An examination of the rationale for limiting harmonic emissions from low-voltage equipment

Connection of nonlinear loads such as diode bridge rectifiers to AC mains systems results in current flow at power harmonic frequencies. Undesired effects arising from the flow of harmonic currents through power system impedances drive regulatory efforts via standards such as IEC/EN 61000-3-2, which is intended to limit emissions at the product level, and IEEE 519, which limits emissions at the facility level. Simplified linear models based on Ohm's law for AC circuits have been used to provide a technical rationale for establishing permissible emission levels given desired maximum voltage distortion. Two such models are critically examined in light of a variety of well-documented cancellation effects. Neither model provides a consistent prediction of voltage distortion levels. This examination leads to the conclusion that regulatory efforts should be guided more by ongoing system monitoring than by theoretical models.