Power transformers are an integral part of the power system. The development of the electric grid system in the structure of Smart Grid requires more informatization. Under such conditions, become important components energy monitoring systems and the status of power transformers. This process is even more important in the case of transformers that have been in operation for a long time, and sometimes outside regulatory terms, which is typical of state networks. In most cases, the existing studies is based on the spectral composition analysis a transformer windings phase current and voltage. The procedure of determining the active and reactive power components of a three-phase transformer in idle mode, as indicators of the state of the active parts, taking into account the harmonics of current and voltage, is proposed. An experimental study of the transformer idling mode was carried out on the laboratory equipment. The primary sequence of the experimental study energy performance a three-phase transformer in idle mode is developed. For this purpose is use Fourier transform, and the active and reactive power index of windings are monitoring is determined by the fundamental harmonics of current and voltage and all measurements harmonics. It is noted in one of the phases that the higher harmonics power generation is provided when the star-delta is connected. In the same case, the current harmonics are greatly enhanced relative to the voltage harmonics. The study results can be used as components of information support for the Smart Grid structure.