Method and apparatus for the detection and location of faults and partial discharges in shielded cables

Abstract

A method and apparatus for locating an incipient fault at a point along the length of an insulated power line includes the application of an excitation voltage at an open end of the power line, and the signal pulse transmitted along the power line to the open end is passed through a high pass filter to remove the portion of the signal which is at a frequency below the excitation voltage and its harmonics. The filtered signal is amplified and passed through a band pass filter to remove a high frequency portion of the signal containing a large proportion of noise relative to the frequency of the partial discharge frequency from the incipient fault. This filtered signal is passed to a digital storage device adapted to be triggered by a signal of a predetermined amplitude, and the triggered digital storage device receives the amplified signal directly from the amplifier and stores digital data concerning amplitude and time for the peaks of the amplified signal for a predetermined period of time. The stored digital data is processed to identify the peaks reflecting the point of partial discharge in the power line.

Description

This application is a reissue application of Serial No. 07/839,914, filed Feb. 21, 1992, now U.S. Pat. No. 5,272,439.19A-RSC18k 19K. The estimation method used in the program is least mean squared error.

SimulPD(sub)

This program is illustrated in FIGS. 20a-20b and is a subroutine of the preceeding program. It simulates a PD emanating from any desired position along the cable. The displayed information is shown in FIG. 20c.

IntpPeak

This program and its displays are illustrated in FIGS. 20a-20c and accepts PD signals recorded (a sequence of peaks), interpolates peak position in the PD signal, and calculates automatically the time locations of the initial three peaks.

Trans,AV

This program is illustrated in FIGS. 22a-22o, and it accepts data obtained by the DSO when operating in the "external trigger" mode. It lines up the first peaks of 40-100 frames of PD signals obtained on repeated triggering, averages the data and places the information on the computer hard disk. This virtual instrument transfers data from Nicolet DSO and performs averaging, and saves the original data and the averaged data on the hard disk. The visual display is seen in FIG. 22p.

Time-Int(sub)

This subroutine is illustrated in FIGS. 23a-23e, and it can be used in conjunction with IntpPeak.vi (item 3) to compute the zero crossing of a PD wavelet and provide a mathematical estimate of the wavelet in order to determine the location of its peak. This virtual instrument can be used to find the zero crossing of a waveform and to obtain the waveform between two peaks (first and second, second and third). Its visual display is illustrated in FIG. 23f.

XcorrCoeff(sub)

This is a subroutine is illustrated in FIG. 24a, and it is used with several of the programs listed previously. It allows a cross correlation function to be developed between two sets of PD signals, and computes the cross correlation coefficient between the two input signals. Its display is illustrated in FIG. 24b.

Open&Read

This program is illustrated in FIG. 25a and it opens a data file and outputs the data into an array in a normalized form. Its display is seen in FIG. 25b.

Although the method of the present invention has previously been described as applicable to cables in residential areas operating at 60 Hertz and subject to broadcast noise, it will be appreciated that it is also applicable to cables carrying current at other frequencies and to use in coping with noise from other sources and other frequencies.

The number of discrete segments of the signal stored by the DSO and processed should be at least 10 and preferably at least 20. There is no real benefit to be obtained by averaging more than 60 segments.

The timeframe for each segment should be on the order of 5-10 microseconds for the length of cables which will normally be found in service, and there is little to be gained by longer periods.

As will be appreciated, the DSO includes circuitry providing a short delay in processing a signal received directly from the amplifier through the isolation transformer. This may be set to account for the time delay required for the same portion of the signal to pass through the band pass filter and trigger the recording function of the DSO. Typically, this will be a fraction of a microsecond.

It can be seen that the instrumentation and the measurement method of the present invention are non destructive since the excitation voltage level may be at or below the normal service voltage of the cable system. Accordingly, it need not introduce destructive electrical stresses into the system. The instrumentation required is designed for field use with cables buried underground, and all measurements may be done in situ, rather than on a cable specimen in the laboratory. Moreover, all measurements may be done with no more than one sensor connected at one end at the same time. In a complex cable system, with multiple lateral branches, there may be a need to conduct measurements at multiple ports: if so, such measurements should be carried out sequentially with only one sensor in use at any time.

