Connection and Integration of Offshore Renewable Energy Farms into UK Power Systems - Multi-Terminal Offshore Connection Architectures Report

Abstract:

The ETI has engaged Sinclair Knight Merz (SKM) to identify the opportunity for the development of innovative solutions for the collection of electrical energy from individual and multiple offshore renewable energy farms, and the transportation of bulk electrical energy from these offshore farms to the onshore power system. The study comprises four main tasks :

1. Offshore renewable scenarios – to define the timeline of the expected volumes of offshore renewable generation capacities
2. State of the art of offshore network technologies – establishment of the current state of the art of offshore network technologies and their prospective future development path
3. Analysis at individual farm level – identification of the challenges and resultant technology opportunities from connection of individual large-scale offshore wind or marine energy farms to the UKgrid system, and recommendations for connection solutions for investigation.
4. Analysis at multiple farm level – evaluation of the optimal architecture(s) that could be developed to collect, manage and transmit back to shore the electrical energy produced by multiple, large-scale offshore renewable energy farms.

Building on the Individual Connection Report (with which this report should be read), the report identifies and assesses options for network architectures for the connection of multiple offshore renewable energy farms – to shore and to each other. Again, it describes the associated challenges and technology development opportunities (to augment those in the earlier reports).

The designs investigated  indicate that where a single HVDC connection is possible, it is the most financially attractive due to the significant capital cost saving over the installation of a second HVDC link. Switched DC arrangements show potential for savings; however, this would hinge on the energy markets and development of VSC converters. It is expected that a single HVDC connection is unlikely to increase beyond 2000MW due to both technical and SQSS limitations; however, interconnecting HVDC offshore nodes at either HVDC or AC provides revenue savings which outweigh the additional capital expenditure. The saving offered by interconnections increases with distance from shore.

For high capacity AC connections, there are inherently a high number of connection circuits, providing high availability and thus making interconnection or multi-terminal designs unattractive. Further, a reduction of export capacity significant enough to impact on capex is likely to result in an increase in lost revenue which makes the option unattractive.

By its nature tidal generation is close to shore where it has been shown that multi-terminal is less attractive due to the short connection distance. Wave generation further from shore has shown to be more attractive for multi-terminal application. There was a marked difference between small capacity developments close to shore and larger capacity developments further from shore. Close to shore multi-terminal is not financially attractive due to the very short connection distance, however further from shore multi-terminal is clearly the preferred option due to significant capex savings.

Gas Insulated Lines (GIL) has been shown to potentially have a very low availability which would make it an unacceptable design option and as such we would not recommend significant investment in that area. Further the very high capacity of the connection would cause significant impact to the onshore grid, it may be necessary to split the connection onshore to spread the impact over a larger area at increased cost. This issue is not expected to arise with AC or HVDC as the individual connections are of reasonable capacities.

Combined national/international interconnectors with offshore farm connections have been shown to be potentially attractive, and as such may warrant further study. As previously identified for HVDC multi-terminal systems, HVDC switchgear and control solutions would be required for such national and international interconnector arrangements.

Author(s):

Sinclair Knight Merz