WO1994028566A1

WO1994028566A1 - Partial discharge passive monitor

Info

Publication number: WO1994028566A1
Application number: PCT/AU1994/000263
Authority: WO
Inventors: Joseph Unsworth; Joseph Stanley Kurusingal; Ronald Edward James
Original assignee: University Of Technology, Sydney; Pacific Power
Priority date: 1993-05-20
Filing date: 1994-05-20
Publication date: 1994-12-08

Abstract

An apparatus for the monitoring of partial discharge in on-line high voltage electrical equipment such as transformers. The apparatus consists in transducer means (11), amplifying means (21) and an analyser means (15). The transducer means (15) converts directed radio frequency waves and ultrasonic pressure waves into electrical signal outputs, which outputs are then amplified and analysed so as to determine if partial discharge is occurring. The apparatus comprises a means for minimising interference (55, 12) in the electrical signal outputs, and is adapted such that the transducer means (15) is capable of being mounted within the wall (12) of the electrical equipment.

Description

PARTIAL DISCHARGE PASSIVE MONITOR

FIELD OF THE INVENTION The present invention relates generally to the detection of partial electrical discharges in high voltage electric equipment. In particular, the invention provides an apparatus for the monitoring of partial discharge in on-line high voltage equipment such as transformers and switchgear.

BACKGROUND ART

High voltage transmission and generator transformers form an essential and integral part of any electrical power distribution and transmission system. Insulation in such transformers typically comprises paper which is wound on the copper windings. In order to enhance stability, the paper is permeated with a dielectric, typically mineral oil, which fills the transformer. Transformer insulation is prone to degradation, the rate of degradation dependent upon the workload and the internal operating environment of the transformer, such as temperature, oil moisture content, oil pH and the like. A degradated insulation can result in increased levels of partial discharge within the transformer. Such increased partial discharge leads to further degradation of the insulation which in turn leads to increasing levels of partial discharge.

Continued degradation of the insulation can result in a serious short-circuit fault or a catastrophic failure due to an explosion of the gases, for example, hydrogen, acetylene and ethylene, produced as chemical by-products of the degradation process. Such failure can result in reduction or loss of supply to the power system, incur considerable expense for the replacement or repair of the transformer and also present a serious risk to nearby personnel and the environment.

Partial discharge in transformers can also occur due to the movement of metallic components, such as fastenings, within the transformer which provide an opportunity for discharges to occur even when there has been none or little degradation of the insulation.

It is known that a partial discharge can produce a number of signals at different locations within a large transformer including a discharge current in neutral, a displacement current through the capacitive tapping of a bushing, a radiated radio frequency (RF) pulse and a radiated ultrasonic pulse.

The level of partial discharge within a transformer provides a means for determining the integrity of insulation within a transformer. Testing of insulation integrity has involved analyses of the dissolved gas concentration in the transformer oil.

Detection of increased partial discharge via gas analysis has the disadvantage that typically monitoring is periodic and can only give a long term trend. As it is suspected that partial discharges of high magnitude develop shortly before a major failure, continuous monitoring of electric equipment while it is kept on-line is very desirable.

DISCLOSURE OF THE INVENTION

According to one aspect, the present invention consists in an apparatus for the monitoring of partial discharge in on-line high voltage electrical equipment containing a dielectric, the apparatus comprising: transducer means for the detection of ultrasonic pressure waves and subsequent production of an electrical signal output corresponding to the detection; transducer means for the detection of radio frequency waves and subsequent production of an electrical signal output corresponding to the detection; a means for minimising interference in the respective electrical signal outputs; a means for amplifying the signal outputs; an analysing means which analyses the amplified signal outputs to detect the occurrence of an ultrasonic pressure wave within a pre-set time from the detection of a radio frequency wave and hence provide an indication of partial discharge; the respective transducer means being arranged to be housed within a wall of the electrical equipment and in contact with the dielectric .

The high voltage electrical equipment preferably comprises power and instrument transformers and switchgear containing a dielectric, the dielectric being preferably mineral oil.

Preferably, the respective transducer means are housed within a common enclosure, the enclosure being positioned within the wall of the electrical equipment and such that one surface of each transducer means is coincident with the interior surface of the wall.

