This links to GNU Octave. Octave is a free open source alternative to MATLAB®. You can type all MATLAB type commands within Octave and get the same effects.
The website has useful resources to learn ATP/EMTP. Again, you need to register for the listserv and ATP to be able to send and receive emails to this group.
An open source project with a free and strong powerflow engine that works with Matlab or Octave. It has most common IEEE test systems readily available as well.
Link to a free version of PSS/E. The software has a limitation of 50 buses only and can be used for academic purposes. You need an email with a .edu domain to be able to get this software.
This website has PowerWorld powerflow cases for the most commonly used IEEE bus systems such as IEEE 118 and IEEE 3000 bus systems. There are also large dynamics cases such as the 2000 ERCOT bus system.
