The paper presents a method for monitoring and diagnosis of the effects yielded by the non-sinusoidal regime over the voltage transformers operating in a power group. The analyzed schematic structure is presented along with all its components. Information on the designing, construction and implementation of a data acquisition system (DAS) are presented, along with the interface for data visualization and processing. Numerical recordings were accomplished by using the DAS. Waveforms were acquired from the primary winding of the transformer used for excitation supplying, corresponding to a power of the main generator of 280 MW. The recorded waveforms were processed by using the Fast Fourier Transform (FFT), respectively the Discrete Wavelet Transform (DWT). Data processing with FFT revealed a significant non-sinusoidal regime for the currents from the main winding of the transformer used for excitation supplying. Over time they affect the voltage transformers. The DWT analysis also revealed a significant non-sinusoidal regime, accompanied by frequent dynamic regimes corresponding to the above mentioned currents, with a negative multiplication effect over the voltage transformers. The waveforms analyzed in this paper were acquired after the substitution of the voltage transformers. The noticed dynamic non-sinusoidal regimes require the analysis of new measures for their diminishing.