What are harmonics in electrical networks
and how do they affect us?
With the
massive installation of equipment based on power electronics (computers,
variable speed drives, inverters ...), most users are faced with the presence
of harmonics in electricity distribution networks. This article will help you
better understand the cause of certain breakdowns and how to avoid them.
Harmonics
are usually defined with the two most important data that characterize them,
which are:
The amplitude:
refers to the value of the voltage or intensity of the harmonic,
The order:
refers to the value of its frequency referred to the fundamental (50 Hz). Thus,
a harmonic of order 3 has a frequency 3 times higher than the fundamental that is
3 * 50 Hz = 150 Hz.
The order
of the harmonic, also referred to as the harmonic range, is the ratio between
the frequency of a harmonic FN and the frequency of the fundamental (50
Hz). n = fn / f1.
Every
electrical network is comprised by a certain amount of charges. When the
current passing through a charge has the same shape as the voltage, this charge
is called linear ; on the other hand, when the shape of the current does not
correspond to the shape of the voltage, the load is called non-linear (see
figures 1 and 2)
But what are harmonics?
The
harmonic currents are the similar components of a periodic electrical current
decomposed in the Fourier series. The harmonics have a frequency that is
multiple (2, 3, 4, 5, ... n) of the fundamental frequency (50 or 60 Hzen
electrical networks). The number "n" determines the range of the
harmonic component. The harmonic component of the range corresponding to
"n" times the frequency of the network is called "harmonic of
the range n".
Example:
for a fundamental frequency of 50 Hz, the harmonic of rank 5 will present a
frequency of 250 Hz.
The
harmonics of even range (2,4, 6, 8 ...) are not usually studied in industrial
environments because they cancel out thanks to the symmetry of the alternating
signal. They are only taken into account in the presence of a continuous
component. On the other hand, single-phase non-linear loads have a spectrometer
in odd-range harmonic components (3, 5, 7, 9 ...), which also happens in
three-phase triangle-connected loads, except that the latter do not have 3-rank
components.
In addition
to the range, the harmonics are classified according to their amplitude
(indicated in% with respect to the fundamental) and their parity (even or odd).
The harmonics, which also have importance in the electromagnetic compatibility,
are part of the disturbances treated in the EN 50160 standard with regard to
the quality of the electrical supply.
Spectrum of harmonic frequencies
An
essential element of the study is the spectrum of harmonic frequencies of the signal;
it is the graphic representation that lists the harmonics present, the signal
in frequency and in amplitude.
It must be
taken into account that in addition to the harmonics indicated above, in the
networks there are also two other types of components superimposed on the
fundamental wave. The inter-harmonics that are characterized by a frequency
that is not multiple of the fundamental (for example: 175 Hz is not a multiple
of 50 Hz), and the infra-harmonics which have a lower frequency than the
network.
The former,
although they are only present in a small quantity, can, for example, disturb
the remote control signals sent by the distributors of electrical energy, while
the latter are usually due to the cycle converters, the arc furnaces or the
speed variators.
