Note: Descriptions are shown in the official language in which they were submitted.
CA 02608216 2007-11-13
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DETECTION, LOCALIZATION AND INTERPRETATION OF PARTIAL
DISCHARGE
FIELD OF THE INVENTION
The invention relates to an apparatus for detection, localization and
interpretation
of partial discharge, for example in an electrical accessory of an underground
=
power distribution network, or in any other kind of electrical equipment
located in
any environment.
BACKGROUND
Power failures may occur at cable joints in underground conduit networks. Some
are due to partial discharges inside the joints and the corresponding
degradation
of their electrical insulation.
Diagnosis of underground cables and their accessories or equipment is
desirable
for safety issues, and for performing predictive maintenance and removing
defective accessories or equipment before failure.
US patents Nos. 6,809,523 (Ahmed et al.), 5,530,364 (Mashikian et al.),
5,767,684
(Steennis), 6,420,879 (Cooke), 6,507,181 (Pakonen et al.), 6,418,385 (Hacker
et
al.), 6,255,808 (Hacker), 6,297,645 (Eriksson et al.), 6,392,401 (Cooke),
5,642,038
(Kim et al.) and CA (Canadian) patent No. 2,455,206 (Wendel et al.) disclose
certain methods and various devices for detecting partial discharges which are
however not much reliable, imprecise, sensitive to noise, rudimentary,
cumbersome, limited to some specific equipment to be tested, require that the
equipment to be tested be out of service, or else difficult to be practically
implemented due to manipulations requiring uncommon dexterity.
Partial discharges may also occur in a variety of electrical devices or
equipment,
and are often forerunners of more serious damages to follow if nothing is done
to
repair or replace the possibly defective equipment.
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SUMMARY
According to an aspect of the invention, there is provided an apparatus for
detection, localization and interpretation of partial discharge that outdo the
apparatuses known in the art.
According to another aspect of the invention, there is provided an apparatus
for
detecting, localizing and interpreting a partial discharge occurring in a
partial
discharge site along an electrical equipment, comprising:
two measurement probes and a synchronization probe installable along the
electrical equipment so that pulses travelling in the electrical equipment are
detectable by the measurement probes and a phase angle in the electrical
equipment is detectable by the synchronization probe;
a control unit connecting to the measurement probes for receiving signals
representative of the detected pulses, and connecting to the synchronization
probe
for acquiring a signal representative of the detected phase angle, the control
unit
having a circuit for selective conditioning of the received signals; and
a digital processing unit connecting to the control unit for acquiring the
signals after selective conditioning as a function of the detected phase angle
and
driving the control unit, the digital processing unit having a correlation
measuring
module for measuring correlation of the acquired signals, a module for
performing
a time-frequency distribution of at least one of the acquired signals, a form
factor
estimating module for estimating a form factor derived from the time-frequency
distribution, and a diagnosis module responsive to results generated by the
correlation measuring and form factor estimating modules for generating a
diagnosis indicative of a detection of a partial discharge and of its
localization
along the electrical equipment.
Preferably, the processing unit further comprises a candidate eliminating
module
for eliminating candidates of diagnosis solutions corresponding to traces in
the
acquired signals derived from detected pulses having out-of-range propagation
delays between the measurement probes.
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A threshold for delays considered as being out-of-range by the candidate
eliminating module may be set by the user. The control unit may further
comprise
a circuit for generating a test signal transmitted to the synchronization
probe that
injects it in the electrical equipment. Thus, the digital processing unit may
determine the propagation delay between the measurement probes from the
acquired signals corresponding to pulses detected by the measurement probes
caused by the test signal injected in the electrical equipment. The threshold
for
delays considered to be out-of-range may then be set as a function of the
propagation delay so determined. The digital processing unit may alternatively
or
concurrently check a configuration of the measurement probes as a function of
the
acquired signals corresponding to the test signal injected in the electrical
equipment, e.g. based on the polarity or the acquired signals.
Preferably, the processing unit further comprises a module for estimating a
probability of error as a function of a ratio between a peak of a maximum of
correlation among other correlation peaks, a warning signal indicative of a
second
probable candidate of diagnosis explanation being produced when the
probability
of error exceeds a preset threshold.
Preferably, the processing unit further detects typical traces of radiation in
the
acquired signals. When it is established that the acquired signals correspond
to
radiation, their processing is stopped and a "radiation" diagnosis is
retained.
Preferably, the correlation measuring module provides the diagnosis module
with
a signal indicative of the polarities of the pulses in the acquired signals, a
correlation factor of the correlated signals, a temporal trace portion of a
higher
amplitude discharge, and a temporal distance between a same discharge sensed
by the measurement probes.
Preferably, the processing unit further comprises a module for estimating an
equivalent bandwidth and a rise time of the higher amplitude discharge, both
provided to the diagnosis module.
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According to another aspect of the invention, there is provided a method for
detecting, localizing and interpreting a partial discharge occurring in a
partial
discharge site along an electrical equipment, comprising:
detecting pulses travelling in the electrical equipment using two
measurement probes spaced from each other along the electrical equipment;
detecting a phase angle in the electrical equipment using a synchronization
probe positioned along the electrical equipment;
performing a selective conditioning of signals representative of the detected
pulses;
acquiring the signals after the selective conditioning as a function of the
detected phase angle;
putting the acquired signals in correlation;
presenting at least one of the acquired signals in a time-frequency
distribution;
estimating a form factor derived from the time-frequency distribution; and
establishing a diagnosis indicative of a detection of a partial discharge and
of its localization along the electrical equipment as a function of results
from the
correlation and the form factor.
Preferably, the method may further comprise: eliminating candidates of
diagnosis
solutions corresponding to traces in the acquired signals derived from
detected
pulses having out-of-range propagation delays between the measurement probes.
The method may further comprise:
generating a test signal transmitted to the synchronization probe that injects
it in the electrical equipment; and
determining the propagation delay between the measurement probes from
the acquired signals corresponding to the pulses detected by the measurement
probes caused by the test signal injected in the electrical equipment, the
threshold
for delays considered to be out-of-range being then set as a function of the
propagation delay so determined.
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The test signal may also be used to check a configuration of the measurement
probes, for example as a function of the polarities of the acquired signals
corresponding to the test signal injected in the electrical equipment.
Among other possible steps of the method are:
estimating a probability of error as a function of a ratio between a peak of a
maximum of correlation among other correlation peaks, a warning signal
indicative
of a second probable candidate of diagnosis explanation being produced when
the
probability of error exceeds a preset threshold;
detecting typical traces of radiation in the acquired signals, their
processing
being stopped and a "radiation" diagnosis being retained when it is
established
that the acquired signals correspond to radiation;
establishing the diagnosis based also on a signal indicative of the polarities
of the pulses in the acquired signals, a correlation factor of the correlated
signals,
a temporal trace portion of a higher amplitude discharge, a temporal distance
between a same discharge sensed by the measurement probes, an equivalent
bandwidth and a rise time of the higher amplitude discharge;
interpolating the acquired signals before correlation;
clusterizing the acquired signals prior to the correlation to group the
acquired signals that are similar.
