ABSTRACT
As microprocessor-based protective relays appeared on the market, the situation changed radically. The algorithm of frequency measurements in digital protective relays is related to the allocation of transition points of a sine wave through the zero value that allows eliminating the effect of sinusoid distortion on the accuracy of the frequency measurement. Relay protection constitutes a major part of any power system that provides for continuous control of the main operation modes of power system elements and generates tripping commands for the failed parts or elements of the system. Faulty operation of relay protection owing to internal malfunctions can lead to the development of massive failures and even to the collapse of the power system with huge attendant financial losses. The digital recorders with high-resolution capabilities in a memory-recording mode enable determining the time intervals of the low-frequency signals with accuracies of parts of a millisecond.
