Science Interviews


Tue, 13th May 2014

High voltage DC

David Hughes, ABB

Listen Now    Download as mp3 from the show Powering up the National Grid

Moving energy along transmission lines comes at a cost because power is lost along Electricity Substationthe lines. But those losses are lower if the electricity is transmitted at higher voltages, which is why the current UK National Grid operates at up to 400,000 volts.

Now the Technology company ABB are working on developing even higher voltage systems that could operate at up to a million volts, which they think could transport power further and more efficiently. David Hughes is working on the project.

Chris - Tell us first of all a little bit about the present national grid that we use at the moment. How does it work?

David - Well, the present national grid was developed in the in-between 30s and the 50s. It was typically around large centralised power stations very close to large population centres. But now, what were seeing is a very different demand, were seeing with the same tidal, the same wind, the same solar, the same biomass. And also, where the load event as in Scotland, to where the consumers are in England, the power has to be moved between Scotland and England.

Chris - Also, I did some back of the envelope calculations and found that you wouldn't actually need to use more than about a fifth or half of the Sahara Desert and cover it with solar panels. In fact, you could generate enough electricity to meet most of the worlds energy demands. But most of the problem comes with getting the energy from the Sahara to where we need it.

David - You are absolutely correct and this has been a big dilemma across the majority of Europe. If you take Europe as an example, you have wind in the north of Europe and you have solar in south of Europe. So, moving solar when the wind doesnt blow up to the northern Europe and moving the wind when the wind does blow, and the sun doesnt shine down to countries like Spain is a different dilemma and a different equation. Obviously, the solutions we have as ABB are being put in place for an integrated European network.

Chris - Well come back to David Hughes in just a second because first, we need to go to Dave Ansells garage because we did a neat little experiment this week to explain why actually jacking the voltage up to very high levels can save you quite a lot of energy when it comes to transporting electricity.

Dave - Power stations arent inside your house so weve got to somehow get that electricity to somewhere where we can use it. The obvious way to do this would be to just transmit it at normal 240 volts like you would do around your house. I've got a little model showing roughly what would happen. What I've got is a power supply which is using 6 volts AC. Its voltage is pushing electricity through some quite long wires to a light bulb which is modelling your washing machine or your lights in your house.

Chris - So, this long wire, that's like the national grid.

Dave - Yeah, in reality, it would be thousands of miles long, but in our case, its only about a meter. But its got lots of resistance. It behaves very similarly.

Chris - And the light bulb is barely glowing.

Dave - This is because as you push current through a wire, you lose lots of energy and the more the current is, the more energy you lose. And so, very, very little of that energy is ending up in the light bulb so its very, very dim.

Chris - Because weve turned most of the energy into heat effectively in the national grid, the transmission wires, before it got anywhere near the light bulb.

Dave - Indeed.

Chris - So, the power stations, if they transmitted electricity at mains voltage, this would be happening. The lights in the houses would be dim and the transmission lines would actually lose most of the energy on the way to the houses.

Dave - That's right. So, to get around this, the power companies use a fundamental piece of physics and this is the amount of power. So the amount of energy you're using per second is the current times of voltage. So, how much basically is flowing times how hard you're pushing it. So, you can transmit the same amount of power either at a low voltage and a high current or at a very, very high voltage in a very low current. Because all the energy lost is actually with the current, the higher the voltage, the lower the losses. So hopefully, the more power gets to your house.

Chris - So, if we were to step up the 6 volts of AC that you're power station is pushing into our national grid and then step it down again to 6 volts of the other end of your miniature national grid, we should lose less energy in the transmission lines because the current is lower. That means the light bulb should be correspondingly brighter.

Dave - That's right. To do that, what I've got is two devices called transformers.  This one will convert that 6 volts up to about 45 volts. And so, this will increase the voltage by a factor of 8 and Ive got another one which will drop it down again back to 6 volts hopefully.

Chris - So, were going to put one of these transformers close to our power station our 6-volt supply and this is going to step the voltage up and therefore, the current will go down to keep the power the same. At the other end of our transmission lines, were going to put a second transformer which is going to step the voltage back down and therefore, the current back up but only in a short supply line into the lamp. So, we should see the light lighting up brighter.

Dave - So, it should be much less energy lost in the national grid and the light should work a lot better.

Chris - Okay, so lets wire this up then. So, you got them on a chalk block for ease of connection and this should give us between 40 and 50 volts out of the 6 volts supply.

Dave - That's the idea.!

Chris - So now, were on to the second transformer. So, this is the step down again at the other end of our transmission lines. This is going to return the voltage the 6 volts hopefully that we began with and at a proportionally high current.  Woow! Now that's quite a stunning difference. Id say that's a good 5 times brighter.

Dave - That sounds about right. There's far, far less energy getting lost because were forcing much less current through the wires. And so, everything is much more efficient and our light bulbs work much better.

Chris - If I were to measure the temperature in the wires, would I therefore see that it was getting less hot with this setup because its losing less energy as heat?

Dave - Yup and this means that the national grid loses probably less than 5% of its energy to transmission like this.

Chris - Thank you very much to Dave Ansell. Well put the pictures of our little experiment on our website as well so you can take a look at the difference the transmitting at high voltage achieved. So David Hughes, that's the physics behind why you want to transmit energy at high voltages. The difference between our system and our experiment and what you're proposing is you're going to use DC direct current rather than alternating current AC why is that?

David - The reason why we use DC again is purely down to the losses as you heard in the previous piece. Basically, when you're moving HVDC over several thousand kilometres...

Chris - That's high voltage DC.

David - Its high voltage DC, that the percentage of power loss is around 3%.  When you compare that to AC, the power that we lose is between 30 and 40%. So, it makes good economic and commercial sense to go for HVDC.

Chris - The good thing about AC as we demonstrated is that you can very easily convert electricity from one voltage to another using a transformer because you have this changing field in your transformer because you've got a changing current. If you use direct current, how do you step the voltage up and down like we do with AC? Are you going to have to run it through some kind of inverter like the people who have solar cells on their roof?

David - Yes. In theory, you're quite correct. What we do is the current in voltage is transformed over DC and then we have large converter situations where we convert back to AC. So historically, when you're moving large currents, you have problems interfacing with the frequency. With DC, it is very, very simple and you just convert straight back to AC.


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