The energy transition in Europe and other parts of the world is characterized by the development and deployment of renewable energy sources such as wind- and solar power. In recent years, the transformation and conversion of renewable energy into sustainable solutions like Power-to-X (PtX), especially hydrogen or e-methanol (but also others), has become more and more important and plays a key role in achieving the decarbonization targets of the energy system. In recent years, many large-scale PtX projects have been announced and discussed, but only a few of them have been realized and are in operation.\\

This paper focuses on the Grid Compliance aspects of co-located generation and demand facilities, mainly based on the example of the Kassø PV + PtX plant in Denmark. The work will briefly introduce the requirements applied to co-located plants in Europe and Denmark (especially NC RfG and NC DCC and upcoming updates) and will outlines challenges during the grid compliance process in the areas of design (grid related capabilities and functionalities), simulation model development and assessment of the simulation results (RMS- and EMT models) as well as grid compliance testing. Furthermore, the paper will provide lessons learned and recommendations for future requirements for co-located facilities.