Emerson Liebert STS2 - Manual (Page 4)

4
Optimized Transfer Option Enhances
Cost-Efficient System Operation
Liebert offers a patented optimized transfer option for the Liebert STS2 that greatly
improves operation when used in primary side switching applications.
See also
White Paper “STS2
Optimized Transfer
(June 4, 2004)” at
www.liebert.com.
The Liebert STS2 can be used in two different types of high-availability dual bus
configurations—as primary or secondary side switches. For primary side switching,
the unit is connected to the primary or input of a downstream transformer. On
secondary side switching the Liebert STS2 is connected to the secondary or output of
two transformers.
One of the main advantages of using primary side switching is lower cost.
These savings are the result of only one power distribution unit, a lower current due
to 480V vs. 208V, and lower installation and wiring cost thanks to use of smaller three
wire cable.
The one drawback of this configuration is the creation of transformer inrush
saturation current each time switching occurs. The downstream transformer can
cause large peak saturation current during automatic transfers. The transformer
saturation is caused by DC-flux built-up during transfer, especially when the sources
are not in phase.
Figures 1 and 2
show results for the
standard STS2 vs.
the optimized STS2
for the same
condition
(alternate source
lags 120 degrees)
respectively. The
optimized transfer
control algorithm
minimizes the
transformer
saturation current
resulting from an
out of phase
transfer.
load voltages
Figure 1 Standard STS2 transfer
transformer fluxes
primary currents