CA2959420A1

CA2959420A1 - Compact apparatus for detecting faulty composite insulators used on electric distribution systems

Info

Publication number: CA2959420A1
Application number: CA2959420A
Authority: CA
Inventors: Christophe Volat; Richard Martin
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-03-01
Filing date: 2017-03-01
Publication date: 2018-09-01

Abstract

This application relates to a novel and compact apparatus for identifying high risk defective composite insulators with conductive or high permittivity defects independently of the position of the defects along the composite insulator. The apparatus is compact, portable, lightweight, and able to be used on energized installed insulators. The method used by this apparatus is simple in its concept as it does not require any calibration on a sound composite insulator in order to provide a reference measurement. The apparatus consists in an electric field probe using several measuring electrodes and a reference electrode. This electric field probe has the form of a circular clamp which, when clamped between the sheds of the composite insulator, permits to measure and detect a non-uniformity in the radial electric field component distribution that is caused by the presence of a conductive or semi-conductive internal defect between the core of the composite insulator and its envelope. The apparatus provides a simple "Go/No Go" output and a simple visualization method of the presence of the defect available by the user via a specific display program compatible with common Cellphone OS technology.

Description

= COMPACT APPARATUS FOR DETECTING FAULTY
COMPOSITE INSULATORS USED ON ELECTRIC
DISTRIBUTION SYSTEMS
BACKGROUND OF THE INVENTION
[001] Composite insulators are sed to mechanically support an electrical conductor while electrically insulating the conductor from the grounded structures.
Composite or non-ceramic insulators (NCI) have been widely used on electrical networks since the 1980s. Composite insulators can be found under multiple designs, materials and manufacturing processes. They are preferred to ceramic insulators because of their low weight, low installation cost, higher resistance, strength as well as higher contamination resistance due to their hydrophobic surface.

[002] However, the most important issue that limits the widespread application of composite insulators is the issue of assessing their in-service condition using actual live line diagnostic methods (LLDM). As reported by the literature [1], no single diagnostic technique has yet emerged that can identify all the possible types of damage that could exist in a composite insulator. This means that a variety of complementary LLDM and procedures must be employed in order to identify the absence of critical defects of composite insulators and to carry out live line work (LLW) safely on overhead lines equipped with composite insulators. This fact seems to be a limited factor in the application on composite insulator on wide scale.

[003] In order to ensure their safety when performing LLW with composite insulators, line men workers need to confirm the electrical integrity of the insulators for the duration of their work. For that, there are currently two identified apparatus which are used by the electrical compagnies all around the word.

[004] The first method used by the line men to verify the electrical integrity of the composite insulators is the apparatus for detecting defective insulators in an insulating column supporting an electrical conductor in a power circuit line [2].
Originally developped to identify defective ceramic insulators in a column made of a plurality of serially connected insulator members, this apparatus, commercially called the positron insulator tester, was later adapted for composite insulators. This apparatus is based on the measurement of the axial component of the electric field in front of the insulator at the extremity of the composite insulator sheds. The apparatus uses a pair of metallic electrodes which are displaced along the composite insulator. A measure is taken at each insulator shed extremity in order to record the distribution of the axial electric field component of the defective composite insulator. When a conductive defect is present, the distribution of the axial electric field component measured at the extremity of the insulator sheds is modified depending on the position and the length of the defect. In order to identify the presence of the defect, this apparatus proposes two operating modes. The first mode is a Go/ No Go mode which can detect immediately the presence of a defect closed to the High voltage (HV) electrode of the composite insulator. However, this function requires the adjustement of a threshold which must be selected and ajusted by the user on power-up depending on the power line voltage. The second mode is the investigation mode which is used to record and display on a Laptop the axial electric field distribution obtained at the tip of the sheds along the insulator. The investigation mode is generally used to validate the alarm obtained with the Go/
No Go mode. For that, the investigation mode must compare the recorded axial electric field distribution of the defective composite insulator to a recorded axial electric field distribution obtained from a sound insulator. The diagnostic is done via a Laptop PC and a specific software which records the data of the apparatus via bluetooth link. The Laptop is not intended to be used by the lineperson on top of the tower and must be used by the supervisor on the ground level.

