# Patent

## Title
Location method for locating a partial discharge emission zone and a device associated with said method

## Abstract

A method of locating a partial discharge emission zone and to the associated device. The method is characterized in that it comprises a step of measuring partial discharge signals by means of four identical measurement channels each including a VHF and/or UHF detector, the four VHF and/or UHF detectors being positioned at the four vertices of a square or rectangle in such a manner that the partial discharge emission zone is determined inside the square or rectangle.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS OR PRIORITY CLAIM
This application is a National Phase of PCT/EP2011/059947, filed Jun. 15, 2011, entitled, “METHOD FOR LOCATING A PARTIAL DISCHARGE TRANSMISSION AREA AND ASSOCIATED DEVICE”, which claims the benefit of French Patent Application No. 10 54772, filed Jun. 16, 2010, the contents of which are incorporated herein by reference in their entirety.
TECHNICAL FIELD AND PRIOR ART
The invention relates to a method of locating a partial discharge emission zone and to a locating device that is suitable for implementing the method.
Partial discharge measurements are performed, in the factory, on power transformers so as to ensure that they are functioning correctly. Depending on the conditions of the surrounding environment, partial discharge measurements may sometimes be disturbed by the presence of interfering discharges coming from electricity sources external to the transformers. The problem thus arises of how to locate those interfering discharges so as to be able to eliminate them.
Various techniques are known from the prior art for locating external discharges.
A first known location technique uses an ultrasonic detector. A first problem encountered by the use of an ultrasonic detector is its sensitivity to disturbances other than those caused by discharges. A second problem is the considerable time period that may be required to locate the source of a partial discharge when said source is situated on a large site.
A second known technique uses an ultra-violet camera. A first problem encountered by the use of an ultra-violet camera is that it is not possible to locate solely discharges that ionize air (corona discharges). Another drawback of that technique is its very high cost.
For both of the above-mentioned prior art techniques, it is also necessary for the discharge activity to be continuous and not sporadic because, otherwise, the detection of discharges is very highly random. That represents another drawback.
The method of the invention does not present the above-mentioned drawbacks.
SUMMARY OF THE INVENTION
The invention relates to a method of locating a partial discharge emission zone, characterized in that it includes:

    
    
        a step of calibrating four substantially identical measurement channels, each including a very high frequency (VHF) detector and/or ultra high frequency (UHF) detector, the step of calibrating providing, for each measurement channel, a calibration data associated with the measurement channel;
        a step of measuring partial discharge signals by means of the four measurement channels, the four VHF and/or UHF detectors being positioned in an (X, Y) plane, a detector D1 is placed at point (0, 0), a detector D2 at point (0, Y2), a detector D3 at point (X3, Y2), and a detector D4 at point (0, X3);
        a step of calculating a time difference TOAD2−TOAD3, corrected using the calibration data resulting from the calibration step, between the reception times of the signals measured by the measurement channels including the detectors D2 and D3, respectively;
        a step of calculating a time difference TOAD4−TOAD1, corrected using the calibration data resulting from the calibration step, between the reception times of the signals measured by the measurement channels including the detectors D4 and D1, respectively;
        a step of calculating a time difference TOAD1−TOAD2, corrected using the calibration data resulting from the calibration step, between the reception times of the signals measured by the measurement channels including the detectors D1 and D2, respectively;
        a step of calculating a time difference TOAD3−TOAD4, corrected using the calibration data resulting from the calibration step, between the reception times of the signals measured by the measurement channels including the detectors D3 and D4, respectively; and
        a step of locating a partial discharge emission zone at the intersection of the straight lines Xd1, Xd2, Yd1, and Yd2, defined in the (X, Y) frame of reference by the following equations:

Xd1=(X3/2)+((TOAD2−TOAD3)/2)×c 

Xd2=(X4/2)−((TOAD4−TOAD1)/2)×c 

Yd1=(Y2/2)+((TOAD1−TOAD2)/2)×c 

Yd2=(Y3/2)−((TOAD3−TOAD4)/2)×c 

    
    


where c is the speed of light.
In an improvement of the invention, the method includes two additional measurement channels that are substantially identical to the four measurement channels, the two additional detectors D5 and D6 of the two additional measurement channels being positioned in a plane that is parallel to the (X, Y) plane, vertically offset relative to the detectors D1 and D2, respectively. It is therefore possible to determine an additional discharge emission zone by means of the four detectors D1, D2, D5 and D6, in a (Y, V) plane, the axes X, Y, V defining a right-handed system of axes [X, Y, V].
Thus, when the plane partial discharge emission zones are simultaneously located in the respective planes (X, Y) and (Y, V), the method of the invention makes it possible to determine whether there exists a partial discharge emission zone in the form of a volume having projections onto the (X, Y) and (Y, V) planes that constitute the determined plane emission zones.



BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention appear more clearly in the following description, made with reference to the accompanying figures, in which:
FIG. 1 shows a device that is suitable for implementing a preliminary calibration step necessary for implementing the method of the invention;
FIG. 2 shows an example of a device for implementing the method of the invention;
FIGS. 3A-3D show measuring signals delivered by the UHF detectors of the device shown in FIG. 2; and
FIG. 4 shows an improvement of the device shown in FIG. 2.



In all of the figures, the same elements have the same references.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
FIG. 1 shows a device that is suitable for implementing a preliminary calibration step necessary for implementing the method of the invention.
As described below on reading the preferred embodiment of the invention, four measurement channels are used to implement the method of the invention. Each measurement channel is constituted, for example, by a high-frequency detector Di (e.g. a UHF detector), a long coaxial cable Ki, an amplifier Ai and a short coaxial cable ki. By way of example, the long coaxial cable Ki has a length in the range 5 meters (m) to 50 m and the short coaxial cable has a length that is substantially equal to 1 m. Other cable configurations between the detector Di and the amplifier Ai are also possible, the cable configurations depending solely on convenience. The high-frequency detector operates, for example, in the UHF frequency band at 300 megahertz (MHz) to 1000 MHz. The high-frequency detector may also operate in a VHF/UHF frequency band, e.g. the 200 MHz-1000 MHz band. The VHF/UHF detectors selected are of the omnidirectional type so as to be able to detect discharges in all directions.
The detectors Di and the amplifiers Ai of the various measurement channels are selected to be identical from one measurement channel to another. In addition, the lengths of the various cables are selected to be identical from one measurement channel to another. The calibration step consists in sending a high-frequency signal SHF to the four UHF detectors from an impulse calibrator CAL (emitting a signal having a rising front of a few picoseconds that excites waves in the 200 MHz-1000 MHz frequency band). An oscilloscope OSC then measures the received electrical signal as a function of time at the end of each measurement channel. The oscilloscope is triggered by one of the four detected signals and the time differences between the various signals are measured. The measurement channels are adjusted (cable length, amplification) until the times differences measured do not exceed a threshold value, e.g. equal to 5 nanoseconds. The measured time differences that are less than the threshold value are stored in memory for subsequent incorporation in the calculations (c.f. the data dcal with reference to FIG. 2).
FIG. 2 shows a device for implementing the method of the invention. The four high-frequency detectors D1-D4 define a square or rectangular surface inside which a presumed partial discharge source is to be located. The measurement channels Di, Ki, Ai, ki (i=1, 2, 3, 4) are all connected to a computer C or, as in the preliminary calibration step, to an oscilloscope. In the remainder of the description, reference is made exclusively to the measurement channels being connected to a computer. The computer C receives as input signals the signals delivered by the four measurement channels. It also receives, as calculation parameters, the previously-measured calibration data dcal and the position data dp that represent the known positions of the four detectors D1-D4 in a reference (X, Y) plane where, for example, the detector D1 is placed at point (0, 0), the detector D2 at point (0, Y2), the detector D3 at point (X3, Y3=Y2) and the detector D4 at the point (X4=X3, 0).
Whenever a partial discharge takes place in the zone defined by the four detectors, the signals measured by the four measurement channels are transmitted to the computer C. The computer C thus determines four time differences corrected by the calibration data dcal namely:

    
    
        TOAD2−TOAD3: difference between the reception times of the signals detected by the measurement channels associated with the detectors D2 and D3, respectively;
        TOAD4−TOAD1: difference between the reception times of the signals detected by the measurement channels associated with the detectors D4 and D1, respectively;
        TOAD1−TOAD2: difference between the reception times of the signals detected by the measurement channels associated with the detectors D1 and D2, respectively; and
        TOAD3−TOAD4: difference between the reception times of the signals detected by the measurement channels associated with the detectors D3 and D4, respectively.
    
    


The zone Z from which the partial discharges are emitted is thus defined by the horizontal straight lines Xd1 and Xd2 and the vertical straight lines Yd1 and Yd2 as defined in the (X, Y) frame of reference by the following equations:

