Harmonic distortion indicators - Crest factor
The crest factor is the ratio between the value of the peak current or voltage(IM or UM) and its r.m.s. value.
- For a sinusoidal signal, the crest factor is therefore equal to [math]\displaystyle{ \sqrt 2 }[/math].
- For a non-sinusoidal signal, the crest factor can be either greater than or less than [math]\displaystyle{ \sqrt 2 }[/math].
The crest factor for the current drawn by non-linear loads is commonly much higher than [math]\displaystyle{ \sqrt 2 }[/math]. It is generally between 1.5 and 2 and can even reach 5 in critical cases.
A high crest factor signals high current peaks which, when detected by protection devices, can cause nuisance tripping.
Examples:
Figure M7 represents the current absorbed by a compact fluorescent lamp.
Ir.m.s. : 0.16A
IM : 0.6A
THDi : 145%
Crest factor: 3.75
Fig. M7 : Typical current waveform of a compact fluorescent lamp
Figure M8 represents the voltage supplying non-linear loads through a high impedance line, with a typical "flat top" distorted waveform.
Vr.m.s. : 500V
VM : 670V
THDu : 6.2%
Crest factor: 1.34
Fig. M8 : Typical voltage waveform in case of high impedance line supplying non-linear loads