According to another aspect of the invention, there is provided a wideband
magnetic probe for detecting pulses traveling in an electrical equipment
caused by
a partial discharge, comprising:
a removable clamp having a conductive loop forming a magnetic sensing
circuit apt to surround a section of the electrical equipment in order to
sense a
signal representing a magnetic component of the pulses traveling in the
electrical
equipment;
a conductive shield covering and electrostatically insulating the conductive
loop, the conductive shield being in open circuit at opposite ends of the
clamp so
that a gap appears between the ends of the clamp;
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a connector closing the circuit of the conductive loop at the ends of the
clamp where the gap is located when the clamp is installed around the
electrical
equipment; and
a connector for establishing an external electrical connection with the
circuit
of the conductive loop.
Preferably, the wideband magnetic probe further comprises an amplifier
circuit,
possibly having a controlled gain, integrated in the conductive shield and
inserted
in the conductive loop in order to filter and amplify the signal.
According to another aspect of the invention, there is provided an apparatus
for
detecting partial discharges in an electrical equipment, comprising:
a measurement probe and a synchronization probe installable along the
electrical equipment so that pulses travelling in the electrical equipment are
detectable by the measurement probe and a signal indicative of a phase angle
in
the electrical equipment is detectable by the synchronization probe;
a control unit connecting to the measurement probe for receiving signals
representative of the detected pulses, and connecting to the synchronization
probe
for acquiring the signal indicative of the phase angle, the control unit
having a
circuit for selective conditioning of the received signals; and
a digital processing unit connecting to the control unit for acquiring the
signals after selective conditioning as a function of an appraisal of the
phase angle
and driving the control unit, the digital processing unit having a
clusterization
module for clusterizing the acquired signals into respective clusters and
producing
signatures characterizing the signals in the clusters, a module for performing
a
time-frequency distribution of the signatures, a form factor estimating module
for
estimating a form factor derived from the time-frequency distribution, a
module for
determining rise times of the signatures, and a diagnosis module responsive to
results generated by the form factor estimating module and the module for
determining rise times for generating a diagnosis indicative of a detection of
partial
discharges and producing a warning signal as a function of the diagnosis.
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According to another aspect of the invention, there is provided a method for
detecting partial discharges in an electrical equipment, comprising:
detecting pulses travelling in the electrical equipment using a measurement
probe positioned along the electrical equipment;
detecting a signal indicative of a phase angle in the electrical equipment
using a synchronization probe positioned along the electrical equipment;
performing selective conditioning of signal representative of the detected
pulses;
acquiring the signals after the selective conditioning as a function of an
appraisal of the phase angle;
clusterizing the acquired signals into clusters and producing signatures
characterizing the signals in the clusters;
presenting the signatures in a time-frequency distribution;
estimating a form factor derived from the time-frequency distribution;
determining rise times of the signatures;
establishing a diagnosis indicative of a detection of partial discharges as a
function of results from the form factor and the rise times; and
producing a warning signal as a function of the diagnosis.
According to another aspect of the invention, there is provided an apparatus
for
detecting partial discharges in an electrical equipment, comprising:
a measurement probe and a synchronization probe installable along the
electrical equipment so that pulses travelling in the electrical equipment are
detectable by the measurement probe and a signal indicative of a phase angle
in
the electrical equipment is detectable by the synchronization probe;
a control unit connecting to the measurement probe for receiving signals
representative of the detected pulses, and connecting to the synchronization
probe
for acquiring the signal indicative of the phase angle, the control unit
having a
circuit for selective conditioning of the received signals; and
a digital processing unit connecting to the control unit for acquiring the
signals after selective conditioning considering the phase angle and driving
the
control unit, the digital processing unit having a clusterization module for
clusterizing the acquired signals into respective clusters and producing time
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CA 02608216 2011-04-12
signatures characterizing the signals in the clusters, a module for
determining
characteristic elements of the signatures, and a diagnosis module responsive
to
results generated by the module for determining characteristic elements for
generating a diagnosis indicative of a detection of partial discharges and
producing a warning signal as a function of the diagnosis.
According to another aspect of the invention, there is provided a method for
detecting partial discharges in an electrical equipment, comprising:
detecting pulses travelling in the electrical equipment using a measurement
probe positioned along the electrical equipment;
detecting a signal indicative of a phase angle in the electrical equipment
using a synchronization probe positioned along the electrical equipment;
performing selective conditioning of signals representative of the detected
pulses;
acquiring the signals after the selective conditioning considering the phase
angle;
clusterizing the acquired signals into clusters and producing time signatures
characterizing the signals in the clusters;
determining characteristic elements of the signatures;
establishing a diagnosis indicative of a detection of partial discharges as a
function of results from the characteristic elements; and
producing a warning signal as a function of the diagnosis.
The following provides an outline of other possibly preferable and non-
restrictive
features of the invention, which will be more fully described hereinafter.
The apparatus is preferably portable, autonomous and apt to perform the
detection
when the electrical line or equipment to be tested is in service. With the
apparatus,
a reliable diagnosis regarding the nature of the discharge site and the health
condition of the equipment may be obtained. The apparatus may be operated from
a remote location from the measurement site and has probes whose installation
is
achieved in a minimum of time to increase the security level of the workers.
Strong
and improved probes are proposed for detecting a partial discharge, in
particular in
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CA 02608216 2011-04-12
the form of wideband magnetic probes comprising a controlled gain amplifier. A
test procedure allows checking the working condition of the probes,
determining
the parameters used to establish the diagnosis, and calibrating the whole
apparatus. The apparatus uses correlation, synchronization and form factor
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estimation procedures on the measured signals in order to establish and
unequaled diagnosis.
The apparatus and the method are highly versatile. They may be used whenever
there is a need to verify if an electrical accessory or equipment is plagued
with
partial discharges. For example, it may be used to check transformers, switch
gears, batteries, capacitors, dielectric containing components, entertainment
or
communication systems, medical equipment, etc., no matter whether some of
their
operating parts are solid, liquid or gas. They may be used to test electrical
equipment located anywhere, underground, overhead, on an airplane, a train, a
vehicle, a boat, in a plant, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
A detailed description of preferred embodiments will be given herein below
with
reference to the following drawings, in which like numbers refer to like
elements.
FIG. 1 is a schematic diagram illustrating a partial discharge in an accessory
located between two wideband probes.
FIG. 2 is a schematic diagram illustrating a partial discharge or signal
coming from
beyond the two wideband probes, and a possible connection of the
synchronization probe.
FIG. 3 is a schematic diagram illustrating a partial discharge in an accessory
located beyond the two wideband probes.
FIG. 4 is a schematic block diagram illustrating a design of the apparatus for
detecting, localizing and interpreting a partial discharge.
FIG. 5 is a schematic diagram illustrating a typical cross section of a power
line
cable.
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FIG. 6A and FIG. 6B are perspective schematic diagrams of a power line cable
and magnetic flux circulating in it.
FIG. 7 is a schematic diagram illustrating field lines around the magnetic
probe.
FIG. 8 and FIG. 9 are schematic diagrams illustrating a wideband magnetic
probe
without and with an amplifier circuit and an external connector.
FIG. 10 is a schematic block diagram illustrating an amplifier circuit
integrated in a
wideband magnetic probe.
FIG. 11 is a schematic block diagram illustrating a control unit (CU)
processing the
signals from the probes.
