Large power-systems for power-flow study
There are models of the Polish power system included in the program Matpower. It is free for download, but you need Matlab to run the program. However, the data files are available as separate files in the standard IEEE-format, when you download the package. 

Matpower is basically a bunch of Matlab-files created for load flow studies. 

The size of the systems range from 2700-3300 buses. Will that suffice?