A Quantitative Comparison of High Efficiency AC vs. DC Power Distribution for Data Centers

Overview

The paper compares the two highest-efficiency distribution architectures: 400/230 V AC with no PDU transformers, and a 380 V DC bus feeding DC-input IT equipment. The method is a simple product of UPS, wiring and IT power-supply efficiencies at a 50% load baseline. Certified UPS figures at 25/50/75/100% load are: Symmetra PX double conversion 95.5/96.3/96.4/96.3%; eco-mode 97.4/98.5/98.8/98.9%; Emerson 380 V DC 93.4/95.1/94.8/94.2%; and LBNL typical legacy double conversion 87.3/88.8/88.8/88.4%.

Wiring loss uses a common design value of 1% at full load; losses vary with the square of load, giving 99.5% wiring efficiency at 50% load for both AC and DC. Delta Electronics power supplies at 20/50/80/100% load reach 85.0/93.2/93.7/93.6% on AC 208 V and 85.5/94.1/94.8/94.5% on DC 400 V, a 0.9% DC advantage at 50% load. The chains at 50% load come to DC 96.5% x 99.5% x 94.1% = 90.35%, AC 96.3% x 99.5% x 93.2% = 89.30%, and AC with the UPS in eco-mode 98.5% x 99.5% x 93.2% = 91.34%. DC is roughly a point ahead of double-conversion AC and behind eco-mode AC.

The second step asks how much of a distribution gain reaches the facility total. For a typical PUE 1.47 site (IT 68%, distribution 5%, proportional A/C losses 13%, lighting 2%, fixed A/C losses 12%), a 1% distribution efficiency gain cuts total consumption by 0.86%, and DC's 1.05% advantage yields only 0.90%. A watt saved in distribution saves about 1.13 W of facility power, because only the proportional cooling losses, about 13% of IT load, scale with it.

The authors conclude that efficiency alone does not justify moving to DC, and that widely quoted 10-30% DC savings compare against obsolete AC designs. For North America, an auto-transformer adapting 480/277 V to 400/230 V needs a transformer kVA of only 17% of system rating and costs about 1% efficiency. In the ETSI DC standard the 380 V DC system is midpoint-grounded, so the maximum voltage to ground is +/- 190 V.

Key points

1. Chain efficiency at 50% load: DC 90.35%, double-conversion AC 89.30%, eco-mode AC 91.34%.
2. Certified UPS efficiency at 25/50/75/100% load: Symmetra PX double conversion 95.5/96.3/96.4/96.3; eco-mode 97.4/98.5/98.8/98.9; Emerson 380 V DC 93.4/95.1/94.8/94.2; LBNL legacy 87.3/88.8/88.8/88.4.
3. Wiring loss is 1% at full load and scales with load squared; 99.5% efficiency at 50% load.
4. Delta PSU efficiency at 50% load: AC 208 V 93.2%, DC 400 V 94.1%; a 0.9% DC advantage.
5. At PUE 1.47, a 1% distribution gain cuts total consumption by 0.86%; DC's 1.05% gives only 0.90%.
6. One watt saved in distribution saves about 1.13 W at facility level (proportional cooling loss about 13% of IT load).
7. An auto-transformer adapting 480/277 V to 400/230 V needs 17% of system kVA and costs about 1% efficiency.
8. ETSI 380 V DC is midpoint-grounded: maximum voltage to ground +/- 190 V.

Using it in design

• Build the power-chain calculator as UPS(load) x wiring(load) x PSU(load), defaulting wiring to 1% loss at full load scaling with load squared.
• Convert distribution savings to facility savings using only the proportional cooling fraction, not by multiplying by PUE.
• Store UPS efficiency at 25/50/75/100% with its mode (double conversion or eco) and a distribution voltage option set: 480/277 AC with PDU transformer, 400/230 AC, 380 V DC midpoint-grounded.
• Flag designs claiming more than about 1-2% savings from DC versus modern 400/230 V AC, and flag North American designs with transformer PDUs as an efficiency loss.

Who should read it

Electrical designers choosing between AC and DC distribution and decision makers who need to test DC savings claims.

Related standards

• ETSI EN 300 132-3-1
• IEC 62040-3

Publication details

Authors

Neil Rasmussen; James Spitaels

Publisher

Schneider Electric

Year

2015

Document number

WP 127

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

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

Citation