The progressive integration of software-based components into the electricity grid has given raise to what is known as Smart Grids. As long as Smart Grids gain on connectivity and automation, new concerns on their safety and security have arisen. It is agreed that non-negligible risks and enlarged impact due to misbehaviors and intrusions exist. Following a model driven paradigm, a method is proposed to reinforce the security of these complex widely distributed systems. The method guides system re-engineering and is based upon timed models. It encompasses reverse engineering, symbolic, and testing techniques to model, analyze, and deploy attack testing. In early stages of the method, a reference timed model to support security analyses is designed via reverse engineering and symbolic execution. During latter stages, the nominal models are enriched so as to specify attack scenarios which are symbolically executed to prove the ability of the system to detect attacker intrusions. In final stages, the attack scenarios are used to specify test cases which are later deployed to test the system. The method and main outcomes are presented relying upon a Smart Grid subsystem analyzed in the scope of a joint academy-industry project.