The instruments and software measure and analyze the spectral content of the electrical noise prevailing at the test site, and utilize line and notch filters to eliminate offending portions of the noise spectrum.

With the trigger synthesized by using the suitably filtered signal, triggering of the DSO occurs only on genuine PD waveforms even when the PD is buried in the noise. The method and apparatus provide electronic and software related SNR (signal to noise ratio) enhancement techniques to produce clean, readily processible PD signals. The individual filtered frames are readily aligned to permit many frames to be averaged to cancel out noise and enhance the desired PD signals.

The adaptive digital signal processing technique requires much less operator intervention and can make use of the transfer function of the cable for added accuracy. Accordingly, it may include construction of an accurate cable model capable of generating signals which simulate actual PD signals emanating from an arbitrary location along the cable. It may also include the development of a cable transfer function by the analysis of the cable response to either an injected low voltage pulse at one end as shown in the aforementioned Mashikian et al Patent, or introduction of an actual PD signal at one end by placing a special defective insulation system (DOE PD cell) and exciting it at a moderate power frequency voltage. In addition, it may include detection and location of the PD site(s) along the cable length through repeated cross correlation operation (performed by software) between the measured waveform and the model of a PD signal emanating from location x=x₁, with x₁ being varied in small increments to cover the cable length.

The procedure incorporates an adjustable parameter K (open loop gain) in the cable model for increased robustness of the algorithm, and a systematic procedure for determining the adjustable parameter K from the measured data. It also enables use of a new technique for finding the actual round trip delay time from the measured data, a new procedure for making the initial estimate for the round trip time from the measured data, and a new technique for estimating the location of the peak of a rounded or attenuated waveform. Lastly, it provides a new position calibrator configuration for coupling pulses into a buried cable, or receiving pulses from the buried cable by means of an antenna like device.

Thus, it can be seen that the method and apparatus of the present invention provide an effective means for determining the location of an incipient fault in a power cable in an environment where the PD signal is obsecured by electrical noise. The apparatus is of relatively simple construction and operation, and the method requires minimal operator intervention.

Claims

1. In a method for locating an incipient fault at a point along the length of an insulated power line, the steps comprising:

(a) applying an excitation voltage at an open end of said power line to produce a partial discharge signal pulse at a fault in the power line;

(b) passing said partial discharge signal pulse transmitted along said power line to an said open end of the line through a high pass filter to remove the portion of said signal pulse which is at a frequency below the excitation voltage and its harmonics;

(c) amplifying the filtered signal pulse to produce an amplified signal pulse;

(d) passing the amplified signal pulse through a band pass filter to remove a high frequency portion of the signal containing a large proportion of noise relative to the partial discharge signal pulse occupying the same frequency band and to provide produce a doubly filtered signal pulse;

(e) passing the doubly filtered signal pulse from the band pass filter to a digital storage device triggered by a doubly filtered signal pulse of a predetermined amplitude;

(f) triggering said digital storage device by a doubly filtered signal pulse of at least said predetermined amplitude, said device when triggered receiving said amplified signal pulse directly from said amplification step and storing digital data concerning amplitude and time of said peaks of the amplified signal pulse for a predetermined period of time; and

(g) processing the stored digital data to identify the peaks reflecting the point of partial discharge in said power line.

2. The method for locating an incipient fault in accordance with claim 1 wherein steps (d), (e) and (f) are repeated to store data from a predetermined number of discrete time periods; and the stored digital data from said predetermined number of time periods is processed.

3. The method for locating an incipient fault in accordance with claim 2 wherein said processing step includes aligning the first peaks of the stored data for said number of triggered time periods.

4. The method for locating an incipient fault in accordance with claim 3 wherein the stored data concerning amplitude for the peaks at the same time periods is averaged to identify the peaks and time for the signals of the point of incipient fault.

