Temporal Causal Diagrams for Diagnosing Failures in Cyber-Physical Systems

Resilient and reliable operation of cyber physical systems of societal importance such as Smart Electric Grids is one of the top national priorities. Due to their critical nature, these systems are equipped with fast-acting, local protection mech-anisms. However, commonly misguided protection actions together with system dynamics can lead to un-intentional cas-cading effects. This paper describes the ongoing work using Temporal Causal Diagrams (TCD), a refinement of the Timed Failure Propagation Graphs (TFPG), to diagnose problems associated with the power transmission lines protected by a combination of relays and breakers. The TCD models represent the faults and their propagation as TFPG, the nominal and faulty behavior of components (including local, discrete controllers and protection devices) as Timed Discrete Event Systems (TDES), and capture the cumulative and cascading effects of these interactions. The TCD diagnosis engine includes an extended TFPG-like rea-soner which in addition to observing the alarms and mode changes (as the TFPG), monitors the event traces (that corre-spond to the behavioral aspects of the model) to generate hy-potheses that consistently explain all the observations. In this paper, we show the results of applying the TCD to a segment of a power transmission system that is protected by distance relays and breakers. 1