THD · Total harmonic distortion
The share of waveform distortion injected by non-linear loads; it is capped at the point of common coupling, which drives active-front-end, filter, and transformer decisions.
Current numbers
5–20% TDDIEEE 519 current-distortion (TDD) limit at the PCC, sliding with the Isc/IL short-circuit ratio; voltage THD capped at 5% (1–69 kV) / 8% (<1 kV)