Safety and Security Dependencies for Gridshield

Present day cyber-physical systems, such as the Smart Grid, lead to the integration of multiple sub-systems into one single intertwined system. Such systems are characterized by many inter-dependencies between these sub-systems. This makes it complex to correctly assess the impact of new defence mechanisms with respect to the safety and security of the system as a whole. Existing formalisms such as fault and attack trees cannot describe the full system complexity.This paper presents a novel integrated model, namely the Attack-Fault-Defence Tree (AFDT), and tools to analyse such cyber-physical systems. The presented visual representation allows experts from various disciplines to discuss system dependencies together. In addition, we also present how minimum cut sets can be derived to formally quantify how the safety and security of the overall system is enhanced with the implementation of new defences. The presented AFDT is applied to the Gridshield concept, a novel defence mechanism to prevent grid overloading in power grids due to simultaneous charging of electric vehicles.