Elimination of Transformer Inrush Currents by Controlled Switching—Part I: Theoretical Considerations

Transformer inrush currents are high-magnitude, harmonic-rich currents generated when transformer cores are driven into saturation during energization. These currents have undesirable effects, including potential damage or loss-of-life to the transformer, protective relay misoperation, and reduced power quality on the system. Controlled transformer switching can potentially eliminate these transients if residual core and core flux transients are taken into account in the closing algorithm. This paper explores the theoretical considerations of core flux transients. Based on these studies algorithms were developed which allow controlled energization of most transformers without inrush current.