferred source through a user configuration setting.
Automatic Retransfer Inhibited
If the system makes five transfers from the preferred source to the alternate source in a five minute
period, automatic retransfers back to the preferred source is disabled.
To clear this alarm condition, you must assign the alternate source as the preferred source.
Also, the UV detection thresholds may need adjustment. See Source Set Points on page 90 for
instructions on setting the UV detection.
This alarm also is cleared if the unit transfers to the preferred source due to an alternate source AC
volt failure (i.e., an emergency transfer.)