[Adam Murphy (OP)]

Andrew got in touch with this one:

"What happens to 'unused' electricity?

I live in Australia and am fairly used to periodic load-shedding when, for instance, the grid can't supply everyone's air-conditioning on very hot days. But what happens when there is very low demand? There is power when you switch a light on, but when it is switched off again, where does that electricity go. It's being generated and fed into the grid, but if everyone switched off their homes at the same time, what would happen to that electricity fed into the wires at the generator end?"

Any shocking revelations to impart?

[Bored chemist]

They reduce to power fed to the generators.

[alancalverd]

In the UK, you (HM Taxpayer) pay windmill owners to not generate electricity on warm days.

[Janus]

Andrew got in touch with this one:

"What happens to 'unused' electricity?

I live in Australia and am fairly used to periodic load-shedding when, for instance, the grid can't supply everyone's air-conditioning on very hot days. But what happens when there is very low demand? There is power when you switch a light on, but when it is switched off again, where does that electricity go. It's being generated and fed into the grid, but if everyone switched off their homes at the same time, what would happen to that electricity fed into the wires at the generator end?"

Any shocking revelations to impart?

The amount of electricity generated at the plant is determined by the load.  If you have a generator spinning like crazy, but is is hooked up to no load, it is generating no current. it isn't until a load is connected and you have a complete circuit that a current flows.  The greater the load, the more the current.  However, the more current the generator needs to supply, the harder it is to turn the generator.  Eventually whatever is turning the generator can't supply enough force to keep the genrator up to speed.

So basically, if everyone just completely disconnected from the power plant, the generators would be just be "turning free" without producing any electricity.

[evan_au]

In Australia, electricity supply and demand is managed by the National Electricit Market (NEM). Equivalents exist in other countries.

This organisation manages electricity generation and consumption by running a continuous auction between those who want to generate electricity, and those who want to consume it. Prices can get very high in peak hour, inviting companies to connect their in-house standby generators into the grid. At low consumption times  (eg 1am-4am), prices fall, and most suppliers disconnect (or are told to disconnect). There always must be some "spinning reserve": generators that are ready to generate, but not actually generating any power - just in case one of the active generators suddenly fails.

Even at low consumption times, there are still a large number of electricity consumers, so one consumer turning off a light globe or air-conditioning unit does not change the situation noticeably.

However, if there is a sudden, large drop in electricity consumption (eg a storm blows over a high-capacity transmission line, blacking out a large part of a city):
- voltages will increase, and resistive loads will automatically consume more current
- grid frequency will increase, as generators start to turn faster
- Grid voltage and frequency are both closely monitored, to ensure they stay within tight ranges.

In the next few years, we can expect to see home power systems monitoring electricity prices continuously, and choosing the cheapest time to charge the electric car, or the home battery system.

See the "Price and Demand" tab at: https://aemo.com.au/energy-systems/electricity/national-electricity-market-nem/data-nem/data-dashboard-nem

PS: Another innovation happening soon in Australia: At times of high electricity prices, large industrial consumers of electricity will be able to offer bids against coal, solar and wind power: Instead of offering to generate more power at $X/MW, industry can offer to cut consumption for $X/MW.
- Rather than "MegaWatts", I have seen this described as "NegaWatts"
- This has some generator companies worried - how can they compete with someone who has to do nothing?
- I wonder if some industries might use this to "game" the system - turn on large loads to push up the price, then offer to be paid to turn them off again...

[hamdani yusuf]

- I wonder if some industries might use this to "game" the system - turn on large loads to push up the price, then offer to be paid to turn them off again...

At what time could they turn on and off the large loads to take advantage out of it?

[evan_au]

At what time could they turn on and off the large loads to take advantage out of it?

Prices are highest during peak hour, so that is when they would be paid most to turn the load off....
Of course, that is also when it costs most to turn the load on.

[alancalverd]

It's already happened in Northern Ireland, under the "cash for ash" scheme where the subsidy for burning wood pellets made it profitable to buy and burn them without doing anything useful with the heat. Pretty much the same with windmills around the UK - it's always windy offshore but most people like to sleep at night, so you get your subsidy for doing nothing.

