Method for broadband ultrasonic detection of electrical discharges The invention relates to a method for detecting and assessing ultrasonic sources with a broadband ultrasonic microphone and a mobile test system, wherein the ultrasonic sources constitute acoustic effects of faults, in particular of electrical partial discharges, which propagate through airborne sound.
According to the invention, a broadband ultrasonic signal from at least one ultrasonic source is measured and at least one suitably selectable freguency range of the ultrasonic signal is evaluated.
The freguency range in which the ultrasonic signal is evaluated — is selected on the basis of the available signal strength which depends on the distance between the ultrasonic microphone and the ultrasonic source.
The invention also relates to an ultrasonic measurement device, in particular an ultrasonic test device, and to the use of the method according to the invention or the ultrasonic measurement device for detecting and assessing electrical faults.
The reliability of electrical systems is the basis for supplying all sectors of society with energy.
An important part of industrial maintenance is therefore oriented to the safety of the supply with electrical energy and of the technical operation of the installations.
The aims of preventative maintenance of electrical installations include finding and assessing evolving insulation faults in order to be able to initiate suitable measures.
Typical problems occur in many items of energy generation, energy transmission and energy distribution eguipment, for example in (high-voltage) cables, switchgear installations, electric motors, generators and transformers.
Insulation faults may result in serious conseguential damage and — failures, culminating in catastrophic situations.
In high-voltage installations, such as high-voltage overhead lines, substations, switchgear cabinets, transformer installations and others, undesirable electrical partial discharges endanger the function and safety.
There are various measurement and test technologies for detecting such undesirable partial discharges.
In addition to measuring the electrical fields that occur in the process or induced voltages, thermographic methods and methods based on ultraviolet radiation are used.
On account of the mechanical effect of electric sparks, acoustic and vibration methods are also suitable, in principle, for detecting the occurrence and localization.
In this case, a distinction is made between internal and external partial discharges.
According to DIN EN 60270, partial discharges are defined as a “locally restricted electrical discharge which only partially bridges the insulation between conductors and can, but need not, occur adjacent to a conductor”. The partial discharge phenomena relevant to maintenance can be grouped into particular types, for example: Internal partial discharges: - Inclusions (micanite insulation) - Treeing (high-voltage cables) External partial discharges: - Corona (fastening elements, overhead lines) - Surface discharge (insulators) Internal partial discharges occur if, as a result of inhomogeneities in materials, high field strengths can be achieved when electrical fields are applied.
Local voltage — flashovers can occur on account of the difference in the dielectric constant.
These local discharges are acoustic sources at the same time.
The sound waves are then propagated in the medium (liguid, solid, surrounding construction). These phenomena are detected by means of methods based on vibration and/or the propagation of structure-borne sound.
The present invention does not relate to these phenomena.
In the case of air-insulated electrical equipment and external partial discharge phenomena occurring in them, the acoustic effects of discharges are forwarded as direct or indirect sound through the propagation in air.
The electrical discharges are generally detected using measurement and test technology that is effective in the ultrasonic range.
Analogue and digital test technology solutions are available for this purpose.
Use is usually made of heterodyne receivers which operate on an analogue basis and have a narrowband ultrasonic microphone and integrated mixer frequency technology which can capture and assess the ultrasound present in a narrow frequency band.
This access is physically not necessary.
However, it can be implemented in a relatively simple and cost-effective manner.
The elementary processes during the electrical discharge take place very quickly.
This means that —— from a physical point of view - a very broad acoustic spectrum is generated.
A typical frequency range during capture is around 40 kHz in practical applications.
The choice of this frequency has pragmatic reasons.
As a result of the use of ultrasound, ambient and interfering noises in the audible range play a smaller role — than when capturing signals in the audible range.
Furthermore, ultrasound is distinguished by the fact that its propagation direction can be tracked well and, as a result, it is possible to locate the source more easily than at lower frequencies.
On the one hand, there is an inexpensive sensor system (ultrasonic distance warning in vehicles) and, on the other hand, the range of ultrasound at 40 kHz is still large enough to manage practical application scenarios.
The detection technology is usually in the form of a portable handheld test device.
Suitable concave mirrors are used to assist with locating the source and to physically amplify the signal.
