Harmonic currents are caused by non-linear loads connected to distribution
systems, i.e. by loads that draw current with a waveform different that that of the
voltage that supplies them.
The most common non-linear loads are equipment including rectifiers, fluorescent
lighting and computer hardware.
In installations with a distributed neutral, non-linear loads may cause significant
overloads in the neutral conductor due to the presence of third-order harmonics.
The order is the ratio
between the harmonic
frequency fn and the
(generally the power
frequency, 50 or 60 Hz):
n = fn / f1
By definition, the
fundamental f1 is order 1
Third-order harmonics (H3)
have a frequency of 150 Hz
(when f1 = 50 Hz).
The presence of third-order harmonics depends on the applications involved.
It is necessary to carry out an in-depth study on each non-linear load to determine
the level of H3:
ih3 (%) = 100 x i3 / i1
i3 = rms current of H3
i1 = rms current of the fundamental
Assuming that H3 is preponderant among harmonics, the THD is close to the value
of H3 (ih3(%)).
There are two decisive factors:
the types of connected devices:
disturbing loads: fluorescent lighting, computer hardware, rectifiers, arc furnaces,
non-disturbing loads: heating, motors, pumps, etc.,
the ratio between the two types of disturbing loads.
Mix of disturbing loads (computers,
UPSs, fluorescent lighting) and non-
disturbing loads (motors, pumps,
Low probability of harmonics
Numerous disturbing loads (computers,
UPSs, fluorescent lighting).
High probability of harmonics
15 % < THD 33%.
Origin of harmonic currents
Estimating THD (total harmonic