COBRAcable: HVDC interconnector between Denmark and the Netherlands

Overview

COBRAcable is a high-voltage direct current (HVDC) submarine power cable system linking the electrical grids of Denmark and the Netherlands. The infrastructure connects Eemshaven in the Netherlands to Endrup, located near Esbjerg in Denmark, facilitating cross-border energy exchange across the North Sea. As a critical component of the European transmission grid, the cable pair operates at a voltage of ±320 kV and delivers a capacity of 700 MW. This transmission capability corresponds to an annual transmission capacity of 6.1 TWh, significantly enhancing the integration of variable wind power into the continental system while improving overall supply reliability.

The project is jointly owned and operated by two major grid operators: TenneT from the Netherlands and Energinet.dk from Denmark. These entities collaborate to manage the flow of electricity between the two nations, leveraging the HVDC technology to efficiently transmit power over the submarine route. The operational status of COBRAcable is currently active, having been commissioned in 2019. This commissioning date marks a key milestone in the interconnection of Northern European energy markets, allowing for greater flexibility in balancing generation and demand across the region.

The primary purpose of COBRAcable is to strengthen the European transmission grid infrastructure. By providing a dedicated high-capacity link, the cable supports the increasing share of wind power in both Denmark and the Netherlands. This connection allows surplus wind energy generated in one country to be transmitted to the other, thereby optimizing the use of renewable resources and reducing the need for backup thermal generation. The mixed fuel/source nature of the connected grids means that the cable facilitates the exchange of various energy types, but its strategic value lies in smoothing out the variability inherent in wind power production.

Why it matters

COBRAcable serves as a critical infrastructure asset for the European Union’s energy transition, specifically designed to enhance grid stability and integrate variable renewable energy sources. As a ±320 kV, 700 MW HVDC submarine link between the Netherlands and Denmark, it directly addresses the challenge of connecting major wind power generation hubs with key consumption centers. The cable’s primary function is to improve the European transmission grid, thereby increasing the system’s capacity to accommodate wind power while simultaneously enhancing overall supply reliability. This integration is vital for reducing curtailment in Denmark, a leading wind energy producer, and delivering clean power to the Dutch market.

Project of Common Interest (PCI) Status

COBRAcable is recognized as a Project of Common Interest (PCI), a designation that highlights its strategic importance for the European energy market. This status facilitates funding, regulatory alignment, and cross-border cooperation between the two primary operators, TenneT and Energinet.dk. The 700 MW capacity of COBRAcable corresponds to an annual transmission capacity of 6.1 TWh, providing a substantial baseline of power flow between the two nations. This volume is significant for balancing the intermittency of wind generation, allowing for more efficient use of existing infrastructure and reducing the need for backup thermal power plants.

Comparison with Other PCI Projects

COBRAcable is one of several major HVDC interconnectors enhancing the North Sea grid. The table below compares COBRAcable with other notable PCI projects, such as Viking Link and Kriegers Flak, highlighting differences in capacity and geographical focus.

While COBRAcable has a capacity of 700 MW, similar to Kriegers Flak, it operates at a lower voltage level of ±320 kV compared to Kriegers Flak’s ±600 kV. Viking Link, with its 1,400 MW capacity, represents a larger scale of interconnection, but COBRAcable’s strategic location between two major North Sea wind producers makes it indispensable for regional grid stability. These projects collectively demonstrate the EU’s commitment to creating a seamless, high-capacity transmission network to support the growing share of variable renewables.

Technical specifications

The COBRAcable operates as a high-voltage direct current (HVDC) submarine power link, utilizing a ±320 kV voltage level to transmit electricity between the Netherlands and Denmark. This configuration supports a total installed capacity of 700 MW, a figure that corresponds to an annual transmission capacity of 6.1 TWh under standard operating conditions. The system is designed to enhance the European transmission grid, specifically aiming to increase the integration of variable wind power while improving overall supply reliability across the North Sea region.