The introduction of digital broadband test technology makes it possible to use the — entire acoustic frequency range.
The assessment can be carried out using suitable display and recording of the ultrasonic signal (amplitude profile, spectrograms, level representations). It is likewise possible to digitally simulate the analogue narrowband methods.
An important element of the implementation based on broadband ultrasonic technology is the complete digitization of the entire signal chain.
It is therefore also possible to integrate and optimize signal processing algorithms.
The propagation of sound in air is known to be subject to freguency-dependent attenuation.
Current everyday experience is that the thunder accompanying lightning can be heard to some extent only at low freguencies.
The higher freguencies occurring during the discharge are absorbed by the air over relatively long distances.
This effect is also effective at distances which are conventional when checking electrical discharges (a few metres to 10 m). The attenuation in air depends on the distance, the temperature and the humidity.
The influence of pressure can be disregarded.
The propagation of sound outdoors is described in DIN ISO 9613 (attenuation of sound during propagation outdoors). The relevant attenuation coefficients are stated there in a comprehensible form.
An exact analytical description is not possible on account of the multiplicity of parameters when testing discharge operations.
However, this is not necessary on account of the accuracy which is required and can be achieved during testing.
A prediction on the basis of the test situation without further measures would amount to a rough estimation.
In the methods which reflect the prior art, the assessment of ultrasonic sources depends more or less randomly on the test distance.
The measurement frequency which has been set (for example 40 kHz) is only randomly suitable.
If the distances — are increased, the amplitudes of the higher frequencies are reduced and the characteristic of the ultrasonic signal can no longer be detected under certain circumstances in the measurement range used.
The methods from the prior art are therefore not suitable for analysing electrical disturbances both from relatively short and from relatively long distances and for comparing the intensities.
The — digital broadband ultrasonic data always contain a spectrum influenced by the distance attenuation for each measurement distance.
In addition, the measurement and test data are not automatically assessed by the previously known solutions.
The assessment must be carried out by the user/tester on the basis of their practical knowledge by viewing the measured values on a screen or writing data in a log or on the basis of an auditory impression.
In 5 particular, the assessment on the basis of the auditory impression is very subjective and depends greatly on the tester’s experience.
Furthermore, not all physically possible situations which can occur can be captured by using analogue narrowband frequency technology.
In contrast, the available digital test technology which assesses frequencies with a bandwidth of up to approximately 100 kHz can provide only raw data.
Integrated or downstream assessment is not available.
The physical specifics of electrical discharges are not taken into account in any of the known solutions.
Electrical discharges are a transient phenomenon, for the capture of which the measurement technology and software are not optimally designed.
It has hitherto not been possible and provision has not been made for different discharge patterns to be distinguished.
US 2016/0341782 Al relates to an apparatus and a method for the steady-state monitoring of power supply devices.
In this case, arcs are detected on the basis of ultrasound by positioning one or more sensors close to a power supply device and recording the ultrasonic signal from the power supply device.
The data are used to
— generate a feature vector by converting the time signal into the frequency domain (by FFT) and breaking it down into individual intervals.
A “frequency sub-band energy value” is calculated and normalized for each interval.
These values then constitute the feature vector.
The feature vector is compared with stored feature vectors which have been detected during the occurrence of arcs.
If this comparison reveals a high degree of similarity, an alarm signal is output.
US 2016/0341782 Al does not disclose any possible way of carrying out tests in a non-steady-state manner at different distances from a device to be tested.
Provision is not made for distance-dependent measurements to be implemented in any way.
Accordingly, the apparatus is not in the form of a portable handheld device either.
US 2011/0252888 Al relates to an ultrasonic handheld device which is used to = monitor the functionality of motor bearings or switchgear cabinets (occurrence of arcs). The handheld device makes it possible to represent the detected ultrasonic signal in the freguency domain by means of FFT.
It is therefore possible for a tester to also assess the freguency spectrum in addition to an acoustic signal.
The handheld device also has a laser pointer which can be used to determine the distance — between the handheld device and an object to be analysed.
KR 101 318 926 BI discloses an ultrasonic measurement device for detecting faults in electrical energy supply installations.
