I’ve read about this on wikipedia, but I don’t really understand what linear- and non-linear loads is and why it can create harmonic currents. How can there be other frequencies than the supply frequency on 50/60Hz?
I’m studying Technology Management and Marine Engineering, so I have some knowledge in electricity.
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Linear load is something that will respond with current in the same shape as you apply voltage. For example, resistor: If you put sinusoidal voltage on it’s terminal, current will also be a sinus. Similarly ideal capacitor, if you apply sinus voltage, current will be sinus with the same frequency, just different phase.
On the other hand, when you look at [voltage – current curve](https://learn.sparkfun.com/tutorials/diodes/real-diode-characteristics) of non linear component (diode), it is visible that this response is non linear. If you apply sinus voltage across ideal diode terminals, then for half cycle current will be 0, and for another half it’s going to be the same as for resistor.
Now, how can we represent this new current graph as function? It’s no longer pure sinus. [Fourier transfrom](https://en.wikipedia.org/wiki/Fourier_transform) tells us, that any function can be represented as sum of sinuses. And to do this, we are using sum of base frequency of our voltage (lets say 50Hz), and higher harmonics (multiples of base frequency, so 100Hz, 150Hz, 200Hz… etc).
Basically, to mathematically describe current curve in circuit with non linear component, we need to add bunch of harmonics to our base frequency.
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