However, this apparatus presents some limitations as reported by the litterature [1]. The proposed apparatus may not detect low severity defects near the end fitting of the composite insulators when there are equipped with a corona ring due to the electric field shielding induced by the corona ring. Also, it was reported that the use of this apparatus is quite demanding in terms of time/cost and expertise as two peoples (the lineman on the tower and the supervisor on the ground level) are required to provide a reliable diagnostic of the insulator integrity and a well trained person to adjust the thresold level of the GO/ NO
GO
mode. Finally, the Go/ NO GO mode does not permit to identify defect at the grounded end of the insulator as well as the defects at floating potential positionned in the middle of the insulator.

[005] The second method used by the line person to verify the electrical integrity of the composite insulators is the apparatus and method for identifying high risk non-ceramic insulators with conductive or high permittivity defects [3], also called polymer insulator tester. The diagnostic method used by this apparatus consists in submitting the defective composite insulator to a high voltage at various frequencies for a pre-determined amount of time to determine a resonance frequency of the defective composite insulator. The measured resonance frequency obtained for the defective insulator is then compare to a calibration result set obtained from a sound composite insulator of the same type that the potentially defective insulator tested. The high voltage of various frequency is applied between two electrodes positionned between the insulator sheds at spaced of about 152 mm. The electrode spacing corresponds to the section legnth of the insulator which can be tested. This method permits to detect high permittivity and conductive defects at any place along the composite insulator. However, with a weight of 2 kg, this apparatus when mounted on a hotstick is quite demanding in terms of manipulation for the lineperson working on the tower.

[006] The above-described apparatus and methods of the prior art presents some additional disadvantages than those mentionned previously. Both of them required to compare the different measurement obtained on the defective insulator with a reference measurement taken on a sound insulator of the same type than the inspected insulator. This implies that the user must be well trained in order to provide a reliable calibration of two mentionned apparatus and consequently to provide a reliable diagnostic of the electrical integrity of the composite insulator under test. Also, the two above-described apparatus of the prior art are not compact in size. Consequently, they are difficult to handle when mounted at the extremity of a hotstick. Moreover, due to their size, the section closed to the fly end of the composite insulator surrounding by a corona ring is not accessible. This can be problematic as this section closed to the HV
electrode of the insulator presents a high probability of internal defects. If this section can not be tested, this could represent a life-threatening situation for the lineperson. Accordingly, there is a need for an apparatus and method that can identify electrical integrity of installed polymer insulators BREF SUMMARY OF THE INVENTION

[007] It is a feature of this invention to provide a novel and compact apparatus which substantially overcomes all of the above-mentioned disadvantages of the prior art, which provides a compact apparatus for identifying high risk defective composite insulators with conductive or high permittivity defects independently of the position of the defects along the composite insulator. The apparatus is compact, portable, lightweight, able to be used on energized installed insulators.
The method used by the detector is simple in its concept which does not require a calibration on a sound composite insulator in order to provide a reference measurement. The apparatus provides a simple "Go/No Go" output and a simple visualization method of the presence of the defect available by the user via a specific display program compatible with common Cellphone OS technology.

[008] According to an aspect of the invention, the apparatus has the capacity to identify the presence of conductive, semi-conductive or high permittivity defects, both internal and external without electrical contact with these defects.
The apparatus is in the form of a circular clamp made of composite insulating material inside which a plurality of measuring electrodes are equally spaced around the diameter of the clamp. These measuring electrodes are surrounded by a coaxial reference electrode at a pre-determined spacing from the measuring electrodes. The measuring electrodes and reference electrode are positionned around the core of the composite insulator, between its sheds, and the voltage of each measuring electrodes referenced to the reference electrode is obtained thanks to the electronic measuring and detection system embeded in the apparatus. The measuring and references electrodes are positionned in a way to measure an image of the radial electric field component distribution around the insulator core.