The ultrasonic measurement device has a narrowband ultrasonic sensor on the front side and a broadband ultrasonic sensor on the rear side.
CN 109 490 730 A likewise relates to an ultrasonic measurement device for detecting faults in electrical energy supply installations.
A measurement device which is intended to be used to detect different electrical discharge types on the — basis of their spectral patterns is proposed.
The present invention is therefore based on the object of detecting and assessing, for example classifying, the acoustic effect of a partial discharge, which propagates through airborne sound, taking into account the effects of the distance between the ultrasonic microphone used and the ultrasonic source.
For this purpose, the invention provides a method for detecting and assessing ultrasonic sources with a broadband ultrasonic microphone, wherein the ultrasonic sources constitute acoustic effects of an electrical partial discharge which propagate through airborne sound, characterized in that a broadband ultrasonic signal from at least one ultrasonic source is measured; at least one frequency range of the ultrasonic signal is evaluated;
and in that the frequency range in which the ultrasonic signal is evaluated is selected on the basis of the distance between the ultrasonic microphone and the ultrasonic source.
The invention also provides an ultrasonic measurement device for detecting broadband ultrasound, having o a broadband ultrasonic microphone; o measurement electronics having a library of cepstra for known electrical partial discharges;
o a user interface;
o optionally a distance meter;
wherein the ultrasonic measurement device is configured to detect and assess the acoustic effects of an electrical partial discharge which propagates through air ultrasound;
and wherein the ultrasonic measurement device is configured to select a freguency range to be evaluated on the basis of a distance between the ultrasonic microphone and a measured ultrasonic source.
The use of the method according to the invention or of the ultrasonic measurement — device according to the invention for detecting and assessing electrical faults, in particular electrical partial discharges, is also described.
Detailed description
Features which are described below for the method according to the invention likewise apply to the ultrasonic measurement device according to the invention and vice versa.
As already described at the outset, ultrasonic sources according to the present invention are sources which emit air ultrasound in a manner caused by electrical partial discharges.
The ultrasonic sources are particularly preferably faults, preferably electrical faults, at which electrical partial discharges occur.
In accordance with the method according to the invention, a broadband ultrasonic signal from at least one ultrasonic source is measured. In one preferred embodiment of the present invention, an ultrasonic signal from precisely one ultrasonic source is measured. In this context, broadband means that frequencies in the range of 10 kHz to 100 kHz, in particular in the range of 20 kHz to 100 kHz, are preferably captured. The ultrasonic signal is preferably sampled at a sampling rate of fs > 200 kHz. According to the invention, at least one frequency range of the ultrasonic signal is evaluated in order to assess the ultrasonic signal. In one embodiment of the present invention, the distance between the ultrasonic source and the ultrasonic microphone is measured. A frequency range of the ultrasonic signal is selected for the evaluation on the basis of the measured distance. With increasing distance from the ultrasonic source, ultrasound of higher frequencies is attenuated to a greater extent than the ultrasonic signal of lower frequencies. With increasing distance, the frequency range which is evaluated is therefore shifted to lower frequencies. In this case, the frequency range for the evaluation depends on the acoustic power of the source and the sensitivity of the — measurement chain and its dynamic range. For distances between the ultrasonic microphone and the sound source of several tens of metres, it has been found that the frequency range for the evaluation is centred around 40 kHz. For distances of up to 10 m, it is found that a frequency range around 100 kHz is most favourable. For ranges around 100 m and above, a frequency range around 10 kHz is evaluated. Alarge distance range between the ultrasonic microphone and the ultrasonic source can therefore be advantageously captured using the present invention. Partial discharges as acoustic sources which act as interference sources can therefore be detected and assessed at a distance from the ultrasonic microphone of below 10 m to a distance of 200 m and more. In one preferred embodiment of the present invention, for a distance between the ultrasonic microphone and the ultrasonic source of up to 10 m, a frequency range of the ultrasonic signal which is between 60 kHz and 100 kHz, preferably between
70 kHz and 100 kHz, particularly preferably between 80 kHz and 100 kHz, is evaluated.
