Designing power systems for co-location data centers

Overview

In this 2018 article in Consulting-Specifying Engineer, Brian P. Martin works through the electrical design decisions for a speculative colocation facility. Topics include project economics, SLA metrics, unknown tenant IT loads, MV topology, generator rating, UPS and PDU trends, metering and redundancy architectures. The article asks how a facility with no known tenants should plan its capacity. Each decision is discussed in terms of cost, flexibility and risk.

The author contrasts loop and radial-with-tie MV systems on the medium-voltage side and distributed-redundant and catcher (reserve) UPS schemes on the redundancy side. He also covers concurrently maintainable versus fault-tolerant definitions and warns against loose Tier language: 'Tier 3 compatible' has no technical definition. Numerical examples include battery runtime shrinking from 10-15 min to 2-5 min, UPS typically operating at 40-66% load, PDU sizes growing from 225/300 kVA to 600 kVA or larger, and a distributed redundant 4+1 example running each unit at 80% when fully loaded.

The article also gives a phased substation example: 66 MVA final load as three 33 MVA transformers in 2+1, and 10 MVA on day one as two 12 MVA in 1+1. On arc flash, it notes that the highest incident energy is commonly at secondary transformers and the line side of secondary switchgear; mitigations are transformer differential, HRG, optical sensors and primary breakers/VFIs. Finally it advises sizing to peak rather than average PUE (typical PUE 1.1-1.8). As a limit, this is a trade-press article, not a standard, and rests on the author's experience.

Key points

1. Battery runtime has shrunk from 10-15 min to 2-5 min; unity power-factor UPS ratings are common; UPS typically operates at 40-66% load.
2. PDU sizes have grown from 225/300 kVA to 600 kVA or larger.
3. Distributed redundant 4+1 runs each unit at 80% when fully loaded; catcher systems range from 2+1 to 6+1 or higher and expand more easily.
4. Phased substation example: 66 MVA final load as three 33 MVA transformers in 2+1; 10 MVA day one as two 12 MVA in 1+1.
5. Highest incident energy is commonly at secondary transformers and the line side of secondary switchgear; mitigate with transformer differential, HRG, optical sensors and primary breakers/VFIs.
6. Size systems to peak PUE, not average; typical PUE 1.1-1.8; 'Tier 3 compatible' has no technical definition.

Using it in design

• Support both distributed-redundant and catcher/reserve UPS topologies with automatic post-failure load checks, warning if any unit exceeds 100% after a single failure.
• Size mechanical-electrical capacity on peak PUE.
• Avoid the label 'Tier compatible' in generated reports; state concurrently maintainable or fault tolerant explicitly instead.
• Plan the substation in modular arrangements such as 2+1 for phased growth.
• Include arc-flash mitigation measures for the secondary transformer side in the design.

Who should read it

Electrical designers, operators and investors of colocation data centres.

Related standards

• NFPA 70
• NFPA 70E
• NFPA 110
• TIA-942
• ASHRAE 90.4
• IEEE C37.20.4-2013
• The Green Grid WP #49

Publication details

Authors

Brian P. Martin

Publisher

Consulting-Specifying Engineer

Year

2018

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