Detuned vs Standard Capacitor Bank: Which One Fits Your Plant
In a detuned vs standard capacitor bank decision, the deciding factor is harmonic load. A standard bank suits a plant with mostly motors and lighting. A detuned bank, with a series reactor on every step, is needed when drives, UPS units or rectifiers make up roughly a quarter or more of the transformer load.
What you receive: after you send your bill and your drive list, a written proposal with the bank type, reactor tuning, rating and price within one working day. Request a quotation
By Eng. Ahmed Ali, Saudi Power Factor Company. Published 9 October 2026.
What is the difference between a detuned and a standard bank?
A standard bank connects capacitors directly to the supply. A detuned bank puts an inductor, called a reactor, in series with each capacitor. The pair is tuned to a frequency below the lowest harmonic your plant produces, so the bank no longer forms a resonant circuit with the supply at a harmonic frequency.
| Feature | Standard bank | Detuned bank |
|---|---|---|
| Series reactor | None | On every step |
| Harmonic loads on the transformer | Low | Significant |
| Resonance risk | Present if drives are added later | Moved below the lowest harmonic |
| Capacitor voltage rating | Supply voltage plus margin | Higher, because the reactor raises the voltage across the capacitor |
| Space and heat | Smaller | Larger, with reactor losses |
| Cost | Lower | Higher |
A detuned bank also absorbs a share of harmonic current. It is not a harmonic filter, so it does not replace one where the distortion limits are strict.
How do 7% and 14% detuned reactors differ?
The reactor's percentage rating compares its impedance with that of the capacitor, and this ratio decides where the step is tuned. On a 60 Hz system the tuning frequency is 60 Hz ÷ √(percentage as a decimal).
- 7% reactor tunes the step to about 227 Hz, which is the 3.8th harmonic. It sits below the 5th harmonic (300 Hz), the main component from six-pulse drives and rectifiers.
- 14% reactor tunes to about 160 Hz, which is the 2.7th harmonic. It sits below the 3rd harmonic (180 Hz), which appears with single-phase loads such as office equipment and lighting.
- 5.67% reactor tunes to about 252 Hz and is sometimes chosen where the 5th harmonic is strong and the 3rd is not.
Choose 7% when drives and rectifiers dominate and the 3rd harmonic is small. Choose 14% when the 3rd harmonic is large. The choice should come from measured data, which a power quality survey provides. The reactors themselves are listed on the detuned reactors page.
Worked example: why does a standard bank fail beside drives?
This is an example to show the method. A plant has a 1,000 kVA transformer with 6% impedance. It runs 180 kVA of variable frequency drives and 120 kVA of UPS units, and it needs 300 kVAr of correction.
| Step | Calculation | Result |
|---|---|---|
| Non-linear load | 180 + 120 | 300 kVA |
| Share of transformer | 300 ÷ 1,000 | 30% |
| Short-circuit power at the transformer | 1,000 ÷ 0.06 | about 16,700 kVA |
| Resonance order of a standard bank | √(16,700 ÷ 300) | about 7.5 |
| Resonance frequency | 7.5 × 60 Hz | about 450 Hz |
| Resonance order with a 7% reactor | 1 ÷ √0.07 | about 3.8 |
The standard bank resonates at about the 7.5th harmonic, which sits close to the 7th (420 Hz) that six-pulse drives generate. The 7th harmonic current is amplified and the capacitors overheat. With a 7% reactor the resonance moves to the 3.8th order, between the 3rd and 5th, where the plant produces little current. Because the non-linear share is 30%, the detuned bank is the right choice. A commonly used guide is that a standard bank is acceptable up to about 15% non-linear load, a detuned bank is advised between roughly 15% and 25%, and above that harmonic filtering needs study. The measured spectrum has the final say.
Which capacitor bank does my plant need?
Answer these in order.
- List your non-linear loads. Include drives, soft starters, UPS units, LED lighting, welding sets and rectifiers.
- Divide their kVA by the transformer kVA. Under about 15% a standard bank is usually acceptable; above it, specify a detuned bank.
- Check for planned additions. Adding drives later turns a safe standard bank into a resonant one, so size reactors now if expansion is planned.
- Measure when in doubt. A week of logging shows the harmonic spectrum and the voltage distortion.
- Choose the reactor. Use 7% for typical drive loads and 14% when the 3rd harmonic is large.
If the answer is a detuned bank, the full range is on our detuned capacitor bank page. If the load changes through the day, the bank is usually built into an automatic APFC panel with the right reactors on every step. Since 2010 Saudi Power Factor Company has held approvals from Saudi Aramco, SABIC and the Royal Commission for Jubail and Yanbu.
Detuned vs standard capacitor bank questions
What is the purpose of a detuned reactor in a capacitor bank?
The reactor moves the resonant frequency of the capacitor and the supply below the lowest harmonic in your plant. This stops harmonic currents from being amplified, which protects the capacitors from overheating, fuse failures and early ageing.
What is meant by a 7% detuned reactor?
It means the reactor impedance is 7% of the capacitor impedance at the supply frequency. This tunes the step to about 3.8 times the fundamental, below the 5th harmonic, which suits plants with six-pulse drives and rectifiers.
Why do detuned banks use higher-voltage capacitors?
A series reactor raises the voltage across the capacitor by about 1 ÷ (1 − p), where p is the reactor percentage. With a 7% reactor the rise is about 7.5%, so a capacitor rated above the supply voltage is chosen to carry it.
Can I convert a standard bank into a detuned one?
Often yes. We check the capacitor voltage rating, add reactors to each step and replace contactors if needed. If the capacitors are old or rated too low, replacing them is more economical than reusing them. Send us the nameplate data and we will assess it.
How long does a detuned panel take to deliver?
Detuned panels take slightly longer than standard ones because the reactors are an extra component to procure and mount. The schedule also depends on the reactor tuning and your drawing approval, and our typical lead time is 3–4 weeks for a detuned panel.
Not sure about the detuned vs standard capacitor bank choice for your plant? Send us your bill with the drive and UPS nameplate data and receive a quotation within one working day.
Email: inquiry@saudipowerfactor.com | Request a quotation