[009] According to an other aspect of the invention, a method of evaluating insulators for defects includes the steps of providing a compact apparatus for identifying high risk insulators having a microprocessor, a plurality of uniformly spaced measuring electrodes, and a reference electrode. The method further includes the steps of engaging the uniformly spaced measuring electrodes and reference electrode around the core of a composite insulator to be tested, measuring the potential of each measuring electrode referenced to the reference electrode to determine the magnitude of the radial component of the electric field at each position of the measuring electrodes, and conducting measurements during a pre-determined amount of time to detect any distortion in the distribution of the radial electric field component around the core of the composite insulator to be tested.
[1] F. Schumk, J. Seifert, I. Gutman and A. Pigini, 'Assessment of the Condition of Overhead Line Composite Insulators', CIGRE WG B2-214, 2012.

[2] G. H. Vaillancourt and F. Risk, 'Apparatus for detecting defective insulators in an insulating column supporting an electrical conductor in a power line', US
patent 4760343 A, 1988.
[3] A. J. Phillips, M. Major, R. Carlton Lynch, P. N. Beverly and S. H. Moins, 'Apparatus and method for identifying high risk non-ceramic (nci) with conductive or high permittivity defects', US 20130043881 Al, 2013.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[010] The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

[011] FIG. 1 illustrates a schematic view of a composite insulator supporting a HV
electrical conductor of an overhead electric line;

[012] FIG. 2A and 2B present a numerical comparison of the distribution of the equipotential lines with and without the presence of a semi-conductive internal defect closed to the HV metallic electrode of an energised composite insulator respectively.

[013] FIG. 3A and 3B illustrates a numerical comparison of the distribution of the radial electric field lines obtained in the vicinity of the HV electrode of an energised composite insulator with and without the semi-conductive internal defect of FIG. 2.

[014] FIG. 4A and 4B presents an example of the distributions of the normalized axial and the radial electric field components (in pm.) around the core of the composite insulator at 3 mm of the surface between the HV electrode and the first shed obtained with and without the semi-conductive internal defect of FIG
2 respectively.

[015] FIG. 5A presents an illustration of an internal defect at the ground electrode of the composite insulator and FIG. 5B presents an example of the distributions of the normalized axial and radial electric field components (in p.u.) around the core of the composite insulator at 3 mm of the surface between the HV
electrode and the first shed obtained with the semi-conductive internal defect of FIG
5A.

[016] FIG. 6 is an example of the perspective view of the apparatus being installed on one end of a hotstick and engaged around the envelope between sheds of said composite insulator;

[017] FIG. 7 is a perspective view of an example of the construction of the apparatus with an example of the arrangement of the measuring and reference electrodes as well as the arrangement of the shielding protection for the electronic circuit of measurement, detection and data transmission;

[018] FIG. 8 presents an exemple of experimental results obtained with the apparauts of FIG. 7 obtained for an internal conductive defect positionned at the HV end of a 69 kV composite insulator.

[019] FIG. 9 is a schematic diagram of the measuring, detection and data transmission circuits associated with and contained within the apparatus;

[020] FIG. 10 presents an example of the type of display from the electronic circuit which permits to visualize the results obtained from FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

[021] Referring now to the drawings and more particularly to FIGS. 1, 2 and 3, there is shown generally a composite insulator 10 supporting an high voltage (HV) electrical conductor 11, as used in an overhead electrical line. At each extremity of the composite insulator, there is a metallic electrode: the HV electrode 12 which mechinally connect the insulator to the electrical conductor at HV
voltage and the ground electrode 13 which mechinally connect the composite insulator to the tower 14 at the ground potential. The composite insulator 10 is formed of a core 15 made of a fiber glass rod on which is molded the envelop 16 with the sheds made of composite materail like silicone.

