Other calculations for amplitudes, voltages, currents, and so forth are equivalent. For a voltage signal, for instance, the ratio of the squares of the RMS voltages is equivalent to the power ratio:
In this calculation, Vn means the RMS voltage of harmonic n, where n=1 is the fundamental harmonic. One can also calculate THD using all harmonics (n=∝):
Other definitions may be used. Many authors define THD as an amplitude ratio rather than a power ratio. This results in a definition of THD which is the square root of that given above. For example in terms of voltages the definition would be:
This latter definition is commonly used in audio distortion (percentage THD) specifications. It is unfortunate that these two conflicting definitions of THD (one as a power ratio and the other as an amplitude ratio) are both in common usage. Fortunately if the THD is expressed in dB then both definitions are equivalent. This is not the case if the THD is expressed as a percentage. The power THD can be higher than 100% and is known as IEEE, but for audio measurements 100% is preferred as maximum, thus the IEC version is used (Rohde & Schwartz, Bruel and Kjær use it).
A measurement must also specify how it was measured. Measurements for calculating the THD are made at the output of a device under specified conditions. The THD is usually expressed in percent as distortion factor or in dB as distortion attenuation. A meaningful measurement must include the number of harmonics included.
For a given input frequency and amplitude, THD+N is equal to SINAD, provided the bandwidth for the noise measurement is the same for both (the Nyquist bandwidth).
Electric goblins in water treatment systems: if your plant is experiencing inexplicable nuisance tripping, inconsistent power quality or extensive power interruptions, harmonic distortion may be the culprit.(Municipal Special Edition)
Oct 01, 2003; Water treatment plants and pumping stations supported by variable-frequency drives (VFDs) may be unwittingly threatened with...