DC overlay systems represent a radical evolution of the legacy of AC interconnected systems developed over the past few decades. Due to significant changes in the type and location of power generation, along with a shift towards a digitalised energy sector, continuing incremental AC interconnection upgrades may not provide the most cost-effective or timely solutions for managing the energy transition.

As shown in the figure above, DC overlay systems can be designed as linear, radial or meshed multi-terminal systems, providing the characteristics of a grid [1]. Two-terminal long-distance DC corridors emerged in the 1960s and, with the rapid advancements in power electronics and control systems, the first multi-terminal, non-meshed, High Voltage Direct Current (HVDC) system was commissioned in the 1990s. Meshed multi-terminal DC grids (MTDC), where multiple power-flow paths exist between two grid terminals, are still being examined at the research level to address integration challenges with AC meshed grids. Once these challenges are resolved, this will create a so-called DC overlay grid. This concept could eventually enable various large electricity networks to be interconnected on a global level. Additionally, a DC overlay grid system can enhance the flexibility of the entire transmission grid, enabling it to cope more effectively with the characteristics of renewable power infeed.

The choice between an extended AC or DC grid depends on a variety of technical, economic and environmental factors. The profitability threshold between the two types of current systems has varied over time depending on the use cases. The first resurgence of a DC system was registered in 1954 in Sweden, when the island of Gotland was connected back to the mainland.

With the increasing need to integrate remote, large-scale offshore renewables and enable controllable connections between countries (markets), DC transmission will become more relevant. Its integration within the current AC system will contribute to achieving a cost-efficient energy transition in several ways.

Major advantages of the integration of DC systems include:

• Increased transmission capacity by leveraging existing AC corridors to create new higher capacity DC corridors, boosting transmission capacity with limited additional environmental and social impact.
• Enhanced power flow control, enabling more effective utilisation of the lines closer to thermal limits.
• Increased ancillary service provision, such as voltage/frequency regulation or power oscillation damping.
• Enhanced flexibility in the overall transmission grid, allowing it to cope with the characteristics of renewable power infeed.
• Potential use of very long extra high voltage DC cables, which are unavoidable in offshore areas and may increase the acceptance of onshore projects.

R&D is accelerating in this field to overcome the technical and regulatory barriers to operating and controlling MTDC systems and integrate them into meshed AC systems. This integration will combine the benefits of AC and DC technologies and open the door to new devices and systems, such as HVDC circuit breakers, HVDC gas-insulated switchgears and flexible DC transmission system devices that can enhance the security, reliability, performance and economics of DC overlay grids.

Concepts such as the North Sea Wind Power Hub [2] already show advanced DC grid layouts complementing the AC onshore system. The Medgrid idea [3] is already linking European, North African and Middle Eastern areas around the Mediterranean region.

More visionary approaches, such as a global grid based on DC backbones, may allow high levels of renewable energy supply to be exchanged in a cost-effective and secure manner.

The bulk of offshore wind energy foreseen by the EC decarbonisation policy goals will be transported to the onshore system using HVDC technology due to the many advantages it offers, including improved controllability, lower costs, etc. The most efficient way to enable large-scale integration of offshore wind is to combine offshore cable connections with the interconnectors between member states. This combination would yield a meshed offshore DC grid that would not only reduce costs and environmental footprint but also provide a reliable power system that is resilient against changing operating conditions, robust against faults and stable due to ancillary services such as grid forming capability (GFC). However, several technical milestones must be achieved to enable the development of an offshore DC grid in the North Sea:

• Interoperability: at present, converter stations from various manufacturers cannot be interconnected because there are no common European specifications and standards. To address this issue, the InterOPERA project [4], funded by the Horizon Europe programme, was initiated in 2023. This project aims to develop the common functional specifications and standard interfaces required to allow different manufacturers’ systems to seamlessly integrate and operate together.
• DC Grid Planning Standards: the integration of multiple synchronous systems via a meshed DC grid will result in the formation of an extensive hybrid AC/DC transmission network. As this system is likely to evolve progressively over the coming decades, the establishment of DC grid planning standards is essential to fully harnessing HVDC technology to improve the reliability and resilience of the overall transmission infrastructure. The HVDC-WISE project [5], launched at the end of 2022 and funded by the Horizon Europe programme, addresses all significant aspects related to the development of DC grid planning standards.
• DC Grid Protection: establishing a large meshed offshore DC grid necessitates the selective isolation of faulty segments to ensure the healthy parts continue to function properly. The challenges of managing faults in DC grids are more significant than those in AC grids due to factors such as the absence of natural current zero crossings, lower impedances and rapid changes in current and voltage during faults. At the end of 2024, a new project, DC PROSECCO, funded by the HE2020 programme, was launched to address grid protection innovations near HVDC converters and congestion management for hybrid AC/DC grids.

