Static transfer switches use solid-state devices to perform transfers. These solid-state devices replace the mechanical operators in the conventional transfer switches. There are two approaches for transfer switches operation; open transition or closed transition. “An open transition is a break-before-make operation where the current to the load is interrupted for a brief period of time. This has the advantage of never paralleling the two sources. A closed transition is a make-before-break operation where the current to the load is not interrupted during the transfer. This has the advantage constantly supplying the load” [1].
The solid-state switches use the high reverse voltage blocking capability of the solid-state devices to make an open transition air break for isolation. STS provides a closed transition to the loads without connecting the two sources in parallel. The control algorithm can control the actual switching characteristics and switch response to achieve this result.
STS can be mainly classified into 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).