The transducer means for detection of ultrasonic pressure waves preferably comprises a piezoelectric element. The piezoelectric element preferably has an upper face and lower face. The piezoelectric element preferably has a resonant frequency of 75 to 300kHz, and more preferably 100-150 kHz, and a maximum operating temperature of 120 C.

The piezoelectric element is further preferably a ceramic/polymer composite with a 1-3 geometry. The ceramic is preferably lead zirconate titanate (PZT) and the polymer is preferably epoxy resin. The upper and lower faces are preferably used as electrodes. The electrodes preferably comprise a conducting adhesive to which is adhered a metal plate. The conducting adhesive is preferably silver loaded polymeric adhesive. The metal plate is preferably brass of 50 microns thickness.

The piezoelectric element is further preferably encapsulated within a substantially oil impervious housing. The housing is preferably comprised of two portions, an upper portion proximate the piezoelectric element comprising epoxy resin and a second lower portion comprising tungsten particle loaded epoxy resin. Electrical connection from each of the electrodes to the exterior of the housing is preferably achieved by means of copper wire, one end of which is soldered to the electrode, the other end of which is soldered to a copper post that exits one face of the housing.

In a preferred embodiment, the transducer means for detection of radio frequency waves is adapted to preferably detect waves radiated from the partial discharge.

In one embodiment, the means for detection of radio frequency waves is a coil pickup. In a preferred embodiment, the means for detection of radio frequency waves comprises a capacitive plate having a capacitance to ground of 100 to 200pF. The plate preferably has an annular geometry. The annular plate is preferably formed from copper laminated glass epoxy printed circuit board. Within the common enclosure, the piezoelectric element is preferably positioned coaxially within the capacitive annular plate and adapted such that the mineral oil surrounds the capacitive plate and all but the one face of the housing encapsulating the piezoelectric element.

The interference minimising means is adapted to ensure that the output signals from each transducer means correspond only to the occurrence of partial discharge within the electrical equipment. Preferably, the interference to both the radio frequency signal and ultrasonic signal output is minimised.

For radio frequency interference, the placement of the transducer unit within the wall of the electrical equipment allows the wall of the electrical equipment to serve as a shield to radio frequency sources located external to the equipment. In the case of ultrasonic signal interference, it has been determined that mechanical vibrations within the transformer, which are the prime source of ultrasonic signal interference, have a frequency less than approximately 75kHz. Preferably, therefore, the means for minimising interference in the ultrasonic signal output comprises a high pass filter having a low cut off frequency of 75kHz.

The amplifying means is preferably housed within the common enclosure and preferably comprises two amplifiers and two cable drivers, one for amplifying the signal output by the radio frequency transducer means, the other for amplifying the signal output by the ultrasonic transducer means. The analyser means in one embodiment can be housed within the common enclosure and preferably comprises an analog signal processor and a digital signal processor for the signal outputs from each of the transducer means . The analog signal processor is preferably an envelope detector. The envelope detector preferably comprises at least a precision rectifier and a low pass filter. The digital signal processor preferably comprises an analog to digital (A/D) converter which facilitates input of the signals to a digital processor for further processing under software control.

The software control preferably monitors the input from the ultrasonic detector. Preferably, if an ultrasonic pressure wave is detected by the ultrasonic transducer means and has a magnitude greater than a pre-set level, the software then examines the input from the radio frequency transducer means to determine if a radio frequency wave was present and detected within a specified time prior to the ultrasonic signal. If a correlation between the radio frequency and ultrasonic signals is observed to repeat greater than a certain number of times, the apparatus determines that a partial discharge has been detected and an alarm signal is generated. The software is preferably adapted such that an alarm output is only generated if the level of partial discharge is greater than that generated by the electrical equipment in normal operation.

In a further embodiment, the location of partial discharge within high voltage electrical equipment is determined by analysis of the signal outputs from at least three of each transducer means housed within the wall of the electrical equipment. If a partial discharge is detected, the analyser means preferably determines the differences in detection time between each of the transducer means . The location of the partial discharge is then preferably determined by calculation and analysis of the distance between each transducer means and the partial discharge site.

