The work presents a one-part model of the transition process in the electricity system. Laplace transform applies mathematical models of signals on digital substations. The author offers a spectral representation of analysis methods on the Laplace transform. The synchronized phasor measurement methods are for determining and filtering the carrier frequency. The results of the frequency analysis are input data for the diagnostics of digital substation devices. We present the developed software of a frequency diagnostic process using FIR-filter and choose an adequate model structure of simulated electrotechnical objects of synchronized phasor measurements. Finally, the article discusses the prospects of applying the proposed approach to solve the problems determining the primary frequency in a digital substation