There are two typical definitions used when dealing with Total Harmonic Distortion. The two types of Total Harmonic Distortion are THD-F and THD-R. They both are figures of merit used to quantify harmonic levels in voltage and current waveforms; however, each one uses a different reference. THD-F is a comparison to the funda- mental and THD-R is a comparison to the signal’s RMS value. For power systems, the THD-F is the most accurate measurement especially when there is high harmonic content. The THD-R measure- ment can be prone to misinterpretation which can easily lead to measurement errors when measuring larger distortions.
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