Three-Port Small Signal Admittance-Based Model of VSCs for Studies of Multi-Terminal HVDC Hybrid AC/DC Transmission Grids

Abstract

Multi-terminal high voltage direct current (HVDC) systems, together with AC transmission systems and voltage source converters (VSCs), form hybrid AC/DC grids with complex dynamic and transient interactions. VSCs characterization taking into account DC- and AC-side dynamics in order to study these interactions is not yet well solved. This paper presents a three-port transfer admittance-based matrix model of VSCs that can be applied for such purpose. It is derived from dq-complex space vectors and characterizes both AC- and DC-side dynamics by relating AC- and DC-side current and voltages in a three-dimensional admittance transfer matrix which considers the VSC outer control loops. The paper also proposes a systematical procedure for studying multi-terminal HVDC hybrid AC/DC transmission grids by the Norton admittance method, where the proposed VSC model can be easily included in the Norton admittance matrix. This procedure allows the study of grid dynamics using impedance-based stability criteria. The proposed model and procedure are applied to a stability study in a multi-terminal HVDC hybrid AC/DC transmission grid. PSCAD/EMTDC simulations are used to validate the application.