This paper describes a new technique that has been developed to facilitate the preparation of frequency and duration models for large composite systems. These models are required for power system reliability and probabilistic production cost evaluations. The technique is based on the sequential composition and state aggregation of component subsystems and composite partial systems. A significant feature is the fact that state aggregation is performed at the subsystem level rather than the final system level. This has important computational consequences since it greatly reduces the number of states that must be stored and manipulated during the computational process.
