In the "Assign Power Integrity Models" dialog box as part of the setup before running PI simulation, for each power plane/shape a reference net must be selected which is usually ground. Leaving no stone unturned for the setup, I noticed that this dialog box is asking for reference plane for ground net too. Going through this dialog box I found that no net can be assigned as reference net to itself, including ground net. What should I do? Leave it as <none>?
If the option is to do nothing and leave it as <none>, then IMHO, the option to assign reference net for GND should be grayed out or Hyperlynx should give the user the ability to make the reference net assignment for GND or any other net of the user's choosing grayed out. This will make the checking of the reference net assignment easier as there are many entries for large designs.
The reference net is the return path for your power net, so it make sense not to be able to assign as return path the same net.
You want to perform the DC drop simulation for your power nets, not for your GND, so you can leave it as none for GND.