[022] When the HV electrical conductor 11 is energised under HV service voltage, the distribution of the equipotential lines 17 that originates on this composite insulator can be represented numerically by FIG. 2A for a safe insulator. For the same composite insulator with a semi-conductive internal defect 18 present between the core 15 and the envelop 16, numerical simulation permits to show that the equipotential lines 17 are modified in the vicinity of the internal defect 18 and closed to the HV electrode 12, as shown in FIG. 2B. When the HV
electrical conductor 11 is energised, an electric field is also present around the composite insulator 10. FIG. 3A presents the distribution of the electric field lines 19 obtained for a safe composite insulator, in the vincinity of the HV
electrode 12. When a semi-conductive internal defect 18 is present, the distribution of the electric field lines 19 is significantly pertubed, as demonstrated by the FIG. 3B, in the vicinity of the position of the internal defect 18.

[023] The perturbation induced by the internal defect can be clearly visualized by the results of FIG. 4A where the distributions of the normalized radial 20 and axial 21 electric field components were computed between the HV insulator electrode and the first shed, on a circular line coaxial to the core 15 of the insulator at 3 mm from the surface of the envelope 16. The results of FIG. 4A clearly illustrate . .

that the presence of the defect leads to a significant perturbation of the both radial 20 and axial 21 electric field components. The perturbation induced in the radial electric field component 20 corresponds to an increase of about 62%.
This can be compared to the decrease of 27% obtained for the axial electric field component 21. The results can be compared to the results of FIG. 4B obtained without internal defect with a variation of less than 12% and 2% in obtained in radial 20 and axial 21 electric field component respectively. The larger variation of the radial electric field component 21 is principally due to the presence of the energized conductor 11 at the HV electrode 14, as illustrated by FIG. 1. These results clearly demonstrate that the radial electric field component 20 is more sensitive to the presence of the internal defect than the axial electric field component 21.

[024] When the internal defect 18 is present at the ground electrode 13 of the composite insulator 10, as illustrated by FIG. 5A, the perturbation induced by the internal defect on the radial 20 and axial 21 electric field components is presented on FIG. 5B. The perturbation induced in the radial electric field component 20 corresponds to an increase of about 56%, which can be compared to the decrease of 38% obtained for the axial electric field component 21.
These results demonstrate that the radial electric field component 20 is also more sensitive to the presence of the internal defect than the axial electric field component 21.

[025] As demonstrated by the numerical results of FIG. 2 to 5, the measurement of the uniformity of the distribution of the radial electric field component 18 around the surface of the composite insulator between the shed of the envelope 16 can be a good indication of the presence of an internal defect between the core 15 and the enveloppe 16 of the composite insulator 10 and this, independantly of the position along the composite insulator 10 of the internal defect 18.
Measuring the uniformity of the radial electric field component 20 around the _ surface of the core between the composite insulator sheds is the fundamental concept of the present invention which general concept is illustrated on FIG.
6.

[026] As illustrated on FIG. 6, the apparatus of the invention has the form of a circular clamp 30 which is engaged between the sheds of the composite insulator 10 and closed on the envelope 16 of the composite insulator in order to perform the measurement of the radial electric field component distribution. The clamp 30 is fixed at the end of a hotstick 23 thanks to a joint 22 which permits to orient the clamp 30 is several directions. The specific geometry of the clamp, as presented in detail in FIG. 7, allows the lineman to easily engage the clamp between the sheds of the envelope 16 of the composite insulator 10 by applying a pressure on the hotstick 23. The compact size and low weight of the clamp allows the lineman to easily perform measurement at predetermined positions along the composite insulator 10.

