Engineering Matters est produit par Reby Media. Cet épisode a été écrit et réalisé par Alex Conacher, avec l'aide de Bernadette Ballantyne.
Transcription
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MUSIC – IRISH, HIGH PITCHED
SOUND – A GOOD WIND COMING IN FROM THE OCEAN, NOT A STORM
Bernadette
You are standing on a clifftop on the western coast of Ireland. As you look out at the ocean, a strong wind is pressing into you.
Alex
The weather front formed on the other side of the Atlantic, where warm air from the Gulf of Mexico presses against colder northerly winds.
Bernadette
It is often windy here, but not always. Tomorrow the weather will be calm, as the front continues eastwards on its way to knocking over garden furniture in the UK.
SOUND – SOFT ELECTRIC WHIRRING, SPINNING BLADES
Alex
The island of Ireland has made good use of its blustery conditions, with 5.5GW of installed wind power as of 2021. This means that, in 2020, wind generated more than 35% of Ireland’s electricity.
Bernadette
This is one of the highest levels of wind power penetrations in the world, which is amazing, but it does have one drawback.
SOUND – WIND DROPS
Alex
What happens when the wind doesn’t blow?
Bernadette
When the risks of intermittent supply become more serious, these need to be mitigated.
Alex (exasperated)
This can be done by having fossil fuel generation on standby (which we don’t want to use) or by using grid storage (which isn’t really practical yet)…
Bernadette
What if there was a way to exploit the free energy from that weather front as it moves across the continent?
Alex
What if Ireland could benefit from the wind in Britain and what if, in return, Britain could benefit when it’s windy in Ireland?
OPENING MUSIC
Alex and Bernadette
Hello and welcome to Engineering Matters, our presenters are Alex Conacher and Bernadette Ballantyne. In this episode we have partnered with WSP to talk about interconnectors.
Bernadette
Specifically HVDC interconnectors – that’s high voltage direct current.
Alex
These are high-capacity electricity systems that can deliver enormous amounts of power over long distances, directly from one electric grid to another. It is a simple idea, but the implications for how we generate, transmit, and consume electricity are enormous.
Bernadette
In this episode we will look at a project to build a new interconnector between Great Britain and Ireland called “Greenlink”.
Alex
We will learn what it takes to build one of these HVDC systems, including laying HV cables under the sea, and discover the benefits they bring to the countries at each end of the link. We’ll also consider the implications of linking together to form a super grid.
Bernadette
But before we get to that, we had better start with first principles…
MUSIC
Alex
For long-distance energy transmission it’s important to keep power losses to a minimum.
Norman
One of the big things we try to do as engineers is not to waste energy by just heating up the wires heating up the atmosphere. If we can increase the voltage of the transmission, we decrease the current, if we decrease the current decrease the loss of energy.
Alex
This is Norman McLeod, he is the Technical Director for HVDC at WSP. and he has been contributing to the evolution of interconnectors for decades.
Norman
I was the director of a team working on one of the many big manufacturing companies that developed 800kV DC technology. The previous threshold had been about 500 kV. But we knew that to get more power over longer distances, we'd have to increase the voltage.
Bernadette
This innovation allows the construction of interconnectors up to 3,000km. Not very useful for Europe, but it is ideal for countries in Asia, South America, and Africa.
Norman
So it suits countries with big power demands that have to cross huge distances.
Alex
And, although the distances in Europe do not require voltages as high as 800kV, a 500kV rating is still considered very high. So that’s the HV part of HVDC.
Bernadette
But why Direct Current and not Alternating?
Norman
Two main reasons for using direct current is that it has lower losses than AC transmission, and is more controllable than AC transmission. AC transmission is how the world works and has worked for the last 140 years. It's very flexible, very adaptable, and it's the currently (pun intended) the universal solution.
