Transient Behavior of Three-Phase Shell Transformers in a Distribution Feeder

This paper presents an investigative study on an overvoltage incident which occurred during a routine scheduled maintenance operation conducted by a local utility on their 24.94 kV distribution network. The network has a three-phase shell transformer connected by an underground cable at its primary. Single phase switching operation was carried out by the local utility´s line men. With two phases remaining energized from a three phase source, overvoltages were observed at the open ended phase terminal of the underground cable. Possible causes of overvoltage such as switching transients, sub-harmonic oscillation, and ferroresonance have been investigated. Alternate Transients Program (ATP) has been utilized for modeling the distribution system of the local utility. Both the linear and nonlinear characteristics of the transformer are considered in the model. The cause and sustenance of this overvoltage has been diagnosed due to ferroresonance in this study. The responses of the system with ferroresonance characteristic waveforms are presented in the paper