FIG. 12 is a schematic block diagram illustrating the main elements involved
in the
signal processing.
FIG. 13 is a schematic block diagram illustrating the use of correlation as a
digital
processing tool.
FIG. 14 is a schematic diagram illustrating a spectrogram of a typical partial
discharge.
FIG. 15 is a picture illustrating a spectrogram of a typical partial discharge
after
application of a time-frequency filter.
FIG. 16 is a picture illustrating an apparatus for detecting, localizing and
interpreting a partial discharge.
FIG. 17 is a picture illustrating a disassembled wideband probe.
FIG. 18 is a schematic diagram illustrating a control unit of the apparatus.
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FIG. 19 and FIG. 20 are schematic diagrams in elevation and in plan view
illustrating a construction of a wideband probe.
FIG. 21 is a schematic diagram illustrating the microcontroller circuit of a
control
unit.
FIG. 22 and FIG. 23 are schematic diagrams illustrating the power supply
circuits
of the wideband probes.
FIG. 24 and FIG. 25 are schematic diagrams illustrating the signal amplifier
and
conditioning wideband (RF) circuits of the wideband probes.
FIG. 26 is a schematic diagram illustrating a signal conditioning circuit of a
synchronization probe.
FIG. 27 is a schematic diagram illustrating power supply circuits of the other
circuits of the control unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As used in connection with this disclosure, the term "signal" represents an
analog
and temporal physical unit, typically of current or voltage type, appearing in
a
continuous form in the time domain.
As used in connection with this disclosure, the term "measurement" represents
a
series of digital discrete samples derived from a signal sampled during a
finite
period.
As used in connection with this disclosure, the term "test" represents a set
of
simultaneous measurements and of available specifications concerning a
physical
event recorded by the apparatus.
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As used in connection with this disclosure, the expression "signal
conditioning"
represents an action carried out by analog electronics prior to signal
digitization.
As used in connection with this disclosure, the expression "signal processing"
represents a procedure comprising mathematical manipulations required to
confirm the presence of a partial discharge, localize the discharge site and
bring
out the corresponding physical features.
As used in connection with this disclosure, the term "computer" represents a
compatible or other PC computer or equivalent electronics having or provided
with
a high speed acquisition card.
As used in connection with this disclosure, the expression "control unit" (CU)
represents an intelligent interface connecting the computer to the probes.
As used in connection with this disclosure, the expression "partial discharge"
represents a spontaneous, simultaneous and short duration local displacement
of
an electric charge over a short distance in a dielectric subjected to an
electric field.
As used in connection with this disclosure, the expression "discharge site"
represents a defect localized in a small volume of a dielectric where one or
more
partial discharges occur most often during application of an electric field
although
also possible under other circumstances.
As used in connection with this disclosure, the expression "power wave"
represents a sinusoidal wave carrying the network or other supplying power at
60
Hz or 50 Hz or another frequency according to the application, for example 400
Hz
in the case of airplanes, etc.
As used in connection with this disclosure, the term "phasor" represents a
phase
angle of the power wave rotating 360 per cycle at the network frequency.
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As used in connection with this disclosure, the term "radiation" represents
any
noise of electric or magnetic nature, having a source external to that of a
partial
discharge, most often characterized by a greater number of oscillations, a
greater
propagation delay, a smaller degree of correlation and an inappropriate
polarity.
As used in connection with this disclosure, the terms "a", "one", "two", etc.,
are to
be construed non-restrictively unless qualifiers such as "single", "sole",
"only" are
specified.
Referring to Figure 16, the apparatus for detecting, localizing and
interpreting a
partial discharge may be built so as to be portable and autonomous. The
apparatus may take the form of a computer 1 provided with a display 3, a
keyboard 5 and a mouse 7, and equipment 9, the whole fitting in a case 11. The
apparatus especially allows detecting and localizing one or more partial
discharge
sites present on a high voltage line accessory of an underground power network
from a manhole giving access to the network. The apparatus also allows
detecting
and indicating a direction of a partial discharge source taking place in
another
manhole linked to the first manhole by the high voltage line. The obtained
information allows a reliable diagnosis of the nature of the discharge site
and of
the health condition of the accessory, e.g. a transformer, a switch gear or
other
possible equipment: The apparatus allows discriminating the partial discharge
signals in presence of several signals from different origins.
Usually, for considerations of cleanliness for the computer 1, of comfort and
of
safety for the user, the computer 1 is operated at a distance of a few meters
from
the measurement site. Depending on the intended use, the probes 4, 6 may be
built to be strong, e.g. for rough manipulations in a dirty and wet
measurement
site. Preferably, the installation of the probes such as the measurement
probes 4,
6 should be achievable in a minimum of time to minimize the exposure of the
worker to risks inherent to the measurement site. Figure 17 illustrates a
possible
construction of the measurement probes 4, 6.
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Partial discharges mainly occur during voltage rises associated to the power
wave,
thus at particular angles of the network phasor. Furthermore, the angular
distribution of the dispersion of these discharges contains important
diagnosis
information since it is a function of the type of the discharge site. Thus,
the
apparatus ascribes an angular position referenced to the network phasor for
each
partial discharge (phase resolve partial discharge). The apparatus may also
control measurements over predetermined angular portions to target certain
discharge sites or obtain an unbiased statistical picture.
Partial discharges have variable durations, as a function of the type of the
discharge site, of the geometry of the accessories and of the distance of each
measurement probe 4, 6. The shortest ones have a rise time in the order of a
few
nanoseconds and sometimes less (in the picoseconds) depending on the
measuring means and the nature of the discharge. The signal is preferably
digitized at one gigasamples or more per second. At this sampling rate, the
dead
time between two discharges represent a very important volume of data to be
digitized and recorded. The digitizing is preferably performed by segments
each
containing a triggering event. This event may be a partial discharge or noise
exceeding the triggering threshold. A high-pass filter 61 (Figure 11) combined
with
switchable RF filters 63 allows reducing the noise level at a point where it
is
possible to control the starting of the sampling on the beginning of a low
amplitude
partial discharge and that despite the presence of electromagnetic noise.
Referring to Figure 1, a partial discharge occurring in an accessory 8 or a
cable 10
generates an electromagnetic pulse propagating in both cable directions. Two
probes 4, 6 located on both sides of a discharge site 13 provide pulses 15, 17
which, once processed and correlated, are indicative of the location of the
discharge site 13. Indeed, in such a case, the polarity is opposite and the
inter-
channel delay is less than the propagation delay of a wave between the two
probes 4, 6.
Referring to Figure 2, a pulse coming from one side or the other side of the
measurement site 19 will appear with the same polarity to both probes 4, 6 and
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with a delay corresponding approximately to the propagation delay of a wave
between both probes 4, 6.
Referring to Figure 3, furthermore, two probes 4, 6 located on a same side of
a
discharge site 13 provide pulses which, once processed and correlated, are
indicative of the direction of the discharge site.
It should be noted that the polarity of the sensed wave is a function of the
orientation of each probe 4, 6. In a network measurement context, pulses
sensed
by the probes 4, 6 differ from one another due to the presence of reflections
on the
surrounding accessories (not illustrated) and due to sensed noise.
