Causes and Control of the Maximum Anticipated Per-unit Transient Overvoltage Factor (T) Used in Energized Work

The determination of the maximum value (TMAX) of the anticipated per-unit transient overvoltage factor (T) used to establish the maximum anticipated transient overvoltage magnitude (TOVMAX) for use during energized work, requires evaluation of the connected system components and control mechanisms available. This evaluation process should consider the various causes and control of T as well as the probability of a triggering event occurring. Knowledge of T, determined for the system conditions at the time of energized work, will allow the appropriate controls to be used for the system and at the work site to safely perform the task. One of the most important factors that determine maximum T is the magnitude of the trapped charge on the line after opening of circuit breakers at both ends. The purpose of this paper is to suggest criteria for an engineering evaluation of the causes and controls of T for calculation of the minimum air insulation distance (MAID) for use during energized work. Methods for reduction of the trapped charge are also discussed.