With more offshore energy generation, there is a need for a transmission grid to transport the electricity as well as to supply offshore production facilities with power in an efficient way. Currently, the construction of offshore networks is hindered by long lead times for the installation of capacity and a lack of regulatory framework, whilst robust technology to operate the network securely & safely must be developed. One option to overcome these challenges are going beyond the current point-to-point connections and establish a meshed offshore transmission grid. A meshed offshore grid connecting offshore energy generation and offshore installations to land could provide significant financial, technical and environmental benefits compared to the current point-to-point connections. The largest number of units that will be connected to the offshore grid will be wind turbines, while other generation types (wave power plants, gas turbines on platforms) will play a minor role. Neither will offshore loads (pumps and compressors at platforms, wind farm internal loads) play a major role. The main share of the produced electric power will not be consumed offshore but transferred to shore via high voltage direct current (HVDC)¹.

Due to long distances, HVDC is the most suitable option to avoid too large power losses. Current offshore HVDC systems are point-to-point systems, connecting offshore installations (power production or power users) directly to shore. To reduce cost, a meshed offshore transmission grid would be preferable, connecting several installations with both power producers and consumers directly without going through the onshore grid². Compared to a point-to-point cable, a meshed grid would have a higher utilisation and reliability.

With the current Voltage Source Converter HVDC converter technology and the current limits of DC cables, we can already build ‘smaller’ multi-terminal systems. However, addressing several challenges is key to enable the development of reliable and affordable meshed HVDC grids. There is a need for speeding up the development of the building blocks of the HVDC grid, by increasing their ratings and achieving cost reductions³:

• HVDC circuit breakers will be essential to establish a reliable system. The development of the first prototypes by different manufacturers is encouraging, but further developments and improvements are needed.
• Offshore platforms, including converter and breakers, will make up a large share of the system cost. New converter and breaker designs should aim at substantially reducing the platform size to achieve important cost reductions.
• Voltage and power ratings for breakers, converters and cables need to increase further to build a ‘supergrid’ with ratings that exceed current-day levels.
• If existing point-to-point links are to be connected to the future HVDC grids, new building blocks such as DC/DC converters need to be developed.

Currently, the high cost of converter technology, a lack of experience with protection systems and fault clearance components hamper the deployment of meshed HVDC offshore grids². Research and development activities are addressing these issues.

Opportunities and market impacts

A meshed grid is expected to lower the cost for adding more power production facilities as well as power users offshore. A meshed offshore grid could improve efficiency of power supply in general and make the energy use for offshore installations more reliable and lower the CO₂ footprint². The development would enable the supply of offshore installations with renewable power in a more efficient way. In addition, the reliability of the power supply is improved as more routes for transporting the power is possible.

An onshore meshed HVDC grid is being built now in China⁴. It is expected to be commissioned by 2020. In the EU, the PROMOTioN project is developing and demonstrating cost-efficient offshore HVDC equipment as well as developing recommendations for a regulatory framework for HVDC offshore grids and financing mechanisms. Given the long planning/permission/construction cycle for typical offshore project (at least five years), a full-fledged meshed offshore grid will probably not be implemented there within the next 10 years. But an earlier shape or part of meshed HVDC grid (with no or less meshing, fewer offshore HVDC converters) would likely be up and running by 2030.

Risks and uncertainties

Offshore platforms, including converter and breakers, will make up a large share of the system cost. New converter and breaker designs should aim at substantially reducing the platform size to achieve important cost reductions.

In addition, there is a need for interoperability guidelines and standards for combining the building blocks from different manufacturers are needed and should depend on system-based functionality requirements³. These are essential for a stepwise development of an HVDC grid. Such standards are still missing since no meshed HVDC grid has been built up to now, while the existing point-to-point links have generally been designed and developed as single products by different manufacturers.

In many cases a meshed offshore grid includes a transnational network, which has regulatory implications and implications for the business models, governance and financing.

Contributors

Main author: Erik Hektor

Editor: Peter Lovegrove

1. Vrana et al. Technical Aspects of the North Sea Super Grid. 2011.
2. PROgress on Meshed HVDC Offshore Transmission Networks – PROMOTioN. 2019.
3. IEEE. HVDC Grids for the European Transmission System: Accelerating the Large-Scale Integration of Renewables. 2017.
4. ABB. ABB enables world’s first HVDC grid in China. 2018.