Impact of Leading Power Factor on Data Center Generator Systems

Overview

The paper explains that power-factor-corrected (PFC) IT power supplies present a capacitive, or leading, load, most noticeably at light load. When the UPS is bypassed, or when a load sits on a non-UPS path, the generator sees that capacitive load directly. The risk appears when the UPS is in bypass or in Tier III designs with one non-UPS path. This risk goes unnoticed in normal operation but can appear during maintenance or a fault, which is why it deserves a separate check at design stage.

The mechanism is that a generator loses voltage regulation when leading reactive current exceeds what its reduced excitation can absorb. Generators typically tolerate leading reactive current up to about 20% of rated current before becoming unstable. Because a UPS input draws less than 6% reactive current, having a UPS in the path shields the generator from the effect. These values mean the designer must read the generator capability curve together with the light-load behaviour of the IT load.

The paper gives a method for estimating leading kVAr from the IT load. The power factor of an IT power supply varies from about 0 at no load to about 0.97 at full load and is leading at light load. Historically, old non-PFC supplies had a power factor of about 0.65 dominated by harmonics. For data-centre loads, a generator capability of at least 20% leading kVAr is recommended. It also offers design and retrofit mitigations and recommends checking for this condition both in design and in operation. One limit is that generator capability depends on manufacturer and model, so the check should be made against the actual capability curve of the machine.

Key points

1. Generators typically tolerate leading reactive current up to about 20% of rated current before instability.
2. A generator capability of at least 20% leading kVAr is recommended for data-centre loads.
3. A UPS input draws under 6% reactive current, so a UPS in the path shields the generator.
4. IT PSU power factor varies from about 0 at no load to about 0.97 at full load and is leading at light load.
5. Old non-PFC supplies had a power factor of about 0.65, dominated by harmonics.
6. Risk appears when the UPS is in bypass or in Tier III designs with one non-UPS path.

Using it in design

• Run a generator check: estimate leading kVAr from the IT kW on the non-UPS or bypass path and the PSU load percentage.
• Flag the result if it exceeds 20% of the generator rated kVA.
• Suggest an inductive load bank, keeping mechanical load on the generator, or a UPS on both paths as mitigation.
• Keep a leading-kVAr capability percentage field in the generator catalogue.

Who should read it

Electrical engineers and operations teams designing generator and UPS paths.

Related standards

• ISO 8528-3
• IEEE 446 (Orange Book)
• NFPA 110

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2018

Document number

WP 200

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