In 2008 European leaders committed to reaching the “three 20 targets”; by 2020 reduce greenhouse emissions by 20%, compared to 1990 levels, and reach 20% of energy consumption generated by renewables. In 2020 the average renewably sourced energy by EU countries was 22.1%, and the percentage of electricity from renewable sources was 37.5%.
One country that has been making good use of its renewable resources is Ireland. In 2020 more than a third of Irelands electrical power was generated from wind power, but as countries become more reliant on these renewable energy sources a new problem arises, what happens when the wind doesn’t blow?
Currently fossil fuel energy generation is ready to pick up the slack but this is not a long term solution, another answer would be to store the power generated in the grid, although that is not yet feasible. There is another solution that is currently being used to ensure the reliability of electricity supply across Europe, high voltage direct current (HVDC) interconnectors.
Sharing electricity
The first ever HVDC interconnector was installed between the Swedish island of Gotland and the Swedish mainland all the way back in 1954. The second link set up in Europe was between Kent and Calais and was capable of transferring 200MW.
But why interconnectors use direct current rather than alternating current, despite almost every country on earth using alternating current? As Technical Director HVDC at WSP Norman McLeod puts it “Using direct current is two things really, it has lower losses than AC transmission, and it is more controllable than AC transmission.” DC is also useful in this situation as it allows two countries using different electrical frequencies two transfer power between each other using DC. Without high voltage direct current interconnectors want parts of Europe would be cut off from exchanging power with their neighbours.
As Norman McLeod explains “So the island of Great Britain, for example, is not connected to France, or Denmark or Belgium, except by HVDC. There are two links to the island of Ireland, which again is not connected except by HVDC, to Great Britain.
This first stage of interconnection gave countries more security. If something went wrong and power was lost then countries could just bring in power from their neighbours and vice versa. This allowed the countries across Europe to share the risk of power outages and correct the issue much faster.
Now with the rise in renewable power sources there has been huge growth in the number of HVDC interconnectors being or planning to be built. The issue is now the reliability of generating electricity, what happens when the wind stops blowing or even is blowing more than needed? After all storing wind energy is not very effective, it must be used as its generated. This means countries with HVDC interconnectors can send excess electricity to neighbouring countries at times of high wind and can get electricity from their neighbours at times of low wind. For example the UK with its interconnections to mainland Europe can sell excess wind generated electricity when there is plenty of wind but also get French generated nuclear energy at times of calmer weather.
Norman McLeod says “If you go back 10 years on the British grid, you would have found maybe three interconnectors, one to Ireland, one to France, and maybe one to the Netherlands. And now you’ll find seven or eight with another 10 in planning and construction, and many more in the future.” One of those interconnectors that has just secured funding and begun construction is Greenlink.
The Greenlink Project
Greenlink will be an HVDC interconnector between Ireland and Great Britain and it has raised the £420 million investment required solely from the private sector, one of the first interconnection projects to do so.
For the Greenlink project two converter stations will be built, the one on the Irish side will be built on Great Island in County Wexford, and the one on British side will be built in Pembroke in Wales. These substations will be where the electrical current will converted between alternating and direct current, depending on which direction the electricity is being sent.
This is how Daniel Abbott, Greenlink Engineering Manager for WSP describes the plan for substations design “The substation there at Great Island is operating at 220kV. And we 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, it would look to anybody who sees it like a building, it’s an indoor environment. And then 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.”
The converter stations on both sides are then connected in-land to the countries respective electrical grids. The Irish grid operated by EirGrid and in Britain the National Grid.
The two stations will be connected to each other by the HDVC cables laid underground. The cables are 150mm in diameter and at the core there are hundreds of thousands of volts but on outside the insulation there are zero volts. Two cables will be laid next to each other, one positive and one negative cable allowing for a complete circuit for the current to flow.
The first challenge is to get the cable from the Irish substation the 22km across roads agricultural land and the beach, the same also has to happen on the Welsh side, although only 6km to the Pembroke substation. To achieve this Greenlink will implement a technology called horizontal directional drilling (HDD).
Daniel Abbott describes the process, “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 and 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 of the engineering ongoing, but it’s looking more likely that we will use steel ducts because of their strength. And the favoured option is to push the duct through from the landward side. 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.”
