Harmonic distortion is found in both the voltage and the current waveform. Most current distortion is generated by electronic loads, also called non-linear loads. These non-linear loads might be single phase or three-phase as into variable speed drives.
The typical non-linear loads are as follows:
- a discharge lighting or other electronic loads;
- a switched mode power supply;
- an electric welding machine;
- a variable speed drive;
- an arc furnace;
- a rectifier inverter.
As the current in these systems is interrupted by a switching action, the current itself contains frequency components that are multiples of the power system frequency. It may happen then that these harmonic components resonate with the natural frequency of the net-transformer-capacitors oscillating circuit. Additional over currents may result, which produce system overloads, damage the capacitors and other components, and trip the safety devices off.