Abstract
Frequency scanning is a practical technique to obtain a small-signal frequency-domain model of complex Power Electronic Systems(PES) by injecting a wide band signal of small amplitude in the time domain simulation of the system. The study of interactions of a PES with a grid may then be done using this model, using stability analysis methods like the Nyquist criterion, provided the model is Linear Time-Invariant(LTI). PES are usually time-periodic systems in the phase variables. For balanced three-phase systems with negligible lower order harmonics, the model of the system in the D-Q variables is time-invariant. The model formulation in Dynamic Phasor variables yields an LTI model for a more general class of systems which includes systems with negative sequence controllers and/or low-order harmonics, but the model is of high order and it is tedious to obtain the frequency scans in these variables. This paper presents a comparative evaluation of the application of DQ and Dynamic Phasor based frequency scanning for grid interaction studies. It is shown that DQ and Dynamic Phasor based frequency scans yield practically identical conclusions although the DQ model is not strictly time-invariant in some situations. This result has practical significance given the smaller size of a DQ based model and the relative ease in obtaining DQ based frequency scans.
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