Harmonics are integral multiples of a fundamental frequency that, when added together, distort the normally sinusoidal waveform. They are usually derived by a Fourier transform of the measured current and/or voltage curves (over time) at multiples of the network's base frequency. They are usually specified as a percentage known as the total harmonic distortion referred to the RMS value of the base current or voltage. For example:
The abbreviations THD-U and THD-V are commonly used for the voltage terms, which are calculated in the same way as THD-I. Harmonics cause numerous problems in power networks and are increasing because they represent a growing problem for modern electrical and electronic equipment.
For more information, consult the technical part of the EPCOS PFC Product Profile. The 3rd, 5th and 7th harmonics, and odd ones up to the 50th harmonic, are most common.