Controlled Switching of Unloaded Power transformer- Colcabamba Case with 500/230/33 kV Auto Transfomer

This paper presents the application of a controlled switching system for energizing a 500/230/33 kV auto transformer with delta connected tertiary winding in Colcabamba substation, Peru. Controlled switching is used to reduce grid voltage drop during no load energization of power transformer due to high inrush current during its charging; specifically, controlled deenergization to lock residual flux at lowest possible level followed by controlled energization considering residual flux to achieve inrush current mitigation. A verified simple and easily applicable approach to estimate residual flux in individual phases based on inrush peaks obtained during first time energization of power transformers is used to further reduce inrush current to best possible levels.