Furthermore, a
partial discharge signal may contain more than one oscillation so that several
correlation peaks may exist when the pulses are compared, thus several
possible
diagnoses. The apparatus thus evaluates which one of the diagnoses is the most
probable by associating a value of likeliness to it and by indicating, if
relevant, the
presence of another diagnosis almost as probable.
The likeliness of the diagnosis is based in part on the configuration of the
probes
4, 6 (relative position with respect to the accessory 8 or other equipment
under
test, location and direction of installation) and the a priori knowledge of
the actual
propagation delays. In this respect, a first test procedure on a portable
testing
bench allows estimating accurately the response of each probe 4, 6, including
the
actual delay. A second test procedure at the measurement site allows measuring
the probe-to-probe propagation delay as well as validating the configuration
of the
test.
Once the test is achieved, followed by validation of the assembly
configuration, the
test may begin. Since the digitization rate is very high with respect to the
available
memory, and since, anyway, the discharges are events of very short duration
and
quite spaced from one another, it is unnecessary to store the whole signal.
Only
the useful portions of the signal may be memorized. Each portion corresponds
to
an overrun of a preset threshold level, the overrun being called "event"
hereinafter.
Many diagnoses may correspond to a given event. During a test, this threshold
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level is progressively lowered until an acceptable probability of events is
observed
for the measurement. When the hidden presence of discharges having lower
amplitudes than the triggering threshold is suspected, this threshold level
must
thus be further lowered with the result that a high rate of events do not
correspond
to discharges. For each test and each threshold, the apparatus records several
bursts of events which may cover one or several cycles at the network
frequency.
For each event, the apparatus displays and stores in memory an automatic
diagnosis obtained from correlations, namely if there is presence of a
saturated
signal, presence of radiation or other inappropriate noise, a discharge
between
both probes 4, 6, a discharge coming from one side or the other side of the
arrangement of probes 4, 6, and presence of a second probable diagnosis
explanation.
To each diagnosis is preferably attributed a likeliness factor based on a
distribution
of possible diagnoses, a factor of correlation between the discharges coming
from
both probes 4, 6, a form factor corresponding to a ratio of the spectral
bandwidth
over the time length of the discharge pulse, the value of the network phasor
at the
time of the event, and an analog (fixed) and digital (variable by the user)
processing parameter set.
The user may select the digital processing to be applied, the burst(s) and the
test(s) to compile and show on the display 3. The results may then be
illustrated in
an unprocessed form (text format list) or through various common statistical
presentation tools, such as 2D or 3D histograms. These results may be exported
for analysis by software applications, such as MicrosoftTM ExcelTm. As a
complement to the diagnosis, the user may have access to various common
digital
and display processing tools, for example a graphical display of the temporal
traces filtered or not, spectrograms and Wigner-Ville and time-frequency
wavelet
distributions. The user may also apply different digital filters and
processings on
the digital temporal traces and return to the statistical display of the
latter (concept
of iterative looping in the diagnosis).
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CA 02608216 2007-11-14 -
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Referring to Figure 4, the apparatus comprises three probes 4, 6, 14 connected
to
a control unit (CU) 16 which is connected to the computer 1. The length of the
probe connecting cables 19, 21, 23 is such that it allows placing the CU 16
and the
computer 1 in a truck or other remote location whereas the probes 4, 6, 14 are
mounted on the power cables 10 and the accessories 8 (Figures 1-3). The CU 16
may be housed inside the computer 1 for saving space and facilitating
connections. However, the CU 16 may as well be located outside the computer 1
and be connected to it by means of an appropriate cable 29.
The purpose of two of the three probes, namely probes 4, 6, is to sense the
electromagnetic pulse generated by a partial discharge, propagating in the
accessories 8 and cables 10 (Figures 1-3). The purpose of the third probe 14
is to
sense the power wave and, when desired, to inject a test signal.
The purpose of the CU 16 is mainly to allow synchronization of an acquisition
window with the power wave, in defining the temporal beginning and end of the
digitization in network phasor degrees. The other functions are the analog
processing of the partial discharge signals (including voltage surge
protection) and
their transmission to the computer 1 in analog form (or digital form if
desired),
supplying power to the probes 4, 6 and controlling their gain, transmitting a
test
signal to the synchronization probe 14, validating that the accessory 8 is
under
voltage and diagnosing a defective probe 4, 6, 14.
The purpose of the computer 1 is to configure the CU 16, to digitize the
signals, to
apply a digital processing on the signals, to carry out the diagnosis, to
display the
measurements and the diagnosis as well as to save the tests in a database and
exchanging the data with other systems.
The measurement method uses three probes, namely two magnetic coupling
wideband probes 4, 6 sensing the partial discharges and a synchronization
probe
14 sensing the 60 Hz wave for extracting the phase angle value from it. Both
magnetic probes 4, 6 may be positioned on both sides of the accessory or at
one
of its ends. The synchronization probe 14 may be of a capacitive, magnetic,
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voltage or current measurement type. The synchronization probe 14 may be
connected to the tested accessory 8 as illustrated in Figure 2, or to another
accessory (i.e. another phase), in which case the phase angle value is
corrected
by 120 to take the interphase shift into account if necessary. The
synchronization probe 14 may be combined to any or each one of the other
probes
4, 6. It may then appear as a distinctive element in the housing of the
combined
probe or be integrated with the wideband part. In the latter case, a signal
separation circuit allows extracting the low frequency signal (close to the
network
frequency) from the wideband signal.
Three calibration procedures are preferably carried out. In one of these
procedures, the CU 16 injects a known signal in the digitizing module of the
computer 1, which allows calibration of the response of the acquisition
card(s)
linked with the filters. In another one of the procedures, the CU 16 injects a
known
signal in the synchronization probe 14 installed on the accessory 8 to be
tested in
order to confirm the configuration of the probes 4, 6 and to calibrate the
probe-to-
probe propagation delay in the cable 10. In another procedure, the CU 16
injects a
known signal in the synchronization probe 14 installed on a portable test
bench
(not shown) in order to calibrate the whole apparatus.
Given the low amplitude of the discharge signal and the length (several
meters) of
the connecting cables 19, 21, 23 of the probes 4, 6, 14, the wideband (many
hundreds of megahertz) magnetic probes 4, 6 advantageously (but not
compulsorily) house a controlled gain amplifier 42 (Figure 9). The illustrated
probes 4, 6 are especially designed to operate with power cables 10 having a
neutral sheath 28 (Figures 5 and 6A) formed of strands. The probes 4, 6
measure
the longitudinal magnetic field generated by the helix so formed. The
measurement method still remains valid for other types of cables (for example,
with a continuous smooth sheath in aluminum or lead) but possibly requires
replacement of the magnetic probes 4, 6 by another type of wideband probe (for
example, a capacitive probe).
17
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The synchronization probe 14 is bidirectional since it is used both for
sensing the
phase of the 60 Hz wave and for injecting a test signal or a calibration
signal in the
cable 10. The probe 14 as illustrated in Figure 2 measures the shunt current
between two electrodes positioned for example using mounting clamps 25, 27,
one on the semiconducting sheath 26 and the other one on the neutral 28.
However, another type of sensing may well be used such as magnetic coupling,
capacitive coupling or voltage measurement.
