RES integration into the energy system requires a continuous adaptation of energy grids and their technologies. Direct current (DC) technologies can have a leading role in this transformation thanks to their flexibility, efficiency and sustainability. HVDC is essential for offshore generation and for the integration of energy islands, while MVDC and hybrid networks (AC/DC) can foster direct RES integration and sector coupling.

While both transmission and distribution grids are currently predominantly AC, DC grids are becoming increasingly important. Especially for long transmission distances the use of HVDC is often advantageous, as there are lower losses over long distances and compensation techniques are not required.

HVDC is therefore also important for connecting offshore wind farms to continental electricity grids and for the integration of "energy islands", a system concept currently being promoted by several Member States. In particular, the offshore wind expansion plans of the countries bordering the North Sea and the new energy island concept are challenging to integrate into the existing grids with current solutions, so that in addition to the necessary investments, there is also a high demand for research.

Moreover, there is a clear trend towards employing MVDC networks and MVDC grids/microgrids are attracting even more interest thanks to the advancements in the power electronics area. For example, large RES can be easily connected to the MVDC networks, and DC loads such as energy storage or electric vehicles are increasingly appearing. In addition, existing AC grids can be effectively coupled using DC. MVDC networks and micro grids, which have different characteristics and applications from HVDC networks, are still in their infancy in terms of research.

This Call Module addresses the main challenges of DC technologies at medium and high voltage level for effective integration of energy islands or high shares of RES into the energy infrastructure. The dynamic response of power systems is more dependent on complex fast-responding power electronic devices, which affects the dynamic behaviour of the power system. Furthermore, meshed DC grids are expected to proliferate at all voltage levels as well as hybrid transmission and hybrid distribution AC/DC networks. In this context, the grids evolve towards multi-terminal, multi-vendor and meshed DC grids. Therefore, new and advanced control and operation principles are required for secure and stable grid operation supported by a higher level of observability, covering all voltage levels and new types of dynamics (e.g. faster dynamics).

In addition, the DC grids must be integrated into the existing grid infrastructures, whereby costefficient solutions are essential. Especially the integration of energy islands into the onshore grids requires not only transnational planning but also the consideration of market aspects, e.g. connection to shore, technology used, design of pricing zones. Furthermore, the costs of the different technologies on all voltage levels must be taken into consideration in order to enable effective hybrid AC/DC grid planning. Not only should optimal solutions be determined in terms of a cost-benefit analysis, but the solutions should also be designed for the long term and the public should be involved.