Xd1=(X3/2)+((TOAD2−TOAD3)/2)×c 

Xd2=(X3/2)−((TOAD4−TOAD1)/2)×c 

Yd1=(Y2/2)+((TOAD1−TOAD2)/2)×c 

Yd2=(Y2/2)−((TOAD3−TOAD4)/2)×c 

Where c is the speed of light in free air (3×108 meters per second (m/s)).
In the preferred embodiment of the invention, the computer C includes a display device that displays on a screen the zone Z defined by the four straight lines Xd1, Xd2, Yd1, and Yd2.
When the partial discharge source is situated substantially at the center of the rectangle or square defined by the position of the four UHF detectors, the zone Z is a zone that is substantially a point that is situated substantially at the center of the rectangle. When the partial discharge source is not at the center of the rectangle, the zone Z is an area of greater or lesser extent that is not centered in the rectangle.
In advantageous manner, when searching for partial discharge in a building of large dimensions (typically 30 m×30 m) where the UHF detectors are placed in the four corners of the hangar, in general a side of the zone Z does not exceed a few meters. Thus, it is generally easy to locate visually the exact position of the partial discharge source in the determined zone Z. When visual location is not possible, the zone Z occupying a small extent, the use of an ultrasonic detector oriented towards the zone Z also enables the discharge source to be located rapidly.
In an example, FIGS. 3A-3D show measuring signals SA-SD delivered by the measurement channels of the device of the invention that are associated with detectors D1-D4, respectively. In FIGS. 3A-3D, the times t1-t4 thus show various times where a single partial discharge is detected by the detectors D1-D4, respectively.
FIG. 4 shows an improved device for implementing the method of the invention. In the improved device, the device of the invention includes six measurement channels. In addition to the four above-mentioned measurement channels, two additional measurement channels that are substantially identical to the above-mentioned four measurement channels are associated with two additional UHF detectors D5 and D6. As already mentioned above, each measurement channel includes a high-frequency detector, e.g. a long coaxial cable, an amplifier, and for example, a short coaxial cable. For reasons of convenience, only the detectors D1-D6 are shown in FIG. 4.
The two additional detectors D5 and D6 are situated vertically above the detectors D1 and D2, respectively, in a plane parallel to the plane defined by the detectors D1-D4. In the same manner as the detectors D1-D4 make it possible to define the (X, Y) plane, the detectors D1, D2, D5, D6 make it possible to define a (Y, V) plane that is perpendicular to the (X, Y) plane. The axes X, Y, V thus define a right-handed system of axes [X, Y, V]. The positions of the detectors D1, D2, D5, and D6 in the (Y, V) frame of reference are such that the detector D1 is placed at the point (0, 0), the detector D2 at the point (Y2, 0), the detector D5 at the point (0, V5), and the detector D6 at the point (Y6=Y2, V6=V5).
As mentioned above, the detectors D1-D4 make it possible to define, in the (X, Y) frame of reference, a plane zone Z at the intersection of the straight lines having respective equations:

Xd1=(X3/2)+((TOAD2−TOAD3)/2)×c 

Xd2=(X3/2)−((TOAD4−TOAD1)/2)×c 

Yd1=(Y2/2)+((TOAD1−TOAD2)/2)×c 

Yd2=(Y2/2)−((TOAD3−TOAD4)/2)×c 

In the improvement of the invention, the detectors D1, D2, D5 and D6 also make it possible to define, in the (Y, V) frame of reference, a plane zone Q at the intersection of the straight lines having respective equations:

Yd3=(Y2/2)+((TOAD5−TOAD6)/2)×c 

Yd4=(Y2/2)−((TOAD2−TOAD1)/2)×c 

Vd1=(V5/2)+((TOAD1−TOAD5)/2)×c 

Vd2=(V5/2)−((TOAD6−TOAD2)/2)×c 

where

    
    
        TOAD5−TOAD6 is the time difference, corrected using the calibration data, between the reception times of the signals detected by the measurement channels associated with the detectors D5 and D6, respectively;
        TOAD2−TOAD1 is the time difference, corrected using the calibration data, between the reception times of the signals detected by the measurement channels associated with the detectors D2 and D1, respectively;
        TOAD1−TOAD5 is the time difference, corrected using the calibration data, between the reception times of the signals detected by the measurement channels associated with the detectors D1 and D5, respectively; and
        TOAD6−TOAD2 is the time difference, corrected using the calibration data, between the reception times of the signals detected by the measurement channels associated with the detectors D6 and D2, respectively.
    
    


It is thus possible to define, in the right-handed system of axes [X, Y, Z], a volume W having a projection on the (X, Y) plane that occupies the area Z and having a projection on the (Y, V) plane that occupies the area Q. The volume W therefore locates the source of the partial discharge.

