There are only three power grids in the entire United States, and Texas has its own.
The states to the east of the Rocky Mountains have one power grid, and the states to the west all the way to the Pacific have one as well. Why did Texas get its own?
Texas got its own power grid because the United States Government wanted to ensure that the factories in the Lone Star State that were producing critical war supplies during WWII would be able to operate even if mainland America were to be invaded.
By having its own power grid, Texas was self-sustainable and has remained that way to this day.