Operation and Control of hybrid AC/DC transmission grids

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.