Drives, UPS systems, rectifiers and LED lighting inject 5th, 7th, 11th and 13th harmonic currents into the network. Because a capacitor's impedance falls as frequency rises, most of these currents flow into the compensation panel.

 

The real danger is resonance. The capacitor bank forms a parallel resonant circuit with the transformer inductance. If the resonant frequency coincides with a harmonic, current and voltage are amplified many times over; the result is swollen capacitors, blown fuses, overheating contactors and a poor reactive power ratio. A harmonic filter reactor solves the problem at its root by shifting this resonance to a safe frequency.

 

What Is a Detuned Filter?

In a detuned filter, a reactor is connected in series with each capacitor step. The reactor percentage (p) is the ratio of the reactor's reactance to the capacitor's reactance: p = XL / XC. The tuning frequency is fr = fn / √p.

On a 50 Hz network, a 7% reactor brings the tuning frequency to about 189 Hz (3.78th harmonic). Above this frequency the LC branch behaves inductively, so it cannot resonate with the 5th and higher harmonics. The aim is not to absorb the harmonics completely, but to protect the capacitor.

 

5.67%, 7% or 14%: Which and When?

In most industrial plants in Türkiye, 7% is the right choice. If the 3rd harmonic is dominant, choose 14%. 5.67% is used only in special cases confirmed by measurement.

• 5.67% reactor: tuning frequency 210 Hz (4.2nd harmonic), capacitor voltage ×1.06 (≈424 V), 440 V capacitor. For sites where the 5th harmonic dominates and there is no 3rd harmonic.

• 7% reactor: tuning frequency 189 Hz (3.78th harmonic), capacitor voltage ×1.075 (≈430 V), 440 V capacitor. General industry with drives, UPS and mixed loads; the most common choice.

• 14% reactor: tuning frequency 134 Hz (2.67th harmonic), capacitor voltage ×1.16 (≈465 V), 480–525 V capacitor. Offices, shopping centres, hospitals, heavy LED and single-phase loads with a significant 3rd harmonic.

The capacitor voltage rise is calculated as 1/(1−p). In a detuned panel, the capacitor rated voltage must therefore always be higher than the network voltage.

 

Choose the Right Reactor in 5 Steps

• 1. Measure: Record at least one week of power quality data at the main panel, including THD values and the harmonic spectrum.

• 2. Identify the dominant harmonic: If 3rd harmonic current is significant, start with 14%; otherwise with 7%.

• 3. Check the transformer and existing capacitors: Calculate the resonant frequency from the transformer rating, short-circuit voltage (uk) and bank rating.

• 4. Determine the capacitor voltage: Choose a 440 V, 480 V or 525 V capacitor according to the reactor percentage, and calculate the kVAr based on actual output.

• 5. Match the reactor to the step: Reactor current, linearity and temperature class must match the capacitor; use reactors compliant with IEC 60076-6.

 

Common Mistakes

• Adding only a reactor in front of a 400 V capacitor: the capacitor runs at overvoltage and its life is shortened.

• Mixing detuned and non-detuned steps on the same busbar: the non-detuned step creates a new resonance point.

• Reading capacitor output from the nameplate: a capacitor rated 440 V delivers fewer kVAr at 400 V.

• Choosing a percentage without measuring: in a site with 3rd harmonics, a 7% reactor will overheat.

• Neglecting ventilation: without a fan and thermostat, capacitor life drops significantly.

 

Frequently Asked Questions

Does a 7% reactor eliminate harmonics completely? No. A detuned filter primarily protects the capacitor and prevents resonance. If IEEE 519 limits must be met, a tuned filter or an active harmonic filter (AHF) is needed.

Can I add reactors to my existing panel? Usually the capacitors also need to be replaced with higher-voltage units; panel space and ventilation must be checked too.

How do I know which percentage to choose? The safest way is a one-week power quality measurement. Based on the results, the reactor, capacitor and step configuration are determined together.

 

The Right Choice with Enersolix

Enersolix provides power quality measurement, reactor and capacitor selection, and manufacture of detuned power factor correction panels from a single source. Request a quote for your project via our Contact page or call +90 533 198 41 75.