The CU 16 controlled by the computer 1 is able to achieve a) the analog
conditioning and protection against voltage and current surges; b) the
switching of
analog filters 63; c) the synchronization of an acquisition over a targeted
portion of
the cycle (0-360 degrees) of the 60 Hz wave through a triggering signal
transmitted to the acquisition card of the computer 1; d) the control of the
gains of
the wideband probes 4, 6 and their power supply; e) the transmission of a test
signal to the synchronization probe 14 to inject this signal in the accessory
8; f) the
transmission of a test signal to the computer 1; g) the diagnosis regarding
the
good working condition of the wideband probes 4, 6 and the transmission of
this
diagnosis to the computer 1; and h) the checking of presence of voltage on the
accessory 8 and the transmission of this state to the computer 1.
Digitization is performed at a very high rate, namely between 1 giga and 10
giga
samples per second for the signals of both wideband probes 4, 6.
In the digital analysis, many diagnosis possibilities are examined so that at
certain
occasions, the apparatus warns the user of a second probable diagnosis
explanation in addition to that presented as the most probable one.
A signal interpolation is preferably achieved before the correlation.
A calculation of the time-frequency form factor, based on the ratio of
bandwidth
over time length, facilitates recognition of the real partial discharges.
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Clusterization (or clustering) can be achieved on the acquired signals so as
to
group signals exhibiting similar characteristics or traits (noise, discharge,
etc.) into
respective clusters and produce signatures characterizing the signals in the
clusters. The signatures may be obtained by working out the means of the
signal
data in the respective clusters or by other possible data manipulations or
operations. Correlation and other processings can then be carried out on the
signatures in order to increase the signal/noise ratio while decreasing the
computation time since less data are thereby processed compared to the raw
data
of the acquired signals.
Table 1 below provides a list of the main elements of information and signals
at
the input of the apparatus and their source.
Table 1
User controls:
- Adjustment of the clock and of the calendar
- Selection of language
- Test parameter and control
- In test mode:
- Description of the test (location, accessories ...)
- Configuration of the probes with respect to the accessories
- Parameters of the measurement
- Start/end of the measurement
- Reject/accept the measurement
- In analysis mode:
- Parameters of the signal processing
- Parameters of the display
Computer links:
- Reception of measurements and diagnoses coming from other sites
Wideband magnetic probes, via the CU:
- Propagation signal having a longitudinal magnetic component and sensed on
the
cable
Synchronization probe:
19
. CA 02608216 2007-11-14
PCTICA 2006, 0308 1 I
14 MARCH " 2007
=
-60 Hz power wave
Table 2 provides a list of the main elements of information at the output of
the
apparatus and their target.
Table 2
User:
- Parameters of the completed test
- Graphical plot of the temporal signal
- Graphical plot of the filtered temporal signal
- Diagnosis
- Calculated delays
Portable computer:
- Writing of the completed tests in a database on a mass memory (disk or
other)
Computer links:
- Transmission of measurements, tests, diagnoses and database elements.
Synchronization probe:
- Injection of a test signal in a cable
Referring to Figures 5, 6A and 6B, the shielded cable 10 on which each
wideband
magnetic probe 4, 6 is installed is made of a central conductor 20 surrounded
by a
semi-conductive sheath 22, an insulating dielectric 24, a second semi-
conductive
sheath 26 and a twisted concentric neutral 28. The resultant of the strands of
the
conductor 20 and of the neutral 28, and the fact that the neutral 28 does not
provide a perfect shielding, are such that a neutral-conductor electromagnetic
propagation wave has ,a non-negligible axial magnetic component 30, as
illustrated
in Figure 6B.
Referring to Figure 7, each magnetic probe 4, 6, having a single conductive
loop
32, senses the axial magnetic component 30 of the wave.
Referring to Figure 8, each wideband magnetic probe 4, 6 looks like a
removable
clamp made of a magnetic sensing conductive loop 32 covered with a conductive
AMENDED SHEET
PCTICA 12096, 0808 1 11
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= ' 14 MARCfL 2007 1.4 03
at?
shield 34 also used as a housing for the probe 4, 6. One purpose of this
shield is
to electrostatically insulate the magnetic sensing loop 32. To prevent current
conduction in the shield which would oppose a magnetic field against the axial
magnetic flux, the shield is in open circuit at the ends of the clamp so that
a gap 36
appears. A connector 38 is used to close the circuit of the magnetic loop 32
at the
ends of the clamp, where the gap 36 is located. The conductive loop 32 senses
the axial magnetic flux 30 while being not much sensitive to the tangential
field 40
(Figure 6) surrounding the cable 10 and to ambient electric fields.
Referring to Figure 9, the magnetic sensing loop 32 is connected to an
amplifier
circuit 42 integrated in the shielded housing of the clamp 34.
Referring to Figure 10, the amplifier circuit 42 comprises a low-pass filter
44, a
preamplifier 46, a power amplifier 48 also acting as an impedance matcher, a
decoupler 50, a gain control unit (GCU) 52, a protection circuit 54 and a
power
supply regulator 56. The decoupler 50 allows the sensed signal to be
transmitted
via the connector 58, to provide the supply current to the regulator 56, and
to
transmit the gain control value received via the connector 58 to the GCU 52.
The
gain of the GCU 52 is adjustable as a function of the required sensitivity.
The gain
control comes from the CU 16 and is transmitted through the cable 19, 21
connected to the probe 4, 6 by the connector 58, this same cable 19, 21
transmitting the signal sensed and amplified by the probe 4, 6 to the CU 16
(see
Figure 4).
Figure 18 illustrates a possible diagram of the amplifier circuit 42 of a
probe 4, 6.
Referring to Figure 19 and 20, the magnetic sensing loop 32 may be made of a
flexible printed circuit on which the amplifier circuit 42 and the connector
58
connectable to the cables 19, 21 are also located.
The cable connecting each wideband probe 4, 6 to the CU 16 may be of various
natures. 50 ohms coaxial cables and double shielded 50 ohms coaxial cables may
be used. This last type of cable is more expensive but it offers a better
protection
21
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against noise. In fact, it is possible to use any type of cable providing a
good
immunity to electromagnetic noise and ground loop currents.
Referring to Figure 11, only the circuit processing the signal of the second
probe 4
received on the connector 67 is illustrated. The circuit of the first probe 4,
replaced
by stipple lines 18, is identical to that of the second probe 6. The CU 16
controlled
by the computer 1 comprises the analog conditioning of the signal of the
wideband
probes 4, 6, the protection against voltage surges on the probes 4, 6 and
current
surge on the power supply of the probes 4, 6. Connectors 69, 71 allow
connecting
the UC 16 to the measurement probe 4 and to the synchronization probe 14. A
decoupling module 65 allows the separation between the signal coming from the
probe 4 and the supply current. A current surge protection circuit 73 is
positioned
downstream from the power supply 75, this circuit 75 being connected to a
microcontroller 77 so that the microcontroller 77 may detect the presence of
the
power supply of the probe 4. The current surge protection limits the supply
current
provided to each probe 4, 6 so that the probe 4, 6 and the power supply 75 are
protected at once. The analog signal first passes through the voltage surge
protection module 79. The purpose of the voltage surge protection is to
maintain
the signal within acceptable voltage limits to protect the probe 4 and the
analog
conditioning circuit 81. The analog conditioning 81 consists in the
application of a
high-pass filter 61 along with an amplification of the signal 83. The
microcontroller
77 may inject a test signal, the synchronization signal or another required
signal by
means of the amplifier module 83. The cut-off frequency of the high-pass
filter 61
may be between 30 kHz and 1.7 MHz (preferably 100 kHz). The purpose of this
filter 61 is to eliminate noise coming from the electric network and radio
broadcasts, mainly in the AM band. The conditioning circuit 81 preferably
comprises a space reserved for the installation of a second optional filter
stage 85
(high-pass, band-pass or low-pass) which would follow after the high-pass
filter 61.
