Liebert 100-1000A user manual General Description, Theory of Operation

Theory of Operation

9.0THEORY OF OPERATION

9.1General 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.

9.1.1Liebert 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.

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.

9.1.2Source 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, 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.

Source selection can also be done remotely using the Remote Source Selection option, if installed.

9.1.3Automatic 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.

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.

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