Practical considerations of Voltage-Source-Inverters Input Impedance Modeling for System Stability Analysis

In the last few years, switched power converters have proliferated in DC distribution applications such as microgrids or transport systems (e.g. more electrical aircraft, electric boats or electric road vehicles). The interaction of multiple controlled converters, operating as sources or loads connected to a common DC bus, degrades and jeopardizes the system stability. Impedance-based stability criteria are well-established methods to analyze the stability of interconnected systems. The complexity involved in obtaining the needed converter input impedance by experimental or analytical methods is usually overcome by using a constant-power-load (CPL) model. However, there are important practical considerations that the makes CPL model does not properly fit the real impedance. This paper proposes and validates an analytical input impedance model for a three-phase VSI considering important practical implementation effects neglected in conventional models. The analysis shows the importance of practical parameters in the accuracy of the model and defines the limits beyond which the CPL model becomes inexact for system stability analysis.