Short answer
Hybrid compensation active filter schemes combine cheap capacitor banks for steady reactive power with an active filter for harmonics and fast reactive changes. The active filter’s current rating covers both duties together, combined in quadrature. In an illustrative case, a 300 A filter on a 400 V bus that needs 200 A for harmonics has about 224 A, or about 155 kvar, left for reactive support, not the 208 kvar its full rating would suggest. If both the capacitor controller and the active filter regulate power factor, the faster filter may take the whole reactive duty and leave the capacitors idle, using up its capacity, or the two may hunt against each other. Roles must be assigned, priorities set in the controls, and the remaining capability checked for when the filter trips. An active filter’s current rating is not a kvar rating.
Sharing the filter’s current
| Use | Current | Equivalent kvar |
|---|---|---|
| All reactive | 300 A | ≈ 208 kvar |
| Harmonics | 200 A | — |
| Remaining reactive | ≈ 224 A | ≈ 155 kvar |
Irated ≥ sqrt(I_h^2 + I_q^2)
Illustrative example. Active filter rated 300 A on a 400 V bus; harmonic correction needs 200 A RMS. Values chosen for demonstration.
I_q ≤ sqrt(300^2 − 200^2) ≈ 224 A
Q ≈ 1.732 × 400 × 224 ≈ 155 kvar
If treated as a pure reactive source, 300 A would be about 208 kvar. The harmonic duty takes the rest.
Role assignment
| Task | Best handled by |
|---|---|
| Steady fundamental kvar | Capacitor banks (detuned) |
| Harmonic currents | Active filter |
| Fast kvar changes between stages | Active filter, limited |
| Unbalance correction | Active filter, if capable |
Detuned banks are preferred so the bank does not amplify harmonics the filter must then correct; see detuning factor in transformer-fed capacitor banks.
Avoiding hunting
- Priority: capacitor controller handles steady kvar; the active filter’s reactive mode limited to a band or disabled.
- Measurement points: controller and filter CTs placed so each sees the right quantity; see active harmonic filter CT placement.
- Time constants: capacitor controller slower, filter faster, with the filter’s reactive output returning toward zero once a stage switches.
- Communication between controller and filter where supported.
Without these, the filter may sit at full reactive output while capacitor stages stay off, leaving no capacity for harmonics.
After a failure
- Filter trips: capacitors remain; check harmonic levels and capacitor duty without the filter; see capacitor overload from background harmonics.
- Capacitor stage fails: filter may be asked for more kvar than its spare capacity.
- Transformer configuration changes: recheck resonance; see harmonic amplification after one transformer is removed.
Voltage steps from capacitor stages are covered in capacitor-bank step size and voltage steps.
What this article does not cover
It does not treat an active filter’s current rating as equal to compensating kvar.
Where this reasoning stops. The quadrature sum is a simplification; manufacturers specify how ratings apply.
Practical takeaway
Assign steady kvar to detuned capacitors and harmonics to the active filter, check the filter’s current split in quadrature, set control priorities to avoid hunting, and verify performance with either device out.
Reference framework
- IEEE 519, Standard for harmonic control in electric power systems.