The Wikipedia article[3] is not entirely clear and has a shocking failure to cite sources, but it claims the the Texas grid connects to the Eastern grid in two place with DC lines, and has an AC connection that has only been activated once in the Houston area.

What I suspect this is referring to is connecting to the SERC[1] area. I can't find a good source, but I suspect the existing connections are to the MRO[2].

1: https://en.wikipedia.org/wiki/SERC_Reliability_Corporation 2: https://en.wikipedia.org/wiki/Midwest_Reliability_Organizati... 3: https://en.wikipedia.org/wiki/Texas_Interconnection#Ties