Alex
But if all you want to do is move power from, for example, a hydropower plant next to a big river a long distance to a load centres, such as a city or an industrial zone…
Norman
…and you don't want to drop off any power in between, you just want a point-to-point connection then HVDC is more efficient, losing less energy along the way. Theoretically, we only need two wires, positive and negative - like a car battery on a grand scale. If you look at an AC transmission network there are always three wires or multiples of the wire. So we save the material.
Bernadette
Direct Current is also more controllable because it’s not an inherent flow of electricity, it only flows when instructed to do so.
Norman
…and we tell it to flow millisecond by millisecond by millisecond. If we don't tell it to it will stop instantly. Whereas with AC, you don't have to tell it anything, it continuously flows because you’ve strung the wires from the generating station to the load centre.
Alex
As well as controlling where and when the power goes, and whether we want to increase or decrease the amount… it’s impossible to overload the scheme by accident. AC is very different, it can be overloaded very easily and great care has to be taken to avoid that.
Bernadette
DC is also good if two countries that use different electrical frequencies want to transmit power between themselves.
Norman
So, if you live in an island, and you want to connect to your neighbour, if you just string an AC cable under the sea, you're forced to operate at the same frequency as the other country, you encounter their problems and system faults, and everything else.
Alex
If you convert to DC in your country, put a DC cable under the sea to your neighbour, then they convert to AC again…
Norman
…you've effectively decoupled the two countries. You can still exchange power both ways but you're no longer constrained to operate at the same frequency, you no longer have the same operational regimes protection regimes, meaning you can do what you want in your own country.
Norman
So the island of Great Britain, for example, is not connected to France, Denmark or Belgium, except by HVDC. There are two links to the island of Ireland, which again are only connected to Great Britain by HVDC. And we're now beginning to connect some of the remote islands with a DC scheme, going as far up as the Shetland Isles, for example.
Bernadette
An HVDC cable contains a high voltage conductor in the centre that carries the current. Then there is insulation, for example a plastic extruded insulation, and outside that there is a grounded screen.
Alex
So, although there are hundreds of thousands of volts in the core, the outside is zero volts. A cable is maybe 150mm in diameter and could be placed anywhere… and you could almost sit on it!
Norman
It'd be a bit warm, so please don't sit on it!
To continue… you have one of these cables, this is the positive cable and beside it you have the negative cable, and the current would simply flow between the two cables. You must always have an electrical circuit positive to negative to make the circuit…
Alex
So with two of these cables, side by side in a trench at the bottom of the ocean, you have an electricity link that will probably last more than half a century.
Norman
The alternative would be an overhead transmission tower with six cables strung from 45-metre-high towers, it's a much larger infrastructure.
Norman
And it's expensive.
Alex
Especially if you were to build those towers down to the sea bed -which is obviously not a realistic concept, so submarine cables are the only way to link two countries.
MUSIC
Bernadette
The first HVDC interconnector was a 98km link from the Swedish island of Gotland to the mainland. It was installed in 1954 and had a voltage of 100kV, and could transfer 20 megawatts of power.
Alex
The second link, in 1961, was 200MW from Kent to Calais, using that initial Swedish technology, from which all interconnectors have descended. Since then ratings and numbers of HVDC links have grown, although it still represents a small percentage of the world’s transmission systems
Norman
The initial driver was twofold. One: interconnecting networks that were not synchronised together.
Norman
Scandinavia, for example, is not connected to Western Europe electrically. So, Finland, Sweden, and Norway are in an island electrical Island, separated from Denmark and the rest of the European grid for a long time.
Alex
From the 1960s onwards we began to connect the Scandinavian and European grids.
Norman
Essentially to share risk. If you have a problem with power, you can bring power from your neighbour and vice versa. These are bilateral exchanges of electric power, so the power can flow either way you wish, and, at the drop of a hat, you could reverse power in milliseconds extremely quickly in an emergency.
Bernadette
And this was the case with the links between Britain and France too. But now there is a new driver.
