Risk-based operating limits for dynamic security constrained electric power systems

Abstract

In this work we develop a method that provides risk-based security assessment in an operating environment considering any type of security violation. Particular emphasis is placed on security constraints associated with dynamic system performance. Our work is motivated by a perception that today's deterministic approach to security assessment often results in costly operating restrictions that are not justified by the corresponding low level of risk. A risk-based approach to security assessment is attractive because it balances the system's reliability cost and reliability worth. Our method allows determination of operating limits based on the risk of insecurity at a given operating point. We characterize the operating point in terms of pre-contingency controllable parameters, the critical parameter set, that most influence the postcontingency system performance. Total risk at a given operating point is obtained summing over all the individual risk associated with defined security violations and their corresponding triggering events. We develop risk expressions that account for fully reliable conventional protection equipment and for main breakers passive and active failures. This dissertation introduces the concept of limiting operating point functions, curves that give limiting values of the critical parameter for various fault locations on a line and characterize the dependency of operating limit on fault type and fault location. The limiting operating point functions combine system stability performance and probability of instability information. This dissertation includes a detailed study on how excitation systems and other parameters affect limiting operating point functions. We also develop, using probability theory, expressions to calculate the conditional probability of insecurity given a fault occurs for thermal overloads and two approaches for computing probability of transient instability; one based on Law of Total Probability and the other on Cartesian products. Finally, we use a modified version of the IEEE Reliability Test System to illustrate risk-based electric power system security assessment and to compare it with traditional deterministic security assessment. We determine operating limits using iso-risk contours drawn in the space of pre-contingency controllable parameters, effectively creating nomograms based on risk. The contours of constant risk in the space of operating parameters provide a risk management tool that allows managers to justify decisions to operate beyond deterministic operating limits when it is economically advantageous to do so.