You might find this hard to believe, but I am regularly asked to explain how electricity works and in particular how the grid works. I’ve been thinking about and answering this question for a while so it’s probably time I formalised a response. Here goes. As always, questions and comments greatly appreciated.
This will be the first post in a series of posts on electricity, how the Australian grid works and is managed, and some of the basics around renewable energy and how it is accounted.
Part 2 – AC/DC and The Grid is available here
Part 3 – Charges and Accounting is available here
I’m going to skip the actual physics of how electricity moves around. It’s too hard, really, and of little consequence. But one point; considering the movement of electrons will not make understanding electricity any easier for you. Just remember ‘electricity’ (whatever that is) flows from areas of high positive potential to areas of lower potential.
Lastly, apologies to the nerds out there, but I’m not going to include any formulae. Unless your maths is pretty strong, they’re unlikely to aid your understanding. In any case, if your math is strong enough that Maxwell’s equations improve your understanding of the world, then this guide is a little below your level…
Electricity always flows in a circuit; from areas of high potential to areas of low potential. In a torch powered by battery, the battery has a high potential end (which the electricity comes out of) and a low potential end, which it flows too. Put things in between (like a lightbulb) and you have the ability to make things happen (formally called work). But without the ability to move from high to low potential, nothing, I repeat NOTHING will happen.
That’s why birds can sit on electric wires without anything happening. When they sit on a wire, it’s HEAPS easier for the electricity to continue through the wire, rather than up one leg and down the other. However, if the bird was wearing, say, a large, floppy, wizards hat made of alfoil, and this hat touched another wire, then our bird is in trouble. Suddenly there’s a nice short path to an area of lower potential so the electricity is going to go there. Fast. The bird would likely explode.
Voltage and Current
As electricity moves around, there are two major descriptors to consider; voltage and current.
Voltage is essentially the desire electricity has to move between two points. Voltage must be measured ‘across’ or ‘between’ two points; it is always a relative measure.
High voltage means the electricity wants to move A LOT. So, as the voltage goes up, the normal restraints of wires become ignored. The electricity wants to move so much it can leap across gaps; the higher the voltage, the larger the gap the electricity can leap across. Lightning has enormously high voltage; the gap in this case being the distance between the sky and the ground. Voltage is also what makes substations dangerous; current is easy to avoid; just don’t touch the wires. But big voltages are a bit scarey. An 11kV (11,000V) transformer could electrocute you from ~30cm away. Spooky.
Household voltage in Australia is 240 Volts (240V). This was chosen to balance safety and power delivery. Higher voltages mean higher rates of energy delivery, but the reverse of that is a bigger kick when you grab the bare wires of the iron one day.
Current on the other hand, is a measure of the amount of energy in the electricity should it start moving. This measure is called an ampere, amp or just A. It’s a bit of an odd one though; current is demanded by an item, not pushed by the electricity. You flick on a 10A kettle, and the kettle is going to draw 10A. Put 2 kettles on the same plug, and they’re going to demand 20A.
When circuit breakers on your house flick, or a fuse is blown, that’s because there have been too many items demanding amps at once. Voltage stays the same around a house; it should be very close to 240V everywhere in your house. But, since current is the movement of energy, put too much of it in a wire and it will melt, then catch on fire, then burn your house down. So, to stop people demanding so much power that they burn their house down, circuits within a house are limited in the amount of current they are allowed to draw. So, 5 powerpoints might be on the same circuit, with the whole circuit protected by one circuit breaker or fuse (circuit breakers are the modern replacement for fuses, but they behave effectively the same). Plug a toaster, kettle, washing machine and bar heater into the same circuit and turn them on all at once and you’re likely to demand too much current. The circuit breaker will ‘trip’ and you’ll be outside with a torch playing around in the fuse box.
When you get electrocuted it’s current that kills you, rather than voltage. You can have quite large voltages (up to kilovolt range and beyond) but extremely low amps and survive a shock (like an electric fence). But, drive the current up above 5 or 10 milliamps (mA) and you can stop a heart.
The Water Analogy
Is the usual way to explain electricity, particularly the basics; ie how voltage and current are related. For this post I’ll just mention it here to try and help fill in some details, but otherwise I’ll use a different analogy altogether; one I invented to explain how the grid works.
Anyway, the water analogy is simply this; in a pipe full of water, the pressure can be considered analagous to voltage and the flow rate, or pipe size, analagous to current.
The whole analogy falls down when you try and think about alternating current anyway, so I’ve never really understood the appeal.
Power and Energy
Now this is a concept I wish more people understood.
This. Stuff. Is. Important.
Power Is an INSTANTANEOUS descriptor of an item’s ability to do work. It is measured in watts, kilowatts (kW), MW, GW… so on. Power is the result of both voltage and current; pick 2 items that operate at the same voltage, one draws 10 amps, the other draws 20 amps. It can be said that the second one is twice as powerful.
I know I said there wouldn’t be any formulae, but this is worth it. Power is the product of voltage and current. If you’ve got a voltage of 240V and current at 10A then power is 2400 Watts. (2.4kW)
Power is a promise, not a guarantee. It is a descriptor of what something is capable of, not what it is doing nor has done. A 2.4kW kettle has the capacity to draw that much power when going absolutely flat out.
Power is both produced (by generators) and used (by machinery, lights etc). In either case though, it can only be used to describe an instant.
Understanding the difference between power and energy is critical for thinking about the electricity grid.
Energy Is power, over a period of time. Electrical energy is generally measured in watthours, kilowatthours, MWh, GWh, etc
One kWh is the amount of energy used if a 1 kW motor runs flat out for 1 hour. An average compact flourescent light globe (CFL) uses 10 watts; run it for an hour and it has consumed 10 watt-hours; 100 hours and it’s consumed a kilowatthour.
Energy is what is measured on your electricity bill. It’s a combination of the rate at which you use power (so brighter lightbulbs use more power) combined with using it for a longer time (you pay more if you leave the lights on). This is usually charged in cents per kWh.
And I’ll leave this one there.
The next step is the grid, which will need to open with a discussion of the difference between AC and DC power, which will obviously lead to discussion about how Nikolai Tesla is one of the sharpest minds ever to have lived. But once we’re talking about AC power, a lot more interesting things can be discussed. So, AC/DC and the Aussie grid should just about be worth a post in its own right.
Following that there’s scope for discussion on how renewables integrate with the grid and how they are accounted for. Thrilling stuff I know, but if more people knew this, my job would be so much easier.