Impedance Modelling and Stability Analysis of Grid Side Converter Under Unbalanced Weak Grid by Harmonic Transfer Matrix

Designed to study linear periodic system, the harmonic transfer matrix (HTM) is convenient to model complex dynamics of AC power electronics system such as building sequence domain impedance. The HTM can describe the mapping relationship of responses of different frequency components in the form of matrix and simplify the model development. The HTM model of GSC impedance built in the assumption of balanced grid is not sufficient to model GSC impedance if grid become unbalance. To include the additional harmonic interaction caused by the imbalance into the GSC impedance model, a HTM model of higher order is required. In this paper an expanded HTM model structure is proposed and it can be conveniently applied in the sequence domain impedance model development of three-phase device under unbalance grid. Based on it, a reduced GSC impedance model under unbalanced grid is proposed and verified by the simulation. The proposed model can guide the controller tuning of GSC and be used in the impedance-based stability analysis in the unbalanced weak grid.