According to a further aspect, the present invention consists in a transducer unit comprising: a means for detection of ultrasonic pressure waves and subsequent production of an electrical signal output; and a means for detection of radio frequency waves and subsequent production of an electrical signal output; wherein the respective detection means are housed within a common enclosure and positioned adjacent to one surface of the transducer unit. BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a preferred embodiment of the present invention will be described with reference to the accompanying drawings in which:

Fig 1 is a schematic view of the apparatus for monitoring of partial discharge in a high voltage transformer; Fig 2 is perspective view from above of one embodiment of the transducer unit;

Fig 3 is a longitudinal cross-section view along the line II-II of Fig 2, illustrating the position of the transducer unit within the wall of a transformer; Fig 4 is an enlarged longitudinal cross-section view of the ultrasonic detection means within the transducer unit; and

Fig 5 is a schematic view of the apparatus illustrating the transducer unit and the components of the analyser means.

An apparatus for the monitoring of partial discharge in on-line high voltage transformers is generally shown as

10 in Fig 1. The apparatus comprises a transducer unit 11 mounted within a wall 12 of a transformer 13 containing mineral oil 14, and an analyser means 15 which modifies and processes the signals output from the transducer unit

11 in order to determine if partial discharge is occurring within the transformer 13.

As depicted more clearly in Fig 2 and Fig 3, the transducer unit 11 comprises an enclosure 20 which surrounds all but one side 30 of the transducer unit 11, and inside which is an annular capacitive plate 16, for the detection of radio frequency waves and a piezoelectric element 17 for the detection of ultrasonic pressure waves . The piezoelectric element 17 is positioned coaxially within the annular plate 16.

The annular plate 16 is mounted upon a support ring 18, which in turn is mounted on a mounting plate 19 that is impervious to the mineral oil 14. Electrical connection from the annular plate 16 to an amplifier and cable driver circuitry 21 is by means of a first copper wire 22 extending from the annular plate 16 to a conducting post 23 fixed within the mounting plate 19 and a second copper wire 24 extending from the post 23 to the input of the amplifier circuitry 21.

The amplified output from the amplifier circuitry 21 exits the enclosure 20 by means of a BNC electrical connector 25 mounted on one face 26 of the enclosure 20. As is depicted more clearly in Fig 4, the piezoelectric element 17 is a 1-3 ceramic 27/polymer 28 composite. The piezoelectric element 17 is encapsulated within a housing 29 which is in turn mounted on the mounting plate 19, the housing having an upper face 31 and a lower face 32. The housing 29 further comprises two portions, a first portion 33 extending from the lower surface 34 of the piezoelectric element 17 to the upper face 31 of the housing 29, and a second portion 35 extending from the lower surface 34 of the piezoelectric element 17 to the lower face 32 of the housing 29.

The first portion 33 is filled with an epoxy resin that is impervious to mineral oil 14 and is transparent to ultrasonic pressure waves, thereby allowing detection of an ultrasonic pulse from a partial discharge. The second portion 35 is tungsten particle loaded epoxy resin that is substantially opaque to ultrasonic pulses . The tungsten loading has two functions . The first function is to ensure no reflection of ultrasonic signals is detected by the piezoelectric element 17 from the mounting plate 19. The second function is to prevent ultrasonic pulses, propagating in the wall 12 of the transformer 13, being transmitted to the piezoelectric element 17.

Each face of the piezoelectric element 17 is an electrode for the accumulation of charge. Each electrode comprises a thin layer of silver loaded epoxy adhesive 36 to which is adhered a brass sheet 37 of 50 microns thickness.

Electrical connection from each of the brass sheets 37 to an amplifier and cable driver circuitry 38 is via an analog high pass filter 55 having a low cut off frequency of 75kHz and is by means of a first copper wire 39, which in turn is soldered to a copper post 41 that exits the housing 29 proximate the lower face 32. Electrical connection through the mounting plate 19 is by means of a conducting post 23 fixed within the mounting plate 19 and a further copper wire 42 extending from the post 23 to the input of the amplifier circuitry 38.

The amplified output from the amplifier circuitry 38 exits the enclosure 20 by means of a BNC connector 43 mounted on one face 26 of the enclosure 20.

