In short. A filter must be chosen for the problem, not the product. The study sets the target distortion, picks a passive, detuned or active solution, sizes it from the spectrum, and rechecks resonance and generator-mode behaviour. Measurement before and after proves it worked.
Filter types
| Type | Best for | Watch for |
|---|---|---|
| Detuned reactor with capacitors | Protecting capacitor banks and shifting resonance | Does not remove much harmonic current |
| Tuned passive filter | Specific dominant harmonic, such as 5th or 7th | Interacts with source impedance; can overload if the system changes |
| Line reactors or chokes | Drive front ends | Voltage drop, limited reduction |
| Active filter | Varied or changing spectra, tight limits | Cost, current rating, generator compatibility |
| Multi-pulse or active front end | New drives and UPS | Needs to be specified at purchase |
Study steps
- State the target: voltage THD, current TDD or individual limits, at utility and generator.
- Use measured or datasheet spectra. See harmonic studies.
- Choose the filter type and size to the dominant harmonics and kVAr budget.
- Re-run the frequency scan with the filter. A tuned filter creates a parallel resonance below its tuning frequency.
- Check current rating, including harmonic current from other sources.
- Check operation on generator, light load and during switching.
- Commission and measure again.
Generator-mode cautions
- A passive filter’s capacitance can push the generator to leading power factor at light load.
- Generator impedance shifts the resonant frequency compared with the utility.
- Active filters must be rated for the generator’s voltage and frequency tolerance and must not hunt with the AVR.
Illustrative example
A plant has a 5th harmonic problem on its drive bus, with a capacitor bank for power factor. The study finds the bank resonates close to the 5th. The fix is a detuned reactor rated for a resonance near the 4th, plus an active filter sized for the remaining 5th and 7th, with a control that disables the capacitor stages on generator supply. Post-commissioning measurement confirms distortion within the target.
Choosing a filter
| Filter type | How it works | Best used when |
|---|---|---|
| Detuned reactor with capacitor | Tunes the bank below the lowest harmonic, avoiding resonance | Power factor correction with harmonics present |
| Passive tuned filter | Series L and C tuned near a harmonic (such as 5th), shunts it | Dominant known harmonic, stable system |
| Active harmonic filter | Injects opposing current in real time | Varying loads, multiple harmonics, generator supply |
| Line reactor or DC choke | Reduces drive current distortion at the source | Individual drives |
| Multi-pulse or active front-end | Cancels low-order harmonics by design | Large drives |
Sizing a detuned reactor (illustrative)
Tuning frequency = fundamental ÷ √(reactor percent ÷ 100); 7 percent reactor gives ≈ 189 Hz (3.78th order)
This keeps the bank’s resonance below the 5th harmonic, so that characteristic harmonics are not amplified.
Interaction with generators
- Passive filters draw leading current at light load, which can destabilise the AVR.
- Active filters are generally friendlier on generator supply, but need generator capacity for their own rating.
- Switch passive filters out on generator supply unless the study approves.
Verification
Re-run the harmonic study with the filter, and measure on site after commissioning. See harmonic studies.
Common mistakes
- Buying a filter before measuring.
- No generator-mode check.
- Assuming a filter is permanent when load changes.
What to send us
Send measurement files and the single-line diagram. Send your application details. See also reactive power studies.
Questions engineers ask
Passive or active?
Passive suits stable, known harmonics. Active suits varied or strict cases.
Can a filter cause problems?
Yes, if it resonates with the source or is overloaded by other harmonics.
How do I verify?
Measure before and after under the same operating conditions.
Does filtering reduce generator size?
It can reduce the derating needed for harmonic heating.
This guide gives general principles. Filters should be designed from measurements.