logo icon

Harmonic Amplification

Harmonic Amplification
Updated: | 6 min read

Technical Articles

Harmonic amplification is an undesired increase in the magnitude of harmonics beyond the level that is being generated in the system. This in turn amplifies the ill effects of the harmonics. The article briefs how the amplification happens in the network and solutions to avoid the amplification. 


Power capacitors are added to the network for improving the power factor. The addition of capacitor results in reduction of system impedance for high frequency harmonics, because capacitive impedance is inversely proportional to frequency (as shown in the figure 1).


Capacitive impedance

Measurement with APFC Panel OFF

Hence the capacitor offers lower impedance for high frequency (250 Hz, 350 Hz, 550 Hz and so on). This will result in increase in the magnitude of harmonic currents. This can be practically seen by measuring harmonics at a particular location in the electrical network with and without power factor correction capacitors (APFC panels).


Following are the snapshots of harmonic measurement done at the main incomer, with and without capacitors

Capacitive Reactance
Measurement with APFC Panel ON and OFF

Figure 2 (Measurement done with APFC Panel OFF):


In this case, the power factor, obviously, will be poor. But the harmonic contents are very much within the limit (overall I-THD around 3.5% and 5th harmonic around 2%)

Figure 3 (Measurement done with APFC Panel ON):


As soon as the power factor correction capacitors are connected, the THD value shoots up. The overall I-THD increased from 3.5% to 22% and 5th harmonic increased from 2% to 17%. The absolute magnitude of 5th harmonic amplified from 16 A to 144 A. The distortions in the waveform also indicate the same.

In the above equation (1), for the same set of harmonic frequencies, on adding more capacitors for PF improvement, the capacitive impedance (XC) will drop further. Again this will result in amplification of the harmonics. If the power factor goes to leading, the amplification will be worse. The unnecessary amplification of high frequency harmonics will damage power capacitors and over heat switchgear, cables and busbars.


In summary, the harmonic amplification will happen because of increase in frequency, excessive addition of capacitors or both the above.

The solutions to prevent harmonic amplification are:


  • By connecting a series inductor, so as to form a detuned filter (series LC), the impedance will increase, when the frequency increases impedance will be high for high frequency harmonics and no amplification will happen. Hence, the THD (with reactor + capacitor) will be less than or equal to the earlier THD levels with no capacitors.
  • By strictly avoiding leading power factor, the excess addition of capacitors can be prevented and hence the amplification because of this can also be avoided. The optimum power factor of 0.97 to 0.99 should be maintained always.
Series Indicator