Research on Availability of Data Center UPS Power Supply System

Overview

This paper by Chai Yongcai, Li Changchun, Li Qun and Yao Jianqiang, published in 2018 at JIMEC 2018 (Atlantis Press), is a case study of a 1,200-cabinet Class A (GB 50174) data centre in China. It compares a 2N dual-bus UPS system with the 'one way (N+1) UPS plus one way mains' dual-bus arrangement allowed by GB 50174-2017. The introduction states that power-system failures account for 72.2% of data-centre failures.

The method is a series-parallel availability model. It uses published component MTBF/MTTR values for grid, transformer, switchboard, ATS, generator, UPS, PDU, battery and STS: for example, transformer 1,484,736 h MTBF and 156 h MTTR, UPS 156,362 h and 8 h, generator 273,298.7 h and 18.58 h, PDU 500,000 h and 1 h, with grid availability of 0.99979779. The model calculates IT-load availability for dual-corded and single-corded loads.

In the 2N dual-bus UPS system, dual-corded IT load availability is 0.9999999952 (eight nines), and single-corded load through a small ATS is 0.9999989952. The one-way (N+1) UPS plus one-way mains scheme gives 0.9999999954 for dual-corded and 0.9999989954 for single-corded loads, equal to or marginally higher than 2N. Removing six 400 kVA UPSs (45% load, 92% efficiency) was estimated to save 658,100 kWh per year and about 11 million CNY in equipment and space.

One caution concerns mechanical loads: supply availability for air-conditioning and other mechanical loads was only about 0.99994, below the 0.99995 usually expected for a T4 facility, so the mechanical power path limits overall site availability. The study's limit is a single case and modelling with published component data; results should not be transferred directly to other sites or grid quality.

Key points

1. Component data (MTBF h / MTTR h): transformer 1,484,736 / 156; high-power ATS 102,093.95 / 5.74; generator 273,298.7 / 18.58; UPS 156,362 / 8; PDU 500,000 / 1; STS 162,022 / 7; grid availability 0.99979779.
2. For the 2N dual-bus UPS system, dual-corded IT load availability is 0.9999999952 (eight nines) and single-corded load via small ATS is 0.9999989952.
3. Supply availability for air-conditioning and other mechanical loads was only about 0.99994, below the 0.99995 expected for a T4 facility, so the mechanical power path limits overall availability.
4. The one-way (N+1) UPS plus one-way mains scheme gives 0.9999999954 dual-corded and 0.9999989954 single-corded, equal to or marginally higher than 2N.
5. Removing six 400 kVA UPSs (45% load, 92% efficiency) was estimated to save 658,100 kWh per year and about 11 million CNY in equipment and space.
6. Power-system failures are cited as 72.2% of data-centre failures.
7. Related documents: GB 50174-2017, ANSI/TIA-942-B-2017 and Uptime Institute Tier Standard: Topology.

Using it in design

• Offer 'UPS + utility (mains) dual bus' as a topology template beside 2N and N+1, with an RBD calculation that shows it can match 2N for dual-corded IT load.
• The availability report must also cover mechanical-load power paths (chillers, CRAH via ATS), since they can limit site availability.
• Provide default component MTBF/MTTR values with source tags.
• Allow region-specific standards (GB 50174 Class A/B/C, TIA-942 Rated 1-4) as selectable rule sets.

Who should read it

Electrical designers and reliability engineers who compare UPS topologies and power-path availability with RBD calculations.

Related standards

• GB 50174-2017
• ANSI/TIA-942-B-2017
• Uptime Institute Tier Standard: Topology

Publication details

Authors

Chai Yongcai; Li Changchun; Li Qun; Yao Jianqiang

Publisher

Proceedings of the 2018 3rd Joint International Information Technology，Mechanical and Electronic Engineering Conference (JIMEC 2018) (Atlantis Press)

Year

2018

Document language

English

Source type

Scientific

Access

Free to read

DOI

10.2991/jimec-18.2018.63

Licence

CC BY-NC 4.0 — share with attribution for noncommercial purposes

Citation