Harmonic Resonance


  • Sometimes harmonic resonance occurs between power capacitor and transformer which may cause high magnification of harmonics. 
  • The resonance can happen at one particular frequency is called as resonance frequency ( fr )  in a system in which inductive reactance       ( XL ) and capacitive reactance ( XC ). 
  • The net impedance becomes low when the inductive reactance XL is equal to capacitive reactance XC
  • The magnitude of the current becomes high particularly at resonance frequency. 
  • The resistance of the network will limit the current. 


The resonance may occur due to following reasons.

  • Series resonance due to external harmonics in the supply system and resonance between capacitor in the electrical system
  • Parallel resonance within a given electrical system and resonance between internal capacitors and inductive loads
  • Typically, the inductive reactance of the system remains constant but the capacitive reactance varies in order to main higher power factor. 
  • The resonance frequency falls, if the capacitor increases as the resonance frequency is inversely proportional the capacitance. 
  • Lower the resonance frequency is dangerous because it may match with any harmonics and causes more damage.

        fr = 1 / 2π LC

Causes of Harmonic Resonance

  • Over heating of bus bar
  • Higher rate of failure of equipment
  • Frequency failure of capacitors
  • Frequent blow of fuses
  • False tripping of MCCBs

Reduction of Harmonic Resonance

  • The harmonic resonance is reduced by DETUNE FILTER which consists of reactor is in series with each capacitor. 
  • The detune filter creates one resonance frequency which offers higher impedance for high frequency harmonics. 
  • The resonance at higher frequency harmonics ( 5th harmonics and above ) can be avoided.



A 500 kVA industrial transformer with percentage impedance % Z = 4.5 with 500 kVA automatic power factor correction panel. Calculate the resonance frequency.




Short circuit power kVAsc = kVA / ( % Z / 100 )

                                           = 500 / ( 4.5/100 )

                                           = 500 / 0.045

                                           = 11,111 kVA

Resonance frequency fr = f × kVAsc / kVAr


f = Supply frequency

fr = Resonance frequency

kVAsc = Short circuit power

kVAr = Rating of capacitor for power factor improvement

Case 1 : 150 kVA capacitor is connected for power factor improvement

Resonance frequency =  50 × 11,111 / 150

                                   = 50 × 8.60

                                   = 430 Hz

This frequency closely matches with the ninth harmonic frequency ( 450 Hz ). There is no resonance occurs in this case

Case 2 : 500 kVA capacitor is connected for power factor improvement

Resonance frequency = 50 × 11,111 / 500

                                   = 50 × 4.71

                                   = 235.5 Hz

This frequency closely matches with the fifth harmonic frequency. The fifth harmonics is least order harmonic with higher magnitude. The resonance at this harmonic may damage the equipment.

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