Scotland has seen numerous renewable energy projects crop up along its coastline over the past few years, and this trend is showing no signs of slowing down, with more than 4GW of offshore wind schemes given consent by the Crown Estate. A further 1.6GW of potential wave and tidal projects is currently agreed, but yet to formally enter the planning system.
With the greatest number of these projects in the far north of Scotland, hundreds of kilometres from populated areas, the task of transmitting the green electricity to homes and cities in the south can become a major engineering challenge.
This is the kind of demanding task that Switzerland-based ABB is well-versed in undertaking, and in a nod to its experience in this field it recently won an order worth more than £500 million to provide the Caithness-Moray transmission link for SSE’s network arm Scottish Hydro Electric Transmission (SHE Transmission).
This mammoth project will see the installation of a subsea cable capable of carrying up to 1,200MW between Caithness and Moray, equivalent to the electricity needs of 2 million Scottish people. With associated reinforcement of the onshore network, which encompasses the laying of two underground cables, work at eight substation sites, two converter stations and two overhead line reinforcement schemes, the project is reported to be the largest investment in the North’s electricity network since the hydro development era of the 1950s. It is set to become operational by 2018.
The cable at the centre of SHE Transmission’s plan uses high-voltage direct-current (HVDC) technology to transport power subsea between converter stations at Spittal in Caithness and Blackhillock in Moray. Well-established technology that has been used commercially since the 1950s, HVDC cables can allow power exchange between two asynchronous networks and can efficiently transmit large volumes of electricity across long distances.
Caithness to Moray: The route for submarine and underground cables will
cover a total transmission length of nearly 160km
Professor Mike Barnes, of the Power Conversion Group at the University of Manchester, says that while the bulk of the country’s power system is alternating current, it cannot compete with DC for this kind of application.
“The point of AC is that you can easily and cheaply convert between voltage levels, which is what you want to build an efficient grid,” he says. “Unfortunately, because you’re changing the power flow direction, you’re pumping energy into and out of that AC system to effect those changes. Over long distances the power needed to do this rises and then you don’t have a lot of spare capacity to send power down that wire – for long distances you have to go DC.”
Barnes adds that HVDC cables are particularly attractive for subsea long-distance bulk transmission projects such as Caithness-Moray. “HVAC is again less attractive than HVDC for long-distance subsea applications as the break-even distance is between 60-100km. At this stage the cheaper DC line cost offsets the higher station costs of HVDC and it becomes the economical option.”
Anders Gullerfelt, head of ABB grid systems and responsible for the delivery of the Caithness-Moray project, says it is confident in the use of the tried-and-tested technology: “ABB pioneered HVDC more than 60 years ago,” he says. “We have hundreds of installations around the world and we are continuously driving our research and development. We will be using our latest HVDC Light technology for the Caithness-Moray project. We have pioneered this technology and 15 installations of this kind have been built by ABB.”
There are at least a further six long-distance HVDC installations operating around the world built by other engineering firms, including Siemens’ 85km cable running under San Francisco Bay.
ABB’s latest HVDC Light cable is based on voltage source converter (VSC) technology, centred around extruded cross-linked polyethylene layers. Gullerfelt says: “This is a cable that is not only lighter than the traditional mass-impregnated technology, so you have fewer joints, but it also doesn’t have any oil insulation. You get more power per kilo out of the cable and the whole make-up of the insulation inside is more environmentally friendly.”
Going underground: ABB’s HVDC Light cable will transmit renewable energy
The Caithness-Moray project will also use two on-land 320kV HVDC cables, a well-established voltage. However, Gullerfelt says that recently ABB reached the 525kV level. “This is a technical breakthrough. We are in a position with this to transmit 2.6GW on a single cable circuit – the parallel is that you only need one cable circuit to supply power to Paris. We see this cable as the future for these kinds of applications.”
However, there are some disadvantages to HVDC, explains Barnes, with the need for converter stations incurring higher costs and energy losses than HVAC transformers – at least over shorter distances. This is because voltage must pass through semiconductor power switches when changing back and forth from AC to DC. However, Gullerfelt stresses that, thanks to insulated gate bipolar transistor (IGBT) technology, which controls the current, losses are kept to a minimum.
“If you put 1,200MW in at one end you want to get 1,200MW out, and that is something we are continuously striving for,” says Gullerfelt. “Percentage of loss depends where you are operating on the system but you are looking at power losses of much less than 4% overall, and that is very good by any comparison.”
IGBT has steadily taken over from traditional thyristor semiconductor devices as they are more flexible in their ability to control the high levels of power that modern cables are tasked with transmitting. Barnes says: “You can tell thyristors when to turn on but not when to turn off. However, with IGBT you can switch them on and off very fast and control that. This is important as thyristors have to be turned off by the network, so you need a strong network, and that limits how fast they can switch. Unlike IGBT, they can’t connect to weak networks and they can’t black start.”
