The algorithms of numerical protective relays are very sensitive to variations in the power system frequency during operation. Even slight changes of the power frequency can lead to large calculation errors in the values for impedances, the power factor, etc. Implementing an adaptive sampling rate requires knowing the actual system frequency at all times with high accuracy. A new frequency measurement method has been successfully implemented. Since the method is not dependent upon zero-crossings of the measured current, it can calculate the frequency after every sample. In addition, the necessary signal-window is approximately 3 cycles, which means that the calculation is faster than all classical methods. The new method has made possible the development of a full-featured generator protection relay that can operate over a frequency range of 15 Hz-70 Hz. The accuracy of the frequency calculation is better then 10 mHz for a nominal frequency of 60 Hz.
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