Practical approaches for AC system harmonic impedance measurements

AC supply system harmonic impedance measurements are discussed. On the basis of a given and known switchable shunt impedance and/or nonlinear load and site measurements, three methods are presented. The first approach assumes a no-load distorted busbar and uses basic circuit theory to calculate the required harmonic impedance. The second method disregards any previous busbar distortion and the harmonic injection but it takes into account any busbar distortion prior to the nonlinear load connection. Using a laboratory model and by comparing harmonic impedance results it is shown that both no-load busbar distortions and the correct measurement of harmonic phase angles may strongly affect the accuracy of calculation