Protective measures for transient overvoltages in motors starting by autotransformers

Starting medium voltage motors via autotransformers can result in high overvoltages owing to improper circuit breakers switching time before getting a significant speed, and multiple reignitions (Ahmed et al., 2005).The purpose of this paper is to show results of the study made to suppress these overvoltages that appear at the neutral point of the autotransformer in motor - autotransformer starting arrangement. This paper is to continue the work done in the paper "Transient Overvoltages in Motors Starting by Autotransformers", published in the scientific magazine, Ain Shams University, Faculty of Engineering, bulletin 40/4, December 2005, for the same authors. A model was built in ATP for simulating motor starting process and to show the autotransformer neutral point overvoltages suppression. Two types were used as suppressing devices; the metal oxide surge arrester and the R-C damper, separately and together. Simplified model for the metal oxide surge arrester was introduced and used to suppress the overvoltages.