Ultrasonic transducer for monitoring acoustic emissions

#The present invention relates, in general, to electrical monitoring devices and, more particularly, to devices for monitoring acoustic events within liquid and/or gas filled apparatus.

#The monitoring of high voltage, oil-filled electrical apparatus, such as transformers, reactors and capacitors, for the occurrence of acoustic events therein, is very desirable from a quality assurance standpoint. Such events could result from partial electrical discharges, magnetic domain switching (Barkhausen effect), high frequency mechanical vibrations, transformer tap changing, rainstorm or thunderstorm effects, and the like. With respect to partial electrical discharges, such discharges can degrade the insulation within the high voltage electrical apparatus causing the eventual breakdown of same. Thus, it is desirable to provide a device for detecting partial electrical discharges before such discharges increase to a level which may cause extensive damage to the insulation within the high voltage apparatus.

#Various devices and methods are available for monitoring the occurrence of partial electrical discharges within oil-filled electrical transformers and high voltage capacitors. For example, a typical prior art method for detecting and locating partial electrical discharges within a transformer uses the electrical signal produced by the discharge. Another approach utilizes a signal responsive to the pressure pulse induced in the oil from the discharge to determine the intensity and approximate location of the partial electrical discharge within the transformer.

#FIG. 1 is a cross-sectional view of the monitoring probe of the present invention.

#FIG. 2 is a cross-sectional view of the monitoring probe of the present invention installed within an oil-filled transformer.

#Referring now to the drawings where the illustrations are for the purpose of describing the preferred embodiment of the present invention and are not intended to limit the invention described herein, FIG. 1 is a cross-sectional view of a probe 10 which can be utilized to monitor and detect acoustic events, such as those resulting from partial electrical discharges within an oil-filled apparatus. The probe 10 includes outer jacket 12 comprising an elongated body portion 14, an end portion 16 having a square, hexagonal or octagonal configuration permitting the gripping of same, and a threaded portion 18 interposed between the elongated body portion 14 and the end portion 16. A blind bore 20 is provided within the elongated body portion 14 and within a portion of the threaded portion 18. A counter bore 22 is provided within the end portion 16 and terminates in the blind bore 20. A portion of counter bore 22 is threaded to receive a mating threaded cap 24. It should be noted that the foregoing outer jacket 12 may be similar to that utilized in so called "thermal wells" which are typically inserted into oil-filled transformers to measure the temperature of the oil therein.

#The blind bore 20 and the counter bore 22 are concentric with a longitudinal axis of the probe 10. The tip 26 of the elongated body portion 14 has a convex configuration forming a lens (fish-eye) whose function will be described later herein. A piezoelectric crystal 28 is received within blind bore 20 and a surface 30 of the crystal 28 contacts the bottom 32 of blind bore 20. A rod 34 having a diameter slightly less than the diameter of the blind bore 20 is received within bore 20 and its end 36 contacts surface 38 of the piezoelectric crystal 28. The opposite end 40 of rod 34 contacts an insulating washer 42 which is received within counter bore 22 and interposed between rod 34 and threaded cap 24 after cap 24 is threadably received within counter bore 22. Tightening of the cap 24 within the outer jacket 12 causes the rod 34 to apply a compressive force to the piezoelectric crystal 28 resulting in a firm electrical contact between end 36 of rod 34 and surface 38 of crystal 28 and between surface 30 of crystal 28 and bottom 32 of blind bore 20. An electrical conductor 44 is connected to end 40 of rod 34 and is received through apertures 46 and 48 in washer 42 and cap 24, respectively, for a connection to a detection device (not shown). Rod 34 and/or elongated body portion 14 can be threaded for all or a portion of their respective lengths to improve the acoustic performance of the probe 10 by suppressing undesired signals, as hereinafter described. Regardless of whether the rod 34 is threaded for all or a portion of its length, strips of tape can be applied to the surface thereof in a spaced-apart relationship to electrically insulate the rod 34 from the outer jacket 12. The end 40 of rod 34 is electrically insulated from the jacket 12 by insulating washer 42.

#Referring now to FIG. 2, the installation of the probe 10 within an oil-filled transformer 60 is illustrated. The probe 10 is inserted through an aperture 62 within a wall 64 of transformer 60 and is threadably retained therein by threaded portion 18 which engages mating threads provided within aperture 62. The position of the aperture 62 within wall 64 is such that elongated body portion 14 of probe 10 is immersed in the oil 66 within the transformer 60, and end portion 16 of probe 10 is positioned adjacent to and exteriorly of the wall 64 of the transformer 60.