- The frequency of triple harmonics is in the multiple of 3, i.e. 3rd harmonic, 9th harmonic and 15th harmonic and so on.
- 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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