Norman
As we begin to think about renewable energy. For example, the UK has great renewable energy resources in terms of wind. When the wind is blowing very strongly, we may have to curtail the wind, because we don't need so much power and we can't store it effectively. One of the problems with electricity is, unlike coal and gas and oil, you can’t easily store it in a bunker somewhere, you have to generate it and use it immediately.
Alex
But now we have interconnectors to Europe, we export our surplus wind to European grid or the Irish grid, and vice versa. And when there is no wind anywhere, perhaps we can buy French nuclear-generated power.
Norman
So it improves the security of supply and allows a greater penetration of renewable energy. One of the main issues with renewable energy, is that it is intermittent. The wind does not always blow and the sun does not all shine. So what do you do when it doesn't? The answer is: exchange with your neighbours.
Bernadette
And in the present day, the number of interconnector projects underway has proliferated enormously.
Norman
If you go back 10 years on the British grid, you would have found, maybe, three interconnectors, one to Ireland, one to France, and possibly one to the Netherlands. Nowadays you'll find seven or eight, with another 10 in planning and construction, and many more anticipated in the future.
Bernadette
And one project that has just achieved financial close and will go ahead, is the Greenlink between Great Britain and Ireland.
MUSIC
Daniel
Greenlink started as the ‘Green Wire project’. At the time, there was a concept that they would actually build two interconnectors, one to Northern Wales and one to the southern part of Wales, where we do now connect.
Alex
This is Daniel Abbott, Greenlink Engineering Manager for WSP.
Daniel
That was to basically act as an export for three gigawatts of wind on the island of Ireland, that would be directly connected into the UK grid. Now that, for various reasons, including its size and complexity, never came to be, but in the…
Daniel
around 2014 2015, the UK Government and then OFGEM expressed an interest for more interconnection. So we created the Greenlink project from that original concept. We took the bit that was desirable and which had a good future which is the interconnector. In this case the one to Pembroke and we created this project from that.
Alex
Daniel is something of an interconnector generalist – he has sight of all of the moving parts on the project and previously worked on the East-West Interconnector (also between Great Britain and Ireland).
Bernadette
The connection points for each end of the link were decided by the Transmission System Operators in the UK and Ireland.
Daniel
So that's a fairly well defined process of you approaching EirGrid in Ireland, National Grid in the UK, and saying, “Hey, I'd like to connect 500MW of capacity into your network, can you have a look at where that would fit best”, and then they return an offer to you for a similar process which is really one of the first steps in developing a project like this.
Alex
And then the hunt begins to find appropriate landfall sites. Daniel feels this bit has gone really well. It is also one of the first interconnectors to be privately financed.
Daniel
We've gone through quite a process with commercial banks to make that possible. And essentially, you know, we've opened up a new asset class to this kind of arrangement. Historically, these kinds of projects have been built by major utilities, like National Grid, here in the UK. And it's an achievement that I'm certainly very proud of, and to have made it possible with solely private investment.
Bernadette
This will make interconnector projects even easier to finance in future, and it doesn’t hurt that the benefits are so clear.
Daniel
Interconnectors can play a great role in the security and stability of our wider transmission networks. I mean, there were the famous events not very many years ago in the UK when we lost generation in large parts of southern England. It was clear that interconnectors could have played a bigger role in reducing the disturbance to the network.
MUSIC
Alex
The components of the Greenlink, starting at the Irish side, begins with an interface with the Irish network which is owned by the Irish Electricity Supply Board and operated by EirGrid.
Daniel
There’s a place called Great Island in County Wexford. The substation there is operating at 220kV. And, on behalf of our grid, we'll build a small substation, which will be a point of interface. So that will be a gas insulated substation, that would look like a building. It's an indoor environment. From there, and immediately adjacent to that, we've got a kind of showcase for the project, which is mirrored on the other side in Pembroke. And that's our converter station. So this sits on a platform which is 185 metres by 100 metres as a rectangular footprint. This is the HVDC control system and the point of conversion from AC alternating current to DC.
