This paper describes a joint research project in the field of grid integration of distributed renewable energy sources. Several research groups cooperate in an interdisciplinary approach. Main focus of this research project is the analysis of both influence and possibilities of multi-agent systems (MAS) which control consumers and renewable energy resources (RES) in future energy systems. Analysis and solutions are provided in the fields of control of power electronics that connects the RES to the grid and the interaction between multiple renewable energy resources in the grid. A detailed modelling of the connecting distribution and overlaying transmission networks is used for the analysis of distributed generation systems and their influence on the different voltage levels of the power grid. A combined simulation environment for different scenarios of single distribution grids controlled by MAS and the overlaid transmission grid in an overall system model has been developed, so both grid areas can be analysed in adequate accurateness. Furthermore the integration of grid and power market in one simulation environment is extended for analyses of frequency stability.