Where Did My Efficiency Go?
Critical Power System Loading & Equipment Operations
Part One of Three

Many data center owners and designers accept that the reduction of power consumption as a core goal for their facility designs and operations. What is routinely missed is that the component selection, critical power (CP) system loading, the electrical system topology and the tier rating itself has a very large influence on the system efficiency an operator will realize.

It’s this actual “efficiency” is viewed as the ratio of the input kW versus output kW. The tuning or management of the electrical system efficiency demands attention to three areas in the CP power train or on the IT platforms. These include the:

• Selection of critical power equipment (today’s blog).
• Configuration and design of the electrical system.
• Selection and deployment of IT equipment.

Today, we’re talking about the selection and loading of the critical power equipment.

What you may come to discover that there are several, naturally-occurring factors that will reduce the system efficiency seen in the facility. The fact is that system efficiency is not linear for the entire spectrum of loading and varies for a different UPS technologies or PDUs transformer types. Newer UPS technologies are now providing a more modular load ramp up, delivering inverter switching modes that smooth the efficiency curve across the modules kW output.

Conspiring against most power system efficiencies are two facts. First, the initial deployment of equipment, unlike an industrial facility, has a slow loading curve, where it may take several years to reach optimal loading in the facility with the installed equipment. This places most facilities in the low-efficiency portions of the UPS performance curves. Second, virtualization and other forms of platform compaction methods, while significantly reducing power consumption, shoves the UPS systems into those poor efficiency performance areas.

Let’s take a look at the new Liebert NXL (not a specific endorsement of Liebert, but it happens to be the clearest chart for you to review. Thank you to Brad Brindley and Jim Marshall from Liebert):

The Liebert data is indicative of double-conversion UPS systems. Line-interactive and rotary systems will have higher efficiency values when compared to double-conversion systems, but the characteristics of the efficiency curves are the same. What you notice is that:

• The efficiency curve is essentially flat from 50% to 90% loading.
• Efficiency drops off the table below 30%. It should be noted that some UPS systems are neither stable nor efficient below 20% loading.
• There’s a minor drop is system and module efficiency above 90%.
• Efficiency for static UPS single modules is always higher than for paralleled systems.
• The load’s power factor has a bearing on the UPS module/system efficiency.

In addition to UPS module construction, system loading has a meaningful affect on your efficiency. The easiest way to raise system loading is to employ either staged modules (that turn on their inverters in stages) or to turn off module in multi-module systems to carry a given load, as long as you do not compromise your tier rating.

Leaving the UPS system for a moment, let’s take a look at PDU efficiency. There are significant differences in the transformers, and these differences constitute a large efficiency gain or loss in the critical power system.

For PDUs, you can see that there’s a meaningful difference between standard and high-efficiency units. Unlike UPS systems, PDU efficiencies are not tightly linked to the CP load – it has more to do with the quality of materials and robustness of the construction of the PDU itself.

Closing this out with the PUE discussion, the UPS module loading and PDU construction efficiencies are kW or output – based considerations. To arrive at the total efficiency for the CP system, we need to examine the input kW influences. This includes load-independent considerations such as battery recharging (or flywheel or other parasitic DC-support loads). Modifying these settings will raise or lower your input kW, and this should only be undertaken with care and advise from your UPS manufacturer. Careful rectifier tuning and battery recharge current can raise the UPS system efficiency.

So, to maximize critical power system efficiency:

• Maintain UPS systems within their optimal loading ranges.
• Utilize high-efficiency PDUs, if transformers are to be used at all.
• Tune UPS systems to raise the output kW/input kW ratio as high as practical.

For CP UPS Module and PDU efficiency, the formula to apply would be:

(UPS module or system output kW efficiency) x (PDU efficiency) = CP system efficiency

Next blog – the elimination of PDUs and critical power system transformers and the rise of EU – type voltages in the United States.