Abstract: A method for measuring a fundamental frequency of a power system comprises sampling voltage and current waveforms associated with the power system, wherein the voltage and current waveforms are characterized by a fundamental frequency component that may vary over time. Next, determine whether, according to a prescribed frequency tracking criterion, a present value of the fundamental frequency can be measured using voltage samples. If so, measure the fundamental frequency using the voltage samples, and set a best available value (“freq”) equal to the measured value based on voltage samples. If the fundamental frequency cannot be measured using the voltage samples, determine whether, according to the prescribed frequency tracking criterion, the present value of the fundamental frequency can be measured using current samples. If so, measure the fundamental frequency using the current samples and set the best available value equal to the measured value based on current samples.