Toward Secure and Reliable Networked Control Systems

Sammanfattning: Security and reliability are essential properties in Networked Control Systems (NCS), which are increasingly relevant in several important applications such as he process industry and electric power networks. The trend towards using non-proprietary and pervasive communication and information technology (IT) systems, such as the Internet and wireless communications, may result in NCS being vulnerable to cyber attacks. Traditional IT security does not consider the interdependencies between the physical components and the cyber realm of IT systems. Moreover, the control theoretic approach is not tailored to handle IT threats, focusing instead on nature-driven events. This thesis addresses the security and reliability of NCS, with a particular focus on power system control and supervision, contributing towards establishing a framework capable of analyzing and building NCS security. In our first contribution, the cyber security of the State Estimator (SE) in power networks is analyzed under malicious sensor data corruption attacks. The set of stealthy attacks bypassing current Bad Data Detector (BDD) schemes is characterized for the nonlinear least squares SE, assuming the attacker has accurate knowledge of a linearized model. This result is then extended to uncertain models using the geometric properties of the SE and BDD. Using the previous results, a security framework based on novel rational attack models is proposed, in which the minimum-effort attack policy is cast as a constrained optimization problem. The optimal attack cost is interpreted as a security metric, which can be used in the design of protective schemes to strengthen security. The features of the proposed framework are illustrated through simulation examples and experiments. As our second contribution, we analyze the behavior of the Optimal Power Flow (OPF) algorithmin the presence of stealthy sensor data corruption and the resulting consequences to the power network operation. In particular, we characterize the set of attacks that may lead the operator to apply the erroneous OPF recommendation and propose an analytical expression for the optimal solution of a simplified OPF problem with corrupted measurements. A novel impact-aware security metric is proposed based on these results, considering both the impact on the system and the attack cost. A small analytical example and numerical simulations are presented to illustrate and motivate our contributions. The third contribution considers the design of distributed schemes for fault detection and isolation in large-scale networks of second-order systems. The proposed approach is based on unknown input observers and exploits the networked structure of the system. Conditions are given on what local measurements should be available for the proposed scheme to be feasible. Infeasibility results with respect to available measurements and faults are also provided. In addition, methods to reduce the complexity of the proposed scheme are discussed, thus ensuring the scalability of the solution. Applications to power networks and robotic formations are presented through numerical examples.