Frequency measurement algorithm for generator-transformer unit adaptive protection

The proposed method of frequency measurement is based on the comparison of the voltage phasor position on the complex-plane every period of the input signal fundamental component. In case of the stable frequency the difference of the phasor layout in the following cycles is equal to zero. In other case the phase shift is proportional to the frequency variation. The voltage phasor phase angle is obtained from the orthogonal components - sinusoidal and cosinusoidal - which are the outputs of the orthogonal filters pair used also for the measurement of the signal amplitude. The correction factor df is added to the previous sample frequency f^(n-l). As a result one obtains frequency f and a following measurement time, which is inversely proportional to the frequency f. Therefore, the main problem is to determine the accurate value of the voltage phasor phase angle in the subsequent cycles of the frequency measurement, which requires the voltage orthogonal components determination. The proposed method of protective signals frequency determination uses the full-cycle, variable data window and may be applied in the adaptive algorithms of the generator-transformer protection.