Topics on Safety and Security of Power Systems

Abstract

In this thesis, we investigate some of the problems concerning the safety and security of power systems. Since the operational safety of the power system itself is a vast area of research, we choose to investigate the problem of identifying transmission line outages in a power system due to their cascading nature and widespread blackouts they can cause. Therefore, it is important to identify such outages in the shortest time possible. With this motivation, we propose a state estimation-based sequential hypothesis testing procedure to localize the failed lines. The primary focus is on single-line outages as these are more frequently occurring failures. The state estimation (SE) is performed using conventional power measurements (SCADA) and synchronized phasor measurement units (PMUs). The idea is that if there is an outage, then this information is embedded in the state estimation results, and by analyzing this information one can infer the topology of the power system. The proposed framework involves various Kalman filter-based state estimation techniques followed by a generalized likelihood ratio testing procedure to locate the failed lines. This work considers both centralized and decentralized state estimation approaches. In a centralized approach, all the information from various parts of the power system is available to the system operator. However, in the decentralized approach, only limited information is considered to reduce the communication and computational overhead. The proposed algorithms are evaluated on the IEEE 14 and the IEEE 118 bus systems, and results show that all the high-risk line failures were identified quickly. Further, simulation results show that the use of PMUs enables accurate and quicker detection than conventional power measurements.

Furthermore, recent cyber-attacks on power grids highlight the necessity to protect the critical functionalities of a control center vital for the secure operation of a grid. Even in a distributed framework, one central control center acts as a coordinator in the majority of the control center architectures. Such a control center can become a prime target for cyber and physical attacks, and, hence, a single point failure can lead to a complete loss of visibility of the power grid. If the control center that runs the critical functions in a distributed computing environment is chosen randomly among the available control centers in a secure framework, the ability of the attacker to cause a single point failure can be reduced to a great extent. To achieve this, we propose a novel distributed hierarchy-based framework to secure critical functions related to the control center. The proposed framework ensures that the data aggregation and the critical functions are carried out at a random location and incorporates security features such as attestation and trust management to detect compromised agents. A theoretical result is provided on the evolution and convergence of the trust values in the proposed trust management protocol. It is also shown that the system is nominally robust so long as the number of compromised nodes is strictly less than one-half of the nodes minus 1. For demonstration, a Kalman filter-based state estimation using phasor measurements is used as the critical function to be secured. The proposed framework’s implementation feasibility is tested on a physical hardware cluster of Parallella boards mimicking the IEEE-5-bus-system. The framework is also validated using simulations on the IEEE 118 bus system. The experimental and simulation results show that the trust for compromised control centers nominally reduces with the number of attestations.