Electrical and Computer Engineering

If the circuit is cut, you have zero load so you have zero current, but the electrical potential is still at 22kMV if the generators are still turning. (Voltage is irrelevant the way I am explaining this though.)

I know nothing about dams or those kinds of power generation systems or what kind of safety systems are in place. However, it shouldn't matter much if we are talking about 20kMV or 20mV as the concepts are scalable.

The chaos that happens if you suddenly cut load is likely going to be mechanical, not electrical. The load of the power grid magnetically "pushes" back against the force of the generator turning. The stronger the load, the harder it is for the generator to physically turn.

Let's imagine for a second that there are no safety systems, ways to divert water flow or any kind of brakes for the generator.

If the load is suddenly cut, the magnetic resistance pushing back on the generator is also immediately cut, effectively making the generator free spinning with no resistance. The generator would likely tear itself apart spinning out of control. (We are likely talking about thousands of pounds of steel and copper that is already spinning and has a fuck ton of kinetic energy.)

I believe large generators actually use electromagnets instead of large solid magnets like smaller generators would use. Unfortunately, I can't speculate how the electromagnets would behave in a situation where the generator was free-spinning. (I can take a shitty guess and say the coils might fail catastrophicly.)

Edit: My point is that current drops to zero and there is no electrical energy to "burn off". If there were solid magnets in use or the electromagnets in the generator were still functional, I would like to speculate that some bits of the generator might get super hot, similar to how an induction stove works. (Your best bet is to cut the water and apply emergency brakes to the generators ASAP, if those even exist.)

Edit2: So, turbines are a thing. The water behind a dam probably isn't directly pushing a generator but some kind of turbine system. I would imagine there is transmission (or sorts) that can engage or disengage the generators.

Converting electrical energy into heat is trivial. The cooling towers associated with nuclear power plants exist for the purpose of dissipating waste heat by evaporating water and/or heating air. (But in the case of nuclear reactors, they're dissipating heat energy that was never converted into electrical energy.) Pumping in cold water from a lake or reservoir, heating it, and pumping it back out can also be used for the same purpose.

Water can absorb relatively large amounts of energy without much change in temperature. If you're thinking specifically of a hydroelectric dam, consider that simply letting the water flow through without turning turbines would cause most of the potential energy in that water to be converted into heat by default. Water at the bottom of a waterfall is warmer (by only about a tenth of a degree for Niagara falls) than water at the top.

GND