A practicable modern type of HVdc control system is modeled dynamically on a digital computer. The simulation interfaces with dynamic simulation of converter action, the ac system and the dc system. Applications have included harmonic studies, fault analysis and multi-terminal dc systems, in addition to control circuit development and optimisation. An example is presented of a multi-terminal system study in the context of dc fault behaviour with dc circuit breakers included. The detailed control simulation can be simplified for certain dynamic studies.