Amprobe DMIII - Page 94

AMPROBE
DMIII MULTITEST
EN-92
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 60 Hz, the second harmonic has
a frequency of 120 Hz, the third harmonic has a frequency of 180 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
stopping 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.