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DO YOU KNOW HOW TO SOLVE PROBLEMS ORIGINATED BY HARMONICS

CONTENT

Guide of quality of Electric Energy
What are the Harmonics?
What causes harmonics?
The Current and tension harmonics cause
The Current Harmonics that affect the feeding source cause
How to face the problems
Other recommendations to reduce Harmonics

GUIDE OF QUALITY OF ELECTRIC ENERGY

Have you ever been witness of harmonic distortion, power cuts, tension oscillations, tension falls and sharp points and transitory phenomenos?

These are some of the characterisitics of a bad quality Electric Energy that cause disturbances to the users. This causes short life to electrical equipment and increases the costs of industrial production.

WHAT ARE HARMONICS?

Harmonics are tension or current waves, which frequencies are higher than the fundamental frequency of the net (60 Hz in Lima).

In general, several waves of different frequencies are present simultaneously, which modify or distort the initial signal.

WHAT ARE THE CAUSES OF THE HARMONICS?

The Harmonics are produced fundamentally of no lineal charges as fluorescent illumination, power sources that feed computers, electronics velocimeters of electrical engines, rectifiers and in general high power electronic equipments.

Likewise, saturable reactance and blast furnaces.

HARMONICS OF TENSION AND CURRENT ORIGINATE

Superficial effect on the conductors, losses and heat higher to normal.
Overheat of the neutral conductors.
Great losses for parasitic currents in the transformers.
Overload the steam engines to correct the Power Phactor.
Great losses and vibration effects in the induction engines.
Great heat in the Transformers.
Noise when the the Senoidal Tension is cero.

THE HARMONIC CURRENTS THAT AFFECT THE FEEDING SOURCE

A fall of the proportional tension to the magnitud and to the frequency of the harmonic current injected by the users.
A distortion of tension that is inverted in proportion to the feeding transformer size (in big transformers, it is allowed a greater quantity of harmonic currents that in a smaller size one)

EXAMPLES

Type of Charge	Generated harmonics	Coments
Arc welding	3 (56%), 5 y 7 (9%)	Current with many harmonics
Arc furnace and alternate current	Unstable variable Spectrum	No lineal asimetric
Cicle converters	Variables	-
Regulators by Pulse width	Variables	Converter Continues - Alterner

EFFECTS

Effects de las Harmonics	Causes	Consequences
Conductors	The harmonic intensities create the value of the quadratic level of the current. The ¨Skin Effect¨ reduces the Effective section of the conductors As the frequency diminishes.	Sudden shots of the protections Over heating Of the Conductors.
Neutral Conductor	Even when the three-phase charge Is balanced, current circulates Through the neutral conductor, Caused by the circulation of The impair harmonics, multiple of 3.	Heating and over intensity in the neutral conductor.
Steam Engines	Impedance of the steam engine Diminishes wirh the increase of The frequency.	Premature ageing, Amplification Of the existent Harmonics.
Transformers	Increase of the value of the Quadratic Level of the Current. The losses by Foucault are Proportional to the square of the Frequency, the losses by hysteresis To the frequency.	Increase of the Heat by Joule Effect. Increase of the losses in the iron of the Transformer And decrease Of Performance.
Engines	Analogous to the transformers And generation of an additional Field to principal.	Analogous to the Transformers More Abnormal Mechanical Vibrations.

HOW TO FACE THE PROBLEMS?

CONDUCTORS

The presence oh harmonic currents increases the Quadratic Value Level of the Current, which causes more losses and overheating. Besides, the alternating current tends to flow towards the external surface of a conductor (¨skin effect¨), effect that is more pronounced to high frequencies, which causes additonal losses and over heating.

The switches-breakers, common magnetic thermal, which use an active mechanism that responds to the effect of heat of the circuit current could be activated due to overheating caused by the harmonics.

Solution:

To avoid the losses and actions caused by the harmonics, it should be used electrical conductors bars of greater dimension and size. Separate the lineal charges from the no lineal charges (that causes harmonics) and substitute the panels and the automatic switches-breakers where is necessary.

NEUTRAL CONDUCTOR

In the case of the three-phase circuits with no lineal charges, the harmonics of impair type (3a, 9a, 15a, etc) are not cancelled, they are added to the neutral conductor (Fig.3), so the current through the neutral conductor can be greater than the current of phase. The danger consists in an excessive overheat of the neutral cable, besides it causes voltage falls between the neutral and earth, greater to the normal.

Solution:

It is recommended that the size of the neutral conductor is double of the conductor of phase when the circuits feed no lineal charges. Optionally, a neutral conductor must be provided for each phase. Likewise, another alternative to block the flow of harmonics that tend to circulate by the neutral is to use transformers with delta-star connection.

STEAM ENGINES

The steam engines of correction of the power phactor due to the harmonic currents flow through the route of low impedance of the steam engines instead of coming back to the distribution transformer. Besides, when the steam engines and the inductances of the power distribution system form a parallel resonant circuit, that has a frequency of close resonancy to an harmonic of present significative charge, as a result the harmonic current can be increased substantially overcharging the steam engines and causing the fuses to blow up.

Solution:

The resonance can be avoided by adding an inductance in series to the steam engine to detune the frequency of resonance of the system or to install a net of correction KVAR specially designed.

TRANSFORMERS AND ENGINES

A transformer that feeds no lineal charges and an engine fed with a voltage and current distorted overheats due to the parasitic currents and the ¨skin effect¨.

This originates additional losses in the transformer and in the engine, this tends to invert its direction, reducing the torsion force and causing vibrations.

Solution:

In the case of transformers, the charge to be applied must be limited, so less power than the nominal is supplied. In the case of induction engines, separate them form the circuits that cause harmonics ( inputs of variable speed, electronic equipments of great size, etc.). The general practice is to limit the total distortion of the voltage per harmonic to a less value of 5%.

OTHER RECOMMENDATIONS TO REDUCE HARMONICS

The recommendations to control the magnitude of the harmonic currents generated can be summarized as follows:

Use of filters tostablish a route of low impedance for the harmonic currents in a way that they can circulate through the filter and not through the feeding.
Use of active filters, that are compensators of active harmonics.
Use of separation transformers that separate the harmonics, multiple of 3 of the feeding source.
Use of reactors of line for alternate current.
Perform a new distribution of charges and electrical balance of installation.
Greater dimension of the transformers and cables to diminish disturbances.
Perform a predictive maintenance of the electrical instalation.
Separate the lineal charges from the no lineal charges.
Perform a continuous scan of the system.

The ¨Quality of Energy¨ that must be managed in all electrical installation is obtained with and adequate design in which we must take into account the installation of the electrical conductors of copper of a caliber, greater to the one specified, with this, in case of an increase of charge that supports the system, we avoid problems that can generate harmonics.

After everything mentioned above, it is clear that the use of modern electronic equipments that make our tasks simple, also increase our production and improve our quality of life, at the same time cause deformations of the waves of the electrical installations called harmonics of tension and current.

With the objective of reducing costs and losses of electrical installations built, we must follow the recommendations mentioned above.

Source: Copper Development Association - South East Asia