Abstract:
In order to detect, localize and interpret a partial discharge occurring in a partial discharge site along an electrical equipment, two measurement probes and a synchronization probe are installed along the electrical equipment. The measurement probes detect pulses travelling in the electrical equipment while the synchronization probe detects a phase angle in the electrical equipment and is usable for calibration purposes. A control unit receives the signals sensed by the probes and conditions them. Digital processing applied on the conditioned signals, involving their correlation, a time-frequency distribution and a form factor estimation, allows establishing a diagnosis indicating a detection of a partial discharge and its localization along the electrical equipment. A wideband magnetic probe may be provided for detecting the pulses traveling in the electrical equipment.

Description:
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. 
     U.S. Pat. No. 6,809,523 (Ahmed et al.), U.S. Pat. No. 5,530,364 (Mashikian et al.), U.S. Pat. No. 5,767,684 (Steennis), U.S. Pat. No. 6,420,879 (Cooke), U.S. Pat. No. 6,507,181 (Pakonen et al.), U.S. Pat. No. 6,418,385 (Hücker et al.), U.S. Pat. No. 6,255,808 (Hücker), U.S. Pat. No. 6,297,645 (Eriksson et al.), U.S. Pat. No. 6,392,401 (Cooke), U.S. Pat. No. 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. 
     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. 
     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. 
     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.       

     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;   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.       

     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.       

     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 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 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 cross section of a power line cable that may be used in accordance with an embodiment of the system described herein. 
         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 partial discharge that may be used in accordance with an embodiment of the system described herein. 
         FIG. 15  is a picture illustrating a spectrogram of a partial discharge after application of a time frequency filter that may be used in accordance with an embodiment of the system described herein. 
         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. 
         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. 
     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. 
     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  FIG. 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.  FIG. 17  illustrates a possible construction of the measurement probes  4 ,  6 . 
     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  ( FIG. 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  FIG. 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  FIG. 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 with a delay corresponding approximately to the propagation delay of a wave between both probes  4 ,  6 . 
     Referring to  FIG. 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 prior 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 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 Microsoft™ Excel™. 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). 
     Referring to  FIG. 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  ( FIGS. 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  ( FIGS. 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, voltage or current measurement type. The synchronization probe  14  may be connected to the tested accessory  8  as illustrated in  FIG. 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  ( FIG. 9 ). The illustrated probes  4 ,  6  are especially designed to operate with power cables  10  having a neutral sheath  28  ( FIGS. 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). 
     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  FIG. 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. 
     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: 
               
               
                   
                 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  FIGS. 5 ,  6 A and  6 B, 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  FIG. 6B . 
     Referring to  FIG. 7 , each magnetic probe  4 ,  6 , having a single conductive loop  32 , senses the axial magnetic component  30  of the wave. 
     Referring to  FIG. 8 , each wideband magnetic probe  4 ,  6  looks like a removable clamp made of a magnetic sensing conductive loop  32  covered with a conductive 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  ( FIG. 6 ) surrounding the cable  10  and to ambient electric fields. 
     Referring to  FIG. 9 , the magnetic sensing loop  32  is connected to an amplifier circuit  42  integrated in the shielded housing of the clamp  34 . 
     Referring to  FIG. 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  FIG. 4 ). 
       FIG. 18  illustrates a possible diagram of the amplifier circuit  42  of a probe  4 ,  6 . 
     Referring to  FIGS. 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 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  FIG. 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 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 . 
     Across a same wire  19 ,  21  ( FIG. 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  ( FIG. 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 microcontroller  77  then warns the computer  1  of it through the digital communication  111 . 
     Referring to  FIG. 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 Δt 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 . 
     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   ⁡     (   τ   )       =       ∑   n     ⁢       x   ⁡     (     t   n     )       ·     y   ⁡     (       t   n     -   τ     )             ,       with   ⁢           ⁢   τ     =       nT   ⁢           ⁢   and   ⁢           ⁢   n     ∈   Z               (   1   )               
T being the sampling period. The sum applies where samples of x(t) and y(t−τ) exist. For a given value of t, when x(t) and y(t−τ), or yet x(t) and −y(t−τ), 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  FIG. 13 , there is shown a schematic block diagram providing details of the correlation module  64  illustrated in  FIG. 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 the delay calculation module  92 , for each correlation peak, the value of the corresponding variable τ is multiplied by the constant required to obtain the delay Δt 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 γ calculation module  104 . This last coefficient: 
                   γ   =         ∑   n     ⁢       x   ⁡     (     t   n     )       ·     y   ⁡     (       t   n     -   τ     )                 ∑   n     ⁢         x   ⁡     (     t   n     )       2     ·       ∑   n     ⁢       y   ⁡     (       t   n     -   τ     )       2                       (   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 Δt  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  FIG. 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  FIG. 13 . The polarity of the signal confirms the test configuration. For example, for the diagram in  FIG. 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  FIGS. 1 and 2 . For the configuration illustrated in  FIG. 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 X i,m    72  filtered in time-frequency of the signal x n . This factor is defined as the moment of inertia in the spectral domain over the moment of inertia in the time domain such as 
                   F   =         ∑   i     ⁢       X     i   ,     m   s         ·       (     i   -   c     )     2             ∑   m     ⁢         x   ^     m     ·       (     m   -     m   s       )     2                   (   3   )               
where
         X i,m     s    is the Fourier transform corresponding to the time slot m s  coinciding with the maximum amplitude of the discharge,   {circumflex over (x)} m  is the time marginal with m as time slot index of the spectrogram such as       