Bernadette
Having converted to DC there is then a 22km underground cable route running beneath rural roads and avoiding agricultural or private land.
Daniel
And that takes us all the way down to a place called Baginbun Bay. There we use a technology called horizontal directional drill. It's a drill which is trenchless and, basically, we use like a fluidised hydraulic drilling pressure to create a duct and a bore that extends about 800m from the beach head out into the ocean. So, having performed those drills, we then transition from an underground cable to a submarine cable. The submarine cable is substantially very similar but, critically, it has an additional layer of steel armour wire, which protects it from any external damage that may occur.
Alex
The armour around the cable protects it from damage as it is laid from the cable installation vessel to the bed of the sea.
Daniel
So the cable is laid on the seabed. That will be done in two campaigns. We will actually do the Welsh side of the project first in 2023 and the Irish side of the project will follow in 2024. So once the cable has been laying on the seabed, we would then come along and bury that cable using trenching tools t and backfill the trench. The joint in the middle is what's described as an ‘Omega’ joint. So it's like the Greek letter omega. And you can imagine pulling up the two ends of the cable from the seabed onto a vessel forming that joint and then setting them down like a looping a piece of string.
Alex
Laying the cable in a trench protects it from fishing damage, such as trawling. However a heavy ship’s anchor can damage a cable even in a trench…
There will be more on the trenching and the middle joint in a minute!
Bernadette
So having transited the 160km across the Irish Sea…
Daniel
…we arrive very attractive beach called Freshwater West- a popular surfing destination in Pembroke in South Wales. There we have to essentially the mirror of the process in Ireland also horizontal directional drilling. We have a much shorter underground cable air of about six kilometres which is substantially on agricultural and private land. We're very keen not to disturb the tourists and visitors to the area as they're there throughout the summer months. The Pembroke substation is a fairly industrial environment set in an otherwise beautiful and tranquil part of Wales. We're surrounded by the Valero oil refinery and the RWE CCGT power plant. So we will nestle in that industrial environment and build the mirror image of what we have in Ireland, which is the point at conversion from DC back to AC. Then a short run of cable will take us into the National Grid Pembroke substation.
Alex
The link will operate at 320kV, which Daniel says has become relatively standard for projects of this kind and allows a power transmission of up to 1,000MW (1GW). However, local network constraints mean that the project will only transmit up to 500MW.
Bernadette
This is because the electricity network in Ireland is relatively weak. A single link providing more power is too risky. Here’s Norman again to explain.
Norman
We can't really afford to have so much power in one circuit because you have to consider what happens if it trips out. Or if there's a fault, you'll have to look at the worst case scenario. Could your system withstand the loss of 2000, 3000 megawatts? For Great Britain the answer would be ‘no’. For Ireland, the answer would definitely be ‘no’, because it's a much smaller network. For Continental Europe, which is a much, much larger network from the French coast to Russia, from Denmark, to the Baltics, and to Greece and so on, I think that the maximum they can lose is about 3000 megawatts.
Alex
An ‘eggs in one basket’ situation. So 500MW is about as much as Ireland would allow in one link, at least for the moment.
MUSIC
Bernadette
Paul O’Rourke is the Construction Director for Greenlink. Like Daniel, he worked on the East-West Interconnector.
Paul
I was the Marine Project Manager on that and, at the end of the job, for the final six months, I was the overall Project Director. There are a lot of similarities between that project and this project. That East-West interconnector project was between about 2008 and 2013. So it's interesting to be back again, 10 years later, building another interconnector a couple of hundred kilometres further to the south.
Alex
In those 10 years, trenching technology has stayed more or less the same, perhaps some of the trenchers have become more powerful…
Paul
Like, for example, the trencher that we're going to use the jet trench on this project which can develop 1.2 megawatts of jetting power, whereas the one that we used on the East West interconnector had 750 kilowatts of fluidising jetting power so that's one thing that’s definitely moved on.
