The static transfer switch (STS) is
is an electrical device that allows instantaneous transfer of power sources to the load. This superior switching time means that if one power source fails, the STS switches to the back-up power source so quickly that the load never recognizes the transfer made. The STS is classified as low voltage STS (voltages up to 600V, current ratings from 200 amps to 4,000 amps) and medium voltage STS (voltages from 4.16KV to 34.5KV).
Fast-acting STS's that can transfer between two power sources in four to 20 milliseconds are increasingly being applied to protect large loads and entire facilities from short-duration power disturbances. These products use solid-state power electronics or "static" switching as compared to electromechanical switches, which are too slow for the application. Electromechanical switches are found in automatic transfer switches (ATS) for transferring building loads to emergency or stand-by generators.
The basic STS unit consists of three major parts:
The STS monitors two power sources and automatically shifts to the better one on sensing the failure or degradation of either source. There is no wear and tear associated with this process.
A LV-STS is often employed synergistically with UPSs. However, it also competes with UPSs and other power quality equipment types for applications. For example
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Data centers typically isolate the critical load and may employ LV-STS with dual-redundant UPSs as the power sources
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An industrial plant may choose to use LV-STS with dual-redundant UPS's as the power sources
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An industrial plant may choose to use LV-STS on isolated groups of critical process equipment
The same industrial user may instead choose a facility-scale system (medium-voltage STS, custom power, microSMES, or BESS) to protect the entire plant, particularly if the critical process cannot be easily isolated from other independent manufacturing steps.
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