Harmonics are mains failures caused by non linear loads (pc, direct current motors, rectifiers, thyristors,...).

These currents appear at frequencies multiple of 50Hz (150Hz, 250Hz, 350Hz, ...). Often we speak of the third  (3 x 50Hz = 150Hz), fifth (5 x 50Hz = 250Hz), ... harmonic. These currents influence and disturb the final shape of the sine.

Due to these harmonics, the current source has to provide more energy than necessary, extra heating is produced and interferences occur in electronic equipment.

To understand better these problems, it is necessary to make a difference between the causes. 

The third harmonic (150 Hz) occurs often in the tertiary sector. The harmonic distortion is due to electronical equipment such as computers, printers, television screens, electronical ballasts in lighting, ...

The fifth, seventh and eleventh harmonic (250, 350, 550 Hz) occur more in industrial applications (drives, DC applications)

To choose the right size of the filter, it is necessary to take some measurements on the installation in order to select the right reactive power of the fundamental component and of the other harmonics.

It is so possible to estimate the value of the parameter of the harmonic current distortion : THD% (total harmonic distortion).

THD is a number that represent the harmonic pollution and it is mathematically given by the next expression :

THD% = sum of the square of all harmonic capacities

                       square of the fundamental value

THD 20% is tolerated. When 35% is overcrossed, it is suitable to look for a solution for the problem.

You can find more information in the rubric Harmonic filters.