## Claims

1. A method of locating a partial discharge emission zone (Z), characterized in that it includes:
a step of calibrating four substantially identical measurement channels, each including a very high frequency (VHF) detector and/or an ultra high frequency (UHF) detector (D1-D4), the step of calibrating providing, for each measurement channel, a calibration data (dcal) associated with the measurement channel;
a step of measuring partial discharge signals by means of the four measurement channels, the four VHF and/or UHF detectors being positioned in an (X, Y) plane, a detector D1 is placed at point (0, 0), a detector D2 at point (0, Y2), a detector D3 at point (X3, Y2), and a detector D4 at point (0, X3);
a step of calculating a time difference TOAD2−TOAD3 between the reception times of the signals measured by the measurement channels associated with the detectors D2 and D3, respectively, said step of calculating being corrected using the calibration data;
a step of calculating a time difference TOAD4−TOAD1 between the reception times of the signals measured by the measurement channels associated with the detectors D4 and D1, respectively, said step of calculating being corrected using the calibration data;
a step of calculating a time difference TOAD1−TOAD2 between the reception times of the signals measured by the measurement channels associated with the detectors D1 and D2, respectively, said step of calculating being corrected using the calibration data;
a step of calculating a time difference TOAD3−TOAD4 between the reception times of the signals detected by the measurement channels associated with the detectors D3 and D4, respectively, said step of calculating being corrected using the calibration data; and
a step of determining the partial discharge emission zone at the intersection of the straight lines Xd1, Xd2, Yd1, and Yd2 defined in the (X, Y) frame of reference by the following equations:


Xd1=(X3/2)−((TOAD2−TOAD3)/2)×c 



Xd2=(X3/2)−((TOAD4−TOAD1)/2)×c 



Yd1=(Y2/2)+((TOAD1−TOAD2)/2)×c 



Yd2=(Y2/2)−((TOAD3−TOAD4)/2)×c 


wherein Xd1 and Xd2 are intersection points on the X-axis of vertical straight lines bounding the partial discharge emission zone, and Yd1 and Yd2 are intersection points on the Y-axis of horizontal straight lines bounding the partial discharge emission zone, and
wherein c is the speed of light.

2. A method according to claim 1 that further includes:
during the calibration step, an additional step of calibrating two additional measurement channels that are substantially identical to the four measurement channels, the additional step of calibrating providing, for each additional measurement channel, an additional calibration data (dcal ) associated with the additional measurement channel;
two additional steps of measuring partial discharge signals by means of the two additional measurement channels, the two additional detectors D5 and D6 of the two additional measurement channels being positioned in a plane that is parallel to the (X, Y) plane, vertically offset relative to the detectors D1 and D2, respectively;
a step of calculating a time difference TOAD5−TOAD6 between the reception times of the signals measured by the measurement channels associated with the detectors D5 and D6, respectively, said step of calculating being corrected using the additional calibration data;
an additional step of calculating a time difference TOAD1−TOAD5 between the reception times of the signals measured by the measurement channels associated with the detectors D1 and D5, respectively, said step of calculating being corrected using the additional calibration data;
a step of calculating a time difference TOAD6−TOAD2 between the reception times of the signals measured by the measurement channels associated with the detectors D6 and D2, respectively, said step of calculating being corrected using the additional calibration data; and
a step of determining a plane zone (Q) at the intersection of the straight lines defined in the equations:


Yd3=(Y6/2)+((TOAD5−TOAD6)/2)×c 



Yd4=(Y2/2)−((TOAD2−TOAD1)/2)×c 



Vd1=(V5/2)+((TOAD1−TOAD5)/2)×c 



Vd2=(V6/2)−((TOAD6−TOAD2)/2)×c 


wherein Y6 and V6 are, respectively, the position of the detector D6 in a (Y, V) frame of reference that is perpendicular to the (X, Y) frame of reference, the axes X, Y, V defining a right-handed system of axes [X, Y, V], and V5 being the position of the detector D5 in the (Y, V) frame of reference, and
wherein Yd3 and Yd4 are intersection points on the Y-axis of straight lines parallel to the V-axis and bounding the plane zone (Q) in the (Y,V) frame, and Vd1 and Vd2 are intersection points on the V-axis of straight lines parallel to the Y-axis and bounding the plane zone (Q) in the (Y,V) frame.

3. A method according to claim 2, wherein, whenever the partial discharge emission zone and the additional partial discharge zone are simultaneously determined, it is verified whether or not there exists a three-dimensional volume (W), said volume having on the (X, Y) plane a projection defined by the emission zone and having a projection on the (Y, V) plane defined by the additional emission zone and, if said volume exists it is determined as being the partial discharge zone.

4. A device for locating a partial discharge emission zone (Q), characterized in that the detectors Dl-D4 are positioned at the four vertices of a square or rectangle outlined in the plane referenced (X, Y), the device further including a computer (C) that determines the location of the emission zone in accordance with the method of claim 1.

5. A device according to claim 4, further including two additional measurement channels that are substantially identical to the four measurement channels, the two additional detectors D5 and D6 of the two additional measurement channels being positioned in a plane that is parallel to the (X, Y) plane, vertically offset relative to the detectors D1 and D2, respectively, the two additional measurement channels being connected to the computer (C).