Emerson STS2 manual Load voltages Transformer fluxes Primary currents, Figures 1

Optimized Transfer Option Enhances

Cost-Efficient System Operation

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.

See also

White Paper “STS2 Optimized Transfer

(June 4, 2004)” at

www.liebert.com.

Liebert offers a patented optimized transfer option for the Liebert STS2 that greatly

improves operation when used in primary side switching applications.

Figure 1 Standard STS2 transfer

load voltages

transformer fluxes

primary currents

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.

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