Uncontrolled energization of power transformers can create large flux asymmetries and saturation of the transformer magnetic core. This saturation results in high current magnitudes with a wide harmonic spectrum and a high direct-current component [1]. Inrush currents can cause false operation of protective relays [2,3], reduce transformer lifecycle [4,5], and usually reduce power quality on the system [6,7]. The magnitudes reached by these currents depend on two principal factors: the point on the voltage waveform at which the switches are closed; and the residual fluxes in the transformer core [8]. Several methods, such as series compensator [9,10], sequential phase energization with a grounding resistor [11,12] and controlled switching [1,8,13,14], have been developed to reduce the inrush current. Among these, the controlled switching that takes into account the core residual flux constitutes the most promising method. Its basic principle is to guarantee that the residual fluxes are equal to the prospective fluxes at the instant of energization. A prospective flux is the steady-state flux if the supply source is already connected to the transformer.
