MGE UPS Systems STS 600A - Manual (Page 19)

Installation and User Manual
1.4 Epsilon STS TM System Characteristics
The Epsilon digital Static Transfer Switch (STS) is a solid state, three-phase, break-before-make, dual position
switch designed to connect a critical three-phase load to one of two separate, independent, synchronized sources
of three-phase power. The STS consists of six pairs of SCR's connected in an AC switch configuration. Each 3-
phase input is fed through a molded case automatic switch which provides short circuit protection by means of a
magnetic trip only. The switch then feeds three pairs of “hockey-puck” SCR's, each pair consisting of two SCR's in
an anti-parallel arrangement. The SCR outputs for each corresponding phase from both inputs are then connected
and fed through an output isolation molded case switch to the output terminals. These SCR's are rated to carry the
full 100% load (continuous rated) while operated as stipulated herein and at the maximum ambient temperature
specified. The source to which the load is normally connected to is designated as the “Preferred” source, and the
other redundant, standby source is the “Alternate” source. Selection of which input source is preferred is user selec-
table from the operator control panel, by control contact inputs, or through the communications port(s).
In normal operation, the load is connected to the preferred source as long as all phases of the preferred source are
within the acceptable limits. Upon failure of the preferred source (more degraded than the alternate source), the
load will be transferred to the alternate source until such time as the preferred source returns to within acceptable
limits. The switching action itself is practically instantaneous and the time involved in the operation is mainly the
sensing time required to determine that a transfer is necessary, normally a small fraction of a cycle.
The STS is controlled by a number of system operating variables which define the operation and performance of
the system. Many of these variables may be adjusted for site-specific criteria.
The determination to transfer from one source to the other involves the evaluation of Source Qualities and System
Operating parameters:
Source Quality Concerns the evaluation of a sources ability to support the critical load by evaluating
the voltage, phase, current and frequency of a source on a real time basis.
System Operating Parameters control transfers between the two sources, given the state of both
sources and the load together with the specified site-specific operating criteria.
During a transfer (or re-transfer), the SCR's of the loaded source are turned off and the SCR's of the previously
unloaded source are turned on in such a way as to prevent any cross-current between sources, even if the sources
have large phase displacement. Normally, transfer between sources with large phase difference is not allowed.
The molded case switches with magnetic trips will provide short circuit protection only – overcurrent protection must be
provided by upstream and/or downstream devices.
Triple-redundant control power supplies, independent gate drive circuits for each sources SCR switch and other
design features provide the optimal level of reliability and immunity to single point failures. The Epsilon STS™ is
also built to withstand up to 100KA of available short circuit current.
A Bypassing scheme is provided. This can be used to manually bypass the AC load directly to either of the two
power sources and isolate the static switching for servicing.
The Epsilon STS™ is available in three current ratings – 200A, 400A and 600A (208-600V range). The 200A model
is natural convection cooled. The 400A and 600A models use forced air cooling by fans (with one redundant fan).
The STS is designed for installation in a room where humidity and temperature can be controlled. The recom-
mended and maximum environmental parameters are listed in Appendix B, of this document.
The Epsilon STS™ is listed for safety by Underwriters Laboratories, Inc. (UL) under UL Standard 1008.
Introduction 1 — 3 86-504004-00 B03