  The theme of this paper is three-phase distribution system modeling suitable
for the Z-Bus load-flow. Detailed models of wye and delta constant-power,
constant-current, and constant-impedance loads are presented. Models of
transmission lines, voltage regulators, and transformers that build the bus
admittance matrix (Y-Bus) are laid out. The Z-Bus load-flow is then reviewed
and the singularity of the Y-Bus in case of certain transformer connections is
rigorously discussed. Based on realistic assumptions and conventional
modifications, the invertibility of the Y-Bus is proved. Last but not least,
the MATLAB scripts that construct the detailed component models for the IEEE
37-bus, IEEE 123-bus, and 8500-node feeders as well as the European 906-bus
low-voltage feeder are provided.
