Capacitor Bank + De-Tuned Reactor Sizing for Pakistani Factories 2026 — Harmonics & PFC
Do I need a de-tuned reactor with my power-factor capacitor bank?
Yes — if your Pakistani factory has ANY VFDs, large rectifiers or significant non-linear load, a plain capacitor bank will fail. VFDs inject 5th, 7th and 11th harmonic currents. A bare capacitor bank's impedance falls with frequency, so it sucks in those harmonics and can resonate with the supply transformer — amplifying the harmonic voltage, overheating and blowing the capacitors (often within 6-12 months) and tripping the bank. The fix is a 7% de-tuned reactor in series with each capacitor step: it shifts the bank's resonant frequency below the 5th harmonic (to ~189 Hz on a 50 Hz system), so the bank corrects power factor without amplifying harmonics. Size the kvar for the PF correction, add 7% reactors per step, and use an automatic controller (JKW5C). Standards: IEC 60831 (caps), IEC 61642 (reactors), IEC 61439 (assembly).
Step 1 — size the kvar
To raise power factor from current PF1 to target PF2 for a load of P kW:
Qc (kvar) = P × [tan(arccos PF1) − tan(arccos PF2)]
- Example: 100 kW at PF 0.75 → target 0.95. tan(arccos0.75)=0.882, tan(arccos0.95)=0.329.
- Qc = 100 × (0.882 − 0.329) = 55 kvar. Install ~55-60 kvar in steps.
Step 2 — split into automatic steps
- Steps let the controller match correction to actual load (light nights vs full days), avoiding leading PF (also penalised).
- Common: 60 kvar as 1×25 + 1×15 + 1×10 + 1×5, or 6×10. More steps = finer control.
- The JKW5C controller switches steps via contactors based on measured PF, with a dead-band + min-step.
Step 3 — add 7% de-tuned reactors (if any VFD/non-linear load)
- A 7% reactor tunes the step's resonance to ~189 Hz, safely below the 5th harmonic (250 Hz).
- The capacitor must be rated for the higher voltage the reactor imposes (e.g. 440V cap on a 400V system).
- Without reactors on a VFD-heavy site: expect resonance, capacitor failure in months, and possible DISCO power-quality penalty.
BoM — 60 kvar de-tuned PFC bank (Pakistani factory)
| Component | Spec |
|---|---|
| Capacitors | BSMJ 440V self-healing, stepped to 60 kvar |
| De-tuned reactors | 7% per step, iron-core |
| PFC controller | JKW5C 6-12 step, auto |
| Switching contactors | Capacitor-duty CJ19/CJX2 per step |
| Protection | MCCB + HRC fuses per step |
| Cooling | Panel fan + grille (reactors run hot) |
Common Pakistani PFC mistakes
- Bare caps on a VFD site — resonance blows them in months. Always 7% reactors.
- 400V caps behind reactors — reactor raises cap voltage; use 440V caps.
- Fixed (un-stepped) bank — overcorrects at light load to leading PF, also penalised.
- General-purpose contactors — capacitor inrush welds them; use capacitor-duty contactors.
- No cooling — reactors + caps in a sealed box in 45°C ambient = early failure.
Related
PFC Capacitor Buyer Guide · MD Penalty Calculation · PQ Audit Checklist