  Impedance-based stability analysis is appealing in the case of Single-Input
Single-Output (SISO) systems. However, in the case of grid-tied voltage source
converter (VSC) systems, dq impedances of source and load (VSC) subsystems are
typically Multi-Input Multi-Output (MIMO) systems in which case the Generalized
Nyquist Criterion (GNC) is required for analyzing the closed loop stability,
which increases the complexity of the analysis. This paper explores further the
coupling between positive and negative sequence impedances, in particular the
dependency and bindings between them. It shows that the couplings in each
subsystem can be compounded into two non-coupled sequence impedances if the
source and load subsystems are viewed as an integrated system instead of as two
separate subsystems. Therefore, two decoupled SISO systems are obtained which
are defined as Augmented Sequence Impedance Networks (ASIN). The stability
analysis of the closed loop system is performed directly on the ASIN with the
principal of Argument. Both numerical and time domain verifications are
presented.
