A large number of power systems can be classified as weak, either due to their isolation or to the poor available interconnection to other stronger systems. In weak systems, large voltage sags are produced when power transformers are energized, owing to their inrush current. Moreover, this operation can achieve high periodicity in weak grids, e.g., in pumped hydro storage plants and in auxiliary diesel units. The voltage dips that are produced have a negative effect on power quality and can even cause to trip certain consumers via undervoltage load shedding. In order to reduce the inrush currents of main power transformers and hence to improve power quality in weak power systems, in this work it is proposed to install a series-connected auxiliary power transformer through which intermediate energization of the main power transformer is carried out before its direct connection to the grid is executed. Simulations of this method have been performed to evidence the minimization of inrush current and further aspects such as breaker closing times and power source paralleling have been analyzed.