Harmonics in HVAC applications - Surprisingly simple ways to reduce harmonic mitigation costs

Overview

The brochure explains the causes and effects of harmonics, then lists the main mitigation options: passive filters, multi-pulse, active front end (AFE) and active filters. Its three rules are to mitigate only as far as regulations require, to design to a THDv target at the point of common coupling (PCC) rather than a per-drive THDi, and to choose by lifecycle losses.

The second rule in practice: regulations set voltage distortion limits, so design to 5% THDv at the PCC rather than specifying THDi of 5% or 8% at each drive. As a rule of thumb, mitigation is usually not needed when drive load is below 40% of total transformer loading, though generator supply needs care.

The worked hospital-ventilation example has 50 drives totalling about 2,100 kW, two 2,500 kVA transformers and a 4,600 kVA backup generator, with IEC/EN 61000-2-4 Class 1 required. Unmitigated THDv is 4.42% in normal mode but 7.05% when one 2,500 kVA transformer feeds both subsystems, failing Class 1; adding a 715 A active filter gives compliance in all modes.

In the cost comparison a central active filter shows about 37% lower operating cost than AFE drives, about 250 kEUR over 10 years at 0.1 EUR/kWh and 80% utilisation. The SiC-based AAF 007 active filter is claimed to have 60% lower losses than comparable filters; that is a manufacturer claim and rests on a single example plant.

Key points

1. Design to 5% THDv at the PCC rather than THDi <= 5% or 8% at each drive.
2. Mitigation is usually not needed when drive load is below 40% of total transformer loading; take care on generator supply.
3. Example: 50 drives, about 2,100 kW, two 2,500 kVA transformers, 4,600 kVA generator; IEC/EN 61000-2-4 Class 1.
4. Unmitigated THDv is 4.42% in normal mode but 7.05% with one transformer feeding both subsystems; a 715 A active filter gives compliance in all modes.
5. Central active filter shows about 37% lower operating cost than AFE drives, about 250 kEUR over 10 years (0.1 EUR/kWh, 80% utilisation).
6. The SiC-based AAF 007 is claimed to have 60% lower losses than comparable filters.

Using it in design

• In the power-quality check compute THDv at the PCC for every supply mode (both transformers, one transformer, generator only); generator mode is usually the worst case.
• Use 5% THDv as the default target (IEC 61000-2-4 Class 1 for sensitive loads; IEEE 519 where applicable).
• Suggest mitigation when VFD load exceeds about 40% of source rating.
• Compare a central active filter against AFE drives on 10-year loss cost.

Who should read it

Electrical and mechanical designers assessing the harmonic impact of cooling-plant drives on transformer and generator supplies.

Related standards

• IEC 61000-2-4
• IEEE 519
• IEC 61000-3-12

Publication details

Authors

Danfoss

Publisher

Danfoss

Year

2024

Document number

AD439926801978en-US0202

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