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
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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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This was first published in May 2010