Multi-port admittance model and SISO stability criterion for oscillatory stability analysis of VSC-based AC/DC interconnected renewable systems

•The VSC is viewed as a three-port circuit with its port dynamics and couplings at ac and dc sides described by a three-dimensional admittance matrix (3DAM).•The principle and steps to transform the 3DAM to a single-input–single-output (SISO) equivalent form are introduced. The SISO model facilitates an intuitive stability analysis with a single Nyquist curve.•The 3DAM is applied to identify the dominate factor of oscillatory instability. In particular, the instability caused by ac-ac couplings and ac-dc couplings is quantitatively distinguished.