[027] As illustrated by FIG. 7, the apparatus of the invention has the form of a circular clamp 30 made of two parts : a fixed part 31 and a moving part 32. The moving part 32 is attached to the fixed part 31 via a hinge 33 containing a metallic spring (not represented on the FIG. 7). The metallic spring permits to maintain the fixed part 31 and moving part 32 in contact in order to close the clamp 30 on the envelope 16 of the composite insulator 10 once the clamp 30 is engaged between the sheds of the composite insulator 10. The extremity of the fixed and moving parts 31 and 32 present the same specific turned up geometry permitting to engage the clamp 30 around the composite insulator envelope 16 when the lineman applies a force on the opposite end of the hotstick 23. The internal diameter of the clamp 30 can be adjusted to the diameter betwwen shed of the composite insulator envelope 16 to ensure that the clamp 30 is well centered around the composite insulator 10. This also permits to the lineman to correctly orient the clamp 30 in a plane perpendicular to the longitudinal axis of the composite insulator as the joint 22 pivotally connected the clamp 30 to the hotstick 23 allows several degrees of freedom for the orientation of clamp 30.

[028] Once the clamp 20 is closed on the envelope 16 between sheds of the composite insulator 10, the uniformity of the radial electric field component around the surface of the envelope can be determined. As illustrated by FIG. 7, the determination of the uniformity of the radial electric field distribution is obtained by using a specific arrangement of electrodes 34 and 35. The measuring electrodes 34 present the same dimension equally spaced on the same perimeter inside the clamp 30 and axially positionned at the same distance than the center of the clamp 30. The reference electrode 35 is composed of two parts to permit the opening of the clamp. These two parts are electrically connected via the electricla link 36. The reference electrode 35 is positionned on a larger perimeter than the measuring electrodes 34 and centered with respect to the center of the clamp 30. In this manner, the distance separated the measuring electrodes 34 and the reference electrode 35 is exaclty the same. The presence of the reference electrode 35 is significant as it permits to provide an uniform reference potential around the measuring electrodes 34 by uniformise the radial component 20 of the electric field distribution around the measuring electrodes 34 which is not really uniform in the vicinity of the HV electrode 12, as demonstrated in FIG. 4B.

[029] The number and the size of the measuring electrodes 34 depends principally on the diameter of the insulator core. In the example of FIG. 7, the clamp 30 presents a configuration of six measuring electrodes 26. The number of measuring electrodes 26 can be adjuted to the diameter between sheds of the composite insulator envelope 26 in order to provide a optimal estimation of the radial electric field component distribution 20.

[030] When a conductive or semi-conductive internal defect 18 is present at the HV
electrode 12 (FIG. 3B) or the ground electrode 13 (FIG. 5A), a local increase of the radial electric field component 20 in the vicinity of the internal defect can be observed, as demonstrated by FIG 4A and 5B. This increase of the radial electric field component 20 leads to an increase of the potential of the measuring electrodes 34 situated on and closed to the internal defect position. This increase can be observed on the FIG. 8 by the experimental measurements obtained with a prototype of a clamp 30 containing six measuring electrodes 34, as presented in FIG. 7. The distribution 39 of FIG. 8 presents the normalized potential distribution of six measuring electrodes 34 with a peak value obtained for the measuring electrode 5 which was positionned directly on the internal defect 18 positionned at the HV electrode 12. The distribution 39 can be compared to the normalized potential distribution 40 obtained for the same position of the clamp 30 but without internal defect. In presence of an internal defect 18 at the HV

electrode 12, an increase of around 65% is obtained in the normalized potential distribution 39, compared to an variation of less than 15% when no internal defect is present. Such significant increase can then be easily detected in order to provide a diagnostic on the presence of a internal defect and, above all, this does not require the use of a comparison with any signature or signal reference obtained for a sound composite insulator, as used by actual apparatus presented in the state of art. The only requirement to provide a simple diagnostic is to detect the non uniformity of the potential distribution 39 obtained between the sheds of the composite insulator 10. This can be achieved in different manners using several statistical tools like the deviation relative to the average of potential measurements obtained. For example, in the case of the results presented on FIG. 8, the maximum deviation obtained is equal to 79% with an internal defect and less than 13% without defect. In this way, by fixing a deviation threshold of around 30%, all the deviations greater than this threshold correspond to the presence of an internale defect. The indication of the presence of an internal defect is provided via a light emitting device indication 38 which turns red or green dependently if the calculated deviation is greater or lower than the fixed threshold value.

