A study undertaken on behalf of the Carbon Trust and Offshore Wind Accelerator by TNEI in the UK suggests that full-scale use of DC arrays on commercial-scale offshore windfarms will require significant further development effort.
TNEI concluded that there are significant timescales involved in the development of individual components as well as the overall system, suggesting that the first full-scale offshore windfarm utilising DC arrays could be more than 10 years away.
The premise of DC arrays is to use medium voltage DC (MVDC) array cables rather than conventional 66 kV or 33 kV AC array cables to connect wind turbines to an offshore substation. This has been a subject of ongoing research by academic and industry bodies looking at cost reduction opportunities for offshore renewables for many years, including the Offshore Wind Accelerator in their original DC Array project in 2011. The TNEI ‘refresh study’ considered the advancements in DC technology in the intervening years.
TNEI’s previous assessment of DC technology identified barrier technologies which required development to enable the implementation of DC arrays. Based on the findings of research and stakeholder engagement, the technology areas requiring the most technical development include integration with wind turbines and platform scale DC/DC conversion.
DC integration with turbines in particular is ‘mission critical’ as it is required for all MVDC array designs. At present there is little activity or serious interest from turbine manufacturers in pursuing the development of DC-connected turbine technology, rather they are focused on optimising their AC-connected turbine offerings. The early progress made by GE who demonstrated a 6 MVA passive boost converter arrangement for use in the base of a WTG tower in 2012/13 has not been significantly progressed or developed by any other parties.
Since the original study, academic investigations have continued in this area, but there has been limited progress from OEMs and developers in terms of product development and full-scale demonstration for some of the key enabling technologies.
These findings are viewed against the background of an ever decreasing levelised cost of wind energy based on existing AC electrical solutions. During the course of the study the 2017 CfD auction for offshore wind cleared at £57.50/MWh for the delivery year 2022/23 compared to £114.39/MWh from the first contracts for difference round in 2015. A rapidly decreasing cost baseline such as this makes it more challenging to encourage technical innovation, as the cost savings appear to be diminishing in comparison with the ‘business as usual’ AC approach
The main obstacle to implementing DC arrays currently is from the commercial side; in particular the greatest risk is uncertainty in the cost and cost reduction potential of key equipment, handling and installation.