[evan_au]

In the USA, Enron owned electrical generators, and made lots of money by various underhanded tricks.
One was scheduling maintenance at multiple generators at once, pushing prices to very high levels for which their remaining generators received an enormous bonus.
See: https://en.wikipedia.org/wiki/Enron_scandal

[Bored chemist]

It's already happened in Northern Ireland, under the "cash for ash" scheme where the subsidy for burning wood pellets made it profitable to buy and burn them without doing anything useful with the heat. Pretty much the same with windmills around the UK - it's always windy offshore but most people like to sleep at night, so you get your subsidy for doing nothing.

Do you understand that the whole point of a subsidy is, in effect, to pay someone to do nothing?

Imagine that for some reason- an in practice, the reason is to get votes- the government wants to transfer the power generation in the country from coal to renewables.

There are a few options.
You can simply issue legal decrees forcing the suppliers to build renewables stations- but that's not popular because it's seen as "heavy handed".
You can have government owned renewables stations- but that's incompatible with the current "public sector = bad; private sector = good" dogma.

Or you can just pay them to do it.

That's a subsidy.
They aren't being paid to produce power; they are being paid to build wind-farms.

They are doing exactly what they set out to do, in a fairly reasonable way.

On a tangentially  related note, the purpose of HS2 is not to take 20 minutes of a trip to London.

[evan_au]

if everyone switched off their homes at the same time, what would happen to that electricity fed into the wires at the generator end?

Let's say that the system is initially in equilibrium: coal is being fed into the boilers at a constant rate, the steam pressure is constant, the turbine is rotating at a constant speed, and the alternator is supplying a constant AC voltage to the grid, and providing a constant torque on the turbine. The load is drawing a constant AC current from the grid.

If the load suddenly drops significantly, the grid current will drop, and the grid voltage will increase (the phase between voltage and current will normally change too). The alternator will see a lower current, and, driven by a constant torque, will start to speed up (increasing the grid frequency). Control systems will see that the grid frequency is increasing, and will reduce the rate of coal feeding into the boiler, which will reduce the pressure in the boiler, so the turbine will apply less torque to the alternator, allowing the grid frequency to level off, and then return to normal.

It takes a while for coal feeders and boiler pressures to stabilize at a new, lower level. In case of severe reduction in power, faster-acting approaches can be used, like partially closing the valve that feeds steam to the turbine.

Just so people's AC clocks aren't put off by an event like this, operators will run the grid slightly below the standard frequency for a while, to compensate for the number of AC cycles they gained during the overfrequency event.

[Bored chemist]

Conversely, if a large surge in demand happens- the cliche reason is the end of a popular sport event leading to everyone going and making the - the system has to compensate.
As Evan has pointed out, there are systems that will regulate the voltage and frequency, but that takes some time..
There's another set of automatic controls. Some large scale users of electricity have an agreement with the supplier where, they get their power at a reduced cost in exchange for agreeing to let the grid controllers disconnect them when there's a spike in demand.
An automatic system disconnects them from the grid if the frequency of the mains drops by some set amount.
These users are things like huge refrigerated warehouses. Cutting the power to the cooling plant for a few minutes won't make any real difference to the temperature, but it gives the power suppliers some leeway.
The clever bit is that the mains frequency is a very accurate indicator of the load on the grid, and it's automatically distribute to all users.

[alancalverd]

It's a great shame that AC clocks seem to be out of fashion, replaced by unreliable battery-operated quartz clocks. Thanks to grid phase control a domestic AC clock will keep time to within a couple of seconds unless the power goes out completely.

The National Physical Laboratory had an original Synchronome system where every room had a clock driven by the national primary standard, driven by huge lead-acid batteries, so everyone knew the time within 30 seconds of GMT (as it then was). But they required MAINTENANCE which is a REVENUE COST , so one fell day they were all replaced by battery operated clocks from some backyard factory in the Far East, and within a week, nobody had any idea of the time.

The same auditor found an old estate car which anyone could use for official business - just bring it back full and claim the fuel on petty cash. Being Government Property it wasn't taxed or insured, but again it was MAINTAINED which is a REVENUE COST, so it was scrapped and any time we needed to transport equipment we had to rent a van, which required a tender exercise and umpteen signatures, but was accountable to your lab budget....

