HT instruments SIRIUS89N User Manual
Page 123
SIRIUS89N
EN - 121
LEGENDA:
1. Fundamental
2. Third Harmonic
3. Distorted Waveform
Effect of the sum of 2 multiple frequencies.
In the mains voltage, the fundamental has a frequency of 50 Hz, the second harmonic has
a frequency of 100 Hz, the third harmonic has a frequency of 150 Hz and so on.
Harmonic
distortion is a constant problem and should not be confused with short events such as
sags, surges or fluctuations.
It can be noted that in (1) the index of the sigma is from 1 to the infinite. What happens in
reality is that a signal does not have an unlimited number of harmonics: a number always
exists after which the harmonics value is negligible. The EN 50160 standard recommends
to stop the index in the expression (1) in correspondence of the 40
th
harmonic.
A fundamental element to detect the presence of harmonics is THD defined as:
1
40
2
2
V
V
THDv
h
h
∑
=
=
This index takes all the harmonics into account. The higher it is, the more distorted the
waveform gets.