When your power grid management interfaces are directly connected to the Internet you must suffer. There's no excuse for that.
There are plenty good reasons. You're being extreme.
The grid management has to be connected to *some* network. That's so you can monitor the health of the grid from a central location, and coordinate a distributed response to events. (Heck, it's also useful if you can connect to control it even when weather conditions make it too hazardous to travel on-site).
 You could do that with suitable VPNing over the public internet. That way you benefit from its extensive reach, its cheap price, its resilience, the rapid repair time that ISPs offer. All you need to build is a network connection from each of your grid nodes to the nearest internet.
 Or you could do it with dedicated leased lines that aren't part of the internet. You'll pay a heck of a lot more, and loads of grid nodes won't have convenient connection.
 Or you could put up your own network. (You're a power-grid so you're used to putting up networks!) But this isn't your core competence, will suffer from longer outages, and will be most expensive.
Bear in mind that every subcontractor who prepares a bid using the public internet will produce a *LOWER* bid with *INCREASED* functionality. The only way that a higher-priced bid will ever win is if they someone demonstrate that the downside costs (in terms of expected cost of future hacks) will be significantly larger than the higher upfront bid. And any such attempted demonstration would be instantly met by the answer "why not use just a secure VPN to get best robustness at the cheapest price?"
So I think that infrastructure like this *can* and *should* be connected to the internet.