R&D by industry and academics

• Technologies [6]

  • Converter Technology: the development of converters will allow higher voltages, higher capacities and lower losses, with cost optimisation via technology standardization.
  • DC/DC Converter and Power Flow Control: DC/DC converters will not only connect different voltage levels but also manage flows on the DC side.
  • Lines and Cables: advancements in technology are expected to reach at least 800 kV, possibly higher depending on cable physics limitations.
  • Switchgear: standardisation of DC switchgear is critical to reach reliable and cost-efficient technologies.
  • Superconductor-Based Technologies: following the successful testing of a 320 kV/3000 MW superconducting cable based on magnesium diboride (MgB2), future development work should address innovative cable and fault limiter solutions for future overlay HVDC grids (small footprint, high efficiency).

• Network operations [6]

  • Operation: several options should be studied, such as a regulated DC system operator managing the HVDC grid only, a merchant DC operator or a territorial operator managing part of the grid related to an area/country.
  • Control: an HVDC grid controller must be developed to provide system-level control, with certain converter controls potentially supervised by the HVDC grid controller. Next, the HVDC grid controller is required to coordinate control actions over the overlay system. The ultra-long distance poses challenges for communication reliability and delays in control coordination. Redundant HVDC grid controllers at different locations can be used to ensure availability. This requires novel technologies to synchronise and seamlessly switch between the redundant HVDC grid controllers, given substantial communication delays. Advanced control methods and strategies must be developed to maintain the stable operation of the overlay HVDC.
  • Protection: the main challenges in protecting an overlay HVDC grid are robust protection algorithms, fault clearing, recovery, managing grounding and unbalanced faults.
  • System digitalisation: the transition to a digital substation environment using IEC 61850 [7] or similar protocols is not obvious, as these protocols are not designed for DC applications.

• Network simulation software and tools [6]

  • Planning: risk-based tools are required to support the optimal design of HVDC grid protection systems, while considering available technologies, the probability of faults in the grid and their impact on both AC and DC systems.
  • Operations: improved HVDC converter models are needed to correctly represent the different control features and operating modes in different time scales.

Standardisation

Grid codes, technical standards and regulatory rules must be adjusted. This is part of the recent EUROBAR initiative, which eight TSOs joined in 2022 [8].

The technology is in line with milestones “Development of network codes for HVDC”, “Alignment of requirements for HVDC, cable and monitoring systems”, “Development and test of critical MT HVDC components” and “HVDC system planning criteria and identification of possible new interconnectors” under Mission 2, milestone “Adaptation of Codes and Procedures (C & P) to the interaction with numerous HVDC On-/Off-Shore” under Mission 3 and milestone “Integration of HVDC links, renewable power plants and offshore installations in the ancillary services markets” under Mission 5 of the ENTSO-E RDI Roadmap 2024-2034.

Examples

For the estimation of the TRL of DC grid systems, the following must be distinguished:
TRL 9 for radial/linear multi-terminal systems.
TRL 6 for meshed multi-terminal systems*.
*This assessment refers only to projects located in Europe. The Zhangbei 4-Terminal project is the world’s first four-station meshed HVDC voltage-sourced converter grid. Hence, the worldwide TRL of this technology is 9.

1. S. Anhaus, P. Düllmann, L. Osterkamp, R. Dimitrovski, P. Mcnamara and J-C. Gonzalez. “A classification framework for HVDC-based transmission grid architectures”, CIGRE Session 2024, Paris, France.

2. “Key concepts.”, North Sea Wind Power Hub.

3. “Our project.” Medgrid.

4. “Project InterOPERA.”, InterOPERA.

5. “Project HVDC-WISE.”, HVDC-WISE.

6. S. Weck, S. Rübergg and J. Hanson. “Planning and design methodology for a European HVDC overlay grid.” 13th IET ACDC, 2017, pp. 1–6, doi: 10.1049/cp.2017.0026.

7. IEC 61850 – Home

8. EUROBAR TSO members, “EUROBAR Scoping paper”, 2022