Amprobe DM-111S Current-Data-Logger - Manual (Page 55)

DMIIIS
EN - 53
In the mains voltage, the fundamental has a frequency of 60 Hz, the second harmonic has
a frequency of 120 Hz, and 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
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.
16.2.2. Limit values for harmonics
EN-50160 fixes the limits for the harmonic voltages, which can be introduced into the
network by the power supplier. In normal conditions, during whatever period of a week,
95% if the RMS value of each harmonic voltage, mediated on 10 minutes, will have to be
less than or equal to the values stated in the following table.
The total harmonic distortion (Thd) of the supply voltage (including all the harmonics up to
40
th order) must be less than or equal to 8%.
Odd harmonics Even harmonics
Not multiple of 3 Multiple of 3
Order
h
Relative voltage %Max
Order h
Relative voltage %
Max
Order
h
Relative voltage % Max
5 6 3 5 2 2
7 5 9 1,5 4 1
11 3,5 15 0,5 6..24 0,5
13 3 21 0,5
17 2
19 1,5
23 1,5
25 1,5
These limits, theoretically applicable only for the supplier of electric energy, provide
however a series of reference values within which the harmonics introduced into the
network by the users must be contained.