What are harmonics in electrical networks and how do they affect us?

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:

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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".

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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.