On the Welsh side the cable arrives on a beach call Freshwater West, a popular surfing location. Daniel Abbot says those on the Greenlink project “are very keen that we don’t disturb the tourists and visitors to the area as they’re there throughout the summer months.”
Trenching the cable
The cable then has to cross 160km under the Irish Sea, the cable will have an added layer of steel armour wire to protect from any external damage. The process of laying the cable on the sea bed will be done in two parts, firstly from the Welsh side in September 2023 followed by the Irish side in beginning the three month process in March 2024.
Despite the extra armour wire for the cable laid under the sea it is still at risk of damage from things like ships anchors. To prevent this it will be put under the sea bed using trenching technology. Paul O’Rourke is the Construction Director for Greenlink and he worked previously as the marine project manager on the East-West Interconnector, the first interconnector between Ireland and Britain which was completed in 2013. O’Rourke explains the trenching process, “So, a very large cable laying ship will lay the cable in a bundle the two power cables plus 320kV minus 320kV and the fibre optic cable laid in a bundle on the seabed, a trenching support vessel, which operates the trenching equipment will come along afterwards… you know, hours or a day or two afterwards it trenching is a slower technique. And the pilots will control the trench or so the trenches are launched from the deck of the ship very carefully into the seabed, and then they’re placed on top of the cable. The cable trencher will pick up the cable very gently and then start to cut the trench and then the cable is gently pushed into the trench by a depressor as the trencher moves forward.”
To create the trenches two different types of trencher will be used when the first installation begins on the Welsh side. One is a fluidising jet, this works by jetting high pressure water into the sea bed and will be used on the majority of the sea bed, in the area that are mostly sandy. The other type is chain cutting trencher that works like a large chainsaw to cut away stronger materials.
For the second stage of the installation starting in Ireland in March 2024 a brand new kind of trencher is being developed. Manufactured by the company Jan de Nul with investment from contractor Sumitomu, the new trencher is expected to be able to perform both jet trenching and chain cutting. If the development is successful the new trencher, nicknamed “The Swordfish” will be used for the first time on the Greenlink project.
Creating the super grid
Once Greenlink is set up it will operate at 320kV which allows for the transmission of up to 1,000 MW (1GW). However the full potential won’t be unleashed and instead Greenlink will transmit only up to 500MW, this is due to the potential issues of losing such large amounts of electrical supply at once if something goes wrong.
“We can’t really afford to have so much power in one circuit. Because you have to consider what happens if it trips out what happens if there’s a fault,” says McLeod, “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.”
There are some potential issues raised by Greenlink and other interconnectors most importantly the impacts of an unplanned outage, which could cause large market imbalances.
Colin D’Arcy, Commercial Director for Greenlink, describes how that process could play out, “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 is an interconnector, 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, you know, 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 that, we have an uninstructed imbalance, we have an imbalance from what we said we would actually deliver. And therefore there’s a commercial penalty.”
As renewables make up a larger part of Britain and Irelands electrical supply, outside of such unplanned outages, interconnectors will play an important role in increasing the grids reliability.
Greenlink will be the third interconnector that can transmit 500MW, which will increase the total supply between the two countries to 1.5GW. Irelands increasing reliance on wind energy for their electrical supply could leave them vulnerable, but with interconnectors in place there is a back up system for when the wind is not blowing.
The same is true for Britain, which itself is becoming more reliant on wind energy and other renewables. Norman McLeod view is that “It gives us more security, if we can link to the French, Belgian, Danish, German, Dutch systems. Just to improve the security. If something if the wind is not blowing, you can import power, if the wind is blowing, you can export power.”
As more interconnectors pop up across Europe, the creation of a continent-wide super grid is emerging. This is not through specific design but as a patchwork of interconnectors emerge across Europe it will be possible to move electrical energy across the continent. Wind generated electricity from Ireland, Britain and Scandinavia can be move to central and southern Europe and solar energy north from the Mediterranean and north African nations can be moved north. This is crucial to allow all these countries to move to renewable sources of energy without risking their grids reliability.
As Norman McLeod puts it “Greenlink is just a one off system, it’s not built in conjunction with anything else. But effectively, there’ll be three links down the Irish Sea. So effectively there’s a mini super grid in the Irish Sea, evolving shall we say. 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 super grid.”
