On impedance modeling of single-phase voltage source converters

This paper explores modeling of ac-side impedance for grid-connected single-phase voltage source converters (VSC). It shows that the conventional impedance, in the form of one-dimensional transfer function, may not completely capture the linearized dynamics of the converter. Resonance analysis using such impedance may fail for oscillations involving multiple frequency components. This problem is addressed by extending the impedance concept and defining a two-dimensional admittance for single-phase VSC. This admittance, in the form of a two-by-two transfer matrix, accurately captures the frequency cross-coupling effect that plays a critical role in low-frequency oscillations. A new form of signal-flow graphs is also introduced to represent the linearized dynamics of the VSC. These graphs visually demonstrate: a) the flow of perturbations through converter, b) why the conventional impedance fails in completely capturing the converter dynamics, c) how the two-dimensional model better captures them, and d) how to extend the concept further to higher dimensions without repeating the modeling process. Proposed two-dimensional and the conventional one-dimensional impedance models are developed for a single-phase VSC and they are validated using detailed circuit simulations.