Improving overall system reliability
In critical applications, two independent sources, each with high MTBF and low MTTR (mean time to repair) should be connected to a static transfer switch.
The automatic transfer system is a critical node in the supply system, since it ensures the connection of independent supplies. It is critical the static transfer switch (STS) is designed to ensure the total transfer time is suitable for the load and ensure that a faulty supply cannot feed or damage the overtaking supply or the load.
Static switches type STS-L accept multiple synchronous three phase supplies and connect one of these supplies to the load. Usually there are two input supplies, A and B, and only A or B are connected to the load. If STS A is connected and fails, the load is statically isolated from supply A.
The control circuit ascertains STS A is disconnected and then proceeds to activate STS B, supplying the load from supply B. Under some conditions supply A can be commutated by supply B. Since this type of static switch utilizes thyristors and since thyristors can only be turned off by bringing zero, either naturally or by external commutation, these types of switches can be relatively slow.
Under normal switching (when the incoming supplies are not in fault conditions and when they are reasonably balanced) the total transfer time is about 0.5msec. It is well known, that under fault conditions of the outgoing supply the time taken by the outgoing switch can be longer than 10msec.
Unless forced commutation can be applied, this time may be too long to be tolerated by the computer load.
Unlike the thyristor STS, where an off command can take in excess of 10msec under some fault conditions to be effected, a gate turnoff thyristor GTO will guarantee a turnoff within a few microseconds.
An Integrated Gate Commutated Thyristor is introduced as the main switching device in STS-i switches. The control is by fiber optic cables. It is to be noted that fast isolation is a very important feature of a transfer switch if all supply faults are to be isolated in time.
By fast we mean under 0.5msec. An IGCT is essentially a gate turn off thyristor and device failures are always by short circuit. This feature allows either fuse or circuit breaker protection and faults can be safely isolated. IGBT's can also be employed, but are limited, for reliable switch design, to 200 A.