Integrated Evaluation of Reliability and Stability of Power Systems

This paper investigates the impacts of transient instability on power system reliability. Traditionally, composite system reliability evaluation has been performed based on steady-state estimation of load curtailments; system dynamics have often been ignored, mostly due to computational complexity. In this paper, three probabilistic transient stability indices are proposed to assess system robustness against dynamic contingencies and to account for system instability in computing reliability indices. A direct method is utilized for transient stability assessment based on computing the energy margin of the system under fault events (energy margins measure the ability of a system to withstand contingencies). Energy margins along with the probability of occurrences of the events are used to update the probabilistic transient stability indices. The dependencies of reliability and stability indices on the fault clearing time are also evaluated. This method is applied on the reduced Western Electricity Coordinating Council and the New England 39 bus test systems. The results indicate the importance of considering the effect of stability in reliability evaluation.

I. Introduction

Modern power systems are increasingly being operated close to their stability limits due to the increase in the penetration level of renewable energy sources, market forces, and recent advances in computation and communication technologies. In composite system reliability evaluation, following a contingency, faulted components are assumed to be disconnected from the grid immediately and the system is assumed to return to a stable state with suitable generation rescheduling for minimum load curtailments. Although the generation rescheduling optimization problem may converge to a feasible solution representing a steady-state operating point, a stable transition to a post-fault Stable Equilibrium Points (SEP) is not guaranteed. Therefore, transient stability assessment is an important factor that should be considered in evaluating the reliability of a given system. However, transient stability assessment is computationally expensive and for this reason system dynamics are often ignored in reliability evaluation.