In one preferred embodiment of the present invention, for a distance between the ultrasonic microphone and the ultrasonic source of between 10 m and 100 m, a frequency range of the ultrasonic signal which is between 20 kHz and 60 kHz,
preferably between 30 kHz and 60 kHz, particularly preferably between 40 kHz and 60 kHz, is evaluated.
In one preferred embodiment of the present invention, for a distance between the ultrasonic microphone and the ultrasonic source of more than 100 m, a frequency range of the ultrasonic signal which is between 10 kHz and 40 kHz, preferably between 10 kHz and 30 kHz, particularly preferably between 10 kHz and 20 kHz, is evaluated.
In one embodiment of the invention, ultrasonic sources up to a distance of 200 m between the ultrasonic source and the ultrasonic microphone are detected and assessed.
In one embodiment of the invention, the at least one frequency range which is used for the evaluation typically has a bandwidth of 10 to 20 kHz.
The electrical faults, in particular electrical partial discharges, are measured from a distance at which the elements in the measurement chain comprising the measurement electronics and the ultrasonic microphone are sensitive enough and the signal caused by the discharge is thereby singled out from the interfering noise (ambient noises, electronic noise). In one embodiment, the threshold value for the electronic noise is approximately 20 dB (based on po = 20 pPa). Signals from determined sources can be reliably captured in the case of a level difference of approximately 5 dB from the noise.
The frequency-dependent attenuation of the ultrasonic signal depends on many factors.
In addition to pure attenuation which could be described well in physical terms, the measurement environment, such as the substrate, reflective environments, vegetation or else humidity, also plays a role.
The distance is therefore preferably considered from the broadband spectrum itself.
The frequency range in which the ultrasonic signal is evaluated is then selected on the basis of the available signal — strength which in turn depends on the distance between the ultrasonic microphone and the ultrasonic source.
The freguency range with the highest signal strength is evaluated in this case.
The distance between the ultrasonic source and the ultrasonic microphone therefore need not necessarily be available to the tester.
The entire ultrasonic signal measured in broadband is advantageously available to a user, with the result that it is possible to also evaluate more than one freguency range or to apply different freguency filters to the measured signal.
The detected time signal is preferably processed in real time.
In one embodiment
— of the present invention, the entire time signal is recorded and stored.
In this case, the data are advantageously also available for data processing at a subseguent time.
Within the scope of the present invention, the variance, and therefore the fluctuation, in the time/freguency properties of the signal is referred to as a pattern.
The pattern is decisively determined by the temporal variation in the signal amplitude.
The time signal waveform is a mixture of stochastic and systematic seguences of acoustic signals.
The actual information relating to faults, in particular relating to electrical partial discharges, is contained therein.
The elementary events are distributed over a wide freguency range in this case.
Within the scope of the
— present invention, partial discharges having an acoustic effect that propagates through air ultrasound are referred to as elementary events.
The time signal can be converted into the freguency domain using known means, for example a Fourier transform.
The Fourier transform can be carried out both on the high-frequency ultrasonic data and on the data rendered audible with a reduced data rate.
A favourable representation in the time/freguency domain is referred to as a spectrogram.
The high-frequency measured ultrasonic time signal is the basis for all further calculations according to the present invention. This high-frequency measured ultrasonic signal per se is not suitable for direct use in testing, but it has been found that, in the case of suitable processing of the high-frequency ultrasonic signal, it can be rendered available for use in testing. In one embodiment of the present invention, a suitably filtered new time signal is generated from the frequency range of the measurement signal that is selected for the evaluation. The new time signal may be generated by means of one of the methods mentioned below. All methods have the feature in common that the modulation characteristic of the ultrasonic signal is retained. All methods are preferably used in real time. a Broadband calculation and representation of the spectra b. Heterodyne signal: one embodiment has a bandwidth of +/-2 kHz around the carrier frequency. Typical applications use a carrier frequency of 40 kHz on account of the available simple sensors. On account of the broadband digital measurement, the carrier frequency and the bandwidth may be varied in any desired manner. It is possible to simultaneously calculate a large number of frequency bands in heterodyne technology. This can also be used for audio output in the audible range. The signal has a lower temporal resolution than the original high-frequency signal. Although heterodyne technology per se is known to a person skilled in the art, the use according to the present method as a filter bank is not familiar to a person skilled in the art and also cannot be implemented in analogue form. This is enabled only by using heterodyne technology in conjunction with an ultrasonic signal detected in broadband according to the present method.
