Static Power iSTS W - 2 System Overview & Operation; Description; Overview

2
. SYSTEM OVERVIEW & OPERATION
2
.1 Description
A
Static Transfer Switch (STS) provides seamless switching between two power sources to provide
interrupted
power
to a critical load.
The
iSTS continuously monitors the supply sources for failure or degeneration and switches automatically to
the
alternate
source as required to protect the load against failure. The break in the supply transition is so short
that
it
is not seen by the critical load. This switching process is undertaken as a break before make
transfer,
preventing
overlapping which can cause large and unpredictable currents. In 2 and 4-pole models, all the
active
conductors
and the neutral are switched. In 3-pole models, only the active phases are switched. Thyristors
are
used
to undertake the switching process.
In
the case of downstream load fault conditions, the fault current drawn from the supply may degrade or
damage
the
supply sources; as a consequence should a fault current exist in the load the iSTS will inhibit a transfer to
the
alternate
source even if this causes source supply degradation or loss. This is to ensure that the fault will not
be
transferred
to the alternate supply with the possibility of degrading both sources.
The
current threshold for isolation is pre-set to approx. 300% of the rated current.
This
unit contains fuses. The fuses are to provide for safe operation even in high fault capacity environments.
It
should
be noted that the fuses are for the protection of the semiconductor switches (SCRs/ Thyristors), not
the
load
. The iSTS does not have any automatic tripping devices, so load discrimination is undertaken in the
primary
and
secondary supply networks.
iSTS W
– User Manual
Page 4
iSTS W
2
.2 Overview
The
picture shows a bottom cable entry iSTS W.
The
maintenance bypass switch and terminal arrangement is reversed for top cable entry models.