Active filter or AFE drive - What is a better option?
When taking care of grid harmonics there are several technical solutions to choose from. You might stand in front of a dilemma: active filter or AFE drive? What is a better choice?
Nowadays there are multiple approaches how to tackle the network harmonic issues. The harmonics can principally be solved on a converter level, such as using AFE drives or DFE drives with multi-pulse rectifier and corresponding multi-winding transformer. Or they can be eliminated by using an active or passive filter.
Active filter is a shunt device. It generates the harmonic spectrum that shall be eliminated – with an opposite phase (180 degree inverted). At low voltage such solution is quite effective. The low voltage active filter (400 V or 690 V) has a considerable bandwidth and can eliminate multiple harmonics including higher orders. At medium voltage, however, the solution looks quite different. The bandwidth is much more limited and elimination of high order harmonics is very challenging.
The advantage of an active filter is that one active filter may be used as a global solution for multiple drives or other harmonic sources. That makes the solution commercially atractive. It allows using simple (cheaper) rectifier with less good waveform and fix it using an active filter. Moreover, active harmonic filter offers some additional “goodies” that are not available in AFE drives (e.g. resonance damping).
AFE drives tend to solve the harmonics on the converter level, i.e. directly at the source. By using suitable modulation several harmonic orders can be eliminated. At medium voltage, as the bandwidth is limited, some AFEs may use a multi-winding transformer as well (typically 12-pulse). Another possible strength is that in some cases AFE drive can eliminate the input transformer in so called direct-to-line configuration.
Besides active filters and AFE drives other proven solutions are DFE multi-pulse rectifier or using a VFD in combination with passive filters. DFE multi-winding rectifiers eliminate harmonics only while the passive filter inherently provides reactive power componsation as well. However, the passive filter is a bulky system and the traditional multi-branch solution requires a larger switchgear with several feeders.
The comparison of active filter, AFE type VFD, DFE drive with multi-winding transformer and a passive filter is shown in Table 1 (full access for premium subscribers).
In order to make the comparison more realistic, we distinguish low voltage and medium voltage levels separately. The advantages of one solution at low voltage can be much compromised at medium voltage so that the picture looks quite different.
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References
[1] COMSYS – Perfecting power, https://comsys.se/
[2] Network harmonics: VFD harmonic spectrum, https://mb-drive-services.com/network_harmonics_vfd_harmonic_spectrum/
[3] Network harmonics: Harmonic filter, https://mb-drive-services.com/network-harmonics-harmonic-filter/
[4] Any drawbacks of active harmonic mitigation? https://mb-drive-services.com/any-drawbacks-of-active-harmonic-mitigation/
