system
Given the following post, respond as if you're commenting on Reddit:
human
How does an entire power grid fail, as in India right now or in the US and Canada in 2003?

I was wondering what exactly causes the failure of an entire power grid, and why fixing it is not as simple as selectively switching off parts of the grid and hooking them up again iteratively. Are there pieces of equipment that are destroyed in the process?
gpt
Nuclear engineer here (I work in a power plant).

There is a lot of talk about cascading failures. To understand why they are such an issue, you need to remember two things. First, heat generated in lines, transformers, breakers, is proportional to current drawn, and second, there is a very strict balance between how much "Load" is on the grid (how much power draw there is) and how much power is being generated on the grid. This balance is both in terms of VARs (imaginary power units) as well as VA (Volt-Amp, Apparent Power). 

So to better understand this, imagine the following situation. You have very little available electrical margin on your grid. Something happens (say a tree hits a line) and causes a fault on a cross connect between two regional areas. This line disconnects the two regions. One region had more power available than load, and the other region had more load than available generation. On region 1, the power stations will runback due to the load reject. If the load reject is severe enough, the power transient will cause the generator to go into a load reject trip to protect itself from the power transients on the grid. This would cause power stations on grid 1 to fail. These stations going down will cause the loads on grid 1 to pull in power from anywhere they can. Say they pull power in from another regional line, but that line now becomes overloaded. Overloading the line causes increased temperatures and leads to failures. These failures can cause other parts of the grid to become unbalanced, and so-on and so-forth. A well designed grid protection system will be able to detect this and stop it from crossing from major regions, but that's not always the case.

The real simple answer is you throw that load balance off, and when power margins are tight to begin with, the grid cannot handle that very well. 

As for why you can't "just fix it". Firstly, big power plants will need to go through their cooldown/heatup cycles. There may be damage caused as a result of the sudden load reject. In the case of nuclear power plants, there are very few plants that can handle a full load reject without a reactor scram, and recovering from a scram usually takes a few days to do equipment inspections and the like. Field equipment like lines, transformers, and switchgear, all need to be inspected and repaired. Finally, when the grid is brought back online, you ALWAYS have to maintain a balance between power and load, and as a result, depending on how bad the outage is, you need to bring it up very slowly and in a very controlled fashion to prevent sudden load transients from putting you back into a total blackout condition.