Infrastructure and Ownership

The transmission line connects Eemshaven in the Netherlands to Endrup, located near Esbjerg in Denmark. The infrastructure consists of a pair of submarine power cables, jointly owned and operated by TenneT and Energinet.dk. The project became operational in 2019, marking a significant milestone in cross-border energy interconnection in Northern Europe.

The technical design of the COBRAcable facilitates the efficient movement of renewable energy, particularly wind power, which is a dominant variable source in the region. The ±320 kV HVDC technology allows for lower electrical losses over the submarine distance compared to alternating current (AC) systems, making it suitable for the specific geographic and load characteristics between Eemshaven and Endrup. The joint ownership structure between TenneT and Energinet.dk ensures coordinated management of the grid assets on both sides of the North Sea.

How does the COBRAcable infrastructure work?

COBRAcable operates as a high-voltage direct current (HVDC) submarine power link, utilizing a ±320 kV configuration to transmit 700 MW of electricity between the Netherlands and Denmark. The infrastructure consists of a pair of submarine cables connecting the Eemshaven converter station in the Netherlands to the Endrup converter station near Esbjerg in Denmark. This HVDC technology is selected for its efficiency in long-distance transmission and its ability to synchronize two alternating current (AC) grids that may operate at slightly different frequencies or phases, thereby enhancing the overall stability of the European transmission grid.

Physical Infrastructure and Converter Stations

The physical layout of COBRAcable is defined by its two primary landing points: Eemshaven in the Netherlands and Endrup in Denmark. At each end, the submarine cables terminate at dedicated converter stations that transform the electrical current between AC and DC formats. The converter station in Eemshaven is situated within the existing energy infrastructure of the Dutch North Sea coast, integrating with TenneT’s broader grid management systems. Similarly, the Endrup station in Denmark connects to Energinet’s national grid, facilitating the import and export of power across the border. The submarine cable pair runs through the North Sea bed, designed to withstand marine environmental conditions while maintaining signal integrity and power flow over the distance between the two countries.

Role in Renewable Energy Integration

The primary functional purpose of COBRAcable is to increase the amount of variable wind power integrated into the European system while improving supply reliability. By linking the wind-rich coasts of Denmark and the Netherlands, the cable allows for the balancing of generation and consumption patterns. When wind speeds are high in one country and lower in the other, power can be transferred efficiently, reducing curtailment and enhancing the utilization of offshore wind farms. The 700 MW capacity of the link corresponds to an annual transmission capacity of 6.1 TWh, providing a substantial conduit for renewable energy exchange.

Future Expansion and Offshore Wind Connections

The design of COBRAcable includes provisions for future connections to offshore wind farms. The infrastructure is structured to allow for the tapping of power directly from offshore wind parks located along the cable route or near the landing points. This modular approach enables the integration of new wind capacity without requiring entirely new transmission corridors, thereby optimizing the use of the North Sea’s wind resources. The joint ownership by TenneT and Energinet ensures coordinated planning for these future expansions, aligning the cable’s operational capabilities with the growing demand for offshore wind power in the region. This forward-looking design supports the long-term goals of the European energy transition by providing a flexible and scalable transmission solution.

Construction and development history

The COBRAcable project was designated as a Project of Common Interest (PCI) to enhance the European transmission grid, aiming to integrate variable wind power and improve supply reliability across the North Sea region. The initiative was jointly owned by Energinet.dk and TenneT, who coordinated the development of this ±320 kV, 700 MW HVDC submarine power cable pair. The project connected Eemshaven in the Netherlands to Endrup near Esbjerg in Denmark, creating a direct link between the two national grids.

Development involved significant contractual agreements with major infrastructure providers. Siemens and Prysmian were key contractors for the cable system, with Siemens typically handling the converter stations and Prysmian providing the submarine cable pairs. The project received EU subsidy allocations as part of the broader PCI framework, which helped finance the cross-border infrastructure. Construction activities included both onshore and offshore phases, with onshore works commencing at the terminal points in Eemshaven and Endrup.

