Where Did My Efficiency Go?
European Voltage and the Elimination of Magnetics

Part Two of Three

In the last post, we pointed out that system loading, when above or below a relatively narrow band, can be actually quite poor. In this post, we’re dealing with the current design fad of delinving power to the IT system at their utilization voltage. While this has been common in the EU, it’s a new approach in the US.

There are several ways this is accomplished:

• Make the substation transfomers the same as the utilization voltage, typically 400/230V. Make the UPS modules 400V, 3P, 3W in and and out. The bypass is at the correct load voltage. We’ll call this the “substation” technique.
• Make the UPS output 400/230V, 3P, 4W and add a transformer on the bypass. We’ll call this the “UPS” technique.

Regardless of the option, the gols is to remove one or two stages of magnetics (either the low-voltage PDU or the input or output transformer on the UPS module). As stated in previous blogs, each time a high-efficiency transformer is utilized, it reduces the system efficiency by 1.5%.

This design technique has a lot of merits, if it’s applied properly. Ok, every design has merits, if it’s applied properly. Some of the caveats to this design include:

• Examine the short circuit and arc flash duty on the bypass. Unlike the internal impedance from the UPS module, there’s basically none on the bypass.
• If a 4W secondary is used on the substation, the harmonics may compel an oversized neutral in excess of current UL ratings on the equipment (i.e. 4000A = 7000A neutral at 173% of line current).
• Efficiency may be recaptured and fualt current mananged by specifying custom-wound PDU, where the impedance is lower. In this case, we would eliminate the output transformer in the UPS module and leave only the input transformer (to keep the DC reference isolated from the line-side AC input).

This is a strong design option, if enacted properly. If not, short circuit current or patchwork compromises may gavely affect your work.

Next post, IT utilization and virtualization and their impact on power consumption.