A capacitor bank corrects power factor by supplying reactive power locally. But on a modern non-linear-load installation, capacitors resonate with harmonics, and the resulting current amplification destroys them. Detuned reactors shift the resonance frequency below the lowest dominant harmonic. The fix is mandatory.
Four ideas to carry into the next specification conversation you have.
Automatic PFC (APFC) banks have multiple capacitor steps controlled by a relay that measures pF and switches steps in/out to maintain target pF (usually > 0.95).
A naked capacitor and the upstream transformer form an LC circuit with a resonance frequency. If a dominant harmonic matches that frequency, current amplifies 5–10×.
189 Hz tuning frequency (for 50 Hz systems) ensures the bank looks inductive to the 5th harmonic and above, preventing resonance with the dominant LED / VFD harmonics.
Spec the kVAR you need for pF correction. Then choose a reactor with the right inductance to give 189 Hz tuning. Then pick capacitors rated for the higher voltage (~525 V) the reactor creates.
Standard detuning factors for 50 Hz systems.
| Detuning factor (p) | Tuning freq | Blocks harmonics |
|---|---|---|
| p = 14% (189 Hz) | 189 Hz | 5th and above |
| p = 7% (211 Hz) | 211 Hz | 5th and above (lighter) |
| p = 5.67% (210 Hz) | 210 Hz | 5th and above |
| p = 8% (177 Hz) | 177 Hz | 5th and above (stricter) |
| p = 4.2% (244 Hz) | 244 Hz | 7th and above |
| p = 3.8% (256 Hz) | 256 Hz | 7th and above |
| No detuning | ~ 250-500 Hz | Risk of 5th/7th resonance |
Five steps to spec capacitors that survive non-linear loads.
Before specifying kVAR, run a Class-A power-quality study for at least 7 days. Identify the dominant harmonics (typically 5th, 7th, 11th, 13th). The detuning factor and capacitor voltage rating depend on this.
For Saudi 50 Hz systems with mixed LED/VFD/IT loads, 14% detuning factor sets the bank's resonance below the 5th harmonic (250 Hz). The bank looks inductive at 5th and above, preventing harmonic amplification.
The reactor adds reactance, raising the voltage across the capacitor above nominal. For 14% detuning on a 415 V system, capacitors must be rated ≥ 525 V. Using 415 V caps in a detuned bank shortens life by 5–10×.
Static APFC (mechanical contactors) works for slow-varying loads. Thyristor-switched (no contactor) responds in 1 cycle for fast loads. Active filters synthesize the correcting current and can handle harmonics simultaneously, most expensive, most flexible.
Capacitors lose μF over years (drying of electrolyte). A bank's pF correction drops gradually. Annual capacitance measurement on each step catches this before the relay over-switches and the contactor fails.
Every NLC APFC ships with 14% detuning standard, 525 V capacitors, K-13 transformer compatibility, and a 7-day harmonic survey included.
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