I was thinking on the drive home for work that the ever increasing number of electric vehicles will likely mean an increasing demand on the energy infrastructure’s of countries.
But what is actually happening in the electricity grid if supply can’t meet demand? Is it simply the devices furthest away from the generation of power won’t receive current?
Whilst this is ELI5, I also wouldn’t mind a slightly more technical answer. Thanks!
In: 9
Not to re-hash what others have said, but in a slightly different way.
It takes a certain amount of physical energy to spin a turbine, be it steam, hydro, wind, etc. These are called prime movers. The more electrical load on the system, the harder these systems have to work to keep the generators spinning at 60 hertz.
An analogy would be pedaling your bike on level ground. You are the prime mover. As the trail becomes steeper and steeper, you have to pedal harder and harder to maintain the same speed. At some point, the slope will overcome your ability to maintain that speed and you will slow down.
So, as others have said, you get to the point where the load is overcoming the capability of the prime mover.
Here’s the more technical part of what is happening. In an induction device (i.e., any motor, like the compressor of an AC), *the impedance of the motor is proportional to its rpms*. Without going too much into detail, as a motor spins, it generates what is called a CEMF (counter electro magnetic force) that opposes the current flow. This is why your lights dim when the AC cuts on. At a dead start (zero rpms) impedance is minimal and you have an extremely high inrush of current, which drops the voltage. This inrush degrades quickly as the motor comes up to normal operating speed, so you normally only see a blink.
Hold that thought.
A motors rpms are syncronized to the frequency of the system. So if the generator slows, and the frequency drops, the motor also slows. This means its impedance drops and it draws more current. When more current is drawn, the load on the generator/prime mover is increased, which causes it to slow more.
Which causes the frequency to drop, and the motor to slow more, which cause it to draw more current, which slows the generator, which slows the motor more, which draws even more current, etc. etc. etc.
It’s called a cascading event. Once it’s underway it goes to shit pretty damn quick.
So the frequency is critical. As others have noted, there are under frequency relays in place that will start operating in two main ways.
In ERCOT, the NERC protocols state that the the generators will not trip in less than 9 minutes once the frequency hits 59.5 Hz. This is that infamous “we were 4 minutes from blackout” that you hear.
Which isn’t quite true.
ERCOT also has a NERC required UFLS (under frequency load shed) protocol that requires each utility to be able to shed AUTOMATICALLY up to 25% of it load (i.e., with no human interaction) in discrete steps of 5%, 10%, and 10%. Each step cuts in at a certain frequency with each step being 59.3 Hz, 58.9 Hz and 58.5 Hz respectively. So if generators had started tripping, there was still a failsafe in place to avoid a total black out.
Make sense?
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