DC grid control through the pilot voltage droop concept - Methods for establishing set-point tracking

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.