Esther1 got it right. The power grid which covers 3/4 of Texas is largely isolated from the rest of the contiguous US & Mexico. Today there are a few small direct current transmission lines, but these are different from the large-capacity alternating currect lines that would be needed to connect the state to the power grid.
The grid originally was completely isolated, and was designed that way to avoid federal oversight of the grid. The interconnected power grids in the other 48 states are under the jurisdiction of FERC, the Federal Electric Regulatory Commission (Texas’s DC connection lines with other states are under FERC’s jurisdiction though). The Texas grid, except for the DC ties, are only under the jurisdiction of the state public utility commission. Of course this wouldn’t have been possible if Texas didn’t have sufficient sources of power generation inside the state, including two nuclear power plants and many natural gas and oil plants.