Got this back yesterday. I had attended the Somervell CAD meeting on August 1, 2016 in which Luminant lodged a protest against the appraisal amount from SCAD.  I did an open records request to ask exactly what Luminant's protest document was. The attorneys involved lodged a protest with the Texas Attorney General's office and I got this back yesterday (I have redacted my name, etc for internet purposes) - PDF

Hah, here's one of the pages, although there are also emails, etc. The other information is clearly proprietary and Luminant does not want to divulge it.

Still waiting for my second public information request, which was to ask what was the protest document that Somervell County filed. It, too, is being contested from being provided to me.