Mitigation methods of magnetizing inrush transient during energization of large power transformer

Energization of unloaded transformer results mainly in magnetizing inrush currents having a high magnitude with harmonic-rich currents, when transformer cores are driven into saturation during energization. The currents cause many undesirable effects on transformer attribute; large dynamic forces inside it, life-span reduction, insulation deterioration and mechanical fatigue in windings support, as well as, problems in protection system and finally reduced power quality of the system. A 230/66 kV Yn/Δ no-load transformer connected to a 230 KV power system was simulated using the PSCAD/EMTDC. The effect of pre-insertion resistors to suppress inrush transients was clearly demonstrated. Without controlled switching the energization may occur at any time on the voltage wave producing high current peak, when the transformer core is driven into saturation. Recent techniques have been presented to reduce inrush current phenomena and to minimize the residual flux during energization of large power transformer. Different mitigation methods were analyzed and evaluated by using Fast Fourier transform (FFT).