  The integration of distributed renewable energy sources and the multi-domain
behaviours inside the cyber-physical energy system (smart grids) draws up major
challenges. Their validation and roll out requires careful assessment, in term
of modelling, simulation and testing. The traditional approach focusing on a
particular object, actual hardware or a detailed model, while drastically
simplifying the remainder of the system under test, is no longer sufficient.
Real-time simulation and Hardware-in-the-Loop (HIL) techniques emerge as
indispensable tools for validating the behaviour of renewable sources as well
as their impact/interaction to with the cyber-physical energy system. This
paper aims to provide an overview of the present status-quo of real-time and
HIL approaches used for smart grids and their readiness for cyber-physical
experiments. We investigate the current limitations of HIL techniques and point
out necessary future developments. Subsequently, the paper highlights
challenges that need specific attention as well as ongoing actions and further
research directions.