The CU 16 comprises a stage of switchable filters 63. In the illustrated
circuit, a
multiplexer 87 selects the signal transmitted to the computer 1. This signal
may be
absent (not connected) 89, the raw signal without passage through a filter 91,
the
22
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signal filtered by a low-pass 93 (preferably set to around 39 MHz) or a
selection
among two other optional filters 95, 97.
The CU 16 allows synchronization of an acquisition over a target portion of
the
cycle (0-360 degree) of the power wave phasor (60 Hz or 50 Hz depending on the
network). The synchronization signals are combined to the signal coming from
the
probe 4 in the conditioning amplifier 83. The resulting signal is transmitted
to the
acquisition card in the computer 1. The acquisition card then starts the
detection of
the discharges a bit prior to or at the showing of the first synchronization
signal
(the acquisition always takes place and the data are continuously recorded in
a
circular buffer) and stops the capture of the partial discharges after passage
of the
end of synchronization signal. An electronic synchronization circuit formed of
a
decoupler 99, a voltage surge protection circuit 101, a low-pass filter 103,
an
amplifier 105 and a band-pass filter 107 is controlled by the microcontroller
77 and
is locked on the phase of the power wave in order to determine times
corresponding to the start and the end of the detection range of the partial
discharges. The decoupler 99 allows the transmission of a test signal to the
synchronization probe 14 without this signal being transmitted to the rest of
the
circuit 101, 103, 105, 107. The signal coming from the synchronization probe
14
passes by the voltage surge protection module 101 to be then directed toward a
low-pass filter 103, preferably set at 1 kHz, to reduce noise present in the
signal.
This filter 103 is followed by an amplifier 105 which supplies a narrow band-
pass
filter 107. The phase at the output of this filter 107 is compared to that of
the input
in order to control the frequency of the filter 107 for the purpose of
preserving a
180 degrees phase-shift in spite of the frequency variations of the electric
network.
A counter (integrated in the microcontroller 77) controlled from the passages
by
zero of the signal at the output of the band-pass filter 107 provides an
estimate of
the phasor value except for a scale factor. The comparison of this last value
with
"start run" and "stop run" commands provides the generation time of the
synchronization signals. Furthermore, in a preferred option, the partial
discharge
signal is communicated to the computer 1 only during the time period
corresponding to this portion using the multiplexer 87.
23
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r
=
Across a same wire 19, 21 (Figure 4), the CU 16 transmits the gain control
signal
and the supply current required by the wideband probes 4, 6. The power supply
module 75 controlled by the microcontroller 77 adjusts the voltage level
corresponding to the required gain. The current surge module 73 monitors the
power supply module 75 and transmits the supply state to the microcontroller
77.
Finally, the decoupler module 65 insulates the supply current from the other
signal
conditioning circuits of the probe 4, 6.
The microcontroller 77 allows the transmission of a test signal to the
synchronization probe 14 for injection of this signal in the accessory 8. The
microcontroller 77 has a circuit that may generate a calibrated pulse of very
short
length, similar to the typical length of a partial discharge, or a pulse train
having a
delay pattern optimizing the correlation 64. After passage of the pulse in an
amplifier (not shown in the Figures), the decoupler 99 directs it in the cable
23
(Figure 4) connected to the synchronization probe 14. The low-pass filter 103
located at the input of the synchronization circuit insulates it in order to
block the
high amplitude pulse.
The CU 16 also allows the transmission of a test signal to the computer 1. An
option resides in replacing one of the optional filters 93, 95 by a voltage
source.
Then, the multiplexer 63 simply has to be controlled in order to select this
signal
for transmitting it to the computer 1.
The CU 16 allows a diagnosis regarding the good working condition of the
wideband probes 4, 6 and transmission of this diagnosis to the computer 1.
When
the protection system 73 of one of the probes 4, 6 is solicited, the
microcontroller
77 is informed of it and then warns the computer 1 of the presence of a fault
through a digital communication link 111.
The CU 16 allows the checking of the presence of voltage on the accessory 8
and
transmission of this condition to the computer 1. In the absence of
appropriate
voltage, or when the synchronization probe 14 is not connected, the
synchronization circuit 99, 101, 103, 105, 107 cannot operate normally. The
24
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microcontroller 77 then warns the computer 1 of it through the digital
communication 111.
Referring to Figure 12, there is illustrated a possible signal processing
sequence
performed by the apparatus to establish a diagnosis. Temporal correlations 64
may be used to determine the polarity of the discharges, the temporal distance
At
between a same discharge sensed by both probes 4, 6, the "gamma" correlation
coefficient, the diagnosis error probability, and the presence of another
plausible
diagnosis possibility.
A dedicated digital algorithm 66 assesses the possibility of the prevailing
presence
of radiation by calculating the number of oscillations in the portion of the
sensed
wave which exceeds the level of local noise. The presence of a high number of
oscillations, namely more than 8 to 15 cycles, is a typical symptom of
radiation.
The calculation of the number of oscillation cycles may be achieved on one or
both
channels 113, 115. The processing stops when radiation (f) is detected.
Another
algorithm 68 calculates the form factor (g) over the channel exhibiting the
highest
discharge amplitude. This form factor (g) corresponds to the ratio of the
spectral
bandwidth over the time length of the discharge pulse. These last two values
are
respectively estimated from the prevailing spectral line and from the temporal
marginal of the partial discharge represented in a time-frequency distribution
70.
This time-frequency. distribution 70 may correspond to a spectrogram 72, a
Wigner-Ville transform 74 or to a wavelet transform 76 of the signal. The time-
frequency distribution 70 may first be subjected to a time-frequency filtering
80
prior to calculation of the form factor (g) to remove the background noise
exhibiting
a substantially constant spectral power in the time domain. The temporal
portion of
the signal of the highest amplitude discharge is transmitted to a module 110
that
estimates the equivalent bandwidth (h) and the rise time (i) of the discharge.
The
results (a) to (i) of the various applied processings form as many potential
symptoms that are submitted to the user for letting him/her deduce a
diagnosis, or
submitted to a diagnosis algorithm 82.
CA 02608216 2007-11-13
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The algorithm of the diagnosis module 82 may, for example, use hard-wired
logic
("and", "or", threshold overrun), neural networks, an expert system, fuzzy
logic, a
genetic algorithm or a combination thereof to process the raw temporal signals
and/or the results of the previously described processings.
The probes 4, 6 are AC coupled, meaning that the DC voltage is not measured.
