How long should power system disturbance site monitoring be in order to be significant?

The author presents the results of a statistical study on power supply disturbances monitored at a computer center over a period of two years. The results of the statistical study are discussed in detail, revealing the statistical patterns of occurrence of power supply disturbances at the site and the problematic nature of defining the duration of site monitoring. The primary objectives of the present work were: (1) to attempt to determine how long site monitoring of power system disturbances should last in order to be significant; and (2) to reveal the importance of having a sound knowledge of the possible patterns of occurrence of power supply disturbances at a selected site prior to initiating the monitoring process. It appears from the statistical analysis of the power-supply disturbances that the two-week monitoring period suggested in the IEEE Orange Book, IEEE Std. 446-1987, will provide an adequate sample provided the monitoring begins with an adequate knowledge of the site´s electrical and system performance characteristics and is immediately initiated after the occurrence of two or more site disturbances