Total Harmonic Distortion

A perfect sine has one frequency. Anything else — a rectifier, a variable-frequency drive, an LED driver, a switching supply — draws current in bursts, and a burst is a sine plus a family of harmonics at multiples of the supply frequency. Total harmonic distortion is the single number that says how much of that family there is: the RMS of everything above the fundamental, divided by the fundamental. Eighteen amperes of harmonic content on a sixty-ampere fundamental is 30 % THD. If your meter gives you total RMS instead of harmonic content, the harmonic part is √(I_rms² − I₁²) — the two are the legs of a right triangle, because harmonics at different frequencies add in quadrature and not arithmetically.

The consequences are practical and mostly thermal. Harmonic current heats conductors without doing work, so a distorted feeder runs hotter at the same real power. The third harmonic and its odd multiples — the triplens — are in phase across all three phases of a wye system and therefore add in the neutral instead of cancelling, which is how a neutral can carry more current than any of the phases feeding it, and why a distorted feeder gets treated as having an extra current-carrying conductor for derating. Transformers feeding electronic loads need K-rating or de-rating for the same reason, and plain power-factor capacitors can resonate with the supply inductance at a harmonic frequency and amplify what they were meant to ignore.

Two things to keep straight. THD is not the same as displacement power factor: a load can sit at cos φ ≈ 1 and still be violently distorted, which is why true power factor accounts for both and why an old-fashioned meter and a modern one disagree on the same load. And current THD read at light load looks alarming for a boring reason — the harmonics stay roughly constant while the fundamental shrinks, so the ratio balloons. That is why IEEE 519 frames its limits as total demand distortion, referenced to the maximum demand current rather than the instantaneous fundamental.