Detuned Reactor Compensation: Should p Be 7 % or 14 %?
detuned reactor compensation: Detuned reactor compensation places a reactor in series with each capacitor stage so that the resonance frequency of the circuit falls below the dominant harmonics present in the system. With a detuning factor of 7 % the tuning frequency is about 189 Hz and with 14 % about 134 Hz; 7 % suits installations dominated by the fifth harmonic, while 14 % is chosen where the third harmonic is also present.
Contents
- Why does resonance occur and what does a detuned reactor do?
- How are the detuning factor and resonance frequency calculated?
- Which value of p suits which installation?
- Capacitor voltage class, losses and reactor heating
- Commissioning, measurement and verification
- Key takeaways
- Frequently asked questions
- Related articles
Installing a plain capacitor bank in a plant that contains harmonics can push the installation into resonance instead of improving power factor, and the result is failed capacitors, blown fuses and rising voltage distortion. Detuned reactor compensation adds a series reactor to each stage, lowering the resonance frequency below the dominant harmonic and making the bank behave inductively above the tuning point. This article covers the choice of detuning factor, the resonance frequency calculation, capacitor voltage selection and the measurements to take during commissioning.
Why does resonance occur and what does a detuned reactor do?
A capacitor bank forms a parallel resonant circuit together with the short-circuit inductance of the supply. If the resonance frequency of that circuit coincides with a component of the plant harmonic spectrum, the corresponding harmonic current is amplified and voltage distortion rises quickly. When a series reactor is added, the stage still behaves capacitively at fundamental frequency but becomes inductive above the tuning frequency set by the reactor, so no low-impedance path remains for harmonic currents. The size of the reactor is expressed as a proportion of the capacitor reactance at fundamental frequency, and that proportion is called the detuning factor p.
How are the detuning factor and resonance frequency calculated?
The resonance frequency is found by dividing the system frequency by the square root of the detuning factor. On a 50 Hz system, 7 % gives about 189 Hz, that is 3.8 times fundamental; 5.67 % gives about 210 Hz, or 4.2 times; and 14 % gives about 134 Hz, or 2.7 times. The selection rule is that the tuning frequency must stay below the lowest dominant harmonic present in the installation. Since the fifth harmonic sits at 250 Hz, both 7 % and 5.67 % remain safe against it; the third harmonic sits at 150 Hz, so only 14 % gives protection against that component.
Which value of p suits which installation?
In industrial plants dominated by drives, rectifiers and three-phase converters the fifth and seventh harmonics prevail, and 7 % detuning is the common and well balanced solution there. Where measurement shows significant third harmonic current, typically in offices, data centres and lighting-heavy installations with many single-phase loads and switch-mode supplies, 14 % is preferred. A value of 5.67 % is an intermediate option for plants with low harmonic levels where resonance risk still cannot be ruled out, and it offers slightly better compensation efficiency. The decision must rest on an actual harmonic measurement; a p value chosen from catalogue assumptions may not deliver the intended effect.
Capacitor voltage class, losses and reactor heating
| Detuning factor p | Approximate resonance frequency (50 Hz) | Typical use |
|---|---|---|
| No reactor (0 %) | Depends on supply impedance, unpredictable | Small installations where harmonics are negligible |
| 5.67 % | About 210 Hz, 4.2 times | Plants with measured fifth harmonic and low distortion |
| 7 % | About 189 Hz, 3.8 times | Industrial plants with drives and rectifiers, the most common choice |
| 14 % | About 134 Hz, 2.7 times | Offices, data centres and lighting loads with third harmonic present |
A series reactor raises the voltage at the capacitor terminals at fundamental frequency, which is why a 400 V system typically uses 440 V or 480 V capacitors for 7 % detuning and 525 V class capacitors for 14 %. A capacitor chosen with too low a rated voltage fails early through dielectric fatigue. Reactors generate additional losses under harmonic currents and noticeably raise the internal heat load of the panel; the thermal calculation must include those losses, and thermally protected reactors with forced ventilation should be provided where required. Capacitor contactors with pre-charging resistors are used for switching, and the discharge period must be complete before a stage is re-energised.
Commissioning, measurement and verification
Before commissioning, a power quality measurement of at least one week at the main supply point is the most reliable way to see the harmonic spectrum and the load profile. During commissioning, voltage distortion, capacitor current and reactor temperature are monitored stage by stage; if the current exceeds the rated value by more than about fifteen per cent, harmonic loading must be assessed. Stage switching sequence and dwell times are set in the control relay and the discharge period must not be violated. Measurement results and settings are recorded in the commissioning report, and capacitance values and reactor connection torques are rechecked at periodic maintenance.
Key takeaways: detuned reactor compensation
- The resonance frequency is set by p — the tuning frequency must stay below the lowest dominant harmonic.
- 7 % addresses the fifth, 14 % the third harmonic — the choice follows the measured spectrum.
- Capacitor voltage class is increased — a 400 V system typically uses 440 to 525 V class capacitors.
- Reactor losses enter the thermal calculation — harmonic current produces extra loss and temperature rise.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Does a detuned reactor filter harmonics?
No. The purpose of a detuned reactor is not to filter harmonics but to prevent the compensation bank from resonating with them. If the harmonic level itself has to be reduced, a tuned passive filter or an active harmonic filter is used; a detuned bank does not replace either solution.
Can I add reactors to an existing plain capacitor bank?
Only after reassessing capacitor voltage class, contactor selection, busbar distances and panel heat load. Because the reactor raises the capacitor terminal voltage, existing 400 V class capacitors are no longer suitable. In practice replacing the complete stage modules is safer and usually more economical.
How do I verify the chosen p value?
During commissioning, measure busbar voltage distortion and the harmonic content of capacitor current with the stage energised. In a correctly selected bank the dominant harmonic current is not amplified and voltage distortion does not rise when a stage comes in. If it does, the tuning frequency is too close to the dominant component and the p value should be reconsidered.
Related articles
- What is Reactive Power Compensation? Benefits of Compensation Panels
- Harmonics and Power Quality: Harmonic Filtering in Switchboards
- Energy Monitoring in Switchgear: Meters, Modbus and SCADA Integration
- All technical articles
Devpan compensation panels are designed by calculating the detuning factor, capacitor voltage class and heat load from the measured harmonic spectrum of the plant. Contact our engineering team for a power quality measurement and a stage plan. Related Devpan solution: DevComp power factor correction panel.
