On the national grid it is. They are separate. There was talk of an AC/DC to DC/AC substation in Oklahoma (Because they are synchronized with each other) to allow transfer of power but I don't know if it ever got built.
Texas is not synced with either the Western or Eastern interconnect. The only way to move power is with back to back HVDC connections which Texas has three of they total about 1500 one North one East and the other is called the railroad it also connects North. There is a 100 megawatt variable frequency AC transformer in Laredo to push or pull power from Mexico also not synced to Texas.
This new link would be a 300 mile HVDC line connecting Texas with Louisiana and Mississippi with another 3000 megawatt of flow capacities. So at most 4600 megawatts in or out...Texas has a summer capacity of over 100,000 megawatts and 90000 in the winter so less than ten percent of the ERCOT grid in other words not a significant impact in a Uri level event where 40,000 megawatts of natural gas, nukes and coal froze out. Read the report it shows why the grid went down in cold detail Texas ignored the 2012 and 2013 events when the industry was told to winterize and they didn’t.
There is not a ‘national grid’ in the USA. Who is coming up with this garbage?