In a perfect world, the voltage and current in an electrical AC power system have a sinusoidal waveform, with specific amplitude, frequency and phase angle. In the real world however, this is seldom the case. If the voltage is measured with an oscilloscope, the sinusoidal curve is always more or less distorted by sinusoidal waves different from the fundamental frequency. These disturbances called harmonics are generated by nonlinear loads in the system. The degree of distortion is dependent on the magnitude of the individual harmonics and can be expressed as Total Harmonic Distortion, THD.

Active Harmonic Filter and Passive harmonic filter are the solutions of harmonics mitigation.