Total harmonic distortion (THD)
From Electrical Installation Guide
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The term THD means Total Harmonic Distortion and is a widely used notion in defining the level of harmonic content in alternating signals.
Definition of THD
For a signal y, the THD is defined as:
This complies with the definition given in standard IEC 61000-2-2.Note that the value can exceed 1.
According to the standard, the variable h can be limited to 50. The THD is the means to express as a single number the distortion affecting a current or voltage flowing at a given point in the installation. The THD is generally expressed as a percentage.
Current or voltage THD
For current harmonics, the equation is:
The equation below is equivalent to the above, but easier and more direct when the total rms value is available:
For voltage harmonics, the equation is:
Relation between power factor and THD
(see Fig. M13)
When the voltage is sinusoidal or virtually sinusoidal, it may be said that:
P = P1 = U1.I1.c'o'sφ1
Consequently:
as:
hence:
Figure M13 shows a graph of 
as a function of THDi.
Fig. M13: Variation in as a function of the THDi, where THDu = 0
