Harmonics and Power Quality

A distorted current contains harmonics at multiples of 50 Hz. Total harmonic distortion:

$THD = \frac{\sqrt{\sum_{h=2}^{\infty} I_h^2}}{I_1}$

Sources: rectifiers in SMPS and LED drivers, VFDs, arc furnaces, UPS systems. Effects: transformer and neutral overheating (triplen harmonics — 3rd, 9th… — add in the neutral of a three-phase four-wire system), nuisance tripping, capacitor bank resonance and meter errors.

Mitigation: passive tuned filters, active harmonic filters, multi-pulse (12/18-pulse) rectifiers, DC chokes in drives and active PFC. Limits are set by standards such as IEEE 519 and IEC 61000-3-2.