Total Demand Distortion (TDD)
Total Harmonic Distortion (THD) is a common method used to measure the level of waveform distortion.
Total Harmonic Distortion (THD) measurements are an effective method of evaluating voltage distortion. However THD can be misleading when evaluating current distortion. THD is the summation of the harmonics presented as a percentage of the fundamental. For voltage THD this will be the summation of the voltage harmonics compared to the fundamental voltage. In a non-faulted condition voltage will always be present so a proper reference always exists.