Construction Timeline

The 700 MW capacity of the COBRAcable corresponds to an annual transmission capacity of 6.1 TWh, providing substantial flexibility for energy trading between the Netherlands and Denmark. The project's completion marked a significant step in the regional interconnection strategy, allowing for more efficient use of wind resources from both countries. The HVDC technology chosen for the link enabled efficient long-distance power transfer with lower losses compared to AC alternatives.

What are the operational performance metrics?

The COBRAcable system entered commercial operation in 2019, establishing a direct current link between the Netherlands and Denmark. The infrastructure consists of a ±320 kV high-voltage direct current (HVDC) submarine cable pair connecting Eemshaven in the Netherlands to Endrup, located near Esbjerg in Denmark. The project is jointly owned and operated by TenneT and Energinet, integrating the national transmission grids of both countries to enhance regional power exchange capabilities.

Capacity and Energy Throughput

The transmission line has a rated capacity of 700 MW. This electrical capacity translates to an annual transmission volume of 6.1 TWh, providing a significant conduit for cross-border energy flows. The primary operational objective of the COBRAcable is to improve the European transmission grid infrastructure. By facilitating greater interconnection, the system increases the integration of variable wind power into the broader energy mix while simultaneously improving overall supply reliability for consumers in both nations.

Operational Incidents and Maintenance

Following its initial commissioning, the COBRAcable experienced a notable operational event in 2020. During this period, a cable fault occurred, necessitating targeted maintenance and repair efforts to restore full functionality. While the grounding data confirms the occurrence of this 2020 fault and the subsequent repair phase, specific technical details regarding the exact nature of the defect, the duration of the outage, or the precise energy transported during the first half-year of operation are not explicitly detailed in the provided source material. The incident highlights the operational realities of maintaining long-distance submarine HVDC infrastructure, where environmental and mechanical factors can impact continuous power flow.

The system remains operational, continuing to serve as a critical artery for North Sea energy transmission. The collaboration between TenneT and Energinet ensures that the cable pair continues to support the growing demand for renewable energy integration across the Nordic and Continental European grids.

Regulatory and financial context

COBRAcable is jointly owned by TenneT and Energinet, the respective national transmission system operators of the Netherlands and Denmark. This joint ownership structure is central to the project’s regulatory and financial framework, reflecting a bilateral agreement designed to integrate the two national grids into a more cohesive European transmission network. The collaboration between TenneT and Energinet facilitates the sharing of infrastructure costs and operational responsibilities, ensuring that the ±320 kV HVDC submarine cable pair serves both national interests while contributing to broader European energy goals.

European Commission and Regulatory Oversight

The European Commission played a significant role in the regulatory context of COBRAcable, recognizing the project as a key initiative to enhance cross-border electricity exchange. The Commission’s involvement helped streamline the approval processes and align the project with the European Union’s broader energy infrastructure priorities. By supporting COBRAcable, the Commission aimed to improve the reliability of the European transmission grid and facilitate the integration of variable renewable energy sources, particularly wind power, into the system.

EU Economic Recovery Plan Subsidy

Under the EU Economic Recovery Plan, COBRAcable received a notable subsidy that contributed to its financial viability. This subsidy was part of a broader strategy to accelerate the deployment of key energy infrastructure projects across the European Union. The financial support helped reduce the economic burden on the joint owners, TenneT and Energinet, and ensured the timely completion of the project. The subsidy underscored the strategic importance of COBRAcable in enhancing the European energy market’s resilience and efficiency.

Financial Structure and Ownership

The financial structure of COBRAcable is characterized by the equal partnership between TenneT and Energinet. This joint ownership model allows for a balanced distribution of financial risks and rewards, fostering a collaborative approach to project management and operation. The 700 MW capacity of the cable, which corresponds to an annual transmission capacity of 6.1 TWh, represents a significant investment in the European energy infrastructure. The financial commitment from both operators reflects their shared vision of improving supply reliability and increasing the share of variable wind power in the European energy mix.