5. The method for locating an incipient fault in accordance with claim 2 wherein said predetermined number of time periods is at least 25.

6. The method for locating an incipient fault in accordance with claim 1 wherein said digital storage device, upon triggering, is operative to record data in that portion of the doubly filtered signal pulse which has passed through said band pass filter to produce the triggering signal.

7. The method for locating an incipient fault in accordance with claim 6 wherein the amplified signal pulse entering said digital storage device is time delayed to enable said doubly filtered signal pulse to trigger said digital storage device.

8. The method for locating an incipient fault in accordance with claim 1 wherein said amplified signal pulse has rounded peaks of reduced amplitude.

9. The method for locating an incipient fault in accordance with claim 8 wherein said processing step includes the estimation of the peaks in the amplified signal pulse prior to rounding.

10. The method for locating an incipient fault in accordance with claim 9 wherein said estimation employs second order equations for increase and decay of the signal amplitude.

11. The method for locating an incipient fault in accordance with claim 1 wherein said processing determines the time location of an initial series of at least two peaks.

12. The method for locating an incipient fault in accordance with claim 11 wherein the initial series comprises three peaks.

13. The method for locating an incipient fault in accordance with claim 1 wherein the processed data is further processed to estimate the position of the point of partial discharge along the length of the power line.

14. The method for locating an incipient fault in accordance with claim 1 wherein there is included an initial step of injecting an artificial partial discharge signal pulse at one end of the power line and processing the signal pulse received at said one end of the power line after multiple reflections at both ends to produce a mathematical model simulating a partial discharge input at said one end for the simulation of a partial discharge at any point along the length of the power line.

15. The method for locating an incipient fault in accordance with claim 14 wherein said step of processing of said digital data from said digital storage device analyzes said stored digital data and said mathematical model to estimate the point of the partial discharge.

16. The method for locating an incipient fault in accordance with claim 1 wherein said power line is carrying current at 60 Hertz ,and said high pass filter removes that portion of the signal partial discharge partial discharge signal pulse which is below about 10 kHz.

17. The method for locating an incipient fault in accordance with claim 1 wherein said band pass filter removed the portion of the amplified signal pulse above about 450 kHz.

18. The method for locating an incipient fault in accordance with claim 1 wherein said time period is at least about 5 microseconds.

19. The method for locating an incipient fault in accordance with claim 1 wherein there are included the steps of:

(a) applying an excitation voltage electric pulse at one end of said insulated power line;

(b) placing a pulse receiver at a point along the length of said power line spaced a known lineal distance from said one end;

(c) receiving the signal said electric pulse generated by said excitation voltage; and

(d) processing said excitation voltage signal received electric pulse to obtain data reflecting representing amplitude and time of signal peaks corresponding to the said known lineal distance.

20. The method for locating an incipient fault in accordance with claim 19 wherein there is included the step of comparing the data obtained from said known distance processing with the data reflecting representing the point of partial discharge to indicate the relative position direction of said point of partial discharge.

21. The method for locating an incipient fault in accordance with claim 1 wherein there are included the steps of preliminarily applying an excitation voltage of zero, amplifying the signal pulse received at said open end, and analyzing said amplified signal pulse to evaluate its frequency components.

22. The method for locating an incipient fault in accordance with claim 21 in which the frequencies obtained from the evaluation of said signal pulse using the zero excitation voltage are utilized to establish the frequencies to be set in digital notch filters for the signal pulse.

23. The method for locating an incipient fault in accordance with claim 22 wherein the frequencies are used to adjust the parameters in the processing of the stored data to create adaptivity the proper digital notch filtering of said stored data during the processing of said data to eliminate undesirable noise frequencies.

24. The method for locating an incipient fault in accordance with claim 1 wherein there are included the steps of:

(a) placing a pulse receiver at said open end of said power line;

(b) applying an excitation voltage at a point along the length of said power line spaced a known lineal distance from said open end thereof;

(c) receiving the signal pulse generated by said excitation voltage; and

(d) processing said excitation voltage signal pulse to obtain data reflecting amplitude and time of signal peaks corresponding to the known distance.

