Renewable Energy Focus is proud to be the official "Innovation in wind technology" media partner for this year's All-Energy event in Aberdeen, taking place on 23-24 May. The All-Energy Exhibition & Conference is the UK's largest renewables event devoted to all forms of clean and renewable energy. It is being held at an important time as the UK continues to assume a dominant role in the offshore wind sector and many companies look to the immense supply chain opportunities this brings.
The idea of a supergrid connecting offshore wind farms in the North Sea with a multiple number of countries is a concept being quietly supported by politicians and pushed by industry groups like Friends of the Supergrid. But how can an offshore supergrid be realised in practice, on the technical side?
As more and more offshore wind farms are built in Northern Europe, the question of transmission becomes increasingly important. Not just in terms of cables and substation build-out, but also how the wind parks will be connected to shore, and resultant electricity traded across countries. A Northern European supergrid, which could effectively create a trading system delivering offshore wind generated-power between different countries is a compelling idea. But technically, this is a mighty – and expensive – challenge.
At the moment, offshore wind farms are connected via inter array cables at 33 kV to a transformer platform that steps up voltage to 132 kV, and then a cable connects this platform to an onshore substation. This is a point-to-point connection, also called a “radial connection,” explains Kjell Eriksson, director of the Energy Programme at Det Norske Veritas (DNV) Research and Innovation.
The most remote offshore wind farms are usually connected using high-voltage direct current (HVDC), which is needed when transporting hundreds of MW (or more) in cables, over distances of more than around 100 km.
The reason HVDC is chosen lies in the high capacitance of alternating current (AC) cables, which generate significant amounts of reactive power. This limits the amount of active power - or MWs – that the cable can supply at the other end, Eriksson explains.
But reactive power is not an issue in HVDC cables and there is, in principle, no technical limit to the distance of a HVDC cable.
HVDC has been in use for many decades, such as the cross-border connections between Norway and Denmark, and more recently between Sweden-Germany and Norway-The Netherlands. Most, if not all of the cross-border interconnections however are based on Line Commutated Converters (LCC), which require strong AC networks in both ends for the converters to operate. An offshore HVDC grid connecting weaker AC grids, such as wind farms and oil and gas installations, will require the use of Voltage Source Converters (VSC) which are not as mature as LCC.
The Skagerak 4 cable connection between Norway and Denmark, scheduled for 2014, will be a VSC-HVDC at 500 kV DC voltage and a capacity of 700 MW, showing that VSC technology is steadily approaching the voltage and capacity requirements for an offshore super grid.
Radial connections are fine with one or two wind farms, but as more and more offshore wind farms are being built, radial connections could become a bottleneck: “When you have many wind farms, each with a radial connection to shore, you cannot exploit the system in the same way you could have done if the wind farms and other offshore installations were connected in a large grid,” Eriksson explains.
One of the problems with radial connections is that if a fault arises, the electricity cannot be transported to shore. If connected in a grid, on the other hand, the electricity could simply be led via a different route in the network. The other issue with radial connections, is that the trade of electricity across borders becomes more difficult: “If you imagine a wind farm such as Dogger Bank which will be built in the North Sea with a capacity of 9 GW, and you only have multiple radial connections to the UK, you have only one place to send the electricity. You cannot exploit the fact that you might need more electricity in Norway one day, or in Germany. You can transport it via a radial connection to the UK, but if you had a grid, you could transport large amounts of electricity back and forth.”
The danger now with the increasing rate of offshore wind developments, Eriksson warns, is that each project focuses on its immediate costs without considering the long-term benefits of building something much more robust and flexible.
Breaking the circuit
Although HVDC technology has been around for decades, there are still technological challenges that must be overcome before an offshore transmission grid can become a reality. One of the real show stoppers is an HVDC circuit breaker:
According to Eriksson, “with alternating current, the voltage and hence the current fluctuates at 50 Hertz, and the circuit breaks when the current passes zero. But with direct current, there is a constant flow, and we are talking about 2000 amperes for a 500 kV DC connection of 1 GW, so [it is not just like] flicking the switch as you would do in your house. What is needed is a DC circuit breaker, and as of today this does not exist commercially,” he says. “It is a real technology show stopper, because if there is a fault on a network [that doesn't have a] DC circuit breaker, you have to take down the whole grid to clear the fault.”
There are prototypes in development, however, for 2000 A DC currents and 500 kV DC voltage, which have been successfully tested on an existing multi-terminal HVDC (MTDC) scheme.
The salty sea
Another challenge is the need to “marinise” existing grid technology. Today, the large transformer stations are onshore, but when going offshore developers are exposed to a very different environment in terms of humidity, temperature fluctuations and vibrations. The industry has to look at materials and how they behave in an offshore environment.
In addition, developers have the same problem faced by wind turbine operators – how to get access for maintenance and repairs. Currently, there are no international standards and practices in existence for offshore transmission equipment - for voltage levels above 35 kV AC and 1.5 kV DC - in other words the entire offshore transmission grid (apart from inter array AC cables in the wind farms).
Eriksson believes that the challenge of marinisation can still be overcome, if the offshore oil & gas industry and the energy sector cooperate on specifications and designs: “As of today, there is no master plan or master specification for an offshore grid. There are many research reports and ideas showing that it makes sense, [and] would bring down costs, but the standards are lacking,” Eriksson warns. “To create such a grid we need equipment that is compatible – one crucial element is the DC voltage level, which should be standardised to avoid excessive DC/DC voltage conversion, but which would bring with it energy losses and additional costs.”
But some progress is already being made in offshore grid technology. According to DNV's Technology Outlook 2020, voltage source converters (VSC) are feasible for offshore platforms due to their compact design, and the fact that they can be connected to weak or passive AC networks such as wind power installations. They provide voltage control and black start capabilities, and could pave the way for multi-terminal DC networks, enabling interconnection of wind farms and countries.
Multi-terminal HVDC (MTDC) technology could also reduce the number of converter stations if the problem of DC circuit breakers is overcome.
DNV is also looking into cabling issues – especially with the development of floating constructions as a concept: “Copper, which has to be used for offshore cables, has poor fatigue qualities and with repetitive load changes, it does not behave as it should,” Eriksson explains.
Floating substations is another technology conundrum that will need to be solved, as HVDC equipment has so far been designed for stable, non-moving environments – it has not been designed to move as a floating construction would.
For the time being, however, most offshore wind installations are fixed to the seabed, and so the issue of movement is not a hot topic - quite yet. On the other hand, designing equipment that will function offshore in terms of the harsh environment is.
Part 2 will look at where policy fits in to the supergrid equation.
About the author: Kari Williamson is a freelance journalist specializing in renewable energy. She is the former Assistant Editor at Renewable Energy Focus and Renewable Energy Focus U.S.