Frequency estimation by demodulation of two complex signals

This paper presents a method for frequency estimation in power system by demodulation of two complex signals. In power system analysis, the @transform is used to convert three phase quantities to a complex quantity where the real part is the in-phase component and the imaginary part is the quadrature component. This complex signal is demodulated with a known complex phasor rotating in opposite direction to the input. The advantage of this method is that the demodulation does not introduce a double frequency component. For signals with high signal to noise ratio, the filtering demand for the double frequency component can often limit the speed of Ihe frequency estimator. Hence, the method can improve fast frequency estimation of signals with good noise properties. The method looses its benefits for noisy signals, where the filter design is governed by the demand to filter harmonics and white noise. The method has been previously published, but not explored to its potential. The paper presents four examples to illustrate the strengths and weaknesses of the method.