25. The method for locating an incipient fault in accordance with claim 24 wherein there is included the step of comparing the data obtained from said known distance processing with the data reflecting the point of partial discharge to indicate the relative position of said point of partial discharge.

26. Apparatus for locating an incipient fault at a point along the length of an insulated power line, comprising:

(a) means for applying an excitation voltage at an open end of the power line to produce a partial discharge signal pulse at a fault in the power line;

(b) means at an said open end of the power line for receiving and analyzing said signal pulse transmitted along the power line to an open end of the power line including:

(i) a high pass filter to remove the portion of the signal pulse which is at a frequency below the excitation voltage and its harmonics;

(ii) an amplifier for amplifying the filtered signal pulse to produce an amplified signal pulse;

(iii) a band pass filter to remove a high frequency portion of the amplified signal pulse containing a large proportion of noise relative to the frequency of the partial discharge signal pulse occupying the same frequency band and to produce a doubly filtered signal pulse;

(iv) a digital storage device triggered by a doubly filtered signal pulse of a predetermined amplitude from the band pass filter, said device receiving the amplified signal pulse directly from the amplifier and storing digital data concerning amplitude and time of the peaks of the amplified signal pulse for a predetermined period of time; and

(c) a processor for processing the stored digital data to identify the peaks reflecting the point of partial discharge in the power line.

27. The apparatus for locating an incipient fault in accordance with claim 26 wherein there is included control means to permit triggering of said storage device and storage of data and said processor processes the data from the predetermined number of periods.

28. The apparatus for locating an incipient fault in accordance with claim 27 wherein the stored data concerning amplitude for the peaks at the same time periods is averaged to identify the peaks and time for the signals of the point of incipient fault.

29. The apparatus for locating an incipient fault in accordance with claim 26 wherein said digital storage device, upon triggering, is operative to record data in that portion of the amplified signal pulse which corresponds to the doubly filtered signal pulse passing through said band pass filter to produce the triggering signal.

30. The apparatus for locating an incipient fault in accordance with claim 29 wherein said apparatus includes time delay means for the amplified signal pulse entering said digital storage device.

31. The apparatus for locating an incipient fault in accordance with claim 29 wherein said processor aligns the first peaks of the stored data for said number of triggered time periods.

32. The apparatus for locating an incipient fault in accordance with claim 26 wherein said band pass filter removes the portion of the amplified signal pulse above about 450 kHz.

33. The apparatus for locating, an incipient fault in accordance with claim 26 wherein the doubly filtered signal pulse from said amplifier has rounded peaks of reduced amplitude, and the processor estimates the peak of the signal.

34. The apparatus for locating an incipient fault in accordance with claim 26 wherein the processor further processes the data to estimate the position of the point of partial discharge along the length of the power line.

35. In a method for locating an incipient fault at a point along the length of an insulated power line buried in a trench in the ground, the steps comprising:

(a) applying an excitation voltage at an open end of said power line to produce a partial discharge signal pulse at a fault in the power line and a signal pulse reflected from the other end of said power line;

(b) processing data with respect to the partial discharge signal pulse and reflected pulse to determine initially the relative position of said fault along the length of said power line;

(c) placing a pulse receiver above the trench in which said power line is buried, at a point spaced a known lineal distance along the length of said trench from said open end of said power line and approximating the initially determined position of said fault along said power line;

(d) applying an electric pulse to said open end of said insulated power line;

(e) detecting by said receiver at said point along the length of the trench the applied electric pulse and the reflection of said electric pulse from said other end of said power line; and

(f) processing said applied and reflected electric pulses detected by said receiver to obtain data representing amplitude and time of the signal peaks corresponding to the known distance along said length of said trench.

36. The method for locating an incipient fault in accordance with claim 35 wherein there is included the step of (g) comparing the data obtained from said processing of said applied electrical and reflected pulses with the data representing said initially determined relative position of said fault to indicate the direction of said fault along the length of said power line relative to the point at which said receiver is placed along the length of said trench.