This paper describes challenges and possibilities related to operation and control of hybrid AC/DC power transmission grids. The aim is to discuss topics that transmission system operators and regulators must assess in order to develop technical standards, grid codes, market regulations and operational procedures for multi-terminal DC grids that connect different synchronous AC systems. The topics include control and operation in a fully integrated power market, solutions for exchange of reserves and possible adverse interactions and consequences of interconnecting AC grids on power system stability. It is shown that there is stronger interaction between AC systems when voltage droop control is applied in converter control, and when inertia support and exchange of primary reserves are provided through the DC grid. With coordinated design and well-tuned controllers, we have found no evidence of adverse interactions. We conclude that in relation to dynamic responses and power flows, it is possible to design control schemes that makes interconnected AC/DC grids behave almost like one AC grid. This property can be exploited in design of future primary and secondary control services, as well as to preferred schemes for market integration.

There are very ambitious plans in Europe for changing the energy infrastructure in order to reduce greenhouse gas emissions. This involves scenarios where renewable energy sources by 2050 will meet almost 100% of the electric power demand. This has spurred offshore wind farm development activities in the North Sea due to the vast wind energy potential in this region and the potential lack of suitable onshore sites in the future. Large-scale wind farms in the North Sea pose grid integration challenges such as the need for long-distance subsea power transmission and tackling the impact of wind power variation on alternating current (ac) grids. These challenges can be properly managed by the use of meshed high-voltage direct current (HVDC) grids. Three of the regional groups (RGs) in the European Network of Transmission System Operators for Electricity (ENTSO-E), namely Regional Group Great Britain (RG-GB), Regional Group Nordic (RG-N), and Regional Group Continental Europe (RG-CE), surround the North Sea area. A meshed North Sea HVDC interconnection between offshore wind farms and these three asynchronous ac grids can also enable market integration of the otherwise separate regions. This, in turn, results in better utilization of generation and transmission infrastructures, improved security of power supply, and maximized utilization of renewable energy resources. In this paper, a test scenario of a meshed North Sea HVDC grid is studied to demonstrate the potential of such a system for enhancing power supply security of the ac grids. Simulation studies of the test case show that with proper control techniques, a meshed North Sea HVDC grid can mitigate the effect of wind power variation by facilitating exchange of primary and secondary reserves between asynchronous ac grids.