Why does there occur an excitation inrush current when the transformer is switched on?

In the transformer coil, the relationship between excitation current and magnetic flux is determined by the magnetization characteristics. The more saturated the iron core is, the greater the excitation current required to generate a certain magnetic flux. Under normal conditions, the magnetic flux in the iron core is already saturated. If the switch is closed under adverse conditions, the maximum magnetic flux density in the iron core can reach twice the normal value, causing severe saturation of the iron core, reducing its magnetic permeability and greatly decreasing the excitation reactance. As a result, the excitation current increases significantly. The current waveform determined by the magnetization characteristics is very sharp, and this surge current can exceed 6-8 times the rated current of the transformer. Therefore, due to the conversion of electrical and magnetic energy in the transformer, the phase angle of the voltage at the moment of switch-on, the degree of saturation of the iron core, etc., determine the occurrence of an excitation inrush current when the transformer is switched on. The magnitude of the excitation inrush current will be affected by the remanence of the iron core and the phase angle of the switch-on voltage