The paper provides substantiation for the idea that interharmonics, though still insufficiently studied in power engineering, are an important element of the complex notion of electromagnetic compatibility of power supply systems. The purpose is to systematize mathematical apparatus for the analysis of interharmonics amplitude-frequency spectrum. Special attention is paid to practical calculations of interharmonics in respect to staple sources of electromagnetic interferences – frequency converters of different types. Certain elements of spectral analysis theory are presented as an efficient tool for the analysis of interharmonics’ levels. The paper analyzes the influence of higher harmonics and interharmonics on the elements of the power supply system at an industrial enterprise, taking into account peculiarities of the construction of external and internal power supply schemes. Typical graphs of electrical loads of the main technological installations are plotted to seek for an individual "trace" in the spectrum of higher harmonic components. The authors present the results of the bench experimental research into the influence of the output frequency of energy converters and the motor shaft torque on the values of higher harmonics and interharmonics. The voltage quality criterion affects the reliability index in the projected transition to decentralized principle of power systems construction and justification of physical mechanisms ensuring rational reliability indicators at low voltage quality. On the basis of the developed dependences, a specified technique is proposed to study electric power losses considering individual schedules of higher harmonics and remoteness of electromagnetic noise source. Recommendations are worked out for the implementation of energy-efficient modes of the enterprise operation, which can be realized by comprehensive consideration of reliability, quality of power supply and optimal flow of reactive power. Recommendations are also provided for the selection of compensating devices based on the number and power of converters.