This paper examines the current status of technology and discusses technical options for developing DC transmission grids. The fast advances in VSC HVDC, the recent offshore VSC projects, the experience with multiterminal HVDC and recent development of fast DC circuit breakers bring large meshed DC grids closer to reality. The most important and most difficult remaining technical challenge is the system level protection of DC grids. The article further discusses some of the ongoing research directions like the use of travelling wave detection for fast protection or deployment of DC/DC converters for isolation of DC faults. One of the main work packages in EU funded Twenties project studies the major prerequisites for operation of DC grids. This project has delivered some major studies of DC grids and two hardware demonstration systems are under development: a mock-up DC grid at University of Lille and fast DC Circuit Breaker at ALSTOM.

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Control of voltage-source-converter high-voltage dc (VSC-HVDC) transmission systems embedded in ac systems is a central issue requiring attention in the context of multi-terminal dc grids. A number of power drooped against dc voltage control concepts have been proposed in the literature for this purpose. The main advantage of droop control in this context is that it facilitates for several converters to share a sudden mismatch in power, e.g., due to a converter trip. However, it is usually less clear how to re-establish set-point tracking in, e.g., power once it has been lost. This paper proposes two different methods that in conjunction can resolve this issue for both small and large disturbances for a variant of power drooped against dc voltage called pilot voltage droop control. The methods are derived from the basic control laws in a straight forward fashion and the accuracy is exemplified by time domain simulations in Digsilent's Powerfactory platform.