Eco-mode: Benefits and Risks of Energy-saving Modes of UPS Operation

Overview

Its central finding is that the saving is smaller than expected. Raising UPS efficiency from 96% to 98.5% (loss from 4% to 1.5%) lowers PUE from 1.54 to 1.50, only about 2.3% of total facility energy, or roughly US$10,000 a year for a 1 MW data center at 50% load and $0.10/kWh. UPS losses and the cooling of their heat are a small slice of the PUE stack: in the installed-base example the UPS accounts for about 9% in its own losses plus about 5% for cooling.

The bypass path is typically 98.0-99% efficient against 94-97% for double conversion, a 2-5% gain. The costs follow: detection, decision, inverter start and static switch opening take 1 to 16 ms, during which the load sees the disturbance. An 8 ms loss can saturate a transformer on restoration and trip breakers. With reactive loads the static bypass SCRs may not clear until the next zero crossing (10 ms at 50 Hz, 8.3 ms at 60 Hz), so gaps under 8 ms cannot be assured without detailed analysis.

Further risks are loss of harmonic isolation, thermal shock to the inverter just when it is needed, extra battery cycling and confused fault clearing. Modern servers ride through if voltage does not drop more than 20% for more than about 8 ms. A vendor-claimed 1.2 ms transfer was not achieved by a measured 275 kVA UPS in standard eco-mode.

In advanced eco-mode the inverter stays energised in parallel; most protection and harmonic filtering returns at a cost of 0.5-1.0% efficiency, reducing the saving to about 1.8% from 2.3%.

Key points

1. Bypass path efficiency is 98.0-99% versus 94-97% for double conversion.
2. UPS efficiency 96% to 98.5% moves PUE from 1.54 to 1.50, about 2.3% energy saving.
3. About US$10,000 a year for 1 MW at 50% load and $0.10/kWh.
4. Transfer takes 1-16 ms; an 8 ms loss can saturate a transformer.
5. Advanced eco-mode costs 0.5-1.0% efficiency and cuts the saving to about 1.8%.
6. Modern servers ride through if voltage does not drop more than 20% for more than about 8 ms.
7. Installed-base PUE 1.93 versus 1.54 for a current design (about 20% less energy).
8. A vendor's 1.2 ms transfer claim was not met by a measured 275 kVA UPS.

Using it in design

• Compute eco savings through the PUE stack (UPS loss plus the cooling of that loss), not from the UPS efficiency delta alone.
• When eco-mode is chosen, warn about downstream transformers, static-switch PDUs or STS, and large motor or VFD loads.
• Require site-specific eco-mode transfer tests and documented sensitivity settings at commissioning.
• Treat eco-mode as a reduced protection class (VFD) in availability analysis.

Who should read it

Electrical designers and operators balancing UPS efficiency against protection.

Related standards

• IEC 62040-3
• IEEE 519
• ITI (CBEMA) curve

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2014

Document number

WP 157

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