Harmonic distortions can occur in electrical systems when non-linear loads are connected to the power supply. These loads include motors, lighting, and other types of equipment. The voltage waveform produced by non-linear loads does not vary smoothly with time—it has peaks at multiples of the basic frequency.

The frequencies of these peaks are called harmonics, and they add to the fundamental frequency being supplied by the generator. The most common harmonic distortion caused by non-linear loads is the third-harmonic current (third harmonic). In this case, the third harmonic of 50 Hz is 100 Hz; it's also called triplen harmonics because its frequency is 3 times higher than 50 Hz. Triplen harmonics cause problems because they cause voltages and currents on transmission lines to be higher than normal voltages and currents at these same frequencies would be without this distortion which means they could cause damage if they aren't dealt with.