Meshed offshore grids are set to power the global energy transition

Our power networks are facing a period of unprecedented changes as they evolve to meet the challenges of the global energy transition. The first change is the need to connect an increasing level of renewable energy resources, especially the offshore wind projects that are developing fast in Europe, USA and China. The second change is the development of subsea interconnectors that enable countries to trade energy and make the optimum use of geographically dispersed renewable energy resources.

To handle these changes, we need to create a new breed of offshore high voltage transmission system. Until recently, the development of offshore electricity infrastructure has been relatively uncoordinated. That means wind farms have been connected individually to shore (with the exception of Germany), while there have been a limited number of point-to-point subsea interconnectors, such as Nordlink, BritNed and the North Sea Link, that link the grids of only two countries.

An alternative is to adopt a more coordinated approach. In this case, several neighboring wind farms might be clustered together to share connections to shore while a number of countries might be linked by multi-terminal interconnectors. Modern technology could even allow this concept to be taken to the next level in the form of a meshed grid. This would comprise “clusters” of wind farms connected to offshore hubs that connect to each other and then to various countries.
 

Increased security of supply

The meshed offshore grid would require the construction of offshore hubs. But fewer cables connected to shore will be needed, although they would need to capable of carrying more power than a conventional export cable for a wind farm. A major benefit of the meshed grid is that, should a fault occur in one connection, it will allow the flow of electricity to be diverted via an alternative cable. This would ensure security of supply for consumers in the same way as the onshore grid, increasing the reliability of offshore wind with consequent economic and energy system benefits.

The need for HVDC

Offshore wind farms are now being planned at long distances from shore, sometimes over 100 km, where the wind is stronger and more predictable. They will also have large power capacities of over 500 MW. At the same time, interconnectors may need to carry over a GW of power over distances of hundreds of km. The distance and power requirements mean that conventional alternating current (AC) connections are not viable. Instead, high voltage direct current (HVDC) technology capable of carrying high power over long distances with minimal losses must be used.

With existing HVDC technology and the current DC cables, it is already possible to build “smaller” multi-terminal systems. However, several challenges need to be addressed for the development of reliable and affordable meshed HVDC grids, not just in terms of technology but also in establishing standards and the regulatory framework.

These issues were explored in the EU-funded Horizon2020 project ‘Progress on Meshed HVDC Offshore Transmission Networks’ (PROMOTioN) that presented its research results in 2020. They showed that hardware-based technologies, such as HVDC circuit breakers and gas insulated substations, are ready for use and can be manufactured industrially immediately. Furthermore, software-based technologies, such as HVDC system control and protection, have been proven to work and to be interoperable, and are considered ready for a real-world pilot.

PROMOTioN called for further research to improve performance and whole system integration. It found that as a first step, real full-scale cross-border pilot projects at sea should be carried out. This will provide practical experience and demonstrate the real-life benefits of multi-terminal grid development. 

Hybrid assets

The PROMOTioN project considers hybrid assets as the first building blocks towards a meshed offshore grid. They combine the connection of offshore wind farms with interconnection between multiple countries. 

Several studies have shown that hybrid connections are more economically beneficial than separate wind farm connections and interconnectors. In addition, hybrid assets reduce the length of cables in coastal waters compared to separate wind farm connections and interconnector cables. This results in reduced environmental impact and less impact on the fisheries sector and shipping activities. 

Several forms of hybrid assets are possible:

I. Existing offshore wind farms (or hubs) that are already connected to their “own” countries are connected to existing wind farms or hubs in other countries. This means that the hub-to-hub connection is constructed later than the hubs themselves. 

II. Offshore wind farms are connected to an existing interconnector (Tee-in). 

III. The entire asset (the offshore wind farm connection and interconnector) is constructed at the same time.

IV. A meshed offshore grid with grid extensions from time to time.

It is envisaged that connection forms I, III and IV will be classed as hybrid assets from the point of construction. However, connection form II (the Tee-in to an interconnector) will first be regulated as a “normal” interconnector and later as an “offshore hybrid asset”’, which entails a different kind of regulation. This could be problematic as the business case for interconnectors and offshore hybrid assets are different. However, the expectation of the PROMOTioN project is that with long-term grid planning and coordination between countries, connection forms I, III and IV will be more likely to happen than form II.
 

Practical progress towards a meshed offshore grid

China has already taken the next step beyond multi-terminal with the construction of a land-based HVDC grid. The Zhangbei project, commissioned by the State Grid Corporation of China (SGCC), has integrated four 535 kV HVDC transmission links, a total of 648 km in length. Each link has a capacity of 3,000 MW. The new grid infrastructure enables interoperability, allowing power to flow in two directions. This has created a flexible grid with transmission losses up to 40% less than conventional AC, while also maintaining grid resilience.

There has also been progress offshore with the first hybrid asset. Together with its project partner, the Danish transmission system operator Energinet, 50Hertz constructed the first offshore interconnector in the Baltic Sea. Known as the Kriegers Flak Combined Grid Solution (CGS), it connects the Danish region of Zealand with the German state of Mecklenburg-Western Pomerania with a capacity of 400 MW.

The link was energized in 2020, when it integrated the two German wind farms Baltic 1 and Baltic 2. The commissioning and integration of the Danish wind farm Kriegers Flak is planned for 2021.

Considering that typical offshore projects involve lengthy planning, permission and construction cycles of five years or more, it will be some time until we see a fully-fledged meshed offshore HVDC grid. However, the building blocks are in place and it is realistic to expect that commercial projects will be in operation by 2030. Nexans, with its expertise in subsea HVDC cable technology that is already rated at up to 525 kV, is well placed to support this exciting development.