I've heard many times that the grid electricity production and consumption must be balanced and imbalances lead to overvoltage or undervoltage. I've never seen an explanation of that phenomena from energy conservation law standpoint.
Suppose I have a grid with exactly one gigawatt generation and exactly one gigawatt consumption. Let's pretend there're no losses - all the conductors are superconductors. So whatever is generated is fully consumed.
Now someone switches his one thousand watts space heater off. Generation now exceeds consumption. Although it's a relatively small increases there's still some imbalance.
What happens? Where does this excess power go?