The input for the external electric power for the amplifier circuitries 21 and 38 is provided by means of a power input connector 44 in the one face 26 of the enclosure 20. The amplified signals from the radio frequency output 25 and ultrasonic output 43 on the transducer unit 11 are transmitted via fibre optic or electrical cables 45 and 46 respectively, as inputs to the analyser means 15 the components of which are depicted in more detail in Fig 5. The input signals to the analyser means 15 are each firstly processed in an analog signal processor 47. The analog signal processor 47 is an envelope detector and comprises at least a precision rectifier 48 and a low pass filter 49. The envelope of the radio frequency signal output from the analog signal processor 47 is then input into a comparator circuit 51. If the signal is greater than a pre-set level it is output from the comparator circuit 51 to a monostable multivibrator 52. The monostable multivibrator 52 lengthens the pulse width of the radio frequency envelope signal from typically 5 microseconds to 100 microseconds . The pulse output from the monostable multivibrator 52 is then, when required, input into an analog to digital converter 53 which converts the pulse to a digital signal ready for further processing in a digital processor housed in a computer 54 under software control.

The envelope of the ultrasonic signal output from the analog signal processor 47 is input directly to the analog to digital converter 53 which converts the ultrasonic pulse to a digital signal ready for further processing in the digital processor housed in the computer 54.

In operation, the program monitors the processed ultrasonic signal input to the computer 54. If an ultrasonic signal of magnitude greater than a pre-set level is detected, the program then examines the corresponding radio frequency input to determine whether any radio frequency signals, received within a pre-set time period (eg: 3 milliseconds) before the ultrasonic signal, correlate with the ultrasonic signal. If there is a correlation between radio frequency and ultrasonic signals, a partial discharge has been detected within the transformer 12 and an alarm signal is generated.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. An apparatus for the monitoring of partial discharge in on-line high voltage electrical equipment containing a dielectric, the apparatus comprising: transducer means for the detection of ultrasonic pressure waves and subsequent production of an electrical signal output corresponding to the detection; transducer means for the detection of radio frequency waves and subsequent production of an electrical signal output corresponding to the detection; a means for minimising interference in the respective electrical signal outputs; a means for amplifying the signal outputs; an analysing means which analyses the amplified signal outputs to detect the occurrence of an ultrasonic pressure wave within a pre-set time from the detection of a radio frequency wave and hence provide an indication of partial discharge; the respective transducer means being arranged to be housed within a wall of the electrical equipment and in contact with the dielectric.

2. An apparatus as in claim 1 wherein the respective transducer means are housed within a common enclosure, the enclosure being positioned within the wall of the electrical equipment and such that one surface of each transducer means is coincident with the interior surface of the wall.

3. An apparatus as in claim 2 wherein the transducer means for detection of ultrasonic pressure waves comprises a piezoelectric element.

4. An apparatus as in claim 3 wherein the piezoelectric element is a ceramic/polymer composite with a 1-3 geometry and having a resonant frequency of 75-300 kHz.

5. An apparatus as in claims 3 or 4 wherein the piezoelectric element is housed within a substantially oil impervious housing.

6. An apparatus as in any one of claims 1-5 wherein the interference minimising means for the ultrasonic signal output comprises a high pass filter having a low cut off frequency of 75kHz and for the radio frequency signal comprises the wall of the electrical equipment.

7. An apparatus as in any one of claims 2-6 wherein the amplifying means and analyser means are housed within the common enclosure.

8. An apparatus as in any one of claims 1-7 wherein the analyser means comprises an analog signal processor and a digital signal processor for the signal outputs from each of the transducer means.

9. An apparatus as in claim 8 wherein the digital signal processor comprises an analog to digital (A/D) converter which facilitates input of the signals to a digital processor for further processing under software control.

10. An apparatus as in any one of claims 1-9 wherein the location of partial discharge within high voltage electrical equipment is determined by analysis of the signal outputs from at least three of each transducer means housed within the wall of the electrical equipment.

11. A transducer unit comprising: a means for detection of ultrasonic pressure waves and subsequent production of an electrical signal output; and a means for detection of radio frequency waves and subsequent production of an electric signal output; wherein the respective detection means are housed within a common enclosure and positioned adjacent to one surface of the transducer unit.

12. A transducer unit as in claim 11 wherein the means for detection of ultrasonic pressure waves comprises a piezoelectric element and the means for detection of radio frequency waves comprises a capacitive plate having an annular geometry, the piezoelectric element positioned coaxially within the capacitive plate and adapted such that the mineral oil surrounds the capacitive plate.