The power supply network provides a sinusoidal voltage, and the load draws a certain amount of current depending on its own impedance. If the response is linear, the relationship between voltage and current is constant—as is the case with a resistive load, where the current and voltage share the same sinusoidal waveform, resulting in no distortion.

When the load is non-linear, the current waveform does not follow the voltage waveform; instead, it depends on the conduction cycles of semiconductor devices, resulting in a non-sinusoidal current waveform. An example of this type of load is the rectifier stage of an adjustable-speed drive (ASD) or variable-frequency drive (VFD).

In power systems, the term “harmonic” refers to the distortion introduced into the supply network’s voltage and current by certain devices. Since the input of an ASD constitutes a non-linear load, the harmonics injected at the point of common coupling must be taken into account.