The auditor was promoted. As, I am sure, was every clown-in-office who ever gave away taxpayer's money for doing nothing.

[Bored chemist]

It's a great shame that AC clocks seem to be out of fashion, replaced by unreliable battery-operated quartz clocks.

There are three clocks in my room as I write this.
One is on the wall, it's a quartz clock, but MSF disciplined.
The second is in my computer- again, a quartz clock, but it's locked, via the internet to the NPL's time server.
And the third is my mobile phone- another quartz clock adjusted as needs be to the correct time via some magic. I don't know if that's GPS or the 4G network.

They all agree to within a few seconds. I can force the PC to resynchronise if I want a better precision.

within a week, nobody had any idea of the time.

Really?
They didn't go to lunch at lunch time?

NPL is still distributing the time, but in 1950 they went over to 19th century tech. and distribute it by radio on 60KHz. And, of course, now they use the web- just like everything else.
If you want a more international flavour, there's GPS which keeps pretty good time.

Of course, they could, if they wanted, call the speaking clock.

But they no longer had a synchronome clock on the wall. Well, that's a pity in a way because they are truly beautiful bits of kit. , but was it a tradition that the taxpayer should really have been funding? How often do you actually need to know the time to  half a minute?

Having heating in the building is a "REVENUE COST" but the bean counters didn't abolish it.
So it's not the nature of the expense that makes the decision.
What they do is look at value. Presumably, the new clocks were cheap and kept "good enough" time.

So, to portray it as auditors making  decisions on the basis of whether a cost is a "revenue cost" or not, is just silly.

[alancalverd]

Presumably, the new clocks were cheap and kept "good enough" time.

For a week, as I said. But as the batteries didn't all expire at the same time, and could be replaced by the occupant of the office rather than the maintenance guy, they became a project cost rather than a general overhead, so made more work for accountants. A bit like getting rid of the quick, accurate professional audio typing pool and getting scientists to type their own letters slowly and badly.

Knowing the time to within 30 seconds is admittedly not much more useful than knowing it to a minute, but a clock that is only right twice a day is money down the drain. And since the canteen closed at 2 pm it was kind of useful to coordinate lunch with the cooks' idea of time.     

[Bored chemist]

I forgot to mention the other clock in my house that keeps good time- the central heating timer.

It keeps good time because, unlike most of the last few posts, it's connected to the electricity supply.

[alancalverd]

Having heating in the building is a "REVENUE COST" but the bean counters didn't abolish it.

Because it is a statutory requirement!

[Bored chemist]

Having heating in the building is a "REVENUE COST" but the bean counters didn't abolish it.

Because it is a statutory requirement!

I guess it must be considered important.
There are also statutory limits on the deviation from the nominal mains frequency.
Are there any limits on deviation from the topic?

[vhfpmr]

Conversely, if a large surge in demand happens- the cliche reason is the end of a popular sport event leading to everyone going and making the - the system has to compensate.
As Evan has pointed out, there are systems that will regulate the voltage and frequency, but that takes some time..
There's another set of automatic controls. Some large scale users of electricity have an agreement with the supplier where, they get their power at a reduced cost in exchange for agreeing to let the grid controllers disconnect them when there's a spike in demand.
An automatic system disconnects them from the grid if the frequency of the mains drops by some set amount.
These users are things like huge refrigerated warehouses. Cutting the power to the cooling plant for a few minutes won't make any real difference to the temperature, but it gives the power suppliers some leeway.
The clever bit is that the mains frequency is a very accurate indicator of the load on the grid, and it's automatically distribute to all users.

There's also pumped storage. Dinorwic empties the turbines with compressed air whilst on standby, and spins the generators at synchronous speed so that when the sluice valves are opened they can go from zero to full power in 16 seconds. It's quite an impressive beastie if you go for the guided tour.

[vhfpmr]

When I went round Bradwell in the late 70s they had two clocks on the wall in the control room, one running of the mains, and a battery one adjusted to the Greenwich time signal. They just adjusted the frequency to keep the clocks reading the same time.