Harmonic spectrum

Principle

Each type of device causing harmonics draws a particular form of harmonic current (amplitude and phase displacement).
These values, notably the amplitude for each harmonic order, are essential for analysis.

Individual harmonic distortion (or harmonic distortion of order h)

The individual harmonic distortion is defined as the percentage of harmonics for order h with respect to the fundamental.
[math]\displaystyle{ U_h(%)=100 \frac{U_h}{U_1} }[/math] 

or

[math]\displaystyle{ i_h(%)=100 \frac {I_h}{I_1} }[/math] 

Harmonic spectrum

By representing the amplitude of each harmonic order with respect to its frequency, it is possible to obtain a graph called the harmonic spectrum.Figure M12 shows an example of the harmonic spectrum for a rectangular signal.

Rms value

The rms value of the voltage and current can be calculated as a function of the rms value of the various harmonic orders.
[math]\displaystyle{ Irms=\sqrt {\sum_{h=1}^\infty I_h^2} }[/math]

and

[math]\displaystyle{ Urms=\sqrt {\sum_{h=1}^\infty U_h^2} }[/math] 

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Fig. M12: Harmonic spectrum of a rectangular signal, for a voltage U (t)

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