Frequency scan based stability analysis of power electronic systems

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Shirinzad, Mahsa
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Frequency scanning is a numerical method for extracting the frequency response of a power electronic system (PES). This technique is performed via injecting a small-amplitude wide-band signal in the time-domain simulation of a PES at steady state. This thesis presents the stability analysis of the interactions between a PES and its ac grid using frequency scanning techniques. Frequency scanning techniques in different domains are used to extract the model of PESs and the ac grids independently. These results are then compared and the advantages and disadvantages of each of these techniques are presented. Using the obtained models of the PESs and the ac grids, two different approaches i.e. the Generalized Nyquist Criterion (GNC) and the Eigenlocus Stability Analysis are applied to access the stability of the combined system. A comparison is given on the use of these methods for stability analysis. The results obtained from the frequency scanning can be used for other applications such as finding the stability margin of the multiple input multiple output (MIMO) systems using the results of the eigenlocus approach. The adverse interactions can then be stabilized by increasing the strength of the ac grid. The approximate stability screening technique based on the positive and negative sequence impedance/admittance can also be used for stability screening in grid-connected PESs. Although this technique is widely used in industry and generally gives correct indications of stability, it can give erroneous results due to the small but existent cross-coupling of frequencies. As a result, this thesis recommends using the DQ-based frequency scanning for stability screening purposes instead of using the positive/negative sequence-based approximate stability screening technique.
Frequency scanning, Stability analysis, Small signal stability analysis, Power system stability, Frequency response, Nyquist stability criterion, Eigenlocus stability, Stability based on positive/negative sequence impedance/admittance