Offshore power generation and transmission re-quires long subsea cables. When using AC, the in-ductance of the cables results in reactive power that contributes to the load of the cables and increases the voltage at busbars. At the same time, offshore power systems are becoming increasingly complex. They are evolving from dedicated collector grids and transmission cables for wind farms through shared use of transmission cables by multiple wind farms to hybrid offshore grids that connect wind farms to multiple countries. This paper discusses operational challenges of off-shore grids and how they are solved with a model-based master controller that solves optimal power flow problems in real-time. The Kriegers Flak Combined Grid Solution serves as example. It transports wind power from the four offshore wind farms (Baltic 1 and 2, Kriegers Flak A and B) with a total capacity of 950 MW to shore. Additionally, it promotes energy trade between Germany and Denmark. At the heart of the connec-tion of these two energy grids through the Baltic Sea there is a Master Controller for Interconnected Operations (MIO). Based on a Modelica model and using the PowerSystems library, it solves several state estimation and optimal power flow problems in real-time. MIO is implemented as geo-redundant system with ABB AbilityTM OPTIMAX® and System 800xA.