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INTRODUCTION

The modernization of industrial processes, the sophistication of electrical machines and equipment has, in recent years, led to significant development in power electronics:

These semi-conductor-based systems (transistors, thyristors, etc.) designed to produce:

• solid state power converters : AC/DC

• rectifiers

• inverters

• frequency converters

• and many other wave train or phase setting control devices. For electrical supplies, these systems represent "non-linear" loads. A "non-linear" load is a load for which the current consumption is not the reflection of the power supply voltage (even though the source voltage on the load is sinusoidal, the current consumption is non-sinusoidal).

Other "non-linear" loads are also present in electrical installations, in particular:

• variable impedance loads, using an electric arc: arc furnaces, welding stations, fluorescent tubes, discharge lamps, etc.

• loads using strong magnetizing currents: saturated transformers, inductors, etc.

The FOURIER series breakdown of the current consumption of a non-linear receiver reveals:

• a sinusoidal term at the supply 50 Hz frequency, the fundamental.

• sinusoidal terms for which the frequencies are multiples of the frequency of the fundamental, the harmonics.
  According to the equation:

Σ : Sum of all the harmonic currents from rank 2 (50 Hz x 2) 
to the last rank n (50 Hz x n).

These harmonic currents circulate in the source and the harmonic impedances of the source produce harmonic voltages according to the equation U_h = Z_h x I_h

Harmonic currents induce most of the harmonic voltages which cause the overall harmonic distortion of the supply voltage.

Note: The harmonic distortion of the voltage generated by manufacturing defects of the alternator and transformer windings is generally negligible.

Harmonics summary