Alex
Trenching is the process by which the cable will be placed below the sea bed. Greenlink will be installed by what is known as post-lay burial.
Paul
So, a very large cable laying ship will lay the cable in a bundle comprising the two power cables plus 320kV minus 320kV and the fibre optic cable laid on the seabed. A trenching support vessel, which operates the trenching equipment, will come along afterwards… hours or a day or two afterwards. Trenching is a slower technique so, we expect there will be a gap of a week or two between the cable laying ship and the trenching support vessel.
Bernadette
The trenching support vessel will have containers on the deck of the ship with remotely operated vehicle pilots in them.
Paul
And the pilots will control the trench as the trenches are launched from the deck of the ship very carefully into the seabed, and then placed on top of the cable. The cable trencher will pick up the cable very gently and start to cut the trench, then the cable is gently pushed into the trench by a depressor as the trencher moves forward.
Alex
Two types of trencher are used. A fluidising jet trencher is more commonly used in sandy areas of the sea bed and works by jetting high-pressure water into the seabed. A chain cutting trencher is used for stiffer materials, and looks like a heavy duty chainsaw.
Bernadette
The ground along the cable alignment is expected to be two-thirds sandy and one-third stiffer materials.
Alex
As Daniel mentioned, the submarine cable will be installed in two campaigns. The first begins in September 2023 and will last three months, the second will be from March to June 2024. The cable jointing process will take place shortly after the second campaign.
Bernadette
That is a delicate, 10-day process that requires perfectly calm weather conditions, so the summer seas offer the best chance for a smooth operation.
Alex
For the second campaign of cable trenching, a completely new kind of trencher is being developed, capable of doing both chain cutting and jet trenching.
Bernadette
The trencher is the result of the contractor Sumitomu investing a lot of money with a manufacturer called Jan de Nul (“yan de null”)
Alex
The finished trencher will be named “The Swordfish” and, if development is successful, it could see its first use on Greenlink.
Paul
And it's bringing some interesting features that we haven't seen another jet trenchers. So we're quite excited about that.
MUSIC
Alex
At each end of the submarine cable, both landfalls will be undertaken by horizontal direction drilling, (or HDD).
Paul
On the Welsh end, which is the longer of the two, we're looking at about 1270 metres of HDD. So there will be two bores, one for each of the power cables when the fibre optic cable will be pulled through one of them along with a power cable. At this point in time, there is still some engineering ongoing, but it's looking more likely that we will use steel ducts because of their strength. The favoured option is to push the dogs through from the landward side. One of the benefits of this is, if you have to pull it from the seaward side, it means having a large offshore spread to conduct this, which is less invasive, more straightforward, and it's cheaper to push the ducts from the landward side.
Bernadette
The Irish side is similar, but around 960m in length.
Paul
One of the constraints we have at the Irish side is the HDD exit points are quite close to a couple of archaeological sites. So, we need to be very careful to avoid them during the exit. But that will be catered for in the HDD design.
Alex
As things stand, geological boreholes have not detected large amounts of rock, which is good for HDD as it’s better suited to softer ground, and work at the convertor station sites began on 10th January 2022, so the project is officially underway.
MUSIC
Bernadette
Once in operation, the main challenges facing the link, besides maintenance and unplanned outages, are commercial
Alex
So here is Colin D’Arcy, Commercial Director for Greenlink to explain.
Colin
When the interconnector is running smoothly, and it participates in the markets it's dispatched then I won't say it’s a straightforward asset to run, but it’s a lot more straightforward to run than, let's say, a generating plant - where you have to buy commodities and start up and shut down frequently, etc. So I suppose when it's running smoothly, it shouldn't be too onerous. We will have trading operations to do in terms of forward trading in particular, FTR’s potentially… things like that. So it will be an ongoing kind of trading aspect.
