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!
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In the AC system we use, it means that the frequency of the current drops. This causes huge problems for anything that has a timer, since timers are calibrated with AC pulses. It also decreases the force received by everything connected to the network.
So, my understanding is a that a overdrawn electrical grid will run “slow”, in the sense it will drop from its normal frequency of 60hz to a slower frequency. the effects of this are…complex, but generally its A Bad Thing.
however, widespread electrical car charging is NOT that likely to cause this, mainly because it causes relatively stable, predictable loads that can be planned around, and power companies can manage power generation around it much easier than, say, several million households in England all getting up from watching TV during an ad break and putting their kettles on at *the same time* (which is a real thing they have to work around)
As is done often, comparing it to plumbing may help with the concept.
As many of us have experienced personally, when you turn on a lot of water faucets, spigots, and/or appliances in your home, you will see a decrease in water pressure from each of those outlets. There comes a point where some of the connected equipment will no longer function properly. There’s no point in running a water sprinkler that’s just dribbling water, for example.
So, when it comes to electricity, the same applies. At some point there’s not enough electrical current to power your devices and they will experience a brownout. Some equipment will fail to operate altogether in this situation. Some things may operate but at a lower capability. As a result, utility companies may then start using techniques to match supply to demand like asking customers to reduce usage through incentives like bill credits. Ultimately, they may be forced to start implementing rolling blackouts where customers have power completely turned off for a period of time so that others won’t be affected.
You’re right that adding a whole lot of EVs to the grid at once could be a problem. But the plan is that as EV adoption increases, so will electrical production. Also, other related technologies like whole home electrical storage exist that serve to mitigate demand from the grid during periods of increased demand which should hopefully prevent widespread problems.
Presuming an AC system, as is used for most electric grids:
The first effect is that the frequency drops. Most of this effect comes from physical turbines that produce most of the power, whether in hydroelectric dam, fossil fuel plant, thermal solar-thermal generator, geothermal generator, or nuclear plant. When there is more demand, it increases the electromagnetic resistance in the generator, thus slowing it down. The opposite happens if there is an oversupply of electricity.
Minor fluctuations of frequency happen all the time, though typically not by much. The US grid runs at 60hz with a tolerance of 0.050hz – that is, it is allowed to normally vary from 59.95hz to 60.05hz. This is also corrected over a longer period to ensure it averages out to 60hz.
More major changes can start to cause problems. While a lot of modern technology doesn’t care much at all what frequency it gets as input, such as computers, others do care: motors will run at a different speed, as will many AC clocks.
If the frequency changed too much, various power plants will start to disconnect. Any given powerplant is only designed to operate inside a fairly tight range and will be forced to disconnect to prevent damage if the frequency goes outside that range. Naturally, having a powerplant disconnect when there already is not enough power will worsen the problem, so that is something you really want to avoid – you’ll basically end with a blackout.
To avoid such a cascading failure, parts of the demand grid will get cut off at some point before that by flipping breakers feeding areas. Generally, this will be done by rolling blackouts. That is, the power company will cut off some neighborhoods for a few hours then switch to a different set of neighborhoods. In this way, everybody gets power for some time, minimizing damage, though nobody will be completely happy. Some areas have, optionally or required, high power draw appliances, such as air conditions, able to be controlled by the power company to balance load.
In other cases, the power company will institute a brownout, reducing the voltage, which naturally results in lower power demand. This is not commonly used as it is much more likely to damage modern equipment than a frequency change or complete power outage.
In a DC system, the entire thing is much simpler: the excessive draw will drop voltage across the system, with the effect being stronger the higher your resistance is.
Overall, any predictable usage is very unlikely to cause problems. The real issue comes down to the hard to predict demand, such as heaters and air conditioners. These can draw a lot of power suddenly and can do so with much less predictability than car charging patterns.
Electricity for the grid is generated on demand. Somewhere there’s an electric motor being spun that generates the electricity that ultimately makes it to your wall socket. That may be for the turbine in a hyro plant, nuclear plant, coal plant, or whatever, but there’s an electric motor (generator) turning and devices are consuming it at an equal rate.
In fact, when you turn on a light bulb, there’s a very vey very small drop in the power on the grid, and those generators need to run just a little harder to compensate. This of course happens on a massive scale, so it’s all averages out over thousands of homes and dozens of generators, but that’s the basic concept.
At a certain point, those generators just can’t meet demand. Maybe it’s the limit of the available source of power (water, steam, wind, etc), but in that scenario, there’s insufficient power generated to meet demand. And thus you get brownouts, or in worse case, blackouts. And that affects everyone connected, not just those further away.
There *are* instances of battery stored power on the grid, but those are rare.
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?
Two things can happen on generator side.
If the generator excitation current is maxed out to provide the rated power, pulling more load from the circuit will cause the voltage to sag.
If the generator excitation continues to rise to try to generate more power to feed more current so the voltage doesn’t sag, the torque required to turn the generator will also increase. if the engine driving the generator cannot produce enough torque, the RPM will decrease and the generator slowly drops out of phase.
Voltage dropping will be noticable as light dimming, stuff like TVs, computers tripping off, users starts to complain about the quality of electrical power, but there is no blackout.
Phasing issues on the other hand is much more worse. Different parts of the grid that have interconnections to share the load must be synced in frequency, phase and voltage. Inconsistencies in frequency will quickly result in phase differences, which will cause massive voltage differences inside each oscillation, overloading the interconnections despite no real power being transmitted.
Therefore the grid operator will be forced to either slow the whole grid frequency down a bit, increase voltage of other parts of the grid to try to feed the overloaded grid, or just cut the overloaded part off.
Slowing the grid down results in the frequency going out of spec, it’s another quality issue and if there are interconnections to other parts of the grid not under his/her control, they will also be forced to disconnect them.
Feeding the grid risks overloading the other plants/transmission lines, especially at the interconnections, because they are typically designed to only carry the load differencesof normal days between the grid, not situations like this when there is a massive shortage. Overloading the grid risks in tripping various parts of it.
If a trip happens, it could be a downward spiral from there, the grid quickly collapses and it may take hours or even days to restart it.
> the ever increasing number of electric vehicles will likely mean an increasing demand on the energy infrastructure’s of countries.
This actually isn’t projected to be a problem. EV’s are typically charged overnight when electrical demand is half of daytime load.
Now, in cold areas with electric heat EV charging may actually be an issue. We don’t know yet because most places are transitioning to heat pumps at same time which halves heating load.
Overall, the issue is overblown IMO because of night time charging and because electrical demand in developed countries is actually dropping as HVAC, electronics, and lighting become more efficient.
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