c. Vocoder signal: this data compression is used to make the ultrasonic signal audible. The temporal dynamics are also retained in the auditory impression. Since this signal type represents a freguency compression of the entire ultrasonic range, bandpass filters which can each be assigned to an ultrasonic freguency band can also be calculated for the vocoder signal. If the ultrasonic signal is broken down into 1, 2, 4, 6 or 12 bands, this also relates, in the scaled form, to the vocoder signal with the similar number of bands. The use of the vocoder method is known to a person skilled in the art.
d. Bandpass-filtered high-frequency signals: a filter bank separates the signal in the time domain into different time signals. On account of the broadband acoustic signals, the information relating to the fluctuation is the same in all frequency bands (with a possibly different amplitude). A frequency band in which there is a sufficiently large amplitude is sufficient for the assessment for qualitative purposes. A representative amplitude value can be used for each frequency band. An averaged amplitude is suitable. One disadvantage of this method variant is that the possibility of spectral calculations is lost in this case. In a further embodiment of the invention, the envelopes of the temporal fluctuations are calculated from the time signal detected using the ultrasonic microphone. This can be achieved by means of a Hilbert transform of the time signal. This procedure is advantageous, in particular, if the spectral information can be dispensed with. Since the envelopes are likewise time functions, these can reflect the (global) amplitude fluctuations in the detected ultrasonic signal and can be used for the — calculation methods a. to d. which have already been described. In one embodiment of the present invention, the envelopes of the temporal fluctuations in the detected ultrasonic signal are calculated and are then used as a new time signal. It order to objectify the assessment, a further signal processing step is inserted, preferably in real time. In time signals with a stochastic character, concealed and repeating structures can be made visible surprisingly well using the cepstrum operation from the field of Fourier analysis. In this case, the signals are based on an inverse frequency. This is then formally a time again and can be understood as meaning a measure of the repetition distance. In one embodiment of the present invention, a cepstrum is calculated for the ultrasonic signal. The cepstrum can be used as a suitable basis for a visual distinction (consideration by testers) or as an input for a further signal processing step for automatic assessment.
During visual assessment by a tester, the assessment can take place once on the basis of the tester’s experience.
However, it is also possible for the calculated cepstrum to be compared with already known cepstra patterns by a tester or automatically using pattern recognition methods, wherein the known cepstra reflect the ultrasonic signals of precisely defined types of electrical partial discharges.
The basis for this is a comprehensive experimental database.
The different types of discharge operations have different forms in terms of their repetition structure.
There are very uniform repetitions through to a virtually stochastic repetition structure.
This structure is characteristic of different variants of electrical partial discharges.
The type of fault can therefore be objectively described from the signal structure using the present method without having to resort to the subjective assessment by experienced testers by means of the auditory impression (heterodyne method) and the visualization of the (audible) audio signal, asis conventional with the methods from the prior art.
The present method therefore advantageously provides a test method for faults, in particular for electrical partial discharges at faults, which enables an objective test independently of the respective tester.
In a further embodiment of the present invention, the cepstrum can also be subsequently calculated with the aid of the stored measurement data.
It has been found that, in the case of relatively long distances, the measurement or assessment frequency can be shifted to considerably lower frequencies.
The assessment can be carried out correctly since the temporal assessment pattern which can be captured with the cepstrum is retained.
In principle, the audible frequencies can also be used in the case of very long distances, but interfering audible sound must be eliminated from the assessment.
Distances of up to several hundred metres can therefore be achieved.
This is not possible with the previously known methods.
The directional effect of sound which becomes worse in the case of longer distances between the ultrasonic source and the ultrasonic microphone can be partially compensated for by the use of a concave mirror.
In one embodiment of the method according to the invention, the directional effect of ultrasound is therefore partially corrected using a concave mirror.
Selecting a suitable frequency band facilitates the testing for the user.
The selection (heterodyne, bandpass filter or broadband) can be made digitally.
If the broadband signal is used and the high-frequency signal is stored, all situations can be subsequently calculated (offline). The invention also provides an ultrasonic measurement device for detecting broadband ultrasound.
