[zeugma]

I read on an energy web site a while back that transformers were the biggest concern. If a good chunk of the grid blows transformers, there were nowhere near enough in reserve to replace all. They were thinking 6 months to 1 year to catch up, whether from foreign sources or new manufacture here in the US. That info was about 3 years ago. Don't know about the present inventory.

Correct. It would probably be much more than just a year. I do not believe that the really big transformers are even manufactured in the United States. Those are almost always one-off jobs custom built for an application. What would happen if suddenly we needed to replace about half or even just a quarter of those all at once?  The backlog would be extensive. It might be a big enough job that it would prompt some domestic manufacturer to enter the market. I don't know what kind of capital investment it would take to bring a manufacturer up to speed on it.

[BigpapaBo]

GE no longer makes any large substation or step-up/step-down transformers in the US. They are manufactured in Mexico, Europe and China.
Westinghouse has the capability to build in the US and overseas.
Siemens and ABB are built in Europe (Germany and Austria) and China.
Waukesha (SPX) still design and manufacture utility scale transformers here in the US.
These are transformers everyone sees at the large switch yards located everywhere.
Almost without exception, every generator has GSU’s ((generator step up transformers) They convert 4Kv-20Kv from the generator unit to utility voltages 138Kv-745Kv. You can bet that those would need to be rewound or replaced.
The knowledge base is still here to build these transformers, but if it happens tomorrow (CME hitting North America) then we would be talking several years or more to bring the capability back online to produce them in the quantities needed.
Then there’s the smaller distribution transformers, HV breakers, protection systems etc

[Jed Eckert]

......It might be a big enough job that it would prompt some domestic manufacturer to enter the market. I don't know what kind of capital investment it would take to bring a manufacturer up to speed on it.

Unfortunately, one of the insulating fluids used in the big transformers (including the ones up on the power pole) was PCB. Nasty stuff. Its use was banned by the Feds around '79-'80. Not sure what they use these days but I suspect whatever it is is probably not worth the effort or expense to jump through the EPA hoops. Hence the "Let somebody else make them" attitude. We do still have a few manufacturers of big transformers but as you stated they don't keep them on the shelf in stock. Build as needed.