How to Reduce / Mitigation Third Harmonic?

  • The frequency of triple harmonics is in the multiple of 3, i.e. 3rd harmonic, 9th harmonic and 15th harmonic and so on.
Sources of Triple harmonics
  • The triple harmonics are generated by single phase non linear load that have common neutral. 
  • These non linear loads are common in office, hospitals, commercial buildings, educational building, IT offices. Some of examples are

( 1 ) Compact Fluorescent Lamp
( 2 ) Electronic ballast
( 3 ) SMPS in the Computer
Triple Harmonic in Neutral Conductor
  • If the load is balanced, the neutral current is zero as the vector sum of all three phase current is zero. 
  • The current flows through neutral conductor in spite of load are balanced due to presence of triple harmonics
  • All the triple harmonics are zero sequence in nature and they are in phase with all the three phases R, Y and B. 
  • The neutral current increases up to 2 times phase current due to presence of third harmonics.

Causes of Triple harmonic
  • Burning of neutral conductor due to overloading on it ( or size of the neutral conductor must be kept higher in order to consider effect of third harmonic )
  • Cable overheating due to skin effect
  • Over heating of distribution transformer
  • Higher eddy current losses as the eddy current losses is directly proportional to square of the frequency
  • Due to heating, it will reduce the life of transformer, switch gear and other switching devices

Methods to Reduce / Mitigation Triple harmonic
  • The Detuned filter ( Type of passive filter consists of series combination of power capacitor and reactor which offer higher impedance for higher frequency harmonics ) is used to reduce the effect of triple harmonic. 
  • The tuning frequency of the detuned filter is 133 Hz, which is below third harmonic frequency ( 150 Hz ). 
  • The amplification of 3rd harmonic is reduced by this filter. However this filter does not eliminate the harmonic completely.
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