The ultrasonic measurement device is preferably an ultrasonic test device which can be accordingly used for test purposes.
According to the present invention, digital measurement technology is used.
For this purpose, the ultrasonic measurement technology must be equipped with a broadband ultrasonic microphone.
This is preferably an ultrasonic microphone which can capture frequencies of 10 to 100 kHz.
In a further embodiment of the invention, this is a microphone which is also designed for high frequencies (up to approximately 100 kHz) and is operated with suitable frequency filters in the digital measurement chain.
Suitable frequency filters are, for example, 1/3 octave filters — or further subdivisions (1/6 or 1/12 octave filters) and freely selectable digital filters.
The ultrasonic microphone is connected to suitable measurement electronics in a measurement chain.
The test device integrates a digitally stored library of cepstra — which reflect the ultrasonic signals of precisely defined types of electrical partial discharges.
The ultrasonic measurement device also has a user interface, with the aid of which a user can navigate and can select certain method steps.
In this manner, it is possible — to initiate a measurement or influence the evaluation of the measurement data.
The measurement result may be advantageously likewise presented on the user interface.
In one preferred embodiment, the user interface is a graphical user interface with touch operation.
The ultrasonic measurement device optionally has a distance meter.
In one preferred embodiment, the ultrasonic measurement device has a distance meter.
The measured distance can then be used to select the frequency range which is intended to be evaluated.
As already described, however, a distance measurement is not absolutely necessary since the information relating to the influence of the distance is already contained in the broadband ultrasonic data.
However, integrating the possibility of a direct distance measurement can be used to confirm the assessment or to ensure the comparability of different measurement positions.
In one embodiment of the invention, the ultrasonic measurement device also has a concave mirror.
This proves to be advantageous, in particular, when the ultrasonic measurement device is at a relatively long distance from the source of the ultrasonic signal or when there are very low sound levels.
The directional effect of ultrasound which becomes worse can be compensated for by using the concave mirror and the signal-to-noise ratio is improved.
The signal improvement is 6 dB.
Sound sources of equal power can therefore be found at approximately twice the distance and assessed.
In this case, the combination of the use of concave mirrors and a frequency band with lower ultrasonic frequencies has a favourable effect on the — ability to test partial discharges at relatively long distances.
The ultrasonic measurement device, in particular ultrasonic test device, is particularly preferably in the form of a handheld device.
It is therefore possible to easily handle the device and transport it in an uncomplicated manner.
The ultrasonic measurement device can therefore be used and applied flexibly as a mobile test system.
The method according to the invention and the ultrasonic measurement device according to the invention are preferably used to detect and assess faults, in — particular electrical partial discharges.
The faults are preferably electrical faults at which partial discharges occur, which partial discharges cause ultrasonic signals which propagate through air ultrasound.
The method and the ultrasonic measurement device are designed, according to the invention, to detect and assess such faults up to a distance of approximately 200 m between the ultrasonic microphone and the ultrasonic source.
The invention is explained in more detail below on the basis of 6 figures and 1 exemplary embodiment.
Figure 1 illustrates a Fourier spectrum of an individual discharge event; Figure 2 illustrates a time/freguency representation of a discharge sequence; Figure 3 illustrates a cepstrum; Figure 4 illustrates the adaptation of the frequency section that is evaluated to the distance-dependent attenuation of the ultrasonic signal; Figure 5 (A) and (B) schematically illustrate an ultrasonic signal and the attenuation of the signal with the distance from the ultrasonic source; Figure 6 illustrates the attenuation of ultrasonic signals in different frequency ranges with the distance from the ultrasonic source.
Figure 1 illustrates a Fourier spectrum of an individual discharge event over a wide frequency range.
The spectrum shows that a significant signal gain (20 dB) in comparison with the frequency (gap) at 25 to 40 kHz can be found at frequencies
— around 50 to 60 kHz.
This can be used to set an optimum measurement frequency or the optimum frequency band.
The optimization must be adapted to different test situations.
Different discharge types and electrode shapes generate different spectral distributions in the Fourier spectrum.
Figure 2 shows the time/frequency representation of a typical discharge sequence.