[031] The potential of each measuring electrode 34 referenced to the reference electrode 35 is obtained via the measuring, amplification, signal treatment and detection and transmission electronic system embeded in the clamp 30 which will be described later. As presented on FIG. 7, the measuring, detection and transmission electronic system positionned in the outer part of the clamp 30 is protected from the electric field and partial discharges present around the composite insulator using a flexible shielding material 37 which acts as a Faraday cage. In order to match the circular shape of the clamp 30, the electronic system is obtained using flexible plastic substrate.

[032] FIG. 9 presents the block diagramm of the measuring, amplification, signal treatment and detection and transmission electronic system. The potential of each measuring electrode 34 is measured using the same electronic circuit associtated to each electrode 34 consisting in: a protection circuit 41, a programmable gain amplifier/attenuator 42 and a analogic/digital convertor (ADC) 43. The amplifier/attenuator 43 is controlled via the microcontroller 44 which can automatically adjust the gain to provide the optimal analogic voltage of the measuring electrode 34 before its digital conversion by the ADC 43.
Once digitized, the potential of each measuring electrode 34 is readed by the microcontroller 44 which computes the deviation or other mathematical tool relative to the average of the potential measurements of the measuring probes and verify if the value is higher or lower than the fixed thresold. If the result is higher, the light emitting device 38 is turned to red which indicates the presence of a defect where the clamp 30 is positionned along the composite insulator 10.
In contrary, the light emitting device 38 is turned to green if the value is lower than the fixed threshold, which also indicated the presence of an applied voltage on the composite insulator. In the same time, the normalized value of each measuring electrode potential is sent via the wireless communication module 45 to a smart device which can be positionned on the other extremity of the hotstick 22 in order to provide the lineman with the graphical display of the measuring electrodes potential, as illustrated by FIG. 10. The measuring, detection and transmission electronic system is powered by the module 46 containing batteries, a charger and a voltage regulator.

[033] FIG. 10A and FIG. 10B present two examples of a display of the potential distribution obtained for a clamp 30 equipped with six measuring electrodes

34. The two displays are constitued of the representation of the potential distribution 39 of the six measuring electrodes 34 numbered form 1 to 6, a circle 40 representing the average of the potential measurements and a circle 41 representing a threshold of 30%.
The display of FIG. 10A clearly illustrates the presence of an intenal defect at the HV
electrode positionned on the measuring electrode 5 which presents the higher potential value. Moreover, this value exceed the threshold 41 which confirms the presence of the internal defect. In the display of FIG. 10B, it can be observed that the distribution 39 is quite uniform and does not exceed the threshold 41, meaning that there is not present of internal defect where the clamp is positionned. The graphics presented on FIG.
10 can be displayed using smart devices like tablets or smart phones which can be linked to the clamp 30 via wireless communications. These displays will assist the lineman to perform the diagnostic of the composite insulator as it permits to the lineman to judge of the uniformity of the distribution of potential of the measuring electrodes 34. This is a diagnotic tool which is complementary to the automatic detection provided by the microcontroller 44 and the visual indication 38, then provided a double verification for the lineman.

Claims

1. A method for assessing, without electrical contact, the electrical integrity of composite insulators with conductive or semi-conductive internal defects, said composite insulators supporting an energized electrical conductor of an overhead line, said method comprising the steps of:
i. positioning an electric field probe in the form of a clamp around the envelope of the composite insulator and between its sheds ii. measuring the distribution of the radial component of the electric field around the composite insulator envelope between the sheds in order to detect a non-uniformity in the radial electric field component distribution that is caused by the presence of a conductive or semi-conductive internal defect between the core of the composite insulator and its envelope, and iii. interpreting the measured radial electric field component values to identify the presence of conductive or semi-conductive internal defect inside the composite insulator, and iv. repeating the steps (i) to (iv) at different positions along the composite insulator, starting from the HV electrode to the ground electrode, in order to locate the position of the conductive or semi-conductive internal defect, estimate the length of said internal defect and provide an assessment of operative condition of said composite insulator.

2. A method according to claim 1, wherein in step (iii) there is further provided the step of generating an audible and/or visual signal each time the presence of said internal defect is detected.

3. A method according to claim 1, wherein in step (iii) there is provided the step of transmitting a measurement signal to visualize the distribution of the radial electric field component around the core of said composite insulator.

4. A compact apparatus for instantly identifying and locating, without electrical contact, the presence of conductive or semi-conductive internal defects along composite insulators supporting an energized electrical conductor of an overhead line, said apparatus has the form of a circular clamp.

5. An apparatus according to claim 5, wherein said circular clamp is made of an assembly of two or more parts of insulating material with specific geometry arrangement which allow the said clam to be easily engaged and centered around the core of the composite insulator between sheds along a plane transverse to the longitudinal axis of said composite insulator.

6. An apparatus according to claim 5, wherein said clamp comprising:
electric field responsive means in the form of specific electrode arrangement to measure the distribution of the radial electric field component around the core of said composite insulator, means for engaging the said clamp around the core of said composite insulator between sheds and maintain the said clamp oriented substantially parallel to a plane transverse to the longitudinal axis of said composite insulator at predetermined positions along said composite insulator, means to measure the potential of the specific electrode arrangement, means to automatically detect the presence of a conductive or semi-conductive internal defect, and means to transmit and visualize the distribution of the radial electric field component values.

7. The apparatus according to claim 7, wherein said electric field responsive means is a specific arrangement of several measuring electrodes and a reference electrode positioned in a manner to measure the radial electric field component distribution around the core of said composite insulator.

8. The apparatus according to claim 8, wherein the measuring electrodes are uniformly spaced and centered on the core of said composite insulator once the clamp is engaged between the sheds and are positioned inside the said clamp in a manner to be oriented along a plane parallel to the longitudinal axis of the composite insulator.

9. The apparatus according to claim 8, wherein the reference electrode is made of two and more parts inserted respectively in each part of the said clamp, positioned in a manner to be oriented along a plane parallel to the longitudinal axis of the composite insulator and electrically connected between them one the clamp is closed.

10. The apparatus according to claim 10, wherein the reference electrode, once the clamp is closed on the core of said composite insulator between sheds, is centered on the said core and axially positioned around the measuring electrodes to provide a uniform floating potential which is used as a reference potential for the measurement of the potential of each measuring electrode.

11. The apparatus according to claim 11, wherein the potential of each measuring electrode referenced to the reference electrode provides a measurement of the distribution of the radial electric field component around the core of said composite insulator.

12. The apparatus according to claim 7, wherein means for engaging the said clamp around the composite insulator core refers to the specific design of the extremity of two parts of the said clamp which permits to easily engage said clamp pivotally connected at one end of an hotstick and to the internal diameter of the said clamp which can be adjusted to the outer diameter of the core of said composite insulator.

13. The apparatus according to claim 7, wherein means to measure the potential of specific electrode arrangement comprises an analogic circuit connected to each measuring electrode, each analogic circuit being connected to a microcontroller which measure the potential of each measuring electrode referenced to the reference electrode.

14. The apparatus according to claim 14, wherein the analogic circuit comprises a protection circuit to protect the microcontroller, an analogic attenuator and amplifier circuits whose gain is automatically adjusted by the microcontroller.

15. The apparatus according to claim 7, wherein means to automatically detect the presence of a conductive or semi-conductive internal defect is a specific algorithm implemented in the microcontroller which uses the potential of each measuring electrode to verify the uniformity of the distribution of the radial electric field component around the core of said composite insulator and activate an indicator and/or a loudspeaking element present on the said clamp to indicate the presence of the internal defect.

16. The apparatus according to claim 7, wherein means to transmit and visualize the distribution of the radial electric field component values comprises a wireless emitter circuit connected to the microcontroller and a wireless receiver connected to an android apparatus to visualize the distribution of the radial electric field component around the core of said composite insulator using a specific visualization program.