However, the digital converters may well exhibit a null offset. Furthermore,
the RF
signal of longer period than the duration of the digitization introduces a DC-
like
offset to which a slope and a parabola are added. These slow variations of the
signal should preferably be suppressed prior to the processing by correlation
64,
and even prior to the calculation of an interpolation. Two types of switchable
high-
pass filters 117, 119 arranged in parallel achieve this function. The first
filter 117
achieves, with the signal, the convolution of a spectral window to then
subtract this
result from the signal. The other filter 119 is similar to a 0 Hz notch
filter. This filter
119 convolutes a distribution obtained from the reverse Fourier transform of a
unitary pulse over the full spectral band to which a spectral window set at 0
Hz has
been subtracted. In this filter 119, the width of the cut-off window as well
as the
signal rejection level in the stopband may be adjusted. These filters 117, 119
eliminate the AM band radio signal.
A FM-TV filter 121 may be connected in series after the high-pass filters 117,
119.
This filter 121 is also of convolutional type with a filter function
consisting of a
reverse Fourier transform of a stopband pattern adjusted, according to the
request, on the various FM and TV bands. It is also possible to choose the
rejection of the FM band, of the FM and TV bands for the channels from 2 to 4,
or
yet of the FM and TV bands for the channels 2 to 4 in addition to the channels
5 to
13. The other filter parameters are the filter order, the rejection level of
the
stopband and the frequency smoothing of the filter pattern.
The correlation consists in achieving (4) the sum of the crossed products of
two
functions such as:
Corr(r)= Ex(tn)=Atn ¨r), with r = nT and nEZ (1)
n
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T being the sampling period. The sum applies where samples of x(t) and y(t-T)
exist. For a given value of t, when x(t) and y(t-t), or yet x(t) and -y(t-t),
show a
similarity, the correlation value reaches a maximum. Let us recall that the
signal is
digitized and quantized: a same waveform digitized with a small delay exhibits
a
different aspect. This remark has a direct relation with the fact that: the
position of
the correlation peak is defined temporally close to a half sampling period;
the
amplitude of the real correlation peak may be substantially different from the
examined peak. The drawbacks of the quantization are minor when the signal is
highly oversampled. Conversely, for a wideband type of signal, interpolation
of the
signal prior to correlation is preferable to minimize these effects. Here,
interpolation increases the number of samples from 2 to 5 times. The
interpolation
function used is the product of the sinc() function by a spectral window (e.g.
Blackman-Harris), but another interpolator may very well be used.
Clusterization depicted by module 62 can be achieved on the acquired signals
so
as to group signals having similar characteristics or traits into respective
clusters
and produce signatures characterizing the signals in the clusters as
aforesaid. The
subsequent processings may then be carried out on the signatures, resulting in
a
greater signal-to-noise ratio and a faster processing as the number of data to
be
processed is reduced.
Referring to Figure 13, there is shown a schematic block diagram providing
details
of the correlation module 64 illustrated in Figure 12. An interpolation 84
takes
place prior to a correlation 64'. Prior to correlation 64', on one of the
channels, the
temporal width of correlation is reduced to the required minimum 86 in order
to
increase the signal/noise ratio. This required minimum corresponds to the
portion
of the signal where it seems to emerge from the noise of a partial discharge.
It is
the channel exhibiting the highest amplitude discharge signal that is selected
for
this decoupling 88. This ensures that the start and the end of the discharge
will be
defined more acutely.
The result of the correlation 64' is a set of positive and negative peaks. A
simple
classification in absolute value of amplitude allows keeping the main peaks
90. In
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the delay calculation module 92, for each correlation peak, the value of the
corresponding variable t is multiplied by the constant required to obtain the
delay
At between both observations of the discharge. This delay is used by a module
94
which eliminates false discharges, namely those which exhibit a delay
exceeding
the propagation time between both probes 4, 6. The propagation time may be
estimated during calibration at the beginning of the test. Another module 96
estimates the probability of diagnosis error based on how much the maximum
correlation peak stands out against the other peaks. When the error
probability
exceeds a preset threshold 98, a warning of the existence of a second probable
candidate of diagnostic explanation is transmitted.
Finally, the data concerning the maximum correlation peak are kept 100 and
transmitted to the polarity estimation module 102 as well as to the
correlation
coefficient y calculation module 104. This last coefficient:
x(rn).Atn ¨)
(2)
y VEõx(t,7)2 . En At r)2
provides an assessment of the similarity between the two compared discharge
signals.
The correlation module 64 also provides the value of the channel-to-channel
delay
At 106 of the partial discharge as well as the temporal trace portion of the
highest
amplitude discharge 108.
The calibration of the propagation delay between the wideband probes 4, 6 is
usually carried out at the start of a diagnosis test. Following the injection
of the test
signal in the synchronization probe 14, the response sensed by each wideband
probe 4, 6 is analyzed to estimate the propagation delay between these two
probes 4, 6. To this effect, a phase and amplitude correction filter (not
shown as
such, but embodied by the computer 1) reshape the signal detected by the probe
located on the farthest side from the synchronization probe 14 (for example in
28
pad MeV ego8 a
= CA 02608216 2007-11-14
14
MARCH 2007 '1 4 .03 .07;
Figure 2 it will be the signal from the first probe 4). The reshaping is
intended to
correct the distortion that the propagation wave has sustained in its passage
across the accessory 8. The reshaping of the signal from the other probe is
optional. Afterwards, the propagation delays are estimated by correlation with
the
signal injected by the synchronization probe 14. This last correlation uses
the
elements 84, 86, 88, 64, 90, 100 and 102 illustrated in Figure 13. The
polarity of
the signal confirms the test configuration. For example, for the diagram in
Figure 2,
the polarity must appear reversed otherwise one of the sensor 4, 6 is inverted
from
the other sensor. The sum of both delay estimations provides an estimate of
the
propagation delay between both wideband probes 4, 6 for the diagrams in
Figures
1 and 2. For the configuration illustrated in Figure 3, the difference between
both
delay estimations is used. The response sensed by the wideband probes 4, 6
derived from the test signal may thus also be used to check the configuration
of
the probes 4, 6 as a function of the polarity of the acquired signals.
The partial discharges are characterized by a short duration wideband pulse.
The
form factor 68 is informative as to what extent the digitized event conforms
to that
last characteristic. The form factor 28 is derived from the spectrogram Ximi
72
filtered in time-frequency of the signal xn. This factor is defined as the
moment of
inertia in the spectral domain over the moment of inertia in the time domain
such
as
1. Xi ms = 0-02
F ____________________________________________________________ (3)
.(m ¨ ms )2
where
Xims is the Fourier transform corresponding to the time slot ms coinciding
with the maximum amplitude of the discharge,
im is the time marginal with m as time slot index of the spectrogram such
as im = ,
c is either equal to zero or equal to the gravity center of Xi such as
c =Ei xi = i/Ei xi .
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Figure 14 illustrates a spectrogram Xim such as i is the frequency index and m
is
the time index, whereas Figure 15 provides the filtered result. In Figure 14,
time is
in x-axis whereas frequency is in y-axis (the shown values are not to scale).
The 3
horizontal rippled lines correspond to undesired radio modulations. In Figure
15,
the 3 radio modulation signals appearing in Figure 14 have been eliminated.
The filter function writes as:
{exp(log(Xim )¨ (Si + x)) when log(X + x)
1 when log(X
4õ,,)<(Si + x) (4)
with
Ek log(Xi,k
= ____________________________________________________ (5)
Ek 1
where k E In 1E .log(Xim _< Marginal threshold in time domain/.
