Harmonic currents

Very often called Harmonic . The electric currents in the electrical communications are of periodicals type, but not always sinusoidal. However, it is known that a periodic wave can be broken up by decomposition into Fourier series into a succession of sinusoidal waves whose frequency is a multiple of the fundamental frequency (in general 50 or 60 Hz). One thus distinguishes between the basic component (at the frequency of the network) of these currents, and the current Harmonique S which comes to be added with the basic component to give to the current its nonsinusoidal form.

In general, the harmonics do not come from the food but from the network customer: the powerplants generating perfectly sinusoidal tensions, the harmonic currents are due to the presence of a nonlinear electric Charge in a Electrical communication. It is said that a load is nonlinear when, subjected to a sinusoidal tension (typically to 50 Hz), it does not absorb a sinusoidal current.

Row of the harmonics

Within the framework of the decomposition in Fourier series of a periodic signal, an electric current is broken up into a sinusoidal signal at the frequency of the network, and summons it (in general infinite) sinusoidal signals, of frequency 2,3,4,5. N time the frequency of the network. It is this number N which one calls the row of the harmonic component.

Particular case of the harmonic of row 2

In general the loads of the network are symmetrical. Under these conditions the harmonics of row 2 are judicious being null.

Particular case of the harmonic of row 3

It is the most frequent case. It is generated amongst other things by the computers, the television sets or other electronics components equipped with circuits Redresseur S in entry. They are particularly harmful for the three-phase electrical communications because of the important currents of neutral then produced. The same applies to the odd harmonics of the third (9th, 15th, 21e, etc) so known under name “harmonics sequence zero.

Interharmoniques

The name of interharmonic returns to harmonics of nonwhole row. Such harmonics are frequent in the light-arc furnaces, but can also be generated by systems of electronics of power like SVC.

Apparatuses generator of harmonics

It was considered often wrongly that the generators of harmonics were primarily industrial. However of measurements on the French electrical communication, showed that the moment when the network is polluted the most by the harmonics is Sunday evening, i.e. at one moment when industries idle, but where the television sets are massively lit.

The television sets or the computers are the principal transmitters of harmonics. Indeed, these apparatuses comprise Redresseur S and capacitive filters composed of diodes, reels and condensers which disturb the tension of the network. The role of these circuits is to transform an alternating voltage delivered by the network into a continuous tension. Being given the number of this equipment during each hearth and their long period of use in one day, these domestic apparatuses emit a current and a harmonic tension which will disturb the sinusoidal tension of the network, even if they do not consume an important power.

In industry

Rectifying Variable speed transmission for engines
Chopper
Welding machines
Light-arc furnace used in the metallurgy

In the domestic uses

Lamp low consumption called to energy saving (electronic food)
fluorescent Tubes
All the apparatuses electric household appliances comprising a food with cutting (TV, video tape recorder, computer, etc)

Effects

Instantaneous effects

The effects of the most known harmonics are the destruction of the condensers and the circuit breakers.

Trading loss per inopportune opening of the Circuit breaker S. (heating, disturbance of the Electric drive of these apparatuses)
Vibrations, noises on the revolving machines.
various electromagnetic Disturbances.

It can be a question of a deterioration of the image for the TV screens or a deterioration of the sound if it is of a HI-Fl chain or a telephone.

Long-term effects

additional Echauffement of the cables and transformers, therefore premature ageing, except if downgrading of the electrical material.

Which limits to tolerate?

limit in tension

One defines the concept of Total Harmonic Distortion (see this article) In certain countries, the supplier of electrical energy engages on the purity of the power supply provided to his industrial customers, via a level of guaranteed THD (typically 2%)

The harmonic tensions can be evaluated:

- individually, according to their relative amplitude (Uh) compared to the fundamental tension U L, where H represents the row of the harmonic

- overall i.e. according to the value of the harmonic rate of distortion THD.

limit while running

In certain case, the supplier of electrical energy can impose maximum levels harmonic currents generated by its customers. These levels are expressed as a percentage fundamental level

Solutions (by order preferably of the owners)

Passive filtering

The goal is to lower the harmonic impedance of the network using filters granted on the frequencies of the harmonics generated by the process. The filters are generally installed in HTA. They can take part in the compensation in energy reactivates. They are condensing overall compounds - coils whose frequency of Résonance is granted on the rows of the generated harmonics.

Generally the harmonics of row 3 are filtered by the network (Transformateur S) and do not require specific filtering. One will on the other hand often envisage a filter granted on row 5.

Active filtering

The active filter is a converter which injects into the network of the of the same harmonics amplitude than those generated by the industrial process, but in opposition of phase. The harmonics generated by the active filter and the process are thus cancelled. It is about a solution which is applicable only in low tension, for industrial facilities of low power.

Modification of the installation

Food of the circuits pollutants and sensitive by 2 distinct transformers

Downgrading installation

See too

technical book Schneider on the harmonics

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