Comparisons of normal and sympathetic inrush and their implications toward system voltage depression

Voltage depression can be initiated by severe inrush phenomenon during energization of large power transformers. Sympathetic inrush, which has not been generally appreciated in terms of its prolonging effects toward voltage depression, is discussed in this paper. Following a short review of the basic principles behind transformer inrush and sympathetic interactions, an EMTP model has been developed for a transmission system, which allows simulations to be performed to evaluate the impacts of energization of transformers on system voltage depression. It can be concluded that the normal inrush determines the magnitude of the initial voltage sag while the sympathetic inrush, which takes place at the latter stage, prolongs the voltage recovery process and consequently will determine the sag duration.