The objective of the electrical systems working group is to increase efficiency and reliability of collecting and transmitting electricity and to increase the cost effectiveness of deploying such systems
The Electrical systems area focuses on reducing the overall cost of electrical systems for offshore wind. The working group commissions R&D projects which optimise the electrical system through investigating and implementing projects that:
As windfarms move further offshore, and turbine ratings continue to increase; considerable challenges and opportunities are presented to reduce the costs of electrical systems through the implementation of innovative solutions. The working group will continue to build on current and future studies to overcome these challenges and capitalise on promising concept and solutions.
Moving from 33kV to 66kV arrays presents a number of benefits, including the ability to connect more turbines per string and the possibility to design inter-array networks in ring layouts rather than in radial strings, thus increasing reliability. The Offshore Wind Accelerator (OWA) managed a number of studies investigating these potential benefits, which eventually led to the launch of the 66kV cable qualification competition and the corresponding discretionary project outlined below.
In order to encourage the development of a competitive 66kV cable market in time for Round 3 projects, the OWA launched a competition for innovative 66kV inter array cables. This resulted in the OWA supporting the testing and qualification of 4 different designs, being developed by JDR, Nexans and Prysmian, with the first cables becoming commercially available from 2015.
The premise of DC arrays is to use medium voltage DC (MVDC) array cables rather than conventional 66kV or 33kV AC array cables to connect each wind turbine to the offshore substation(s). 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 OWA in their original DC Array project in 2011.
On behalf of the Carbon Trust and Offshore Wind Accelerator (OWA), TNEI undertook a fresh investigation into the feasibility of DC arrays as a means of connecting offshore wind farms. This Refresh Study considers the advancements in DC technology in the intervening years.
Transmission infrastructure typically accounts for 10-20% of the capital cost of offshore wind. A large element of these costs are related to the development, manufacturing and installation of offshore substation platforms (OSPs) which are needed to convert the array voltage (33kV or 66kV) to a higher level (155kV or 220kV). Offshore converter platforms which are needed for HVDC transmission are even more costly. It is self-evident that any reduction in platform size and weight offers a massive potential for cost reduction. If the total weight of topsides and substructures could be kept below 1,000t each, smaller and less costly installation vessels could be used (a large cost area of the overall cost). Modular concepts may unlock scale effects while reducing the number of platforms by hosting transformers and converters on a common substructure could also bring costs down significantly. The Offshore Wind Accelerator program has started various projects to investigate innovative concepts and identify the most promising solutions.
Offshore Transformer Module (OTM)
The OTM concept reduces an OSP down to its bare essentials and places the substation components on smaller substructures. Multiple modules are utilised to create a capacity similar to existing traditional OSPs installed in offshore wind farms today. The solution would remove the need for large heavy lift vessels to install the large OSP foundations and their topsides, and open up the potential to utilising smaller and more cost-effective vessels. In 2015, the Offshore Wind Accelerator (OWA) launched a study to further investigate the benefits of the OTM concept, and a discretionary project to support market introduction of OTM technology. The objective of the OTM discretionary project is to share knowledge of the first full-scale development of the OTM module for the Beatrice offshore wind farm in Scotland. The project will investigate the engineering and regulatory challenges faced during design and refinement and ultimately allow a better industry understanding and acceptance of this cost saving solution.
Integrated offshore substations
Another OWA study examined the possible opportunities for cost reduction in transmission assets by combining two 500MW HVAC substations and one 1GW HVDC converter platform on a single jacket substructure. The key findings of this study is that a combined HVAC+HVDC topside for a 1GW platform could be delivered weighing less than 10,000 tonnes using currently proven HVDC technology being offered by a number of OEMs. This presents a significant opportunity for reduction in CAPEX and OPEX due to the leaner cost and service requirements from having one platform instead of three.
A prior OWA study focussed upon the optimisation of AC systems for offshore wind farms by investigating the cost reduction potential of new AC technologies (midpoint compensation and LFAC). The study found that although midpoint compensation may extend the range of applicability for 50Hz HVAC technologies, this could be limited by transients and low-frequency resonance effects that occur due to the long AC cables (i.e. export) and the required transformers. This can impact the overall WTG, wind farm control and network stability causing over-voltages and heating. Additionally, emissions and amplification of high voltage harmonics present a major risk for the operation and grid connection of offshore wind farms. These aspects are being addressed within this OWA’s ‘DH50Hz’ study which is investigating advanced harmonics and transients modelling methodologies, harmonics mitigation measures, and is supporting the creation of better standards for harmonic modelling the future.
Cross section of an inter-array cable