Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
1998-08-14
2001-01-23
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C336S145000
Reexamination Certificate
active
06178386
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to preventing transformer breakdowns and, more particularly, to a method for determining the early location of faults in transformers to enable repair before their catastrophic failure.
Power utilities utilize large numbers of transformers in their power distribution system, and the failure of such transformers can cause power outages or substantial power fluctuation.
Frequently, transformer failure occurs as the result of breakdown in the insulation in the transformer tanks. A partial discharge (PD) occurs due to the breakdown of a small part of the transformer insulation, generally caused by the inclusion of moisture or the presence of a cavity. This activity results in an instantaneous shunting of a small partial capacitance which burgeons into catastrophic failure of the entire insulation. It has been recognized that incipient failures are frequently reflected by partial discharge pulses (PD) generated at the areas of insulation breakdown, and that, if such locations can be accurately identified through partial discharge signals, repairs can be made expeditiously and relatively economically to prevent catastrophic failure.
When a PD takes place within the confines of a transformer, it emits mechanical stress waves with subsequent resonance in the frequency range of 50-350 kHz. Sensors which are placed on the outside surface of the transformer tank, can detect the waves which may have propagated from the PD source through the internal core, winding, insulation materials and oil to the transformer tank wall. The waves detected by a piezoelectric sensor on the transformer tank wall have a waveform.
This signal often manifests a bursty nature and is a non-stationary random process. It also represents an admissible kernel representation for using wavelet transform (WT) in appropriate signal processing algorithms. High time-bandwidth, spread spectrum signals that experience time-scaling are difficult to decompose with narrowband analysis, such as Fourier transform, due to its sinusoidal kernel, which approximates the scaling effect with a Doppler shift. However, the WT, utilizes a more general analysis kernel, or mother wavelet.
In “
Electrical Power Transformers
” by R. H. Shertukde et al, Proceedings ICSPAT, Boston, October 1996, pages 1229-1986, there are described apparatus and methods for achieving online monitoring of transformers to identify incipient faults, and there is proposed a technique for utilizing wavelet transform techniques for increasing the accuracy of locating the incipient fault. Although such technology has been proposed as providing a more reliable measurement, the costs associated with the electronics to achieve the desired processing of a large number of signals from the multiplicity of ultrasonic sensors needed has represented a substantial impediment to widespread usage as has been the need to process the signals effectively.
It is an object of the present invention to provide a novel method for detecting and determining the position of partial discharge signals in a transformer tank.
It is also an object to provide such a method which can be produced readily, reliably and relatively economically.
Another object is to provide novel apparatus to conduct the detection and position determination.
A further object is to provide such apparatus which may be fabricated from readily available components at a reasonable cost to enable its widespread use.
SUMMARY OF THE INVENTION
It has now been found that the foregoing and related objects may be readily attained in a method for determining the position of a partial discharge fault in a transformer tank in which supersonic sensors are secured on a transformer tank at a multiplicity of points spaced about the periphery thereof. A threshold amplitude and a frequency range for pulse vibrations to be evaluated are established, and supersonic vibrations are sensed at the multiplicity of points. The signals from the sensors are transmitted to an interface at which the signals from the multiplicity of points are multiplexed, synchronized and localized, following which they are transmitted to a processor.
The multiplexed signals are then processed by steps including:
1. Initially, there is a determination of the existence of triggering pulse vibrations exceeding the established threshold amplitude and within the established frequency range.
2. A wavelet transform is then conducted on the multiplexed signals from the sensors at the multiplicity of points, two at a time with one signal being the signal from the first sensor found to provide a signal above the threshold amplitude. This wavelet transform provides both frequency and time domain.
3. A scaling factor and translation parameters associated with the frequency of the vibrations are applied to obtain estimates of the time delays for the triggering pulse vibrations detected at the multiplicity of points.
4. These time estimates from the transformed and scaled signals are then evaluated to determine the position of the partial discharge fault which generated the triggering pulse vibrations.
The processing step initially generates a triggering signal to the interface upon sensing a signal of greater amplitude than the predetermined value and initiates the further processing steps to determine the position of a fault.
In the processing step, the multiplexed signal are passed into a data acquisition board which samples signals in several channels at a predetermined minimum sampling frequency per channel which is determined by the Nyquist Criterion. A clock circuit is employed to time the signals being processed in the multiplexing and processing steps.
REFERENCES:
R.H. Shertukde, H.M. Shertukde “Manufacture of Fault Diagnostic Device for Electrical Power Transformers (FD2-EPT)”.
Hemchandra M. Shertukde, Hisham Alnajjar, Uday Prabhune “Fault Detection Device For Electrical Power Transformers Using Noval DSP Scheme”.
Alnajjar Hisham
Shertukde Hemchandra M.
Assouad Patrick
Pepe & Hazard LLP
Raymond Edward
The University of Hartford
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