The Electricity Series P2 – AC/DC and the Grid

This is part 2 of what is likely to be a gripping trilogy of posts on how electricity is made and moved around Australia. Part 1: The basics is available here.Part 3 – Charges and Accounting is available here

Today we wind up the technicality a bit with a discussion of the differences between AC and DC power and why AC was chosen to power the grid. Once you’ve got your head around that we’ll move into how the grid is managed and why.

The Two Different Flavours of Current

In the early days of electricity research and designing networks for cities (late 1800’s), something of a war went on between Thomas Edison and Nikola Tesla, two pretty stout minds in the field of electrical research.

Edison, seriously famous American inventor was in favour of direct current (DC) for distributed networks, mostly because he held patents in most of the DC equipment.

Tesla, famous Serbian engineer (who emigrated to the States to work) favoured alternating current and legend has it the two men banged heads pretty vigorously over this one. I would have liked to be a fly on the wall then! Those same legends also have it that they were both awesomely eccentric and stubborn men too.

This begs the question: Why is it even important enough to argue about?

Direct Current and Alternating Current

When electricity passes through an object, say a light globe, it interacts with the materials in it and converts some electricity into another form of energy. It’s not overly relevant, but I’m pretty confident there is ALWAYS heat lost as part of this interaction, plus some of what you actually want, such as light in the case of the light globe.

In a direct current circuit the electricity flows (as always) from high potential to low potential, and this direction is constant. Batteries are the classic example of DC operation. As discussed in P1, one part of the battery is high potential, the other low potential, and the energy essentially drains from high to low, like letting water out of the bath. There is no mechanism for it to come back the other way. Once the high potential end is no longer higher than the low end, you’ve got a flat battery.

But DC doesn’t necessarily need to complete its circuit through a battery, it just needs to flow from high to low. If the connection is good enough and the potential difference great enough DC will flow straight into the ground. This is exactly how lightning works.

Alternating Current is a slipperier concept altogether. But, once you’ve got your head around it, the mechanisms governing the rest of the grid make a lot more sense.

Rather than going in the same direction all the time, Alternating Current, as suggested, switches direction periodically, a characteristic of the power known as frequency. We’ll come back to that. So, in the lightbulb example above, rather than, say, flowing always in a clockwise direction, the current flows clockwise, then deteriorates to zero, goes back the other way, then back through zero. A graph of it looks like this:

This is the sort of power that comes out of the walls in your house. You poke a plug into the wall and the top 2 pins complete a circuit through the device and the rest of the electrical network. Considering the lightbulb again, the electricity comes from one pin, through the bulb, to the other pin, then drops to zero, turns around and comes back again, through the bulb and to the original pin, where the voltage drops to zero again. Imagine this whole cycle as one full wavelength. That happens 50 times a second in the Australian grid; meaning the frequency is 50 hertz (Hz). During each wavelength the bulb goes from off to full brightness, back to off, then back to full brightness, then darkness on the return journey. But our eyes can’t perceive this as it happens 100 times a second (2 bright spots per wave, 50 waves per second), where I believe our eyes struggle to detect flicker about 30Hz, somewhere around there.

The frequency of electricity has some tangible influences on our experience of it, so 50Hz isn’t arbitrary. In early networks where electricity was applied to replace bullock and horse drawn devices, grinding, slow traction was required, which is provided by lower frequencies, around 16-24Hz. But, once this was applied to street lighting the flicker caused by such low frequency electricity was considered unacceptable. At the other end of the spectrum, higher frequencies move more energy, faster, but with a side effect of increased radiation. This would make your mobile phone reception even worse than it already is. Planes often use very high frecuency AC because they don’t care about your phone reception. Most grids worldwide operate around 50-60Hz.

The Grid

I carried on for a long time about frequency there because the concept is important. Particularly when operating an electricity network.

It’s easy to imagine how AC and frequency works with just a one generator/one load model, but as the number of generators and loads go up, it gets complex fast. Here’s where my new analogy comes in.

Every wave cycle on the grid, EVERY generator is pushing the same direction and every load is using that generation in the same direction. Then the current reverses and everyone pushes in the opposite direction, and all the loads use electricity in the opposite direction. This means the actual location of the generator is of very little relevance. It’s all about the combined effort pushing in the same direction.

One way to think of it is one of those old school, properly dangerous, merry-go-rounds from suburban childhoods:

This one will do.

Imagine that anyone pushing the MGR is a generator and anyone wanting to get on is a load.

Assume it always spins at the same frequency, like the grid. As people get on, the mass increases and the size of the pushes needs to be bigger to maintain frequency.

