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?
Best Answer
For tiny variations of load, the grid voltage changes slightly, which causes the power consumption of all of the other loads to change accordingly.
For example, if your 1 GW load suddenly becomes 0.999999 GW (-1 ppm), it only takes a voltage rise of +0.5 ppm to bring the consumption back up to 1 GW. This is enough of a rise to cause the power that was being consumed by the switched-off load to be distributed across all of the other loads.