Your questions answered: Electrical systems and data center efficiency

Electrical systems play an important role in data center energy efficiency. Questions not answered in the Feb. 18, 2016, webcast are addressed here.

02/25/2016


Brian Rener, PE, LEED AP, SmithGroupJJRBrian Rener, PE, LEED AP, SmithGroupJJR, Chicago; and Ken Kutsmeda, PE, LEED AP, Jacobs, Philadelphia, respond to topics that were not covered in the Feb. 18, 2016, webcast Critical Power: Electrical systems and data center efficiency.

Question: Given the low cost of copper, does it make sense to provide feeders larger than NFPA 70: National Electrical Code (NEC) requires to reduce the line losses?

Brian Rener: Ideally it makes sense to limit long distances for either 480 or 415 V feeders. For the numerous branch circuits to data racks, it makes sense to use busway, which will have lower losses then cables.

Question: Why not include the efficiency gained by using medium-voltage (MV) UPS for critical power feeding 5 kV or 15 kV to 415 V transformers for critical loads, and 5 kV or 15 kV to 480 V for mechanical loads?

Brian Rener: Using MV as the primary distribution makes sense from an energy and capacity standpoint. Locating the 415 or 480 V step-down transformers as close as possible to the data or mechanical loads to limit losses.

Kenneth Kutsmeda, PE, LEED AP, Jacobs, PhiladelphiaQuestion: Because IT loads vary on a daily, weekly, and seasonal basis for different types of data centers (financial, enterprise, etc.), is there a source of information to help model the energy use?

Brian Rener: The Green Grid recommends relying on an annual average energy use for calculating power usage effectiveness (PUE).

Question: Power distribution unit (PDU) transformers provide a local neutral grounding point and total harmonic distortion (THDi) triplens handling. How can this be handled when PDU transformers are not used?

Brian Rener: Triplens will result in neutral loading and also transformer loading. If we consider using overhead busway connected to a 415 V substation, then the busway can be specified with oversized neutrals and the substation transformer can be K rated or oversized.

Question: Are there many early adopters to the eco mode operation with UPSs?

Ken Kutsmeda: Like with other devices, eco mode generally means some sort of toned-down operation where certain functions are not available to consume less energy. In my experience, not many were willing to reduce reliability or add risk in order to save money on energy. They spent the money on the uninterruptible power supply (UPS) and wanted to use it as it was intended. Most of the major UPS manufacturers modified their eco mode to make it more reliable. We are starting to see more use the advanced eco mode, but mostly in scenarios where it is a “catcher” type UPS or a system with full 2N redundancy (only one of the two systems is on eco mode).

Question: If you do not have the metering infrastructure to measure information technology (IT) and facility energy usage over time, is there a preferred approach for taking "spot" measurements to estimate PUE?

Brian Rener: The Green Grid specifies the location and intervals required to report PUE. The lowest, or “basic” level of metering would require meter readings at the utility input and the UPS outputs. This should be measured monthly.

Question: Are there challenges in designing a European distribution system in order to achieve the higher voltage architecture in data centers?

Ken Kutsmeda: Yes, the two main challenges are fault current and circulating neutral currents. Impedance in the transformer limits the let-through current and reduces the available fault current on the secondary side of the transformer. Systems with larger UPS and large bypass feeders tend to have high available fault currents. By removing the transformer impedance, you eliminate the fault current limiter so the downstream system will see higher fault currents. Make sure the equipment is properly rated. Another recommendation: use multiple smaller systems instead of large multi-module systems to keep the fault current down. The other issue is due to the multiple neutral paths. Certain breakers sum the currents to determine if there is a fault. With a 415 V system you have to carry the neutral through the system (UPS, bypass, maintenance bypass) so there will be multiple natural paths and only one phase path. Current may not sum correctly causing nuisance tripping. Recommend using 4-pole breakers so the neutral is open as well as the phase conductors.                                

Question: With a 380 Vdc system, are IT power supplies available that support this? Also, would that not increase the chance of a 380 Vdc shock for someone working at the rack?

Ken Kutsmeda: Yes, there are 380 Vdc power supplies available. They are more expensive than the standard 110 to 240 V power supplies, but the cost has come down since the industry standardized on 380 V. The chances are no different than working with 120 or 240 V; in most cases at the rack it is a plug connection to the power supply, so it is low-risk type of work.

Question: Can you please explain fault calculation discussion for 415 V systems?

Ken Kutsmeda: Impedance in the transformer limits the let-through current and reduces the available fault current on the secondary side of the transformer. Systems with larger UPS and large bypass feeders tend to have high available fault currents. By removing the transformer impedance you eliminate the fault current limiter so the downstream system will see higher fault currents. You’ll need to make sure equipment is properly rated. Recommended: multiple smaller systems instead of large multi-module systems to keep the fault current down.



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