A Demodulation Technique For Power System Local Frequency And Voltage Measurements
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The demodulation technique of two complex signals is used to estimate the power system local frequency and voltage phasor. The first complex signal is generated from the three-phase voltage waveform by using the ab-transformation, while the second complex signal is assumed to have a one per unit amplitude and to rotate in a direction opposite to the original three-phase voltage signals with nominal frequency wo(negative sequence signal). Use of the demodulation technique does not introduce a double frequency component, which limits the speed of the frequency estimator especially, if the signal has a high signal to noise ratio. Two closed form formulae are derived based on the least error squares (LES) algorithm, for system frequency as well as its phase angle. It is shown that using such a technique, a very short data window size can be used to measure the power system frequency. To be specific, a minimum of two samples data window is enough for the noise free signals.