Frequency tracking function in numerical protection devices

This paper presents a sampling frequency tracking principle that is effective from 10 Hz to 90 Hz. Tracking functionality is essential for modular relays where a single protection terminal can be equipped with expansion modules for multiple current transformers (CTs) and voltage transformers (VTs) connected to circuits where the frequency can differ significantly. In applications with process bus, a single relay may be subscribed to the sampled measured values (SMV) provided by merging units from different vendors.

This paper discusses protection applications with multiple distributed generation resources where up to six frequency tracking groups can be implemented in one relay. Frequency tracking functionality is beneficial for motor protection applications since a slowing motor gradually decreases voltage and frequency on the disconnected circuit. The authors also discuss measured value acquisition process in the numerical devices, and behavior of the sampling frequency tracking algorithm.