WO2003044516A1

WO2003044516A1 - Device and method for acoustic detection and localization of defects

Info

Publication number: WO2003044516A1
Application number: PCT/NO2002/000357
Authority: WO
Inventors: Lars Lundgaard
Original assignee: Sintef Energiforskning As
Priority date: 2001-10-31
Filing date: 2002-10-03
Publication date: 2003-05-30
Also published as: AU2002335563A1; NO20015325A; NO313848B1; NO20015325D0

Abstract

A transducer for acoustic detection and localization of sound generating defects comprising two spatially separated sensors (1), each coupled to a multiplier (6) for multiplying the signals from the sensors (1), and an output port for transferring the multiplied signal to a suitable instrument (8) or a suitable indicator. In a method the said transducer is used for acoustic detection and localization of sound generating defects in machines, in particular electrical partial discharges in high voltage transformers comprising at least the placing of an acoustic transducer at several locations on the surface of an object to be examined. Detection of the acoustic signal which the transducer senses is performed for each position of the transducer and stored together with position data. The transducer is for each position rotated mechanically through a plurality of angles. At each position the value of the acoustic signal which the transducer senses is detected and stored. The detected and stored signal values are analyzed for at each position of the transducer to identify the angle corresponding to the largest signal value, in order to determine the localization of the acoustic source in a plane.

Description

Device and method for acoustic detection and localization of

^' This invention relates to a transducer for acoustic cie_fects detection and localization of sound generating defects in machines, particularly electrical partial discharges in high voltage transformers . Partial discharges indicate defects and are an unwanted condition in high voltage isolation systems which may influence the material and safety of the plant . In order to evaluate the risk it is important to be able to determine the position of the defect by the use of external measurements.

Various methods are known for finding the position of the defects, and are often based on acoustic measurements. In the publication by Lars E. Lundgaard og Walter Hansen: "Location of discharges in power transformers using external acoustic sensors.", Sixth international symposium on high voltage engineering, New Orleans, LA, USA, August 28- September 1, 1989 there is described measurement of acoustic waves appearing from partial discharges, and the propagation of the acoustic waves through a medium. Other examples of measuring techniques:

- Multiple acoustic sensors and triangulation.

- Electrical detection of a source combined with acoustic measurements of development with time.

- Movement of an acoustic sensor until maximum signal is achieved.

- Use of two sensors and oscilloscope, where one of the sensors is moved until the signals appear simultaneously on the oscilloscope.

The disadvantages of the known solutions is that they either are complex both in use and equipment or, using only one sensor channel, in that it is hard to achieve sufficiently high accuracy.

It is hence a purpose of this invention to provide acoustic measurement equipment for the measurement of partial discharges which is simple to use, but which also provides sufficiently accurate determination of position.

These purposes are achieved by a transducer as described above and which is characterized as given in the independent claim 1. The purpose is also achieved by a method as given in the independent claim 6.

In this manner a measuring equipment and method is achieved which is very simple in use, which achieves high accuracy in localization of defects, and which in addition can be produced from easily available equipment such that costs can be kept low. The invention will be described below with reference to the attached figures which illustrate the invention by way of examples.

Figure 1 illustrates one embodiment of the invention. Figure 2A illustrates the position of two sensors placed in relation to three noise sources on a plate.

Figure 2B illustrates in a table the measured signals in relation to the position on a plate.

Figure 3 illustrates the peak value of the combined signal in relation to the difference in time of arrival at the sensors .

Figure 1 shows a preferred embodiment of the invention comprising two sensors 1 which are each placed in association with a magnet 2 in order to give optimum contact with a magnetizable surface. The sensors can be of any available type of acoustic sensor with a frequency response which suits the relevant measurements. The choice of type of sensor will have to be determined form apparatus to apparatus. Normally, however, acoustic emission sensors with a wideband characteristic and sensitivity peak in the range 20-200 kHz will be used.

The sensors 1 and the associated magnets 2 are held in a predetermined mutual distance with a bar 3. For practical purposes the bar will typically be 30-60 cm long. In some cases it might be envisaged that the distance can be altered between measurements, for example in order to take into account local space considerations or the like, but the distance is substantially constant during measuring sequences to localize defects. Each sensor of the preferred embodiment is connected to a preamplifier 4 for the amplification of the signal form the sensor, and a filter 5 for the removal of undesired frequency components from the signal . The type of filter may- vary with the application but will typically comprise a band-pass filter in order to separate distinct frequency components. In some cases the filter 5 may be integrated in the preamplifier 4 or be omitted, as a certain degree of filtering may also be performed in the sensors themselves. In applications on transformers the low-pass filters of 50 kHz or no filter seem to be suitable.

