The Quality of U.S. Commercial AC Power

From May 1977 through September 1979, the Bell System conducted a study of commercial ac power disturbances ¿ To determine their nature and extent ¿ To devise statistical ways of predicting their occurrence ¿ To reduce or eliminate computer downtime through selection of appropriate, cost-effective power-conditioning equipment for use throughout the System. For a total of 270 months, field data was gathered from 24 Bell System data-processing sites. A statistical model, a Polya probability density function, best fit the overall observed data. It allowed predictions to be made as to the rate and duration of the various electrical disturbances distributed across the sites. However, statistical cluster tests of the data indicated that the overall model could not be used to predict disturbance activity at any specific site because of the high variability among the data from site to site. A modified Polya function, applied to observed data from specific sites, yielded more accurate predictions of the rate at which power disturbances will occur at various probability levels. A simple "look-up" table was devised to eliminate the need for tedious calculations. This table makes it possible to determine, based on the measured average number of disturbances occurring at a specific site each month, the maximum number of each kind of disturbance to be expected per year at a number of probability levels. The study indicated that different types of power-conditioning equipment yield varying reductions in the percentages of power disturbances.