Colin
But, really, the main thing that will kick in is if there is an unplanned outage, and, I suppose, dealing with the technical consequences of that, potentially working with the contractors to mobilise, you'll also have your commercial implications from those in the markets who are disrupted by the imbalances, and that needs to be traded out.
Alex
So what are uninstructed imbalances?
Colin
When you are dispatched in the energy system in the energy market, you have effectively committed to generating or supplying an amount of energy. So, in our case, we have potentially committed to importing, for example, 500 megawatts into the UK. So on a day where there's surplus generation in Ireland, because of wind or solar, the price should be lower here than in the UK, therefore, the markets will deem that the flow should be exporting in that direction. If the interconnector then trips, or fails to deliver, we have an uninstructed imbalance, which is an imbalance from what we said we would actually deliver. Therefore there's a there's a commercial penalty and you have to trade that out effectively, to deal with that uninstructed imbalance,
Alex
Because the market expected that energy to flow and it hasn’t - another generator needs to be ramped up to deal with the imbalance.
Colin
Therefore there are commercial implications for…
Colin
…the party that doesn't deliver as instructed
MUSIC
Bernadette
Greenlink will be the third 500MW interconnector between Great Britain and Ireland, bringing the total power supply up to 1.5GW.
Alex
Here’s Norman again.
Norman
From the Irish perspective, that's quite a lot of interconnection to the UK grid. Ireland is becoming more and more dependent on wind energy and renewable energy. So the penetration they have in their grid is amongst the highest in the world, in terms of how dependent they have become on wind energy. So it improves the security of their supply if they are linked to the British grid, which is a mix of wind, solar, gas and nuclear.
Norman
So, if you want to have a country with a huge penetration of wind power, it's probably best to have a backup system, which, essentially in this case, is a link to your neighbour. The same holds true for Great Britain as we have more and more wind power on our system. It gives us more security. If we can link to the French, Belgian, Danish, German, Dutch systems just to improve that security. If the wind is not blowing, you can import power, if the wind is blowing, you can export power. So really, from my perspective, Greenlink is driven by the need to ensure the security of supply in Ireland because of the high penetration of wind power.
Alex
When a continent begins to link together electrically, it could become what is known as a “super grid”. There is no technical definition for when it happens, but a large and robust enough grid can behave very differently.
Norman
It's really around the use of renewable energy. How do you run an area as large as Western Europe, which becomes heavily dependent on renewable energy? The wind is blowing strongly in the north and west. The people don't live in the north and west they live in the centre. The sun is shining very strongly in the south around the Mediterranean and North Africa. And again, the population centres 1000 or 2000 kilometres north of that. The waters in Norway provide hydroelectricity other nations with big mountain ranges and so on….
Alex
So how do you interlink all of these disparate generation sources across, what is quite a large area in Western Europe, from the Ural Mountains to the Atlantic coast, from the north of Norway, to the south of Italy
Norman
It's a big area, it's not quite Africa, but it's big enough. So the concept of the European Supergrid was born to try to exploit that diversity of renewable energy sources and improve the security of everybody’s supply. So you're not dependent on the sun shining 12 hours a day in your country, because, for the other 12 hours a day, you can import wind power from somewhere with a surplus of wind, like in Scotland, for example. There's a lot of wind in Scotland, but not a huge population. Whereas there's a huge population in Central Europe.
Bernadette
But not a lot of wind potential. The upergrid is a way of thinking how we could integrate all of these systems, whereas at the moment we are building individual systems.
Norman
Greenlink is just a one off system, it's not built in conjunction with anything else. But effectively, there will be three links down the Irish Sea. So effectively there’s an evolving mini supergrid in the Irish Sea. Nobody designed it as such, the owners of the three links are quite different. Two of them are private companies. One is the Irish transmission company but, de facto, we're slowly but surely building the European Supergrid.
Alex
And enabling a stable European energy market, with a high penetration of renewables.
Bernadette
Hopefully taking the continent one step further to ending our reliance on polluting fossil fuels and unreliable suppliers.
End of transcript.
