Comparison of Static and Rotary UPS

Overview

The comparison covers double-conversion, delta-conversion and flywheel-backed static units against rotary units, both motor-generator/battery and diesel engine-coupled. The main argument is efficiency at realistic part load. Most UPSs run below 80% load, typically 45-55%, and redundant UPSs typically run at 30 to 50%. Rotary units carry higher fixed losses, so static wins in that range. A worked example at 35% load puts double-conversion static at 95% efficiency against 90% for an average rotary. For a 2 MW load at $0.13/kWh over 10 years, losses are $1,195,740 versus $2,528,136. Including heat removal raises the net difference to $2,664,792 and the static option avoids 12,298 metric tonnes of carbon emissions.

Energy storage changes generator behaviour: battery strings typically give five to 15 minutes or more, while a flywheel typically supplies only 8 to 10 seconds, so the generator must start for every outage. Per IEEE 493, diesel generators fail once per 74 start attempts, and rotary units may need over 15 seconds to reach nominal torque. Sample component MTBF: rectifier/charger 100,000 h, battery 120,000 h, inverter 70,000 h, static switch 475,000 h.

On investment, a rotary solution can be a 40% larger up-front cost than a similar static unit and is typically not modular. On service, flywheel bearing replacement in a diesel rotary UPS has an MTTR averaging 24-36 hours and needs a lifting device for 3 to 5 tons. The conclusion is that rotary becomes competitive mainly at multi-megawatt scale.

Key points

1. Most UPSs run below 80% load, typically 45-55%; redundant UPSs at 30-50%.
2. At 35% load: double-conversion static 95% vs average rotary 90%.
3. 2 MW, $0.13/kWh, 10 years: losses $1,195,740 (static) vs $2,528,136 (rotary).
4. With heat removal the difference is $2,664,792; static avoids 12,298 metric tonnes of carbon.
5. Batteries give 5-15 minutes or more; a flywheel only 8-10 seconds.
6. Rotary can cost 40% more up front and is typically not modular.
7. Sample MTBF: rectifier 100,000 h, battery 120,000 h, inverter 70,000 h, static switch 475,000 h.
8. Per IEEE 493 a diesel generator fails once per 74 starts; flywheel bearing MTTR is 24-36 hours.

Using it in design

• Store UPS efficiency as a curve with at least 25/50/75/100% load points, plus storage type (battery, flywheel) and ride-through seconds.
• Evaluate losses at the per-module load implied by the redundancy topology (2N gives at most 50%) and add the cooling penalty to lifecycle cost.
• Require a generator start probability check for ride-through below 30 s.
• When rotary is chosen, require weight/floor loading, lifting provision, ventilation and noise fields.

Who should read it

Electrical designers choosing UPS technology and comparing lifecycle cost.

Related standards

• IEEE 493
• IEC 62040-3

Publication details

Authors

Carl Cottuli

Publisher

Schneider Electric

Year

2011

Document number

WP 92

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