Apart from the installation and maintenance in harsh conditions, and components needing to withstand extreme weather, the biggest challenge with offshore wind is bringing the power to the shore and to the grid.
With an offshore wind capacity of 22,072 MW from 5,047 grid-connected wind turbines across 12 countries, Europe is on top of its game for offshore wind energy. Even with an expected 30% slow down in wind energy installation this year owing to COVID-19. Integrating the offshore installations seamlessly with the grid is critical for the continued expansion of offshore wind power.
Why HVDC transmission systems?
As of 2019, there are 39 voltage source converter-type High Voltage Direct current (VSC HVDC) systems globally that have been running since 1999. And the markets are just opening up. Greece, for instance, is gaining a lot of interest these days with it opening up more offshore energy transmission systems.
HVDC is popular because it is a problem solver. It addresses the technical snag affecting this offshore wind transmission – energy loss because of long cables. Though electrical power generated as AC power and is used in the form of AC, wind farms at sea or far from the mainland depend on HVDC as it does the job better than HVAC (high-voltage alternating current).
Edge of HVDC over HVAC in transmitting bulk power
In a conventional HVAC transmission line, there is a limitation of distance covered, as it should be a break-even distance. HVDC transmission lines aren’t limited by breakeven distance. There is also the fact of fewer conductors required that brings the overall cost down. In the HVAC system, energy is transmitted in the form of three-phase AC power, with two conductors needed at max. Fewer conductors also result in the smaller size of the towers, which again makes for a lesser tower cost.
Challenges with HVDC
Maintenance and Safety of Long Cables
In terms of sheer length, the Viking Link (Energinet, National Grid) is the undisputed winner as of now, covering 740km on completion in 2023. Then there is the North Sea Link (NSL) that connects UK and Norway in a 730km-long subsea HVDC owned by Statnett and National Grid which on completion in 2021 will connect wind farms and offshore oil and gas platforms. Both these lines on completion are longer than NorNed (580 km) the current longest submarine power cable in the world. The other much-awaited COBRAcable connecting Denmark and Netherland (Energinet, TenneT) is 325 km long.
The Viking Link connecting UK and Denmark is 740 kms long and will be completed in 2023
Though HVDC cables are better for long-distance transmission of energy with less energy loss, maintenance and service could be a pain. There is also a question of whether there are any environmental implications of laying down long HVDC cable.
The overall expense for setting up HVDC cables is much higher compared to conventional transmission lines. Take the North Sea Link project, for instance. It cost €2 billion. The Nemo Link, (a 50:50 joint venture between National Grid and Belgian transmission system operator Elia) saw National Grid investing £2.1 billion. The COBRAcable – €580 million. The Viking Link power cable under construction estimates £1.8bn ($2.2bn). Even the shortest submarine HVDC power cable capable (51 km) producing 1 GW – ElecLink – is capped at around £490m.
The cost of the North Sea Link project is €2 billion
Is it future-proof technology?
Recently the UK government realized that its current approach to building and designing offshore transmission is outdated for its big ambitions of targeting from 30 gigawatts to 40 GW by 2030. Also, is the power grid maturing along with the transmission system? The recent case of Germany, where its grid was too weak to handle all the renewable power generated offshore, is a sign that it may not be so.
As with any complex projects, there is a massive amount of detailed design work, system studies, risk assessments and planned supply chain and maintenance detailing. We have already covered some fault clearing strategies for making your HVDC reliable.
The Cost of Downtime
The unplanned outrage of the Western Link HVDC. The Western Link brings renewable energy from Scotland to Wales and England through a subsea cable more than 300km in length. It went down and remained offline for 18 days at the onset of this year. The incident resulted in National Grid ESO fork out £31 million by Balancing Mechanism payments for its wind farm operators. With more subsea cables planned soon, what can you do to mitigate issues?
Western Link HVDC was down for 18 days and cost National Grid £31 million in balancing mechanism payments
The Case of Interconnectors
Though interconnectors make a meshed grid a reality, it is not seen as lucrative by many. Case in point, the 1GW BritNed interconnector. It earned a profit of €111 million; a profit margin of 50%. This business case can’t be a global example since profit margins fluctuate based on electricity prices. Other than the complex technical side of interconnectors, there is also the multinational compliance approval it requires.
Interested to know more about the best practices in offshore power transmissions, cabling and HVDC links? Then, we have got a virtual conference for you! Join our upcoming conference on Subsea Power Transmission & Cabling 2021 to gain insights with our unique long peer-to-peer interaction model.