More than anything, this illustrates the importance of frequency in a grid. Anyone that tries to push at the wrong time, or in the wrong direction is going to get belted. Ditto any load that tries to get on out of synch with the grid.

One of the hardest things in renewable (in fact any) energy generation on a small scale is synchronisation. The grid is so big and ‘heavy’ (momentum is a useful similie for a high-energy grid) that anything trying to connect will BECOME the frequency of the grid; there’s no way a small generator can influence the frequency. So, get it wrong at your peril. There’s no faster way to make a motor explode than trying to run it out of phase with the grid.

Managing the Grid

The network (and we’re talking about the Australian East coast, Tassie and SA, there’s no connector across the Nullabor) is managed by AEMO; The Australian Energy Market Operator. Their job is 2 fold; make sure the network keeps running and to organise the market for electricity.

Keeping the network running, their most important task is maintaining frequency. If the frequency gets weird you could wreck a lot of generators very quickly, crashing the network not just momentarily but for a LONG time. They manage this by bringing generators on and booting loads off, if the need arises. It’s managed in real time, with the accounting done after the fact.

So, Why AC Again?

This is the part where Tesla become a genius and Edison a stubborn old bastard looking to protect his patents and income.

A national network MUST be AC, so that the distribution of generators and loads is less crucial. That way you can hook in anywhere and contribute; DC you always need to be upstream of the load. Tesla knew this and pursued it accordingly.

So the grid is AC for a couple of major reasons;
1. Distributed generators and loads. As discussed extensively above.
2. It’s EASIER to generate AC power from rotating machinery. I could go into the physics, but it might make your head explode?

There are others, but they’re getting a little esoteric.

Bonus Question

So, what’s 3-phase power then?

Once you’ve got your head around AC power, you can see there will be times there’s effectively no power. This is overcome with 3-phase power.

The power described in the examples above is 1-phase; just one wave, wobbling back and forth all by itself. You can improve the flat spots, by making machinery that uses 3 different circuits at once; 3 phases. They don’t come down the same wires, but in separate ones, co-ordinated by the generators. Each wave is a 1/3 of a wavelength behind the other, as shown below:

Assuming for simplicity’s sake that a wave at the top of its curve is maximum power, see that 3 waves gives a lot more time at full power? In the Merry-go-round analogy it effectively means 3 times as many handles to push; more opportunities to provide energy. This is why 3-phase welders are the envy of shed oriented men everywhere.

The Postscript

I’ve wrestled with this post for an age. I know this stuff pretty well, but I always think about it in terms of maths; integrals and rates of changes. How’d this go for you? It’s big and difficult stuff, but sort of important. Well it’s important to me anyway. But once you know this, it refutes some of the common misconceptions about the grid; like Dave’s (totally valid) question about the location of generators and the type of electricity he gets. It doesn’t matter where you are, or what electricity you use. It’s all about the energy contracts and who you’ve chosen to go with.

And so, next post will look at how you are charged for energy, how big users are charged for their connection (it’s different to residential users) and how renewables are calculated. I will also answer the question of how Greenpower contributes in a MRET world. That’ll get the punters in!

Part 3 also available here.

Questions, as always, appreciated.

EB

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About evcricket

Extreme gardener, engineer and bird nerd.
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6 Responses to The Electricity Series P2 – AC/DC and the Grid

  1. Bernie says:

    While I understand and aggree that AC for general distribution is curently the easiest and most effiecent to implement. I am struggling to come to terms with this statement

    “A national network MUST be AC, so that the distribution of generators and loads is less crucial. That way you can hook in anywhere and contribute; DC you always need to be upstream of the load. Tesla knew this and pursued it accordingly. ”

    Why cant DC sources be placed anywhere in a DC newtork? In a distributed model to combine DC sources would be easier to implement as there is no need to maintain syncronisation of the frequency and phase of the supply.

    Bernie

  2. This is a great post and may be one to be followed up to see what goes on

    A mate sent this link the other day and I will be desperately awaiting your next write. Keep on on the superb work.

  3. Matt says:

    Cool, I never studied distribution at uni. I think lightbulbs don’t technically go completely off each cycle as there is a little bit of glow in the hot coil bit in the middle for a few milliseconds isn’t there? But I agree if we could see at 50Hz they would flicker. (I think the ‘framerate’ of the eye is about 18Hz so you need to have at least 36Hz according to Nyquist)

    Also, Tesla is the electric Jesus! (great video Kat)

  4. Kat says:

    I can’t really let this slip by without directing you to the drunk history of Tesla and Edison:

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