  This paper considers unbalanced multiphase distribution systems with generic
topology and different load models, and extends the Z-bus iterative load-flow
algorithm based on a fixed-point interpretation of the AC load-flow equations.
Explicit conditions for existence and uniqueness of load-flow solutions are
presented. These conditions also guarantee convergence of the load-flow
algorithm to the unique solution. The proposed methodology is applicable to
generic systems featuring (i) wye connections; (ii) ungrounded delta
connections; (iii) a combination of wye-connected and delta-connected
sources/loads; and, (iv) a combination of line-to-line and
line-to-grounded-neutral devices at the secondary of distribution transformers.
Further, a sufficient condition for the non-singularity of the load-flow
Jacobian is proposed. Finally, linear load-flow models are derived, and their
approximation accuracy is analyzed. Theoretical results are corroborated
through experiments on IEEE test feeders.
