Analysis of Power Quality Parameters at Comparing UPS of Different Technologies

Overview

The study asks how UPS technology affects the power quality delivered to the supplied network and load. Two three-phase 380 V, 60 Hz UPS units were measured: UPS A, a 75 kVA offline unit feeding a research laboratory with a capacitive load character, and UPS B, a 40 kVA online double-conversion unit feeding a data centre. Voltage and current THD, individual harmonics up to the 41st, power factor, voltage unbalance and frequency were logged with a power-quality analyser and compared with Brazilian PRODIST Module 8 limits.

On the online 40 kVA UPS, input current THD was 33.89%, 46.92% and 23.31% per phase while output current THD was 5.93%, 7.20% and 5.79%. Output voltage THD was 0% on phases A and C and 0.87% on phase B, voltage unbalance 0.09%, frequency about 60.00 Hz and output power factor 0.97. On the offline 75 kVA unit, output current THD stayed high at 24.62%, 25.13% and 21.77%, with 3rd harmonics reduced but 5th, 7th and 9th increased, which the authors attribute to internal components resonating with the load.

The offline UPS output voltage unbalance factor was 3.91%, the only parameter that failed PRODIST limits; its output power factor was 0.95 (PRODIST range 0.92-1.00). The paper explains that offline UPS transfer delays are usually a few milliseconds, whereas online double conversion has no transfer delay but lower efficiency.

The conclusion is that a UPS installed to improve power quality must be online; an offline UPS does not improve network quality. Limits: only two units and two different loads were compared, and because rating and load type differ, the comparison does not isolate the topology effect alone. The authors stress the importance of a UPS labelling programme.

Key points

1. Online 40 kVA UPS (data centre): input current THD was 33.89%, 46.92% and 23.31% per phase; output current THD was 5.93%, 7.20% and 5.79%.
2. Online UPS output voltage THD was 0% on phases A and C and 0.87% on phase B; output voltage unbalance 0.09% and frequency about 60.00 Hz.
3. Offline 75 kVA UPS: output current THD stayed high at 24.62%, 25.13% and 21.77%, with 3rd harmonics reduced but 5th, 7th and 9th increased, attributed to internal components resonating with the load.
4. The offline UPS output voltage unbalance factor was 3.91%, the only parameter that failed PRODIST limits.
5. Output power factor: offline UPS 0.95, online UPS 0.97 (PRODIST range 0.92-1.00).
6. Offline UPS transfer delays are usually a few milliseconds; online double conversion has no transfer delay but lower efficiency.

Using it in design

• Choose online double-conversion UPS for loads where power quality improvement is expected; an offline UPS passes harmonics and unbalance through to the load side.
• Account for UPS input current THD (here 23%-47%) in grid- and generator-side filter and sizing decisions.
• For offline UPS installations, verify output voltage unbalance and the resonance-driven rise of 5th, 7th and 9th harmonics by measurement before commissioning.

Who should read it

Electrical engineers selecting UPS topology and assessing harmonic impact on the supply side.

Document contents

1. I. Introduction
2. II. Literature Review
3. III. Tests
4. IV. Results Analysis
5. V. Conclusion
6. VI. Author's Contribution
7. VII. References

Related standards

• ANEEL PRODIST Module 8

Read the document

Publication details

Authors

Rafael Provenzano Jou Alves; André Abel Augusto; Marcio Zamboti Fortes

Publisher

ITEGAM- Journal of Engineering and Technology for Industrial Applications (ITEGAM-JETIA) (GN1 Sistemas e Publicacoes Ltd.)

Year

2021

Volume

7

Issue

28

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.5935/jetia.v7i28.747

Licence

CC BY 4.0 — share with attribution

Licence evidence

doi.org

Citation