What happens when too much electric power is fed into the grid?

I’ve been reading about the great need for improvement in energy storage because supply and demand is so volatile. What exactly happens when too much energy is generated, or too little, and there isn’t sufficient storage?

I guess if there’s too little supply then everyone’s voltage drops and there may even be intentional blackouts to prevent prevent inconsistent voltage. Clear enough. But what about when there’s too much supply? Are there people constantly monitoring the “too much” light and throttling back generators when necessary (or scrambling to find somewhere else to route the current)?

The first thing that happens when supply and demand are mismatched is that the frequency changes. On a time scale of seconds, it is all the rotating mass in generator and motor rotors everywhere that absorbs or releases energy. This happens faster than any throttling.

I think some of the sources that can change the fastest, like gas turbines, are the ones that get throttled fastest. It’s important to have some diversity of response time in the system so that it can adapt on various time scales.

No, you can’t supply “too much energy.” What happens is that when demand drops, it gets easier for the generators to turn, so they tend to speed up, which causes their automatic governors to close the throttles to the associated turbines, thereby reducing steam demand.

This may be true for a simple system, but in a large scale industrial system with many sources of power, many sources of demand and significant transmission lines, ‘too much energy’ is possible.

My [limited] understanding is that there is some ‘slack’ in the system’ so changes don’t have to be instantaneous. Some power can be kept in the lines and demand customers can experience a limited amount of oversupply.

However, within minutes supply is shrunk to meet demand. To accomplish this, several steps can be taken. Large generators are instructed to reduce their generation (say nuclear plants or coal plants, where instructing them to completely switch off would be extremely costly and potentially dangerous), small generators are instructed to shut off their generation (say smaller natural gas generation plants or individual wind generation towers) and in emergencies, breakers can be tripped that immediately cut off power supply. In theory, demand customers could also be instructed to ramp up their power use in an emergency, but I imagine that would be a last ditch effort to preserve important infrastructure that could be damaged by power overloads.

My point is that the “too much energy” is never actually produced.

If electrical demand decreases, it takes less power to drive the generators in each individual power plant, so the generators start to increase speed (and frequency). Automatic governors automatically decrease the driving force (by closing steam throttles, the throttles on diesel generators, etc.), or the generators would overspeed until they tripped or were damaged.

After all, all of the energy that is produced has to go somewhere.

What does it matter that the frequency goes up or down?

induction and synchronous AC motors will run at incorrect speeds, for one.

Transformers can also have problems if the frequency drops too much since this will cause saturation, which will dramatically increase the current drawn (due to economy, most transformers are designed to operate close to saturation, which otherwise means a bigger core and more windings). Although this isn’t as much of a problem (unless the frequency is MUCH lower than normal) for most modern equipment with switching power supplies, which effectively run off of DC (you’ll see something like 85-264 vac / 47-63 Hz on the back). Many AC powered clocks also use the power line as a reference, so they were made with a stable power line frequency in mind.

Also, for the OP, one way to see for yourself how load affects generator speed is to take a small DC motor and spin it without a load (a generator is basically a motor in reverse, although this won’t work with all types of motors), then try the same with the terminals shorted; it will be much harder to turn. Otherwise, nothing happens if the generator tries to produce “too much” power; it’s like a battery with nothing connected. As for how they control power supply/demand, they essentially do as you described, throttling generating stations, so-called load following and peaking generating plants, with the latter having the fastest response. These supply power in addition to that from base load plants, which meet the minimum requirements and are usually slow to start-up/shut down. Storage is largely hydropower, such as using “excess” electricity to fill a reservoir then emptying it during increased demand.

The short answer is that if there is extra power, they just stop making so much. The long answer is that an electric grid has numerous ways of dealing with fluctuations. Here’s just the tip of the iceberg.

There are three types of power plants. Base plants which run at full output 24/7, peak plants which only run during the day when power demand is at it’s greatest, and load following plants which adjust their output according to demand. If demand suddenly drops, the load following plant can cut production, as can all the others if demand drops that much.

They can actually store some extra power in the grid itself. Home voltage in the US is 120V, but can be anywhere from, say, 115-125. Electric devices are built to handle a certain range of voltages because they simply can’t keep the grid at exactly 120V.

So if the line is at 120V and someone unplugs a city block, they may adjust power production or they may just let it go and let the grid voltage change by .0001 volt. I guess it depends on what their computer is programmed to do.