As a function of the duration of the digitization and the duration of a
typical
discharge, the marginal threshold in the time domain is adjusted to obtain the
fraction of the time slots in which noise is mainly observed. It is thus
preferable to
have digitized data before occurrence of the discharge in order to have a
measurement of noise. In the algorithm, the adjustment of the marginal
threshold
is achieved by trial and error by reducing the search range by a factor of two
at
each trial. For each tested threshold value, there is obtained a ratio of time
slots
below this threshold. The iterative adjustment stops when the obtained ratio
is
close to the one required.
Thus, Si provides a plausible estimate of the mean spectral density of noise
(in
dB) during the discharge. The constant x is expressed in decibel and allows
keeping only the signal emerging of x decibels from noise. The result is an
adapted noise levelling at each spectral line.
PCTICA 2006/ 0908 II
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=
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MARCH = 24:167. 1 4 .83 .07
The same algorithm can be applied on the spectral amplitude or power rather
than
on the logarithm of Xi,m (i.e. log(*) is replaced by * and exp(*) by *).
However, the
version using the logarithm is much more efficient.
The signals of close partial discharges are characterized by higher amplitude
lobe
followed by a small number of highly damped oscillations. There sometimes
appears a half-cycle of oscillation prior to the higher amplitude lobe: this
pre-peak
oscillation has a low amplitude and is not considered. The rise time is thus
defined
as being the passage from 5% to 95% of the rise of the higher amplitude lobe.
From the portion of the signal containing the higher amplitude discharge, the
module 110 (Figure 12) applies a Fourier transform and estimates the typical
frequency (or characteristic frequency) of the discharge. A particularity of
the
digital processing is that there is spectral interpolation by addition of
zeros on both
sides of the signal in time domain before application of the Fourier
transform. The
inverse of the typical frequency, multiplied by a calibration coefficient,
provides an
estimate of the rise time. The advantage of this calculation is that it is
based on the
whole of the points of the discharge signal in the time domain: the result is
thus
stronger. to noise.
Referring to Figures 21 to 27, there are shown possible schematic diagrams of
the
electronics of the CU 16. Figure 21 more specifically relates to the circuit
of the
microcontroller 77. The channels for the wideband probes are respectively
attributed the names "yellow channel" and "blue channel". There is seen in the
Figure that there are two power supply measurements for these two channels
(pins 19 to 22 of U1): a voltage measurement upstream of the fuse and a
measurement downstream of the fuse. These measurements allow detecting a
malfunction of one of the probes 4, 6. The circuit also comprises a connector
CN1
for programming purposes and another connector COM1 for RS-232
communications. One of the analog outputs (pin 3) of the microcontroller (U1)
is
used to inject a test signal in the wideband probes 4, 6 while the other (pin
2) is
used to inject a test signal in the synchronization probe 14.
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Figures 22 and 23 illustrate power supplies for the wideband probes 4, 6. The
output of a power supply simultaneously contains the required current for
operation of the electronics of the probe 4, 6 and the signal indicating the
controlled gain to the electronics of the probe 4, 6. This signal containing
the gain
control is a voltage superimposed on the supply voltage, voltage which is
controlled by the microcontroller U1. The circuit of a power supply is thus
formed
of a reference voltage source (U14/U13), a digital/analog converter (U8/U9), a
voltage adder (U5) and a voltage regulator (U10/U11).
Figures 24 and 25 illustrate schematic diagrams of the amplifier and
conditioning
wideband circuits (i.e. RF) of the signals from the wideband probes 4, 6. On
the
left hand side, there is seen the presence of the injection of the power
supply
signal of the probe 4, 6 through an inductor (L2/L3) and the presence of a RF
coupling capacitor (C27/C31). The purpose of these two components is to
decouple the power supply circuit from the amplifier circuit. This signal and
supply
shunt filter is followed by a protection and by a high-pass filter (PBLP_39),
itself
followed by an impedance matching stage (U16, U17/U20, U19) and by filters
(PBLP_39) arranged in parallel and switchable. The switching is controlled by
the
microcontroller (U1) via 4 bits. On the upper left hand side, there is shown
an input
for the injection of the test signal for the "zero". A last amplifier stage
(U18/U22)
transmits the processed signal in a 50 Ohms line.
Figure 26 illustrates the conditioning of the signal of the synchronization
probe 14.
The circuit begins on the right hand side with a shunt filter (C15) between
the test
signal (RF) transmitted to the probe and the signal coming from the probe (60
Hz).
This last signal, after passage in an inductor (L1), is clipped close to the
passages
by zero. The signal so clipped is directed in a low pass filter and a narrow
band
pass follower filter. This last filter is connected to a low pass filter
identical to the
first low pass filter in order to preserve the same delay. These signals
filtered by
the low pass filters are transmitted to the microcontroller (U1) that controls
the
response of the band pass filter so that a 360 degrees phase shift is observed
between the outputs of the two low pass filters. A last amplifier circuit
shown at the
bottom of the Figure provides the microcontroller (U1) with the sinusoidal
signal
32
CA 02608216 2007-11-13
WO 2006/122415 PCT/CA2006/000811
filtered and in phase with that of the synchronization probe 14. This signal
is used
by the microcontroller (U1) to estimate the 60 Hz phase as a function of time.
Figure 27 shows the various power supply circuits of the circuits of the
control unit
16.
Referring to Figures 4 and 12, a scaled down version of the apparatus may be
used to quickly check if an electrical equipment is plagued with partial
discharges,
and thus requires further testing with the more sophisticated version of the
apparatus or should be approached with caution. The scaled-down version of the
apparatus may have only one measurement probe 4 (or 6) instead of two, and the
digital processing unit 1 may be reduced to the clusterization module 62, the
module 70 for performing a time-frequency distribution (of the clusterized
signals),
the form factor estimating module 68, the module 110 for determining rise
times
(of the clusterized signals), and the diagnosis module 82 which in that case
is
responsive to results generated by the form factor estimating module 68 and
the
module 110 for determining rise times, and produces a warning signal as a
function of the diagnosis to report the detection of partial discharges if
necessary.
All the electronics of the digital processing unit 1 and of the control unit
16 related
to the second measurement channel (when two measurement probes are used)
may thus be omitted. Preferably, in the scaled down version, the calibration
probe
14 will be such that it does not require any contact with the equipment to be
tested
to provide an indication of the phase angle in the equipment. Depending on the
type of probe 14 used in this respect, it may be necessary to appraise the
phase
angle based on a signal indicative of the phase angle detected by the probe 14
(for example, a measurement of current). As the phase angle is mainly used for
timing and tracking purposes in the scaled down version of the apparatus, it
will be
unimportant if the appraised phase angle does not coincide with the real phase
angle in the equipment provided that the phase error remains approximately
constant. Preferably, phase resolved information is used to match clusters of
opposite polarity discharges. For diagnostic purposes, probability of
discharge
presence is increasing with the existence of two clusters of opposite
polarities.
33
CA 02608216 2013-07-02
The scope of the claims should not be limited by the preferred embodiments set
forth hereinabove, but should be given the broadest interpretation consistent
with the description as a whole.
34
