Cyber-physical modeling and cyber-contingency assessment of hierarchical control systems

Online closed-loop hierarchical control systems (HCSs) are widely used in power-system operation. Like typical cyber-physical systems, the contingencies on the cyber side of an HCS may lead to inappropriate control commands, which will influence the physical power system. To evaluate the degree to which these inappropriate control commands influence the power system, we propose a cyber-physical equivalent model for HCSs. In this model, the HCS cyber network is abstracted to a directed graph consisting of data nodes and directed branches, and connectivity is described by using a node-branch incidence matrix. Using this strategy, we can describe the general information flow in an HCS using mathematical equations on the basis of which quantitative evaluation can be carried out. Furthermore, by using existing operation records, several kinds of typical cyber-contingencies are also modeled on the basis of which cyber-contingency assessment (cyber-CA) can be implemented by using a model-based approach. Considering the computational efficiency, such an approach keeps only key characteristics of the information flow rather than all features of the cyber network. In the case study, a coordinated secondary-voltage control system is studied as an example. The physical impacts of various cyber-contingencies on different data transmission and processing modules are compared. The results show that the model-based method provides improved efficiency compared with conventional simulation-based methods while maintaining accuracy.