Abb Power Td & Company

Joseph P Benco

James Stoupis

David G Hart

Yi Hu

Ratan Das

Cliff Downs

Frederick P Perfect

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 (&ldquo;freq&rdquo;) 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. If the present value of the fundamental frequency cannot be measured using the voltage or current samples, determine whether previously measured values of the fundamental frequency based on current are stable according to a prescribed stability criterion and, if so, set the best available value to a value based on the previously measured values for a limited period of time.

Current based frequency tracking method and apparatus

A protective relay system comprises current and voltage transducers 10, filters 12, and a multiplexor 14, the latter outputting an interleaved stream of analog phase current and voltage signal samples, as well as neutral current samples. The analog multiplex output by the multiplexor 14 is digitized by an analog-to-digital converter 16. The output of the analog-to-digital converter 16 is fed to a digital signal processing block 18. The multiplexor necessarily introduces a time-skew between the successive samples for each channel and also introduces a time-skew between the respective channels. The system corrects the sample-to-sample time-skew for each channel, and then derives current and voltage phasors from the time-skew corrected data.

Double interpolation anti-skew compensation of sampled analog data

A method and system for detecting the peak of respective half-cycles of a sinusoidal waveform. The sinusoidal waveform is rectified and only sample points above a first threshold are analyzed. The peaks of each half-cycle are only accepted if they are above a second, greater threshold. Peaks below the second threshold are not accepted. The peaks themselves are detected by starting a counter at the first sample in the rectified waveform which is above the first threshold value. Once the rectified waveform descends below the first threshold, the counter is stopped and the maximum value of the previous k samples is the peak value, where k is the counter value for the successive samples above the first threshold. The peak value so determined is rejected if it is less than the second threshold value. On the other hand, when the peak value is above the second threshold, it is averaged with the last valid peak value (above the second threshold). Thus, the V/Hz value remains the same until a new, valid peak is determined. A preferred embodiment implements such a digital peak detector in a digital relay system for detecting overexcitation of a power system.

Digital peak detector

A method for estimating phasors and tracking the frequency of a signal during frequency ramping is provided. The method uses a variable N-point DFT periodically to compute one or more phasors based on data acquired from one or more sampled signals. The period between DFT computations is a predetermined number of sample periods. After each DFT computation, the change in phasor angle between the current phasor estimate and the most previous phasor estimate is determined and used to estimate the instantaneous frequency of the signal. The current instantaneous frequency estimate and the most previous instantaneous frequency estimate are averaged to compute an average cycle frequency. In addition, a number of discrete frequencies and corresponding DFT windows based on a fixed sampling rate and a predetermined fundamental frequency of the signal are defined and used in estimating the instantaneous frequency. Once the average cycle frequency is determined the DFT window is adjusted by setting it equal to the DFT window corresponding to the discrete frequency closest to the average cycle frequency. A generator protection system analyzing voltage and current signals output from a generator is also provided. The voltage and current signals are monitored by respective voltage and current sensors to produce a plurality of signals representative of voltage and current characteristics of the generator. The system comprises components for receiving and sampling signal input, and for processing the samples according to the inventive method.

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Germansville, PA, United States of America

Joseph P Benco