Refining the Concepts of Small and Large Disturbances for Stability Analysis of Power Systems

The definitions of small and large disturbances adopted by many reliability organizations and transmission system operators are in harmony with the IEEE/CIGRE definitions. However, in many academic and industry stability studies differing definitions are used. This paper is an attempt to refine the concepts of small and large disturbances. After providing a working classification of common definitions, along with representative examples, the paper discusses inconsistencies, contradictions and ambiguities in the definitions and their impact on the interpretation of study results. The paper redefines disturbances and proposes a new term of modest disturbance to harmonize the mathematical definitions and operational definitions used in time-domain simulations. It is recommended that conceptual and operational definitions be reconciled, to the extent possible, and that each simulation-based stability study describe and explain the definitions it adopts paying special attention to the interpretation of study findings.