Grid stability and security of supply are the two major critical issues for reliable operation of the DC Grids (DCGs) and of the AC systems to which they are connected. DCGs may necessitate control systems capable of ensuring operation with adequate security margins in steady-state and during major network disturbances, initiated in the DC or AC side. Such systems may need to take into account the potential interactions between DC and AC side, including security constraints of the integrated AC-DC grid for real-time operation, and avoid conflict between the control functions or objectives in both sides during network disturbance.
The TWENTIES project aims at developing and demonstrating network architectures with power system technologies and novel network architectures. Within the large-scale demonstration “DEMO 3: DC Grid” of the project , the benefits and impacts of meshed HVDC networks were studied in the context of the North Sea area, in comparison with the current approach of radial DC connection of wind farms.
It was demonstrated by dynamic simulation that reactive power capability of onshore terminals can be used to improve voltage stability of the AC networks during normal operation and AC network disturbances.
Small signal stability can also be enhanced through appropriate converter controls.
In a preventive way, AC system stability can be improved by shifting power injections on the mainland AC network.
As for the impact of a DC grid on operation of AC system distance protection, mis-coordination might occur for existing AC protection close to the onshore converters, whatever the DC transmission structure, due to the reactive power control of onshore Voltage Source Converters (VSC). This issue is related to the under-reach problem of distance protections (a zone 2 fault which might be viewed in the zone 3 range). Adaptive protections have been proposed to overcome this problem.
 TWENTIES project: see www.twenties-project.eu/