Influence of Harmonics on Broadband AC Grid Impedance Identification for Stability Studies

The determination of the grid impedance for stability studies with broadband signals is a popular method. The disturbing influences of harmonic emissions caused by non-linear loads or power electronics can distort these measurements and thus corrupt the calculated grid impedance. This paper presents an in-depth analysis of these distorting influences by a simulative comparison of the estimated grid impedance in a base scenario without harmonics and a scenario with harmonics. It is shown that the harmonics lead to false resonances, corruption of the impedance phase and additional coupling of the impedance elements. In order to prove that this distortion in the measurements and the resulting corruption of the impedance is not only a problem in the simulations but also in reality, the impedance estimation is carried out on an industrial grid with a high proportion of power electronics.