Data processing: generic control systems or specific application – Specific application – apparatus or process – Electrical power generation or distribution system
Reexamination Certificate
1998-04-15
2001-01-09
Sheikh, Ayaz R. (Department: 2781)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Electrical power generation or distribution system
C700S292000, C700S293000, C700S297000, C700S298000, C702S059000, C702S064000, C702S066000, C702S072000, C702S077000, C361S080000, C361S086000, C361S088000
Reexamination Certificate
active
06173216
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to protective relaying, and more particularly to a microprocessor- or DSP-based protective relay with an improved cosine filter function.
BACKGROUND OF THE INVENTION
Electrical transmission lines and power generation equipment must be protected against faults and consequent short circuits, which can cause a collapse of the power system, equipment damage, and personal injury. It is the function of the protective relays, which monitor AC voltages and currents, to locate line faults and initiate isolation by the tripping of circuit breakers. Protective relays generally perform one or more of the following functions: (a) monitoring the system to ascertain whether it is in a normal or abnormal state; (b) metering, which involves measuring certain electrical quantities for operational control; (c) protection, which typically involves tripping a circuit breaker in response to the detection of a short-circuit condition; and (d) alarming, which provides a warning of some impending problem. Fault location, e.g., is associated with the protection function and involves measuring critical system parameters and, when a fault occurs, quickly making a rough estimate of the fault location and of certain characteristics of the fault so that the power source can be isolated from the faulted line. Thereafter, the system makes a comprehensive evaluation of the nature of the fault.
Modern protective relays employ microprocessors and/or digital signal processors (DSPS) to process the voltage and current waveforms measured on the protected transmission line (the term “transmission line” as employed herein is intended to cover any type of electrical conductor, including high power conductors, feeders, and transformer windings). Such processing may include the computation of a DFT. For example, U.S. Pat. No. 5,592,393, Jan. 7, 1997, titled “Method and System for Providing Protective Relay Functions,” describes a system that uses the DFT function to compute instantaneous values of fundamental, second harmonic and fifth harmonic components. U.S. Pat. No. 5,172,329, Dec. 15, 1992, “Microprocessor Digital Protective Relay for Power Transformers,” describes a system that uses the DFT function to compute voltage and current phasors.
The conventional DFT exhibits poor performance if the input signal contains a decaying DC component having a continuous frequency spectrum. Therefore, the DC signal component, or offset, is typically filtered out of the input signal before the DFT function is carried out. There are a number of the methods to deal with such DC offset, including the use of (1) a digital mimic circuit, (2) half-cycle and full-cycle compensation, (3) a parallel filter, and (4) a cosine filter. However, certain problems are associated with each of these methods. U.S. patent application Ser. No. 08/811,646, filed Mar. 5, 1997, “Protective Relay With Improved DFT Function,” discloses an improved DFT function in which the decaying DC components are subtracted from the normal current and voltage phasors to yield modified phasors that are free of the effects of the decaying DC components. The present invention is directed to an alternative approach involving the use of an improved cosine filter.
Since distance relaying involves the use of voltage and current phasors to determine whether a fault is in the protected zone, it is imperative that the phasor estimates be as accurate as possible. The voltage signal may contain high-frequency components that can be filtered using an anti-aliasing filter and through least squares methods such as the DFT. However, the current may contain a decaying DC component what will cause the DFT to erroneously calculate the current phasor. The current phasor estimate will typically be inflated; therefore, the ratio of the voltage to the current will yield a small impedance value, causing the relay to trip when the fault is actually beyond the relay setting. This phenomenon is known as relay over-reach since the relay reaches beyond the setting and trips. Those skilled in the art recognize that the cosine filter provides one mechanism for removing the DC component of the current and thereby improving the current phasor estimate. Unlike the DFT, the cosine filter estimates only the cosine component of the current signal. The complete DFT is obtained by using two estimates shifted by 90 degrees, yielding the cosine and sine components. The cosine filter has been demonstrated for four samples per 60 Hz cycle. The principle is proven by approximating the decaying DC current term by the first two terms in the exponential series (a ramp function) as described in greater detail below. The object of the cosine filter is to sum the DC component such that it sums to zero. This concept can be extended to higher sampling rates. The problem is to select the cosine filter offset angle such that the DC ramp input sums to zero. The present invention is directed to providing a novel, improved cosine filter.
SUMMARY OF THE INVENTION
Accordingly, a primary object of the present invention is to provide an improved cosine filter and protective relay utilizing the improved cosine filter. The invention is especially intended for protective relaying applications in which accurate voltage and current phasors must be derived.
A method or system for deriving a phasor representation of a current or voltage waveform in accordance with the present invention comprises the steps of, or means for, measuring time-domain samples (v(k), i(k)) of the waveform; computing, on the basis of the samples, a phasor representation of the waveform, the phasor representation being computed on the basis of a cosine filtering process; and performing a prescribed protective relaying function using the phasor. The prescribed relaying function may include, e.g., fault typing and/or fault location, although many other applications for phasors are known. In a preferred embodiment of the invention, the phasor representation is by a method comprising steps S
1
through S
5
, as follows: (S
1
) applying an N-point cosine filter to a window of data points x(0), x(1), . . . x(N−1), wherein the data points comprise measured and digitized current and/or voltage samples; (S
2
) applying an N/2-point cosine filter to the same window of data points, but using only odd-numbered points x(0), x(2), x(4), etc.; (S
3
) applying an N/4-point cosine filter to the same window of data points, using only a subset of the data points used in step S
2
; (S
4
) applying an N/8-point cosine filter to the same window of data points, using a subset of the data points used in step S
3
; and (S
5
) using a least squares fit to estimate the phasor.
Other features of the invention are disclosed below.
REFERENCES:
patent: 5172329 (1992-12-01), Rahman et al.
patent: 5343404 (1994-08-01), Girgis
patent: 5406495 (1995-04-01), Hill
patent: 5428549 (1995-06-01), Chen
patent: 5453903 (1995-09-01), Chow
patent: 5537327 (1996-07-01), Snow et al.
patent: 5592393 (1997-01-01), Yalla
patent: 5671112 (1997-09-01), Hu et al.
patent: 5796629 (1998-08-01), Calero et al.
patent: 5909656 (1999-06-01), Yang
patent: 5995911 (1999-11-01), Hart
Schweitzer, III, et al., “Filtering for Protective Relays”, 19th Annual Western Protective Relay Conference, Spokane, Washington, Oct. 20-22, 1992, 1-24.
Zegarra, “Microprocessor-Based Time-Overcurrent Relay: Development, Simulation, Design and Testing”, A Thesis Presented to the Faculty of the Graduate College of Ohio University, Jun., 1979, 1-145.
Hart David G.
Novosel Damir
Vu Khoi
ABB Power T&D Company Inc.
Jean Frantz Blanchard
Sheikh Ayaz R.
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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