For the embodiments of the invention it is important that the sensors 1, preamplifiers 4 and filters 5 are as similar as possible.

The signals from the sensors are then transmitted to a signal multiplier 6 which sends a multiplied signal to a measuring instrument 8. Generally any kind of peak value indicating instrument or oscilloscope may be used provided the bandwidth is sufficiently large compared to the characteristics of the sensors. In example, the instrument AIA (Acoustic Insulation Analyzer) from Transinor may be used. Figure 2A shows schematically two sensors 1A, IB placed in relation to three noise sources 7A,7B,7C, where one of the sources 7B are located in a plane equidistantly in relation to the sensors 1A,1B. In Figure 2B it is illustrated, from the left, how the signals from source 7A, to the left in the table, do not arrive at the sensors 1A,1B simultaneously. The product of the two signals will then be zero. The corresponding will happen with the signal from source 7C to the right . The signals from the centre source 7B, however, arrive simultaneously at the sensors. The multiplied signal will then be the product of two signals which are not equal to zero, which yields a detectable signal which is sensed by the measuring instrument which is connected to the multiplier circuit 6.

Figure 3 illustrates how the peak value of the multiplied signal changes with the time difference Δt between the signals from the sensors.

The transducer may in one embodiment of the invention be rotatably suspended, such that the sensor may be placed in one place and rotated until the angle of maximum signal is achieved from the multiplier 6 and into the measuring instrument is found. The acoustic source then lies in a first plane midway between the sensors . The measuring instrument coupled to the multiplier may be of any type which may provide an indication of the peak value of the multiplied signal such that an operator may move the measuring equipment over a search area until the highest value is achieved. In order to achieve a better localization the transducer is moved to a new location. The transducer is then rotated again until the angle where maximum signal is achieved from the multiplier 6 and into the measuring instrument is found. This may, for example, be achieved by storing corresponding values of the measured signal and the angle of the transducer during rotation of the transducer. By analysis of the detected and stored signal values the angle corresponding to the maximum signal value may be identified. The acoustic source is now localized in a second plane and will lie where the first and second planes intersect .

Measurements at further positions will give a further improved determination of position of the acoustic source. The filtering which is performed may, as mentioned, be of various types depending on the local conditions, for example in that frequency ranges which contain a lot of noise are suppressed or in that the propagation properties of the materials which is to propagate the sound is taken into account .

Claims

C l a i m s

1. Transducer for acoustic detection and localization of sound generating defects c h a r a c t e r i z e d i n that it comprises to spatially separated acoustic sensors (1) , each coupled to a multiplier (6) for multiplying the signal from the sensors, and an output port for transferring the multiplied signal to a suitable instrument (8) or suitable indicator.

2. Transducer according to claim 1, c h a r a c t e r i z e d i n that the signal from each sensor (1) is transferred via a filter (5) for removing chosen signal components.

3. Transducer according to claim 1 or 2 , c h a r a c t e r i z e d i n that the signal from each sensor (1) is transferred via an amplifier (4) for amplifying chosen parts of the signal .

4. Transducer according to claim 1, c h a r a c t e r i z e d i n that the sensors (1) are mounted with a constant spaced-apart relationship on a common fixture (3) .

5. Transducer according to claim 1, c h a r a c t e r i z e d i n that it is rotatably suspended.

6. Method of acoustic detection and localization of sound generating defects in machines, in particular electrical partial discharges in high voltage transformers comprising at least placing an acoustic transducer at several locations on a surface of an object to be examined, detection and storage of the value of the acoustic signal sensed by the transducer at each position of the transducer together with position data,

c h a r a c t e r i z e d i n that at each position the transducer is rotated mechanically through a plurality of angles, at each position the value of the acoustic signal which the transducer senses is detected and stored for each angle of the transducer, a transducer is applied which comprises to spatially separated acoustic sensors (1) , each coupled to a multiplier (6) where the signals from the sensors are multiplied, and an output port where the multiplied signal is transferred to a suitable instrument or a suitable indicator, and the detected and stored signal values are analyzed for at each position of the transducer to identify the angle corresponding to the larest signal values in order to thereby determine the location of the acoustic source in a plane.

7. Method according to claim 6 c h a r a c t e r i z e d i n that the signal from each sensor (1) is transferred via a filter (5) for removing chosen signals.

8. Method according to claim 6 or 7 c h a r a c t e r i z e d i n that the signal from each sensor (1) is transferred via an amplifier (4) for amplification of chosen parts of the signal.

9. Method according to claim 6 c h a r a c t e r i z e d i n that the sensors (1) are mounted in a constant spaced-apart distance on a common fixture (3) .