Sequence Impedance Modeling and Converter-Grid Resonance Analysis Considering DC Bus Dynamics and Mirrored Harmonics

Sequence impedance models have been developed for two-level voltage source converters by assuming constant dc bus voltage and ignoring coupling over frequency effects. There have been efforts to overcome these limitations but the resulting models are very complex and the formulated system models require generalized Nyquist criterion for stability analysis. This paper presents a new modeling and system analysis method that solves these problems. The method starts by modeling the converter current response at different frequencies to a perturbation in the ac terminal voltage while keeping the dc bus voltage constant, and to a perturbation in the dc terminal voltage while keeping the ac terminal voltage constant. The individual transfer functions are then connected to build the overall converter models that include both the effects of dc bus dynamics and coupling over frequency. For converter-grid system stability analysis, the effects of coupling over frequency are shown to be equivalent to an additional impedance in parallel with the sequence impedance. The developed models are validated individually as well as in system resonance analysis.