Transformers are designed and rated to deal with "non-linear" loads (such as those found in computer power supplies) by the K-Factor system, which ranges from K-1 (none) to K-40 (best). Non-linear loads cause harmonics, which in turn cause additional losses in the transformer. In the data center floor-type PDU, the commonly available choices are K-1, K-13 and K-20.
The K-1 transformer is not designed for non-linear loads but is available as the lowest-cost choice. K-13 is a somewhat better choice and is priced a bit higher. K-20 is the preferred choice for data centers but carries the highest price tag. However, since the data center load is almost entirely non-linear, the long-term operating costs are lower. The K-20 transformer losses will typically be 1.5% or less versus the 2.5-3.0 % of a K-1 unit. This can add up to a substantial amount in a 24/7 environment. For example, in a 300 kVA floor-level PDU a 1% difference in losses can amount up to 3 kW at full load. This does not sound like much, but since there are 8,760 hours in a year, a 3 kW savings represents 26,280 kWH annually.
At 11.5 cents per kWH, this is approximately $3,000 per year in direct cost savings. In addition, since the floor-level PDU/transformer is within the data center, there is approximately another 1 to 2 kW of additional energy expended to cool the extra 3 kW of transformer heat load. The result is 4-5 kW or $4,000 to $5,000 in potential annual savings for a K-20 versus a K-1-rated transformer for each 300 kVA of load. In a typical 300 kVA PDU, the difference in price between a K-1 and a K-20 transformer is approximately $4,000 to $5,000. In essence, the K-20 transformer will not only perform better, it will pay for itself within 12 months (of course, the ROI is directly dependant on the utility costs). Yet like many capital expenditure versus operating expenditure purchasing decisions, the group that is responsible for building the data center can lower its up-front cost (by buying a K-1 PDU) without regard to the long-term operating expenses.
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ABOUT THE AUTHOR: Julius Neudorfer has been CTO and a founding principal of NAAT since its inception in 1987. He has designed and managed communications and data systems projects for both commercial clients and government customers.
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