What happens to excess electricity generated going in to a grid?
A good question. The excess of generator drive power over generator load will cause all of the generators on the grid to start speeding up.
For a small over-power, there will be time for the mechanical steam valves and water valves to start closing, and reducing power to the generators, which will slow them back to nominal speed.
For a large over-power event, let's say there's a generating station at the end of a long feeder, and the feeder opens for some reason, then a set of large (very large) resistors will be switched onto the station as a safety load, until the power input to the generators can be throttled down.
A few years ago, I read the report of a test of such a bank of resistors, where the old cast-iron ones (which absorb the energy by heating up) were replaced by sheet stainless steel ones (which were much less massive and had to dissipate the power to the air). I'll see if I can find it again and link to it. The test went on for 30 seconds, which sounded like the length of time they expected could pass before they shut down the steam input to the turbines.
Further to Neil's answer:
For tiny amounts of over-power (e.g. when a machine is switched off) the excess power is consumed by the remaining connected loads.
During over-power, all loads are exposed to a small amount of excessive voltage, and thus they generally draw more current and dissipate more power. These tiny fluctuations are usually not regulated out. Rather, grid statistics provide for a generally constant power consumption over short terms, and thus constant voltage.
With a sufficiently high number of loads (large neighbourhoods...) the fluctuation is a statistical effect with small variance. The voltage variation is small enough so as not to cause damage. With a low number of loads (a portable generator), controlling and maintaining the output voltage for fluctuating loads is paramount.
As an aside, active loads (regulated loads, like LED, controlled motors...) generally respond differently and do not draw proportionately more current. Their power-draw rigidness does not ameliorate the over-voltage.
The duration of the over voltage is determined by the lag in controlling the primary power (up or down), and/or the lag in detecting this and shunting-in dummy loads.
The primary power lag is small for non-mechanical power supplies like solar cells. Switching off power is as fast as switching an array of power transistors or relays.
For mechanical supplies, like impeller-based generators (steam, diesel, hydro) the lag is a matter of mechanical momentum: how rapidly can you accelerate or decelerate the rotation of a heavy lump of metal.
A whole different question is maintaining the efficiency of power generation. Where does excess generated primary power go (heat, steam, water)?
Primary power delivery is controlled by the primary power consumption (e.g. the amount of fuel burn) times the efficiency or efficacy in transferring the mechanical power to electrical power.
The primary power consumption is relatively more difficult to regulate. For rapid momentary excess of power, primary power is "let off" through by-pass vents in the generator. This however results in an immediate loss of efficiency. For long term power reduction, generators are operated at lower power or they are "taken off the grid".
So back to the question, where does the excess power go?
- into existing loads (connected appliances, esp. passive loads)
- into dummy loads (shunted-in as needed)
- bled off (by-pass of mechanical power)
- regulated out (reduced primary power)