     
       
         
           
             
               
                 
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       FIG. 14  illustrates a spectrogram X i,m  such as i is the frequency index and m is the time index, whereas  FIG. 15  provides the filtered result. In  FIG. 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  FIG. 15 , the 3 radio modulation signals appearing in  FIG. 14  have been eliminated. 
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                     Y     i   ,   m       =     {           exp   ⁡     (       log   ⁡     (     X     i   ,   m       )       -     (       S   i     +   χ     )       )               when   ⁢           ⁢     log   ⁡     (     X     i   ,   m       )         ≥     (       S   i     +   χ     )               1           when   ⁢           ⁢     log   ⁡     (     X     i   ,   m       )         &lt;     (       S   i     +   χ     )                       (   4   )               
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     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, S i  provides a plausible estimate of the mean spectral density of noise (in dB) during the discharge. The constant χ is expressed in decibel and allows keeping only the signal emerging of χ decibels from noise. The result is an adapted noise levelling at each spectral line. 
     The same algorithm can be applied on the spectral amplitude or power rather than on the logarithm of X i,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  ( FIG. 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  FIGS. 21 to 27 , there are shown possible schematic diagrams of the electronics of the CU  16 .  FIG. 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 U 1 ): 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 CN 1  for programming purposes and another connector COM 1  for RS-232 communications. One of the analog outputs (pin  3 ) of the microcontroller (U 1 ) 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 . 
       FIGS. 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 U 1 . The circuit of a power supply is thus formed of a reference voltage source (U 14 /U 13 ), a digital/analog converter (U 8 /U 9 ), a voltage adder (U 5 ) and a voltage regulator (U 10 /U 11 ). 
       FIGS. 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 (L 2 /L 3 ) and the presence of a RF coupling capacitor (C 27 /C 31 ). 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 (U 16 , U 17 /U 20 , U 19 ) and by filters (PBLP_ 39 ) arranged in parallel and switchable. The switching is controlled by the microcontroller (U 1 ) 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 (U 18 /U 22 ) transmits the processed signal in a 50 Ohms line. 
       FIG. 26  illustrates the conditioning of the signal of the synchronization probe  14 . The circuit begins on the right hand side with a shunt filter (C 15 ) 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 (L 1 ), 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 (U 1 ) 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 (U 1 ) with the sinusoidal signal filtered and in phase with that of the synchronization probe  14 . This signal is used by the microcontroller (U 1 ) to estimate the 60 Hz phase as a function of time. 
       FIG. 27  shows the various power supply circuits of the circuits of the control unit  16 . 
     Referring to  FIGS. 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. 
     While embodiments of the invention have been illustrated in the accompanying drawings and described above, it will be evident to those skilled in the art that changes and modifications may be made therein without departing from the essence of this invention.