The U.S. is physically too large for a single grid. History and politics also factor into it. But basically, North America has two main grids (eastern and western) which are shared between the U.S. and Canada, and three smaller grids, Alaska, Texas, and Quebec. Texas maintains a separate grid because they don’t want to be subject to U.S. Federal regulations and can retain control over their power grid at the state level.
The Northeast and Southwest U.S. are somewhat overloaded, and therefore have a greater risk of a cascade failure, especially in the summer when the electrical load is greatest (due to everyone running their air conditioners).
Here is how a cascade failure works. Imagine that you have 4 power companies, A, B, C, and D. All four of these tie their systems together into one big grid. C can’t produce enough power for all of their customers, so they buy power from D. Now what happens if D experiences a major generator outage? D’s system goes down, because it can’t supply enough power to everyone on their system. C depends on D, so C starts to go down, but B is tied to the same grid so C starts drawing power from B. But B can’t handle the extra load, so B starts to go down. Now A ends up supplying power to B, C, and D, but it can’t handle all of that load, so it starts to go down too. So the failure starts with D, then cascades to C, B, and A, and everyone is in the dark.
Protection systems are supposed to prevent the failure from cascading across the entire grid. So ideally, if D goes down, it’s going to take C with it, but the protection systems should isolate the grid between B and C so that A and B remain running. But those protection systems don’t always work.
Every few decades, there is a major cascade failure somewhere. Then the power folks come up with protection systems that they claim will prevent the failures from cascading all throughout the grid, and then a few decades later, another failure shows that their protections weren’t as effective as they thought they were, and a few decades later it repeats.
The last major cascade failure in the U.S. was in 2003 when overloaded power transmission lines sagged from overheating and contacted trees, creating a short circuit. The protective systems that should have limited the failure didn’t work (due to a software bug), power wasn’t re-routed to limit the blackout, and much of the northeastern U.S. ended up in the dark.
To me, the event in Spain and Portugal sounds like a cascade failure.
There are all kinds of possible causes, everything from one relatively minor failure that the protection systems didn’t catch in time causing everything to go dark, to potentially terrorism or just a plain old stupid software bug in the control systems. It will be interesting to see what they end up finding as the root cause.
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