EricGibbs, the reason why I was replacing the diodes for voltage sources/batteries is because that's what we were taught to do.
We learned 3 models for diodes:

1 - replace the diode for a shortcut (ideal model) (when it's ON)
2 - replace the diode for a voltage source Vd = 0.7V (2nd approximation) (when it's ON)
3 - replace the diode for a voltage source Vd = 0.7V and an internal resistance of Rf = xxx ohm (linear approximation) (when it's ON)

The text underlined is an hard translation of the names we use in Portuguese to refer to each model!

 

When you replace the zener with one of the models that is valid when it is regulating, you just have to be sure to take into account that your simulation is only valid when the zener is regulating.