I. Introduction
The transformer is an essential component of the transmission and distribution system. Transformers represent one of the key components in utility systems requiring a constant monitoring. For this reason, it is necessary to improve the techniques and methods for keeping good function of the network that helps to prevent transformer faults. The machine electric life is a decisive factor for managing the risk associated with the network reliability and of course provides a quality server directly impacting the availability. Currently are executed various preventive tests in energized transformers which use technics as ultrasound, power factor, physics and chemical analysis.