The use of multi-terminal HVDC grids is a promising option to tackle the coming challenges in long distance power transmission, but the combined operation of HVDC and AC grids will bring certain challenges. The dynamic interaction from the fast controllable converter station with the rotational inertia dependent AC grids has to be investigated in detail, as well as how the converter stations will be controlled in the static and dynamic case. This paper will focus on the dynamic case and proposes a decentralized controller structure to control the HVDC converter power set-points. The proposed controller does not rely on real-time communication nor on a slack bus. A simulation study is presented to demonstrate the implementation of the controller.