The smart grid is positioned as the next generation energy distribution system which incorporates distributed renewable energy and digital communication infrastructure. The Smart Grid is considered by many as a complex Cyber-Physical System, a system with tight integration between its modular cyber and physical processes interacting in a networked environment. Traditionally, the method of testing Smart Grid automation systems is based on device-based testing and this is largely influenced by the bottom-up design methods that are used widely in the engineering of the substation automation system. However, with the advent of the Smart Grid and the introduction of distributed based automation systems, requirement-driven testing of the whole automation system may be more beneficial than device based testing. In this paper, a scripted co-simulation platform is introduced with the purpose of automating the testing of functional requirements in distributed substation automation systems. The scripted co-simulation testing framework is illustrated on a CIGRE case study where the distributed automation system is validated against the functional requirements.