In this example, different frequency bands could be selected in order to represent the temporal fluctuation.
The cepstrum in Figure 3 shows the temporal repetition pattern of discharge operations.
The different types of discharge operations have different forms in terms of their repetition structure.
There are very uniform repetitions through to a virtually stochastic repetition structure.
This structure is characteristic.
According to the present method, the type of fault, in particular the electrical partial discharge,
is objectively described with the aid of the signal structure.
This assessment is presently implemented, with the methods from the prior art, subjectively by experienced testers by means of the auditory impression (heterodyne method) and its visualization.
Figure 4 shows the selection of the optimum frequency range for the evaluation of the measurement signal on the basis of the distance between the ultrasonic microphone and the ultrasonic source.
The ultrasonic signals 1 to 5 were measured with increasing distance.
The distances between the ultrasonic microphone and the sound source were as follows: In this case, the distance between the ultrasonic measurement device and the ultrasonic source was shortest for measurement curve 1 and longest for measurement curve 5. The frequency-dependent attenuation of the ultrasonic signal with increasing distance can be clearly seen.
Higher frequencies are attenuated to a considerably greater extent than lower frequencies.
Frequency range A is selected for the evaluation for the measurement of curve 1 with the shortest distance between the ultrasonic measurement device and the ultrasonic source, since the — lowest interfering sound can also be expected here.
In contrast, frequency range B is selected for the evaluation for curves 2 and 3, since the attenuation of the ultrasonic signal is considerably lower here than in range A.
An evaluation in range A is no longer possible at all for curves 4 and 5, since almost no signal can be measured for these frequencies.
Ultrasonic signals in this frequency range are virtually completely attenuated in the air on account of the long distance between the ultrasonic source and the ultrasonic measurement device.
However, an evaluation for the measurement of this distance is still possible for frequency range
C.
According to the invention, ultrasonic signals from electrical partial discharges can also therefore be detected and assessed over long distances.
Figure 5 (A) schematically illustrates an ultrasonic signal.
The entire ultrasonic signal is measured in broadband, that is to say in the frequency range of 10 kHz to
100 kHz, using the apparatus according to the invention.
When further processing the ultrasonic signal, filters can be used to select which frequency range of the entire signal is intended to be used for the further evaluation.
Filters of 20 kHz to 40 kHz, 40 kHz to 60 kHz, 60 kHz to 80 kHz and 80 kHz to 100 kHz are shown.
These filters are used only for illustration; it goes without saying that another filter width or another frequency range can also be selected.
For example, it is also possible to filter the ultrasonic signal in the frequency range of 50 kHz to 80 kHz.
Figure 5 (B) schematically illustrates the attenuation of different frequency components of the ultrasonic signal with increasing distance between the ultrasonic source and the ultrasonic microphone.
It can clearly be seen that, for signal components with higher frequencies, the signal is attenuated to a greater extent with increasing distance.
With increasing distance, it is therefore useful to evaluate signal components of the measured broadband ultrasonic signal which comprise lower frequency ranges.
Figure 6 illustrates the data from the measurement of broadband ultrasonic signals for different distances between the ultrasonic source and the ultrasonic microphone.
Exemplary embodiment 1
Measurements of an ultrasonic signal from an electrical partial discharge were carried out, for example, for distances of between 4 m and 20 m.
As already described, the method can be used up to distances of up to 200 m.
A broadband ultrasonic signal in the frequency range of 10 kHz to 120 kHz was recorded for each distance using an apparatus according to the invention.
The broadband ultrasonic signal was then processed further using a filter of 20 kHz to 120 kHz and the sound pressure for this distance was determined.
The measured broadband ultrasonic signal was likewise evaluated using frequency filters in the range of 80 kHz to 100 kHz, in the range of 60 kHz to 80 kHz, in the range of 40 kHz to 60 kHz and in the range of 20 kHz to 40 kHz and the sound pressure level was calculated in each case.
The sound pressure levels for the evaluation in the individual frequency ranges on the basis of the distance are illustrated in Figure 6. It can clearly be seen that signals in the higher frequency range have a lower sound pressure level than signals in the lower frequency range with increasing distance.
This is caused by the greater attenuation with increasing frequency.