The goal is to analyze the impact of the disturbance voltages on the impedances of the harmonics in the positive and negative sequence. Therefore, I have implemented a Multiple Run Simulation and I am changing the frequency in the single line voltage sources to multiples of my base frequency, e.g. 50 Hz. Moreover, I change the phase in the second and third voltage source to +120° and -120°, respectively, to disturb the system in the negative or positive sequence. The goal is to analyze the impact of the disturbance voltages on the impedances of the harmonics in the positive and negative sequence (via FFT). You can find a screenshot of the grid and the results attached. Unfortunately, I have very large impedances in the negative sequence (when I disturb the system in the positive sequence and vice versa) when the frequencies of the disturbance voltage sources are couples of 400 Hz, although I shouldn´t have a (large) impedance during a stimulation in the positive sequence in the negative sequence. It looks like as if there is a resonance for these frequencies in the grid but I do not know where/why. Furthermore, I recognized that I can improve the results when I reduce the Solution Time Step. Do you have any idea what the problem is?
Thanks a lot in advance!