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Impedance models are widely used in assessing small signal stability ofgrid-tied voltage source converter (VSC) systems. Recent research has proventhat impedance models of grid-tied VSC in both dq and sequence domains aregenerally Multi-Input Multi-Output (MIMO) systems, and the generalized Nyquistcriterion has to be applied for stability analysis to these MIMO systems.However, finding Single-Input and Single-Output (SISO) equivalents for thissystem is always appealing because of the simplicity and the convenience forphysical interpretation when assessing the stability, compared to MIMO systems.This paper presents two types of SISO impedance models of grid-tied VSC system,one is derived from the strong grid assumption, and the other is from theclosed-loop equivalence. The accuracy of these models is assessed with respectto the measured impedances in PSCAD/EMTDC, and their effects on the stabilityassessment were analyzed as well. It is proven that the accurate SISO modelgives identical result as the MIMO (matrix-based) impedance model with respectto stability analysis. However, the reduced SISO model may lead to wrongresults if the bandwidth of phase locked loop is large.

Accurate and reduced SISO Sequence Impedance Models of Grid-tied Voltage Source Converter for Small Signal Stability Analysis