In addition, IGBT devices need far less equipment, such as reactive power supports and filters, to make them grid compatible, so the footprint of the converter station is much smaller, therefore reducing costs.
Gullerfelt says: “It is not simply a question of turning it on and off. You also need the capability to handle dynamic situations if there is a fault, either on the in-feed AC side, or the DC side. This control system is pretty much the brain of it all. When a fault is detected it will not stop transmission but isolate the faulty part of the system, take down the power and enable the fault to clear, then very quickly pick it up again.”
Graeme Barclay, project director at SSE, says that while HVDC is a proven and reliable technology the firm has had little experience of it and is preparing for any challenges that may arise when integrating the system into the AC grid. “It’ll operate in a more efficient manner because it is the latest technology but we expect some glitches too,” he says. “We will run system model analysis and go through various connotations and scenarios to ensure appropriate control and protection measures are in place to minimise anomalies once it is energised.”
Laying the groundwork: Projects like Caithness-Moray could pave the way for a pan-European super-grid
However, currently there are more practical challenges to overcome, with preliminary on-land works now under way. This has involved trial horizontal directional drilling at the north of Caithness to ‘route prove’ for the HVDC landing points for the subsea cable. ABB will drill further exploratory boreholes at five places along the cable route to identify existing utilities and verify the suitability of these locations for conducting horizontal directional drilling.
Engineers have also been planning the most optimal route for the subsea cable based on geological and sub-bottom surveys. Once completed, Ecosse Subsea Systems will carry out trenching works, using a scar plough to excavate the proposed cable route on the seabed. A cable-laying vessel will follow and the scar plough will then go back over the route in a different configuration to back-fill the trench.
The north end of the on-land route will provide logistical challenges, says Barclay, with the 30km of cable having to run through arable land and, more importantly, Caithness rock and stretches of dense, deep peat.
“The problem with peat,” he says, “is that not only do you have to remove it but you also have to dispose of it at appropriate specified landfill sites. That gives us a logistical issue as the amount of peat could be quite a significant quantity, between 5,000 and 10,000m3. Taking cognisance of the location, environment and available handling facilities, this could be a significant operation in its own right. In terms of rock quantity it can be tens of thousands of cubic metres depending on the final route of the cable and how the strata lies along the route.”
Reinforcement of the onshore transmission network will run in parallel to the laying of the on-land cables to improve the existing network. This will see the old 132kV overhead line between Dounreay and Spittal replaced with a 275kV overhead line, via a new substation to the south of Thurso. An additional 132kV overhead line will be built between Spittal and Mybster, where the substation will be extended.
This kind of work to strengthen the grid is vital, explains Gullerfelt, as SSE plans to transmit a lot of this power further south, which could otherwise put strain on the existing system.
Barnes says: “What you’re effectively doing is building another super-highway to go from the north of Scotland to the south – you’re providing another path for electricity to flow. This way you can send more power down rather than congest the network.”
As more renewable energy projects plan to connect to the grid, HVDC cables are becoming a desirable solution to transmit the additional power. One HVDC connector link is already running down the west coast of England, and another is planned for the east coast. But Barclay says that the Caithness-Moray project is one of the largest of its kind so far, and many will be watching its progress closely as the idea of a pan-European super-grid continues to be discussed.
Barnes says: “We would benefit from a pan-European super-grid as very often there is cheap electricity that we could use, such as from hydropower in Norway, but we are limited in the capacity to connect to it. The only technology that would allow us to do that is IGBT HVDC. Projects like Caithness-Moray that educate us on this technology have a real benefit to an affordable low-carbon future.”
Norway and Germany will swap green energy
The Norwegian and German power grids will be able to share green energy directly following the installation of two HVDC interconnector cables off the coast of Norway and Denmark, with a total length of more than 700km.
French cable firm Nexans will design, manufacture and install the two 525kV mass impregnated non-draining HVDC cables at depths down to 450m. The £360 million scheme is to become part of a larger European interconnector project called NordLink.
The 1,400MW NordLink voltage source converter HVDC project is a collaboration between Statnett, TenneT and the German bank KfW. Surplus wind and solar power produced in Germany will be exported to Norway, and hydroelectricity from Norway will be exported to Germany.
Dirk Steinbrink, Nexans senior executive vice-president, says: “The NordLink project will be Nexans’ largest subsea power cable contract to date in terms of length and value, and we look forward to continuing our close working relationship with TenneT, KfW and Statnett on this important project. We are excited to be involved in the NordLink which is a further step towards a complete integration of the European power grid.”