Reliability Calculations Including the Effects of Overloads and Maintenance

One of the difficult problems traditionally associated with reliability calculations has been the development of accurate and simple methods for representing the combined effects of (1) device outages due to extemal forces, (2) outages contingent on the load level and other outages, and (3) planned outages for maintenance. This paper shows how system loads, maintenance outages, and forced outages can be represented as two-stage, statistically independent renewal processes. The approach leads to a simplified method for calculating the steady-state rates and durations of the contingencies of interest to the system planner. The paper also shows how these methods are generally more accurate than methods using availability load models and briefly examines the effects of load growth on reliability. Two sample problems using the models are also included.