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This paper present three methods for tracking the fundamental frequency of power systems in digital protection relaying, which is based on numerical differentiation, hardware and digital filtering respectively. The numerical differentiation method is adaptive only to pure sinusoidal cases, and hardware and digital filtering method is used not only for pure sinusoidal cases but also non-sinusoidal cases. The numerical differentiation method is characteristic of high accuracy, lesser time and computation. The hardware and digital filtering method employs hardware and digital filter to decompose the fundamental component out of the non-sinusoidal signals, and then uses numerical differentiation algorithm to track the fundamental frequency.