Internal resonance for power transformer interacted with cable transients

Power transformers may be subjected to switching surges or fault events that may be responsible for internal transformer resonance. This paper deals with two scenarios of cable-transformer resonance. The first resonance case is due to single-phase earth fault, the second resonance is due to capacitor energization. These two situations lead to excessive transient overvoltages in the secondary side of the power transformer. The effect of various factors on these transient overvoltages is studied. The study is performed on three-phase 155 MVA power transformer. Fast Fourier transform is used for analysis and defining resonance frequencies. Also, phase-plane diagram and discrete wavelet transform are used for internal resonance feature extraction. The results confirm overvoltage occurrence on the low voltage side of power transformer behind cable due to cable-transformer resonance.