Static Transfer Switch ( STS )

The static transfer switch (STS) is a static AC Switch designed and used to transfer, automatically. This works for single-phase or three-phase loads. This consists of back-to-back thyristors connected in parallel. As shown in the figure. The STS functions like a mechanical switch i.e. double pole input one pole out Or three pole input one pole output. Many of such combinations can be achieved automatically. If the main source supplying the loads fails, then STS transfers to the other source automatically.

The STS shall provide an effective solution to the following needs

◗ complete separation of the two sources and of the associated distribution systems;
◗ source redundancy with no-break transfer of the loads, in existing installations;
◗ separation of loads to avoid mutual disturbances.

The STS thus improves the availability of energy, while facilitating operation and maintenance, in installations supplying sensitive loads. The static transfer switch shall comprise two inputs, called Source 1 and Source 2, supplied by two power sources.
The first source is termed as the Preferred source and the second is the alternate source. The STS output shall be connected to a critical load. In the event of a power failure, the STS shall automatically transfer from one source to the other in less than a 5 to 10mes.