Convergence of the Z-Bus Method for Three-Phase Distribution Load-Flow   with ZIP Loads
  This paper derives a set of sufficient conditions guaranteeing that the load-flow problem in unbalanced three-phase distribution networks with wye and delta ZIP loads has a unique solution over a region that can be explicitly calculated from the network parameters. It is also proved that the well-known Z-Bus iterative method is a contraction over the defined region, and hence converges to the unique solution. 
Mohammadhafez Bazrafshan and Nikolaos Gatsis
math.OC cs.SY