Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1999-03-05
2001-10-02
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C324S142000, C324S076390, C324S076110, C324S076490, C714S799000
Reexamination Certificate
active
06298309
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to digital protective relaying and metering systems, and more particularly to methods and apparatus for measuring the fundamental frequency of the voltage and current waveforms used by such systems to determine the number of samples per cycle when a fixed rate of sampling is used.
BACKGROUND OF THE INVENTION
Digital protection devices are used in power systems to protect electrical generators and other system components, such as transformers and transmission lines, using voltage and current waveforems. Such digital protection devices may be embodied in a generator protection unit (GPU), protective relay, or the like. For example, a typical GPU estimates voltage and current phasors that are determined from the “fundamental” frequency of the system, which is based upon voltage waveforms, and from such phasors detects various fault conditions that may require a circuit breaker to be tripped. The phasors are also used for metering and other power system control functions. Abnormal conditions may also be detected through relatively large changes in the generator's operating of fundamental frequency. Accordingly, a technique for frequency tracking is important for both deriving accurate voltage and current phasors and for monitoring fault conditions in the generator by detecting a significant change in frequency. Further background information concerning the present invention may be found in U.S. Pat. No. 5,832,413, Nov. 3, 1998, “Generator Protection System and Method for Phasor Estimation and Frequency Tracking During Frequency Ramping,” the disclosure of which is hereby incorporated by reference in its entirety.
In typical protection systems of the type discussed above, frequency is calculated using only measured system voltages. There are scenarios, however, in which the voltages are not available and hence the frequency cannot be calculated. This in turn prevents the calculation of many system-wide values needed for protection and metering.
SUMMARY OF THE INVENTION
The present invention makes use of current in addition to voltage to measure or calculate a power system's fundamental frequency. Briefly, in accordance with the present invention, upon detecting the unavailability of voltage (typically three-phase analog voltages), analog current inputs are used to measure the fundamental frequency. In the event that voltage values are not available and the analog currents become unstable, a “history” mechanism may be advantageously employed to maintain a correct frequency for a short period of time.
A presently preferred implementation of a method in accordance with the present invention is used to measure a fundamental frequency of a power system. The inventive method comprises the step of sampling voltage and current waveforms associated with the power system. The next step is to determine whether, according to a prescribed frequency tracking criterion, a present value of the fundamental frequency can be measured using voltage samples and, if so, to measure the fundamental frequency using the voltage samples, and to 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, the next step is to determine whether, according to the prescribed frequency tracking criterion, the present value of the fundamental frequency can be measured using current samples and, if so, to measure the fundamental frequency using the current samples and to set the best available value (“freq”) 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, we determine whether previously measured values of the fundamental frequency are stable according to a prescribed stability criterion and, if so, set the best available value (“freq”) to a value based on the previously measured values. The best available value of the fundamental frequency is used in subsequent calculations that include performing a Discrete Fourier Transform (DFT) using the best available value for the fundamental frequency.
These and other aspects of the present invention are explained below.
REFERENCES:
patent: 5721689 (1998-02-01), Hart
patent: 5736847 (1998-04-01), Van Doorn et al.
patent: 5809045 (1998-09-01), Adamiak et al.
patent: 5832413 (1998-11-01), Benco
patent: 5832414 (1998-11-01), Hart
patent: 6058134 (2000-05-01), Toivonen
patent: 6185508 (2001-02-01), Van Doorn et al.
Benco Joseph
Das Ratan
Downs Cliff
ABB Power T&D Company Inc.
Duong Khoi Hung
Hoff Marc S.
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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