Two-Port Characterization and Transfer Immittances of AC-DC Converters—Part I: Modeling

This two-part paper presents a general method to model three-phase ac-dc converters in the frequency domain for power system stability studies. In the developed approach, an ac-dc converter is treated as a two-port network connecting an ac and a dc power system, and is characterized by a set of self and transfer immittances. The transfer immittances describe two types of small-signal coupling that are unique to three-phase ac-dc converters: The first type describes the coupling between the ac and dc port and involves a frequency shift by the fundamental; the second type models the coupling over frequency in the ac port current response and involves a frequency shift by twice the fundamental. Together with the self-immittances of each port, the developed models can be used as building blocks to support impedance-based stability analysis of various power systems that involve such converters, including ac, dc, and hybrid ac-dc power generation, transmission and distribution systems. Part I of the paper explains the two-port modeling approach and develops the associated models for a two-level voltage-source converter. Part II presents applications of the developed models for stability analysis of different systems.