Data center power strategies

Overview

The article is an expert piece by Kenneth Kutsmeda in Consulting-Specifying Engineer, published in 2013. It compares the legacy medium-voltage, 480 V, UPS and 208/120 V distribution chain with 415/240 Vac, 480/277 Vac, 600 Vac and 380 Vdc alternatives. The comparison criteria are losses, fault current, arc flash and practicality. The author first quantifies component efficiencies and the cost of distribution losses. Electrical distribution losses average about 12% of total data centre energy, and for a 2,000 kW IT load the losses cost roughly $280,000 a year. A double-conversion UPS is about 84% efficient at 25% load and 94% at 100%; flywheel and standby types are about 94% and 99%.

The alternatives are then assessed one by one. 415/240 Vac eliminates the PDU transformer and cuts losses an estimated 5-7%, but raises available fault current and arc-flash exposure. 380 Vdc is estimated to cut losses 8-10% and is most cost-effective in Tier IV, where the rectifier can serve as a redundant bypass. 600 Vac reduces copper and fault current but yields no efficiency gain.

The article also presents a theoretical 2 MW, 2N case study: against 208/120 V, 415 Vac gave 12% CAPEX and 20% OPEX savings and 380 Vdc 14% CAPEX and 28% OPEX savings. The author flags the engineering challenges of each option, such as high fault current at 415 V and arc extinction at dc. Limit: the results rest on a theoretical case.

Key points

1. Electrical distribution losses average about 12% of total data centre energy; a 2,000 kW IT load costs roughly $280,000 a year in losses.
2. Double-conversion UPS about 84% at 25% load and 94% at 100%; flywheel and standby types about 94% and 99%.
3. 415/240 Vac eliminates the PDU transformer and cuts losses an estimated 5-7%, but raises fault current and arc-flash exposure.
4. 380 Vdc is estimated to cut losses 8-10%; most cost-effective in Tier IV where the rectifier can serve as redundant bypass.
5. 600 Vac reduces copper and fault current but yields no efficiency gain.
6. 2 MW 2N case: 415 Vac gave 12% CAPEX and 20% OPEX savings and 380 Vdc 14% and 28%, versus 208/120 V.

Using it in design

• Offer distribution-voltage options (208/120, 415/240, 380 Vdc) with automatic loss and fault-current comparison.
• When 415/240 V is selected, prompt for short-circuit and arc-flash studies and high-impedance transformer options.
• Use part-load UPS efficiency curves rather than a single efficiency value.
• Assess arc extinction at dc and equipment availability separately for 380 Vdc.

Who should read it

Design engineers and consultants choosing a data centre electrical distribution architecture.

Related standards

• NEMA TP1
• IEEE 1100-2005
• NFPA 70E
• ETSI EN 300 132-3

Publication details

Authors

Kenneth Kutsmeda

Publisher

Consulting-Specifying Engineer

Year

2013

Document language

English

Source type

Expert / community

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation