ABB Guide to Harmonics with AC DrivesGuide to Harmonics
with AC Variable Frequency Drives

Contents

Guide to Harmonics with AC Drives

Definitions

S: Apparent power
P: Active power
Q: Reactive power
Rsc: Short circuit ratio is defined as the short circuit power of the supply at PCC to the nominal apparent power of the equipment under consideration. Rsc = Ss / Sn.
w1: Angular frequency of fundamental component w1 = 2*p*f1, where f1 is fundamental frequency (eg. 50Hz or 60Hz).
n: Integer n = 2, 3, … �. Harmonic frequencies are defined as wn = n*w1.
In: RMS-value of n:th harmonic component of line current.
Zn: Impedance at frequency n*w1.
%Un: Harmonic voltage component as a percentage of fundamental (line) voltage.
THD: Total Harmonic Distortion in the input current is defined as:

where I1 is the rms value of the fundamental frequency current. The THD in voltage may be calculated in a similar way. Here is an example for the 25 lowest harmonic components with the theoretical values:

THD = 29%
PWHD: Partial weighted harmonic distortion is defined as:
PCC: Point of Common Coupling is defined in this text as such a point of utility supply which may be common to the equipment in question and other equipment. There are several definitions of PCC in different standards and even more interpretations of these definitions in literature. The definition chosen here is seen as technically most sound.
PF: Power Factor defined as PF = P/S (power / voltampere) = I1 / Is * DPF (With sinusoidal current PF equals to DPF).
DPF: Displacement Power Factor defined as cosf1, where f1 is the phase angle between the fundamental frequency current drawn by the equipment and the supply voltage fundamental frequency component.